1. Higher Chemistry Controlling the Rate
NEW LEARNING
Potential Energy Diagrams
Standard Enthalpy Changes
Catalysts and Activation Energy
REVISION
Reactions monitored and graphs interpreted
Average Rate of Reaction Calculated from a Graph

2. Starter Task
Complete Quick Test 1 in your booklet and we will review as a class. You have 10 minutes!
S3 Revision

3. Answers Quick Test 1 1a. slower b. faster c. faster d. Faster
2a. Reactant particles are absorbed onto the surface where their internal bonds are weakened and they are held in the correct position. This lowers the energy required for a successful collision. When the product molecules are formed they leave the active site.
b. Increase the surface area of the catalyst and therefore increase the number of active sites.
c. The active sites become permanently blocked.
Ask 3, Then Me!
Mark your answers and check with neighbours to see why you got any wrong.
S3 Revision

4. Lesson 6: Potential Energy Diagrams
Today we will learn to
Draw and interpret potential energy diagrams for endothermic and exothermic reactions.
We will do this by
Investigating different temperature changes in reactions and showing them on a diagram.
We will have succeeded if
We can predict the energy change in a reaction from the potential energy diagram.

5. Potential Energy Potential Energy Diagrams
During a chemical reaction the chemical potential energy of the molecules change as they turn from reactants into products.
As the reactants collide and come together they form an unstable activated complex; with bonds within the reactant molecule beginning to break and new bonds forming to make the products. The more unstable the molecules the higher the potential energy.
A potential energy diagram can show these changes and allow us to determine whether the reaction is exothermic (gives out energy) or endothermic (takes in energy).

6. Potential Energy
If the products have a lower potential energy than the reactants the reaction will be exothermic and if the products have more energy than the reactants the reaction will be endothermic (see below):

7. Potential Energy To calculate the enthalpy change use ∆H = Hp—Hr
In industry it is important to know whether your reaction is going to be endothermic or exothermic as this will have an effect on the how much energy you will need to supply to get a good rate of reaction or take away to keep the reaction under control.
To calculate the enthalpy change use
∆H = Hp—Hr
Activity 1.6 Exothermic and Endothermic Reactions
Copy and complete the details from the pupil booklet into your jotter for each experiment. Answer the questions from your booklet after each experiment.

8. Energy Changes in Chemical Reactions
Chemical reactions involve a change in energy which often results in the loss or gain of heat energy (exothermic/endothermic reactions)
The heat energy stored in a substance is called its Enthalpy ( H ).
The difference between the enthalpy of the reactants and the enthalpy of the products in a reaction is the Enthalpy Change ( ∆H ):
∆H is measured in kJ per mole (kJ mol-1)
∆H = H(products) – H(reactants)

9. Potential Energy Diagrams
We can show the energy changes involved in exothermic and endothermic reactions by using potential energy diagrams.
Exothermic Reactions
Reactions which give out heat energy are called exothermic reactions.
The products have less enthalpy (potential energy) than the reactants and the temperature of the surroundings increases.

10. ∆H is always negative for
exothermic reactions
E.g. the combustion of fuels
Reaction Pathway
Activated Complex
Potential energy or
Enthalpy
(kJ mol-1)
Reactants
Products
From this diagram we can work out:
The activation energy (Ea) which is needed to start the reaction.
The change in enthalpy between the reactants and products (∆H) = the energy given out by the reaction.

11. Endothermic Reactions
Reactions which absorb heat energy from the surroundings are called endothermic reactions.
The products have more enthalpy than the reactants and the temperature of the surroundings decreases.
Reaction Pathway
Activated Complex
Potential energy or
Enthalpy
(kJ mol-1)
Reactants
Products

12. From this diagram we can work out: The activation energy (Ea)
The change in enthalpy between the reactants and products (∆H) = the energy taken in by the reaction.
∆H is always positive for
endothermic reactions

13. Pyramid Exit Task Question you have about the lesson
Things you have been reminded of today
Things you have learned today

14. Starter Activity Woosh bottle Magnesium Ribbon Burning Ice packs
Activity 1.7 : Exciting Exothermic and Endothermic Reactions
Your teacher may demonstrate to you some exciting exothermic and endothermic reactions.
Woosh bottle
Magnesium Ribbon Burning
Ice packs

15. Lesson 7/8: Controlling Reactions
Today we will learn to
Control rate and prevent the consequences of an out of control reaction.
We will do this by
Considering the causes and effects of the Bhopal disaster.
We will have succeeded if
We can identify ways to prevent reactions stopping or getting out of control.

16. Lesson 7: Controlling Reactions
Controlling the Reaction
Heating chemicals is an expensive business and so Chemical Engineers look to find ways to heat chemical reactants without having to burn fuels or use electricity. The best way to do this is to use the heat from an exothermic reaction to heat the reactants. A complicated system of pipes will bring the feedstock chemicals near to the reactor so that they are heated up before entering the reactor to react.

