Question

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

Answer 1

Assuming your looking at the following equations:
1) 2Ag+(aq) + Ni(s) ------> 2Ag(s) + Ni2+ (aq)

2) Fe(s) + Mn2+(aq) ------> Fe2+(aq) + Mn(s)

3) 2Al(s) + 3Pb2+(aq)-----> 2Al3+(aq) +3Pb(s)

4) Ca2+(aq) + Zn(s) -------> Ca(s) + Zn2+(aq)


Occuring spontaneously in the reverse direction means that
it's nonspontaneous in the forward reaction

Ecell = Ecathode-Eanode

Spontaneous when Ecell negative, when cathode potential is greater than anode's

nonspontaneous when anode potential is greater than cathode's.


1. Ag is getting reduced and Ni is getting oxidized, so, in the table,
Ni (anode) should be above Ag(cathode) in order to be nonspontaneous.

The reaction is spontaneous. Does not occur spontaneously in the reverse direction.

2. Fe is oxidized and Mn is being reduced.

Fe (anode) should be higher than Mn(cathode) = yes

Occurs spontaneously in the reverse direction.

3. Al oxidized, Pb reduced.

Al should be above Pb, no.

Does not occur spontaneously in the reverse direction.

4. Zn oxidized, Ca reduced.

Zn should be above Ca, yes.

Occurs spontaneously in the reverse direction.







* tip: usually, to see if it's spontaneous in a forward reaction,

I skim the equation to see which one is going through reduction/oxidation.

then, I expect to see reduction half reaction to be located above the oxidation reaction.

Ecathode - Eanode = Ecell

+ (positive) - (less positive) = will always be positive!

Answer 2

For answering spontaneity questions, you need a Data Booklet. This contains information on the E values of particular reactions. For a reaction to bespontaneous, the overall E cell value (E reduction minus E oxidation) has to be greater than zero ie positive.

Ca2+ + 2e --> Ca (-2.87V)
Zn2+ + 2e --> Zn (-0.76V)
Hence, to have a positive reaction, we would have to have calcium oxidized and hence, the reaction from Ca to Ca2+ is spontaneous. This means that in thefirst reaction above, the reverse reaction is spontaneous.

Ag+ + e --> Ag (+0.80V)
Ni2+ + 2e --> Ni (-0.25V)
Note that it doesn't matter whether 2Ag are used or 200Ag, the E values are independent of coefficients. Hence, to have a positive reaction, we would haveto have Ni oxidized and hence the reaction from Ni to Ni2+ is spontaneous. This means that in the second reaction above, the forward reaction isspontaneous and the reverse reaction not.

Fe2+ + 2e --> Fe (-0.44V)
Mn2+ + 2e --> Mn (-1.18V)
Hence, to have a positive reaction, we would have to have Mn oxidized and hence the reaction from Mn to Mn2+ is spontaneous. This means that in the thirdreaction above, the reverse reaction is spontaneous.

Al3+ + 3e --> Al (-1.66V)
Pb2+ + 2e --> Pb (-0.13V)
Hence, to have a positive reaction, we would have to have Al oxidized and hence the reaction from Al to Al3+ is spontaneous. This means that in the lastreaction above, the forward reaction is spontaneous and hence the reverse reaction is not spontaneous.

Hope I was of help!