Question

What balanced equation represents a redox reaction?

Answer 1

The key to identifying oxidation-reduction reactions is recognizing when a chemical reaction leads to a change in the oxidation number of one or more atoms.

You have probably learned the concept of oxidation number. It is nothing more than a bookkeeping system used to keep track of electrons in chemical reactions. It is worthwhile to re-memorize the rules, summarized in the table below.

1. The oxidation number of an atom in an element is zero. Thus, the atoms in O₂, O₃, P₄, S₈, and Al all have an oxidation number of 0.

2. The oxidation number of a monatomic ion is the same as the charge on the ion. Thus, the oxidation number of sodium in the Na⁺ ion is +1, for example, and the oxidation number of chlorine in the Cl⁻ ion is -1.

3. The oxidation number of hydrogen is +1 when it is combined with a nonmetal. Hydrogen is therefore in the +1 oxidation state in CH₄, NH₃, H₂O, and HCl.

4. The oxidation number of hydrogen is -1 when it is combined with a metal. Hydrogen is therefore in the -1 oxidation state in LiH, NaH, CaH₂, and LiAlH₄.

5. Silver and the metals in Group 1 form compounds in which the metal atom is in the +1 oxidation state.

6. The elements in Group 2 form compounds in which the metal atom is in the +2 oxidation state.

7. Oxygen usually has an oxidation number of -2. Exceptions include
   peroxides such as H₂O₂ and the O₂²⁻ ion.

8. The Group 17 elements form binary compounds in which the more electronegative atom is in the -1 oxidation state.

9. The sum of the oxidation numbers of the atoms is equal to the charge on the molecule or ion.

You should memorize these rules.

Let’s apply these rules to decide which if the following equations are redox reactions.

AgNO₃ + NaCl → AgCl + NaNO₃
On the left, the oxidation numbers are: Ag +1; O -2; N +5; Na +1; Cl -1
On the right, the oxidation numbers are: Ag +1; Cl -1; Na +1; O -2; N +5
No oxidation numbers change. This is not a redox reaction.

BaCl₂ + K₂CO₃ → BaCO₃ + 2KCl
On the left, the oxidation numbers are: Ba +2; Cl -1; K +1; O -2; C +4
On the right, the oxidation numbers are: Ba +2; O -2; C +4; K +1; Cl -1
No oxidation numbers change. This is not a redox reaction.

CuO + CO → Cu + CO₂
On the left, the oxidation numbers are:; O -2; Cu +2; C +2
On the right, the oxidation numbers are: Cu 0; O -2; C +4
Cu changes from +2 to 0; C changes from +2 to +4. This is a redox reaction.

I₂ + 5HOBr → 2IO₃⁻ + 5Br⁻ + 7H⁺
On the left, the oxidation numbers are:; I 0; H +1; O -2; Br +1
On the right, the oxidation numbers are: O -2; I +5; Br -1; H +1
I changes from 0 to +5; Br changes from +1 to -1. This is a redox reaction.

4Ag⁺ + Cr₂O₇²⁻ + H₂O → 2Ag₂CrO₄ + 2H⁺
On the left, the oxidation numbers are:; Ag +1; O -2; Cr +6; H +1
On the right, the oxidation numbers are: Ag +1; O -2; Cr +6; H +1
No oxidation numbers change. This is not a redox reaction.

Now that you have successfully worked through the above equations, you should be able to identify whether or not a given equation represents a redox reaction.