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J. W. Hill, R. H. Petrucci, T. W. McCreary, & S. S. Perry General Chemistry 4th edition

Problem 18.27

Use the half-reaction method to balance the following redox equations in acidic solution.

(a) Fe2+(aq) + Cr2O72–(aq) Fe3+(aq) + Cr3+(aq)

(b) S8(s) + O2(g)SO42–(aq)

(c) Fe3+(aq) + NH2OH2+(aq) Fe2+(aq) + N2O(g)






Answer:

(a) Fe2+(aq) + Cr2O72–(aq) Fe3+(aq) + Cr3+(aq)

oxidation:

[ Fe2+(aq)Fe3+(aq) + e ] × 6

reduction:

Cr2O72–(aq) + 14 H+(aq) + 6 e2 Cr3+(aq) + 7 H2O(l)

net:

6 Fe2+(aq) + Cr2O72–(aq) + 14 H+(aq)6 Fe3+(aq) + 2 Cr3+(aq) + 7 H2O(l)



(b) S8(s) + O2(g)SO42–(aq)

oxidation:

S8(s) + 32 H2O(l)8 SO42–(aq) + 64 H+(aq) + 48 e

reduction:

[ O2(g) + 4 H+(aq) + 4 e2 H2O(l) ] × 12

net:

S8(s) + 12 O2(g) + 8 H2O(l)8 SO42–(aq) + 16 H+(aq)



(c) Fe3+(aq) + NH2OH2+(aq)Fe2+(aq) + N2O(g)

oxidation:

[ Fe3+(aq) + eFe2+(aq)] × 4

reduction:

2 NH2OH2+(aq)N2O(g) + 6 H+(aq) + H2O(l) + 4 e

net:

4 Fe3+(aq) + 2 NH2OH2+(aq)4 Fe2+(aq) + N2O(g) + 6 H+(aq) + H2O(l)