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Balance the redox reaction between Fe2+ and MnO4^- in acidic solution.

Chemistry

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Step-by-step solution

8 steps
  1. Separate the reaction into two half-reactions.

    Oxidation half-reaction: Fe2+→Fe3+\text{Fe}^{2+} \rightarrow \text{Fe}^{3+}
    Reduction half-reaction: MnO4−→Mn2+\text{MnO}_4^- \rightarrow \text{Mn}^{2+}

  2. Balance atoms other than oxygen and hydrogen.

    For the oxidation half-reaction, Fe atoms are already balanced.

    Fe2+→Fe3+\text{Fe}^{2+} \rightarrow \text{Fe}^{3+}

    For the reduction half-reaction, Mn atoms are already balanced.

    MnO4−→Mn2+\text{MnO}_4^- \rightarrow \text{Mn}^{2+}
  3. Balance oxygen atoms by adding H2O\text{H}_2\text{O}.

    Oxidation: No oxygen atoms.

    Fe2+→Fe3+\text{Fe}^{2+} \rightarrow \text{Fe}^{3+}

    Reduction: Add 4H2O4\text{H}_2\text{O} to the right side to balance the 4 oxygen atoms on the left.

    MnO4−→Mn2++4H2O\text{MnO}_4^- \rightarrow \text{Mn}^{2+} + 4\text{H}_2\text{O}
  4. Balance hydrogen atoms by adding H+\text{H}^+ (since the reaction is in acidic medium).

    Oxidation: No hydrogen atoms.

    Fe2+→Fe3+\text{Fe}^{2+} \rightarrow \text{Fe}^{3+}

    Reduction: Add 8H+8\text{H}^+ to the left side to balance the 8 hydrogen atoms in 4H2O4\text{H}_2\text{O} on the right.

    8H++MnO4−→Mn2++4H2O8\text{H}^+ + \text{MnO}_4^- \rightarrow \text{Mn}^{2+} + 4\text{H}_2\text{O}
  5. Balance the charge by adding electrons (e−\text{e}^-).

    Oxidation:
    Left side charge: +2+2
    Right side charge: +3+3
    Add 1e−1\text{e}^- to the right side to balance the charge.

    Fe2+→Fe3++e−\text{Fe}^{2+} \rightarrow \text{Fe}^{3+} + \text{e}^-

    Reduction:
    Left side charge: (+8)+(−1)=+7(+8) + (-1) = +7
    Right side charge: +2+2
    Add 5e−5\text{e}^- to the left side to balance the charge.

    5e−+8H++MnO4−→Mn2++4H2O5\text{e}^- + 8\text{H}^+ + \text{MnO}_4^- \rightarrow \text{Mn}^{2+} + 4\text{H}_2\text{O}
  6. Multiply the half-reactions by appropriate integers to equalize the number of electrons.

    The oxidation half-reaction has 1e−1\text{e}^-. The reduction half-reaction has 5e−5\text{e}^-.
    Multiply the oxidation half-reaction by 5:

    5(Fe2+→Fe3++e−)5(\text{Fe}^{2+} \rightarrow \text{Fe}^{3+} + \text{e}^-) 5Fe2+→5Fe3++5e−5\text{Fe}^{2+} \rightarrow 5\text{Fe}^{3+} + 5\text{e}^-

    The reduction half-reaction remains unchanged:

    5e−+8H++MnO4−→Mn2++4H2O5\text{e}^- + 8\text{H}^+ + \text{MnO}_4^- \rightarrow \text{Mn}^{2+} + 4\text{H}_2\text{O}
  7. Add the two balanced half-reactions and cancel common species.

    5Fe2++5e−+8H++MnO4−→5Fe3++5e−+Mn2++4H2O5\text{Fe}^{2+} + 5\text{e}^- + 8\text{H}^+ + \text{MnO}_4^- \rightarrow 5\text{Fe}^{3+} + 5\text{e}^- + \text{Mn}^{2+} + 4\text{H}_2\text{O}

    Cancel 5e−5\text{e}^- from both sides:

    5Fe2++8H++MnO4−→5Fe3++Mn2++4H2O5\text{Fe}^{2+} + 8\text{H}^+ + \text{MnO}_4^- \rightarrow 5\text{Fe}^{3+} + \text{Mn}^{2+} + 4\text{H}_2\text{O}
  8. Verify that atoms and charges are balanced.

    Atoms:
    Mn: 1 on left, 1 on right
    O: 4 on left, 4 on right
    Fe: 5 on left, 5 on right
    H: 8 on left, 8 on right
    Charge:
    Left side: 5(+2)+8(+1)+(−1)=+10+8−1=+175(+2) + 8(+1) + (-1) = +10 + 8 - 1 = +17
    Right side: 5(+3)+(+2)+0=+15+2=+175(+3) + (+2) + 0 = +15 + 2 = +17
    Both atoms and charges are balanced.

    The balanced equation is:

    MnO4−+5Fe2++8H+→Mn2++5Fe3++4H2O\boxed{\text{MnO}_4^- + 5\text{Fe}^{2+} + 8\text{H}^+ \rightarrow \text{Mn}^{2+} + 5\text{Fe}^{3+} + 4\text{H}_2\text{O}}
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