Aims

Class work 26 September 2007

Collision TheoryLO’s• How does size affect how fast?

Write it down!! Page 59© C Meyer

All• Know the speed of a reaction

depends on collisions of particles• Know surface area affects speedMost• List the factors that affect rate• Explain collision theorySome• Use collision theory to explain the

effect of surface area on rate

Aims

• You could have 2 different shape graphs showing increasing amount of product or the decreasing amount of reactant.

Am

ount

of

prod

uct

Time

Am

ount

of

reac

tant

Time

Steep gradient Fast reaction

Shallow gradient Slow reaction

Steep gradient Fast reaction

Shallow gradient Slow reaction

• An equation tells you what?

How does size affect how fast?CH4 + O2 CO2 + H20

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

m3

Particles have tocollide hard enough

before they canreact

O2CH4

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

New bonds form

Old bonds start to break

Activation energy

• 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 energyactivation energy)to start breaking the old bonds before a reaction can occur.

Reactants

Activation energy

needed to break existing

bonds

Energy is given out as new bonds

form

• Which reaction is faster and why– A 10 collisions per sec– B 50 collision per sec

How does size affect how fast?

m2

Title – Collision theory and Activation energy

• Write a sentence to explain collision theory and activation energy

• More collision in a set time fasterParticles have to

collide hard enough before they can

react

O2CH4

Activation energy

needed to break

existing

bonds

• 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?

How does size affect how fast?

Time, temp, concentration and volume of acid,Mass and area of marble chips

Independent – Dependent –

Control –

area marble chipschange in mass on balance

all the restm5

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 chipsResults• Draw up table and plot graph

SAFETYAcid - goggles 20 mins

How does size affect how fast?

Tidy UP!!!

• Stack items tidily• Clean up bench/floor• Sit down when finished

• 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.

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

Very fast

Slow

How does size affect how fast?

Conclusion• Use your

knowledge of collision theory to explain why the small chips react faster

What have I learnt?

m1

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

The End