NEET Study Notes for Chemical Kinetics: The study of rate, mechanism, and factors that affect chemical reactions is called chemical kinetics. The mechanism of chemical reaction, its characteristics, rate of reactions can be understood by the broad study of chemical kinetics.

• The speed at which a reaction occurs is known as the rate of the reaction. The ratio of increase in product concentration per unit time, and decrease in reactant concentration per unit time is also the rate of reaction.
• Factors like temperature, catalyst, solvent, the concentration of chemicals, solvents, and chemical nature affect the rate of reaction.
• Chemical Kinetics forms a part of Physical Chemistry in NEET Chemistry Syllabus.. With an overall weightage of 32%, from Physical chemistry, at least 2-3 questions can be expected from this unit in NEET 2022.

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NEET Study Notes on Chemical Kinetics: Important Topics

Rate of Reaction

Rate of Reaction can be defined as the change in the combination of reactant or product in an interval of time. Let the substance be A such that the change in concentration is [A]

[A]= Final Concentration- Initial Concentration

For Reactant- Initial Concentration>Final Concentration, thus rate of change will be negative

For Product- Final Concentration>Initial Concentration, thus rate of change will be positive

Average rate (Product) = 

Average rate (Reactant) = 

The rate of reaction is never negative.

• The rate of Reaction is always positive. It is proportional to the combination of reactants.
• The quantity of reactant or product as a function of time is used to determine the rate of reaction.
• S.I unit of concentration of the chemical is Mol L^–1

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Instantaneous Rate

When the reaction proceeds, the reactant concentration keeps decreasing. This results in decreasing rate of reaction. Thus the rate of reaction is given by

where dx is the small change, in the interval of time dt.

Average Rate and Instantaneous Rate

• Average Rate- For Average rate, the difference between reactions, products, and time is taken into consideration from the start of the reaction to the end.
• Instantaneous Rate- For instantaneous rate, the difference between chemical concentration is during a limited time of the reaction. This concentration of chemicals is analyzed at two times during the reaction.

What is Relation between Average and Instantaneous Rate

Instantaneous Rate= 

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Factors Affecting Rate of Reaction

Reactant Nature-

The less the arrangement of bonds, the faster reactions occur whereas, with larger bond arrangements, the reactions occur slowly.

Temperature-

In a homogenous reaction, the rate of reaction increases and becomes double when the temperature increases by 10C.

Reactant Concentration-

In the absence of catalyst at a stable temperature, the rate of reaction increases increase in the concentration of reactant. The rate of Reaction in concentration terms is the rate equation/rate law. This rate can also be expressed as Rate ∝ [A]x [B]y where x and y are stoichiometric coefficients of A and B reactants

Surface Area-

The rate of reaction increases when the surface area is large.

Catalyst-

The rate of the chemical reaction is increased when there is the presence of a catalyst.

Order of Reaction-

Order of reaction can be defined as the sun of the reactants power that is expressed in terms of rate expression. For Rate of Reaction, order of the reaction. Consider an equation,

Rate law equation is 

Order of reactant A is x

Order of reactant B is y

The overall order of the reaction is x+y

The main features of Order of Reaction are:

• Order of Reaction is dependent upon the molecules that undergo change in concentration.
• Order of Reaction of value > or equal to 3 is rare.

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Pseudo First Order Reaction

In pseudo First order Reaction, there are 2 molecules on the side of the reactant, this one excess reactant has a behavior of a first-order reaction.

Half-Life of Reaction

The time taken by reactant concentrations to reduce in half in comparison to initial concentration is called the Half-Life of a reaction.

What is the Half-Life Period for a Reaction?

Half-Life Period for the nth-order reaction is

Half-Life Period of zero (t_1/2) = 

Therefore the time taken for completing half of the reaction is directly proportional to the concentration of reactants initially.

Molecularity of Reaction-

when the participating atoms, ions, and molecules simultaneously collide for a successful reaction, it is termed as molecularity of Reaction. The value of molecularity of reaction is always a whole number, it cannot be more than 3 or less than 1.

