202

Unit 2
Elements of Chemical Change

2-2. WRITING CHEMICAL EQUATIONS

At this point, you have seen several examples of chemical equations and should be familiar with the symbols used in an equation. We will now examine the process of writing an equation when we are given a verbal description of the reaction. One general rule that must be kept in mind is that there will always be the same number and kinds of atoms in the products of a reaction as in the reactants. This is because matter can neither be created nor destroyed in a chemical reaction and atoms always combine in certain proportions. When given a written verbal description of a chemical reaction, the following steps are used to write the equation for the reaction.

  1. Write the symbols for all elements involved.
  2. Write the correct formulas for any compounds and check for diatomic molecules. (Some elements never exist as single atoms but only as diatomic molecules. These elements can be identified from their names, which end in -gen or -ine. The common diatomic molecules are hydrogen (H2), nitrogen (N2), oxygen (O2), chlorine (Cl2), fluorine (F2), and bromine (Br2).)
  3. Balance the equation by placing coefficients where appropriate. Remember that there must be equal numbers of atoms of each kind on both sides of the equation. In this step, the subscripts that were used in writing the correct formulas cannot be changed.

EXAMPLE

For application of these steps, consider this description of a reaction. Calcium metal and water react to yield calcium hydroxide and hydrogen gas.

(1)    Write the symbols for all elements involved.

Ca, O, H

(2)    Write the correct formulas for any compounds and check for diatomic molecules.

Ca + H2O Ca(OH2 + H2

(3)    Balance the equation by placing coefficients where appropriate. Look at the number of atoms of each element in the products and reactants.

REACTANTS PRODUCTS
1 Ca    1 Ca
1 O 2 O
2 H 4 H

It is apparent here that there are twice as many oxygen and hydrogen atoms in the products as reactants. How can this equation be balanced to give equal numbers of atoms on both sides? Fill in the coefficients of the molecules in the equation below.

_____ Ca + _____ H2O _____ Ca(OH2 + _____H2

Since there are twice as many hydrogen and oxygen atoms on the right as on the left, if we could double the numbers of these atoms on the left, we would have a balanced equation. This can be done by placing a two in front ofH2O . All the other coefficients would be one (if there is no coefficient, we assume it is one, so there is no need to write it in front of each molecule).