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NCERT Class 12 Chemistry Chapter 4 Notes Chemical Kinetics- Download PDF Notes

NCERT Class 12 Chemistry Chapter 4 Notes Chemical Kinetics- Download PDF Notes

Edited By Shivani Poonia | Updated on Jul 02, 2025 05:44 PM IST

The most recent topic in today's world is nuclear power. Have you ever wondered why some reactions, like nuclear explosions, happen within a fraction of second while some reactions take years to happen like rusting of iron? Well, this depends on the rate of reaction, and in this NCERT chapter of Chemical Kinetics you will learn everything about it. Similarly, Food preservation is something that we see every day. Fresh fruits and vegetables spoil due to interactions between microbes and oxygen in the air. Storing them in the refrigerator slows down these processes, allowing them to last longer. In medicine, how rapidly a drug dissolves in the body influences its effectiveness. All this involves study of rate of reaction, and hence, chemical kinetics.

This Story also Contains
  1. NCERT Class 12 Chapter 3 Class Notes: Chemical Kinetics
  2. Chemical Kinetics Previous Year Question and Answer
  3. Approach to Solve Questions of Class 12 Chemistry Chapter 3 Chemical Kinetics
  4. CBSE Class 12 Chemistry Chapter-wise Notes
  5. Subject Wise NCERT Exemplar Solutions
  6. Subject-Wise NCERT Solutions
  7. NCERT Books and Syllabus
NCERT Class 12 Chemistry Chapter 4 Notes Chemical Kinetics- Download PDF Notes
NCERT Class 12 Chemistry Chapter 4 Notes Chemical Kinetics- Download PDF Notes

Chemical Kinetics is the branch of Chemistry that investigates how quickly certain reactions occur. It investigates what influences the rate of reactions and how compounds transform into new products. It has the potential to improve industrial processes, reduce pollution, and potentially contribute to the development of life-saving drugs.

Background wave

In the NCERT Notes of class 12 Chemical kinetics helps in scoring well in CBSE board exams by simplifying complex concepts and formulas. It is well-organised and concise, which makes it ideal for last-minute study and recapping before exams. These NCERT notes align with the updated CBSE syllabus covers all the important topics like Molecularity of reaction, Integrated rate equations, Temperature Dependence of the Rate of a Reaction, etc. This NCERT Chapter Chemical Kinetics includes some very important topics which are frequently asked in JEE and NEET, like Arrhenius equation, Temperature Dependence of Rate Constant, Rate Law and Order of Reaction.

Also, students can refer,

NCERT Class 12 Chapter 3 Class Notes: Chemical Kinetics

In this chapter, you’ll learn about different factors affect reaction rates, Step-by-step derivations, Important formulas,Diagrams and graphs, how to calculate rate of reaction mathematically, and how to interpret experimental data using integrated rate laws. Below is a structured breakdown of each concept from the NCERT textbook to help you master the topic with clarity.

Rate of a Chemical Reaction

The rate of a reaction can be defined as the change (decrease or increase) in the concentration of a reactant or product in unit time. The rate of a reaction can be given as:

Average rate

The average rate of reaction can be defined as the change in the concentration of a reactant or product in a definite time interval (Δt). It is denoted by the symbol rav.

Consider a hypothetical reaction:

Reactant (M) Product ( N )

The rate of disappearance of reactant-

rav= Decrease in concentration of M Time taken rav=[M]Δt

A minus sign indicates a decrease in concentration.

The rate of appearance of the product-

rav= Increase in concentration of NΔtrav=+[N]Δt

Instantaneous rate

The instantaneous rate of reaction can be defined as the change in the concentration of a reactant or product at a particular instant of time. It is denoted by the symbol rinst ..

For an infinitesimally small instant of time Δt0,

Therefore,

rinst=d[M]dt=d[P]dt

The overall rate of reaction

For an overall rate of reaction, the rate of disappearance of any of the reactants (or the rate of appearance of products) is divided by their corresponding stoichiometric coefficients.

