What is the purpose of half equations in balancing redox equations?

To show the oxidation and reduction processes separately.

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How can half equations be deduced from a redox equation?

By identifying the atoms that have changed their oxidation number.

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What is a disproportionation reaction?

A chemical reaction where atoms of the same element are both oxidised and reduced.

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Describe how to balane each of the following when writing a half equation:

  1. the number of oxygen atoms
  2. the number of hydrogen atoms
  3. overall charge

1Add one or more H2O molecules to the side containing the fewer number of oxygen atoms.

1Add one or more H+ ions to the side containing the fewer number of hydrogen atoms.

3Add e- to the side with the higher positive charge.

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What is a half equation?

A half equation is an equation that shows the gain or loss of electrons in a redox reaction.

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Describe the position of the electrons in a reduction half equation.

The electrons (e-) are to the left of the arrow.


For example, in the half equation Na+ + e- ➔ Na, a sodium ion gains an electron and is reduced.

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Describe the position of the electrons in an oxidation half equation.

The electrons (e-) are to the right of the arrow.


For example, in the half equation Na ➔ Na+ + e-, a sodium atom loses an electron and is oxidised.

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