Question

E° (cal.) E° (exp.) 1. Fe(s)| Fe2+ (1 M) || Cu2+ (1 M) | Cu(s) 0.775 V 2. Pb(s) Pb2+ (1 M) || Cu2+ (1 M) Cu(s) 0.461 V 3. Sn(s) | Sn2+ (1 M) || Cu2+ (1 M) | Cu(s) 0.472 V 4. Zn(s) | Zn2+ (1 M) || Cu2+ (1 M) Cu(s) 1.095 V 5. Zn(s) | Zn2+ (1 M) || Cu2+ (1 M + NH3) | Cu(s) 0.928 V a) The Nernst equation for the cell in line 5 above is: E = Eº 0.0592 log 2027 [Cu2+] What has caused the change in its emf compared to the cell in line 4 above? (Hint: Compare the color of the solution in the cathode half-cell with the color of the 1M. CuSO4 stock solution.)

Answer 1

Copper(II) forms co-ordination complex with ammonia ligand in aqueous medium. The reaction is shown below.

[Cu(H₂O)₆]SO₄ + 4 NH₃ (aq) $\rightarrow$ [Cu(NH₃)₄(H₂O)₂]SO₄ + 4 H₂O(l)

The tetraamine-Cu(II) complex is dark blue in color. Therefore the cathode cell appears darker compared to 1M CuSO₄ stock solution. Due to the above reaction effective Cu²⁺ ion concentration in the half cell decreases below 1M as a result, E^o decreases.