Energy and Chemical Dynamics
CHE-5042-2
Learning Guide
CHE-5042-2
LEARNING GUIDE
Gases
Chemical Reactions 1: Energy and Chemical Dynamics Chemical Reactions 2: Equilibrium and Oxidation–Reduction
The three Learning Guides are accompanied by the workbook, Experimental Activities of Chemistry, which covers the “experimental method” component of the program.
scolaires du Québec.
Project Coordinator: Jean-Simon Labrecque (SOFAD) Project Coordinator: Mireille Moisan (first edition) Coordinator: Céline Tremblay (FormaScience)
Author: André Blondin
Illustrators: Gail Weil Brenner (GWB)
Jean-Philippe Morin (JPM)
Content Revisors: Céline Tremblay (FormaScience) (French Version) Stéphanie Belhumeur (English Version)
Layout: I. D. Graphique inc. (Daniel Rémy)
Translator: Claudia de Fulviis
Linguistic Revisor: Patricia Fillmore
Proofreader: Gabriel Kabis
First Edition: November 2000
September 2008
forbidden without the written permission of a duly authorized representative of the Société de formation à distance des commissions scolaires du Québec.
Legal Deposit – 2000
Bibliothèque et Archives nationales du Québec Bibliothèque et Archives Canada
ISBN 978-2-89493-192-9
GENERAL INTRODUCTION
OVERVIEW ... 0.10 HOW TO USE THIS LEARNING GUIDE ... 0.10 Learning Activities ... 0.11 Exercises ... 0.11 Self-evaluation Test ... 0.12 Appendices ... 0.12 Materials ... 0.12 CERTIFICATION ... 0.13 INFORMATION FOR DISTANCE EDUCATION STUDENTS ... 0.13 Work Pace ... 0.13 Your Tutor ... 0.13 Homework Assignments ... 0.14 CHEMICAL REACTIONS 1: ENERGY AND CHEMICAL DYNAMICS ... 0.15
CHAPTER 1 – HEAT: ENERGY IN MOTION ... 1.1 1.1 ENERGY ... 1.3 Forms of Energy ... 1.4 Chemical Energy in Action ... 1.6 Potential Energy ... 1.7 Energy Conversions ... 1.8 Conservation of Energy ... 1.13 1.2 HEAT ... 1.15 Kinetic Molecular Model of Matter ... 1.16 Thermometers and Heat Transfers ... 1.21 Experimental Activity 1: Heat Transfers ... 1.22 Mercury Thermometers ... 1.23 Thermal Equilibrium ... 1.26 Temperature Scales ... 1.26 The Mechanical Equivalent of Heat ... 1.29 Heat and Thermal Energy ... 1.30 1.3 HEAT EXCHANGES ... 1.33 Experimental Activity 2: Final Temperature of a Mixture ... 1.33 Specific Heat Capacity ... 1.34 Sea Breezes and Land Breezes ... 1.36 Heat Exchange Equation ... 1.37 Calorimeters ... 1.38 Applications ... 1.39
This is a preview of:
- the introduction; and - the first chapter.
Boiling ... 1.48 1.4 TECHNICAL APPLICATIONS ... 1.51 The Bread Oven ... 1.51 Re-entering the Atmopshere ... 1.52 Geysers ... 1.52 Key Words in This Chapter ... 1.54 Summary ... 1.54 Review Exercises ... 1.56
CHAPTER 2 – DISSOLUTION: AN ENERGY PHENOMENON ... 2.1 2.1 MIXTURES AND AQUEOUS SOLUTIONS ... 2.3 Mixtures ... 2.3 The Water Molecule ... 2.5 Capillary Rise ... 2.9 2.2 DISSOLUTION AND SOLUBILITY ... 2.10 Molecular Dissolution ... 2.10 Ionic Dissolution ... 2.12 Hydration ... 2.14 Salt Waters ... 2.16 Electrolytes ... 2.16 Solubility and Precipitation ... 2.18 Precipitation ... 2.20 2.3 DISSOLUTION: AN ENERGY PHENOMENON ... 2.23 The Process of Dissolution ... 2.25 Molar Heat of Solution ... 2.28 Experimental Activity 3: Molar Heat of Solution ... 2.30 2.4 SOLUTIONS IN EVERYDAY LIFE ... 2.31 Gases in Aqueous Solutions ... 2.31 The Senses of Taste and Smell ... 2.31 Pharmaceuticals, Beauty Products and Per fumes ... 2.33 Other Solvents ... 2.34 Key Words in This Chapter ... 2.35 Summary ... 2.35 Review Exercises ... 2.37
CHAPTER 3 – CHEMICAL REACTIONS AND ENERGY ... 3.1 3.1 HEAT OF REACTION ... 3.3 Definition and Convention ... 3.4 Measuring the Heat of Reaction ... 3.7 Change in Enthalpy ... 3.9
3.2 COMBUSTION REACTIONS ... 3.18 Rapid Combustion and Slow Combustion ... 3.18 A Little Bit of History ... 3.21 Fossil Fuels: A Useful Commodity ... 3.24 Industrialization and Social Changes ... 3.27 A Technical Application ... 3.29 Fossil Fuels: The Drawbacks ... 3.31 Carbon Dioxide (CO2) ... 3.31 Carbon Monoxide (CO) ... 3.34 Nitrogen Oxides and Sulphur Dioxide ... 3.37 A Few Possible Solutions ... 3.37 3.3 HESS’S LAW ... 3.39 Experimental Activity 4: Hess’s Law ... 3.41 Summation of Heats of Reaction ... 3.41 Application of Hess’s Law ... 3.44 Hess’s Law and Chemical Bonds ... 3.48 Key Words in This Chapter ... 3.53 Summary ... 3.53 Review Exercises ... 3.55
CHAPTER 4 – THE RATE OF CHEMICAL REACTIONS... 4.1 4.1 PROGRESS OF A CHEMICAL REACTION ... 4.3 Rate of a Reaction ... 4.4 Using Graphs to Represent Reaction Rates ... 4.8 Reaction Rate As a Function of Time ... 4.10 4.2 DETERMINING FACTORS ... 4.20 Experimental Activity 5: Rate of a Chemical Reaction ... 4.21 Nature of Reactants ... 4.21 Concentration of Reactants ... 4.23 Pressure and Gaseous Reactants ... 4.24 Temperature ... 4.25 Sur face Area ... 4.27 Catalysts ... 4.30 An Overview ... 4.34 Preparation of a Reference Solution ... 4.34 Effect of Concentration on the Rate of Reaction ... 4.34 Effect of Temperature on the Rate of Reaction ... 4.35 Key Words in This Chapter ... 4.37 Summary ... 4.37 Review Exercises ... 4.39
Kinetic Energy of Molecules ... 5.4 A Nornal Distribution for Large Populations ... 5.6 Maxwell-Boltzmann Distribution ... 5.10 5.2 CONSEQUENCES OF THE MAXWELL-BOLTZMANN DISTRIBUTION ... 5.16 Temperature ... 5.16 Threshold Energy and Activation Energy ... 5.18 Other Applications ... 5.24 Kinetic Energy in Our Lives ... 5.24 5.3 ENERGY AND THE MECHANISM OF REACTION ... 5.27 Reaction Mechanism ... 5.27 Catalysts ... 5.32 Spontaneity of Reactions ... 5.36 Key Words in This Chapter ... 5.40 Summary ... 5.40 Review Exercises ... 5.42
CONCLUSION
SELF-EVALUATION TEST ... C.5 ANSWER KEY ... C.13 CHAPTER 1 – Heat: Energy in Motion ... C.13 CHAPTER 2 – Dissolution: an Energy Phenomenon ... C.27 CHAPTER 3 – Chemical Reactions and Energy ... C.36 CHAPTER 4 – The Rate of Chemical Reactions ... C.49 CHAPTER 5 – Energy and the Rate of Reaction ... C.61 ANSWER KEY TO THE SELF-EVALUATION TEST ... C.68 APPENDIX A – The International System of Units (SI) ... C.73 APPENDIX B – Mathematical Prerequisites ... C.75 Ratios and Proportions ... C.75 Formulas ... C.76 APPENDIX C – Chemical Prerequisites ... C.78 Balancing Equations ... C.78 Calculating Molar Mass ... C.81 APPENDIX D – Table of Figures ... C.83 BIBLIOGRAPHY ... C.87 GLOSSARY ... C.89 INDEX ... C.97
GENERAL INTRODUCTION
OVERVIEW