Such that for a reaction,

aA+bBcC+dD

The overall rate is given as:

1ad[A]dt=1bd[B]dt=1c+d[C]dt=1d+d[D]dt

Or

[A]aΔt=[B]bΔt=+[C]cΔt=+[D]dΔt

Factors Influencing Rate of a Reaction

Major factors that influence the rate of reaction are:

  • Concentration of reactants

  • Temperature

  • Catalyst

  • Rate Law

Concentration of reactants

When the rate of reaction is represented in terms of the concentration of the reactants is known as rate law. It is also known as the rate equation or rate expression.

Consider the following reaction:

aA+bBcC+dD

The rate law for this reaction is:

Rate [A]x[B]y

Where x and y represent order w.r.t to A and B and may or may not be equal to a and b.

The differential form of the rate law is:

d[R]dt=k[A]x[B]y

Order of a reaction

The order of the chemical reaction is the summation of powers raised to the concentration of the reactants in the rate law.

For a reaction,

aA+bBcC+dD

The rate law is:

Rate [A]x[B]y

Therefore, the order of the reaction is x+y.

The order of a reaction can be 0,1,2,3 and fraction.

Order can never be a negative number.

Units of rate constant

REACTION

ORDER

UNITS

Zero-order reaction

0

mol 1 time 1

First-order reaction

1

time1

Second-order reaction

2

l mol1 time 1

Third-order reaction

3

l2 mol2 time 1

Molecularity of a reaction:

  • The sum total of species reacting or (molecules, atoms, ions) taking part in a chemical reaction is called the molecularity of a reaction.
  • The reaction can be unimolecular when one reacting species yields product(s).
  • The reaction can be bimolecular when two reacting species are involved.
  • The reaction can be trimolecular when three reacting species are involved.
MolecularityNo. of reactantsExample Reaction
1 (Unimolecular)OneO3O2+O
2 (Bimolecular)TwoNO+O3NO2+O2
3 (Termolecular)Three2NO+O22NO2

Integrated rate Equations

  • Zero-order reactions

Consider the reaction,

A → P

The rate law of this reaction is given by:

Rate = k[A]0

On integrating the above equation we get,

[A]t=[A]0kt

The graph of the above equation can be plotted as:

1644828448034

  • First-order reaction

Consider the reaction,

A → P

The rate law of this reaction is given by:

Rate = k[A]

On integrating the above equation we get,

ln([A]0[A]t)=kt

The exponential form of a first-order reaction is given as:

[A]t=[A]0ekt

Let us consider a typical first order gas phase reaction

A( g)B( g)+C( g)


Let pi be the initial pressure of A and pt the total pressure at time ' t '. Integrated rate equation for such a reaction can be derived as Total pressure pt=pA+pB+pC (pressure units)

pA,pB and pC are the partial pressures of A,B and C , respectively.
If x atm be the decrease in pressure of A at time t and one mole each of B and C is being formed, the increase in pressure of B and C will also be x atm each.

A( g)B( g)+C( g)

Initial conditions (at t=0 ):

  • Partial pressure of A=pi atm
  • Partial pressure of B=0 atm
  • Partial pressure of C=0 atm

At time t :

  • Partial pressure of A=(pix) atm
  • Partial pressure of B=x atm
  • Partial pressure of C=x atm

pt=(pix)+x+x=pi+xx=(ptpi) where, pA=pix=pi(ptpi)=2piptk=(2.303t)(logpipA)=2.303tlogpi(2pipt)

The following equation can be used to plot graphs of first-order reactions:

k=1tln([A]0[A]t)

1644828448531

log[A]0[A]t=kt2.303

1644828448711

The Half-life of a reaction:

The time in which the concentration of a reactant is decreased to one-half of its initial concentration is known as the half-life of a reaction. Half-life is represented as t1/2.

  • For zero-order reactions

t1/2=[A]02k

  • For first-order reactions

t1/2=0.693k

Note that for zero-order reaction t1/2[A]0. For the first-order reaction, t1/2 is independent of [A]0.

