Chemical Kinetics: MCQs and Answers with explanation

1. The rate of reaction------------as the reaction proceeds.

a) Increases
b) Decreases
c) Remains the same
d) May increase or decrease

... Answer is b)
Explanation: Rate of reaction is directly proportional to concentration of reactants. As reaction proceeds, the concentration of reactant decreases, so rate of reaction also decreases.

2. The unit of the rate constant is the same as that of reaction in------------order reaction.

a) First
b) Second
c) Third
d) Zero

... Answer is d)
Explanation: In zero order reaction, the rate of reaction is independent of concentration of reactant, So rate of reaction and rate constant are equal, so their units are also same. Consider a reaction, A .......> B If this reaction is a zero order, then rate equation can be derived as follow: Rate of reaction ∝ [A]⁰ Rate of reaction = K [A]⁰ Rate of reaction = K

3. For the reaction,

the rate law for the reaction is

a) Rate = K[A]²[B]
b) Rate = K[A][B]
c) Rate = K[C]
d) None of these

... Answer is a)
Explanation: Rate equation or rate law depends upon the slow step. The slow step of this reaction involves two molecules of reactant A and one molecule of reactant B, so rate equation can be written as Rate = K[A]²[B]

4. For the reaction

The expression for the rate law is Rate = K[A]², the order of reaction in B is

a) First
b) Second
c) Third
d) Zero

... Answer is d)
Explanation: The rate law for the reaction is Rate = K[A]² As rate of reaction depends upon the concentration of reactant A and does not depend upon the concentration of reactant B. So, order of reactant in B is zero.

5. The activation energy for a reaction can be

a) Increased by increasing the         temperature
b) Increased by decreasing the temperature
c) Decreased by increasing conentration of reactants
d) None of these

... Answer is d)
Explanation: The activation energy has no relationship with temperature and concentration. However, increase in temperature results in increasing the number of molecules having activation energy.

6. Rate law for the reaction R-X + H₂O ---------> R-OH + HX is, Rate = K[R-X]. The rate of reaction will be doubled when

a) Concentration of H₂O is doubled
b) Concentration of R-X is reduced to half
c) Concentration of both R-X and H₂O is doubled
d) None of these

... Answer is c)
Explanation: The rate of reaction will be double when concentration of R-X is doubled. i.e., Let, "R" is the initial rate of reaction R = K[R-X] When [R-X] is doubled, new rate will be "R^/" R^/ = K 2 [R-X] R^/ = 2 K[R-X] R^/ = 2 R Hence, K[R-X] = R So, new rate R^/ is the two times the initial rate. However, concentration of water is not involved in the rate law, new rate is independent of concentration of water if it is increased by two times, three times etc.

7. The rate of catalyzed reaction is independent of the concentration of

a) Reactant
b) Products
c) Catalyst
d) None of these

... Answer is c)
Explanation: The rate of catalyzed reaction is independent of the concentration of catalyst. This is beacuse catalyst is used in very small quantity which changes the rout of chemical reaction. Increase in concentration of catalyst does not make any difference. 

8. If a reaction proceeds in such a way that order of reaction is independent of the reactants concentration, the overall order of reaction would be

a) First
b) Second
c) Third
d) Zero

... Answer is d)
Explanation: For zero order reaction, the rate of reaction is independent of chemical reaction. Thus, Rate = K

9. Reactions with high activation energy are usually

a) Fast

b) Slow
c) Exothermic
d) Reversible
... Answer is b)
Explanation: Reactions with high energy of activation are usually slow. This is because, with increase in activation energy, the number of molecules possesing activation energy are decreased. Thus reaction rate become slow.,

10. In a reversible reaction, a catalyst lowers the activation energy of the

a) Forward reaction.
b) Reverse reaction
c) Forward as well as reverse reaction
d) Forward reaction but increase for the reverse reaction

... Answer is c)
Explanation: A catalyst has no effec to position of equilibrium, however it helps chemical equilibrium to be establish very quickly. It equally speeds up both forward and reverse reaction because it lowers energy of activation for forward and reverse reaction by the same amount. Some important MCQs

11. The rate equation determined experimentaly for this reaction: is Rate = K[(CH₃)₃CBr], Hence it is which of the following?

a) Fractional order
b) Pseudo first order
c) First order
d) Second order

... Answer is b)
Explanation: A chemical reaction which becomes first order due to excess of a reactant or reactants as compared to other reactant, the rate of reaction becomes independent of that reactant, is known as pseudo first order reaction. In above example, reaction between (CH₃)₃CBr and water is second order, but rate equation does not include concentration of water in it. Above reaction is pseoudo first order reaction, as water is present in excess.

12. The rate of first order reaction depends on :

a) Concentration of one reactant
b) Concentration of two reactant
c) Concentration of three reactant
d) Independent of initial concentration

... Answer is a)
Explanation: The rate of the first order reaction depends upon only one reactant.

13. The two steps involved in the gas phase reaction

are shown below

What is the rate equation for the overall reaction?

a) Rate = K[X]⁰[Y]¹
b) Rate = K[X]⁰[Y]²
c) Rate = K[X]¹[Y]¹
d) Rate = K[XY]¹[Y]¹

... Answer is c)
Explanation: Rate equation depends upon the reactants involved in slow step or rate determining step. Hence, fist step involves one mole of X and one mole of Y. So, rate is first order with respect to X and first order with respect to Y. Overall order of reaction is second order. So correct option is C.

14. When the change in concentration is 6 x 10^-4mol.dm^-3. and the time for that change is 10 second. The rate of reaction will be

a) 6 x 10^-3mol.dm^-3.sec^-1
b) 6 x 10^-2mol.dm^-3.sec^-1
c) 6 x 10^-4mol.dm^-3.sec^-1
d) 6 x 10^-5mol.dm^-3.sec^-1

... Answer is d)
Explanation: Rate of reaction can be determined as follow.

15. Unit of K in first order reaction is ?

a) s^-1
b) moles.dm³
c) moles.dm₃.s^-1
d) moles^-2.dm³.

... Answer is a)
Explanation: Rate of a chemical reaction is measured in change in concentration (mol.dm_-3) of reactant or product per unit time (s).