4 a) (6 points) In the 1960’s the following mechanism for reaction of H2(g) with
ID: 931308 • Letter: 4
Question
4 a) (6 points) In the 1960’s the following mechanism for reaction of H2(g) with I2(g) to form HI(g) was proposed. I2(g) 2I(g) fast RXN 1
H2(g) + I(g) H2I(g) fast
RXN 2 H2I(g) +I(g) 2HI(g) slow RXN 3
The experimental rate law was determined to be rate = k [H2][I2]. Is this mechanism correct?
b) (4 points) Using the above mechanism, and the activation energies of 1, 0.5 and 75 kJ mol-1, for steps 1, 2 and 3 respectively, determine the temperature required to increase the rate of the reaction by a factor of 10 relative to the rate at 298 K.
Explanation / Answer
4.a) The given rate law based on the mechanism given is correct.
b) ln(k2/k1) = Ea/R[1/T1 - 1/T2]
with,
k2/k1 = 10
(i) Ea = 1 kJ/mol
R = 8.314 J/K.mol
T1 = 298 K
T2 = ? [temperature needed to increase rate of reaction by factor of 10]
we get,
ln(10) = 1000/8.314[1/298 - 1/T2]
T2 = -63.34 K
When,
(ii) Ea = 0.5 kJ/mol
we get,
ln(10) = 500/8.314[1/298 - 1/T2]
T2 = -28.63 K
when,
(iii) Ea = 75 kJ/mol
we get,
ln(10) = 75000/8.314[1/298 - 1/T2]
T2 = 322.533 K
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