Balancing Equations (DP IB Chemistry): Revision Note

Philippa Platt

Written by: Philippa Platt

Reviewed by: Richard Boole

Updated on

Balancing 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:

    • Do not change any of the formulae

      • When polyatomic ions (like SO42- or NO3-) appear on both sides, do not split them into individual atoms unless needed

    • 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:

Mg + O2 → MgO

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

 

Mg

O

Reactants

1

2

Products

1

1

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)

Examiner Tips and Tricks

Some elements are diatomic in their natural state and must be written with a subscript 2 in equations:

  • H2

  • N2

  • O2

  • F2

  • Cl2

  • Br2

  • I2

Worked Example

Write a balanced symbol equation for the following equation

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

Answer 

Step 1: Write out the symbol equation:

Zn  + CuSO4  → ZnSO4 + Cu

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

 

Zn

Cu

S

O

Reactants

1

1

1

4

Products

1

1

1

4

The equation is already balanced

Step 3: Use appropriate state symbols in the equation

Zn (s)  + CuSO(aq)  → ZnSO(aq) + Cu (s)

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Philippa Platt

Author: Philippa Platt

Expertise: Chemistry Content Creator

Philippa has worked as a GCSE and A level chemistry teacher and tutor for over thirteen years. She studied chemistry and sport science at Loughborough University graduating in 2007 having also completed her PGCE in science. Throughout her time as a teacher she was incharge of a boarding house for five years and coached many teams in a variety of sports. When not producing resources with the chemistry team, Philippa enjoys being active outside with her young family and is a very keen gardener

Richard Boole

Reviewer: Richard Boole

Expertise: Chemistry Content Creator

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.

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