1. EXOTHERMIC AND ENDOTHERMIC REACTIONS
Reaction profiles

2. Objectives
To draw reaction profiles for exothermic and endothermic reactions
To understand the meaning of activation energy

3. SIMPLE INTRODUCTION
Exothermic means “ Heat out” Heat is given out to the surroundings
Endothermic means “Heat in” Heat is taken in from the surroundings
All reactions are either exothermic or endothermic

4. SOME REACTIONS ARE OBVIOUSLY EXOTHERMIC
e.g. methane gas burning is obviously exothermic
e.g. The reaction between magnesium and copper oxide is obviously an exothermic reaction
HOWEVER it isn’t quite as simple as this
All reactions are in fact a mixture of exo and endo parts
If we say a reaction is exothermic, then what this really means is that the exo part of the reaction is greater than the endo part of the reaction

5. To put it simply:
When we say a reaction is exothermic what we are really saying is that the overall reaction is exothermic
Similarly:
When we say a reaction is endothermic what we are really saying is that the overall reaction is endothermic

6. This clearly needs some extra explanation
Imagine an example using money instead of energy
Imagine you are investing money into an investment fund in a bank
We will show the results of the investment over time on a graph
On the graph paying in money shown by  direction payback money shown by  direction

7. “paid in” money shown by  direction “
“payback money” shown by  direction
Money in fund
Time
Would this be a good investment?

8. This is a good investment because you have more “payback” than “pay in”
Money
in fund
Paid back
Paid in
Time

9. Take an actual example. How much money would you make on this deal?
Answer = £2,000
Money
in fund
£5,000
£7,000
The profit is the difference between the two arrows
Time

10. A MEMBER OF STAFF ACTUALLY MADE AN INVESTMENT WHICH COULD BE REPRESENTED BY THS GRAPH
Money
in fund
£3,500
£5,000
How did he feel about this?
Time

11. Overall £5,000 paid in and £3,500 paid back
So overall loss, or drop of £1,500

12. What if you made more than one investment at the same time?
Imagine you invest £ 5,000 and £4,000
Imagine the paybacks are £2,000 and £8,000
How do you feel – was this a good deal? OR
Total investment = £5,000 + £4,000 = £9,000
Total payback = £2,000 + £8,000 = £10,000 So

13. Back to Chemistry- Energy instead of Money
In chemistry the “paying in” part of reactions is breaking bonds between atoms. e.g. breaking up hydrogen molecules H H H  H 
2 separate atoms
1 molecule 
Breaking bonds always involves a “paying in” of energy and is represented by an  direction on the energy graph. We call this an investment of energy

14. In chemistry the “paying back” part of reactions is making bonds between atoms. e.g. the making of a hydrogen molecule from two hydrogen atoms  H  H H H
2 separate atoms 
1 molecule
This is shown by a  direction on an energy graph

15. Let’s now apply this to an actual example
Magnesium reacts with iron oxide to make magnesium oxide and iron
Mg + FeO = MgO + Fe Mg  Fe O  Mg O  Fe
This bond must break
This bond must be made
This involves “pay in” of energy
This involves a “pay back” of energy

16. On an energy graph this will look like:
Mg + FeO = MgO + Fe
Energy “PAY IN” to break the FeO bond
Energy “PAY BACK” on making the MgO bond
Energy
Time
More “PAY BACK” energy than “PAY IN” energy – this is a GOOD energy deal.
A “GOOD” energy deal is Exothermic

17. An Exothermic reaction
Mg + FeO = MgO + Fe
This is the ENDO’ part of the reaction
This is the EXO’ part of the reaction
Energy
Time
We say this is an exothermic reaction (overall) since the exothermic part is greater than the endothermic part

18. Working out the overall energy change
400kJ/mole
330kJ/mole
70kJ/mole
The overall energy change is given by the difference between the lengths of the two arrows
The overall energy change is
70kJ/mole
In exam questions, you will be given energy values so that you can work out the lengths of the arrows
We say that ∆H = - 70kJ/mole

19. What sort of reaction would have this energy graph and why?
Time
This sort of reaction would be ……………………… because the energy ………………. in to …………………… bonds is …………… than the energy ………… back from ……………………… bonds.
The ………………………………………. part of the reaction is greater than the ……………………………………. part.

20. The activation energy Ea is the amount of
energy the reactant particles need when
they collide with each other in order to react.

21. Exothermic reactions - a summary
Activation energy
Energy
Reactants
Products
Progress of reaction
In exothermic reactions the energy of the reactants is greater than the energy of the products

22. Endothermic reactions - a summary
Energy
Activation energy
Products
Reactants
Progress of reaction
In endothermic reactions the energy of the products is greater than the energy of the reactants
The activation energy is the minimum energy required to break the bonds and start the reaction off

23. Exothermic reactions and catalysts
A.E. without cat
Energy
A.E. with catalyst
Reactants
Products
Progress of reaction
A catalyst lowers the activation energy. This helps the bonds to break more easily so the reaction works faster at that temperature.
NOTE that the overall energy change is the SAME

24. How would a catalyst change the energy graph of an endothermic reactions?
A. E. no cat
Energy
Products
A.E. with catalyst
Reactants
Progress of reaction
Again the reaction would go faster because the activation energy has been lowered
Again note that the overall energy change is the same