Question B6 Total for Question B6: [15 marks] The isomerisation of CH3NC to CH,C
ID: 553199 • Letter: Q
Question
Question B6 Total for Question B6: [15 marks] The isomerisation of CH3NC to CH,CN in the gas phase shows kinetics that are dependent on the overall pressure. The data for the observed rate constant (kun) as a function of pressure, as well as the Lindmann mechanism which has been proposed to explain the observed data are provided below. (M is a molecule that does not take part in the overall chemical reaction) CH.NC + M CH.NC* + M F 101 CH,NCCH,CN CH-NC- CH3CN 101 10 103 Pressure/Torr © 2010 Pearson Education, Inc a. Derive the rate law for the proposed mechanism using the steady state approximation 5 marksl b. At low pressure, which expression is the rate law reduced to? Correlate this to the data presented in the graph. 5 marksl Based on the reaction mechanism (and the rate law), what overall reaction order is observed at low pressure (eg. 10 Torr) and at high pressure (eg. 104 Torr)? Relate this to the data. c. 5 marksExplanation / Answer
ANSWER:
(A) The rate of the reaction is given as
Rate = k2[CH3NC*] =====1)
[CH3NC*] is an intermediate for which we can use steady state approximation, i.e rate of formation of [CH3NC*] is equal to its rate of decomposition to product.
Hence k1[CH3NC][M] = k-1 [CH3NC*][M] + k2[CH3NC*]
Since M does not take part in overall reaction we did not show it in above expression.
k1[CH3NC][M] = {k-1[M] + k2 }[CH3NC*]
[CH3NC*] = k1[CH3NC] [M] / {k-1[M] + k2 }
Sunbstituting the value of [CH3NC*] in equation 1)
Rate = k2k1[CH3NC][M] / {k-1[M] + k2 } = Kuni [CH3NC] ====2)
Kuni = k2k1[M] / {k-1[M] + k2 } ======3)
(b) At sufficient low pressure k-1[M] << k2 , (because there will be lesser number of collisions) , so k-1[M] can be neglected in denominator. In that case we have
Rate = k1[CH3NC][M]
The reaction behaves as second order.From the graph it is clear that at low pressure the rate increases more with increase in pressure than at high pressure.
(C) At high pressure k-1[M] >> k2 Hence k2 can be neglected in dnominator.
Rate = k2k1[CH3NC][M] / k-1[M] = k2k1[CH3NC] / k-1
The rate is first order.