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

Problem 18.44

Predict whether a spontaneous reaction will occur in the forward direction in each of the following. Assume that all reactants and products are in their standard states.

(a) Sn(s) + Co2+(aq)Sn2+(aq) + Co(s)

(b) 6 Br(aq) + Cr2O72–(aq) + 14 H+(aq)2 Cr3+(aq) + 7 H2O(l) + 3 Br2(l)




Answer:

(a) Sn(s) + Co2+(aq)Sn2+(aq) + Co(s)

Sn(s)Sn2+(aq) + 2 e  Eooxidation = +0.137 V

Co2+(aq) + 2 eCo(s)  Eoreduction = –0.277 V

Eocell = +0.137 + –0.277 = –0.140 V < 0, therefore nonspontaneous.



(b) 6 Br(aq) + Cr2O72–(aq) + 14 H+(aq)2 Cr3+(aq) + 7 H2O(l) + 3 Br2(l)

2 Br(aq)Br2(l) + 2 e  Eooxidation = –1.065 V

Cr2O72–(aq) + 14 H+(aq) + 6 e 2 Cr3+(aq) + 7 H2O(l)  Eoreduction = +1.33 V

Eocell = –1.065 + 1.33 = +0.27 V > 0, therefore spontaneous.