3. * (10 marks) The electrochemical cell Zn(s) | Zr\" (? M) 11 Ni2+ (1.00 M) I N

Question

3. * (10 marks) The electrochemical cell Zn(s) | Zr" (? M) 11 Ni2+ (1.00 M) I Ni(s) is constructed using a completely immersed zinc electrode that weighs 32.68 g and a nickel electrode immersed in 575 mL of 1.00 M Ni2(a) solution. A steady current of 0.0715 A is drawn from the cell as the electrons move from the zinc electrode to the nickel electrode. Zn2 (a)2e > Zn(s0.76 V Ni2+ (aq) + 2 e--> Ni(s) E° =-0.23 V a. Which reactant is the limiting reactant in this cell? b. How long,in hours, does it take for the cell to be completely discharged? c. What mass has the nickel electrode gained when the cell is completely discharged? d. What is the concentration of the Ni2+(aq) when the cell is completely discharged?

Explanation / Answer

(Reduction Potential) Eo(red) of Ni2+|Ni(s)=-0.23 V

(Reduction Potential) Eo (red) of Zn2+(aq)| Zn(s) =-0.76 V

(Reduction Potential) of Ni2+|Ni(s) > (Reduction Potential) Eo (red) of Zn2+(aq)| Zn(s)

So,Cathode : Ni2+|Ni(s) (reduction half-rxn):

Ni2+(aq) +2e- --->Ni(s)

Anode:(oxidation half-rxn):

Zn(s) --->Zn2+(aq)+2e-

Net cell Rxn: Ni2+(aq)+Zn(s) --->Zn2+(aq)+Ni(s)

Eo(cell)=Eo(cathode)-Eo(anode)=-0.23V-(-0.76V)=0.53V

a) mol of Zn(s) =32.68g/molar mass of Zn=(32.68g/65.38g/mol)=0.041 mol

mol of Ni2+(aq)=0.575L*1.00M =0.575mol

Ni2+(aq)+Zn(s) --->Zn2+(aq)+Ni(s)

mol Ni2+/mol Zn=1:1

As Zn(s) is present in lesser amount so Zn is the limiting reactant.

b)Current(I)=Charge passed(Q)/time(t)

As time(t)=Q/I= nF/I ,F=faraday=96485C/mol,n=mol of electrons exchanged =2mol

t=(2mol)*(96485C/mol)/0.0715A=2698881.118 s=2698881.118s(1h/3600s)=750hours [A=C/s]

c) mol of imiting reactant reacted=0.041 mol

So,mol of Ni(s) added=0.041 mol

mass of Ni(s) added to Ni electrode=0.041 mol *molar mass of Ni=0.041 mol*(58.693g/mol)=2.406 g

d)Ni2+ was reduced to Ni(s) so it reduced by 0.041mol

Initial mol of Ni2+=0.575mol

Ni2+ (remaining)=0.575mol -0.041mol=0.534 mol

[Ni2+]aq remaining=0.534 mol/0.575L=0.929M

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