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1 Solutions 2 Electrochemistry 3 Chemical Kinetics 4 The d-and f-Block Elements 5 Coordination Compounds 6 Haloalkanes and Haloarenes 7 Alcohols, Phenols and Ethers 8 Aldehydes, Ketones and Carboxylic Acids 9 Amines 10 Biomolecules

3 Chemical Kinetics

3.1 Rate of a Chemical Reaction 3.2 Factors Influencing Rate of a Reaction 3.3 Integrated Rate Equations 3.4 Temperature Dependence of the Rate of a Reaction 3.5 Collision Theory of Chemical Reactions

Rate of a Chemical Reaction

NCERT Reference: Chapter 3 – Page 80

Quick Notes

  • Rate of a Reaction: The change in concentration of a reactant or product per unit time.
  • Average Rate = Δ[Concentration]/Δt
  • Instantaneous Rate = rate at a specific time, obtained using a tangent to the concentration-time curve.
  • Unit: mol L−1 s−1
  • For a given reaction: NCERT 12 Chemistry rate expression for aA + bB → cC + dD showing normalized rates with stoichiometric coefficients.

Full Notes

Chemical kinetics is the branch of chemistry that deals with the speed or rate at which a chemical reaction occurs and the factors affecting this speed.

The rate of a chemical reaction is a measure of how quickly reactants are consumed or products are formed during a chemical reaction.

Definition of Rate of Reaction

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

Let’s consider a general reaction: R → P
Where [R] and [P] represent the concentrations of reactant and product respectively.

Average Rate:

NCERT 12 Chemistry kinetics diagram for average rate showing concentration change over a time interval Δt with reactant R converting to product P.

The negative sign indicates that the concentration of the reactant is decreasing with time.

Instantaneous Rate:

An instantaneous rate refers to the rate of reaction at a specific point in time.

NCERT 12 Chemistry instantaneous rate diagram showing tangent drawn to concentration–time curve to obtain rate at a specific time.

This is mathematically obtained as the slope of the tangent to the concentration vs. time graph at that point.

NCERT 12 Chemistry graph showing slope of tangent on concentration versus time curve used to determine instantaneous rate.

Units of Reaction Rate

For a reaction involving concentration measured in mol L−1 and time in seconds, the unit of rate is:
mol L−1 s−1
However, other units such as mol m−3 s−1 may also be used depending on the context.

Rates in Reactions Involving Multiple Reactants and Products

For a general reaction: aA + bB → cC + dD

NCERT 12 Chemistry rate expression for aA + bB → cC + dD showing normalized rates with stoichiometric coefficients.

This expression accounts for the stoichiometry of the reaction and ensures a consistent rate value regardless of which species is being observed.

For Example:

Consider: 2NO2(g) → 2NO(g) + O2(g)
Then, Rate = − (1/2)(d[NO2]/dt) = (1/2)(d[NO]/dt) = (d[O2]/dt)
This ensures that the rate calculated from the disappearance of NO2 is equal to the rate calculated from the formation of NO or O2, properly adjusted for stoichiometry.

Summary