Welcome to the course entitled Chemical Reactions 1: Energy and Chemical Dynamics, which is part of the Secondary V Chemistry program. This program comprises the following three courses:
CHE-5041-2 Gases
CHE-5042-2 Chemical Reactions 1: Energy and Chemical Dynamics
CHE-5043-2 Chemical Reactions 2: Equilibrium and Oxidation—Reduction The three main components of the Chemistry program are related content, the experimental method and the history-technology-society perspective. Whereas the experimental method is developed in the workbook Experimental Activities of Chemistry, the related content and the history-technology-society perspective are covered in the three Learning Guides accompanying the three courses which must be taken in sequential order.
Chemical Reactions 1: Energy and Chemical Dynamicsis the second in the set of three Learning Guides. It is divided into five chapters corresponding to the five terminal objectives of the program.1This Guide is to be used in conjunction with the workbook Experimental Activities of Chemistry. You will find references to the appropriate sections of the Workbook throughout the Guide.
The course Chemical Reactions 1: Energy and Chemical Dynamicswill help you gain a better understanding of chemical dynamics and the energy transfers involved in chemical reactions, together with the related technical applications, social changes and environmental consequences.
HOW TO USE THIS LEARNING GUIDE
This Guide is the main work tool for this course and has been designed to meet the needs of adult students enrolled in individualized learning programs, or distance education courses.
Each chapter covers a certain number of themes, using explanations, tables, illustrations and exercises designed to help you to master the different program objectives. A list of key words, a summary and review exercises are included at the end of each chapter.
1. The terminal objectives and associated objectives are listed at the beginning of each chapter.
The conclusion contains a summary covering all the courses in the program along with a self-evaluation test. It also includes an Answer Key for the self-evaluation test, for the exercises in each chapter and for the review exercises. A glossary with definitions of the key words, a bibliography, appendices and an index are also provided in the conclusion. You may wish to consult the books and publications in the bibliography for further information on the topics covered in this course.
Learning Activities
The Guide contains theoretical sections as well as practical activities in the form of exercises. The exercises come with an Answer Key.
Start by skimming through each part of the Guide to familiarize yourself with the content and the main headings. Then read the theory carefully:
– Highlight the important points.
– Make notes in the margins.
– Look up new words in the dictionary.
– Summarize important passages in your own words, in your notebook.
– Study the diagrams carefully.
– Write down questions relating to ideas you don’t understand.
Exercises
The exercises come with an Answer Key, which is located in the coloured section at the end of the Guide.
• Do all the exercises.
• Read the instructions and questions carefully before writing your answers.
• Do all the exercises to the best of your ability without looking at the Answer Key.
Reread the questions and your answers, and revise your answers, if necessary. Then check your answers against the Answer Key and try to understand any mistakes you made.
• Complete each chapter before doing the corresponding review exercises. Doing these exercises without referring to the Guide is a good way to prepare for the final examination.
Self-evaluation Test
The self-evaluation test is a step that prepares you for the final evaluation. You must complete your study of the course before attempting to do it. Reread your notebook and the definitions of the key words in the chapters. Make sure you understand how they relate to the course objectives listed at the beginning of each chapter. Then do the self-evaluation test without referring to the main body of the Guide or the Answer Key. Compare your answers with those in the Answer Key and review any areas you had difficulty with.
Appendices
The appendices contain a review of some concepts you should be familiar with before beginning the course. The complete list of appendices appears in the table of contents.
Materials
Have all the materials you will need close at hand:
• Learning material: this Guide and a notebook in which you will summarize important concepts relating to the objectives (listed in the introduction of each chapter). You will also need to use your periodic table and the workbook Experimental Activities of Chemistry.
• Reference material: a dictionary.
• Miscellaneous material: a calculator, a pencil for writing your answers and notes in your Guide, a coloured pen for correcting your answers, a highlighter (or a pale- coloured felt pen) to highlight important ideas, a ruler, an eraser, etc.
CERTIFICATION
To earn credits for this course, you must obtain at least 60% on the final examination which will be held in an adult education centre.
The evaluation for the course Chemical Reactions 1: Energy and Chemical Dynamics is divided into two separate parts.
Part Iconsists of a two-hour written examinationmade up of multiple-choice, short- answer and essay-type questions. This part is worth 75%of your final mark and deals with the objectives covered in this Guide. You may use a calculator.
Part II is designed exclusively to evaluate the experimental method. It will be held in the laboratory during one 90-minute session. It is worth 25%of your final mark and deals with the course objectives covered in Section B of Experimental Activities of Chemistry.
INFORMATION FOR DISTANCE EDUCATION STUDENTS
Work Pace
Here are some tips for organizing your work:
• Draw up a study timetable that takes into account your personality and needs, as well as your family, work and other obligations.
• Try to study a few hours each week. You should break up your study time into several one- or two-hour sessions.
• Do your best to stick to your study timetable.
Your Tutor
Your tutor is the person who will give you any help you need throughout this course.
