CBSE Class 12 Chemistry Chapter 4 Important Questions – Free PDF Download
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Important Questions Class 12 Chemistry Chapter 4 – Chemical Kinetics
CBSE Class 12 Chemistry Important Questions Chapter 4 – Chemical Kinetics
3 Marks Questions
1. For the reaction A+B 
C+D, the rate of reaction doubles when the concentration of A doubles, provided the concentration of B is constant. To what order does A enter into the rate expression?
Ans. A+B C+D
Rate = 1 when [A] = 1 ———- 1)
Rate = 2 when [A] = 2———–2)
Dividing equation 2) by 1)
21
The reaction is first order reaction.
2. A chemical reaction 2A 
4B+C in gas phase occurs in a closed vessel. The concentration of B is found to be increased by 
in 10 second. Calculate (i) the rate of appearance of B (ii) the rate of disappearance of A?
Ans. 2A 4B +C
i) Rate of disappearance of B
= 
= 
ii) 
= 
= 2.5 
3. For the following reactions, write the rate of reaction expression in terms of reactants and products?
i) 
ii)
Ans.
| In terms of reactant | In terms of products |
i)  |  |
 |  |
| II ) In terms of reactant | In terms of product |
R1 =  |  |
 |
– 
4. The reaction 
was studied and the following data were collected :
| S.no (mol/L/min) |  | Rate of disappearance of  (mol/L/min |
| 1. |  |  |
| 2. |  |  |
| 3. |  |  |
Determine
i) The order
ii) The rate law.
iii) Rate constant for the reaction.
Ans. Let the order of reaction be x
i) From the data –
———-1)
———-2)
———-3)
Dividing 1) by 2)
(1.36) = 
X=1
The order of reaction with respect with respect to 
is 1
ii) Rate law 
iii) Rate constant , K = 
= 
= 
5. The following experimental data was collected for the reaction:
| Trial | Intial conc. Of | [NO] mol/L | Initial Rate,(mol/L/s) |
| 1 | 0.10 | 0.010 |  |
| 2 | 0.10 | 0.030 |  |
| 3 | 0.20 | 0.030 |  |
Construct the rate equation for the reaction.
Ans. Order of NO is 2
Rate law = 
6. Draw a graph for
a) Concentration of reactant against time for a zero order reaction.
b) Log [Ro]/ [R] against time for a first order reaction.
Ans.
7. In general it is observed that the rate of a chemical reaction doubles with every 
rise in temperature. If this generalization holds for a reaction in the temperature range295K to 305K, what would be the activation energy for this reaction? 
Ans. 
= 
= 51855.2 J/mol (log2 = 0.3010)
8. The rate constant for a reaction is 
at 
and 
at
.
Calculate the value of activation energy for the reaction
?
Ans. 
9. Plot a graph showing variation of potential energy with reaction. coordinate?
Ans.
10. The conversion of molecules X to Y follows second order kinetics. If concentration of X is increased to three times how will it affect the rate of formation of Y?
Ans. The reaction 
follows second order kinetics.
Therefore, the rate equation for this reaction will be:
Rate = 
(1)
Let [X] = a 
, then equation (1) can be written as:
= 
If the concentration of X is increased to three times, then [X] = 
Now, the rate equation will be:
Rate = 
Hence, the rate of formation will increase by 9 times.
11. A first order reaction has a rate constant 
. How long will 5 g of this reactant take to reduce to 3 g?
Ans. From the question, we can write down the following information:
Initial amount = 5 g
Final concentration = 3 g
Rate constant = 
We know that for a 1storder reaction,
= 444.38 s
= 444 s (approx)
12. For the reaction
, the concentration of a reactant changes from 0.03 M to 0.02 M in 25 minutes. Calculate the average rate of reaction using units of time both in minutes and Pseconds.
Ans. Average rate of reaction 
13. For the reaction: 
the rate = 
with 
. Calculate the initial rate of the reaction when 
, 
. Calculate the rate of reaction after [A] is reduced to 
.
Ans. The initial rate of the reaction is
Rate =
When [A] is reduced from 
to 
, the concentration of A reacted = 
Therefore, concentration of B reacted 
Then, concentration of B available, 
After [A] is reduced to , the rate of the reaction is given by,
Rate =
14. A reaction is second order with respect to a reactant. How is the rate of reaction affected if the concentration of the reactant is
(i) doubled (ii) reduced to half?
Ans. Letthe concentration of the reactant be [A] = a
Rate of reaction, 
(i) If the concentration of the reactant is doubled, i.e. [A] = 2a, then the rate of the reaction would be
= 4 R
Therefore, the rate of the reaction would increase by 4 times.
(ii) If the concentration of the reactant is reduced to half, i.e. 
, then the rate of the reaction would be 
Therefore, the rate of the reaction would be reduced to 
15. A reaction is first order in A and second order in B.
(i) Write the differential rate equation.
(ii) How is the rate affected on increasing the concentration of B three times?
(iii) How is the rate affected when the concentrations of both A and B are doubled?
Ans. (i) The differential rate equation will be
(ii) If the concentration of B is increased three times, then
Therefore, the rate of reaction will increase 9 times.
(iii) When the concentrations of both A and B are doubled,
Therefore, the rate of reaction will increase 8 times.
16. Calculate the half-life of a first order reaction from their rate constants given below:
(i) 
(ii) 
(iii) 4 years-1
Ans. (i) Half life, 
(approximately)
(ii) Half life, 
= 0.35 min (approximately)
(iii) Half life,
= 0.173 years (approximately)
17. The half-life for radioactive decay of 14C is 5730 years. An archaeological artifact wood had only 80% of the 14C found in a living tree. Estimate the age of the sample.
Ans. Here, 
It is known that,
= 1845 years (approximately)
Hence, the age of the sample is 1845 years.
18. The rate constant for a first order reaction is 60 s-1. How much time will it take to reduce the initial concentration of the reactant to its 1/16thvalue?
Ans. It is known that,
(approximately)
Hence, the required time is 
.
19. A first order reaction takes 40 min for 30% decomposition. Calculate 
.
Ans. For a first order reaction,
Therefore, of the decomposition reaction is
= 77.7 min (approximately)
20. Consider a certain reaction A Products with 
. Calculate the concentration of A remaining after 100 s if the initial concentration of A is 
.
Ans.
T= 100 s
Since the unit of k is 
, the given reaction is a first order reaction.
Therefore, 
= 
(approximately)
Hence, the remaining concentration of A is .
