6.4
Information in Chemical Equations
What is a cookie recipe doing in a science textbook (Figure 1)? Believe it
or not, recipes and balanced chemical equations have a lot in common. A
recipe provides more than just a list of required ingredients. It provides the
quantities of each ingredient. It also tells us the conditions under which the
ingredients are combined. Recipes also tell you how much product to expect.
Chemical equations give us similar information (Table 1).
Turn on the oven to 170 °C.
Place the first five ingredients in the mixing
bowl, one at a time, mixing well after each
addition. In a cup, dissolve the baking soda in
the hot water, then add it to the mixing bowl.
Add the flour, oats, and chocolate chips. Mix
well. Drop in spoonfuls onto cookie sheets.
Bake in the oven until starting to turn golden.
Makes about 50 cookies.
1 cup butter at room temperature
1 ½ cups brown sugar
2 eggs
1 teaspoon vanilla essence
1 teaspoon salt
½ teaspoon baking soda
1 tablespoon hot water
1 ½ cups unbleached flour
1 ½ cups rolled oats
1 ½ cups chocolate chips
Figure 1 A cookie recipe has some
features in common with a chemical
equation.
Table 1 Comparison of Recipes and Chemical Equations
Recipe
Chemical equation
See Figure 1.
2 H2(g) + O2(g) → 2 H2O(g)
starting materials
ingredients list (e.g., eggs and
butter)
chemical symbols for the reactants
(to the left of the arrow)
(e.g., 2 H2(g) and O2(g))
conditions of starting
materials
directions (e.g., butter at room
temperature)
state symbols: (s), (l), (g), (aq)
(e.g., 2 H2(g))
proportions of
starting materials
quantities in ingredients list
(e.g., 2 eggs, 1 cup butter)
coefficients of reactants
(e.g., 2 H2(g))
instructions for
combining materials
directions (e.g., mixing well after
each addition)
plus sign (+) between the formulas
of the reactants, indicating that
reactants must come into contact
(e.g., 2 H2(g) + O2(g))
resulting product
title (e.g., Chocolate Chip
Oatmeal Cookies)
chemical symbols for the products
(to the right of the arrow)
(e.g., 2 H2O (g))
proportions/
quantities of product
final sentence (e.g., Makes about
50 cookies.)
coefficients of products
(e.g., 2 H2O (g))
Information
communicated
Now you know what the various parts of a balanced chemical equation
represent. The next stage is to learn how to balance chemical equations.
Balancing Chemical Equations
To learn how to balance a chemical equation, let’s work through some
Sample Problems. To make things simpler when we are learning to balance
equations, we will not use state subscripts for now.
NEL
6.4 Information in Chemical Equations
233
sAMPlE PRoBlEM 1
Balancing a Chemical Equation
Write the balanced chemical equation for the reaction of magnesium with oxygen.
step 1 Write the word equation for the reaction.
magnesium + oxygen → magnesium oxide
step 2 Replace each chemical name with the correct chemical formula. (This is the
skeleton equation.)
Mg + O2 → MgO
step 3 Count the number of atoms of each type on either side of the arrow.
Mg + O2 → MgO
1 Mg atom 1 Mg atom
2 O atoms 1 O atom
step 4 Multiply the formulas by an appropriate coefficient until all the atoms are
LeaRning Tip
Better Balancing
When balancing a chemical equation,
start by balancing the element(s) that
occurs only once on both sides of the
equation. Also, leave any substances
appearing as elements in the equation
until the end since they can be balanced
without affecting any other atom types.
Keep checking as you go along because
balancing one element may “unbalance”
another.
balanced. Keep checking whether the numbers of each type of atom on both
sides are balanced.
• MgO (on the right) must be multiplied by the coefficient 2 to balance the
oxygen atoms.
Mg + O2 → 2 MgO
• Mg (on the left) must now be multiplied by the coefficient 2 so that there are
two Mg atoms on both sides.
The final balanced chemical equation is
2 Mg + O2 → 2 MgO
Practice
Write a balanced chemical equation for the reaction between potassium and bromine to
make potassium bromide.
sAMPlE PRoBlEM 2 Balancing a Chemical Equation
Methane gas, CH4, burns in oxygen to produce carbon dioxide and water (Figure 2).
Write a balanced chemical equation for this reaction.
step 1 methane + oxygen → carbon dioxide + water
step 2 CH4 + O2 → CO2 + H2O
step 3 CH4 + O2 → CO2 + H2O
1 C atom
4 H atoms
2 O atoms
1 C atom
2 H atoms
3 O atoms (2 + 1)
step 4 Check whether the numbers of each type of atom on both sides are balanced.
• Carbon is already balanced.
• H2O must be multiplied by 2 to balance hydrogen atoms.
CH4 + O2 → CO2 + 2 H2O
1 C atom
1 C atom
4 H atoms
4 H atoms
2 O atoms
4 O atoms (2 + 2)
234
Chapter 6 • Chemicals and Their Reactions
Figure 2 Methane makes up about
80 % of the natural gas burned in a
natural gas furnace.
NEL
• O2 must be multiplied by 2 to balance oxygen atoms.
CH4 + 2 O2 → CO2 + 2 H2O
1 C atom
1 C atom
4 H atoms
4 H atoms
4 O atoms
4 O atoms
All the atoms balance, so the final balanced
chemical equation is
CH4 + 2 O2 → CO2 + 2 H2O
(Figure 3)
H2O
O2
+
CH4
+
O2
CO2
Figure 3
H2O
Practice
Write a balanced chemical equation for the reaction between oxygen and a hydrocarbon
called pentane, C5H12, to produce carbon dioxide and water.
