How do scientists in different countries, speaking different languages,communicate the results of their experiments to each other (Figure 1)?How do they represent information about elements, compounds, andchemical equations? You have learned that chemical substances can berepresented with names andformulas. You have alsolearned that chemicalreactions can be representedby word equations. Can wedescribe chemical reactionsin symbolic form?

A skeleton equation is arepresentation of a chemicalreaction in which theformulas of the reactants areconnected to the formulas ofthe products by an arrow.Consider the example of theburning of methane. We candescribe this reaction in aword equation as follows:

methane + oxygen → carbon dioxide + water

We can then write a skeleton equation by replacing each name with aformula:

CH4 + O2 → CO2 + H2O

However, there is a problem. The Law of Conservation of Massstates that the mass of the reactants equals the mass of theproducts. In other words, atoms can be neither created nordestroyed in a chemical change. If we look at the reactants andproducts, we can record the numbers of atoms of each type in atable like Table 1.There is an apparent imbalancebetween the numbers of atomsin the reactants and thenumbers of atoms in theproducts. We have seeminglycreated an oxygen atom anddestroyed two hydrogen atoms(Figure 2).

We cannot change thetypes or formulas of themolecules. So how can wesolve this imbalance?

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6.56.5

Balancing Chemical Equations

Figure 1

A chemical equation is written the same way in any language, making it universally understandable.

Type of atom Reactants Products

C 1 1H 4 2O 2 2 + 1 = 3

Figure 2

The numbers of atoms of reactants and products are out of balance. The total masses of reactants andproducts are also out of balance. The see-saw tilts to the product side because of the greater mass.

La réaction du méthane, brulant avec l’oxygène, produit de l’eau etdu gaz de dioxyde de carbone.

CH4 + 2O2 —> 2H2O + CO2

Methane burns in air by reacting with oxygen to produce waterand carbon dioxide gases.

CH4 + 2O2 —> 2H2O + CO2

CH4 + 2O2 —> 2H2O + CO2

Table 1 Combustion of Methane

Understanding Chemical Reactions 227

The answer is to changethe numbers of moleculesrather than theirformulas. If we add anoxygen molecule to thereactants and a watermolecule to the products,this balances the equation(Figure 3).

CH4 + O2 + O2 → CO2 + H2O + H2O

An equation in which the reactants and the products contain equalnumbers of atoms of each type is a balanced chemical equation. Theusual way to write a balanced equation is to use coefficients. A coefficientis a number written in front of a chemical symbol or formula. It indicatesthe number of atoms or molecules of that substance. The coefficients areshown in red in the following equation.

CH4 + 2O2 → CO2 + 2H2O

Note that by balancing an equation, the mass of the reactants will be equalto the mass of the products.

How to Balance an Equation

Let’s look at an example to see how an equation can be balanced. Ironreacts with oxygen to form magnetic iron oxide (Fe3O4) (Figure 4). Whatis the balanced chemical equation for this reaction?

Step 1. Write the word equation for the reaction.

iron + oxygen → magnetic iron oxide

When iron reacts with oxygen, itforms two oxides. Magnetic ironoxide is an equal mixture ofiron(II) oxide (FeO) and iron(III)oxide (Fe2O3). Add up the atomsto get the formula Fe3O4.

Did You Know ?

Figure 4

Iron reacts with oxygen to form rust or magnetic iron oxide.

Figure 3

The numbers of atoms are now in balance. The mass of the reactants also equals the mass of the products.

228 Chapter 6

Step 2. Write the skeleton equation by replacing each name with a correctformula.

Fe + O2 → Fe3O4

Step 3. Count the numbers of atoms of each type in reactants and products.

The numbers of atoms may be recorded in a table (Table 2).

Step 4. Multiply each of the formulas by the appropriate coefficients tobalance the numbers of atoms.

To balance the three iron atoms on the right side, multiply the iron atomson the left side by 3. To balance the four oxygen atoms on the right side,multiply the oxygen atoms on the left side by 2. Check that the atoms oneach side are balanced.

3Fe + 2O2 → Fe3O4

A balanced chemical equation has been written. The formulas areunchanged, and the numbers of atoms are balanced.

The same steps are used to balance equations that involve morecomplex molecules. For example, what is the balanced chemical equationfor the reaction of magnesium metal with nitric acid?

