Changes in the Rate of Reaction

The rate of reaction describes how quickly reactants are converted into products. Several conditions can speed up or slow down this process. Each factor works by influencing how often and how effectively particles collide.

1. Temperature

Effect Increasing temperature raises the reaction rate; lowering temperature slows it down.

Reason At higher temperatures, particles move faster, collide more often, and more collisions have enough energy to overcome the activation energy barrier.

2. Concentration (in solutions) and Pressure (in gases)

Effect Higher concentration of reactants—or higher pressure for gases—results in a faster reaction. Lower concentration or pressure slows it down.

Reason More particles in the same space leads to more frequent collisions.

3. Surface Area (for solids)

Effect A solid broken into smaller pieces or powdered form reacts faster than a single large chunk.

Reason More surface is exposed to reactants, increasing the number of collision sites.

4. Catalysts

Effect Catalysts speed up reactions without being used up.

Reason They provide an alternative pathway that has a lower activation energy, so a greater proportion of collisions lead to reaction.

(Enzymes are biological catalysts.)

5. Nature of Reactants

Some substances naturally react more quickly than others due to the type of bonds and mechanisms involved.

Examples Ionic reactions in solution are fast; reactions requiring the breaking of strong covalent bonds are slower.

6. Light (for photochemical reactions)

For reactions that depend on light, increasing light intensity increases the rate.

Reason Light supplies energy needed to initiate or accelerate the reaction.

7. Inhibitors

These are substances that slow down a reaction.

ReasonThey interfere with reactant particles or catalyst activity, reducing the number of successful collisions.

Collision Theory Overview

Reaction rate depends on:

• How frequently particles collide

• Whether they collide with enough energy

• Whether they collide in the correct orientation

A factor that increases the number of effective (successful) collisions increases the rate.

Measuring Reaction Rate

Reaction rate can be monitored through:

• Gas volume produced

• Change in mass

• Loss of reactant

• Appearance of product

• Change in color or conductivity