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NCERT Exemplar Class 12 Chemistry Solutions Chapter 4 Chemical Kinetics

NCERT Exemplar Class 12 Chemistry Solutions Chapter 4 Chemical Kinetics

Edited By Shivani Poonia | Updated on Jun 24, 2025 01:36 PM IST | #CBSE Class 12th
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CBSE Class 12th  Exam Date : 15 Jul' 2025 - 15 Jul' 2025

Do you notice that there are some reactions around us that happen instantly, like the explosion of a cracker, whereas there are few that take years to happen, like the rusting of iron? Why does that happen? Well, this depends on the rate of reaction happening, and the study of the rate of reaction is known as Chemical Kinetics. This chapter also talks about the factors affecting the rate of reaction, like concentration, temperature, catalyst, etc.

This Story also Contains
  1. NCERT Exemplar Class 12 Chemistry Solutions Chapter 4: MCQ (Type 1)
  2. NCERT Exemplar Class 12 Chemistry Solutions Chapter 4: MCQ (Type 2)
  3. NCERT Exemplar Class 12 Chemistry Solutions Chapter 4: Short Answer Type
  4. NCERT Exemplar Class 12 Chemistry Solutions Chapter 4: Matching Type
  5. NCERT Exemplar Class 12 Chemistry Solutions Chapter 4: Assertion and Reason Type
  6. NCERT Exemplar Class 12 Chemistry Solutions Chapter 4: Long Answer Type
  7. Class 12 Chemistry NCERT Chapter 4: Higher Order Thinking Skills (HOTS) Questions
  8. Approaches to Solve Questions of Chapter 4
  9. Major Topics Of NCERT Exemplar Class 12 Chemistry Chapter 4
  10. Formulas Of NCERT Class 12 Chemistry Chapter 4
  11. NCERT Exemplar Class 12 Chemistry Solutions Chapter-wise
  12. NCERT Chapter-wise Solutions for Class 12 Chemistry
  13. NCERT Solutions for Class 12 Subject-wise
  14. NCERT Exemplar Solution for Class 12 subject-wise
NCERT Exemplar Class 12 Chemistry Solutions Chapter 4 Chemical Kinetics
NCERT Exemplar Class 12 Chemistry Solutions Chapter 4 Chemical Kinetics

Chemical kinetics has applications in various fields including industrial chemical processes, enzymatic activity, and pharmaceutical drug development. To obtain a complete understanding of the chapter, students will be required to go through the NCERT and its exemplar problems. In this article, we have also included Higher Order Thinking Skills (HOTS) questions along with a structured approach to solve the questions of the chapter. This helps students gain a clearer vision of how to learn the chapter effectively, while the HOTS questions give them an idea of the type of difficult and thought-provoking questions they may encounter.

The NCERT Exemplar solutions of this chapter are designed and structured by our experts, which cover the basic concept of kinetics and complex topics. Students can refers to these Exemplar solutions of Chemical Kinetics, they will be able to solve complex problems including rate equations and mechanisms, and exam-oriented numerical problems with confidence and accuracy. Students can also refer to NCERT Exemplar Class 12 Chemistry Solutions to strengthen their concept and probelm solving ability for other chapters also.This confidence and accuracy will help them to excel in CBSE Boards and competitive exams like NEET, JEE etc.

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NCERT Exemplar Class 12 Chemistry Solutions Chapter 4: MCQ (Type 1)

These Exemplar solutions of chapter 4 of class 12 help in quick revision and better exam preparation.

Question 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.

Answer:

The answer is the option (iii). Activation energy for the reaction is lowered by a catalyst.

Question 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.

Answer:

The answer is the option (iii). Catalyst only eases the threshold for a reaction without participating directly in it. So, it has no effect on heat evolved or absorbed during reaction.

Question 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 the probability of collision.
(iv) using catalyst.

Answer:

The answer is the option (ii). According to Arrhenius equation, activation energy can be calculated by measuring the rate constants at two different temperatures.
log(k2k1)=Ea2.303R(1T11T2)

Where, Ea=Activation energy
T1=lower temperature
T2=Higher temperature
k1=rate constant at temperature T1
k2=rate constant at temperature T2

Question 4:

Consider Fig. 4.1 and mark the correct option.
(i) The activation energy of forward reaction is E1 + E2 and the product is less stable than reactant.
(ii) The activation energy of forward reaction is E1 + E2 and product is more stable than reactant.
(iii) The activation energy of both forward and backward reaction is E1 + E2 and reactant is more stable than the product.
(iv) The activation energy of the backward reaction is E1 and the product is more stable than reactant.


Answer:

The answer is the option (i) Eaforward=E1+E2
As reactants have lower energy than the products, the products are less stable.

Question 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 _________.

