# Chemical Reaction Engineering Questions and Answers – Reaction Order and Elementary Rate Law

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This set of Chemical Reaction Engineering Multiple Choice Questions & Answers (MCQs) focuses on “Reaction Order and Elementary Rate Law”.

1. Molecularity refers to an elementary reaction.
a) True
b) False

Explanation: Elementary reaction occurs in single reaction step with no intermediates and molecularity is the number of particles involved in the reaction.

2. Molecularity is ___________ value and order is ___________ value.
a) an integer, a fractional
b) a fractional, an integer
c) both fractional
d) both integers

Explanation: Order is empirical, hence can be a fractional value but molecularity refers to the mechanism and is applicable only for elementary reactions.

3. The overall order of reaction for –rA = kCA0.7CB1.3 is _______
a) 0
b) 1
c) 2
d) 3

Explanation: Order w.r.t A is 0.7 and B is 1.3.Hence, Overall order is 2.
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4. What is the relative rate of reaction for the given reaction.
2A + 9B → 7C + 4D
a) –rA = -9rB=7rC=–4rD
b) –rA =-rB/9=rC/7=rD/4
c) –rA =-rB/9=7rC=4rD
d) –rA = 9rB=-7rC=–4rD

Explanation: Relative rate of reaction can be obtained from the ratio of stoichiometric co -efficient.

5. The units of rate constant of nth order is ___________
a) time-1
b) concentration(n-1)
c) time-1 concentration(1-n)
d) time-1 concentration(n-1)

Explanation: General expression for the rate constant is time-1 concentration(1-n).

6. The unit for concentration equilibrium constant for the following reaction is _________
2A + 9B ←→7C + 4D
a) $$(\frac{mol}{dm^3})$$2
b) $$(\frac{mol}{dm^3})$$9
c) $$(\frac{mol}{dm^3})$$-4
d) $$(\frac{mol}{dm^3})$$-3

Explanation: Concentration eqm constant for a reversible reaction is
aA + bB → cC + dD is
K = $$\frac{cC^c cD^d}{cA^a cB^b}$$ unit is $$(\frac{mol}{dm^3})$$c+d-a-b when it is applied to the above equation we get -3.

7. The overall reaction order for A + B → C + D is two. It implies that ______
a) reaction may be non-elementary and molecularity is two
b) reaction may be elementary and molecularity is two
c) reaction is non-elementary and molecularity is two
d) reaction is elementary and molecularity is two

Explanation: Order is equal to molecularity when reaction is elementary.

8. Reaction corresponding to stoichiometric equation is called __________
a) Non-elementary
b) Elementary
c) Heterogeneous
d) Homogeneous

Explanation: Elementary reactions are the ones with single reaction step and the reaction rate corresponds to stoichiometric equation.

9. The rate constant for a first order reaction depends on _____________
a) Pressure
b) Temperature
c) Concentration
d) Time

Explanation: The reaction rate constant depends on temperature only.

10. The mlecularity of the following single step reaction is ___________
aA + bB → pP
a) a+b-p
b) a+b+p
c) a+b
d) a-p

Explanation: The above single step reaction is an elementary reaction. Molecularity is the number of molecules participating in the reaction.

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