He or she will answer your questions and correct and comment on your homework assignments.
Don’t hesitate to contact your tutor if you are having difficulty with the theory or the exercises, or if you need some words of encouragement to help you get through this course. Write down your questions and get in touch with your tutor during his or
her available hours. The letter included with this Guide or that you will receive shortly tells you when and how to contact your tutor.
Your tutor will assist you in your work and provide you with the advice, constructive criticism, and support that will help you succeed in this course.
Homework Assignments
In this course, you will have to do three homework assignments: the first after completing Chapter 2, the second after completing Chapter 4, and the third after completing Chapter 5. Each homework assignment also contains questions on the experimental method you studied in Experimental Activities of Chemistry.
These assignments will show your tutor whether you understand the subject matter and are ready to go on to the next part of the course. If your tutor feels you are not ready to move on, he or she will indicate this on your homework assignment, providing comments and suggestions to help you get back on track. It is important that you read these corrections and comments carefully.
The homework assignments are similar to the examination. Since the exam will be supervised and you will not be able to use your course notes, the best way to prepare for it is to do your homework assignments without referring to the Learning Guide and to take note of your tutor’s corrections so that you can make any necessary adjustments.
Remember not to send in the next assignment until you have received the corrections for the previous one.
CHEMICAL REACTIONS 1: ENERGY AND CHEMICAL DYNAMICS
Forest fires, sudden frost, melting snow, respiration, the shuttle launch and archery all involve changes that have one thing in common—energy. While we cannot observe energy directly, we perceive it as light, heat, motion and noise, among others things.
Energy takes different forms and is named according to the way it manifests itself.
For instance, we speak of mechanical energy to describe the thrust, firing or rotation of a mechanical part, of kinetic energy for the movement of molecules or other bodies, of radiant energy for light, and of potential energy for all forms of stored energy.
Associated with hot and cold sensations, heat is transferred between systems when they are at different temperatures. We might say that heat is a mode of energy transfer.
Heat and changes in temperature are therefore closely linked. Heat can either be absorbed or released during a chemical reaction. For instance, energy is released when we burn natural gas to heat our homes, whereas enormous amounts of energy are consumed in aluminum production.
The last topic covered in the course entitled Gaseswas the energy balance of chemical reactions. The second course, Chemical Reactions 1: Energy and Chemical Dynamics, examines energy transfers from a broader perspective as well as the rate of reactions and the factors that affect it. The third course, Chemical Reactions 2: Equilibrium and Oxidation—Reduction, provides an in-depth study of two categories of chemical reactions.
Chapter 1 of this Guide analyzes energy transfers between two liquids that are mixed together. Energy transfers depend on the amounts and types of liquids involved, and can be observed through changes in temperature. The first chapter also reviews the heating curve of a substance and gives a more detailed explanation for rises in temperature and phase changes.
Chapter 2 deals with dissolution reactions, and describes them macroscopically, that is, according to what can be observed with the naked eye. They are then examined at the molecular level, using the kinetic molecular model of matter, which is an expanded version of the model based on the kinetic theory of gases. You will learn why water is the most common solvent in nature and the solvent of choice in the laboratory. This chapter focuses on the energy involved in dissolutions.
Chapter 3 deals with the energy associated with chemical reactions. It also reviews the energy balance involved in the dissociation and formation of bonds. In this course,
however, the subject of energy balance is examined in greater detail. The energy diagrams provide more information about the progress of reactions, and the sum of the energies involved in the steps of a reaction yields the overall energy of the reaction.
This analysis will be based on combustion reactions.
Chapters 4 and 5 examine kinetic chemistry or the rate of chemical reactions. This rate depends on the nature of the reactants, their concentration, their surface area and on environmental conditions such as temperature, pressure and acidity. Catalysts speed up reactions, but do not alter the nature of the products. Speed is part and parcel of any process involving energy. The fifth and last chapter in this course examines the relationship between energy, reaction rate and the various factors that affect this rate. It also introduces the content of the next course.
As in the first two Guides, a table of contents diagram at the beginning of each chapter shows you how the chapter fits into the course as a whole. The content of the chapter you are about to begin is in bold type and in larger characters, whereas the content of completed chapters is in italics. For example, the table of contents diagram for Chapter 2 is reproduced below. The section for Chapter 2 is in bold type and the content of Chapter 1 is in italics and smaller type. You will find this diagram a very useful tool as you go through the course.
Good luck!
1. Heat: Energy in Motion Energy: forms, conser vation Heat: model, thermometer Heat exchanges Energy in phase changes Applications
2. Dissolution: An Energy Phenomenon Mixtures, solutions Molecular and ionic dissolution Solubility, precipitation Heat of solution Solutions in everyday life
5. Energy and the Rate of Reaction Maxwell-Boltzmann distribution Energy threshold and activation energy Reaction mechanisms
Spontaneity of reactions
CHEMICAL REACTIONS 1:
Ene rgy a
nd Chemical Dyna mics
4. Rate of Chemical Reactions Rate of reaction
Graphs
Determining factors
3. Chemical Reactions and Energy Heat of reaction Rapid and slow combustion Fossil fuels Hess’s law
CHAPTER 1
HEAT: ENERGY IN MOTION
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Terminal Objective 1
To analyze the energy transfers that occur in phase changes and mixtures of substances at different temperatures.
Intermediate Objectives
1.1 To recognize the different forms of energy acting in the phenomena obser ved in their environment.
1.2 To associate a macroscopic phenomenon with corresponding changes occurring at the atomic or molecular level.
1.3 To describe a heat transfer in terms of kinetic energy and the variation in temperature.
1.4 To classify physical and chemical phenomena according to whether they represent endothermic or exothermic reactions, on the basis of obser vations.
1.5 To determine, through experimentation, the factors that influence the final temperature of a mixture.
1.6 To establish relationships between the definition of specific heat capacity and its units of measurement.
1.7 To describe the energy transfers produced during phase changes of a pure substance.
1.8 To describe briefly how Joule established a relationship between heat and mechanical energy.
1.9 To give examples of energy conversions involving heat.
1.10 To solve problems related to energy transfers that occur during phase changes and mixtures of substances at different temperatures.
