Change in concentration in equilibrium caused by addition

1. Jun 30, 2016

TT0

1. The problem statement, all variables and given/known data

Cu2+ ions react with Fe2+ ions according to the following reaction.

Cu2+ + 2Fe2+ --> Cu + 2Fe3+

At equilibrium, the concentration of Cu2+ ions is not changed by the addition of
1. A) Cu2++

2. B) Fe2+

3. C) Cu

4. D) Fe3+.
2. Relevant equations

3. The attempt at a solution

I chose A through elimination.

B won't work because the equilibrium will shift to the right if Fe2+ is added.

C and D won't work because the equilibrium will shift to the right.

Therefore A seems the most correct. However, if Cu2+ is added, then the equilibrium will also shift to the right. This means that it shifts to the right until the Cu2+ concentration is back to what it was before which seems impossible. Could someone shed some light on this?

2. Jun 30, 2016

Staff: Mentor

Can you write formula for the equilibrium constant for this problem?

3. Jun 30, 2016

TT0

$$\frac {[Cu][Fe^{3+}]^2} {[Cu^{2+}][Fe^{2+}]^2}$$

This is the equilibrium constant. (took me a while to get the latex working)

4. Jun 30, 2016

Staff: Mentor

Not exactly. Solid copper is not dissolved, so it doesn't have a concentration.

5. Jun 30, 2016

TT0

I see, so since it isn't dissolved, adding solid copper wouldn't change it's concentration. But won't it will still increase the backwards rate of reaction so it will still affect the Cu2+ concentration? This is the best I could come up with from the comment.

6. Jul 1, 2016

Staff: Mentor

Changes in the amount of solid change both forward and backward reaction speed in exactly the same way, the net effect cancels out.

That's why there are no solids in the equilibrium formula (we assume activity of the solids equals exactly 1).

7. Jul 1, 2016

TT0

I see, so as a rule, is this correct?

If solid reactants or products is added to a system already in equilibrium, the concentrations of all chemicals will stay the same; the reaction rate will increase equally in both directions.

Thank you very much!

8. Jul 1, 2016

Staff: Mentor

And the equilibrium won't shift in any direction.

9. Jul 1, 2016

TT0

Cheers! This is useful knowledge!