(i)k=2.303tlogPiPix

(ii)k=2.303tlogPi2PiPt

(iii)k=2.303tlogPi2Pi+Pt

(iv)k=2.303tlogPiPi+x

Answer:

The answer is the option (ii)

A(g)

B(g)

C(g)

Initial Pressure:

Pi

0

0

Pressure after time (t):

(Pi - x)

x

x

Pi=Pi+x
x=PtPi
PA=Pressure of A after time t=Pix=Pi(PiPi)

=2PiPt
k=2.303tlog[A]0[A]

=2.303tlogPi2PiPt

Question 6:

According to Arrhenius equation rate constant k is equal to AeEa/RT. Which of the following options represents the graph of ln k vs 1T?

Answer:

The answer is the option (i) According to Arrhenius equation:
k=AeEaRT
ln(k)=ln(AeEaRT)
lnk=lnAEaRT

Question 7:

Consider the Arrhenius equation given below and mark the correct option. k=AeEa/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

Answer:

The answer is the option (iv). Increase in T leads to an increase in EaRT and thus, an increase in k. Similarly, a decrease in Ea leads to an increase in EaRT and thus, an increase in k.

Question 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.

(i) Average rate up to 40 seconds is V3V240
(ii) Average rate up to 40 seconds is V3V24030
(ii) Average rate up to 40 seconds is V340
(iv) Average rate up to 40 seconds is V3V14020

Answer:

The answer is the option (iii) Average rate of the reaction can be determined by change in concentration of H2 upon time from (0,0) to (40,V3)=V30400=V340

Question 9:

Which of the following statements is not correct about an 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 the molar concentration of the reactants in the rate law expression.

Answer:

The answer is the option (iii). The summation of powers of the reactant’s concentration is used for determining the order of reaction. The order of a reaction is always equal to the sum of the stoichiometric coefficients of reacting species in the balanced chemical equation.

Question 10:

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

(i)V5V25030

(ii)V4V25030

(iii)V3V24030

(iv)V3V14020

Answer:

The answer is the option (ii). Instantaneous rate is the rate of a reaction at a particular moment of time. Option (ii) does not show instantaneous rate of reaction.

Question 11:

Which of the following statements is correct?
(i) The rate of a reaction decreases with passage of time as the concentration of reactants decreases.
(ii) The rate of a reaction is the 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 the concentration of the reactant(s).

Answer:

The answer is the option (i). The rate of a reaction is defined as the change in concentration of the reactant per unit time. Therefore, it is dependent upon the concentration of reactants.

Question 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)

(i)Δ[Br]Δt=5Δ[H+]Δt

(ii)Δ[Br]Δt=65Δ[H+]Δt

(iii)Δ[Br]Δt=56Δ[H+]Δt

(iv)Δ[Br]Δt=6Δ[H+]Δt

Answer:

We can rewrite the rate law expression for the given equation
5Br(aq)+BrO3(aq)+6H+(aq)3Br2(aq)+3H2O(l)

15Δ[Br]Δt=Δ[BrO3]Δt=16Δ[H+]Δt=13Δ[Br2]Δt

Δ[Br]Δt=Δ[BrO3]Δt=56Δ[H+]Δt

Δ[Br]Δt=56Δ[H+]Δt

(iii)Δ[Br]Δt=56Δ[H+]Δt

Question 13:

Which of the following graph represents the exothermic reaction?

(i) (a) only
(ii) (b) only
(iii) (c) only
(iv) (a) and (b)

Answer:

The answer is the option (i). In an exothermic reaction the reaction activation energy of product should be higher than that of reactant whereas the enthalpy of reactant should be greater than that of products.

Question 14:

The rate law for the reaction A+2BC is found to be
Rate =k[A][B]
The 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

Answer:

The answer is the option (i). Rate constant of a reaction is independent of the concentrations of the reactants.

Question 15:

Which of the following statements is incorrect about the collision 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 determine the rate of reaction.
(iii) Collision of atoms or molecules possessing sufficient threshold energy results into the product formation.
(iv) Molecules should collide with sufficient threshold energy and proper orientation for the collision to be effective.

Answer:

The answer is the option (iii) For a reaction to occur, collision theory mandates several conditions that are:
(i) Collision of molecules must take place with sufficient threshold energy and should be effective.
(ii) They should have proper orientation.

Question 16:

A first-order reaction is 50% completed in 1.26×1014s. How much time would it take for 100% completion?
(i)1.26×1015s
(ii)2.52×1014s
(iii)2.52×1028s
(iv)infinite

Answer:

The answer is the option (iv). Since whole of the substance never reacts, reaction is considered 100% complete only after an infinite time that cannot be calculated.

Question 17:

Compounds ‘A’ and ‘B’ react according to the following chemical equation.
A(g)+2B(g)2C(g)
The concentration of either ‘A’ or ‘B’ was 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 Concetration of [A]/molL-1

Initial Concentration of [B]/molL-1

Initial rate concentration of [C]/molL-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

Answer:

The answer is the option (ii)
We know that Rate of a reaction is =k[A]x[B]y
On doubling the concentration of B, keeping the concentration of A constant, the rate of formation of C increases by a factor of four. Thus, the rate of reaction is directly proportional to the square of concentration of B. However, on doubling the concentration of A, the rate of formation of C becomes the double of the initial value. Thus the rate is directly proportional to the first power of concentration of A. Hence Rate = =K[A][B]2

Question 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.