Our third example shows how to balance equations involving polyatomic
ions. In these cases, consider the polyatomic ion as one unit rather thanC06-F12-UBOS10SB.ai
as
individual atoms.
sAMPlE PRoBlEM 3 Balancing an Equation Involving
Polyatomic Ions
Ontario
SB
Zinc metal reacts in a silver
nitrateScience
solution 10
to produce
zinc nitrate and silver metal.
0-17-635528-6
Write a chemical equation for this reaction. (See Table 1 in Section 5.9 for a list of
polyatomic ions.)
C06-F12-UBOS10SB
FN
CrowleArt Group
CO
step 1 zinc + silver nitrate → zinc nitrate + silver
Deborah Crowle
step 2 Zn + AgNO3 → Zn(NO3)2 + Ag
1st pass
Pass
step 3 Zn + AgNO3 →Approved
Zn(NO3)2 + Ag
1 Zn atom
1 Ag+ ion
1 NO3– ion
2+
1 ZnApproved
ion
Not
1 Ag atom
2 NO3– ion
step 4 Check whether the numbers of each type of atom on both sides are balanced.
Because each polyatomic ion generally stays intact, you can count polyatomic
ions in the same way as you count atoms.
• Zinc and silver are already balanced.
• AgNO3 must be multiplied by 2 to balance nitrate ions.
• Zn + 2 AgNO3 → Zn(NO3)2 + Ag
1 Zn atom
1 Zn2+ ion
+
2 Ag ions
1 Ag atom
2 NO3– ions
2 NO3– ions
• Ag+ must be multiplied by 2 to balance the number of silver atoms.
The final balanced chemical equation is
Zn + 2 AgNO3 → Zn(NO3)2 + 2 Ag
Practice
Write a balanced chemical equation for the reaction between iron(III) nitrate and
sodium hydroxide to produce iron(III) hydroxide and sodium nitrate.
NEL
6.4 Information in Chemical Equations
235
RESEARCH THIS
gaS FURnaCe TeCHniCian
SKILLS: Researching, Evaluating
SKILLS HANDBOOK
4.A.
Many careers involve chemical reactions and the delicate balance
between the reactants. Consider the responsibilities of the people
who install, service, and repair gas furnaces. If the reactants
(natural gas and oxygen) are not available in the appropriate
ratios, the outcome could be deadly.
1. Research this career, paying particular attention to the
training required.
2. Research the main dangers of badly installed or poorly
maintained gas furnaces.
B. Write balanced chemical equations for at least two chemical
reactions that furnace technicians must know about. K/ U T/ I
C. Write a paragraph explaining why this would, or would not,
be a good career for you. C A
Go to NElsoN sCIENCE
SUMMARY
in
•Chemicalequationscontaininformationabout
which substances are reactants and which are
products, and the ratios of these substances.
•Chemicalequationsobeythelawofconservation
of mass.
CHECK
2. What is the difference between a skeleton equation and a
balanced chemical equation? K/U
3. Consider this skeleton equation: HI → H2 + I2
K/U
(a) Which of the following two equations is the correct
balanced chemical equation?
2 HI → H2 + I2
or
H2I2 → H2 + I2
(b) Explain your answer.
4. (a) What is the difference between a subscript and a
coefficient in a chemical equation?
(b) Which is the only one that can be changed as you
balance a skeleton equation? Why? K/U
5. Write a balanced chemical equation for each of the
following reactions taking place in water: K/U
(a)
(b)
(c)
(d)
•Coeffi
cientscanbeaddedbeforechemical
formulas in a chemical equation to make the
numbers of atoms in reactants and products
balance.
YOUR LeaRning
1. What strategy for balancing chemical equations did you find
particularly useful? Share this strategy with a classmate
who is having difficulty with balancing equations. K/U C
potassium iodide → potassium and iodine
magnesium + silver nitrate → silver + magnesium nitrate
sodium + water → hydrogen + sodium hydroxide
lead(II) nitrate + sodium chloride →
lead(II) chloride + sodium nitrate
6. Octane, C8H18, is a compound in gasoline. Octane burns in
oxygen to produce carbon dioxide gas and water vapour. T/I
(a) Write a balanced chemical equation for this reaction
(b) How many carbon dioxide molecules are produced for
every octane molecule that burns?
236
A. List the educational background and training required to be
a gas furnace technician. T/I
Chapter 6 • Chemicals and Their Reactions
7. Balance the following skeleton equations if they are not
already balanced. (It does not matter if you do not know the
names of all the compounds, but you must recognize the
polyatomic ions. You do not need to add state symbols.) T/I
(a) Ca + Cl2 → CaCl2
(b) K + Br2 → KBr
(c) H2O2 → H2O + O2
(d) Na + O2 → Na2O
(e) N2 + H2 → NH3
(f) NH4OH + HBr → H2O + NH4Br
(g) CaSO4 + KOH → Ca(OH)2 + K2SO4
(h) Ba + HNO3 → H2 + Ba(NO3)2
(i) H3PO4 + NaOH → H2O+ Na3PO4
(j) C3H8 + O2 → CO2 + H2O
(k) Al4C3 + H2O → CH4 + Al(OH)3
(l) FeBr3 + Na → Fe + NaBr
(m) Fe + H2SO4 → H2 + Fe2(SO4)3
(n) C2H6 + O2 → CO2 + H2O
8. Ammonium dichromate, (NH4)2Cr2O7, is an orange solid
that releases nitrogen gas and water vapour when it is
heated. The green solid produced in this reaction is a toxic
form of chromium oxide. K/U T/I
(a) Write a word equation for this reaction.
(b) When 2.5 g of ammonium dichromate is heated, the
mass of nitrogen and water vapour released is 1.0 g.
What is the final mass of the solid product?
NEL