1. Lesson 4.2 Collision Theory
AQA Additional Science
Chemistry 2
Lesson 4.2 Collision Theory

2. Aims Class work 26 September 2007 Collision Theory LO’s
How does size affect how fast?
Write it down!!
Page 59
© C Meyer

3. Aims
All
Know the speed of a reaction depends on collisions of particles
Know surface area affects speed
Most
List the factors that affect rate
Explain collision theory
Some
Use collision theory to explain the effect of surface area on rate

4. You could have 2 different shape graphs showing increasing amount of product or the decreasing amount of reactant.
Amount of product
Time
Amount of reactant
Time
Steep gradient Fast reaction
Shallow gradient Slow reaction
Steep gradient Fast reaction
Shallow gradient Slow reaction

5. How does size affect how fast?
CH4 + O2  CO2 + H20
An equation tells you what?
What products are made, not how fast
If I turn on the gas in air, what 2 things are needed to get it to react
Particles have to
collide hard enough
before they can
react O2
CH4 m3

6. Chemical reactions occur when particles of reactant collide with enough energy to react.

7. Activation energy
Energy is given out as new bonds form
Chemical reactions involve the formation of bonds between atoms but often before new bonds can be formed old ones have to be broken.
This means that there has to be enough energy (activation energy)to start breaking the old bonds before a reaction can occur.
Activation energy needed to break existing bonds
New bonds form
Reactants
Old bonds start to break

8. How does size affect how fast?
Which reaction is faster and why
A 10 collisions per sec
B 50 collision per sec
Title – Collision theory and Activation energy
Write a sentence to explain collision theory and activation energy
More collision in a set time  faster
Particles have to
collide hard enough
before they can
react O2
CH4
Activation energy needed to break existing bonds m2

9. 25 Mins

10. How does size affect how fast?
Let’s do an experiment to look at surface area
Kit – Marble chips, acid, conical flask, balance
What are the variables in the experiment?
What are you going to do to make it a fair test?
Time, temp, concentration and volume of acid,
Mass and area of marble chips
Independent –
Dependent –
Control –
area marble chips
change in mass on balance
all the rest m5

11. How does size affect how fast?
SAFETY
Acid - goggles
20 mins
Title – Effect of area
Take 1g of small chips, place in conical flask with 25ml acid. Add cotton wool plug.
Place on balance and record change in mass
Repeat with 1g of large chips
Results
Draw up table and plot graph

12. Tidy UP!!! Stack items tidily Clean up bench/floor
Sit down when finished

13. 50 Mins

14. How does size affect how fast?
The reactions of solids can clearly only take place at the surface of the solid.
If we break a solid into smaller pieces we get more area and a faster reaction.
Molecules collide with the surface of the solid
Extra surface for molecules to collide with.

15. How does size affect how fast?
If we grind up a solid to a powder we massively increase the surface area.
We therefore massively increase the rate of any reaction
Slow
Very fast

16. What have I learnt? Conclusion
Use your knowledge of collision theory to explain why the small chips react faster m1

17. Speeding up reactions
Anything that increases the chance of effective collision increases the rate (speed) of reaction. What 4 things will speed up a reaction:
Increased surface Area
Increased concentration
Increased temperature
Use of a catalyst

18. The End