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Reduction and Oxidation Reactions

Introduction

Predict what might happen when a piece of copper wire in a solution of 2% AgNO3.

If you try this experiment, you will initially see that the copper is a shiny copper color and the solution is clear. In less than one hour the solution is light blue and the wire is covered with shiny silver needles. What happened?

Copper metal became copper ions in solution and silver ions became silver metal.

Cu(s) + Ag+(aq) ---> Cu2+(aq) + Ag(s)       (unbalanced)

The Cu(s) loses electrons to become Cu2+(aq) ions and the Ag+(aq) ions gain electrons to become Ag(s).

Reactions that involve the exchange of electrons are called reduction and oxidation (redox) reactions. When a chemical species loses electrons we say that it is oxidized, and when a chemical species gains electrons we say that it is reduced.

The Cu(s) loses electrons to be oxidized to Cu2+(aq).
The Ag+(aq) gain electrons to be reduced to Ag(s).

We can break the reaction into the following two half reactions.

Cu(s) ---> Cu2+(aq) + 2e-

Ag+(aq) + e- ---> Ag(s)

What would you predict if you placed a piece of Ag metal in a solution of Cu2+?

Since we observed that the reaction of Ag+ and Cu is spontaneous, we would not expect the reverse reaction to be spontaneous. So no reaction occurs between Ag metal and Cu2+.


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