Answer:

The answer is the option (ii)
A catalyst increases the rate of both the forward and backward reaction to the same extent. Hence, option a is correct.
ΔG=-RT ln Q where Q is the reaction quotient

Reaction quotient is the relation between the concentration of reactants and products. Thus the gibbs free energy is unaffected by the addition of catalyst. It does not affect the equilibrium of the reaction since equilibrium constant depends only on the concentration of reactants and products.

Question 19:

The value of rate constant of a pseudo first order reaction ____________.
(i) depends on the concentration of reactants present in a 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.

Answer:

The answer is the option (ii). The value of rate constant reaction is dependent on the concentration of reactants present in excess in a pseudo first order reaction. This is because the excess reactants do not get altered much while during the reaction

Question 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 ?

Answer:

The answer is the option (ii) A->B

With time, there is an exponential decrease in the concentration of reactants and an increase in the concentration of products.

NCERT Exemplar Class 12 Chemistry Solutions Chapter 4: MCQ (Type 2)

Below is the list of NCERT Exemplar question and answers:

Question 21:

In the following questions two or more options may be correct.
Rate law cannot be determined from the balanced chemical equation if _______.

(i) the reverse reaction is involved.
(ii) it is an elementary reaction.
(iii) it is a sequence of elementary reactions.
(iv) any of the reactants is in excess

Answer:

The answer is the option (i, iii, iv). From a balanced chemical equation rate law can only be determined if it is an elementary reaction.

Question 22:

In the following questions two or more options may be correct.
Which of the following statements are applicable to a balanced chemical equation of an elementary reaction?

(i) Order is the same as molecularity.
(ii) Order is less than the molecularity.
(iii) Order is greater than the molecularity.
(iv) Molecularity can never be zero.

Answer:

The answer is the option (i, iv). The order and molecularity are same for an elementary or single step reaction with molecularity of a reaction never equal to zero.

Question 23:

In the following questions two or more options may be correct.
In any unimolecular reaction ______________.

(i) only one reacting species is involved in the rate-determining step.
(ii) the order and the molecularity of the 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.

Answer:

The answer is the option (i, ii). Only one reactant is involved in rate determining step in an unimolecular reaction and the order and molecularity of rate determining or slowest step are equal to one.
AB
Rate=k[A]

Question 24:

In the following questions two or more options may be correct.
For a complex reaction ______________.

(i) order of an overall reaction is the same as molecularity of the slowest step.
(ii) order of an overall reaction is less than the molecularity of the slowest step.
(iii) order of an overall reaction is greater than molecularity of the slowest step.
(iv) molecularity of the slowest step is never zero or non-integer.

Answer:

The answer is the option (i, iv).
The answer is the option (i) Order of overall reaction is same as the molecularity of slowest step for a complex reaction.
The rate of overall reaction is dependent on total number of molecules involved in slowest step of the reaction making molecularity of the slowest step equal to order of overall reaction.

Question 25:

In the following questions two or more options may be correct.At high pressure, the following reaction is zero order.2NH3(g)1130KPlatinum catalystN2(g)+3H2(g)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.

Answer:

The answer is the option (i, iii, iv) The rate of reaction is not dependent on the concentration of ammonia in a zero-order reaction

Question 26:

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

Answer:

The answer is the option (i, iv) Activated complex is an intermediate formed at the highest level of energy (Ea) of the system. Some energy is released with the decomposition of this unstable complex to give products. If the reaction is reversible, reactants might be formed.

Question 27:

In the following questions two or more options may be correct.
According to Maxwell Boltzmann distribution 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.
(iii) most probable kinetic energy increases at higher temperatures.
(iv) most probable kinetic energy decreases at higher temperatures.

Answer:

The answer is the option (i, iii) Maxwell Boltzmann distribution of kinetic energy can be described by a graph plotted with fraction of molecules on y-axis versus kinetic energy on x-axis.

Most probable kinetic energy is the kinetic energy of maximum fraction of molecule. The peak of the graph shifts forward and downward with an increase in temperature.
This implies that the most probable kinetic energy increases and fraction of molecules with most probable K.E decreases with an increase in temperature.

Question 28:

In the following questions two or more options may be correct.
In the graph showing Maxwell Boltzman distribution of energy, ___________.

(i) area under the curve must not change with an 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 the increase in the temperature, curve broadens and shifts to the right-hand side.

Answer:

The answer is the option (i, iv) Area under the curve should remain constant with an increase in temperature as the probability must be 1 for all the time. As the temperature rises, the curve broadens out and shifts to the right-hand higher energy value side.

Question 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 an increase in temperature.
(iv) Rate of reaction decreases with decrease in activation energy.

Answer:

The answer is the option (i, iv). According to Arrhenius equation, the rate of reaction increases with increase in temperature and decrease in the activation energy. It gets double with every 10° change. As the rate of reaction increases, there is an exponential increase in the rate constant of the reaction.

