Kinetics of Decomposition of HI

Kinetics of Decomposition of HI

Kinetics of Decomposition of HI

Kinetics of Decomposition of HI

2HI ---h𝜈→ H2 + I2
The possible mechanism of the above reaction is-
i. HI + h𝜈 ---k1→ H + I
ii. H + HI ---k2→ H2 + I
iii. I + I ---k3→ I2
where, k1, k2 and k3 are rate constants.
The rate of formation of decomposition of HI is given by-
−d[HI]/dt = k1Iabs + k2[H][HI] -----Equation-1
d[H]/dt = k1Iabs + k2[H][HI] Applying SSA we get-
0 = k1Iabs + k2[H][HI]
k2[H][HI] = k1Iabs -----Equation-2
Substituting the value of k2[H][HI] in equation-1 we get-
−d[HI]/dt = 2k1Iabs -----Equation-3
The quantum yield of the reaction is given by-
Φ = Rate of disappearance of HI/Rate of absorption of light
or, Φ = (−d[HI]/dt)/k1Iabs
or, Φ = 2k1Iabs/k1Iabs
or, Φ = 2
In this case, Φ decreases as the reaction proceeds. This is due to the fact that as iodine accumulates, the thermal reaction
H + I2 ---k4→ HI + I
becomes significant.
If this reaction is also induced in the mechanism, then the steady state approximation for [H] gives-
d[H]/dt = k1Iabs − k4[H][I2] = 0 -----Equation-4
[H] = k1Iabs/(k2[HI] + k4[I2]) -----Equation-5
Substituting the value of [H] in equation-1 from equation-5 we get-
Kinetics of Decomposition of HI
As the reaction proceeds, [I2] increases and hence the quantum yield decreases.

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