Chemical Kinetics

What is chemical kinetics?

The branch of chemistry which deals with the study of the reaction rates is known as Chemical Kinetics.

The word is derived from the Greek word ‘Kinesis‘ meaning movement. In fact, in all chemical reactions, the reacting species move and collide to form the products. Chemical kinetics also involves the investigation of reaction mechanisms, the conditions of concentration, temperature, pressure, and catalyst which control the rates of different reactions.

Table of contents

• Classification of reactions on the basis of reaction rate
• Factors affecting the reaction rate
• Related topic on chemical kinetics
• FAQs

Classification of reactions on the basis of reaction rate

Different chemical reactions occur at different rates on the basis of rates, the chemical reactions are broadly divided into three categories.

1. Very fast or instantaneous reactions
2. Very slow reactions
3. Moderate reactions

(a) Very fast or instantaneous reactions:

• These reactions are so fast that they occur as soon as the reactants are brought together.
• These reactions involve ionic species.
• These reactions take about 10^-14 to 10^-16 seconds to complete.

Examples

(i) Precipitation of AgCl when solutions of silver nitrate and sodium chloride are mixed.

AgNO₃ + NaCl → AgCl + NaNO₃

(ii) Precipitation of BaSO₄ when solutions of barium chloride and sulphuric acid are mixed.

BaCl₂ + H₂SO₄ → BaSO₄ + 2HCl

(iii) Neutralisation of an acid with a base when their aqueous solutions are mixed.

HCI + NaOH → NaCl + H₂O

Note:

• These reactions are extremely fast, it is therefore not possible to study their rates.
• The spontaneous reactions may not be fast reactions.

(b) Very slow reactions

• These reactions are extremely slow.
• They may take months together to show any measurable change at room temperature.

Examples:

• Fermentation of sugar in the presence of yeast takes more than a month.
• Formation of coal and petroleum under earth’s crust.
• The roasting of iron once started, may not be completed even after a year.

Note:

• These reactions are not feasible to study the rates of reaction.

(c) Moderate reactions

• These reactions proceed at a limited rate and their rates can be measured.

Examples

(i) Decomposition of hydrogen peroxide: 2H₂O₂ → 2H₂O + O₂

(ii) Decomposition of nitrogen pentoxide: 2N₂O₅ → 2N₂O4 + O₂

(iii) Hydrolysis of an ester : CH₃COOC₂H₅ + NaOH → CH₃COONa + C₂H₅OH

(iv) Inversion of cane sugar in aqueous solution.

C₁₂H₂₂O₁₁ + H₂O → C₆H₁₂O₆ + C₆H₁₂O₆

Factors affecting the reaction rate

The rates of chemical reactions are influenced by a number of factors. These are:

(i) Concentration of reactants:

The rate of a chemical reaction is proportional to the concentration of the reacting species taking part in the reaction. It is maximum to start with and slowly decreases since the concentration of the reacting species decreases accordingly. In case of reversible reactions, the rate of chemical reaction can be studied separately for both the forward and backward reactions. In case of gaseous reactions, rate of reaction increases with increase in pressure.

(ii) Temperature

Rate of reaction increases with increase in temperature. The energy of the reactant species increases with the increase in temperature and so will the number of collisions. It has been observed that in most of the cases, about 10 degree increase in temperature makes the reaction rate double.

(iii) Presence of catalyst

In many chemical reactions, the reaction rate is enhanced by certain foreign substances called catalysts. They don’t consume in the reaction and also do not undergo any change in chemical characteristics.

The catalyst increases the rate of the reaction by providing a new reaction mechanism to occur within a lower activation energy.

(iv) Nature of reactants

The nature of reacting species may also influence the reaction rate. For example, combustion of nitric oxide (NO) is faster as compared to carbon monoxide(CO).

2NO (g) + O₂ (g) → 2NO₂ (Fast)

2CO (g) + O₂ (g) → 2CO₂ (Slow)

(iv) Surface area

An increase in surface area provided more opportunities for the reactants to come in contact or collide with each other resulting an increased reaction rate.

Example: we use granulated zinc metal in laboratory while preparing hydrogen gas on reacting with dilute hydrochloric acid or dilute sulphuric acid. Actually, granulated zinc has a greater surface area available for the attack by the acid than a lump of zinc. Therefore, it reacts at a faster rate.

(v) Exposure to radiation

The photochemical reactions are carried out in the presence of sunlight.

In these reactions, the photons of light are the source of energy which helps in breaking the bonds in the reacting molecules so that they can react and form product,

Related topics on Chemical kinetics

• Rate of reaction
• Rate law and rate constant
• Collision theory of chemical reaction
• Order and molecularity of reaction
• Pseudo First order reaction
• Zero order reaction
• First order reaction
• Second order reaction
• Half life period
• Activation energy
• Arrhenius equation