Factors which affect the rates of chemical reactions
Dr. Davinder Bhachu
Teacher
Change to What Do We Mean by “Rate of Reaction”?
The rate of a chemical reaction describes how quickly reactant particles are transformed into product particles.
Some reactions take place almost instantly, while others unfold over days, months, or even years.
Examples:
Slow reaction: Copper metal gradually turning dull when it oxidises in moist air.
Moderate reaction: A piece of zinc reacting with dilute acid to release hydrogen gas steadily.
Very fast reaction: Flash powder igniting to produce a burst of light.
The Four Major Factors That Influence Rate
1. Temperature
2. Concentration (or pressure, for gases)
3. Surface area of solid reactants
4. Catalysts
Each factor changes the number or success rate of particle collisions.
No answer provided.
Collision Theory
Collision Theory
Key principle
For a reaction to occur, particles must:
1. Collide with each other.
2. Collide with enough energy to break old bonds and form new ones.
Why rate changes
Anything that increases:
How often particles collide, or
How energetic those collisions are, will make the reaction go faster.
Increasing the Frequency of Collisions
Temperature
Explanation
When the temperature rises:
Particles move faster.
Faster movement means they meet more often.
More frequent collisions → faster reaction.
Concentration (or Pressure for Gases)
Explanation
In a more concentrated solution, there are more reactant particles in the same space.
More particles → more chances of collisions.
For gases: increasing pressure compresses particles together, raising collision frequency.
Surface Area
Explanation
When a solid reactant is broken into smaller pieces or ground into powder:
More particle surfaces are exposed.
Reactant particles in the solution or gas phase can collide with the solid more easily.
More exposed area → more collisions → faster reaction.
Increasing the Energy of Collisions
Activation energy
Reactions only occur when collisions have at least a minimum amount of energy known as the activation energy.
Why temperature matters again
Increasing temperature:
Not only makes collisions more frequent,
It also makes a higher proportion of collisions have energy equal to or greater than the activation energy.
Thus, temperature has a double effect on rate.
Rate and Proportionality
The rate of a reaction is directly proportional to the number of successful collisions each second.
Example
During the reaction between sodium thiosulfate and hydrochloric acid, the time until the solution clouds over is recorded at various temperatures. If increasing the temperature from 20 °C to 30 °C cuts the reaction time in half, then the frequency of successful collisions has roughly doubled.