Pseudo-first-order reactions:

The reactions in which the actual order is different from the expected rate order predicted using rate law are called pseudo-first-order reactions.

CH3COOC2H5+H2OH+CH3COOH+C2H5OH

The rate law of the reaction:

Rate =k[CH3COOC2H5][H2O]

Expected order = 2

The concentration of water does not get altered much during the reaction as it is in excess and can be considered constant.

Rate =k[CH3COOC2H5]

Actual order = 1

Therefore, the reaction behaves as a first-order reaction, and such reactions are called pseudo-first-order reactions.

Table of Integrated Rate Laws for the Reactions of Zero and First Order

OrderReaction TypeDifferential Rate LawIntegrated rate lawStriaght line plotHalf-lifeUnits of k
0RPd[R]/dt=kkt=[R]0[R][R] vs t[R]0/2k conc time 1 or molL1 s1
1RPd[R]/dt=k[R][R]=[R]0ekt or kt=ln{[R]0/[R]}ln[R] vs tln2/ktime 1 or s1

Temperature Dependence of the Rate of a Reaction

The fact that the rate of a chemical reaction is dependent on temperature can be explained by the Arrhenius equation.

The mathematical form of the Arrhenius equation is given below:

k=AeEa/RT

where A is the Arrhenius factor / the frequency factor / the pre-exponential factor.

According to Arrhenius, a reaction can take place only when two (or more) molecules collide to form an unstable intermediate. This intermediate exists for a very short time and then breaks up to form product(s).

1644828450009

The activation energy (Ea) is the mandatory energy required to produce the intermediate activated complex (C). The reaction coordinate depicts the energy change profile when reactants change into products.

The minimum energy which the colliding molecules required to collide effectively is called threshold energy.

Threshold energy = Activation energy + Energy of the reactants

1644828450204

The rate of reaction depends on the temperature as it is found that with the rise in temperature by 10°, the rate constant is nearly doubled.

Temperature coeff icient =( Rate constant at T+10)( Rate constant at T)

At a particular temperature, the fractions of molecules are plotted against corresponding kinetic energies, a graph given below is obtained.

The peak of the curve represents the kinetic energy possessed by the maximum fraction of molecules and is called the most probable kinetic energy.

1644828450827

Here, the yellow area shows the fraction of additional molecules which react at t+10 while the grey area shows the fraction of molecules reacting at t.

Taking ln on both sides of the following equation:

k=AeEa/RTln(k)=EaRT+ln(A)

Arrhenius equation for two different temperatures can be modified as:

logk2k1=EaRT[T2T1]T1T2

Effect of Catalyst:

  • A catalyst is a substance that alters or changes the rate of a reaction without undergoing any permanent chemical change.

  • According to intermediate complex theory, a catalyst forms temporary bonds with the reactants, forming an intermediate complex. This is how a catalyst participates in a chemical reaction.

  • The catalyst reduces the energy difference known as activation energy between products and reactants which in turn lowers the potential energy barrier.

  • A catalyst catalyses to the same extent the forward as well as the backward reactions so that the equilibrium state remains same but equilibrium is reached faster.

1644828451210

Collision Theory of Chemical Reactions

  • According to the collision theory of chemical reactions, the reactant molecules are supposed as hard spheres, and the reaction takes place when molecules collide with each other.

A+B Products

  • The collision frequency (Z) can be defined as the number of collisions per second per unit volume of the reaction mixture.
  • Activation energy also affects the rate of chemical reactions.
  • Only effective collisions lead to the formation of products.
  • The proper orientation of reactant molecules in space leads to bond formation whereas due to improper orientation no products are formed.
  • The factor for proper orientation during effective collisions is called the probability or steric factor (P).

k=PZABeEa/RT

Chemical Kinetics Previous Year Question and Answer

Question: 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

Answer:

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

Hence, the correct answer is option (4).

