JEE Main Study Notes for Chemical Kinetics: Chemical Kinetics is the branch of science that explains the rate of the reaction, factors affecting the rate of reaction mechanism, etc. The chemical reactions are completed at different times, based on the nature of reactants and products and conditions under which the reaction is run. Chemical kinetics is also referred to as reaction kinetics, which is mainly the study of rates of chemical processes.

The questions related to Chemical Kinetics are straightforward and in case candidates are well aware of the concepts, they can easily answer all the questions correctly. Around 1-3 questions are asked on the topic of Chemical Kinetics in JEE Main. 

JEE Main Study Notes for Chemical Kinetics include some important topics such as the rate of a chemical reaction, molecularity of a reaction, zero-order reactions, first-order reactions, half-life reactions, pseudo-first-order reactions, collision theory of chemical reactions. Candidates will find detailed information on JEE Main Study Notes for Chemical KInetics in this article. Check JEE Main Chemistry Syllabus

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What is Chemical Kinetics?

• Chemical Kinetics is the branch of science that explains the rate of the reaction, factors affecting the rate of reaction mechanism, etc.

• The chemical reactions are completed at different times, based upon the nature of reactants and products and conditions under which reaction is run.

• Some reactions like explosive reactions happen with a fraction of a second while the rusting of a wrecked ship may take centuries to complete.

• The chemical reaction includes breaking of bonds in reactant molecules and making of bonds in product molecules.

• A reaction that includes the breaking of a weak bond(s) is higher than the one included in the breaking of a strong bond at a given temperature.

• Chemical kinetics is also referred to as reaction kinetics, which is mainly the study of rates of chemical processes.

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Parameters of a Reaction and Factors Affecting Rate

What is the rate of a Chemical Reaction?

The rate of a chemical reaction means the speed at which a chemical reaction takes place. With the change in the reaction, a change is also witnessed in the concentration of the reactants and products. The rate at which the concentration of the reactant or product keeps changing according to the time is being referred to as the rate of a chemical reaction.

For a chemical reaction, A (reactant) ----- > B (product)

Rate of a reaction (R.O.R) is mentioned as:

R.O.R is equal to

• Increase in the concentration of product /time taken

• Decrease in the concentration of reactant/time taken.

• Rate of disappearance of A=-d[A]/dt

Rate of the appearance of B is equal to d[B]/dt

The rate of disappearance is negative because the concentration of the reactant keeps on decreasing with proceeds of the reaction while the rate of appearance is positive in case the concentration of the product increases.

In a chemical kinetic, mostly two kinds of rate of reactions are witnessed including

• The average rate of a reaction: It means the rate of change in concentration of the reactant or product per unit time.

• Instantaneous rate of a reaction: It is explained as the rate of change according to reactant or product at a particular instant of time.

Example

Ques: Which of the following statements for the order of the reaction is wrong?

A Order can be explained experimentally

B Order of a reaction is equal to the total power of concentration terms in the differential rate law

C Order reaction will not be affected with the stoichiometric coefficients of the reactants

D Order cannot be fractional

What are the reasons affecting the rate of a Chemical reaction?

The rate of a chemical reaction is mostly affected by some factors mentioned below:

Nature of the reactants: The rate of a reaction is different for different reactions. The reason for this is the fact that energy required or released due to breaking or formation of different bonds is not the same. The rate of a reaction is dependent on the number of collisions witnessed by the reactant molecules.

Temperature: The rate of a reaction keeps increasing with an increase in temperature as the kinetic energy of the molecules keeps increases with an increase in temperature. The increase in kinetic energy results in an enhancement of movement of the molecules resulting in an increased collision and increased rate of reaction as well.

Presence of a catalyst: A catalyst changes with a change in the speed of a reaction.

The surface area of the reactants: With an increase in surface area of the reactants, the rate of a reaction also increases. The reason for this is that with the change in surface area, the chances of collision also increase greatly.

The concentration of reactants: With an increase in the concentration of the reactants, the rate of a reaction also increases. The ‘Law of Mass Action’ assists in studying how concentration affected the reaction rate which states that ‘the rate of a reaction is directly proportional to the product of the concentration of the reactant rose to the power of stoichiometric coefficient.’ But this law has have limitations as it doesn’t hold good for all the reactions and so another law was established known as the rate law.

As per the rate law, for a reaction, A + 2B ----- > C

Rate is equal to k [A]X [B]Y where k is the rate constant and x, y is the order of a reaction while x, y might change.

