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Unit

CHEMICAL KINETICS
CHEMICAL
4
I. Multiple Choice Questions (Type-I)
1. The role of a catalyst is to change ______________.
(i) gibbs energy of reaction.
(ii) enthalpy of reaction.
(iii) activation energy of reaction.
(iv) equilibrium constant.

2. In the presence of a catalyst, the heat evolved or absorbed during the reaction
___________.
(i) increases.
(ii) decreases.
(iii) remains unchanged.
(iv) may increase or decrease.

3. Activation energy of a chemical reaction can be determined by _____________.

(i) determining the rate constant at standard temperature.
(ii) determining the rate constants at two
temperatures.
(iii) determining probability of collision.
(iv) using catalyst.
4. Consider Fig. 4.1 and mark the correct option.
(i) Activation energy of forward reaction is
E 1 + E2 and product is less stable than
reactant.
(ii) Activation energy of forward reaction is Fig. 4.1
E 1+E2 and product is more stable than
reactant.

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(iii) Activation energy of both forward and backward reaction is E 1+E2 and
reactant is more stable than product.
(iv) Activation energy of backward reaction is E1 and product is more stable
than reactant.

5. Consider a first order gas phase decomposition reaction given below :

A(g) ⎯→ B(g) + C(g)
The initial pressure of the system before decomposition of A was pi. After lapse
of time ‘t’, total pressure of the system increased by x units and became ‘pt’
The rate constant k for the reaction is given as _________.

2.303 pi
(i) k= log
t pi − x

2.303 pi
(ii) k= log
t 2 pi − pt

2.303 pi
(iii) k= log
t 2pi + pt

2.303 pi
(iv) k= log
t pi + x

6. According to Arrhenius equation rate constant k is equal to A e –E a / RT . Which

1
of the following options represents the graph of ln k vs ?
T

(i) (ii)

(iii) (iv)

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7. Consider the Arrhenius equation given below and mark the correct option.

k = A e –Ea / RT
(i) Rate constant increases exponentially with increasing activation energy
and decreasing temperature.
(ii) Rate constant decreases exponentially with increasing activation energy
and decreasing temperature.
(iii) Rate constant increases exponentially with decreasing activation energy
and decreasing temperature.
(iv) Rate constant increases exponentially with decreasing activation energy
and increasing temperature.

8. A graph of volume of hydrogen released vs time for the reaction between zinc
and dil.HCl is given in Fig. 4.2. On the basis of this mark the correct option.
V3 − V2
(i) Average rate upto 40s is
40

V3 − V2
(ii) Average rate upto 40 seconds is
40 − 30

V3
(iii) Average rate upto 40 seconds is
40

V3 − V1
(iv) Average rate upto 40 seconds is
40 − 20 Fig. 4.2

9. Which of the following statements is not correct about order of a reaction.

(i) The order of a reaction can be a fractional number.
(ii) Order of a reaction is experimentally determined quantity.
(iii) The order of a reaction is always equal to the sum of the stoichiometric
coefficients of reactants in the balanced chemical equation for a reaction.
(iv) The order of a reaction is the sum of the powers of molar concentration
of the reactants in the rate law expression.

10. Consider the graph given in Fig. 4.2. Which of the following options does not
show instantaneous rate of reaction at 40th second?

V5 − V 2
(i)
50 − 30

V4 − V 2
(ii)
50 − 30

V3 − V2
(iii)
40 − 30

V3 − V1
(iv)
40 − 20

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11. Which of the following statements is correct?

(i) The rate of a reaction decreases with passage of time as the concentration
of reactants dereases.
(ii) The rate of a reaction is same at any time during the reaction.
(iii) The rate of a reaction is independent of temperature change.
(iv) The rate of a reaction decreases with increase in concentration of
reactant(s).

