# chemistry - exam practice help!!

posted by Anya

Given the table below predict the numerical value of the standard cell potential for the reaction:
2 Cr(s) + 3 Cu2+(aq) 2 Cr3+(aq) + 3 Cu(s)

Half Reaction E (volts)
(1) Cr3+ + 3 e- Cr E= -0.74
(2) Cr3+ + e- Cr2+ E=-0.41
(3) Cr2O72- + 14 H+ + 6 e- 2 Cr3+ + 7 H2O
E= 1.33
(4) Cu+ + e- Cu E= 0.52
(5) Cu2+ + 2 e- Cu E= 0.34
(6) Cu2+ + e- Cu+ E= 0.16

how do i do this?

options:

a ) -0.4
b) 2.5
c) 1.08
d) 0.417
e) -1.08

(all units are volts)

i tried

E(products) - E(reactants) but it didn't work, i didn't get any of the options. :S

1. DrBob222

First, you have no arrows. How can you possibly know which are products and which are reactants. I think you do yourself a disservice by not including an arrow.
For 2Cr + 3Cu^2+ ==> 3Cu + 2Cr^3+
Divide this into its two half cells.

Cr(s) ==> Cr^3+ + 3e
Cu^2+ + 2e ==> Cu(s)

Now look at the half cell voltages given to you. 1 is the one you want for Cr. That is
Cr3+ + 3 e- ==> Cr E= -0.74 which is just the reverse of your half cell. Therefore, the voltage for the reverse reaction is +0.74.

For the Cu, you want #5
Cu2+ + 2 e- ==> Cu E= 0.34
Eocell is Eox + Ered = 0.74+0.34 = ?

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