Question 19.2: Half-Reaction Method of Balancing Aqueous Redox Equations in...
Half-Reaction Method of Balancing Aqueous Redox Equations in Acidic Solution
Balance the redox equation:
Fe ^{2+}(a q)+ MnO _4 ^{-}(a q) \longrightarrow Fe^{3+}(a q)+ Mn ^{2+}(a q)
| 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: Fe ^{2+}(a q) \longrightarrow Fe^{3+}(a q)
Reduction: MnO _4^{-}(a q) \longrightarrow Mn ^{2+}(a q) |
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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 other than H and O are balanced, so proceed to balance H and O.
Fe ^{2+}(a q) \longrightarrow Fe^{3+}(a q)
MnO _4^{-}(a q) \longrightarrow Mn ^{2+}(a q)+4 H_{2}O (l)
8 H^{+}(aq)+MnO _4^{-}(a q) \longrightarrow Mn ^{2+}(a q)+4 H_{2}O (l) |
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| 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.) | Fe ^{2+}(a q) \longrightarrow Fe^{3+}(a q) +1e^{-}
5 e^{-}+ 8 H^{+}(aq) +MnO _4^{-}(a q) \longrightarrow Mn^{2+}(a q)+4 H_{2}O (l) |
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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. | 5 [Fe ^{2+}(a q) \longrightarrow Fe^{3+}(a q) +1e^{-}]
5 Fe ^{2+}(a q) \longrightarrow 5 Fe^{3+}(a q) +5 e^{-}
5 e^{-}+ 8 H^{+}(aq) +MnO _4^{-}(a q) \longrightarrow Mn^{2+}(a q)+4 H_{2}O (l) |
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| Step 6 Add the two half-reactions together, canceling electrons and other species as necessary. | 5 Fe ^{2+}(a q) \longrightarrow 5 Fe^{3+}(a q) +\cancel{5 e^{-}}
\underline{ \cancel{5 e^{-}}+ 8 H^{+}(aq) +MnO _4^{-}(a q) \longrightarrow Mn^{2+}(a q)+4 H_{2}O (l)} \\ 5 Fe ^{2+}(a q) + 8 H^{+}(aq)+MnO _4^{-}(a q) \longrightarrow 5 Fe^{3+}(a q) +Mn^{2+}(a q)+4 H_{2}O (l) |
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| Step 7 Verify that the reaction is balanced with respect to both mass and charge. |
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