Factors Affecting Reaction Rates and Rate Laws

Factors Affecting Reaction Rates

Concentration

Increasing the concentration increases the probability of a collision between reactant particles. With more particles in the same volume, collisions are more frequent, speeding up the reaction rate. Conversely, a lower concentration leads to fewer collisions and a slower reaction rate.

Surface Area

If a solid reactant or catalyst is broken down into smaller pieces, the rate of reaction increases. This is because smaller pieces of the same mass have a greater surface area, providing more opportunities for reactant particles to collide with the surface and react.

Types of Rate Laws

Differential Rate Law: Describes how the rate of a reaction depends on the concentration of reactants (often simply called the rate law).

Integrated Rate Law: Describes how the concentration of reactants changes over time.

For each type of differential rate law, there is a corresponding integrated rate law and vice versa. Rate laws help us understand reaction mechanisms.

Order of Reactions

Zero Order Reaction

In a zero-order reaction, the rate is independent of the reactant concentration. The rate law can be written with the exponent of the reactant concentration as 0:

Rate = k[reactant]⁰ = k(1) = k

First Order Reaction

A first-order reaction's rate is linearly dependent on the concentration of only one reactant. This means that if the concentration of the reactant doubles, the rate of the reaction also doubles.

Second Order Reaction

A second-order reaction depends on the concentrations of either one second-order reactant or two first-order reactants. The rate is proportional to the square of the concentration of one reactant or the product of the concentrations of two reactants.

Elementary Reactions

An elementary reaction is a chemical reaction that occurs in a single step. The rate law of an elementary reaction can be determined directly from the stoichiometry of the reaction.

• Elementary reactions involving the simultaneous collision of three or more particles are rare.
• The process by which elementary reactions occur is called the reaction mechanism.
• Substances that are neither reactants nor products but are involved in the reaction mechanism are called intermediates.