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
2 Marks Questions
1. Define the terms – i) Order of a reaction
ii) Molecularity of a reaction.
Ans. i) Order of a reaction.
The sum of powers of the concentration of the reactants in the rate law expression is called order of that reaction.
ii) Molecularity of a reaction.
The number of reacting species which must collide simultaneously in order to bring about a chemical reaction is called molecularity of a reaction.
2. What are elementary and complex reactions?
Ans. The reactions taking place in one step are called elementary reactions whereas when a sequence of elementary reactions, called mechanism , gives us the product , it is called complex reaction.
3. Differentiate between order and molecularity of a reaction?
Ans.
ORDER OF A REACTION | MOLECULARITY OF A REACTION |
| It is sum of the concentration terms on which the rate of reaction actually depends or it is the sum of the exponents of the concentrations in the rate law equation. | It is the number of atoms, ions or molecules that must collide with one another simultaneously so as to result into a chemical reaction. |
| It need not be a whole number i.e. it can be fractional as well as zero. | It is always a whole number. |
| It can be determined experimentally only and cannot be calculated. | It can be calculated by simply adding the molecules of the slowest step. |
| It is for the overall reaction and no separate steps are written to obtain it. | The overall molecularity of a complex reaction has no significance. It is only slowest step whose molecularity has significance for the overall reaction. |
| Even the order of a simple reaction may not be equal to the number of molecules of the reactants as seen from the unbalance equation. | For simple reactions, the molecularity can usually be obtained from the Stoichiometry of the equation. |
4. Determine the overall order of a reaction which has the rate law 
Ans. 
Order = x +y
50 order = 
i.e; reaction is fourth order reaction.
5. What are the units of a rate constant of a
a) First order reaction
b) nth order reaction.
Ans. i) First order reaction –
Units of rate constant, 
ii) For nth order reaction
the units of rate constant, 
6. What is instantaneous rate of a reaction? How is it determined?
Ans. The rate of a reaction at a particular moment of time is called instantaneous rate of a reaction. For a reaction A 
B 
inst = 
Where dt = the smallest possible time interval 
7. For the chemical decomposition of
, its initial concentration is 0.8420 mol/L and final concentration is 
in 2 hours. What is the average rate of this reaction?
Ans. Rate of reaction = 
= 
= 
.
8. In the expression of rate of reaction in terms of reactants, what is the significance of negative sign?
Ans. While writing the expression for rate of a reaction is terms of reactants, there is a negative sign which indicates a decrease in concentration of reactants with time.
9. For the reaction 
was found to be
. Determine the value of 
in atm /s during this period of time?
Ans. From the equation 
= 
= 
10. The rate Law for the reaction A+B C is rate
. What would the
reaction rate be when concentration of both A and B are doubled?
Ans. 
——————1)
——————2)
Dividing 2) by 1)
= 
= 
The rate of reaction increases eight times.
11. Write the integrated rate equation for –
i) zero order reaction.
ii) first order reaction.
Ans. i) Zero order reaction – 
ii) First order reaction K = 
Where 
is the initial concentration
R is concentration at time t.
12. From the graph below
i) Identify the order of reaction.
ii) What will be the unit of rate constant?
Ans. From the graph
i) Reaction is first order reaction
ii) The unit of rate constant will be
.
13. What is the use of integrated rate equation?
Ans. Use of integrated rate equation –
1. The value of rate constant can be known when concentration of reactant at different times are known-
2. Order of a reaction can be determined by the knowledge of reaction concentration at different times.
14. For first order reaction – A B Write
(1) Differential rate law.
(2) Integrated rate law.
Ans. For the reaction A B, if the order =1
(i) Differential rate law is 
(ii) Integrated rate law is 
15. The rate constant for the first order decomposition of 
at 
is
. It the initial concentration of is
, How long will it take to drop the concentration to
?
Ans. t = 
= 
= 
= 46.22 min.
16. Write Arrhenius equation.
Ans. Arrhenius equation
Where K = rate constant , T = absolute temperature
= Activation energy, R = gas constant.
17. If the activation energy of a reaction is zero, will the rate of reaction still depend on temperature?
Ans. 
When Ea = 0
K = A
Rate of reaction will not depend upon the temperature if activation energy is zero.
18. How does collision theory explain formation of products in a chemical reaction?
Ans. According to collision theory, the reactant molecules are assumed to be hard spheres and reaction occurs when these molecules collide with each other. The collisions in which molecules collide with sufficient kinetic energy (called threshold energy) and proper orientation, lead to formation of product. Here activation energy and proper orientation of the molecules determine the criteria for effective collision & hence the rate of a reaction.
19. From the fig. (1)
(a) Calculate 
for the reaction, activation
energy for forward reaction.
(b) Identify the curve for catalysed reaction.
(c) what is the energy of activation in the
presence of catalyst?
Ans. (a) From the figure-
Energy of reactions 
Energy of products, 
Change in energy, 
= 50-150 = 100KJ/Mol
Threshold energy, 
Activation energy, 
= 350-150
= 200 KJ/Mol.
(b) The curve B is for catalysed reaction.
( C) In the presence of catalysts ,
Threshold energy 
Activation energy, 
= 100 KJ/Mol.
20. The activation energy of reaction is 75.2 KJ/mol in the absence of a catalyst and 50.14 KJ/Mol in the presence of a catalyst. How many times will the reaction grow in the presence of a catalyst, if the reaction proceeds at?
Ans. Let the rate constant in the absence of catalyst be
, Let the rate constant in the presence of catalyst be
Activation energy in the absence of catalyst, 
Activation energy in the presence of catalyst,
.
= 4.391.
Or 
21. In a reaction, 
Products, the concentration of A decreases from 
to 
in 10 minutes. Calculate the rate during this interval?
Ans. Average rate 
= 0.005 mol L – 1 min – 1
= 5 ×10 – 3M min – 1
22. For a reaction,
’Product; the rate law is given by, 
. What is the order of the reaction?
Ans. The order of the reaction 
= 2.5
23. Time required to decompose 
to half of its initial amount is 60 minutes. If the decomposition is a first order reaction, calculate the rate constant of the reaction.
Ans. We know that for a 1st order reaction,
It is given that 
24. What will be the effect of temperature on rate constant?
Ans. The rate constant of a reaction is nearly doubled with a 
rise in temperature. However, the exact dependence of the rate of a chemical reaction on temperature is given by Arrhenius equation,
Where,
A is the Arrhenius factor or the frequency factor
T is the temperature
R is the gas constant
is the activation energy
25. Mention the factors that affect the rate of a chemical reaction.
Ans. The factors that affect the rate of a reaction areas follows.
(i) Concentration of reactants (pressure in case of gases)
(ii) Temperature
(iii) Presence of a catalyst
