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Balance the following redox reactions: a) Zn (s) + Ag+(aq) → Zn2+(aq) + Ag (s) b) Cu2+(aq) + Cl-(aq) → Cu (s) + Cl2(g) c) Pb2+(aq) + Br-(aq) → Pb (s) + Br2(aq) d) HCl (aq) + MnO2(s)→Cl2(g) + Mn2+(aq) This reaction takes place in an acidic medium.

Chemistry

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Step-by-step solution

17 steps

Here are the balanced redox reactions:

Part a· 4 steps

Zn (s) + Ag+(aq)→Zn2+(aq)+Ag (s)\text{Zn (s) + Ag}^{+}(\text{aq}) \rightarrow \text{Zn}^{2+}(\text{aq}) + \text{Ag (s)}

  1. Separate into half-reactions.

    Oxidation: Zn (s)→Zn2+(aq)\text{Zn (s)} \rightarrow \text{Zn}^{2+}(\text{aq})
    Reduction: Ag+(aq)→Ag (s)\text{Ag}^{+}(\text{aq}) \rightarrow \text{Ag (s)}

  2. Balance atoms and charge for each half-reaction.

    Oxidation: Zn (s)→Zn2+(aq)+2e−\text{Zn (s)} \rightarrow \text{Zn}^{2+}(\text{aq}) + 2\text{e}^{-}
    Reduction: Ag+(aq)+e−→Ag (s)\text{Ag}^{+}(\text{aq}) + \text{e}^{-} \rightarrow \text{Ag (s)}

  3. Equalize electrons by multiplying the reduction half-reaction by 2.

    2Ag+(aq)+2e−→2Ag (s)2\text{Ag}^{+}(\text{aq}) + 2\text{e}^{-} \rightarrow 2\text{Ag (s)}

  4. Add the half-reactions and cancel electrons.

    Zn (s) + 2Ag+(aq)→Zn2+(aq)+2Ag (s)\boxed{\text{Zn (s) + 2Ag}^{+}(\text{aq}) \rightarrow \text{Zn}^{2+}(\text{aq}) + 2\text{Ag (s)}}

Part b· 3 steps

Cu2+(aq)+Cl−(aq)→Cu (s) + Cl2(g)\text{Cu}^{2+}(\text{aq}) + \text{Cl}^{-}(\text{aq}) \rightarrow \text{Cu (s) + Cl}_{2}(\text{g})

  1. Separate into half-reactions.

    Oxidation: Cl−(aq)→Cl2(g)\text{Cl}^{-}(\text{aq}) \rightarrow \text{Cl}_{2}(\text{g})
    Reduction: Cu2+(aq)→Cu (s)\text{Cu}^{2+}(\text{aq}) \rightarrow \text{Cu (s)}

  2. Balance atoms and charge for each half-reaction.

    Oxidation: 2Cl−(aq)→Cl2(g)+2e−2\text{Cl}^{-}(\text{aq}) \rightarrow \text{Cl}_{2}(\text{g}) + 2\text{e}^{-}
    Reduction: Cu2+(aq)+2e−→Cu (s)\text{Cu}^{2+}(\text{aq}) + 2\text{e}^{-} \rightarrow \text{Cu (s)}

  3. Electrons are already equal.

    Add the half-reactions and cancel electrons.

    Cu2+(aq)+2Cl−(aq)→Cu (s) + Cl2(g)\boxed{\text{Cu}^{2+}(\text{aq}) + 2\text{Cl}^{-}(\text{aq}) \rightarrow \text{Cu (s) + Cl}_{2}(\text{g})}

Part c· 3 steps

Pb2+(aq)+Br−(aq)→Pb (s) + Br2(aq)\text{Pb}^{2+}(\text{aq}) + \text{Br}^{-}(\text{aq}) \rightarrow \text{Pb (s) + Br}_{2}(\text{aq})

  1. Separate into half-reactions.

    Oxidation: Br−(aq)→Br2(aq)\text{Br}^{-}(\text{aq}) \rightarrow \text{Br}_{2}(\text{aq})
    Reduction: Pb2+(aq)→Pb (s)\text{Pb}^{2+}(\text{aq}) \rightarrow \text{Pb (s)}

  2. Balance atoms and charge for each half-reaction.

    Oxidation: 2Br−(aq)→Br2(aq)+2e−2\text{Br}^{-}(\text{aq}) \rightarrow \text{Br}_{2}(\text{aq}) + 2\text{e}^{-}
    Reduction: Pb2+(aq)+2e−→Pb (s)\text{Pb}^{2+}(\text{aq}) + 2\text{e}^{-} \rightarrow \text{Pb (s)}

  3. Electrons are already equal.

    Add the half-reactions and cancel electrons.

