Chemistry

# Chemical Kinetics: Rate Laws Equations and Examples

Chemical kinetics is the branch of chemistry that deals with the study of:

• Rates of the chemical reactions.
• Factors affecting the rate of reactions.
• The mechanism through which reactions proceeds.
• The optimum condition for maximum yield of products.

## The rate of Reaction in chemical kinetics

The rate of reaction is defined as “ The quality of reactants consumed or quantity of products produced per unit time”.

### Formula

Where,

dx = small change in the concentration of reactants or products.

dt = small change in the time.

### Derivation of Rate Constant and Rate Expression

Consider the following hypothetical reaction:

A            +        B             →       P

According to the law of mass action,

Rate of Reaction  = K [A]  [B]

This is the rate expression and where ‘K’ is the rate constant.

#### Specific Rate Constant

When the concentration of each reactant is unity, that is, 1mol / dm3, so the rate constant is known as the ‘Specific Rate Constant’.

As we know:

So, the rate of a reaction when the molar concentration of each reactant is unity is known as the specific rate constant.

### Rate Equation OR Rate Law

An experimentally determined mathematical expression that relates the molar concentration of reactants to the reaction rate is known as a rate equation or rate law.

##### Characteristics of Rate Constant (K)
• Its value remains constant at the constant temperature.
• Its value varies with temperature.
• Its value is independent of the concentration of reactants i.e. its value remains constant even if the concentration of reactants is altered.

Types of Reaction Based on Reaction Velocity

On the basis of their rates  or velocity, there are three types of reactions which are as follows:

1. The reaction is proceeding at a slow rate.
2. Reactions proceed at a high rate.
3. Reactions proceed at a moderate rate.
##### Reaction Proceeding at a slow rate

These are the reactions, which proceed at an extremely slow speed and take a longer time for their completion. It is difficult to determine experimentally the rates of such chemical reactions.

###### For Example
• Rusting of iron
• Formation of Diamond in Earth’s crust.
##### Reaction proceeds at high rates

These are the reactions, which proceed with high speed and take a very short time for their completion. It is impossible to determine experimentally the rates of such chemical reactions.

##### Reaction Proceed At Moderate Rates

These are the reactions, which proceed at experimentally measurable rates, i.e. with a limited (moderate) speed, and are completed at the most, in a few hours.

### Order of Reaction

“It is the sum of all the exponents of the concentration of reactants involved in the rate equation”.

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