KEY2CHEM

Factors that Influence Reaction Rate

The rate of a reaction is measured as the concentration of reactants converted into products per unit time. A variety of factors, including concentration of reacting particles, physical state of the reactants, and reaction temperature, influence the reaction rate.

Molecules must collide to react

In order to react, molecules must first collide. As the number of molecules increases, the number of possible collisions (the collision frequency) increases, which increases the reaction rate. Similarly, molecules must mix in order to collide, so increasing the surface area of reacting particles increases the probability of collision, which in turn increases rate of reaction.

Molecules must collide with sufficient energy

In addition to colliding with one another, molecules must collide with sufficient energy in order to react. Increasing temperature increases kinetic energy of particles, which increases their probability of colliding with sufficient energy. Additionally, increasing temperature increases average molecular speed; when particles move faster, they are more likely to collide and react.

Example 1.

An increase in reactant concentration _________ reaction rate.

A. increases

B. decreases

C. does not change

Solution

A. increases

Increasing reactant concentration increases the collision frequency, which increases reaction rate.

Example 2.

In order for molecules to react, they must collide with sufficient _______.

A. gravity

B. energy

C. inertia

Solution

B. energy

In order for colliding molecules to react, they must collide with sufficient energy. This energy barrier to react is called the activation energy.

Example 3.

A scientist runs a chemical reaction under several trials of experimental conditions. However, the data table is not labeled clearly. Which statement correctly identifies the experimental results?

A. The reaction rate is higher at a higher temperature.

B. The reaction rate is higher at lower reactant concentration.

C. The reaction rate is higher at lower temperature.

Solution

A. The reaction rate is higher at a higher temperature.

Increasing temperature increases reaction rate, due to an increase in collision frequency and collision energy.