    Pb2+(aq)+2Br−(aq)→Pb (s) + Br2(aq)\boxed{\text{Pb}^{2+}(\text{aq}) + 2\text{Br}^{-}(\text{aq}) \rightarrow \text{Pb (s) + Br}_{2}(\text{aq})}

Part d· 7 steps

HCl (aq) + MnO2(s)→Cl2(g)+Mn2+(aq)\text{HCl (aq) + MnO}_{2}(\text{s}) \rightarrow \text{Cl}_{2}(\text{g}) + \text{Mn}^{2+}(\text{aq}) (acidic medium)

  1. Separate into half-reactions.

    Oxidation: Cl−(aq)→Cl2(g)\text{Cl}^{-}(\text{aq}) \rightarrow \text{Cl}_{2}(\text{g})
    Reduction: MnO2(s)→Mn2+(aq)\text{MnO}_{2}(\text{s}) \rightarrow \text{Mn}^{2+}(\text{aq})

  2. Balance atoms other than O and H.

    Oxidation: 2Cl−(aq)→Cl2(g)2\text{Cl}^{-}(\text{aq}) \rightarrow \text{Cl}_{2}(\text{g})
    Reduction: MnO2(s)→Mn2+(aq)\text{MnO}_{2}(\text{s}) \rightarrow \text{Mn}^{2+}(\text{aq})

  3. Balance O atoms by adding H2O\text{H}_{2}\text{O}.

    Reduction: MnO2(s)→Mn2+(aq)+2H2O (l)\text{MnO}_{2}(\text{s}) \rightarrow \text{Mn}^{2+}(\text{aq}) + 2\text{H}_{2}\text{O (l)}

  4. Balance H atoms by adding H+\text{H}^{+} (acidic medium).

    Reduction: MnO2(s)+4H+(aq)→Mn2+(aq)+2H2O (l)\text{MnO}_{2}(\text{s}) + 4\text{H}^{+}(\text{aq}) \rightarrow \text{Mn}^{2+}(\text{aq}) + 2\text{H}_{2}\text{O (l)}

  5. Balance charge by adding electrons.

    Oxidation: 2Cl−(aq)→Cl2(g)+2e−2\text{Cl}^{-}(\text{aq}) \rightarrow \text{Cl}_{2}(\text{g}) + 2\text{e}^{-}
    Reduction: MnO2(s)+4H+(aq)+2e−→Mn2+(aq)+2H2O (l)\text{MnO}_{2}(\text{s}) + 4\text{H}^{+}(\text{aq}) + 2\text{e}^{-} \rightarrow \text{Mn}^{2+}(\text{aq}) + 2\text{H}_{2}\text{O (l)}

  6. Electrons are already equal.

    Add the half-reactions and cancel electrons.
    2Cl−(aq)+MnO2(s)+4H+(aq)→Cl2(g)+Mn2+(aq)+2H2O (l)2\text{Cl}^{-}(\text{aq}) + \text{MnO}_{2}(\text{s}) + 4\text{H}^{+}(\text{aq}) \rightarrow \text{Cl}_{2}(\text{g}) + \text{Mn}^{2+}(\text{aq}) + 2\text{H}_{2}\text{O (l)}

  7. Convert to the full molecular equation using HCl\text{HCl} as the source of H+\text{H}^{+} and Cl−\text{Cl}^{-}, and forming MnCl2\text{MnCl}_{2} from Mn2+\text{Mn}^{2+} and spectator Cl−\text{Cl}^{-} ions.

    We need 4H+4\text{H}^{+} and 4Cl−4\text{Cl}^{-} (2 for oxidation, 2 for MnCl2\text{MnCl}_{2}) from 4HCl4\text{HCl}.

    4HCl (aq) + MnO2(s)→Cl2(g)+MnCl2(aq)+2H2O (l)\boxed{\text{4HCl (aq) + MnO}_{2}(\text{s}) \rightarrow \text{Cl}_{2}(\text{g}) + \text{MnCl}_{2}(\text{aq}) + 2\text{H}_{2}\text{O (l)}}
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