Rate law

Rate Law or Rate Equation is a mathematical expression that describes the relationship between the reaction rate of a chemical reaction and the concentration of its reactants. It plays a crucial role in the field of chemical kinetics, which studies the rates of chemical processes and the factors affecting them. The rate law enables chemists to predict how changes in conditions affect the speed of a reaction.

Formulation of Rate Laws

The general form of a rate law is expressed as:

\[ \text{Rate} = k[\text{A}]^m[\text{B}]^n \]

where:

• \(\text{Rate}\) is the reaction rate,
• \(k\) is the rate constant,
• \([\text{A}]\) and \([\text{B}]\) are the concentrations of the reactants,
• \(m\) and \(n\) are the reaction orders with respect to each reactant.

The reaction order (\(m + n\)) indicates how the rate is affected by the concentration of reactants. It is determined experimentally and can be a whole number, a fraction, or zero.

Types of Rate Laws

There are two main types of rate laws: differential rate laws and integrated rate laws.

• Differential Rate Laws describe how the rate of reaction depends on the concentration of reactants.
• Integrated Rate Laws provide a relationship between the concentrations of reactants and time.

Determining Rate Laws

Rate laws are not deducible from the balanced chemical equation and must be determined experimentally. This is typically done using the method of initial rates, which involves measuring the reaction rate at the very beginning of the reaction, when the concentrations of reactants are known.

Applications of Rate Laws

Understanding and applying rate laws is essential for controlling industrial chemical processes, in environmental modeling to predict the degradation of pollutants, and in pharmacokinetics to understand how drugs are metabolized in the body.

Limitations

Rate laws are specific to a particular reaction mechanism and conditions such as temperature and pressure. They do not apply if the reaction mechanism changes.

See Also

• Chemical Kinetics
• Reaction Mechanism
• Catalysis
• Arrhenius Equation