1. Heat: Energy in Motion Energy: forms, conservation Heat: model, thermometer Heat exchanges Energy in phase changes Applications
2. Dissolution: An Energy Phenomenon Mixtures, solutions Molecular and ionic dissolution Solubility, precipitation Heat of solution Solutions in ever yday life
5. Energy and the Rate of Reaction Maxwell-Boltzmann distribution Energy threshold and activation energy Reaction mechanisms
Spontaneity of reactions
CHEMICALREACTIONS 1:
Ene rgy a
nd Chemical Dyna mics
4. Rate of Chemical Reactions Rate of reaction
Graphs
Determining factors
3. Chemical Reactions and Energy Heat of reaction Rapid and slow combustion Fossil fuels Hess’s law
Have you ever wondered about all the different ways in which we use the word
“energy”? You are no doubt aware that this term has several meanings. For instance, in the expression “My child is full of energy,” it refers to the ability to move and do things; however, when we say that “Solar energy powers satellites that orbit the earth,”
it refers to light; and in “Water heaters consume a lot of energy,” it refers to the source—
more often than not electric—which heats the water.
In this chapter, we will review the various forms of energy and pay particular attention to heat. We will explore the concept of temperature in greater detail, explain heat transfers at the molecular level and represent them by means of a mathematical relation. We will then examine the energy associated with phase changes more closely and end with an overview of a few applications of heat energy.
1.1 ENERGY
Human beings have used what we now call energysince the dawn of time and have tried to harness it in different ways. At first, humans used energy simply to eat, keep warm and bask in the Sun’s light. They then went on to “tame” fire and invent the wheel, levers and other, more complex devices, such as windmills and watermills.
More recently, humans have mastered the use of certain mixtures of explosive substances: this has contributed to the development of firearms, combustion engines and dynamite, used to dig mines and construct roads through mountains.
Humans are still actively involved in the quest for technical advances to meet their ever-changing needs—the construction of completely automated plants and more effective power-generating stations are a few examples. Others include launching satellites into orbit around the Earth, developing the Moon’s mineral resources and exploring the planet Mars, projects which have been undertaken by NASA1 and other space agencies worldwide.
Although the word is used frequently, “energy” remains a difficult concept to grasp.
Strictly speaking, energy is defined as the capacity for doing work, or the capacity for producing an effect such as movement or light, among other things.
1. National Aeronautics and Space Administration: the organization that oversees the entire American space program.
In Canada, the Canadian Space Agency fulfills the same function.
Energy cannot be observed directly. Rather, we observe its manifestations, the main ones being light, heat and motion. Our eyes take in light in a very precise manner and very small corpuscles2 in the skin detect sensations of warmth or cold. And of course, our eyes, ears and muscles can detect motion.
Exercise 1.1
Complete the following table by naming sources of energy that are detected as light, heat or motion. Write two sources for each form of energy indicated.
Form Sources
Light Le soleil, une ampoule électrique allumée, une flamme, etc.
nnn
Heat L’élément électrique d’un rond de poêle allumé, d’un grille-pain, d’une bouilloire, d’un radiateur-plinthe ou d’une ampoule électrique ; le compost en décomposition, une flamme, etc.
Motion L’énergie d’un trampoline compressé, d’un arc bandé, d’un élan de bâton de golf ou de baseball ; une voiture, un vélo ou un train en mouvement, un enfant qui cour t, la rotation des aiguilles d’une horloge, etc.
FORMS OF ENERGY
Forms of energy are usually named after their source or the process of transformation that produces it. For instance, mechanical energyis produced by the movement of mechanical parts in machinery and engines which cut, strike or launch objects; solar energy includes visible light, ultraviolet rays and the heat released by the Sun;
gravitational energy is associated with the force of gravity which causes objects to fall to the ground; nuclear energy is produced by the fusion or fission of atomic nuclei and fuels nuclear power stations and aircraft carriers; magnetic energy causes two magnets to repel each other and aligns the electron stream that produces the image on a television screen; electrical energy lights our homes and powers engines of all kinds; wind energy refers to the wind’s capacity to turn the blades of a mill;
hydraulic energy is provided by rivers, waterfalls and tides; muscular energy refers to a muscle’s capacity to move an arm or a leg; electromagnetic energy includes visible light, X-rays, microwaves and other types of electromagnetic radiation.
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2. Ruffini’s corpusclesdetect sensations of warmth and Krause’s corpusclesdetect sensations of cold.
More specifically, kinetic energyis the name given to the energy associated with the straight-line or spinning motion of an object. By contrast, heat, which is also named calorific energy, involves a transfer of energy between two systems.
In subsequent chapters, we will focus on a form of energy called chemical energy, so named because it is associated with chemical reactions that, as you may recall, change the nature of substances. For instance, the combustionof propane gas releases heat that comes from the chemical energy contained in the propane and oxygen. Thus, the bond energy3discussed at the end of the last course is chemical energy.
Exercise 1.2
State the form of energy (wind energy, nuclear energy or hydroelectric energy) that best corresponds to the descriptions given below.
The electrical energy flowing out of a generator that is driven by a rotating turbine powered by a water fall.
The light, heat and kinetic energy released by the fission of heavy and unstable atomic nuclei such as those of uranium 235.
The mechanical energy generated by the movement of air across the Ear th’s sur face.
3. Lalancette, Pauline and M. Lamoureux, Gases(Chemistry, Secondary V), Chapter 1, Learning Guide produced by SOFAD.
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Benjamin Franklin (1706-1790)4
Born in the Unites States and the fifteenth child in a family of English immigrants, Benjamin Franklin was a self-educated and ver y talented man. In 1752, Franklin per formed his famous experiment that involved going out during a thunderstorm and flying a kite with a metal key attached to it. He wanted to prove that lightning was similar to electricity. Without knowing it, he was taking an enormous risk. Franklin is an excellent example of the long line of curious-minded, ingenious people who
were determined to explore nature.
Franklin invented the stove that bears his name, the Franklin stove, the lightning conductor (a metal rod placed on buildings to protect them against lightning) and bifocal lenses. He also developed a theor y to explain the absorption of heat. A master of many trades, Franklin was elected to Pennsylvania’s legislative assembly and helped draft the American Declaration of Independence, which he signed at the age of 70!
Chemical Energy in Action
Chemical energy is behind the powerful thrust needed to propel space rockets into orbit. In American space shuttles, the central external tank fuels three engines placed in a triangular arrangement at the back of the shuttle. Chemical energy is released by the intense reaction between hydrogen and oxygen.5In order to save space, the two gases are first liquefied and stored separately in refrigerated tanks. The formation of water vapor occurs, and the acceleration produced by the chemical energy that is released causes it to be forcefully expelled from the rocket. Guided by the nozzles, the expelled vapour propels the rocket. The shuttle’s three main engines provide a thrust of more than two million newtons,6 or, in other words, the thrust needed to propel the equivalent of 165 cars into orbit! At the outset, the shuttle weighs about 2 500 tonnes. The white trail behind the rocket as it rises into the sky results from the condensation of water vapour, which cools upon contact with air.
The general combustion reaction for hydrogen is:
2 H2 + O2 → 2 H2O + energy
4. A light bulb indicates additional information: this information is not part of the course as such and will not be covered on the final examination.
