[TwoPlusTwo]

Hi, can anyone tell me what I'm doing wrong here?

"Calculate the equilibrium constant for the redox reaction that would take place in the following instance, and use that number to determine whether a reation would occur or not.

A piece of copper wire in a 1.0 M solution of Pb(NO3)2 (aq)"

Standard reduction potential for the half reactions is given:

Copper: 0.337 V
Lead: -0.126 V

So first I have to find the cell potential, using this formula:

E(cell)=E(red)-E(ox)

As I understand it, the half reation with the most positive reduction potential needs to be first, in this case copper:

E(cell)=0.337 V - (-0.126 V) = 0.463 V

But this is apparently wrong. The solution to the problem says I have to switch the two numbers, like this:

E(cell)=(-0.126 V) - 0.337 V = -0.463 V

Why is that?

[FreeTheBee]

What would a possible (or perhaps not) reaction be, looking at your starting materials?

[TwoPlusTwo]

I would guess something like this:

Cu (s) + Pb(2+) (aq) ---- Cu(2+) (aq) + Pb (s)

[rabolisk]

As I understand it, the half reation with the most positive reduction potential needs to be first, in this case copper:

This results in E(cell) always being nonnegative (at least zero). That is, this yields a reaction that is spontaneous. The opposite direction would be nonspontaneous.

[TwoPlusTwo]

As I understand it, the half reation with the most positive reduction potential needs to be first, in this case copper:

This results in E(cell) always being nonnegative (at least zero). That is, this yields a reaction that is spontaneous. The opposite direction would be nonspontaneous.

So how do I know which order to choose when calculating the cell potential?

[rabolisk]

As I understand it, the half reation with the most positive reduction potential needs to be first, in this case copper:

This results in E(cell) always being nonnegative (at least zero). That is, this yields a reaction that is spontaneous. The opposite direction would be nonspontaneous.

So how do I know which order to choose when calculating the cell potential?

Context. This problem gives you the reaction (in words) and asks you whether or not the reaction is spontaneous in the direction it is written, and to justify it quantitatively. It does not ask you to find out which direction is spontaneous and to justify it quantitatively. i.e. they already give you a direction, which is copper being oxidized and lead being reduced.

Note that E(cell)= E(red)- E(ox) is always correct. What's not always correct is the idea that E(red) must correspond to the half reaction with the more positive reduction potential. E(red) is the reduction potential of the half reaction containing the element that is reduced. In this case, it is lead, because that is how the reaction is written (or described).

[TwoPlusTwo]

As I understand it, the half reation with the most positive reduction potential needs to be first, in this case copper:

This results in E(cell) always being nonnegative (at least zero). That is, this yields a reaction that is spontaneous. The opposite direction would be nonspontaneous.

So how do I know which order to choose when calculating the cell potential?

Context. This problem gives you the reaction (in words) and asks you whether or not the reaction is spontaneous in the direction it is written, and to justify it quantitatively. It does not ask you to find out which direction is spontaneous and to justify it quantitatively. i.e. they already give you a direction, which is copper being oxidized and lead being reduced.

Note that E(cell)= E(red)- E(ox) is always correct. What's not always correct is the idea that E(red) must correspond to the half reaction with the more positive reduction potential. E(red) is the reduction potential of the half reaction containing the element that is reduced. In this case, it is lead, because that is how the reaction is written (or described).

Great! That clears it up a whole lot for me. Thanks!