Introduction to Half-Reactions

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Consider the following chemical reaction:

We can consider this as happening in two steps:

1. Calcium is being oxidized from Ca to Ca2+
   
2. Zinc is being reduced from Zn2+ to Zn
   
   

We can write each of these two processes separately, as half-reactions:

Notice how one atom is losing electrons (getting oxidized) while one is gaining electrons (getting reduced).

Question (from Nelson Chemistry 12)

For each of the following, write and label the oxidation and reduction half-reaction equations. Ignore spectator ions.

Answer

Note: in each case the number of electrons lost in the oxidation = the number gained in the reduction.