5. “Propellant” is the term used to designate one or more substances that react chemically to produce the energy required to propel rockets into space. Hydrogen and oxygen constitute the main propellants for space shuttles.
6. The newton is the unit of force in the International System of Units (SI).
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a) Hydrogen and oxygen form the space shuttle’s liquid propellant which is contained in compartments within the central external tank. The side rockets contain the solid propellant.
b) The central rocket engine is composed of tanks and fuel pumps.
H2and O2combustion takes place in the three engines attached to the back of the shuttle.
Potential Energy
Regardless of its form, energy may be stored in such a way that it can be recovered at a later time. Any form of stored energy that is ready for use is called potential energy. We will now look at three examples that will help us better understand this concept: the propellant in a rocket, a match and an electric battery.
Let’s consider the two liquids (oxygen and hydrogen) stored in the refrigerated tanks of a rocket that is about to be launched. Until the rocket has been launched, the liquids remain in their respective chambers, the combustion reaction does not occur and no energy is released. Upon ignition, however, the two liquids combine in the combustion chamber, the reaction takes place and the energy released propels the rocket into space. Prior to the reaction between the two liquids, the potential chemical energy was stored in the substances, ready to be released.
External tank
Oxygen tank O2(l)
H2(l)
H2 + O2 H2 O
+ Energy
Hydrogen tank
Combustion chamber Nozzles Solid
propellant rocket
Figure 1.1 - Liquid propellant rocket
Fuel pump
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a) b)
}
A match in a matchbox will not light up by itself. By striking the match, we release the energy contained in the substances that cover the match head. More precisely, the potential chemical energy which is stored in the substances that make up the match (sulphur, potassium chlorate, etc.) and in the oxygen in the air, is activated as soon as the match is rubbed against a rough, specially prepared surface. We then perceive heat and light.
When an electric battery is connected to a circuit, the substances in the battery will react chemically to produce the energy needed to light a lamp or drive an engine.
The substances cannot react if the battery is not connected. The battery therefore contains potential chemical energy.
All forms of energy can be described as potential, provided the energy is not activated, that is, as long as it cannot be perceived.
Exercise 1.3
Briefly explain in your own words why we can say:
a) that a stone on the edge of a ravine has potential energy.
b) that a drawn arrow has potential energy.
ENERGY CONVERSIONS
The potential energy stored in a battery can be released in different forms depending on whether the battery is used to power a flashlight, the motor of a toy car or any other device. We speak of a conversion when energy changes form, that is, when it is converted or changed from one form to another.
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Let’s consider an archer who shoots an arrow into the sky at a 45° angle. Between the moment the archer takes aim and releases the arrow and the moment the arrow hits the target, energy is converted from one form to another several times.
Figure 1.2 - An arrow in flight
The archer uses his muscles to release the arrow at a 45° angle above the horizontal plane.
Muscular energy is transferred from the archer to the arrow. The arrow curves downwards and strikes a target about 30 m away. Energy is then transferred from the arrow to the target;
a part of this energy serves to drive the arrow into the wood and the rest is converted into heat.
Let’s look at this example more closely. Read on and try to imagine what happens when the arrow is released. You will see that throughout the arrow’s flight, energy, whether stored or active, is converted from one form to another.
By inhaling air and digesting his food, the archer provides his cells with the sugar and oxygen they need to release the chemical energy that will allow him to contract his muscles (muscular energy) and draw the bow (the bow stores potential mechanical energy). The slow combustion of sugar in the cells converts the potential energy stored in the sugar and oxygen into muscular energy that can be used to draw the bow.
When the arrow is released, the bow straightens out (mechanical energy of rotation) and pulls on the bowstring that then propels the arrow upwards at a 45° angle (kinetic energy). As the arrow rises into the air, the energy in the arrow serves to counteract the gravitational force that is pulling it towards the ground. As it gains height, however, the arrow stores potential gravitational energy. At the highest point along its
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trajectory,7 the arrow changes direction and gains speed as it starts to fall (it acquires kinetic energy). In short, the energy stored in the bow has been converted into the kinetic energy of the arrow, which in turn is converted into potential gravitational energy as the arrow rises and then converted again into kinetic energy as the arrow falls.
The arrow eventually hits and becomes embedded in its target (mechanical energy).
Immediately after impact, the arrow’s metallic tip will feel slightly warm (heat). In this case, the kinetic energy of the arrow has been converted once again, but this time into heat and work, since the arrow has become lodged in the target.
You no doubt noticed in the description you have just read that energy takes a different form with each conversion. For discussion purposes, imagine that in each case a quantity of energy is transferred from a source to a receptor, which in turn becomes the source for the next energy conversion. In this way, we have a continuous chain of energy conversions.
Let’s now go back to our example of an arrow in flight. This time, however, we will use the concepts of source and receptor. This exercise will allow you to become familiar with these two terms used often in this chapter. Figure 1.3 illustrates the description of the sequence of energy conversions.
Food is the primary source of energy. The potential energy that it contains is transferred to the first energy receptor, namely, the archer’s muscles that contract. These then become the source of power needed to draw the bow, which is the new receptor. The drawn bow then becomes the source of the motion of the bowstring, which in turn becomes the new receptor. The bowstring then propels the arrow, which receives the bowstring’s energy. The arrow uses up this energy by gaining altitude. At the same time, however, it stores potential gravitational energy. This energy is recovered when the arrow falls towards the ground. Upon impact, the target and the materials that make up the arrow absorb the arrow’s energy. The partial perforation of the target and the heating up of the arrow’s tip are proof that they have become the new receptors.
In short, at all moments during the arrow’s path, we have been able to identify a source of energy and at least one receptor which absorbs the source’s energy.
7. An arrow’s path always follows a more or less elongated parabola, depending on the arrow’s initial velocity and angle. The vertex is the highest point on the parabola.
Figure 1.3 - An unbroken chain of energy conversions
In the second step in the energy chain, the archer’s (source) muscular energy is transferred to the bow (receptor) in the form of potential mechanical energy. In turn, the bow becomes a source, and its energy is transferred to the arrow (receptor). Thus, each person or object participating in the action is both a receptor and a source of energy. The target is the last receptor in the sequence.
Let’s consider a second example that is similar to the arrow in flight. The figure below illustrates a shotgun being fired.
Figure 1.4 - Trigger mechanism of a shotgun
Detail of a shotgun’s trigger mechanism
When the gunman presses the trigger, the shotgun’s firing pin strikes the detonator (cap) of the cartridge. The heat causes the gunpowder to explode. The shots are forced into the barrel and then
through the air, before reaching the target.
