Deducing Stoichiometry

Section 1 of 7Types of equation

Word equation

• Word equation Uses full names with reactants on the left and products on the right, joined by an arrow meaning produces.
• Examplesodium hydroxide + hydrochloric acid → sodium chloride + water.

Symbol equation

• Symbol equation Uses formulae and coefficients so that the numbers of each type of atom are equal on both sides.
• Balancing follows from the fact that atoms cannot be created or destroyed.

State symbols

• State symbols (s) solid, (l) liquid, (g) gas and (aq) aqueous, meaning dissolved in water.

Section 2 of 7Balancing

1

Write the correct formulae for all reactants and products.

2

Count the atoms of each element on each side.

3

Add coefficients in front of formulae, one element at a time, never altering subscripts.

4

For combustion of organic compounds balance carbon first, then hydrogen, then oxygen.

5

Add state symbols and check each element once more.

Section 3 of 7Ionic equations (HT only)

1Full equation

• Full equation Zn(s) + CuSO₄(aq) → ZnSO₄(aq) + Cu(s).

2Split aqueous ionic compounds

• Split aqueous ionic compounds Write each as its separate ions: Zn(s) + Cu²⁺ + SO₄²⁻ → Zn²⁺ + SO₄²⁻ + Cu(s).

3Cancel spectators

• Cancel spectators Ions that appear unchanged on both sides, here SO₄²⁻, take no part in the reaction.

4Net ionic equation

• Net ionic equation Zn(s) + Cu²⁺(aq) → Zn²⁺(aq) + Cu(s). Solids, liquids and gases are never split into ions.

Section 4 of 7Deducing the equation from masses (HT only)

1

Calculate the M_r of each substance in the reaction.

2

Divide each reacting mass by its M_r to get moles.

3

Divide all the mole values by the smallest to find a ratio.

4

If the ratio is not whole numbers, multiply every value by the same factor, for example 1 : 2 : 2.5 becomes 2 : 4 : 5.

5

Use the whole numbers as the coefficients to write the balanced equation.

Section 5 of 7Worked example (HT only)

Substance	Mass	Mr	Moles	Ratio
CH3OH	64 g	32	2	2
O2	96 g	32	3	3
CO2	88 g	44	2	2
H2O	72 g	18	4	4
Equation				2CH3OH + 3O2 → 2CO2 + 4H2O

Section 6 of 7Key terms

Term	Meaning
Stoichiometry	The numbers of moles of each substance shown by the coefficients in a balanced equation.
Spectator ion	An ion present in a solution that does not take part in the reaction.
Ionic equation	An equation showing only the ions and particles that actually react.
Aqueous	Dissolved in water.
Coefficient	A number in front of a formula showing how many units of it react or form.

Section 7 of 7Exam technique

1

Altering the formula of a compound to make atoms balance instead of changing the coefficient.

2

Splitting solids, liquids and gases into ions in an ionic equation.

3

Leaving ratios such as 1 : 1.5 unconverted to whole numbers.

4

Cancelling ions that are not identical on both sides, including their charge and state.