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

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

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

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

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

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

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

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

25 Mins

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

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

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

50 Mins

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.

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

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