Chemical Kinetics

Notes

Chapter 3: Chemical Kinetics

Chemical Kinetics is the branch of physical chemistry which deals with the rate or speed of a chemical reaction, its mechanism and the factors affecting the rate of reaction.

Classification of Chemical Reactions on the Basis of Rate

1. Very Fast Reactions

These reactions are completed in a very short time, approximately 10^-16 to 10^-14 second.

Generally, ionic reactions such as neutralisation and precipitation reactions are very fast.

Examples:

NaOH + HCl → NaCl + H₂O

AgNO₃ + NaCl → NaNO₃ + AgCl↓

It is practically difficult to calculate the rate of such very fast reactions.

2. Very Slow Reactions

These reactions require a very long time for completion, such as days, months or years.

Examples:

• Rusting of iron
• Conversion of graphite into diamond

It is not practically useful to calculate the rate of such extremely slow reactions.

3. Moderate Reactions

These reactions are completed in a measurable period of time, such as minutes or hours.

Most reactions studied in chemical kinetics are moderate reactions.

Examples:

H₂O₂ → H₂O + O₂

CH₃COOC₂H₅ + H₂O → CH₃COOH + C₂H₅OH

Rate of Reaction

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

It is generally represented by r.

The rate of reaction is always expressed as a positive quantity.

Rate of Reaction = Change in concentration or pressure of reactant/product ÷ Time taken for the change

For concentration:

r = ΔC/Δt

For pressure:

r = ΔP/Δt

Unit of rate of reaction:

mol L^-1 time^-1 or M time^-1

Types of Rate of Reaction

1. Average Rate of Reaction

The rate of reaction measured over a relatively large time interval is called the average rate of reaction.

Consider the reaction:

A → B

The average rate of disappearance of A is:

Average rate = -Δ[A]/Δt

or

Average rate = -([A]₂ - [A]₁)/(t₂ - t₁)

The average rate of appearance of B is:

Average rate = Δ[B]/Δt

or

Average rate = ([B]₂ - [B]₁)/(t₂ - t₁)

2. Instantaneous Rate of Reaction

The rate of reaction measured over a very small time interval is called the instantaneous rate of reaction.