Chemical energy (foods)
Muscular energy
Potential mechanical
energy (bow)
Mechanical energy of
rotation (bowstring)
Kinetic energy (arrow rises)
Potential gravitational energy (arrow
on top)
Kinetic energy (arrow falls)
Mechanical energy + heat (arrow,
target)
→ → → → → → →
GWB
Firing pin
Shots Detonator
Powder
Gun barrel
Firing chamber Spring release
mechanism
Firing pin spring
IDG
GWB
Exercise 1.4
The following six steps describe the firing of shots with a shotgun. The steps are not in chronological order.
• Released gases force the shots to travel down the barrel of the shotgun.
• A finger presses the trigger.
• The firing pin hits the detonator of the cartridge.
• The shots travel through the air and hit the target (a bottle).
• The bottle shatters and the pieces are projected into different directions.
• The heat released by the impact initiates combustion of the gunpowder in the cartridge case.
Complete the following table by answering questions a) and b). The first line in the table has been completed for you.
a) Place the steps in the order in which they occur.
b) For each step, describe the energy conversion that takes place and identify the energy source and receptor.
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CONSERVATION OF ENERGY
Just as it is difficult to believe that a rabbit actually disappears inside a magician’s hat (just because we cannot see it does not mean that it no longer exists!), it is equally difficult to believe that energy can be destroyed. Rather, we observe, as have the leading scientists of our time, that energy is converted into work or other forms without losing its force. This principle, which was implicit in the examples we have looked at so far,
No. Step
1. A finger presses the trigger.
2. Le percuteur du fusil frappe le détonateur de la balle.
3. La chaleur dégagée par l’impact amorce la combustion de la poudre à canon dans la douille.
4. Les gaz libérés poussent les plombs dans le canon.
5. Les plombs voyagent dans l’air et frappent la cible.
6. La bouteille éclate et les morceaux sont projetés dans différentes directions.
Conversion Muscular energy in the finger →mechanical energy in the release mechanism Énergie potentielle du ressor t → énergie mécanique + chaleur Énergie potentielle chimique des substances →chaleur + énergie cinétique des molécules de gaz Énergie cinétique des molécules →énergie cinétique des plombs de chasse
Énergie cinétique des plombs →travail d’impact sur la bouteille + chaleur Énergie cinétique des morceaux de bouteille → impact sur les sur faces environnantes + chaleur
Source Muscles in the finger
Mécanisme de détente (ressor t)
Détonateur
Gaz libérés
Plombs
Morceaux de bouteille
Receptor Release mechanism (spring)
Détonateur de la balle
Poudre
Plombs
Bouteille
Sur faces environnantes
is called the law of conservation of energy. It states that in a closed system, the total amount of energy remains constant, regardless of the conversions it undergoes.
In other words, the total amount of energy is the same (it is conserved) before and after the conversion.
Law of conservation of energy:
Total energy before conversion = Total energy after conversion
Remember that a system is a collection of objects that form a whole. In the example of the bow and arrow, the archer, his bow, the arrow, the ground, the ambient air and the target are all part of a system. The description implied that when the archer contracted his muscles, all the chemical energy released by the cells was transferred to his muscles, which in turn transferred all of this energy to the bow, arrow and target. The same assumption applies to each energy conversion in the chain. We therefore intuitively applied the law of conservation of energy. As we mentioned above, this law states that in a closed system the total amount of energy remains constant.
However, because we wanted to keep the description simple, we omitted certain details.
For example, when the arrow falls, friction with the air produces a small amount of heat that is transferred to the air. The rest of the arrow’s potential energy is converted into kinetic energy. The law of conservation of energy still applies, but if we wanted to be more precise, we would write:
Potential gravitational energy = Kinetic energy (arrow) + Heat (air)
Energy is conserved, since the two receptors, namely, the arrow and the air, absorb all the potential energy. In fact, in real life situations, several receptors often share the energy, even though only one of the receptors is relevant to the action being studied.
Exercise 1.5
Jehane Benoît describes the preparation of hard-boiled eggs as follows: “Place the eggs in a saucepan and cover them with cold water. Place a lid on the pan and bring the water to a boil over medium heat. As soon as the water starts to boil, remove the saucepan from the source of heat and wait from 3 to 10 minutes before removing the eggs from the water, depending on whether you like your eggs soft-boiled or hard- boiled.”8
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8. Benoît, Jehane, La nouvelle encyclopédie de la cuisine(Montréal: Les messageries du Saint-Laurent Ltée, 1971), p. 283. (translation)
Using this recipe, illustrate the principle of conservation of energy by either writing a description or drawing a diagram. Keep in mind all the receptors, including those that are not directly involved in cooking the eggs.
1.2 HEAT
Heat, which is also called calorific energy, is one of the most common manifestations of energy. Examples of heat are abundant: the Sun heats beaches, the electromagnetic energy of microwaves heats and cooks our food, and so on. You may have already noticed that the head of a nail is warm after it has been struck vigorously with a hammer. If not, try it—but watch your fingers!
Now that you have felt this heat, how would you explain it? In other words, how does the hammer’s kinetic energy heat the nail? Do you have any idea? Write it down in the next exercise. We will come back to this exercise later and you will have a chance to complete your answer then, if necessary.
Exercise 1.6
Try touching the head of a nail that you have just hit forcefully with a hammer. How can you explain the heat emanating from the nail? In other words, what happened inside the nail? Give an explanation based on what you currently know about energy.
In order to provide an adequate answer to the preceding exercise, we have to try to imagine what happens inside the nail. But first we will review the model of matter that we developed in the previous course.9
KINETIC MOLECULAR MODEL OF MATTER
First, let’s look again at the kinetic theory of gases which, as you may recall, describes the model of an ideal gas. We will then apply this theory to all matter, whether solid, liquid or gaseous. This model will help us form a mental picture of the fundamental structure of matter, given that it cannot be observed directly.
At the molecular level, the structure of matter is invisible and cannot be observed with an ordinary microscope. The purpose of the model is therefore to provide a microscopic representation of matter. The model suggests images and types of behaviours that provide an explanation for those properties of matter we can perceive with our senses and measure with instruments. These observable properties are said to be macroscopic. The term “microscopic” refers to invisible phenomena and the behaviour that the model attempts to describe and explain. By contrast, the term “macroscopic”refers to properties and phenomena that are normally visible to the naked eye. This will be better understood if we consider the macroscopic and microscopic views of a gas confined in a cylinder.
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9. Lalancette, Pauline and M. Lamoureux. Gases(Chemistry, Secondary V), Chapter 1. Learning Guide produced by SOFAD.
Figure 1.5 - Gas confined in a cylinder
a) The gas appears uniform and translucent.
b) The gas is composed of very small particles (atomsor molecules) that are separated from each other by great distances, that move freely in all directions and that do not attract
or repel one another. Note that this drawing is not to scale.
