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Summary Rules of Balancing Redox Reactions

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Summary Rules of Balancing Redox Reactions

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  • June 1, 2022
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Balancing Redox Reactions Using the Half Reaction Method

Many redox reactions occur in aqueous solutions or suspensions. In this medium most of the reactants and
products exist as charged species (ions) and their reaction is often affected by the pH of the medium. The
following provides examples of how these equations may be balanced systematically. The method that is used is
called the ion-electron or "half-reaction" method.

Example 1 -- Balancing Redox Reactions Which Occur in Acidic Solution

Organic compounds, called alcohols, are readily oxidized by acidic solutions of dichromate ions. The following
reaction, written in net ionic form, records this change. The oxidation states of each atom in each compound is
listed in order to identify the species that are oxidized and reduced, respectively.




An examination of the oxidation states, indicates that carbon is being oxidized, and chromium, is being reduced.
To balance the equation, use the following steps:

1. First, divide the equation into two halves; an oxidation half-reaction and reduction half- reaction by
grouping appropriate species.

(red.) (Cr2O7)-2 ----> Cr+3
(ox.) C2H6O ----> C2H4O

2. Second, if necessary, balance both equations by inspection. In doing this ignore any oxygen and
hydrogen atoms in the formula units. In other words, balance the non-hydrogen and non-oxygen atoms
only. By following this guideline in the example above, only the reduction half-reaction needs to be
balanced by placing the coefficient, 2 , in front of Cr+3 as shown below.

(red.) (Cr2O7)-2 ----> 2 Cr+3=
(ox.) C2H6O ----> C2H4O
(as there are equal numbers of carbon atoms on both sides of this equation, skip this step for this half-
reaction. Remember, in this step, one concentrates on balancing only non-hydrogen and non-oxygen
atoms)

3. The third step involves balancing oxygen atoms. To do this, one must use water (H2O) molecules. Use 1
molecule of water for each oxygen atom that needs to be balanced. Add the appropriate number of water
molecules to that side of the equation required to balance the oxygen atoms as shown below.

(red.) (Cr2O7)-2 ----> 2 Cr+3 + 7 H2O
(ox.) C2H6O ----> C2H4O

(as there are equal numbers of oxygen atoms, skip this step for this half-reaction)

4. The fourth step involves balancing the hydrogen atoms. To do this one must use hydrogen ions (H+).
Use one (1) H+ ion for every hydrogen atom that needs to balanced. Add the appropriate number of
hydrogen ions to that side of the equation required to balance the hydrogen atoms as shown below

(red.) 14 H+ + (Cr2O7)-2 ---> 2 Cr+3 + 7 H2O

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