[CSG]

One of the questions in my textbook is to see whether the following reactions are spontaneous or not:

Pb(s) + Fe²⁺(aq)

Br₂(aq) + 2Cl^-(aq)

According to the text book they are not spontaneous. I worked out the cell potential for each of them and they are both positive, so I put them both down as spontaneous...

How do you know whether or not they are spontaneous? Also, is the fact that they are not in full equations significant?

[Borek]

I worked out the cell potential for each of them and they are both positive, so I put them both down as spontaneous...

Show how.

[CSG]

Ok so this is what I did:

for Pb(s) + Fe²⁺(aq)

Oxidation half reaction: Fe(s)   Fe²⁺(aq) +2e^-                         +0.45V

Reduction half reaction: Pb²⁺(aq) + 2e^-   Pb(s)                        -0.13V
...
                                 Fe(s) + Pb²⁺(aq)   Pb(s) + Fe²⁺(aq)    +0.32V

I assuming that the products on the R.H.S of the equilibrium refers to the question. Have I done it correctly?

[FreeTheBee]

Wouldn't the question be whether the components given, lead to a spontaneous reaction?

[CSG]

The exact wording was: "determine if the following reactions are spontaneous or not."

[Borek]

for Pb(s) + Fe²⁺(aq)

Oxidation half reaction: Fe(s)   Fe²⁺(aq) +2e^-                         +0.45V

Reduction half reaction: Pb²⁺(aq) + 2e^-   Pb(s)                        -0.13V

You were asked about mixture of Fe²⁺ and Pb⁰, only thing that can happen is lead getting oxidized and iron getting reduced. Reaction will be spontaneous if the potential of oxidizer is higher than the potential of the reducing agent (E_ox - E_red > 0).

[CSG]

Thanks, but I still do not understand why both of those reactions are not spontaneous according to the answers in my textbook.

[Borek]

I think you have missed fact that my description of what may happen accroding to given initial composition is exactly opposite to the reactions you wrote.

[CSG]

Are you saying that Pb is a better reducing agent than Fe?

[FreeTheBee]

"determine if the following reactions are spontaneous or not."
Pb(s) + Fe2+(aq)

You are looking if the exact opposite, Fe(s) + Pb2+(aq), would lead to a spontaneous reaction.