1. Exothermic and
Endothermic
Reactions

2. Chemical Reactions
 All
chemical reactions involve bond breaking
and bond forming.
 Energy is needed to break bonds and
released when bonds are formed.
 Chemical reactions are accompanied by a
change in energy, mainly in the form of heat.

3. Exothermic and Endothermic
Reactions
 A reaction
in which heat is given out is
exothermic.
 A reaction
in which heat is taken in is
endothermic.

4. Combustion of Hydrocarbons
 When
hydrocarbons are burned carbon
dioxide, water vapour and large amounts of
heat are produced.
 Hydrocarbons (particularly alkanes) are
excellent fuels e.g. methane, propane,
butane and petrol.

5. Heat of Reaction
 The
heat of reaction, ΔH, of a chemical
reaction is the heat in kilojoules released or
absorbed when the number of moles of
reactants indicated, in the balanced equation
describing the reaction, react completely.
 For an exothermic reaction ΔH is negative(-)
 For an endothermic reaction ΔH is positive(+)

6. Bond Energy
 Bond
energy is the amount of energy in
kilojoules needed to break one mole of
bonds of the same type, all species being in
the gaseous state.
 The average C-H bond energy in methane is
412kj mol-1 i.e. E(C-H) = 412kj mol-1 .
 The energy of a particular bond type can
vary.
 It is usual to quote the average bond

7. Heat of Combustion
 The
heat of combustion of a substance is the
heat change in kilojoules when one mole of
the substance is completely burned in
excess oxygen.

8. Bomb Calorimeter
 Used
to find accurate values for heats of
combustion.
 Sample is ignited using an electrically heated
wire.
 Excess oxygen ensures complete
combustion of the sample.
 Heat produced by the rapid and complete
combustion of the sample heats the water
and the temperature is recorded.

9. Bomb Calorimeter
SCC Science Dept

10. Bomb Calorimeter
 The
apparatus is well insulated to prevent
heat loss.
 The amount of heat used to start burning can
be calculated and removed.

11. Heat of Combustion of Different
Fuels
 Methane
(natural gas)
 Propane (LPG)
 Hydrogen
 Petrol (octane)
-890 kj mol-1
-2219 kj mol-1
-286 kj mol-1
-5470 kj mol-1

12. Heat of Formation
 The
heat of formation is the heat change in
kilojoules, when one mole of a substance is
formed from its elements in their standard
state.

13. Law of Conservation of Energy
 The
law of conservation of energy states that
energy cannot be created or destroyed, but
only changed from one form to another.

14. Hess’s Law
 Hess’s
Law states that the heat change of a
reaction depends only on the initial and final
states of the reaction and is independent of
the route by which the reaction may occur.
 for a reaction described by a given equation,
the heat of reaction equals the sum of the
heats of formation of the products of the
reaction less the sum of the heats of
formation of the reactants of the reaction.
 ΔH = ΔH (products) - ΔH (reactants)