Revision: Chemical Kinetics Chemistry Science (English Medium) Class 12 CBSE

Zero order reaction is the reaction whose rate is independent of the reactant concentration and remains constant throughout the course of the reaction.

The reactions that have higher order true rate law but are found to behave as first order are called pseudo first order reactions.

\[\ce{CH3COOCH3 + H2O - CH3COOH + CH3OH}\]

The time in which concentration of reactant becomes half of its initial concentration is called half Life. It is denoted by `t_(1/2)`.

A chemical reaction in which the rate of reaction depends solely linearly on the concentration of one ingredient is referred to as a first-order reaction.

A first-order reaction is a reaction whose rate depends upon the first power of the concentration of reactants, i.e., the rate is directly proportional to the concentration of reactants.

Half life of a reaction is defined as the time required for the reactant concentration to reach one half of its initial value.

Activation energy is the lowest energy necessary to commence a chemical reaction by disrupting the bonds of reactant molecules and creating the activated complex or transition state. It signifies the energy threshold that must be surmounted for a reaction to transpire. Activation energy is typically represented as E_a.

Activation energy may be defined as the excess energy that the reactant molecules (having energy less than the threshold energy) must acquire in order to cross the energy barrier and to change into the products.

The rate of reaction at a particular instant of time is called instantaneous rate of reaction.

A reaction whose rate is independent of the concentration of reactants is called zero order reaction.

The constant A in Arrhenius equation which represents the frequency of effective collisions is called frequency factor.

The branch of chemistry which deals with the study of rate of chemical reactions and the factors affecting them is called chemical kinetics.

The change in concentration of a reactant or product per unit time is called rate of reaction.

The minimum amount of energy required by reacting molecules to form products is called activation energy.

The intermediate unstable species formed during a reaction at the top of the energy barrier is called activated complex.

A substance which increases the rate of a reaction without itself undergoing permanent chemical change is called catalyst.

The rate of reaction measured over a given finite interval of time is called average rate of reaction.

A reaction whose rate is directly proportional to the first power of the concentration of a reactant is called first order reaction.

The time required for the concentration of a reactant to become half of its initial concentration is called half-life of a reaction.

A reaction which is actually of higher order but behaves as a first order reaction because one reactant is present in large excess is called pseudo first order reaction.

The mathematical expression that relates the rate of reaction with the concentration of reactants is called rate law.

The number of molecular collisions occurring per second per unit volume is called collision frequency.

A collision in which molecules collide with sufficient energy and proper orientation to form products is called effective collision.

The minimum kinetic energy required by colliding molecules for an effective collision is called threshold energy.

The proportionality constant present in the rate equation at a given temperature is called rate constant.

The sum of the powers of the concentration terms in the rate law expression is called order of reaction.

The number of reacting species that collide simultaneously in an elementary reaction is called molecularity.

\[\mathrm{Rate}=\frac{\text{Concentration}}{\mathrm{Time}}\]

Unit = mol L⁻¹ s⁻¹

Rate = k

Integrated form:

[R] = [R]₀​ − kt

\[k=\frac{[R]_0-[R]}{t}\]

Units of k = mol L^⁻¹ s^⁻¹

\[r=-\frac{\Delta[R]}{\Delta t}=\frac{\Delta[P]}{\Delta t}\]

\[r=-\frac{d[R]}{dt}\]

Rate = k[R]

Integrated form:

ln[R] = ln[R]₀ ​− kt

\[k=\frac{2.303}{t}\log\frac{[R]_0}{[R]}\]

Units of k = s⁻¹

For reaction:

aA + bB → Products

Rate = k[A]^x[B]^y

Order = x + y

Statement:
For a first order reaction, the half-life period is constant and independent of the initial concentration of reactant.

Expression:

\[t_{1/2}=\frac{0.693}{k}\]

Important Points:

• Half-life does not depend on initial concentration.
• Used in radioactive decay and decomposition reactions.
• Helps in determination of rate constant.

Statement:
At a constant temperature, the rate of a chemical reaction is directly proportional to the product of the molar concentrations of the reactants, each raised to a power, which may or may not be equal to their stoichiometric coefficients.

Mathematical Expression:
For a reaction

aA + bB → Products

Rate = k[A]^x[B]^y

Where:

• k = rate constant
• x and y = orders with respect to A and B
• x + y = overall order of reaction

Important Points:

• Order is determined experimentally.
• Order may be zero, whole number, or fractional.
• Order is not necessarily equal to stoichiometric coefficients (except for elementary reactions).

The rate of a chemical reaction at a given temperature is proportional to the product of concentrations of reactants, each raised to a power.

Mathematically,

Rate = k[A]^x[B]^y

Where:

• k = rate constant
• x, y = orders with respect to reactants
• x + y = overall order

Note: Order is determined experimentally.

Statement:
The rate constant of a reaction increases exponentially with increase in temperature and is given by the Arrhenius equation.

Mathematical Expression:

\[k=Ae^{-E_a/RT}\]

Taking logarithm:

\[\ln k=-\frac{E_a}{RT}+\ln A\]

Where:

• A = frequency factor
• Ea = activation energy
• R = gas constant
• T = temperature

Important Results:

• A plot of ln k vs 1/T is a straight line.
• Slope = −Ea/R
• Intercept = ln A
• Increase in temperature increases rate constant.

Statement:
According to collision theory, a chemical reaction occurs only when reacting molecules collide with sufficient energy and proper orientation.

Expression for Rate:

\[\mathrm{Rate}=Z_{AB}e^{-E_a/RT}\]

Including orientation factor:

\[\mathrm{Rate}=PZ_{AB}e^{-E_a/RT}\]

Where:

• ZAB = collision frequency
• Ea = activation energy
• P = steric factor

Important Points:

• Not all collisions are effective.
• Effective collisions require:

1. Energy ≥ activation energy
2. Proper orientation