Module 3

Lesson 8 – Collision theory

ObjectivesMust• Recall that reactions happen at different speeds and we call this the “rate

of reaction”• State that surface area, concentration, temperature and catalysis all

affect the rate of reaction. Should• Describe how an increase in the pressure of a gas, increasing its

concentration, may increase the rate of a reaction involving gases. Could• Explain qualitatively, in terms of collision theory how rates of reaction

depend on the concentration of solutions and pressures of gases.

Starter – how fast does a reaction go?

• Some reactions are very slow• Some proceed much more quickly

• For example…

Speed of reaction

Demonstration

• Reactions can be so fast that you don’t get burned…

• http://www.youtube.com/watch?v=1asvpQ9bcrY&feature=related

Rate of reaction

How fast a reaction proceeds is just another way of describing the reaction “rate”

We are going to listen to a song about rates of reaction.

As you listen write down the answers to the following questions…

• Why would we want to control the rate of reaction?

• What are the five factors that can affect rate of reaction?

• How could we measure rate of reaction in an experiment?

Factors which affect rate

• Temperature• Concentration• Pressure• Surface area• Catalysts

How can we measure the rate?

• Volume of gas produced• Change in mass• Colour change• Precipitate

The method depends on the reaction

Investigation

• The reaction between sodium thiosulphate and hydrochloric acid produces a precipitate of sulphur. At high concentrations this is instant, at lower concentrations it takes a measurable amount of time.

• Year 10 helped me to investigate this using a range of concentrations of solutions

• Play movie.

Collision theory

• In order to react, particles must collide with each other

• When they collide they must be in the correct orientation AND must have sufficient energy.

• If we can make more collisions happen then the rate of reaction will increase

• How can we do this?

Speeding up reactions

• Anything that increases the chance of effective collision increases the rate (speed) of reaction. Factors include:

• Increased surface Area

• Increased concentration

• Increased temperature

• Use of a catalyst

Gas pressure and rate

• When the pressure of a gas is increased the molecules of the gas are forced closer together.

• This effectively increases the concentration of gas particles.

• Increasing pressure of a gas therefore has the same effect as increasing concentrations of solutions – it increases the rate of reaction in the gas phase.

• Increasing pressure of solutions has limited effect – why do you think this is?

• Look at the sealed syringes of air and water to help you.

Practical – Activity sheet 28

• You will be investigating the rate of reaction of calcium carbonate (limestone) and acid using the mass lost method and gas evolved method

• You are provided with two different sizes of marble chips and four different concentrations of acid, balances and gas syringes.

• Two groups will do gas syringes measurements and two groups will measure mass lost.

Past paper question

Mark scheme