A single millilitre of gas contains billions upon billions of particles.
Figure 1.5 shows a gas confined in a cylinder with an immobile piston in the top. At the molecular level, the model provides the following explanation for what we see:
the gas particles are in motion and collide with the walls of the container and the piston. The numerous collisions between the particles and the underside of the piston prevent it from falling, even though its weight pulls it down.10
The model of an ideal gas, as it was described in the previous course in connection with the kinetic theory of gases, can be summed up by the following hypotheses.
• All gases are composed of very small particles, either atoms or molecules, separated by a vacuum. The distance between the particles is large compared to their size.
• The particles of a gas are independent, that is, they do not attract or repel one another.
a) Macroscopic view b) Microscopic view
10. Bernoulli (1728) held that the pressure exerted by a gas on the walls of its container is due to the billions of collisions between the molecules of the confined gas and the walls (kinetic theory of gases). This view, which confirms Boyle’s law on the compressibility of gases, also accounts for the fact that the temperature of a gas is related to the motion of its particles and therefore to its kinetic energy. From this point onwards, the foundation for the microscopic interpretation of heat had been laid.
• The particles of a gas are in constant motion (translation, rotation and vibration).
They collide regularly with one another or with the walls of the container in which they are confined.
• The average kinetic energy of the particles is a function of the temperature of a gas. An increase in temperature will cause the molecules of the gas to become more agitated. Conversely, a decrease in temperature will cause them to become less agitated.
Matter exists not only in the gaseous state, but also in the liquid and solid states. As a result, the hypotheses outlined above do not provide a satisfactory explanation for the three states of matter at the microscopic level. If the ideal gas model states that gas particles neither attract nor repel one another, how can we explain the solidity of a candy or a stone?
It may be useful to look at an example of a solid to help us answer this question. Try to imagine the structure of a penny at the microscopic level.
Figure 1.6 - A penny
a) Macroscopic view b) Microscopic view (to be completed)
Exercise 1.7
Take a penny and observe it carefully. It is made of copper. Complete Figure 1.6b by drawing a microscopic representation of the penny as you imagine it.
The coin is a solid. It keeps its shape and it is difficult to cut. In your opinion, can the model of the kinetic theory of gases explain these properties? Can your drawing explain them? Let’s take a closer look.
Since the coin keeps its shape, we can conclude that the particles—the atoms of copper—stick together and occupy fixed positions in relation to one another.
According to this description, although the atoms are agitated, their motion is limited to vibration movements. Furthermore, it is difficult to cut the coin, which, on a molecular level, means that it is difficult to separate the atoms. We can conclude from this that an attractive force(cohesive force) is keeping the atoms together, acting somewhat like a glue. This force explains why the atoms remain together and why it is difficult to separate them. We can use the same explanation for other solids since it accounts for the solidity of a stone just as well as that of a grain of sugar.
The model of a solid that we developed in the preceding paragraph is therefore not consistent with all the hypotheses outlined earlier, which let us recall, are valid for gases. The second hypothesis is particularly inexact. However, by reformulating these hypotheses, we can obtain a more general model that describes the behaviour of gases, liquids and solids. Because this model is based on the kinetic theory of gases, we will refer to it as the “kinetic molecular model of matter.” It can be summed up by the following hypotheses.
Kinetic Molecular Model of Matter
• All matter is composed of very small particles (atoms or molecules) separated by a vacuum. In a gas, the particles are far apart from one another, whereas in a liquid and in a solid, they are close to one another.
• There are forces of attraction between the particles of a substance. These forces are negligible in gases but strong in liquids and solids.
• All the particles are in constant motion, be it translation, rotation, vibration, or a combination of the three.
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• The average kinetic energy of the particles of a substance is a function of its temperature. An increase in temperature will cause the particles to become more agitated. Inversely, a decrease in temperature will cause them to become less agitated.
Before you continue, you may find it useful to compare these hypotheses with those given for gases earlier. Most scientists today use this model of matter to explain many common phenomena. You may want to refer to it often to help you understand what cannot be observed by the naked eye. The table below compares the macroscopic properties of gases, liquids and solids and the corresponding microscopic view provided by the model. Take the time to study it carefully.
Figure 1.7 - The three states of matter: properties and model
MACROSCOPIC
GAS LIQUID SOLID
PROPERTIES
Shape Indefinite Indefinite Definite
Volume Indefinite Definite Definite
Compressibility High Negligible Negligible
MODEL
GAS LIQUID SOLID
(microscopic view) Diagram
Distance between molecules Large Ver y small Ver y small Principal types of movements Vibration, rotation Vibration Vibration
and translation and rotation Attractive forces
between the molecules No Yes Yes
Order
No No Yes
Exercise 1.8
Consider again the nail that heats up when it is struck with a hammer (Exercise 1.6).
A nail is made up of atoms of iron.
a) Use the kinetic molecular model of matter to explain the presence of heat in the head of the nail.
b) Compare your answer with the answer you gave in Exercise 1.6.
THERMOMETERS AND HEAT TRANSFERS
The phenomena that involve a change in temperature are numerous. The nail that is hit with a hammer is just one example. The nail heats up (its temperature rises) because the hammer transferred a part of its energy to the nail. Consider a second example: a drop of alcohol on your skin produces a sensation of coolness as it evaporates. How can we explain this? The temperature of the skin’s surface decreases because the skin provides the energy necessary to evaporate the alcohol. A transfer of energy has occurred between the skin and the alcohol. In the following experimental activity, you will prepare a series of mixtures and, for each one, you will observe whether a change in temperature occurs. You will also determine the direction of the energy transfer.
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Experimental Activity 1: Heat Transfers
In this first activity, you will observe a series of physical and chemical changes. For each one, you will determine whether or not a heat transfer has occurred. In most cases, you will use a thermometer to detect the changes in temperature.
This is your first “hands-on” experience with the scientific method in this second chemistry course. You will explore different types of mixtures, compare the results obtained and interpret them while remembering to take into account the thermometer’s degrees of accuracy. Allow approximately 30 minutes to carry out all the steps in the experiment. Although the steps are relatively simple, you must be meticulous in order to obtain meaningful results.
All of the information you need to carry out this activity is given in Section B of the workbook Experimental Activities of Chemistry. Enjoy your work!
When a physical or chemical change causes the temperature of its surroundings to increase, it is said to be exothermic. For instance, the dissolution of NaOH, which you observed in the experimental activity, is exothermic. By releasing heat, it acted as a source of energy and transferred heat to the solution (receptor).
Inversely, when a physical or chemical change produces a decrease in energy, energy is absorbed from the surroundings or from an external source and the change is said to be endothermic. For instance, ice melting is an endothermic change because energy is absorbed in the process. In the experiment you conducted, the surrounding water (source) provided the energy and the ice was the receptor. The temperature of the water decreased because it gave up some of its energy to melt the ice.
