Simple Redox Reactions
A half-reaction is simply one which shows either reduction OR oxidation, but not both. Here is a simple example of redox reaction (not balanced):
""Ag+ + Cu --> Ag + Cu2+""
It has BOTH a reduction and an oxidation in it. That is why we call it a redox reaction, from REDuction and OXidation.
What you must be able to do is look at a redox reaction and separate out the two half-reactions in it. To do that, identify the atoms which get reduced and get oxidized. Here are the two half-reactions from the above example:
""Ag+ --> Ag""
""Cu --> Cu2+ ""
The silver is being reduced, its oxidation number going from +1 to zero. The copper's oxidation number went from zero to +2, so it was oxidized in the reaction. In order to figure out the half-reactions, you MUST be able to calculate the oxidation number of an atom.
Keep in mind that a half-reaction shows only one of the two behaviors we are studying. A single half-reaction will show ONLY reduction or ONLY oxidation, never both in the same equation.
Also, notice that the reaction is read from left to right to determine if it is reduction or oxidation. If you read the reaction in the opposite direction (from right to left) it then becomes the other of our two choices (reduction or oxidation). For example, the silver half-reaction above is a reduction, but in the reverse direction it is an oxidation, going from zero on the right to +1 on the left.
There will be times when you want to switch a half-reaction from one of the two types to the other. In that case, rewrite the entire equation and swap sides for everything involved. If we needed the silver half-reaction to be oxidation, we would write Ag --> Ag+ rather tha just doing it mentally.
OK, back to the next step, which is both half-reactions must be balanced. However, there is a twist. When you learned about balancing equation, you made equal the number of atoms of each element on each side of the arrow. That still applies, but there is one more thing: the total amount of charge on each side of the half-reaction MUST be the same.
When you look at the two half-reactions above, you will see they are already balanced for atoms with one Ag on each side and one Cu on each side. So, all we need to do is balance the charge. To do this you add electrons to the more positive side. You add enough to make the total charge on each side become EQUAL.
To the silver half-reaction, we add one electron:
""Ag+ + e¯ --> Ag ""
To the copper half-reaction, we add two electrons:
""Cu --> Cu2+ + 2e¯ ""
One point of concern: notice that each half-reaction wound up with a total charge of zero on each side. This is not always the case. You need to strive to get the total charge on each side EQUAL, not zero.
One more point to make before wrapping this up. A half-reaction is a "fake" chemical reaction. It's just a bookkeeping exercise. Half-reactions NEVER occur alone. If a reduction half-reaction is actually happening (say in a beaker in front of you), then an oxidation reaction is also occuring.
For combination of the two half-reactions, see:
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.