Question 19.1: Half-Reaction Method of Balancing Aqueous Redox Equations in...
Half-Reaction Method of Balancing Aqueous Redox Equations in Acidic Solution
Balance the redox equation:
Al(s) + Cu^{2+}(aq) \longrightarrow Al^{3+}(aq) + Cu(s)
| Half-Reaction Method of Balancing Aqueous Redox Equations in Acidic Solution | |||||||||
| GENERAL PROCEDURE | |||||||||
| Step 1 Assign oxidation states to all atoms and identify the substances being oxidized and reduced. |
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| Step 2 Separate the overall reaction into two half-reactions: one for oxidation and one for reduction. | Oxidation: Al(s)\longrightarrow Al^{3+}(aq)
Reduction: Cu^{2+}(aq) \longrightarrow Cu(s) |
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| Step 3 Balance each half-reaction with respect to mass in the following order:
• Balance all elements other than H and O. • Balance O by adding H_{2}O. • Balance H by adding H^{+}. |
All elements are balanced, so proceed to the next step. | ||||||||
| Step 4 Balance each half-reaction with respect to charge by adding electrons. (Make the sum of the charges on both sides of the equation equal by adding as many electrons as necessary.) | Al(s)\longrightarrow Al^{3+}(aq) +3 e^{-}
2 e^{-}+ Cu^{2+}(aq) \longrightarrow Cu(s) |
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| Step 5 Make the number of electrons in both half-reactions equal by multiplying one or both half-reactions by a small whole number. | 2[Al(s)\longrightarrow Al^{3+}(aq) +3 e^{-}]
2 Al(s)\longrightarrow 2 Al^{3+}(aq) +6 e^{-} 3[2 e^{-}+ Cu^{2+}(aq) \longrightarrow Cu(s)]
6 e^{-}+ 3 Cu^{2+}(aq) \longrightarrow 3 Cu(s) |
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| Step 6 Add the two half-reactions together, canceling electrons and other species as necessary. | 2 Al(s)\longrightarrow 2 Al^{3+}(aq) +\cancel{6 e^{-}} \\ \cancel{6 e^{-}}+ 3 Cu^{2+}(aq) \longrightarrow 3 Cu(s) \\ \overline{ 2 Al(s) + 3 Cu^{2+}(aq) \longrightarrow 2 Al^{3+}(aq) + 3 Cu(s)} | ||||||||
| Step 7 Verify that the reaction is balanced with respect to both mass and charge. |
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