Class 12 NCERT Solutions Chemistry Chapter 4 – Chemical Kinetics

Chemical Kinetics Class 12 Chemistry NCERT Solutions Chapter 4

Chemical Kinetics Class 12 Chemistry NCERT Solutions Chapter 4 is a topic that defines chemical reactions, variables, and mechanisms. It includes both the chemical reaction and the physical process. This chapter describes in detail all the aspects that students need to understand the concept and fair well in their examination. The Chemical Kinetics Class 12 Chemistry NCERT Solutions Chapter 4 provided by Extramarks will help students with that.

Chemical Kinetics deals with chemical reactions, factors, and mechanisms. It is closely related to the chemical reaction and physical process. Chemical kinetics Class 12 is categorized into swift, prolonged, and moderate reactions based on their varying rate.

Students learn various subtopics related to chemical kinetics like dependence on the rate of concentration, integrated equations, collision theory, catalyst and Arrhenius equation under class 12 Chemistry chapter 4 solutions.

NCERT Solutions class 12 Chemistry Other Related Chapters

The key topics covered in NCERT solutions class 12 Chemistry chapter 4 include

Students can refer to these exercises by clicking on the respective topic. Given below is a brief of all the exercises under NCERT solutions class 12 Chemistry chapter 4.

4.1 Introduction

Chemical kinetics, also known as reaction kinetics, helps us understand the rates of reactions and how certain conditions influence them. It further helps to define the characteristics of a chemical reaction by gathering and analyzing information about the mechanism of the reaction.

4.2Rate of Chemical Reaction

Under this exercise of NCERT solutions class 12 Chemistry chapter 4, students will learn about

The Rate Law: The rate law (also called the rate equation) for a chemical reaction is an expression that shows a relationship between the rate of the reaction and the concentrations of the reactants participating in it. 

Expression: For a  chemical reaction given by aA + bB → cC + dD

a, b, c, and d are the stoichiometric coefficients of the reactants or products. 

The rate equation for the reaction is obtained as below.

Rate: ∝ [A][B]y ⇒ Rate = k[A][B]y

Where

• [A] & [B] represents the concentrations of the reactants A and B.
• x & y represent the partial reaction orders for reactants A & B (that may or may not be equal to their stoichiometric coefficients a & b).
•  ‘proportionality constant is the rate constant of the reaction.

It’s key to understanding that the expression of the rate law for a specific reaction can only be determined experimentally. The rate law expression cannot be achieved from the balanced chemical equation (since the partial orders of the reactants are not necessarily equal to the stoichiometric coefficients). Students can refer to Chemical Kinetics Class 12 Chemistry NCERT Solutions Chapter 4 by Extramarks for more information on the ‘rate of chemical reaction’.

Reaction Orders: The combination of the partial orders of the reactants in the rate law expression equals the total order of the reaction.

If Rate = k[A]x[B]y, the total order of the reaction (n) = x+y

The order of a reaction gives insight into the change in the rate of the reaction that can be anticipated by increasing the concentration of the reactants. 

For example,

• If this reaction is a zero-order reaction, doubling the reactant concentration will not affect the rate of reaction.
• If it is a first-order reaction, doubling the reactant concentration will double the rate of reaction.
• In the case of second-order reactions, doubling the concentration of the reactants will quadruple the overall rate of reaction.
• For third-order reactions, the tot rate increases by eight times when the reactant concentration is doubled. 

Rate Constants: Rearranging the rate of the equation, the value of the rate constant ‘k’ is 

shown as below k = Rate/[A]x[B]y

Hence, the units of k (assuming that concentration is represented in mol.L-1 or M and time are represented in seconds) can be calculated via the following equation.

k = (M.s-1)*(M-n) = M(1-n).s-1

4.3 Factors affecting the rate of reaction

Under exercise 4.3 of NCERT solutions class 12 Chemistry chapter 4, students will learn about 

The Nature of reactant: The nature of bonding in the reactants determines the rate of a reaction. The ionic compounds react faster than covalent compounds due to the energy requirement in covalent compounds to cleave the existing binds. 

The reaction between ionic compounds:

AgNO3 + NaCl –> AgCl + NaNO3

                             Precipitation of AgCl

Temperature: The rate of reaction increases with the increase in temperature due to an increase in average kinetic energy, increasing the number of molecules having more incredible energy than the threshold energy and consequently increasing the number of effective collisions. The rate of a reaction is increased by 100% or doubled with a 10oC rise in temperature.

Pressure: An increase in partial pressure increases the number of collisions. Therefore, the rate of reactions involving gaseous reactants increases with the increase in partial pressures.

Catalyst: As per Chemical Kinetics Class 12 Chemistry NCERT Solutions Chapter 4, a catalyst increases the rate of reaction by giving an alternative path with lower activation energy (Ea’) for the reaction to proceed.

The concentration of reactants: Increasing concentration increases the number of collisions and the activated collisions between the reactant molecules. According to the collision theory, the rate is directly proportional to the collision frequency. Consequently, the rate of a reaction increases with the rise in the concentration of reactant.

Surface area: According to Chemical Kinetics Class 12 Chemistry NCERT Solutions Chapter 4, the rate of reaction increases with an increase in the surface area of a solid reactant.

4.4 Integrated Rate of Equations

The integrated rate of equations expresses the concentration of the reactants in a chemical reaction as a function of time. Thus, such rate of equations can be employed to check how long it would take for a given percentage of the reactants to be consumed in a chemical reaction. It is essential to observe that different order reactions have different integrated rates of equations.

