1. Learning Objective
To be able to:
Describe how the rate of a reaction is changed, using collision theory
Key Words:
Energy, collisions, particles

2. Which jelly will dissolve quicker? Can you explain your answer?
Monday, 05 August 2019
Collision Theory
Date and title in books. You need a pen, pencil & ruler.
Starter Activity – 5 minutes
Which jelly will dissolve quicker?
Can you explain your answer?
One solid piece or smaller pieces

3. Explain how the rate of reaction is changed
Target 9-1
Identify ways we can speed up a reaction
(F/H)
Target 4-9
Describe how a reaction happens, using collision theory
(H)
Target 5-9
Explain how the rate of reaction is changed
Extension: Application of Knowledge
Target 8/9
Use mathematical analysis to explain why the rate of reaction has changed
Learning Outcomes

4. Particles Colliding Hydrogen Chlorine Reactant particles collide
Reactant particles move towards each other
Product particles move apart

5. percentage completion of reaction
Slower and slower!
Reactions do not proceed at a steady state. They start off at a certain speed, then get slower and slower until they stop.
As the reaction progresses, the concentration of reactants decreases. This reduces the frequency of collisions between particles and so the reaction slows down. 0%
fast
25%
slower
75%
very slow
100%
stopped
reactant A
reactant B
product
percentage completion of reaction

6. Changing the rate of reactions
Anything that increases the number of successful collisions between reactant particles will speed up a reaction.
What factors speed up reactions?
Increased temperature;
increased concentration of dissolved reactants, and increased pressure of gaseous reactants;
increased surface area of solid reactants;
use of a catalyst.

7. Temperature The higher the temperature, the faster the particles move.
This means they have more kinetic energy and so are more likely to react.
They also move faster at higher temperature and so collide more frequently.
So increasing the temperature increases the rate of reaction.

8. Temperature and particle collisions
Boardworks GCSE Additional Science: Chemistry
Changing Reaction Rates
Teacher notes
This simulation illustrates how increasing the temperature increases the number of collisions between particles.
Explaining changes in the rate of reaction in terms of the collisions between reacting particles is higher tier for OCR Gateway GCSE Additional Science.

9. Surface area
Any reaction involving a solid can only take place at the surface of the solid.
If the solid is split into several pieces, the surface area increases.
slow rate
This means that there is an increased area for the non-solid reactant particles to collide with.
The smaller the pieces, the larger the surface area. This means more collisions and a faster rate of reaction.
fast rate

10. Surface area and particle collisions
Boardworks GCSE Additional Science: Chemistry
Changing Reaction Rates
Teacher notes
This simulation illustrates how increasing the surface area increases the number of collisions between particles and solid reactants
Explaining changes in the rate of reaction in terms of the collisions between reacting particles is higher tier for OCR Gateway GCSE Additional Science.

11. Increasing concentration

12. Increasing Concentration
The more concentrated a solution, the more solute particles there are in a given volume.
This means that it is more likely that reactant particles will collide with one another.
More collisions, the faster the rate of reaction.

13. Reactions, particles and collisions
Reactions take place when particles of reactants collide with a certain amount of energy.
This energy is called activation energy, and is different for each reaction.
The rate of a reaction depends on two things:
the frequency of collisions between particles;
the energy with which particles collide.
If particles collide with less energy than the activation energy, they will not react. The particles will just bounce off each other.

14. Activation Energy
Chemical reactions can occur only when reacting particles collide with each other and with sufficient energy.
The minimum amount of energy that particles must have to react is called the activation energy (more next lesson!)