From Word Equations to Balanced Chemical Equations

25 min

Why do cakes rise?

A baker in Nyamirambo mixing dough for cakes or amandazi adds a spoonful of baking powder. In the hot oven the powder breaks down and gives off a gas; the bubbles are trapped in the dough, and it rises. A food chemist describes this in words: "When sodium hydrogencarbonate is heated, it decomposes into sodium carbonate, water and carbon dioxide." In this lesson you will bring together everything from Units 5 and 7 to turn a sentence like that into a balanced chemical equation.

Figure 5.1: Five steps from a description to a balanced chemical equation. Step 4 is where most mistakes are made.

Table 5.1: Formulae you will use again and again

Substance	Formula	Substance	Formula
water	$\mathrm{H_2O}$	hydrochloric acid	$\mathrm{HCl}$
carbon dioxide	$\mathrm{CO_2}$	sulfuric acid	$\mathrm{H_2SO_4}$
carbon monoxide	$\mathrm{CO}$	nitric acid	$\mathrm{HNO_3}$
ammonia	$\mathrm{NH_3}$	sodium hydroxide	$\mathrm{NaOH}$
methane	$\mathrm{CH_4}$	calcium hydroxide	$\mathrm{Ca(OH)_2}$
glucose	$\mathrm{C_6H_{12}O_6}$	calcium carbonate	$\mathrm{CaCO_3}$
ethanol	$\mathrm{C_2H_5OH}$	sodium chloride	$\mathrm{NaCl}$

Work out any other formula from valencies (Unit 5, Lesson 5), and remember the diatomic elements: $\mathrm{H_2}$ , $\mathrm{N_2}$ , $\mathrm{O_2}$ , $\mathrm{F_2}$ , $\mathrm{Cl_2}$ , $\mathrm{Br_2}$ , $\mathrm{I_2}$ .

Worked example

Problem. Write a balanced equation, with state symbols, for baking powder: "When sodium hydrogencarbonate is heated, it decomposes into sodium carbonate, water and carbon dioxide."

Solution.

Word equation: sodium hydrogencarbonate → sodium carbonate + water + carbon dioxide.
Formulae: Na valency 1 with $\mathrm{HCO_3}$ valency 1 gives $\mathrm{NaHCO_3}$ ; Na 1 with $\mathrm{CO_3}$ 2 gives $\mathrm{Na_2CO_3}$ . Skeleton: $\mathrm{NaHCO_3 \rightarrow Na_2CO_3 + H_2O + CO_2}$ .
Balance: sodium is 1 against 2, so write $\mathrm{2NaHCO_3}$ . Recount:

Atom	Left	Right
Na	2	2
H	2	2
C	2	$1 + 1 = 2$
O	6	$3 + 1 + 2 = 6$

4. States: the solids stay solid; in a hot oven water is given off as steam.

$\mathrm{2NaHCO_3(s) \rightarrow Na_2CO_3(s) + H_2O(g) + CO_2(g)}$

Worked example

Problem. Iron wool heated in chlorine gas burns to form brown iron(III) chloride. Write the balanced equation.

Solution. Word equation: iron + chlorine → iron(III) chloride. Iron(III) has valency 3 and chloride 1, so the formula is $\mathrm{FeCl_3}$ ; chlorine is diatomic. Skeleton: $\mathrm{Fe + Cl_2 \rightarrow FeCl_3}$ .

Chlorine is 2 against 3; the smallest number both divide into is 6, so write $\mathrm{3Cl_2}$ and $\mathrm{2FeCl_3}$ . That gives 2 Fe on the right, so write 2Fe:

$\mathrm{2Fe(s) + 3Cl_2(g) \rightarrow 2FeCl_3(s)}$

Check: Fe 2 = 2; Cl 6 = 6.

Worked example

Problem. An aluminium saucepan is scrubbed with a cleaner containing hydrochloric acid, and bubbles of hydrogen appear. Aluminium chloride forms in solution. Write the balanced equation.

Solution. Word equation: aluminium + hydrochloric acid → aluminium chloride + hydrogen. Formulae: Al; HCl; Al valency 3 with Cl valency 1 gives $\mathrm{AlCl_3}$ ; $\mathrm{H_2}$ . Skeleton: $\mathrm{Al + HCl \rightarrow AlCl_3 + H_2}$ .