Integrated Rate of Equation for Zero-Order Reactions: With reference to Chemical Kinetics Class 12 Chemistry NCERT Solutions Chapter 4, Integrated Rate of Equation for Zero-Order Reactions is given by:

kt = [R]0 – [R] (or) k = ([R]0 – [R])/t

[R]0 is the initial reactant concentration ( t = 0)

[R] is the reactant concentration at a time ‘t.’

and the Rate constant is k

The Integrated Rate Equation for First-Order Reactions: The integrated rate law for first-order reactions is:

kt = 2.303log([R]0/[R]) (or) k = (2.303/t)log([R]0/[R])

Integrated Rate Equation for Second-Order Reactions: The integrated rate of the equation is:

kt = (1/[R]) – (1/[R]0)

More details of the Integrated Rate of reaction are described in NCERT Solutions Class 12 Chemistry Chapter 4.

What is Half-Life Reaction?As per Chemical Kinetics Class 12 Chemistry NCERT Solutions Chapter 4, the half-life of a chemical reaction can give as the time taken for the concentration of a given reactant to reach 50% of its initial concentration (i.e. the time taken for the reactant concentration to get half of its initial value). It is denoted by the symbol ‘t1/2‘ and is usually represented in seconds.

Half-Life Formula: It is important to note that the formula for the half-life of a reaction varies with the order of the reaction.

• For a zero-order reaction, the mathematical expression that can be employed to determine the half-life is: t1/2 = [R]/2k
• For a first-order reaction, the half-life is given by: t1/2 = 0.693/k
• For a second-order reaction, the formula for the half-life of the reaction is 1/k[R]0

Where

• t1/2 is the half-life of the reaction (unit: seconds)
• [R]0 is the initial reactant concentration (unit: mol.L-1 or M)
• k is the rate constant of the reaction (unit: M(1-n)s-1 where ‘n’ is the reaction order)

Students may refer to Chemical Kinetics Class 12 NCERT Solutions for more information on this exercise. 

4.5 Pseudo order reaction

The reaction that appears to be an nth order reaction but belongs to some different order is called Pseudo order reaction.

Referring to Chemical Kinetics Class 12 NCERT Solutions, a pseudo-first-order reaction is a chemical reaction between two reactants participating in a chemical reaction and, therefore, should be a second-order reaction. But it resembles a first-order reaction due to the presence of reactants in negligible quantities. 

Let R` + R“ –> P

Rate = k[A]1[B]1

Order of reaction = 2.

Let us consider another reaction,

CH3Br + OH−→ CH3OH+Br−

The rate law for this reaction is

Rate = k [OH−][CH3Br]

Rate = k [OH−][CH3Br] = k(constant)[CH3Br]=k′[CH3Br]

As only the concentration of CH3Br would change during the reaction, the rate would solely depend upon the changes in the CH3Br reaction. 

4.6 Arrhenius equation

Under this exercise of Chemical Kinetics NCERT solutions class 12 Chemistry chapter 4, students will learn about the Arrhenius equation. The formula used to calculate the energy of activation and justify the effect of temperature on the rate of reaction is called Arrhenius Equation.

The formula is,

              K = A e-Ea/RT                  

Where

k = Rate constant

A= Frequency factor

e = mathematical quantity

Ea= activation energy

R = gas constant

T = kelvin temperature

The above relation was created by Swedish chemist Svante Arrhenius and hence named after him.

ln K = ln A – Ea/(2.303RT)

Equation of a straight line with slope = –Ea /R.

When Ea = 0 , Temperature = Infinity

e-Ea/RT =Boltzmann factor.

For a chemical reaction, the rate constant gets doubled for a rise of 10° temperature due to the Arrhenius Equation. 

The NCERT solutions class 12 Chemistry chapter 4 exercise 4.6 offers more examples. 

K = Ae-Ea/RT

Taking log on both sides of the equation

Ln k = ln A – Ea/RT

Compared with the equation of a straight line

y= mx+c,  

[m= slope of the line

c= y-intercept]

So we have

y = ln k

x = 1/T

m = -Ea / R

c = ln A

           Image:   Plotting k Vs (1/T)

4.7 Collision theory

As per NCERT solutions class 12 Chemistry chapter 4 Exercise 4.7 collision theory, the molecules collide with significant kinetic energy to create a chemical reaction. The molecules of the reacting species collide through space in a rectilinear motion. The rate of a chemical reaction is proportional to the number of collisions between the molecules of the reacting species. Chemical Kinetics Solutions provided by Extramarks is very easy to understand.

The molecules must be correctly oriented. Rate of successful collisions ∝ Fraction of successful collisions X Overall collision frequency. The number of collisions per second per unit volume of the molecules in a chemical reaction is called collision frequency (Z).

Let A+B –> C + D

Rate = ZABe-Ea/RT

Here ZAB = collision frequency of A and B.

In many reactions Rate = P ZABe-Ea/RT

Where p= steric factor, which considers the proper orientation of the molecules participating in a chemical reaction. More information is provided in Chemical Kinetics NCERT Solutions class 12 Chemistry chapter 4, provided by Extramarks.

Chemical Kinetics Class 12 NCERT Solutions Article Link

Chemical Kinetics Class 12 Chemistry NCERT Solutions Chapter 4 is explained in detail by the experts of Extramarks. In addition to chapter 4, students can access NCERT Solution for all other Chemistry chapters of class 12. Furthermore, students can click on the likes provided below to access the study material of different classes. 

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