Chemical Kinetics

Along with feasibility and extent, it is equally important to know the rate and the factors controlling the rate of a chemical reaction for its complete understanding. For example, which parameters determine as to how rapidly food gets spoiled? How to design a rapidly setting material for dental filling? Or what controls the rate at which fuel burns in an auto engine? All these questions can be answered by the branch of chemistry, which deals with the study of reaction rates and their mechanisms, called chemical kinetics. The word kinetics is derived from the Greek word ‘kinosis’ meaning movement.

1.0Classification of Chemical Reaction on The Basis of Rate

On the basis of rate, chemical reaction are broadly divided into three categories:

Very fast or instantaneous reactions:

Generally, these reactions involve ionic species are known as ionic reactions. These reactions take about 10^-14 or 10^-16 seconds for completion. So, it is almost impossible to determine the rate of these reactions.

Examples:

AgNO₃ + NaCl ⎯→ AgCl ↓  +  NaNO₃

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BaCl₂ + H₂SO₄ ⎯→ BaSO₄ ↓ + 2HCl

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HCl + NaOH     ⎯→ NaCl + H₂O

Very slow reactions: 

These reactions proceed very slowly, may take days or months to show any measurable change at room temperature.

Examples:

• Rusting of iron.
• Reaction between H₂ and O₂ to form H₂O at ordinary temperature in absence of catalyst.
• CO + 2H₂ CH₃OH

Moderate or slow reactions: 

This type of reactions proceed with a measurable rates at normal temperature and we can measure the rate of these reactions easily. Mostly these reactions are molecular in nature.

Examples:

• Decomposition of H₂O₂

2H₂O₂ → 2H₂O + O₂

• Decomposition of N₂O₅

2N₂O₅ → 4NO₂ + O₂

• Hydrolysis of ester

CH₃COOC₂H₅ + NaOH → CH₃COONa + C₂H₅OH

• Inversion of cane sugar in aqueous solution
• Reaction of NO with chlorine

NO + Cl₂ → NOCl₂