Preidict which of the following reactions would not procees spontaneously at `298 K`

Options

A. `Zn^(2+)(aq.)+H_(2)underset(larr)(rarr) Zn(s)+2H^(+)(aq.)`
B. `Cd(s)+Fe^(2+)(aq.)underset(larr)(rarr)Cd^(2+)(aq.)+Fe(s)`
C. `Cd^(2+)(aq.)+Fe_(s)underset(larr)(rarr) Cd(s)+Fe^(2+)(aq.)`
D. `Co(s)+Fe^(2+)(aq.)underset(larr)(rarr)Co^(2+)(aq.)+Fe(s)`

Answer

B

Explanation

For a reaction that is spontaneous at standard condi-tions, `E^(@)` must be positive. A negative value of `E_("cell")^(@)` indicates that the reverse of the reaction written would be spontaneous at standard conditions.
Refer to the table of standard reduction potentials and choose the two appropriate half reactions :
`{:(Zn^(2+)(aq.)+2e^(-)hArr Zn(s)" "E^(@)=-0.76V),(2H^(+)(aq.)+2e^(-)hArrH_(2)(g)" "E^(@) =-0.00 V),(Cd^(2+)(aq.)+2e^(-)hArr Cd(s)" "E^(@)=-0.40 V),(Co^(2+)(aq.)+2e^(-)hArr Co(s)" "E^(@)=-0.28V),(Fe^(2+)(aq.)+2e^(-)hArr Fs(s)" "E^(@) =-0.44 V):}`
Since `E_(Cd^(2-)//Cd)^(@) gt E_(Fe^(2-)//Fe)^(@), Fe^(2+)` ions can't oxidize Cd i.e. the reaction (2) is nonspontaneous.