Under standard conditions for all concentrations, the following reactions is spontaneous at `25^@C`
`O_2(g)+4H^+ (aq)+4Br^(-) (aq) to 2H_2O (l) +2Br_2 (l), E^@=0.16V`
IF `[H^+]` is adjusted by adding a buffer of 0.10 M NaOCN
and 0.10 M HOCN `(K_a=3.5 times 10^-4)` what value will `E_(cell)` have and will the reaction be spontaneous at this `[H^+]`?

Explanation

For a buffer solution of HOCN and NaOCN we have
`[H^+]=K_a times ([HOCN])/([NaOCN]) =3.5 times 10^-4 times 0.1/0.1=3.5 times 10^-4`
Thus,
`E_(cell)=E_(cell)^@-0.0591/4 log""1/([H^+]^4[Br^-]^4)`
`=0.16-0.0591/4 log ""1/((3.5 times 10^-4)^4 (1.0)^4)`
`=-0.044V`
Negative value of `E_(cell)` suggests that the given cell reaction is nonspontaneous when `[H^+]` is adjusted by a buffer solution.