Combining half-reactions

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Here are the two half-reactions to be combined:

Ag+ + e¯ ---> Ag
Cu ---> Cu2+ + 2e¯

Here is the rule to follow:

the total electrons MUST cancel when the two half-reactions are added.

Another way to say it:

the number of electrons in each half-reaction MUST be equal
when the two half-reactions are added.

What that means is that one (or both) equations must be multiplied through by a factor. The value of the factor is selected so as to make the number of electrons equal.

In our example problem, the top reaction (the one with silver) must be multiplied by two, like this:

2Ag+ + 2e¯ ---> 2Ag

Notice that each separate substance is increased by the factor amount. Occasionally, a student will multiply ONLY the electrons by the factor. That is incorrect.

When the two half-reactions are added, we get:

2Ag+ + 2e¯ + Cu ---> 2Ag + Cu2+ + 2e¯

With two electrons on each side, they may be canceled, resulting in:

2Ag+ + Cu ---> 2Ag + Cu2+

This is the correct answer. Notice that there are two silvers on each side and one copper. Notice also that the total charge on each side is +2. It is balanced for both atoms and charge. Sometimes, I am asked if the order matters, if the Cu could be first on the left-hand side. The answer is that the order does not matter. There happen to be certain styles about where particular substances are put in the final answer, but these are only styles. They do not affect the chemical correctness of the answer.

Practice Problems

Separate into half-reactions, balance them and then recombine.

1) Sn + Cl2 ---> Sn2+ + Cl¯

2) Fe2+ + I3¯ ---> Fe3+ + I¯


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