A half-reaction is the part of an overall reaction that represents, separately, either an oxidation or a reduction. Two half-reactions, one oxidation and one reduction, are necessary to completely describe a redox reaction.
W hen a nickel strip < Ni (s) >is placed in an aqueous solution of copper(II) sulfate < Cu 2+ , SO 4 2- >, an immediate reaction occurs. Copper metal begins to deposit on the strip. The only source for metallic copper in this system is the copper(II) ions in solution. What is happening to the copper(II) ions to cause them to change into elemental copper? Let's represent this fascinating observation with a chemical equation:
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W e have just written a half-reaction! In this equation, the copper(II) ion is being reduced < 2+ 0 >. When we balance a half-reaction, we first balance the mass of the participating species (atoms, ions, or molecules) and then the charge. In this case, the mass is balanced by adding a copper (atom or ion) to each side. To balance the charge, electrons are added. Notice the addition of 2 electrons to left side of the above equation. These electrons are necessary to reduce a copper(II) ion to metallic copper.
I f copper(II) ion is being reduced, what is being oxidized? Another way to ask this question is 'where are those electrons coming from?'. Nickel, of course! What is the half-reaction for the oxidation of metallic nickel? Nickel must release 2 electrons to form the nickel(II) ion as shown in the following equation (remember that we balance the mass and then the charge):
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Take a moment to work the following questions on a piece of paper. When finished, click on the question mark to reveal the answer.
Write a half-reaction showing Zn being oxidized to Zn 2+ .
Write a half-reaction showing Br 2 being reduced to Br .
Did you remember to balance the bromine (first) in the last question? Let's solve another one.
C admium is a silvery, crystalline metal resembling zinc. A large part of the cadmium produced in the United States is used in electroplating metals, such as iron and steel, to protect them from corrosion. The process usually involves the half-reaction shown below. Balance this equation so that we can preserve our steel. Be careful of the cyanide!
B efore moving to the next page you should know how to write and balance a simple oxidation and reduction half- reaction .