Step 1. Write the word equation for the reaction.

magnesium + nitric acid → hydrogen + magnesium nitrate

Step 2. Write the skeleton equation by replacing each name with a correctformula.

Mg + HNO3(aq) → H2 + Mg(NO3)2

Equation Balancing for “Smarties”Try This

ActivityUse candies to represent the process of balancingequations. For example, consider the balancedchemical equation for the combustion of methanein oxygen to produce carbon dioxide and water:

CH4 + 2O2 → CO2 + 2H2O

• Start with one black (or brown) candy, fouryellow candies, and four red candies. Arrangethe candies to represent the reactants. Forexample, methane could be a black candywith four yellow candies just touching it, andthe two oxygen molecules could each be twored candies just touching.

• You can represent the chemical reaction bymixing all the candies together.

• The products could then be represented witha carbon dioxide molecule (one black candywith two red candies) and two watermolecules (each molecule is one red candywith two yellow candies).

• Use the candies to represent other chemicalreactions that you have encountered in thischapter. For a challenge, try to represent theburning of ethane (C2H6) in oxygen gas toproduce carbon dioxide and water.

Type of atom Reactants Products

Fe 1 3O 2 4

Table 2 Rusting of Iron

The following subscriptsare used to indicate thestate of each substance:(s) indicates a solid;(l) indicates a liquid;(g) indicates a gas;and (aq) indicates anaqueous solution (in water).

Understanding Chemical Reactions 229

Understanding Concepts1. (a) Why is the following equation not balanced?

N2 + H2 → NH3

(b) The following is an attempt to balance the aboveequation. What is wrong with the way that the equationis balanced?

N2 + H3 → N2H3

2. Copy the following skeleton equations into your notebook.Then balance the equations:

(a) Na + Cl2 → NaCl

(b) K + O2 → K2O

(c) H2 + O2 → H2O

(d) H2 + Cl2 → HCl

(e) N2 + H2 → NH3

(f) CO + O2 → CO2

(g) Al + Br2 → AlBr3

(h) N2H4 +O2 → H2O + N2

(i) CH4 + O2 → CO2 + H2O

3. For each of the following, write the correct skeletonequation, and then balance it to form a chemical equation:

(a) copper(II) oxide + hydrogen → copper + water

(b) lead(II) nitrate + potassium iodide →lead(II) iodide + potassium nitrate

(c) calcium + water → calcium hydroxide + hydrogen gas

(d) lead(II) sulfide + oxygen → lead + sulfur dioxide

(e) hydrogen sulfide → hydrogen + sulfur

4. Imagine that you are an engineer trying to determine howmuch air had to be supplied to burn gasoline in a carengine. Assuming that gasoline is heptane (C7H16), the wordequation is

heptane + oxygen → carbon dioxide + water vapour

(a) Write the skeleton equation for the reaction.

(b) Balance the equation by adding coefficients asnecessary.

(c) How many molecules of oxygen are required for everymolecule of heptane that burns?

Making Connections5. Nitrogen oxides are a group of air pollutants produced by

internal combustion engines in automobiles. Thesepollutants are formed by the reaction of atmosphericnitrogen (N2) and oxygen (O2) to form various combinations,including NO, NO2, N2O4, N2O3, and N2O5. Write balancedchemical equations to represent the production of each ofthese substances.

1,2,3 What balanced chemicalequations represent thereactions in your Challenge?

Challenge

Step 3. Count the numbers of atoms of each type in reactants and products.This example is complicated by the polyatomic nitrate ion.

The compound magnesium nitrate contains a total of six oxygen atomsbecause there are two NO3 groups, each of which has three oxygen atoms.The numbers of atoms may be recorded in a table (Table 3).

Step 4. Multiply each of the formulas by the appropriate coefficients tobalance the numbers of atoms.

To balance the number of hydrogen atoms, the coefficient 2 is placed infront of the HNO3 molecule. Note that this coefficient affects thenumber of nitrogen and oxygen atoms as well.

Mg + 2HNO3(aq) → H2 + Mg(NO3)2

The equation is now balanced.

Type of atom Reactants Products

Mg 1 1H 1 2N 1 2O 3 6

Work the WebVisit www.science.nelson.com andfollow the Science 10, 6.5 links toweb sites that show how to balancechemical equations. Choose areaction and show it as a wordequation, a skeleton equation, and asa balanced equation.

Table 3 Magnesium Reacts with Nitric Acid