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1.5Principles of Chemistry

Chemical Formulae, Equations, Calculations — Edexcel Linear - OL (Chemistry) Revision Notes & Topical Questions

15 min read 21 parts Lesson 13 of 25 1 quiz

📚 Edexcel Linear - OL (Chemistry) Hub / Topical Revision
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Lesson summary

Word and Symbol Equations, Balancing and Relative Mass — EDEXCEL LINEAR – OL (CHEMISTRY) notes: Write word and balanced symbol equations with state symbols, calculate relative formula masses and use conservation of mass.In a chemical reaction new substances form, but the same ato…

  • law of conservation of mass
  • Worked example 1
  • Worked example 2
  • Worked example 3
  • Worked example 4
  • Coefficient

Part 1 of 5Why equations balance

In a chemical reaction new substances form, but the same atoms are present before and after: they simply join up in different ways. Atoms cannot be created or destroyed, so the total mass of reactants equals the total mass of products. This is the law of conservation of mass, and it is why symbol equations must be balanced.

Part 2 of 5Word equations

A word equation uses the full names of the substances, with reactants on the left and products on the right, joined by an arrow that is read as "to form" or "produces".

Word equations
Conditions or a catalyst can be written above the arrow.
Method for exam questions: pick out the reactants and products, put them in the right order (the question may give products first),
  • and add any substance that is implied.
StatementWord equation
Ammonia reacts with nitric acid to form ammonium nitrateammonia + nitric acid to ammonium nitrate
Iron(II) hydroxide and sodium sulfate form when iron(II) sulfate and sodium hydroxide reactiron(II) sulfate + sodium hydroxide to iron(II) hydroxide + sodium sulfate
Carbon in coal burns in air to produce carbon dioxidecarbon + oxygen to carbon dioxide ("burns in air" means it reacts with oxygen)

Part 3 of 5Writing and balancing symbol equations

First write the correct formula of every substance.

Writing and balancing symbol equations
The non-metals H2, N2, O2, F2, Cl2, Br2 and I2 exist as molecules.
Then balance by changing the numbers in front of formulae (coefficients), never the small numbers inside a formula,
  • because changing H2O to H2O2 creates a different substance.
1
Write the unbalanced equation with correct formulae.
2
Count each element on both sides, working from left to right.
3
Treat an unchanged group, such as NO3, as one unit.
4
Balance elements that appear alone last.
5
Recheck every element and add state symbols (s), (l), (g), (aq).

Examples

  • S(s) + O2(g) to SO2(g);
    • NaOH(aq) + HCl(aq) to NaCl(aq) + H2O(l);
    • 2Fe2O3 + 3C to 4Fe + 3CO2;
    • 2Al + 3CuO to Al2O3 + 3Cu;
    • MgO + 2HNO3 to Mg(NO3)2 + H2O (the two nitrate groups and two hydrogens balance together).

Part 4 of 5Relative atomic and formula mass

Ar (relative atomic mass) is the average mass of an atom of an element, found from the isotopes and their abundances. Mr (relative formula mass) is the sum of the Ar values for all atoms in the formula. Ar values are on the Periodic Table as the larger number.

SubstanceCalculationMr
H2O2 x 1 + 1618
K2CO32 x 39 + 12 + 3 x 16138
Ca(OH)240 + 2 x 16 + 2 x 174
(NH4)2SO42 x 14 + 8 x 1 + 32 + 4 x 16132
Mg(NO3)224 + 2 x 14 + 6 x 16148

Because mass is conserved, the sum of the Mr values of reactants (multiplied by their coefficients) equals that of products.

Part 5 of 5Worked examples

  1. Write a balanced equation with state symbols for magnesium reacting with hydrochloric acid to give magnesium chloride and hydrogen. [2]2 marks

  2. Balance: C2H6 + O2 to CO2 + H2O. [2]2 marks

  3. Calculate Mr of (NH4)2SO4. [2]2 marks

  4. State what the law of conservation of mass means. [1]1 mark

Key terms

TermMeaning
ReactantStarting substance in a reaction.
ProductSubstance formed.
CoefficientNumber in front of a formula in a balanced equation.
State symbol(s), (l), (g) or (aq).
A(r)Relative atomic mass.
M(r)Relative formula mass.

Avoid these mistakes

1
Changing subscripts to balance.
2
Forgetting to write diatomic elements as molecules.
3
Not treating groups like NO3 as a unit.
4
Omitting state symbols or writing them incorrectly.

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