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Balancing Chemical Equations

Science: KS3, topic 2.6.4

Balancing Chemical Equations

Chemical equations must be balanced to make sure that there are the same number of atoms of each element before and after a reaction. The reaction of hydrogen with oxygen to make water will be used as an example of how to do this:

Unbalanced equation

Unbalanced equation

  • The unbalanced symbol equation for this reaction is:
  • H2 + O2 → H2O
  • The equation is unbalanced because there are 2 atoms of oxygen on the left hand side of the equation, but only 1 on the right hand side.
Still unbalanced equation

Still unbalanced equation

  • We can increase the number of oxygen atoms on the right hand side by adding an H2O molecule:
  • H2 + O2 → 2H2O
  • However, the equation is still unbalanced as there are now more hydrogen atoms on the right hand side than on the left hand side.
Balanced equation

Balanced equation

  • The final step is to add an H2 molecule to the left hand side:
  • 2H2 + O2 → 2H2O
  • Our equation is now balanced.

Test yourself on Balancing Chemical Equations

  • H2 + O2 → 2H2O
    Why is this equation unbalanced?

  • Select the true statement in each pair.

  • Why is 2H2 + O2 → 2H2O balanced but H2 + O2 → 2H2O is not?

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