Chlorine: 3 on the right, so write $\mathrm{3HCl}$ . But then H is 3 on the left and 2 on the right: odd against even.
Double everything that is odd: $\mathrm{6HCl}$ and $\mathrm{2AlCl_3}$ , which needs 2Al; hydrogen is now 6 on the left, so write $\mathrm{3H_2}$ .

$\mathrm{2Al(s) + 6HCl(aq) \rightarrow 2AlCl_3(aq) + 3H_2(g)}$

Atom	Left	Right
Al	2	2
H	6	$3 \times 2 = 6$
Cl	6	$2 \times 3 = 6$

Worked example

Problem. Kalisa writes this equation for sodium reacting with water: $\mathrm{Na + H_2O \rightarrow NaOH + H}$ . Evaluate his equation and correct it.

Solution. The formulae of Na, $\mathrm{H_2O}$ and NaOH are right, and the atoms do add up. But hydrogen gas is diatomic, so the product must be $\mathrm{H_2}$ , not a single H atom. With $\mathrm{H_2}$ on the right, H is 2 on the left and 3 on the right; doubling Na, $\mathrm{H_2O}$ and NaOH fixes it. He also left out the state symbols.

$\mathrm{2Na(s) + 2H_2O(l) \rightarrow 2NaOH(aq) + H_2(g)}$

Check: Na 2 = 2; H 4 = 4; O 2 = 2.

Warning

The five most common mistakes: (1) writing H, O or Cl instead of $\mathrm{H_2}$ , $\mathrm{O_2}$ , $\mathrm{Cl_2}$ ; (2) a wrong formula from wrong valencies, such as $\mathrm{AlCl}$ or $\mathrm{MgCl}$ ; (3) changing a subscript to balance; (4) not recounting after adding a coefficient; (5) writing (aq) for water, which is (l).

Tip

Formulae first, numbers last. Get every formula right before you think about balancing, and never touch a formula again once it is written.

Cognito: Writing Word and Symbol Equations and Balancing Equations (6 min). Watch how each word equation is turned into symbols before any numbers are added.

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Practice

Write a balanced equation with state symbols: calcium burns in oxygen to form calcium oxide.
Potassium reacts with water to form potassium hydroxide solution and hydrogen gas. Write the balanced equation with state symbols.
Photosynthesis: carbon dioxide + water → glucose + oxygen. Write the balanced chemical equation.
Ethanol from banana beer burns in air to give carbon dioxide and water. Write the balanced equation with state symbols.
Black copper(II) oxide dissolves in warm dilute sulfuric acid to give blue copper(II) sulfate solution and water. Write the balanced equation.
In a blast furnace, iron(III) oxide reacts with carbon monoxide to form molten iron and carbon dioxide. Write the balanced equation with state symbols.
Umutoni writes $\mathrm{Mg + HCl \rightarrow MgCl + H}$ . Find two mistakes and write the correct balanced equation.

Show answersHide answers

$\mathrm{2Ca(s) + O_2(g) \rightarrow 2CaO(s)}$
$\mathrm{2K(s) + 2H_2O(l) \rightarrow 2KOH(aq) + H_2(g)}$
$\mathrm{6CO_2(g) + 6H_2O(l) \rightarrow C_6H_{12}O_6(aq) + 6O_2(g)}$
$\mathrm{C_2H_5OH(l) + 3O_2(g) \rightarrow 2CO_2(g) + 3H_2O(l)}$
$\mathrm{CuO(s) + H_2SO_4(aq) \rightarrow CuSO_4(aq) + H_2O(l)}$
$\mathrm{Fe_2O_3(s) + 3CO(g) \rightarrow 2Fe(l) + 3CO_2(g)}$
Magnesium chloride is $\mathrm{MgCl_2}$ (Mg valency 2), and hydrogen gas is $\mathrm{H_2}$ : $\mathrm{Mg(s) + 2HCl(aq) \rightarrow MgCl_2(aq) + H_2(g)}$ .

1Why do cakes rise?

Why do cakes rise?