17. Lesson 7: Controlling Reactions
Sometimes it is very important to keep an exothermic reaction cool either because if it got too hot it could produce so much heat that it exploded or it is in equilibrium and the position of equilibrium favours the forward reaction when it is cooler. Water from rivers can be used to remove heat from a reaction vessel and then the hot water is cooled using cooling towers.
Controlling the temperature of the reacting chemicals is vitally important and if care is not taken horrific explosions can occur when exothermic reactions cause the rate of reaction to get faster and faster until the pressure inside the reaction vessel gets so high that an explosion occurs.

18. Bhopal
Activity 1.8 : Bhopal—when reactions get out of control. Watch the 3 video clips about the chemical reactions occurring and answer the questions on the worksheet.

19. Activation energy
The Activation Energy is the ‘energy barrier’ which must be overcome before the reactants can change into products.
Fast Reaction Slow Reaction
The size of the Activation Energy will control how fast or slow a reaction is. The higher the Activation Energy (or ‘barrier’) the slower the reaction.
Energy
Products
Reactants Ea
Energy
Products
Reactants Ea
Small Activation Energy
Large Activation Energy

20. If the activation energy is high, very few molecules will have enough energy to overcome the energy barrier and the reaction will be slow.
E.G Combustion of Methane
CH O2 CO H2O
This is a very exothermic reaction. At room temperature, no reaction occurs as too few reactant molecules have sufficient energy to react when they collide.
Striking a match provides the molecules with enough energy to overcome the barrier- it supplies the Activation Energy.
Once started, the energy given out by the reaction keeps it going.

21. The Activated Complex
When particles collide with the required Activation Energy (& geometry), the activated complex is formed.
The activated complex is an unstable intermediate arrangement of atoms formed as old bonds are breaking and new bonds are forming.
Energy is needed to form the activated complex as bonds in the reactants may need to be broken, or charged particles brought together.
Activated Complex
Reactants
Products

22. As the activated complex is very unstable it exists for a very short period of time. From the peak of the energy barrier the complex can lose energy to form either the products or the reactants again.
The higher the enthalpy change (∆H), the more unstable the activated complex.

23. Catalysts and Activation Energy
A catalyst provides an alternative pathway for the reaction with a lower activation energy.
N.B The catalyst has no effect on the enthalpy change, ∆H ∆H

24. Definition
Choose three new words you have learnt in this topic and write dictionary definitions.

25. Lesson 9: Fuel for Thought
Today we will learn to
Measure the enthalpy change in an exothermic reaction.
We will do this by
Using Eh = cmT then calculating enthalpy of combustion for an alcohol.
We will have succeeded if
We can calculate standard enthalpy from a temperature change in a reaction.

26. Fuel for Thought
Measuring the Enthalpy Change of an Exothermic Reaction
During an exothermic reaction, such as combustion, energy is released to the surroundings. The amount of energy can be calculated by absorbing the heat released with water. Water is used because it will always gain the same amount of energy for a given mass and temperature change, this is known as the specific heat capacity of water.
The following equation is used:
Eh = cmΔT
where Eh = energy gained or lost by the water (kJ)
c = specific heat capacity of water = 4.18 kJ0C-1kg-1
m = mass of water (kg)
ΔT= change in temperature of water (0C)

27. Standard Enthalpy Changes
A standard enthalpy change defines the conditions under which the enthalpy change has been measured.
This is generally the heat taken in or given out by one mole of a substance in its normal state at 1 atmosphere pressure and room temperature (298K).
Standard Enthalpy of Combustion
The energy given out when 1 mole of a substance burns completely in excess oxygen.
e.g. The enthalpy of combustion of propane is the energy change in the reaction:
C3H8(g) O2(g) 3CO2 (g) H2O(l)
(1mole)
∆H = kJ mol-1
(see data booklet pg9)
Exothermic reaction so ∆H is negative

28. Standard Enthalpy of Solution
The energy released or absorbed when 1 mole of a substance dissolves completely in excess water.
e.g. The enthalpy of solution of lithium fluoride is the energy change in the reaction:
Li F(s) Li+ (aq) + F-(aq)
(1 mole)
Standard Enthalpy of Neutralisation
The energy released when one mole of water is formed during the neutralisation of an acid.
e.g. The enthalpy of neutralisation for the reaction between hydrochloric acid and sodium hydroxide is the energy change associated with the reaction:
HCl(aq) NaOH(aq) NaCl(aq) + H2O(l)

29. Calculating Enthalpy Changes by Experiment
The enthalpy change of a reaction can be determined by measuring the temperature change a reaction causes in a known mass of water.
The following equation can then be used to calculate the change in heat energy:
Where,
Eh = heat energy gained or lost by the water (kJ)
c = specific heat capacity of water = 4.18 kJ kg-1 oC-1
m = mass of water (kg)
∆T = change in temperature of the water (oC)
Eh = c m ∆T

30. Fuel for Thought

31. Fuel for Thought Activity 1.9 : Fuel for Thought Challenge
A chemical company needs to heat its reactants to get them to react. It needs to decide which fuel to use in its burners, they can have ethanol which they can buy for £15 per kg or methanol which costs £12 per kg.
Which is the best fuel for the company to use?
You may use any of the chemicals and equipment given to you.

32. Tell me three things...
you have done well you would like to find out more about you know now that you didn’t know 50 minutes ago