CIE AS Chemistry (9701) exams from 2022

Revision Notes

1.2.4 Balancing Equations

Balancing Equations

  • A symbol equation is a shorthand way of describing a chemical reaction using chemical symbols to show the number and type of atom
  • A word equation is a longer way of describing a chemical reaction using words to show the number and type of atom

Balancing equations

  • During chemical reactions, atoms cannot be created or destroyed
  • The number of each atom on each side of the reaction should therefore be the same
    • Eg. the reaction needs to be balanced
  • When balancing equations remember:
    • Not to change any of the formulae
    • To put the numbers used to balance the equation in front of the formulae
    • To balance firstly the carbon, then the hydrogen and finally the oxygen in combustion reactions of organic compounds
  • When balancing equations follow the following the steps:
    • Write the formulae of the reactants and products
    • Count the numbers of atoms in each reactant and product
    • Balance the atoms one at a time until all the atoms are balanced
    • Use appropriate state symbols in the equation
  • The physical state of reactants and products in a chemical reaction is specified by using state symbols
    • (s) solid
    • (l) liquid
    • (g) gas
    • (aq) aqueous

Ionic equations

  • In aqueous solutions ionic compounds dissociate into their ions
  • Many chemical reactions in aqueous solutions involve ionic compounds, however only some of the ions in solution take part in the reactions
  • The ions that do not take part in the reaction are called spectator ions
  • An ionic equation shows only the ions or other particles taking part in a reaction, without showing the spectator ions

Worked example: Balancing equations

Atoms, Molecules & Stoichiometry Worked example - Balancing equations, downloadable AS & A Level Chemistry revision notes

Answer

  • Step 1: Write out the symbol equation showing reactants and products

Mg + O2 → MgO

  • Step 2: Count the numbers of atoms in each reactant and product

Atoms, Molecules & Stoichiometry Worked example - Balancing equations table, downloadable AS & A Level Chemistry revision notes

  • Step 3: Balance the atoms one at a time until all the atoms are balanced

2Mg + O2 → 2MgO

    • This is now showing that 2 moles of magnesium reacts with one mole of oxygen to form two moles of magnesium oxide

 

  • Step 4: Use appropriate state symbols in the fully balanced equation

2Mg(s) + O2(g) → 2MgO(s)

Worked example: Equations (balancing & ionic)

Atoms, Molecules & Stoichiometry Worked example - Equations (balancing & ionic), downloadable AS & A Level Chemistry revision notes

Answer 1

  • Step 1: To balance equation, write out the symbol equation showing reactants and products

Zn  + CuSO4  → ZnSO4 + Cu

  • Step 2: Count the numbers of atoms in each reactant and product. The equation is already balanced

Atoms, Molecules & Stoichiometry Worked example - Equations (balancing & ionic) table, downloadable AS & A Level Chemistry revision notes

  • Step 3: Use appropriate state symbols in the equation

Zn(s)  + CuSO4(aq)  → ZnSO4(aq) + Cu(s)

Answer 2

  • Step 1:  The full chemical equation for the reaction is

Zn (s)  + CuSO4 (aq)  → ZnSO4 (aq) + Cu (s)

  • Step 2:  Break down reactants into their respective ions

Zn (s)  + Cu2+SO42- (aq)  → Zn2+SO42- (aq) + Cu (s) 

  • Step 3:  Cancel the spectator ions on both sides to give the ionic equation

Zn (s)  + Cu2+SO42- (aq)  → Zn2+SO42- (aq) + Cu (s)

Zn (s)  + Cu2+(aq)  → Zn2+ (aq) + Cu (s)

Author: Francesca

Fran has taught A level Chemistry in the UK for over 10 years. As head of science, she used her passion for education to drive improvement for staff and students, supporting them to achieve their full potential. Fran has also co-written science textbooks and worked as an examiner for UK exam boards.
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