Question

2. Determine the values of E°cell and ΔG° for the following reactions.

(a) O2(g) + 4 I–(aq) + 4 H+(aq) → 2 H2O(l) + 2 I2(s) E°cell _________ ΔG°__________

(b) 4 IO3–(aq) + 4 H+(aq) → 2 I2(s) + 2 H2O(l) + 5 O2(g) E°cell _________ ΔG°_________

Answer 1

a) Oxidation half reaction : 4 I ^- (aq) $\rightarrow$ 2 I ₂ (s) + 4 e ^-

Reduction half equation : O 2 (g) + 4 H ⁺ (aq) + 4 e ^-$\rightarrow$ 2 H₂O (l)  

Overall reaction : O 2 (g) + 4 H ⁺ (aq) + 4 I ^- (aq) $\rightarrow$   2 H₂O (l) +  2 I ₂ (s)

E ⁰ cell for the overall reaction is E ⁰ cell = E ⁰ reduction - E ⁰ oxidation  

$\therefore$ E ⁰ cell = 1.229 V - 0.535 V = 0.694 V

We have relation , $\Delta$ G ⁰ = - n F  E ⁰ cell

Where n = no. of moles of electrons transferred in the overall reaction.

We have, n = 4 , F = 96485 C / mol &  E ⁰ cell = 0.694 V

$\therefore$$\Delta$G ⁰ = - 4 $\times$ 96485 $\times$ 0.694 =- 267842 J / mol =- 267.8 k J / mol

ANSWER :  E ⁰ cell = 0.694 V & $\Delta$ G ⁰ = - 267.8 k J / mol

b)

Oxidation half reaction : 10 H₂O (l)    $\rightarrow$ 5 O ₂ (g) + 20 H ⁺ (aq) + 20 e ^-

Reduction half equation : 4 IO₃^- (aq) + 24 H ⁺ (aq) + 20 e ^-$\rightarrow$ 2 I ₂ (s) + 12 H₂O (l)

Overall reaction : 4 IO₃^- (aq) + 4 H ⁺ (aq) $\rightarrow$ 5 O ₂ (g) +  2 I ₂ (s) + 2 H₂O (l)  

E ⁰ cell for the overall reaction is E ⁰ cell = E ⁰ reduction - E ⁰ oxidation  

$\therefore$ E ⁰ cell = 1.210 V - 1.229 V = - 0.019 V

We have relation , $\Delta$ G ⁰ = - n F  E ⁰ cell

Where n = no. of moles of electrons transferred in the overall reaction.

We have, n =20 , F = 96485 C / mol &  E ⁰ cell = - 0.019 V

$\therefore$$\Delta$G ⁰ = - 20 $\times$ 96485 $\times$ (- 0.019) = 36664 J / mol =36.664 k J / mol

ANSWER :  E ⁰ cell = - 0.019 V & $\Delta$ G ⁰ = 36.664 k J / mol