For application of these steps, consider this description of a reaction. Calcium

metal and water react to yield calcium hydroxide and hydrogen gas.

a. Write the symbols for all elements involved.

Ca, O, H

b. Write the correct formulas for any compounds and check for diatomic

molecules.

Ca + H2O → Ca(OH)2 + H2

↑

c. 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.

H2O → _____

Ca +

Ca(OH)2 + ________ 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 of H2O. 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).

We have implied that all reactions only go in the direction of the products, but this

is not always the case. Sometimes as products are formed, they react with one another

or decompose to form the reactants. Thus, the reaction is going in both directions at the

same time, and if allowed to continue indefinitely, would result in a constant amount of

products and reactants. Reactions that go in both directions are called equilibrium

reactions, and when the rate of formation of product is the same as the rate of formation

MD0803

2-6

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