Question 30:

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

Answer:

The answer is the option (ii, iv)
(i) On adding the catalyst to a reaction, medium rate of reaction increases with decreasing activation energy of molecule.
(ii) The enthapy change of reaction remains unchanged due to a catalyst. In both the catalysed and uncatalysed reaction, energy of reactants and products are alike.

Question 31:

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

Answer:

The answer is the option (i, iv). On plotting [R] against t, a straight line is obtained with slope =k and intercept equal to [R]0from the equation
[R]=kt+[R]0
The rate is directly proportional to the zero power of concentration in a Zero-order reaction.
Rate = d[R]dt=k[R]0

Question 32:

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

Answer:

The answer is the option (i, iv) For a first order reaction
t12=0.693k and

rate =d[R]dt=k[R]
Sincek=2.303tlog[R0R] the slope for a graph between log log[R0R] versus t is k2.303

NCERT Exemplar Class 12 Chemistry Solutions Chapter 4: Short Answer Type

Below is the list of NCERT Exemplar short answer type question and answers:

Question 33:

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

Answer:

In a biomolecular reaction:
A+BProduct
where the rate is given by k[A][B] …………. (i)
If the value of [B] is very high, rate law can be represented as Rate =k[A]
where, k=k[B]
The order of reaction will thus be one.

Question 34:

Write the rate equation for the reaction 2A+BC if the order of the reaction is zero.

Answer:

2A+BC
If the order of reaction is zero, the rate equation can be written as
Rate =k[A]o[B]o

Question 35:

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

Answer:

The rate of this reaction is measured as a function of initial concentration for the determination of the rate law. This is done either by keeping the concentration of one of the reactants constant while changing the other or by changing the concentration of both the reactants. The rate law can be determined from the concentration dependence of rate.

Question 36:

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

Answer:

In elementary reactions the value of order and molecularity is equal.

Question 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?

Answer:

Rate=[A]α
27Rate=k[3A]α
Dividing both the equations we get α=3
Therefore the order of reaction is 3

Question 38:

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

Answer:

In case of a zero order reaction
R=[R]0kt
For the reaction to achieve completion [R]=0
kt=[R]0ort=[R]0k

Question 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.

Answer:

Given that the rate of reaction is k[A][B]32
Order =1+32=52
The order of elementary reaction cannot be fractional.
Hence, This indicates that the reaction is not an elementary
reaction.

Question 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?

Answer:

For a certain reaction, large fraction of molecules has energy more than the threshold energy, yet the rate of reaction is very slow due to lack of proper orientation

Question 41:

For a zero-order reaction will the molecularity be equal to zero? Explain.

Answer:

Molecularity of a reaction cannot be equal to zero in any case.

Question 43:

The reaction between H2(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.

Answer:

2H2(g)+O2(g)2H2O(l)
The reaction between H2 and O2 to form water does not take place at room temperature because the activation energy of the reaction is very high.

Question 44:

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

Answer:

The rate of a reaction increases with a rise in temperature as larger fraction of colliding particles can cross the energy barrier (i.e. the activation energy).

Question 45:

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

Answer:

Oxygen does not burn by themselves in spite of being available in plenty in air as the activation energy or Ea for combustion reactions of fuels is extremely high at room temperature.

Question 46:

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

Answer:

The low possibility of more than three molecules colliding simultaneously makes the probability of reaction with molecularity higher than three very rare.

Question 47:

Why does the rate of any reaction generally decrease during the reaction?

Answer:

We know that the rate of a reaction is dependent on the concentration of reactants. During the course of the reaction, the concentration of reactants keeps decreasing because the reactants keep getting converted to products.

Question 48:

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

Answer:

Gibbs free energy determines the thermodynamic feasibility of a reaction. For spontaneous reaction, ΔG should be negative whereas
Activation energy determines the kinetic feasibility of the reaction.
DiamondGraphitewhereΔG=ve

Question 49:

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

Answer:

In the redox titration of KMnO4 and oxalic acid we heat the oxalic acid solution before starting the titration to increase the rate of reaction as it is very slow.

Question 50:

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

Answer:

Molecularity can be defined as the number of molecules involved in the process. It cannot be less than one.

Question 51:

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

Answer:

A complex reaction involves many elementary reactions. Numbers of molecules in each elementary reaction can vary leading to different molecularity at each step. Therefore, the molecularity is not applicable in case of overall complex reaction. Whereas the slowest step is responsible for the experimental determination of the order of a complex and is therefore, applicable even in the case of complex reactions.

Question 52:

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

Answer:

As reactions can be multi-stepped, the rate of reaction is determined by slowest step
KClO3+6FeSO4+3H2SO4KCl+3H2O+3Fe2(SO4)3
Given the above equation, it may seem like a reaction with an order of ten, but in reality this is a second order reaction. Order is determined via experiments and expresses the dependence of the rate of reaction on the concentration of reactants.