Question: Half life of zero order reaction A product is 1 hour, when initial concentration of reaction is 2.0 mol L1. The time required to decrease concentration of A from 0.50 to 0.25 mol L1 is:

(1) 0.5 hour

(2) 4 hour

(3) 15 min

(4) 60 min

Answer: For zero order reaction

 Half life =A02k60 min=22kk=160M/min

Now

At=Aoktt=AoA1k=0.50.251/600.25×60t=15 min

Hence, the correct answer is option (3).

Question: Which among the following is a false statement?

  1. Rate of zero order reaction is independent of initial concentration of reactant
  2. Half-life of a zero order reaction is inversely proportional to the rate constant.
  3. Molecularity of a reaction may be zero.
  4. For a first order reaction, t1/2=0.693/k.

Answer: Statement (1). Rate of zero order reaction is independent of initial concentration of reactant.

This statement is correct because for zero order reaction

Rate =k

Means rate is independent of concentration.

Statement 2. Half-life of a zero order reaction is inversely proportional to the rate constant.

This statement is correct for zero order reaction because,

t1/2=[A]02k

Statement 3. Molecularity of a reaction may be zero.

This statement is incorrect because Molecularity is the number of molecules involved in the reaction. It is always a positive integer. It can never be zero.

Statement 4: For a first order reaction, t1/2=0.693/k.

This statement is true because for first-order reaction, the half-life is :

t1/2=0.693k

Statement (1). Rate of zero order reaction is independent of initial concentration of reactant.

This statement is correct because for zero order reaction

Rate =k

Means rate is independent of concentration.

Statement 2. Half-life of a zero order reaction is inversely proportional to the rate constant.

This statement is correct for zero order reaction because,

t1/2=[A]02k

Statement 3. Molecularity of a reaction may be zero.

This statement is incorrect because Molecularity is the number of molecules involved in the reaction. It is always a positive integer. It can never be zero.

Statement 4: For a first order reaction, t1/2=0.693/k.

This statement is true because for first-order reaction, the half-life is :

t1/2=0.693k

Hence, the correct answer is option (3).

Approach to Solve Questions of Class 12 Chemistry Chapter 3 Chemical Kinetics

The following are the points that can help you build a good approach to solve the questions effectively and efficiently:

1) 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.

7) Solve questions from NCERT textbook and NCERT exemplar as questions from these textbooks are asked directly in boards and competitive exams.

CBSE Class 12 Chemistry Chapter-wise Notes

The hyperlinks of NCERT class 12 notes of each chapter are give below:

Subject Wise NCERT Exemplar Solutions

The hyperlinks of NCERT Exemplar Solutions of all subjects are given below:

Subject-Wise NCERT Solutions

The hyperlinks of NCERT Solutions of all subjects are given below:

Frequently Asked Questions (FAQs)

1. What is chemical kinetics?

Chemical kinetics is the branch of chemistry that deals with the rates of chemical reactions and the factors that affect these rates. It helps us understand how fast a reaction occurs and what influences that speed, such as concentration, temperature, and the presence of catalysts.

2. How do we determine the rate of a chemical reaction?

The rate of a chemical reaction can be determined by measuring the change in concentration of reactants or products over time. For example, if we start with a known concentration of a reactant and measure how much of it reacts in a specific time interval, we can calculate the reaction rate.

3. What factors affect the rate of a chemical reaction?

Several factors influence the rate of a chemical reaction, including:
1. Concentration of reactants, Higher concentration generally increases the rate.
2. As temperature increases, reactant particles move faster, increasing the rate.
3.A greater surface area of solid reactants leads to more collisions and a higher rate.
4. Substances that speed up reactions without being consumed in the process.

4. What is the role of catalysts in chemical reactions?

Catalysts are substances that increase the rate of a chemical reaction by lowering the activation energy required for the reaction to proceed. They help in forming transition states more easily, allowing the reaction to occur faster without being consumed in the process.

5. Can you explain what activation energy is?

Activation energy is the minimum energy required for a chemical reaction to occur. It is the threshold that reactants must overcome to transform into products. The higher the activation energy, the slower the reaction tends to be, as fewer reactant molecules will have enough energy to initiate the reaction.

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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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