The process of bonding together atoms and separating atoms that are already bonded together is called a chemical reaction. A chemical reaction causes a change in the properties of atoms or to a collection of atoms, but the atoms remain unchanged because of a change in the electron configuration. For example,
NaCl + Sodium is chemically unstable because sodium has one valence electron and needs eight valence electrons to be chemically stable.
Chlorine is chemically unstable because Chlorine has seven valence electrons and needs eight valence electrons to be chemically stable.
Chemical reaction causing an iconic bond
Na Sodium donates one valence electron to chlorine. Sodium’s valence shell has eight electrons, making sodium chemically stable.
Cl Chlorine receives the electron from sodium. Chlorine’s valence shell has eight electrons, making chlorine chemically stable.
Donates Fig. 2-5.Sodium donates one valence electron to chlorine in a chemical reaction that forms the compound known as salt.
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a chemical reaction occurs when a sodium atom is combined with a chlorine atom; the property of the resulting chemical compound is table salt. If the sodium chloride (table salt) compound were broken down into its chemical ele-ments, you would see that the atoms of sodium and chlorine remain unchanged.
Theoretically a chemical reaction can be reversed if the conditions are opti-mal. A chemical reaction that is reversible is called a reversible reaction. (see Fig. 2-6)
In practical use, same reactions can do this much easier than others. Some of these reversible reactions occur due to the instability of the reactants and prod-ucts, while others will only reverse under special conditions. Examples of spe-cial conditions could be the presence of water or the application of heat.
Notation Description
Na+ The plus superscript indicates a positive ion.
Cl− The negative superscript indicates a negative ion.
Na++Cl−→NaCl The plus sign indicates synthesizing (combining) two particles. The right arrow indicates that a chemical reaction occurs towards the product.
NaCl →Na++Cl− Decomposing (breaking up) a molecule or chemical compound.
NaOH +HCl →NaCl +H2O Exchange reaction where a chemical compound is de-composed into its chemical elements and those chemi-cal elements are synthesized into a new compound.
Here, sodium hydroxide (NaOH) and hydrochloric acid (HCl) form salt (NaCL) and water (H2O).
Na++ Cl−←→NaCl Reversible reaction is noted with a right arrow over a left arrow.
C – C Single covalent bond.
C = C Double covalent bond.
C ≡ C Triple covalent bond.
H2O A subscript following a chemical symbol indicates the number of atoms (two hydrogen atoms). If no subscript is used, then it is implied there is one atom (here, one oxygen atom).
Table 2-2. Commonly Used Chemical Notations
The type of reaction that occurs can further describe a chemical reaction.
There are three types of chemical reactions:
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Synthesisreaction: Two or more atoms, ions, or molecules are bound to form a larger molecule. A synthesis reaction combines substances called reactants to form a new molecule, which is called a product. Areactantis a substance that reacts in a reaction and the productis the result of a reaction. In Na++ Cl−→NaCl, sodium and chlorine are reactants and sodium chloride is the product of this reaction. A synthesis reaction in a living organism is referred to as an anabolic reactionor anabolism. These are metabolic pathways.•
Decompositionreaction: A reaction that breaks the bond between atoms in a molecule or chemical compound. In NaCl →Na++Cl−, sodium chloride is broken up into its chemical elements sodium and chlorine. A decomposition reaction in a living organism is called a catabolic reactionor catabolism.•
Exchange reaction: A reaction that is both a synthesis reaction and a decomposition reaction, where a chemical compound is decomposed into its chemical elements and those chemical elements are synthesized into a new chemical compound. In NaOH +HCl →NaCl +H2O, sodium hydrox-ide (NaOH) and hydrochloric acid (HCl) enter into an exchange reaction to form salt (NaCL) and water (H2O).A chemical reaction theoretically can be reversed, but in practice some reac-tions create an unstable chemical compound that might require special con-ditions to exist for the reverse reaction to happen. Those special concon-ditions required to reverse a reaction appear below the arrow in the reaction notation.
Above the arrow appears any special condition that must exist for the synthe-sized reaction to occur. In Fig. 2-6, a temperature of 250° C is the special con-dition for the synthesized reaction to occur and absolute zero is necessary for the decomposition reaction to occur.
A catalyst is a substance that speeds up the rate of a chemical reaction by decreasing the energy needed to run the reaction without changing the reactants or products. Enzymes are an example of a biological substance that acts as cat-alysts to speed up a reactor rate.
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Fig. 2-6. In theory all chemical reactions are reversible. In practice these are called reversible reactions.
X + Y heat XY water