NCERT Exemplar Class 12 Chemistry Solutions Chapter 4: Matching Type

Below is the list of NCERT Exemplar matching type question and answers:

Question 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)

(a) 1 st order

(ii)

(iii)

(b) Zero order

(iv)

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Answer:

i(a),ii(b),iii(b),iv(a)
For zero order reaction rate equation may be written as,

[R]=kt+[R0]
which denotes a straight line equation similar to y=mx+c
on transforming [R][R0]t=k
k=[R][R0]T
k=rate
Rate = k[t]0
rateα[t]0
For a first order reaction dxdtα[concentration]
Graph between rate and concentration may be drawn as,

k =2.303tlog[R]0[R]
kt2.303=log[R]0[R]
kt2.303=log[R]0log[R]
log[R]=(k2.303)t+log[R]0

Question 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

(iii)

Second half life of first order reaction

(c)

by lowering the activation energy

(iv)

eEa/RT

(d)

is same as the first

(v)

Energetically favourable reactions are sometimes slow

(e)

total probability is one

(vi)

Area under the Maxwell- Boltzmann curve is constant

(f)

refers to the fraction of molecules with energy equal to or greater than activation energy.

Answer:

(i) → (c) (ii) → (a) (iii) → (d) (iv) → (f) (v) → (b) (vi) → (e)
(i) Catalyst can change the rate of reaction by decreasing the (Ea) or activation energy.
(ii) Molecularity cannot be fraction or zero. No reaction is possible if molecularity is zero.
(iii) The first and second half-life of first order reaction are same since half-life is temperature dependent. ‘
(iv) In Arrhenius equation, eEaRT denotes the fraction of molecules with K.E equal to or greater than activation energy.
(v) The activation energy and proper orientation of the molecules together determine the criteria for effective collision in collision theory.
(vi) Since total probability of molecules must be equal to one at all times, area under the Maxwell-Boltzmann curve remains constant.

Question 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)

Ordibarily rate of conversion is imperceptible

(iii)

Average rate

(c)

long duration of time

Answer:

(i b), (iia), (iiic)
(i) The conversion of diamond to graphite does not take place under ordinary condition.
(ii) Instantaneous rate of reaction is calculated for a very short time interval.
(iii) Average rate of reaction is calculated for a long duration of time.

Question 56:

Match the items of Column I and Column II.

Column I

Column II

(i)

Mathematical expression for rate of reaction

(a)

Rate constant

(ii)

Rate of reaction for zero order reaction is equal to

(b)

rate law

(iii)

Units of rate constant for zero order reaction is same as that of

(c)

order of slowest step

(iv)

Order of a complex reaction is determined by

(d)

rate of a reaction

Answer:

(i b), (ii a), (iii d), (iv c)
(i) Mathematical expression for rate of reaction is termed as rate law.
(ii) Rate for zero order reaction is equal to rate constant r=k[A]
(iii) Unit of rate constant for zero order reaction is same as that of rate of reaction.
(iv)The slowest step or the rate determining step helps in finding the order of a complex reaction.

NCERT Exemplar Class 12 Chemistry Solutions Chapter 4: Assertion and Reason Type

Below is the list of NCERT Exemplar assertion and reason type question and answers:

Question 57:

In the following question, a statement of Assertion The answer is the option (i) followed by a statement of Reason (R) is given. Choose the correct option out of the following choices:
Assertion The answer is the option (i): Order of the reaction can be zero or fractional.
Reason (R): We cannot determine order from balanced chemical equation.

(i) Both Assertion and Reason are correct and the reason is the correct explanation of Assertion.
(ii) Both assertion and reason are correct but Reason is not the correct explanation of Assertion
(iii) Assertion is correct but Reason is incorrect.
(iv) Both Assertion and Reason are incorrect.
(v) Assertion is incorrect but Reason is correct.

Answer:

The answer is the option (ii) Order of a reaction may be zero or fractional and rate law expression can be used to determine it.

Question 58:

In the following question, a statement of Assertion The answer is the option (i) followed by a statement of Reason (R) is given. Choose the correct option out of the following choices:
Assertion The answer is the option (i): Order and molecularity are same.
Reason (R): Order is determined experimentally, and molecularity is the sum of the stoichiometric coefficient of rate determining elementary step.

(i) Both Assertion and Reason are correct and the reason is the correct explanation of Assertion.
(ii) Both assertion and reason are correct but Reason is not the correct explanation of Assertion
(iii) Assertion is correct but Reason is incorrect.
(iv) Both Assertion and Reason are incorrect.
(v) Assertion is incorrect but Reason is correct.

Answer:

The answer is the option (v). While balanced stoichiometric equation is used to calculate molecularity order can only be measured through experiments.

Question 59:

In the following question, a statement of Assertion The answer is the option (i) followed by a statement of Reason (R) is given. Choose the correct option out of the following choices:
Assertion The answer is the option (i): The enthapy of reaction remains constant in the presence of a catalyst.
Reason (R): 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.

