# Chemical Reaction Engineering Questions and Answers – Reaction Rates – 1

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This set of Chemical Reaction Engineering Multiple Choice Questions & Answers (MCQs) focuses on “Reaction Rates – 1″.

1. The factors affecting the rate of homogeneous reaction are ____
a) Temperature only
b) Pressure only
c) Reactant Concentration only
d) Temperature, pressure and reactant concentration

Explanation: The rate constant in the rate expression is a function of temperature. Increasing pressure decreases the collisions between reactant molecules. This decreases the rate of reaction. Increasing the initial concentration of reactant increases the reaction rate. Hence, reaction rate is controlled by temperature and pressure conditions and initial concentration of reactant.

2. Which of the following is the correct expression for the rate of a reaction?
a) $$\frac{Moles \, of \, product \, formed}{Volume \, of \, fluid×Time}$$
b) $$\frac{Moles \, of \, reactant \, initially \, taken}{Volume \, of \, fluid×Time}$$
c) $$\frac{Moles \, unreacted}{Volume \, of \, fluid×Time}$$
d) $$\frac{Moles \, of \, product \, formed}{Volume \, of \, fluid}$$

Explanation: Rate of the reaction is the number of moles of the reactant component disappearing per unit volume per unit time. The moles of the product formed are the number of moles of the reactant species disappearing during the course of reaction.

3. The rate of cellular reactions expressed as moles A disappearing/(m3.s) is in the range of ____
a) 10-3-10-4
b) 102
c) 0.1
d) 109

Explanation: The cellular reactions in living organisms occur very slowly. Hence, the rate is of the order 10-3-10-4 moles disappearing/(m3.s).
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4. For the reaction aA+ bB → rR, which of the following is true?
a) $$\frac{-r_A}{a}$$=$$\frac{-r_B}{b}$$ ≤ $$\frac{r_R}{r}$$
b) $$\frac{-r_A}{a}$$ ≥$$\frac{-r_B}{b}$$ ≤ $$\frac{r_R}{r}$$
c) $$\frac{-r_A}{a}$$=$$\frac{-r_B}{b}$$ ≤ $$\frac{r_R}{r}$$
d) $$\frac{-r_A}{a}$$≤$$\frac{-r_B}{b}$$ ≤ $$\frac{r_R}{r}$$

Explanation: The rates of degradation of reactants and formation of product are related to each other as $$\frac{-r_A}{a} = \frac{-r_B}{b} = \frac{r_R}{r}$$, where a, b and c denote the stoichiometry. The rate of disappearance of any reactant in turn is the rate of formation of products.

5. The rate of a reaction cannot be expressed in terms of ____
a) Volume of reacting fluid
b) Volume of solid
c) Amount of product
d) Interfacial surface

Explanation: The rate of the reaction can be expressed in terms of unit volume of reacting fluid, based on unit interfacial area and unit volume of solid. The amount of product formed has no influence on the reaction rate.

6. Order of a reaction is ____
a) Number of molecules of reactants taking part in the reaction
b) Power of any one of the reactant concentrations
c) Concentration of intermediate species formed
d) Sum of the powers of the concentrations of all the reactants

Explanation: The order of a reaction with respect to a given substance is the power to which its concentration term is raised in the rate equation. If there is more than one reactant molecules involved in the reaction, then the order of the reaction is the sum of the powers of concentrations of all reacting species in the rate expression.

7. The rate expression for a homogeneous reaction comprises of ____
a) A concentration dependent term only
b) A temperature dependent term only
c) A concentration dependent term and a temperature dependent term
d) A catalyst weight dependent term

Explanation: (-rA) = kCn,
Where,
k: Rate constant
C: Concentration
Rate constant, k is a function of temperature.

8. The number of molecules involved in an elementary reaction is termed as ____
a) Molecularity
b) Order of reaction
c) Unimolecular reaction
d) Rate of reaction

Explanation: Molecularity of an elementary reaction is the number of molecules of the reactant taking part in the reaction. Order of a reaction is the sum of the powers of the concentrations of all the reactants. Unimolecular reactions involve only one reactant. Rate of a reaction is the number of moles of the reactant component disappearing per unit volume per unit time.

9. Which of the following is true about elementary reactions?
a) They occur in multiple steps
b) They are complex in nature
c) Order of elementary reaction is an integer
d) All first order reactions are elementary

Explanation: The order of an elementary reaction can never take fractional values. Elementary reactions occur in a single step. First order reactions can be elementary or non- elementary.

10. The unit of frequency factor in Arrhenius equation is ____
a) $$\frac{mol}{m^3}$$
b) Same as that of rate constant
c) $$\frac{mol}{m^3×s}$$
d) mol

Explanation: The unit of frequency factor in Arrhenius equation is same as that of rate constant.
Arrhenius equation is, k = A e-Ea/RT,
k: Rate constant
A: Frequency factor
The exponent term in Arrhenius equation gives an integer. Hence, unit of A depends on that of k.

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