12. Which of the following expressions is correct for the rate of reaction given
below?
5Br (aq) + BrO3 (aq) + 6H (aq) ⎯→ 3Br2(aq) + 3H2O(l)
– – +

Δ[Br − ] Δ[H+ ]
(i) =5
Δt Δt
− +
Δ[Br ] 6 Δ[H ]
(ii) =
Δt 5 Δt
− +
Δ[Br ] 5 Δ[H ]
(iii) =
Δt 6 Δt
− +
Δ[Br ] Δ[H ]
(iv) =6
Δt Δt

13. Which of the following graphs represents exothermic reaction?

(a) (b)

(c)

(i) (a) only

(ii) (b) only
(iii) (c) only
(iv) (a) and (b)
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14. Rate law for the reaction A + 2B ⎯→ C is found to be

Rate = k [A][B]
Concentration of reactant ‘B’ is doubled, keeping the concentration of ‘A’
constant, the value of rate constant will be______.
(i) the same
(ii) doubled
(iii) quadrupled
(iv) halved
15. Which of the following statements is incorrect about the collison theory of
chemical reaction?
(i) It considers reacting molecules or atoms to be hard spheres and ignores
their structural features.
(ii) Number of effective collisions determines the rate of reaction.
(iii) Collision of atoms or molecules possessing sufficient threshold energy

T
results into the product formation.
(iv) Molecules should collide with sufficient threshold energy and proper
ER
orientation for the collision to be effective.

16. A first order reaction is 50% completed in 1.26 × 1014 s. How much time
would it take for 100% completion?
(i) 1.26 × 1015 s
C

(ii) 2.52 × 1014 s

(iii) 2.52 × 1028 s
N

(iv) infinite
17. Compounds ‘A’ and ‘B’ react according to the following chemical equation.
©

A (g) + 2 B (g) ⎯→ 2C (g)

Concentration of either ‘A’ or ‘B’ were changed keeping the concentrations of
one of the reactants constant and rates were measured as a function of initial
concentration. Following results were obtained. Choose the correct option for
the rate equations for this reaction.

Experiment Initial Initial Initial rate of

concentration concentration formation of
of [A]/mol L–1 of [B]/mol L–1 [C]/mol L–1 s–1

1. 0.30 0.30 0.10

2. 0.30 0.60 0.40
3. 0.60 0.30 0.20

(i) Rate = k [A]2 [B]

(ii) Rate = k [A] [B]2
(iii) Rate = k [A] [B]
(iv) Rate = k [A]2 [B]0
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18. Which of the following statement is not correct for the catalyst?
(i) It catalyses the forward and backward reaction to the same extent.
(ii) It alters ΔG of the reaction.
(iii) It is a substance that does not change the equilibrium constant of a
reaction.
(iv) It provides an alternate mechanism by reducing activation energy
between reactants and products.

19. The value of rate constant of a pseudo first order reaction ____________.
(i) depends on the concentration of reactants present in small amount.
(ii) depends on the concentration of reactants present in excess.
(iii) is independent of the concentration of reactants.
(iv) depends only on temperature.

20. Consider the reaction A B. The concentration of both the reactants and
the products varies exponentially with time. Which of the following figures
correctly describes the change in concentration of reactants and products
with time?

(i) (ii)

(iii) (iv)

II. Multiple Choice Questions (Type-II)

Note : In the following questions two or more options may be correct.

21. Rate law cannot be determined from balanced chemical equation if _______.
(i) reverse reaction is involved.

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(ii) it is an elementary reaction.

(iii) it is a sequence of elementary reactions.
(iv) any of the reactants is in excess.
22. Which of the following statements are applicable to a balanced chemical
equation of an elementary reaction?
(i) Order is same as molecularity.
(ii) Order is less than the molecularity.
(iii) Order is greater than the molecularity.
(iv) Molecularity can never be zero.
23. In any unimolecular reaction ______________.
(i) only one reacting species is involved in the rate determining step.
(ii) the order and the molecularity of slowest step are equal to one.
(iii) the molecularity of the reaction is one and order is zero.
(iv) both molecularity and order of the reaction are one.
24. For a complex reaction ______________.
(i) order of overall reaction is same as molecularity of the slowest step.
(ii) order of overall reaction is less than the molecularity of the slowest
step.
(iii) order of overall reaction is greater than molecularity of the slowest step.
(iv) molecularity of the slowest step is never zero or non interger.
25. At high pressure the following reaction is zero order.
1130 K
2NH3(g) ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯
Platinum catalyst
→ N (g) + 3H (g)
2 2

Which of the following options are correct for this reaction?

