Question

Which of the following redox reactions do you expect to occur spontaneously in the forward direction?

__ Ni(s)+Pb2+(aq) ? Ni2+(aq)+Pb(s)
	__ Al(s)+3Ag+(aq) ? Al3+(aq)+3Ag(s)
	__ Cd2+(aq)+Co(s)?Co2+(aq)+Cd(s)
__	Fe2+(aq)+Cd(s)?Fe(s)+Cd2+(aq)

Answer 1

The general rule for any redox reaction is the species on top of the standard reduction potential chart will displace the species below it. Thus, the species above will act as reducing agents and itself gets oxidized, while the species at the bottom will act as the oxidizing agent and will get reduced in the process.

--------------------------------------------------------------------------------------------------------------------------

The species can be arranged according to its standard reduction potential values as,

Al3+(aq) + 3e- ----> Al(s) [E^ored = -1.662 V]

Fe2+(aq) + 2e- ----> Fe(s) [E^ored = -0.44 V]

Cd2+(aq) + 2e- ----> Cd(s) [E^ored = -0.40 V]

Co2+(aq) + 2e- ----> Co(s) [E^ored = -0.28 V]

Ni2+(aq) + 2e- ----> Ni(s) [E^ored = -0.25 V]

Pb2+(aq) + 2e- ----> Pb(s) [E^ored = -0.126 V]

Ag+(aq) + 1e- ----> Ag(s) [E^ored = +0.7996 V]

--------------------------------------------------------------------------------------------------------------------

Thus looking at the data provided above we can say that, the displacement of species in aqueous solution will follow the following order,

Al > Fe > Cd > Co > Ni > Pb > Ag

Thus, Al has highest potential to get oxidized and Ag has the highest potential to get reduced.

Now looking at the options,

(a) Ni(s)+Pb²⁺(aq) ---> Ni²⁺(aq)+Pb(s) - Spontaneous in the forward direction.

In this reaction, Ni is displacing Pb from the solution and getting oxidized. This is a possible by the above explanation. Ni is placed higher in the potential chart then Pb and therefore would easily displace Pb from solution. Ni will act as reducing agent and Pb an oxidizing agent in the redox reaction.

(b)   Al(s)+3Ag⁺(aq) -----> Al³⁺(aq)+3Ag(s) - Spontaneous in the forward direction.

In this reaction, Al is displacing Ag from the solution and getting oxidized. This is a possible by the above explanation. Al is placed higher in the potential chart then Ag and therefore would easily displace Ag from solution. Al will act as reducing agent and Ag an oxidizing agent in the redox reaction.

(c) Cd²⁺(aq)+Co(s) ------> Co²⁺(aq)+Cd(s) - Not-Spontaneous in the forward direction.

In this reaction, Co is displacing Cd from the solution and getting oxidized. This is a impossible reaction by the above explanation. Co is lower in the potential chart then Cd and thus would not displace Cd from solution.

(d) Fe²⁺(aq)+Cd(s) -----> Fe(s)+Cd²⁺(aq) - Not-Spontaneous in the forward direction.

In this reaction, Cd is displacing Fe from the solution and getting oxidized. This is a impossible reaction by the above explanation. Cd is placed lower in the potential chart then Fe and thus would not displace Fe from solution.