(i) Both Assertion and Reason are correct and the reason is the correct explanation of Assertion.
(ii) Both assertion and reason are correct but Reason is not the correct explanation of Assertion
(iii) Assertion is correct but Reason is incorrect.
(iv) Both Assertion and Reason are incorrect.
(v) Assertion is incorrect but Reason is correct.

Answer:

The answer is the option (i) ΔH = Activation Energy of forward reaction – Activation Energy of reverse reaction.
The heat of reaction remains unaltered by the catalysts because it does not involve directly in the reaction and only affects the activation energy reactions. The enthalpy of reaction remains unchanged due to the addition of catalyst.

Question 60:

In the following question, a statement of Assertion The answer is the option (i) followed by a statement of Reason (R) is given. Choose the correct option out of the following choices:
Assertion The answer is the option (i): All collision of reactant molecules lead to product formation.
Reason (R): Only those collisions in which molecules have correct orientation and sufficient kinetic energy lead to compound formation.

(i) Both Assertion and Reason are correct and the reason is the correct explanation of Assertion.
(ii) Both assertion and reason are correct but Reason is not the correct explanation of Assertion
(iii) Assertion is correct but Reason is incorrect.
(iv) Both Assertion and Reason are incorrect.
(v) Assertion is incorrect but Reason is correct.

Answer:

The answer is the option (v). Only the molecular collisions with sufficient kinetic energy and proper orientation bring out the formation of product.

Question:61

In the following question, a statement of Assertion The answer is the option (i) followed by a statement of Reason (R) is given. Choose the correct option out of the following choices:
Assertion The answer is the option (i): Rate constants determined from Arrhenius equation are fairly accurate for simple as well as complex molecules.
Reason (R): Reactant molecules undergo chemical change irrespective of their orientation during collision.

(i) Both Assertion and Reason are correct and the reason is the correct explanation of Assertion.
(ii) Both assertion and reason are correct but Reason is not the correct explanation of Assertion
(iii) Assertion is correct but Reason is incorrect.
(iv) Both Assertion and Reason are incorrect.
(v) Assertion is incorrect but Reason is correct.

Answer:

The answer is the option (iii). The criteria for an effective collision and rate of chemical reaction is determined together by activation energy and proper orientation of molecules in the collision theory.

NCERT Exemplar Class 12 Chemistry Solutions Chapter 4: Long Answer Type

Below is the list of NCERT Exemplar long answer type question and answers:

Question 62:

All energetically effective collisions do not result in a chemical change. Explain with the help of an example.

Answer:

Products are only formed via effective collisions, which means molecules need to collide with sufficient kinetic energy and should have proper orientation. It is a required condition to break molecular bonds, which give way to formation of new bonds i.e. – products.
Taking the example of reaction of bromomethane with hydroxyl ion, we can see that improper orientation does not let the reaction complete.

Question 63:

What happens to most probable kinetic energy and the energy of activation with increase in temperature?

Answer:


On increasing the temperature, the curve flattens out as there are more molecules with higher kinetic energies. Because of this shift, the most probable kinetic energy (the modal value) also increases slightly.

Question 64:

Describe how does the enthalpy of reaction remain unchanged when a catalyst is used in the reaction?

Answer:

Catalyst doesn’t participate in the reaction but increases the speed of reaction. The “Intermediate complex formation theory” claims that the reactants combine with the catalyst to form highly active radical complex. These complex compounds then decompose to create the product and the catalyst.
The intermediate formed with catalyst possesses much lower potential energy than the intermediate formed in the normal reaction, lowering the activation energy required. The following diagram shows the impact of using the catalyst on the potential energy and thus, the activation energy.

Question 65:

Explain the difference between instantaneous rate of a reaction and average rate of a reaction.

Answer:

The change in concentration of reactants or products per unit time is the average rate of reaction.
Average rate=Δ[R]ΔT=Δ[P]ΔT
It is calculated over a large time frame and, in most scenarios, will not give the rate of reaction for future. Instantaneous rate is the actual rate of reaction at a given instance and can be represented as follows:

Instantaneous rate =limΔT0Δ[R]ΔT=limΔT0Δ[P]ΔT=d[R]dT=d[P]dT

=limΔT0Δ[P]ΔT

=d[R]dT=d[P]dT
While average rate can be measured for multi-step and elementary reaction, instantaneous rate can’t be measured.

Question 66:

With the help of an example explain what is meant by pseudo first order reaction?

