Balancing redox reactions using the half reaction method

Question

Al(s)+Ag+(aq)→Al3+(aq)+Ag(s)

Zn(s)+Sn2+(aq)→Zn2+(aq)+Sn(s)

Mg(s)+Cr3+(aq)→Mg2+(aq)+Cr(s)

Answer 1

Oxidation half reaction:

Al(s) ==> Al³⁺(aq)

Al ===> Al³⁺ + 3e- ... balance the charge

Reduction half reaction:

Ag⁺ ===> Ag(s)

Ag⁺ + e- ==> Ag ... balance the charge

Multiply reduction half reaction by 3 to equalize the electrons used in the oxidation half reaction and add the two half reactions together:

Al ===> Al³⁺ + 3e-

3Ag⁺ + 3e- ==> 3Ag

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Al(s) + 3Ag⁺(aq) ==> Al³⁺(aq) + 3Ag(s) ... overall balanced equation

Hopefully you can do the others using the same basic approach.