Schaum's Outline of College Chemistry by Jerome Rosenberg
Author:Jerome Rosenberg
Language: eng
Format: epub
Publisher: McGraw-Hill Education
Published: 2013-08-18T16:00:00+00:00
Half-reaction method
The half-reaction method is a way of balancing oxidation-reductions reactions by the recognition of oxidation and reduction with separate reactions. Included in the reactions are the number of electrons that move and the nature of movement (gain or loss). The steps for this technique are:
1. Identify those atoms that are being oxidized and those being reduced.
2. Write the half-reaction for reduction.
a. Make certain that the atom being reduced is balanced (the same number on both sides).
b. Add electrons to the left side; those electrons are taken up by the reduced atom.
c. If necessary for the purposes of balancing the half-reaction,
i. Use H+ and H2O, if in an acid solution, to balance the equation.
ii. Use OH– and H2O, if in a basic solution, to balance the equation.
3. Write the half-reaction for the oxidation.
a. Make certain that the atom being oxidized is balanced (the same number on both sides).
b. Add electrons to the right side; those electrons are released by the oxidized atom.
c. If necessary for the purposes of balancing the half-reaction,
i. Use H+ and H2O, if in an acid solution, to balance the equation.
ii. Use OH– and H2O, if in a basic solution, to balance the equation.
The result of processes 1–3 will yield balanced separate half-reactions from both the standpoint of atomic particles and the net charges of the sides (left charge = right charge).
4. Half-reactions can be added to produce a net reaction, which is the oxidation-reduction reaction. However, this summation cannot be performed unless the electron numbers are the same on both sides of the reaction; by agreement among chemists, electrons are not written into summation reactions. The way in which adjustments are made is to preserve the ratio of coefficients in the individual balanced half-reaction by multiplying all of the participants in an equation by the same number. The goal is to have the same number of electrons on opposite sides of the half-reactions. The electrons will then algebraically cancel when the half-reactions are added. Since the summation equation should not have coefficients divisible by a common factor, it is customary to choose numbers that will yield the least number of electrons for cancellation.
5. Add up the half-reactions and cancel those participants that are the same chemical entity on both sides, as well as the electrons. Notice that there will be no electrons in the summation reaction, nor will there be water, hydrogen ion, or hydroxide ion on both sides of the summation reaction.
6. It is a good practice to make certain that all matter balances on the left side of the reaction against the right side. Also, check that the net charge of the left side of the reaction is the same as the right. And the last check before pronouncing the equation correctly balanced and written is to make certain that the coefficients cannot be divided by the same factor. Naturally, if any of the checks indicate a problem, review the steps and correct the problem. Don’t guess—either the equation is properly balanced or not!
Master the half-reaction method before studying electrochemistry.
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