# Chemical Reaction Engineering Questions and Answers – Reaction Rates – 2

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This set of Chemical Reaction Engineering Interview Questions and Answers focuses on “Reaction Rates – 2”.

1. For a homogeneous reaction of order n, the unit of rate constant, k is ____
a) concn-1
b) $$\frac{conc^{-1}}{time}$$
c) $$\frac{conc^{-1}}{time^{1-n}}$$
d) $$\frac{conc^{1-n}}{time^{-1}}$$

Explanation: For any order n, (-rA) = kCAn
$$\frac{mol}{m^3×s} = k.(\frac{mol}{m^3})^n$$
Hence the unit of k is $$\frac{conc^{1-n}}{time^{-1}}.$$

2. The unit of rate constant, k for a first order reaction is ____
a) (time)-1
b) mol. time
c) $$\frac{mol}{m^3}$$
d) $$\frac{mol}{m^3×s}$$

Explanation: Rate of 1st order reaction, (-rA) = kCAn
Unit of rate = $$\frac{mol}{m^3×s}$$
Concentration, CA is in $$\frac{mol}{m^3}$$

Hence, unit of k is $$\frac{\frac{mol}{m^3×s}}{\frac{mol}{m^3}}$$ = (sec)-1.

3. Which among the following reactions is elementary?
a) A + B → R; (-rA) = k CACB
b) 2A + B → R; (-rA) = k CA2
c) 2A + B → R; (-rA) = k CACB2
d) A + B → R; (-rA) = k CB

Explanation: Elementary reactions are the ones in which rate equation corresponds to stoichiometry. For the reaction A + B → R, 1 mole of A and 1 mole of B take part in the reaction. Hence, the rate equation has to be
(-rA) = k CA1CB1.

4. An example of pseudo first order reaction is ____
a) Hydrolysis of an ester to a carboxylic acid using water
b) NO2 + CO → NO + CO2
c) N2O5 → NO2 + 1/2 O2
d) 2NO + H2 → N2O + H2O

Explanation: Pseudo first order reactions are the ones in which the concentration of one of the reactants is used in excess to convert higher order reactions to 1st order. As water is used as solvent in esterification reaction, its concentration is in huge excess compared to the ester. Thus, the reaction is pseudo first order for the ester.

5. The study of rates at which chemical reactions occur and the effect of various parameters on the rate are termed as ____
a) Chemical Technology
b) Chemical Sciences
c) Chemical Kinetics
d) Fluid Mechanics

Explanation: Chemical Kinetics is the study of rate of chemical reaction and the effect of parameters like temperature, pressure, concentration, etc. on reaction rates. Kinetic studies help in obtaining information about reaction mechanism and transition states.

6. The rate of reaction is rapid for which of the following reactions?
a) Cellular reactions
b) Reactions in coal furnaces
c) Reactions in rocket engines
d) Gas reactions in porous catalysts

Explanation: The rate of reaction in rocket engines is in the range 105-108. The order of decrease in rates is, Reactions in rocket engines > Reactions in coal furnaces > Gas reactions in porous catalysts > Cellular reactions.

7. For the reaction 3A+2B → R, which of the following expressions is valid?
a) $$\frac{-r_A}{3}$$=$$\frac{-r_B}{2}$$=$$\frac{r_C}{1}$$
b) $$\frac{-r_A}{3}$$=$$\frac{-r_B}{2}$$=$$\frac{r_C}{1}$$
c) $$\frac{-r_A}{1}$$=$$\frac{-r_B}{1}$$=$$\frac{r_R}{1}$$
d) $$\frac{-r_A}{3}$$=$$\frac{-r_B}{4}$$=$$\frac{r_C}{1}$$

Explanation: For the reaction aA + bB → rR, $$\frac{-r_A}{a}$$=$$\frac{-r_B}{b}$$=$$\frac{r_R}{c}$$. In the reaction 3A+2B → R, a=3, b=2 and r=1.

8. Which among the following reactions is elementary?
a) Ester formation from ethanol and acetic acid
b) Formation of Hydrogen Bromide
c) Decomposition of acetaldehyde to give methane and carbon monoxide
d) Formation of Hydrogen Iodide

Explanation: Ester formation is an elementary reaction. The rate equation for esterification reaction corresponds to the reaction stoichiometry.

9. The molecularity of the reaction A+2B → R, (-rA) = k CACB2 is ____
a) 2
b) 1
c) 3
d) 0

Explanation: In the reaction A+2B → R, 1 molecule of A and 2 molecules of B react. Hence, molecularity = 3.

10. Order of the reaction (-rA) = kCAa.CBb is ____
a) a
b) b
c) a-b
d) a+b 