Example: What is the order and molecularity of a reaction?

Order of a reaction is explained as the sum of the power of the concentration or pressure terms present in the rate law expression.

For any reaction,

aA + bB ----- > cC + dD , the rate law is expressed as:

Rate = k [A]x[B]y where order of a reaction(n) = x + y , k = rate constant for the reaction, [A] and [B] are the concentration of the reactants.

Example

Ques: The rate of reaction between two reactants A and B lowers by a factor of 4 in case the concentration of reactant B increases twice. The order of reaction with respect to reactant B will be

A 2

B 2

C 1

D 1

Molecularity of a Reaction

The number of molecular species that are involved in an elementary chemical reaction explains the molecularity of a reaction. The elementary reaction is explained in a single step only. In case one molecular species is included in the reaction, then the reaction is unimolecular, if two then bimolecular and so on.

The main area of difference between order and molecularity of a reaction is mentioned below:

Example

Ques: A reaction including two different reactants can never be

A Unimolecular reaction

B First order reaction

C Second order reaction

D Bimolecular reaction

A. Zero-Order Reactions

Zero-order reaction means the reaction at which the rate of the reaction is equal to zero power of the concentration of the reactant.

For the reaction, A --- > Products,

Rate (-d[A]/dt)= k [A]0 = k

or d[A] =- kdt

On combining the above equation and changing the concentration from [A]0 at time t= 0 sec to [A]t at time t=t sec, the value of rate constant ‘k’ becomes:

Here [A]0 and [A]t are starting and the last concentrations.

B. First-Order Reactions

First-order reaction means the reaction at which the rate of the reaction is equal to the first power of the concentration of the reactant.

For the reaction, A --- > Products,

Rate (-d[A]/dt)= k[A]1 = k[A]

or d[A]/[A] =- kdt

On integrating the above equation and changing the concentration from [A]0 at time t=0 sec to [A]t at time t=t sec, the value of rate constant ‘k’ becomes:

Where [A]0 and [A]t are the starting and last concentrations.

C. Half - life of a Reaction

Half-life of a reaction ( t1/2) refers to the time in which the concentration of the reactant is reduced to half of its initial value.

For a zero order reaction,

at time ‘t’=t1/2 , [A]0 =[A]0/2.

So, rate constant ‘k’ becomes, k = [A]0/2t1/2

or t1/2= [A]0/2k

D. Pseudo First Order Reactions

The reaction in which one of the reactants is present in large excess concerning to the other reactant and conforms to the first-order reaction even after the presence of one more reactant in the reaction is termed as the pseudo first-order reaction. Hydrolysis of sucrose and ester are a few examples of such kind of reactions.

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Example

Ques: In an endothermic reaction, X → Y with activation energies Eb and Ef respectively for the backward and forward reactions, respectively. In general, Eb means

A Eb < Ef

B Eb > Ef

C Eb = Ef

D there is zero fixed relation between Eb and Ef

How temperature results in changing the Rate of Reactions?

• The rate of reaction increases due to an increase in temperature
• The rate of a reaction generally increases with the increase in temperature. In some reactions, the temperature increases by 10 K, the rate of reaction doubles or triples.

How temperature will effect the rate constant

The effect of temperature on the rate constant of a reaction was explained by Arrhenius. Arrhenius introduced an equation showing the relation as:

------ >(1) where A is the frequency factor, Ea is the activation energy, R is the gas constant, T is the temperature and k is the rate constant.

Taking log on both sides of the equation (1), we get,

What is the effect of temperature on the rate constant?

The effect of temperature on the rate constant of a reaction was given by Arrhenius. He proposed an equation to show the relation as:

------ >(1) here A is the frequency factor, Ea is the activation energy, R is the gas constant, T is the temperature and k is the rate constant. Taking log on both sides of the equation (1), we get,

Collision Theory of Chemical Reactions

According to this theory, collision happens between the reactant molecules that result in bringing a chemical change undergoing breaking of bonds and forming new bonds. The collisions that give rise to chemical reaction is being referred to as effective collision.

• The collision frequency is referred to as the total collisions that occur after every second per unit volume of the reaction mixture. In case, the following conditions are fulfilled, it will result in an effective collision.
• The colliding molecule must possess energy equal to or more than the threshold energy (ET).
• The reacting molecules must be in the proper orientation.
• The rate of a reaction is equal to f x z where z is equal to collision frequency, f is equal to the fraction of effective collision.

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