Answer:

Pseudo-first order reaction is a reaction where one reactant has a very high concentration and during the course of the reaction, it doesn’t change significantly. The reaction proceeds as a first order reaction in such a case. Take for instance the hydrolysis of ethyl acetate and inversion of cane sugar
Hydrolysis of ethyl acetate:
CH3COOC2H5+H2OH+CH3COOHt=00.01 mol10 mol+C2H5OHt=t0 mol9.99 mol0 mol
Rate of reaction=k[CH3COOC2H5]

Where, k=k[H2O]

Inversion of cane sugar:

C12H22O11+H2OH+C6H12O6+C6H12O6
Rate of reaction=k[C12H22O11]
Where, k=k[H2O]

Class 12 Chemistry NCERT Chapter 4: Higher Order Thinking Skills (HOTS) Questions

Question 1: Consider the following statements related to temperature dependence of rate constants. Identify the correct statements,

A. The Arrhenius equation holds true only for an elementary homogenous reaction.
B. The unit of A is same as that of k in Arrhenius equation.
C. At a given temperature, a low activation energy means a fast reaction.
D. A and Ea as used in Arrhenius equation depend on temperature.
E. When Ea >> RT. A and Ea become interdependent.
Choose the correct answer from the options given below:

(1) A, C and D Only

(2) B, D and E Only

(3) B and C Only

(4) A and B Only

Solution:

k=AeEa/RT
If Ea↓:k
k and A has same units.

Hence, the correct answer is option (3).

Question 2: In a first order decomposition reaction, the time taken for the decomposition of reactant to one fourth and one eighth of its initial concentration are t1 and t2( s), respectively. The ratio t1/t2 will :

(1) 43

(2) 32

(3) 34

(4) 23

Solution:

t1=t14=1klnA0 A04=1kln4t2=t18=1klnA0 A08=1kln8t1t2=ln4ln8=2ln23ln2=23

Hence, the correct answer is option (4).

Approaches to Solve Questions of Chapter 4

To effectively solve questions from Chapter 4, Chemical Kinetics, students can follow the approaches given below:

1) While solving problems related to Chemical kinetics, students first need to understand the concept of the Rate of Reaction.

  • Understanding the difference between average rate, instantaneous rate, and initial rate.
  • Rate law expression Rate =k[A]m[B]n
  • Identification of the order of reaction.

2) Then, students need to determine the Rate Constant(k)

  • Rate constant for zero order reaction: k= Rate
  • Rate constant for first order reaction: k=1tln([A]0[A])
  • Rate constant for second order reaction: k=1t(1[A]1[A]0)

3) Plot graphs for different orders of reaction:

  • Zero order reaction: Concentration vs time
  • First order reaction: ln[A] vs time
  • Second-order reaction: Plot 1[A] vs. time

4) Analysis of Arrhenius Equation

  • k=AeFnRT

5) Calculation of Half-life:

  • For Zero order reaction: t1/2=[A]02k
  • For first order of reaction: t1/2=0.693k
  • For second order reaction: t1/2=1k[A]0

6) Read every question carefully and practice questions again and again.

Major Topics Of NCERT Exemplar Class 12 Chemistry Chapter 4

  • Rate of a Chemical Reaction
  • Factors Influencing Rate of a Reaction
  • Dependence of Rate on Concentration
  • Rate Expression and Rate Constant
  • Order of a Reaction
  • Molecularity of a Reaction
  • Integrated Rate Equations
  • Zero Order Reactions
  • First Order Reactions
  • Half-Life of a Reaction
  • Temperature Dependence of the Rate of a Reaction
  • Effect of Catalyst
  • Collision Theory of Chemical

Formulas Of NCERT Class 12 Chemistry Chapter 4

Here is a list of important formulas from Class 12 NCERT Chemistry Chapter - Chemical Kinetics:
1. Rate of Reaction

 Rate =Δ[ Reactant ]Δt=Δ[ Product ]Δt

2. Average Rate

 Average rate =Δ[ Concentration ]Δt

3. Instantaneous Rate

 Instantaneous rate =d[ Concentration ]dt|t

4. Rate Law (Rate Equation)

 Rate =k[ A]m[ B]n

Where m+n= order of the reaction.

5. Units of Rate Constant (k)

 Units of k=(mol1n Ln1 s1)

Where n= overall order of the reaction.

6. Integrated Rate Law for First Order Reaction

k=1tln[R]0[R] or ln[R]=ln[R]0kt

7. Half-Life for First Order Reaction

t1/2=0.693k

8. Integrated Rate Law for Zero Order Reaction

[R]=[R]0kt or k=[R]0[R]t

9. Half-Life for Zero Order Reaction

t1/2=[R]02k

10. Arrhenius Equation

k=AeEaRT

Where:

  • k= rate constant
  • A= frequency factor
  • Ea= activation energy
  • R= gas constant
  • T= temperature (in Kelvin)

NCERT Exemplar Class 12 Chemistry Solutions Chapter-wise

Here is a list of NCERT Exemplar chapter-wise solutions:

NCERT Chapter-wise Solutions for Class 12 Chemistry

Here is a list of NCERT chapter-wise solutions:

NCERT Solutions for Class 12 Subject-wise

Here is a list of NCERT subject-wise solutions given below:

NCERT Exemplar Solution for Class 12 subject-wise

Here is a list of NCERT exemplar solutions subject-wise solutions given below:

NCERT Books and NCERT Syllabus

Here is list of NCERT books and syllabus of class 12:

Frequently Asked Questions (FAQs)

1. What is the focus of Chapter 4 in the NCERT Exemplar for Class 12 Chemistry?

Chapter 4,  "Chemical Kinetics," focuses on the study of rates of chemical reactions, factors affecting these rates, and the mechanisms of reactions. It covers concepts such as reaction rate, rate laws, order of reactions, and the effect of temperature on reaction rates.