(i) Rate of reaction = Rate constant
(ii) Rate of the reaction depends on concentration of ammonia.
(iii) Rate of decomposition of ammonia will remain constant until ammonia
disappears completely.
(iv) Further increase in pressure will change the rate of reaction.
26. During decomposition of an activated complex
(i) energy is always released
(ii) energy is always absorbed
(iii) energy does not change
(iv) reactants may be formed
28. According to Maxwell Boltzmann distributon of energy, __________.
(i) the fraction of molecules with most probable kinetic energy decreases
at higher temperatures.
(ii) the fraction of molecules with most probable kinetic energy increases
at higher temperatures.

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(iii) most probable kinetic energy increases at higher temperatures.

(iv) most probable kinetic energy decreases at higher temperatures.

28. In the graph showing Maxwell Boltzman distribution of energy, ___________.

(i) area under the curve must not change with increase in temperature.
(ii) area under the curve increases with increase in temperature.
(iii) area under the curve decreases with increase in temperature.
(iv) with increase in temperature curve broadens and shifts to the right
hand side.

29. Which of the following statements are in accordance with the Arrhenius
equation?
(i) Rate of a reaction increases with increase in temperature.
(ii) Rate of a reaction increases with decrease in activation energy.
(iii) Rate constant decreases exponentially with increase in temperature.
(iv) Rate of reaction decreases with decrease in activation energy.

30. Mark the incorrect statements.

(i) Catalyst provides an alternative pathway to reaction mechanism.
(ii) Catalyst raises the activation energy.
(iii) Catalyst lowers the activation energy.
(iv) Catalyst alters enthalpy change of the reaction.

31. Which of the following graphs is correct for a zero order reaction?

(i) (ii)

(iii) (iv)

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32. Which of the following graphs is correct for a first order reaction?

(i) (ii)

(iii) (iv)

III. Short Answer Type

33. State a condition under which a bimolecular reaction is kinetically first order
reaction.

34. Write the rate equation for the reaction 2A + B ⎯→ C if the order of the reaction
is zero.

35. How can you determine the rate law of the following reaction?
2NO (g) + O2 (g) ⎯→ 2NO2 (g)

36. For which type of reactions, order and molecularity have the same value?

37. In a reaction if the concentration of reactant A is tripled, the rate of reaction

becomes twenty seven times. What is the order of the reaction?

38. Derive an expression to calculate time required for completion of zero order
reaction.

39. For a reaction A + B ⎯→ Products, the rate law is — Rate = k [A][B]3/2

Can the reaction be an elementary reaction? Explain.

40. For a certain reaction large fraction of molecules has energy more than the
threshold energy, yet the rate of reaction is very slow. Why?
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41. For a zero order reaction will the molecularity be equal to zero? Explain.

42. For a general reaction A ⎯→ B, plot of concentration of A vs time is given in

Fig. 4.3. Answer the following question on the basis of this graph.
(i) What is the order of the reaction?
(ii) What is the slope of the curve?
(iii) What are the units of rate constant?

Fig. 4.3

43. The reaction between H 2(g) and O2(g) is highly feasible yet allowing the gases
to stand at room temperature in the same vessel does not lead to the formation
of water. Explain.

44. Why does the rate of a reaction increase with rise in temperature?

45. Oxygen is available in plenty in air yet fuels do not burn by themselves at
room temperature. Explain.

46. Why is the probability of reaction with molecularity higher than three
very rare?

47. Why does the rate of any reaction generally decreases during the course of
the reaction?

48. Thermodynamic feasibility of the reaction alone cannot decide the rate of the
reaction. Explain with the help of one example.

