201

Unit 2
Elements of Chemical Change

2-1. CHEMICAL REACTIONS

As a provider of health care, you will not be required in most cases, to write and balance chemical equations. You will, however, be using and/or seeing the effects of chemical reactions on a daily basis. Chemical reactions are frequently used to explain various concepts of pharmacology and physiology. Consider drugs. All drugs are chemicals and any pharmacological reference you consult will refer to the chemical changes drugs undergo in the body. Consequently, it is essential that you have a basic knowledge of what a chemical reaction involves and how that chemical reaction can be expressed as a chemical equation.

Definite Composition. When atoms combine, they do so in definite ratios of intact atoms to produce compounds with definite composition. Note that this combination is by number of atoms, not by weights of atoms. What the individual atoms happen to weigh is not important. Atoms do not know what they weigh. When they do interact and combine, it is always as whole particles, and the particle-to-particle or atom-to-atom ratio can always be expressed in simple, whole numbers. Chemical changes do not split atoms into fractional pieces. This is the reason we are able to write a formula such as HCl for the compound hydrochloric acid. Hydrochloric acid is always formed from one atom of hydrogen and one atom of chlorine. Since a chemical reaction is merely a change in matter, and matter consists of atoms or molecules, we can discuss chemical reactions by talking about interactions of individual molecules or atoms.

Chemical Equations. In discussing a chemical reaction, it would be very cumbersome to write it out in the same manner as we state it verbally. To get around this problem, chemists have developed chemical equations. Chemical equations are abbreviated ways of writing chemical reactions. They save much writing and effort and give at least as much information as a verbally stated reaction. Chemical equations show:

Chemical Symbols. In writing chemical equations, we use a number of symbols. The most common symbols are shown below with their meanings.

SYMBOL MEANING
D Heat (a form of energy)
"Yields," indicates direction of reaction
Given off as a gas
Given off as a precipitate

As we illustrate several types of reactions, the uses of these symbols will become apparent.

Types of Reactions. There are four types of chemical reactions, which are possible: Combination reactions, decomposition reactions, single replacement reactions, and double replacement reactions.

Combination reactions. A combination reaction can be represented by the chemical equation A + B AB (one atom of A plus one atom of B yield one molecule of AB). A specific example of this type of reaction is the combination of a metal with oxygen to yield a metallic oxide.

2Mg + O2 2MgO

This equation tells us that two atoms of magnesium and one molecule of oxygen react to form two molecules of magnesium oxide.

Decomposition reactions. The general equation representing decomposition reactions is AB A + B. Here is an example:

CaCO3 CaO + CO2 D

This equation tells us that calcium carbonate will yield calcium oxide and carbon dioxide. The D also tells us that this reaction occurs when heat is applied to calcium carbonate. The indicates that the carbon dioxide is given off as a gas

Single replacement reactions. The general equation for a single replacement reaction is A + BC AC + B. An example is:

Zn + CuSO4 ZnSO4 + Cu

This equation tells us that one atom of zinc and one molecule of cupric sulfate yield one molecule of zinc sulfate and one atom of copper.

Double replacement reactions. The most commonly occurring reaction is the double replacement reaction. The general equation for this reaction is AB + CO AD + CB. Double replacement reactions can be further subdivided into several classes. The most common of these classes are the precipitation reaction, the acid-base reaction, and the oxidation-reduction reaction. An example of the precipitation reaction is:

BaCl2 + Na2SO4 2NaCl + BaSO4

This equation tells us that one molecule of barium chloride reacts with one molecule of sodium sulfate to yield two molecules of sodium chloride and one molecule of barium sulfate as a precipitate. Acid-base and oxidation-reduction reactions will be covered later.