Calculate the Δr G° and the equilibrium constant for the reaction
2Cr(s) + 3Cd2+ (aq) 2Cr3+ (aq) + 3Cd(s)
(Given E°Cr3+/Cr = 0.74 V, E°Cd2+/ Cd = – 0.40 V)


            E°cell = E°Cd2+/Cd - E°Cr3+/Cr          = - 0.40 - (0.74) = +0.34 V
∴         rG° = -nFE° = -6 × 96500 × 0.34 = -196860 J     K = Antilog nE°0.059 = Antilog6×0.340.059          = Antilog (34.5762)  K = 3.77 × 1034.

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Calculate the cell e.m.f. and ΔG for the cell reaction at 298 K for the cell
Zn(s) | Zn2+ (0.0004 M) || Cd2+(0.2 M) | Cd(s)
(Given E°Zn2+/Zn = 0.763 V, E°cd 2+ / cd = 0.403 Vat 298 K, F = 96500 C mol–1)


(i)    According to nernst equation:
          E = E°-0.0591nlogZn2+(aq)Cd2+ (aq)E°cell = E°Cd2+/Cd-E°Zn2+/Zn          = (0.403) - (-0.763) = 0.36 V
          [Zn+(aq)] = 0.0004 M,  [Cd2+(aq)] = 0.2 M,  n = 2
                        E=(0.36)-(0.0591 V)2log0.00040.2  = 0.36  - (0.0591 V)2×(-2.69990) = 0.36 V + 0.08 = 0.44 V
(ii)     
          G = -nFEcellE°cell = 0.44 V, n = 2 mol, F = 96500 c mol-1G = -2 (mol) × (96500 C mol-1) × (0.44 V)        = - 84920 CV = -84920 J.

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State whether the following statements are true (T) or false (F):

A.

According Kohlrausch law the value of Λ°of an electrolyte AB2 which dissociates as A2++ 2B is given by λ°A+ + λB

B.

The amount of electricity required to deposit 1 mol of aluminium from a solution of AlClwill be 3F.

C.

The molar conductivity of an electrolyte increases as concentration decreases.

D.

During the discharging of lead storage battery the density of H2SO4 decreases.

E.

Daniell cell converts the heat released during the combustion of H2 to an electrical energy.

A. FALSE
B. TRUE
C. TRUE
D. TRUE
E. FALSE
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State whether the following statements are true (T) or false (F):

A.

A cell is feasible if its Ecell is negative.

B.

The cell constant is independent of the concentration of the electrolyte in the conductivity cell.

C.

In recharging of lead storage battery sulphuric acid is produced.

D.

 The half-cell H+ (0.5)/1/2 H2 (g) (1 atm)/Pt has zero electrode potential.

E.

The dependence of electrode potential for the electrode Mn+/ M with concentration under STP condition is given by expression.
E=E0+0.06nlog Mn+


A. FALSE
B. TRUE
C. TRUE
D. FALSE
E. FALSE
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Calculate the cell emf and AG for the cell rection at 25°C for the cell
Zn(s) | Zn2+ (0.0004 M) || Cd2+ (0.2 M) | Cd(s) E° values at 25°C, Zn2+/Zn = – 0.763 V, Cd2+ / Cd = – 0.403 V F = 96500 C mol–1, R = 8.314 J K–1 mol–1

Applying nernst equation :
E°cell = E°cathode - E°anode = -0.403 - (-0.763) =0.36 V
     Ecell = E°cell + 0.059nlogCd2+Zn2+
or  
                    Ecell = 0.36+0.059nlog0.20.004= 0.36+0.0592log 500         = 0.36+0.0592×2.6990Ecell = 0.4396 V
Now      rG = -nFE
or           rG = -2×96500×0.4396 = -84842.8 J mol-1          =  84.84 kJ mol-1
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