49. Why in the redox titration of KMnO4 vs oxalic acid, we heat oxalic acid solution
before starting the titration?

50. Why can’t molecularity of any reaction be equal to zero?

51. Why molecularity is applicable only for elementary reactions and order is
applicable for elementary as well as complex reactions?

52. Why can we not determine the order of a reaction by taking into consideration
the balanced chemical equation?

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IV. Matching Type

Note : In the following questions match the items of Column I with
appropriate item given in Column II.

53. Match the graph given in Column I with the order of reaction given in Column II.
More than one item in Column I may link to the same item of Column II.
Column I Column II

(i)

(ii) (a) Ist order

(iii) (b) Zero order

(iv)

54. Match the statements given in Column I and Column II

Column I Column II
(i) Catalyst alters the rate of reaction (a) cannot be fraction or zero
(ii) Molecularity (b) proper orientation is not there
always

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(iii) Second half life of first order reaction (c) by lowering the activation
energy
–E /R T
(iv) e a (d) is same as the first
(v) Energetically favourable reactions (e) total probability is one
are sometimes slow
(vi) Area under the Maxwell (f) refers to the fraction of
Boltzman curve is constant molecules with energy equal
to or greater than activation
energy

55. Match the items of Column I and Column II.

Column I Column II
(i) Diamond (a) short interval of time

(ii) Instantaneous rate (b) ordinarily rate of conversion is

imperceptible

(iii) Average rate (c) long duration of time

56. Match the items of Column I and Column II.

Column I Column II
(i) Mathematical expression for (a) rate constant
rate of reaction

(ii) Rate of reaction for zero order (b) rate law

reaction is equal to

(iii) Units of rate constant for zero order (c) order of

reaction is same as that of slowest step
(iv) Order of a complex reaction is (d) rate of a reaction
determined by

V. Assertion and Reason Type

Note: In the following questions a statement of assertion followed by a statement
of reason is given. Choose the correct answer out of the following choices.
(i) Both assertion and reason are correct and the reason is correct explanation
of assertion.
(ii) Both assertion and reason are correct but reason does not explain
assertion.
(iii) Assertion is correct but reason is incorrect.
(iv) Both assertion and reason are incorrect.
(v) Assertion is incorrect but reason is correct.

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57. Assertion : Order of the reaction can be zero or fractional.

Reason : We cannot determine order from balanced chemical equation.

58. Assertion : Order and molecularity are same.

Reason : Order is determined experimentally and molecularity is the
sum of the stoichiometric coefficient of rate determining
elementary step.

59. Assertion : The enthalpy of reaction remains constant in the presence of

a catalyst.
Reason : A catalyst participating in the reaction, forms different
activated complex and lowers down the activation energy but
the difference in energy of reactant and product remains the
same.

60. Assertion : All collision of reactant molecules lead to product formation.

Reason : Only those collisions in which molecules have correct
orientation and sufficient kinetic energy lead to compound
formation.

61. Assertion : Rate constants determined from Arrhenius equation are fairly
accurate for simple as well as complex molecules.
Reason : Reactant molecules undergo chemical change irrespective of
their orientation during collision.

VI. Long Answer Type

62. All energetically effective collisions do not result in a chemical change. Explain
with the help of an example.
63. What happens to most probable kinetic energy and the energy of activation
with increase in temperature?
64. Describe how does the enthalpy of reaction remain unchanged when a catalyst
is used in the reaction.
65. Explain the difference between instantaneous rate of a reaction and average
rate of a reaction.
66. With the help of an example explain what is meant by pseudo first order
reaction.

