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.
You are viewing: Which Equation Represents A Redox Reaction
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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.
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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.
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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.
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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.
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Silver and the metals in Group 1 form compounds in which the metal atom is in the +1 oxidation state.
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The elements in Group 2 form compounds in which the metal atom is in the +2 oxidation state.
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Oxygen usually has an oxidation number of -2. Exceptions include peroxides such as H₂O₂ and the O₂²⁻ ion.
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The Group 17 elements form binary compounds in which the more electronegative atom is in the -1 oxidation state.
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The sum of the oxidation numbers of the atoms is equal to the charge on the molecule or ion.
You should memorize these rules.
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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.
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