2. What is the difference between a zero-order and a first-order reaction?

In a zero-order reaction, the reaction rate is independent of the concentration of the reactants, while in a first-order reaction, the reaction rate is directly proportional to the concentration of one of the reactants. The rate law for a zero-order reaction is given by rate = k, while for a first-order reaction, it is given by rate = k[A].

3. What are some key factors that affect the rate of a chemical reaction as discussed in this chapter?

Key factors affecting the rate of a chemical reaction include:

  • Concentration of reactants: Higher concentration usually increases reaction rate.
  • Temperature: Increasing temperature typically increases reaction rate.
  • Presence of a catalyst: Catalysts speed up reactions without undergoing permanent changes.
  • Surface area of reactants: Larger surface area can lead to a faster reaction rate.
4. How is the order of a reaction determined, and why is it important?

The order of a reaction is determined experimentally and is defined as the sum of the powers of the concentration terms in the rate law expression. It is important because it helps in understanding how changes in concentrations of reactants affect the reaction rate, allowing chemists to predict the behavior of the reaction under various conditions.

5. What is chemical kinetics?

Chemical kinetics is the branch of chemistry that deals with the rates of chemical reactions and the factors that affect them. It helps us understand how fast reactions occur and the mechanisms through which they take place.

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A block of mass 0.50 kg is moving with a speed of 2.00 ms-1 on a smooth surface. It strikes another mass of 1.00 kg and then they move together as a single body. The energy loss during the collision is

Option 1)

0.34\; J

Option 2)

0.16\; J

Option 3)

1.00\; J

Option 4)

0.67\; J

A person trying to lose weight by burning fat lifts a mass of 10 kg upto a height of 1 m 1000 times.  Assume that the potential energy lost each time he lowers the mass is dissipated.  How much fat will he use up considering the work done only when the weight is lifted up ?  Fat supplies 3.8×107 J of energy per kg which is converted to mechanical energy with a 20% efficiency rate.  Take g = 9.8 ms−2 :

Option 1)

2.45×10−3 kg

Option 2)

 6.45×10−3 kg

Option 3)

 9.89×10−3 kg

Option 4)

12.89×10−3 kg

 

An athlete in the olympic games covers a distance of 100 m in 10 s. His kinetic energy can be estimated to be in the range

Option 1)

2,000 \; J - 5,000\; J

Option 2)

200 \, \, J - 500 \, \, J

Option 3)

2\times 10^{5}J-3\times 10^{5}J

Option 4)

20,000 \, \, J - 50,000 \, \, J

A particle is projected at 600   to the horizontal with a kinetic energy K. The kinetic energy at the highest point

Option 1)

K/2\,

Option 2)

\; K\;

Option 3)

zero\;

Option 4)

K/4

In the reaction,

2Al_{(s)}+6HCL_{(aq)}\rightarrow 2Al^{3+}\, _{(aq)}+6Cl^{-}\, _{(aq)}+3H_{2(g)}

Option 1)

11.2\, L\, H_{2(g)}  at STP  is produced for every mole HCL_{(aq)}  consumed

Option 2)

6L\, HCl_{(aq)}  is consumed for ever 3L\, H_{2(g)}      produced

Option 3)

33.6 L\, H_{2(g)} is produced regardless of temperature and pressure for every mole Al that reacts

Option 4)

67.2\, L\, H_{2(g)} at STP is produced for every mole Al that reacts .

How many moles of magnesium phosphate, Mg_{3}(PO_{4})_{2} will contain 0.25 mole of oxygen atoms?

Option 1)

0.02

Option 2)

3.125 × 10-2

Option 3)

1.25 × 10-2

Option 4)

2.5 × 10-2

If we consider that 1/6, in place of 1/12, mass of carbon atom is taken to be the relative atomic mass unit, the mass of one mole of a substance will

Option 1)

decrease twice

Option 2)

increase two fold

Option 3)

remain unchanged

Option 4)

be a function of the molecular mass of the substance.

With increase of temperature, which of these changes?

Option 1)

Molality

Option 2)

Weight fraction of solute

Option 3)

Fraction of solute present in water

Option 4)

Mole fraction.

Number of atoms in 558.5 gram Fe (at. wt.of Fe = 55.85 g mol-1) is

Option 1)

twice that in 60 g carbon

Option 2)

6.023 × 1022

Option 3)

half that in 8 g He

Option 4)

558.5 × 6.023 × 1023

A pulley of radius 2 m is rotated about its axis by a force F = (20t - 5t2) newton (where t is measured in seconds) applied tangentially. If the moment of inertia of the pulley about its axis of rotation is 10 kg m2 , the number of rotations made by the pulley before its direction of motion if reversed, is

Option 1)

less than 3

Option 2)

more than 3 but less than 6

Option 3)

more than 6 but less than 9

Option 4)

more than 9

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