Question

For a voltaic cell based on the reaction below, what reaction happens at the anode?
2Ni3+(aq)+Cd(s)→2Ni2+(s)+Cd2+(aq)
   Ni3+(aq)+e-→Ni2+(aq)
   Ni2+(aq)→ Ni3+(aq)+e-
   Cd2+(aq)+2e-→Cd(s)
   Cd(s)→Cd2+(aq)+2e-

A student measures the voltage across different metal electrodes placed in a voltaic cell. Using a copper cathode, the voltages when using a first a tin, then gold anode are positive and negative, respectively. The order of chemical reactivity of the solid metals must be
   Au>Cu>Sn
   Au=Sn
   Sn>Cu>Au
   Au=Sn>Cu

Answer 1

1)
At anode oxidation takes place.
Here Cd is being oxidised to Cd2+

So, reaction at anode is:
Cd -> Cd2+ + 2e-

Answer: option 4

2)
When copper is cathode and tin is anode, Eo cell is positive.
So, reaction is spontaneous.
So, Sn reduces Cu.
So, Sn is more reactive than Cu

When copper is cathode and gold is anode, Eo cell is negative.
So, reaction is non spontaneous.
So, Au can’t reduce Cu.
So, Cu is more reactive than Au

Answer: Sn > Cu > Au