Balanced Equations (Edexcel AS Chemistry)

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Writing Balanced Equations

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

Balancing equations

  • During chemical reactions, atoms cannot be created or destroyed
  • The number of each atom on each side of the reaction must therefore be the same
    • E.g. 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

Worked example

Balance the following equation:

magnesium + oxygen magnesium oxide

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 react with 1 mole of oxygen to form 2 moles of magnesium oxide

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

2Mg (s) + O2 (g) → 2MgO (s)

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, and not the spectator ions

Worked example

1. Balance the following equation

zinc + copper(II) sulfate → zinc sulfate + copper

2. Write down the ionic equation for the above reaction

Answer 1:

Step 1: To balance the 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)

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Richard

Author: Richard

Expertise: Chemistry

Richard has taught Chemistry for over 15 years as well as working as a science tutor, examiner, content creator and author. He wasn’t the greatest at exams and only discovered how to revise in his final year at university. That knowledge made him want to help students learn how to revise, challenge them to think about what they actually know and hopefully succeed; so here he is, happily, at SME.