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ANSWERS
I. Multiple Choice Questions (Type-I)

1. (iii) 2. (iii) 3. (ii) 4. (i) 5. (ii) 6. (i)

7. (iv) 8. (iii) 9. (iii) 10. (ii) 11. (i) 12. (iii)
13. (i) 14. (ii) 15. (iii) 16. (iv) 17. (ii) 18. (ii)
19. (ii) 20. (ii)

II. Multiple Choice Questions (Type-II)

21. (i), (iii), (iv) 22. (i), (iv) 23. (i), (ii) 24. (i), (iv)

25. (i), (iii), (iv) 26. (i), (iv) 27. (i), (iii) 28. (i), (iv)

29. (i), (ii) 30. (ii), (iv) 31. (i), (iv) 32. (i), (iv)

III. Short Answer Type

33. Bimolecular reaction becomes kinetically first order when one of the
reactants is in excess.
34. Rate = k [A] 0[B]0 or Rate = k
35. See page no. 99 of NCERT textbook for Class XII.
36. If the reaction is an elementary reaction, order is same as molecularity.
37. Three, because rate = k [A]3
38. [R] = [R]0 – kt
for completion [R] = 0

[R ]0
∴ t=
k
39. During an elementary reaction, the number of atoms or ions colliding to
react is referred to as molecularity. Had this been an elementary reaction
the order of reaction with respect to B would have been 1, but in the
3
given rate law it is . This indicates that the reaction is not an elementary
2
reaction.
40. Apart from the energy considerations, the colliding molecules should also
have proper orientation for effective collision. This condition might not be
getting fulfilled in the reaction.
41. No, the molecularity can never be zero or a fractional number.

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42. (i) Zero (ii) –k (iii) mol L–1 s–1

43. This is because activation energy for the reaction is very high at room
temperature.
44. At higher temperatures, larger fraction of colliding particles can cross the
energy barrier (i.e. the activation energy), which leads to faster rate.
45. The activation energy for combustion reactions of fuels is very high at
room temperature therefore they do not burn by themselves.
46. The probability of more than three molecules colliding simultaneously is
very small. Hence possibility of molecularity being three is very low.
47. The rate of a reaction depends on the concentration of reactants. As the
reaction progresses, reactants start getting converted to products so the
concentration of reactants decreases hence the rate decreases.
48. Thermodynamically the conversion of diamond to graphite is highly feasible
but this reaction is very slow because its activation energy is high.
49. The reaction between KMnO4 and oxalic acid is very slow. By raising the
temperature we can enhance the rate of reaction.
50. Molecularity is the number of molecules taking part in an elementary
step. For this we require at least a single molecule leading to the value of
minimum molecularity of one.
51. A complex reaction proceeds through several elementary reactions.
Numbers of molecules involved in each elementary reaction may be
different i.e., the molecularity of each step may be different. Therefore,
discussion of molecularity of overall complex reaction is meaningless. On
the other hand, order of a complex reaction is determined by the slowest
step in its mechanism and is not meaningless even in the case of complex
reactions.
52. Balanced chemical equation often leads to incorrect order or rate law. For
example the following reaction seems to be a tenth order reaction.
KClO3 + 6FeSO4 + 3H2SO4 ⎯→ KCl + 3H2O + 3Fe2 (SO 4) 3
This is actually a second order reaction. Actually the reaction is complex
and occurs in several steps. The order of such reaction is determined by
the slowest step in the reaction mechanism. Order is determined
experimentally and is confined to the dependence of observed rate of
reaction on the concentration of reactants.

IV. Matching Type

53. (i) → (a) (ii) → (b) (iii) → (b) (iv) → (a)

54. (i) → (c) (ii) → (a) (iii) → (d) (iv) → (f)

(v) → (b) (vi) → (e)

55. (i) → (b) (ii) → (a) (iii) → (c)

56. (i) → (b) (ii) → (a) (iii) → (d) (iv) → (d)
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V. Assertion and Reason Type

57. (ii) 58. (v) 59. (i) 60. (v) 61. (iii)

VI. Long Answer Type

62. Hint: Proper orientation of molecule should be explained in detail.

63. Hint : • Flattening of curve and shifting of maxima towards high energy
value.
• Area under the curve beyond the activation energy increases.
64. Hint : • Enthalpy is a state function.
• Difference in energy between reactants and product is constant.
65. See NCERT textbook for Class XII.
66. See NCERT textbook for Class XII.

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