We can also say that the molecularity of reaction is the molecule numbers of reactants that participate in a specific step of the reaction, this concentration may or may not be varied. Some examples are:

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Rate Constant

Consider the reaction of products, A and B, at a specific temperature, rate of reaction is

where k= rate constant

Factors affecting Rate Constant

• Temperature- The relationship between rate constant and temperature is provided by

Arrhenius equation: k = 

where A is frequency factor

Ea is activation energy

If we compare the rate constant at different temperatures

Log k2 / k1 = Ea / 2.303R [ T2 - T1 / T1T2]

• Presence of Catalyst- Rate Constant is affected by the positive catalyst that lower activation energy.

What is the difference between Rate Constant and Rate of Reaction?

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Rate Law Equation

The relation between the rate of reaction and concentration of reactants is called the rate law equation. For example for a reaction

aA + bB  cC + dD

Rate [A]^x [B]^y — This is rate law

Integrated Rate Equation

A differential equation or rate equation, is combined to establish a relationship between rate constant, the concentration of chemicals, and other such measurable experimental values. At different points in time.

Zero Order Reaction

In a Zero order reaction, the rate of reaction is independent of reactant concentration. The concentration of reactants does not change with time and the Rate of reaction is always equal to the rate constant.

On doubling the concentration of reactants, the time for the reaction will also double. Thus for zero, order reaction

First Order Reaction

The rate of reaction is directly proportional to the first power of concentration of reaction

Log [R]1 / [R]2 = k (T2 - T1) / 2.300

What is Collision Theory?

Due to the collision between different molecules that react, a chemical reaction occurs. For this reaction, the two essential things are

• The Arrangement of Reacting molecules in order.
• Reacting molecules should contain minimum energy.

This minimum energy that reacting molecules possess for collisions to take place in an effective manner is called threshold energy.

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Activation Energy

The energy that molecules possess in excess of average potential energy, for the occurrence of collision to form a product is known as activation energy. Activation Energy can be determined by

Activation Energy= Threshold energy- Average potential energy

When the activation energy is low, the rate of reaction is fast

When the activation energy is high, the rate of reaction is low

Collision Frequency

The total collisions that occur between molecules that react per second per unit volume is called collision frequency. Collision Frequency is represented by z

Rate of Reaction= f×z

where z is the collision frequency, f is the fraction of collisions

Rate = = 

NEET Sample MCQs on Chemical Kinetics

Question: What is expressed by rate of reaction?

1. Products formed in reaction
2. How slow or fast does reaction occur
3. Reactants that participate in reaction
4. None of the above

Answer: How slow or fast reaction occurs

Question: What is the S.I. unit of the rate constant of zero-order reaction?

1. mol L-1 s-1
2. L mol-1 s-1
3. L2 mol-2 s-1
4. s-1

Answer: mol L-1 s-1

Question: When are the instantaneous rate and average rate of reaction is equal to ?

1. In the middle
2. In the start
3. In the end
4. When the rate of reaction in two-time intervals is zero

Answer: When the rate of reaction in two-time intervals is zero

Question: What is the order of reaction when the rate of reaction is equal to rate constant/

1. Third-order
2. Second-order
3. Zero-order
4. First-order

Answer: Zero-order reaction

Question: When a catalyst varies, which is a chemical reaction?

1. Activation energy
2. Internal Energy
3. Entropy
4. Enthalpy

Answer: Activation Energy

Question: Which catalyst is used in the Haber process for ammonia manufacture?

1. Alumni
2. Platinized asbestos
3. Iron with the promoter of molybdenum
4. Copper Oxide

Answer: Iron with a promoter of molybdenum

Question: What is incorrect about the order of reaction?

1. Assessing order can only be done experimentally
2. Stoichiometric coefficient does not affect the order of reactants
3. The sum of power to which concentration terms are raised in the rate law equation is the order of reaction
4. Order of reaction is always whole number

Answer: Order of reaction is always the whole number.

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