In most of the changes studied in the activity, the thermometer was used to determine whether a heat transfer had occurred and, if so, the direction of the energy flow. Have you ever wondered how exactly a thermometer works? How does it measure temperature?
A thermometer is a common instrument used to determine whether a system is hotter or colder than another by means of the temperature displayed on a scale. Remember that temperature is associated with the kinetic energy of the molecules of a substance.
If the temperature rises, the kinetic energy of the molecules rises and, inversely, if the temperature decreases, the kinetic energy also decreases. To better understand how a thermometer works, let’s take a look at what happens at the microscopic level.
Mercury Thermometers
A mercury thermometer consists of a glass bulb filled with mercury attached to a graduated tube whose inside diameter is as fine as a hair (capillary tube). Widely used in the chemistry laboratory, thermometers are usually graduated in degrees Celsius. According to this scale, water boils at 100°C, ice forms at 0°C and the temperature of the human body is 37°C.
Figure 1.8b shows a microscopic view of a thermometer. To keep the diagram simple, the solid walls of the bulb and of the capillary tube are represented as a single layer of tightly packed glass particles firmly held together. The bulb contains liquid mercury (Hg) whose atoms are less tightly packed than the glass molecules. The crowded mercury atoms are in continuous motion, colliding with each other and with the molecules that make up the glass wall. However, because the glass molecules are held in place more firmly, they do not separate but vibrate in fixed positions, thus retaining the atoms of Hg.
Figure 1.8 - Mercury thermometer
a) b)
a) Macroscopic view: the base of the thermometer consists of a bulb attached to a long graduated capillary tube.
b) Schematic diagram of a thermometer at the molecular level. To keep the diagram simple, only a few particles have been represented. In fact, a thermometer is made
up of billions upon billions of particles.
Capillar y tube
Wall of bulb
Mercur y atoms Bulb
Immerse the bulb of a thermometer in a hot gas, such as the vapour escaping from a saucepan of steaming vegetables, for instance. Be careful! The temperature of the vapour can go up to 130°C. You may want to use a candy thermometer to conduct this experiment.
Figure 1.9 - Thermometer immersed in a hot gas
a) b)
a) The bulb of the thermometer is immersed in the vapour escaping from the saucepan.
b) Bombarded by vapour molecules, the bulb of the thermometer heats up.
The atoms of mercury (Hg) that it contains become more agitated and tend to occupy more space.
The liquid mercury expands and rises in the capillary tube attached to the bulb.
What happens at the microscopic level? The vapour molecules, which are very agitated, bombard the walls of the bulb, transmitting a part of their kinetic energy to the glass.
The collisions cause the glass molecules to become agitated. In turn, the glass molecules transmit energy to the Hg atoms. As they become more agitated, the distance between the Hg atoms increases and, as a result, the mercury will try to occupy a greater volume.
We say that the mercury expands. The Hg atoms then move into the only available space, the opening of the capillary tube, and we see the mercury rise. The height reached by the mercury depends on the energy that has been transmitted to it.
Exercise 1.9
Now place the thermometer in the freezer. The thermometer’s bulb is now immersed in cold air, whose molecules have a lower average kinetic energy than that of the mercury atoms in the bulb.
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Describe what happens:
a) at the macroscopic level.
b) at the microscopic level.
What happens when the molecules in the thermometer reach the same level of agitation as those in the medium in which the thermometer is immersed? This is a specific case with very important implications. If we immerse a thermometer in a gas whose molecules are moving at the same speed as the molecules in the thermometer’s bulb, then the molecules in the thermometer and those in the ambient gas cannot accelerate each other’s movements. In this case, there is no transfer of energy between the gas and the bulb. The mercury level does not change and the thermometer displays a constant temperature.
The manner in which the mercury moves inside the thermometer is of special significance. When it rises, the bulb absorbs energy; when it drops, the bulb loses energy. When it is stable, there is no transfer of energy from the thermometer’s bulb to the outside. The temperature of the thermometer then equals the temperature of its surroundings.
Exercise 1.10
A thermometer is placed in a bowl of water and, after a few minutes, the level of the mercury stabilizes. Then it suddenly starts to climb. How would you interpret this?
Thermal Equilibrium
A thermometer that is immersed in a liquid for any length of time will display a constant temperature. The temperature will remain constant provided the surrounding conditions remain the same, that is, the water does not cool down or heat up. We say that the thermometer is in thermal equilibriumwith the liquid. In other words, the thermometer and the liquid are at the same temperature. At the microscopic level, the degree of molecular motion is, on average, the same in both the thermometer and the liquid.
We can therefore use a thermometer to detect heat transfers, or transfers of kinetic energy between molecules. If a system comes in contact with an energy source, the mercury in the thermometer rises. Inversely, if the system gives up energy to a receptor, then the temperature drops and the mercury contracts.
Temperature can be expressed according to various scales. The most commonly used ones are the Celsius, Kelvin and Fahrenheit scales. Let’s review here some of the highlights in the history of thermometers.
Temperature Scales
As we have seen, our skin detects sensations of hot and cold. These sensations are therefore very familiar to us. The need to define these sensations and quantify them more objectively led scientists such as Galileo (circa 1592), Torricelli (circa 1672) and especially Fahrenheit (1714) and Celsius (1742) to determine fixed reference points between which a numerical scale could be established.
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Delancé, in 1688, and Newton, in 1701, came up with the first scales for measuring degrees of heat. For instance, Newton had arbitrarily set 0° as the point at which snow melts, 12° as the temperature of the human body and 34° as the point at which water boils vigorously.
Fahrenheit chose the coldest temperature obtained with a mixture of snow and ammonia salt for the zero on his thermometer, and the boiling point of mercury, or 600°, for the highest point. He then divided the interval between these two points into 600 equal divisions. On this scale, water freezes at 32°, water boils at 212° and the temperature of the human body is 98.6°.
Today, Celsius’ reference points have been widely adopted and integrated into the International System of Units (SI). The zero on the Celsius scale was obtained by immersing the thermometer in ice water and the 100° mark was obtained by immersing the thermometer in boiling water at standard atmospheric pressure (101.3 kPa).
Figure 1.10 - The most commonly used temperature scales
The reference points on the Celsius scale are the freezing and boiling points of water, set at 0°C and 100°C respectively. On the Fahrenheit and Kelvin scales, the freezing point of water is 32°F
and 273 K respectively. Note that the Kelvin scale has only positive values.
373
273
0 212
32
−459 100
˚C ˚F K
0
−273 Absolute 0
All molecular movement ceases.
