1. Exothermic and Endothermic Reactions
Linking Energy Profile Diagrams to Thermometer Readings

2. Candidates should be able to:
(a) describe the meaning of enthalpy change in terms of exothermic (ΔH negative) and
endothermic (ΔH positive) reactions
(b) *represent energy changes by energy profile diagrams, including reaction enthalpy
changes and activation energies (see 6.1(c))
(c) describe bond breaking as an endothermic process and bond making as an
exothermic process
(d) *explain overall enthalpy changes in terms of the energy changes associated with
the breaking and making of covalent bonds
(e) describe combustion of fuels as exothermic, e.g. wood; coal; oil; natural gas;
hydrogen
(f) describe hydrogen, derived from water or hydrocarbons, as a potential fuel for use
in future, reacting with oxygen to generate electricity directly in a fuel cell (details of the construction
and operation of a fuel cell are not required)
(g) name natural gas, mainly methane, and petroleum as sources of energy

3. THE CONCEPT OF ENTHALPY CHANGE
In chemical reactions, we can measure changes in terms of energy.
We cannot measure the total amount of energy in the beginning, we also cannot measure the total amount of energy in the end.
We can measure the change in energy, we call this change the enthalpy change (or heat of reaction).

4. Exothermic Reaction
Reactants +
25oC
Products
25oC
Initial temperature

5. Exothermic Reaction
25oC

6. Exothermic Reaction time Reactants give out energy to the surroundings
temperature
32C
25C
time
Reactants give out energy to the surroundings

7. Exothermic Reaction Reactants give out energy to the surroundings
Temperature of surroundings increase
Energy of reactants decrease
Energy of
reactants
Temperature
(surroundings)
Products
32C
Reactants
ΔH is negative
25C

8. Energy Levels Exothermic reactions Energy is given out
The products have less energy than the reactants
Combustion and neutralisation are exothermic

9. Energy Level Diagrams
Exothermic reactions
energy
course of reaction

10. Energy Level Diagrams Exothermic reactions energy reactants
course of reaction

11. Energy Level Diagrams Exothermic reactions energy reactants products
course of reaction

12. Energy Level Diagrams Exothermic reactions energy given out
∆H is negative
energy
reactants
products
course of reaction

13. Endothermic Reaction Reactants + 25oC Products 25oC
Initial temperature

14. Endothermic Reaction
25oC

15. Endothermic Reaction time
temperature
25C
19C
time
Reactants take in energy from the surroundings

16. Endothermic Reaction Reactants take in energy from the surroundings
Temperature of surroundings decrease
Energy of reactants increases
Energy of
reactants
Temperature
(surroundings)
25C
Products
ΔH is positive
Reactants
19C

17. Energy Level Diagrams Endothermic reactions Energy is taken in
The products have more energy than the reactants
The energy is taken in from the surroundings

18. Energy Level Diagrams
Endothermic reactions
energy

19. Energy Level Diagrams
Endothermic reactions
energy
course of reaction

20. Energy Level Diagrams Endothermic reactions energy reactants
course of reaction

21. Energy Level Diagrams Endothermic reactions energy products reactants
course of reaction

22. Energy Level Diagrams Endothermic reactions energy products
energy taken in
∆H is positive
reactants
course of reaction

23. Endothermic reactions
Summary Table
Exothermic reactions
Endothermic reactions
Energy is given out to the surroundings
Energy is taken in from the surroundings
∆H is negative
∆H is positive
Products have less energy than reactants
Products have more energy than reactants

24. Endothermic processes
EXOTHERMIC & ENDOTHERMIC PROCESSES
Examples:
Exothermic processes
Gas  Liquid  Solid
Combustion of fuels (e.g. wood; coal; natural gas)
Neutralisation reactions ( acid + base)
Endothermic processes
Solid  Liquid  Gas
Dissolving of ammonium chloride or ammonium sulphate crystals in water
Thermal decomposition
CaCO3  CaO + CO2

25. ∆H How much energy is given out or taken in?
Energy is needed to break chemical bonds
Energy is given out when bonds are made
∆H is the difference between the energy needed to break the bonds in the reactants, and the energy given out when new bonds are made in the products

26. Bond energies The energy needed to break a chemical bond
Different chemical bonds have different bond energies
Chemical bond
Bond energy, kJ/mole
H―H
436
O=O
498
O―H
464

27. Bond energies 2H2 + O2  2H2O H O H O H O O H Chemical bond
Bond energy, kJ/mole
H―H
436
O=O
498
O―H
464
Stage 1:
Bond breaking H O H O
+436kJ
+498 kJ
+436kJ
Stage 2:
Bond forming
-464kJ H O O H

28. Working out ∆H Summary The energy values have units of kJ/mole
In the exam, you will be given the energy values and all the bonds to make or break
Energy goes in to break bonds
Energy goes out when bonds are made