# Chemical Reaction Engineering Questions and Answers – Reducing the Size of Kinetic Models

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This set of Chemical Reaction Engineering Questions and Answers for Campus interviews focuses on “Reducing the Size of Kinetic Models”.

1. For the parallel reaction A → B and A → C, of rate constants k1 and k2 respectively, both reactions of order 1, the rate expression is given as ____
a) (-rA) = k1CA + k2CA
b) (-rA) = k1CA – k2CA
c) (-rA) = k2CA
d) (-rA) = k1CA

Explanation: For a zero order reaction, $$\frac{-dC_A}{dt}$$ = k1CA + k2CA
A forms two products: B and C. The concentration of A decreases as the reaction progresses.

2. The rate determining step of a series of reactions is the one ____
a) That is fastest
b) That is slowest
c) That does not contribute to the reaction
d) That does not occur

Explanation: Rate determining step is the slowest of all the reaction steps. The rate determining step is important in deriving the rate equation of a chemical reaction.

3. An assumption of Steady State Approximation is ____
a) Reaction occurs without the formation of intermediates
b) Catalyst does not accelerate reaction rate
c) One of the intermediates in the reaction is consumed as quickly as it is generated
d) Equilibrium state is attained at the end of a reaction

Explanation: The steady-state approximation is a method used to derive the rate expression. It is based on the assumption that one intermediate in the reaction mechanism is consumed as fast as it is generated.
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4. The reaction involving oxidation of nitric oxide to nitrous oxide follows the mechanism ____
a) $$\frac{-dO_2}{dt}$$ = k[NO2]2[O2]
b) $$\frac{-dO_2}{dt}$$ = k[NO2][O2]
c) $$\frac{-dO_2}{dt}$$ = k[NO2]2[O2]2
d) $$\frac{-dO_2}{dt}$$ = k[NO2][O2]2

Explanation: In the first step, 2 molecules of NO form N2O2. Further oxidation of N2O2 is the slowest step and it is the rate determining step.

5. The order of the reaction involving the conversion of ozone to oxygen is ____
a) Zero order
b) First order
c) Second order
d) Third order

Explanation: The rate of decomposition of ozone is:
-rO3= k[O3]2[O2]-1
Order = 2-1 = 1

6. At very low concentrations of azomethane, its decomposition follows which order?
a) First order
b) Zero order
c) Third order
d) Second order

Explanation: The rate expression for decomposition of azomethane is:
rN2 = $$\frac{k_1 k_3 C_{Azo}^2}{k_3+k_2 C_{Azo}}$$
Where, k1, k2 and k3 are rate constants of the 3 intermediate reactions involved in azomethane formation.
At low concentration, k2 CAzo << k3
Hence, rN2=k1CAzo2.

7. State true or false.
Gas phase decomposition of N2O follows first order mechanism for low concentrations of N2O and second order mechanism for high concentrations of N2O.
a) True
b) False

Explanation: The rate expression is, rN2 = $$\frac{k_1 NO_2^2}{1+kNO_2}$$
At low concentrations of N2O, kNO2 << 1 and the reactions follows second order. At high concentrations of N2O, kNO2 >> 1 and the reaction follows first order.

8. State true or false.
Decomposition of nitrogen pentoxide follows second order mechanism, assuming that the kinetics is controlled by a single step mechanism.
a) True
b) False

Explanation: The rate of N2O5 decomposition is given by, r = k[N2O5].
Hence, the reaction follows first order.

9. Formation of hydrogen iodide from the corresponding elements is a ____
a) Single step mechanism
b) Two step mechanism
c) Three step mechanism
d) Four step mechanism

Explanation: In the slow step, iodine decomposes to give 2[I]. In the consequent step, hydrogen reacts with 2[I] to give hydrogen iodide.

10. In the formation of phosgene from carbon monoxide and chlorine, which of the following is rate determining?
a) Decomposition of chlorine
b) Formation of COCl.
c) Reaction of COCl. with Cl2
d) Decomposition of CO2

Explanation: Reaction of COCl. with Cl2 is the slow reaction. Hence, it is the rate determining step.

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