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THERMOCHEMISTRY
ENERGETICS/ENTHALPY
INTRODUCTION
All reactions require energy to break bonds in the reactants,
and all reactions give off energy when new bonds form to
make products
The difference in energy required to break the bonds and to
make the new bonds can tell you whether the reaction is
endothermic or exothermic.
PRIOR KNOWLEDGE FROM GCSE
Before you start remind yourself of what you already should
know:
• Atoms bond together to fill their outer shells of electrons
• Atoms may gain or lose electrons to get full outer shells of
electrons
• Chemical reactions may give out heat – exothermic reactions.
• Chemical reactions may take in heat energy from their
surroundings – endothermic reactions
4.1 Endothermic and exothermic reactions
Thermochemistry is the study of heat changes during chemical
reactions. When a chemical reaction takes place bonds must be broken
and new bonds must be formed.
Breaking bonds uses energy from the surroundings;
forming bonds gives off heat energy to the surroundings.
During a reaction there is always an overall change in
energy.
The energy may be in different forms – light, electrical
or, most usually, heat.
Define the terms endothermic and exothermic.
Endothermic reactions are reactions that take in
heat from the surroundings by the end of the reaction.
Exothermic reactions are reactions that have given
out heat by the end of the reaction.
It is always the case that a reaction that is endothermic
in one direction is exothermic in the reverse direction.
ENERGY PROFILE DIAGRAM OF EXOTHERMIC REACTIONS
The diagram below shows an energy profile diagram for an exothermic
reaction
In an exothermic reaction the
products have less energy than the
reactants, so energy is given out
Also in an exothermic reaction more
bonds are made than are broken, so
overall energy is given out
Activation energy (Ea) – is the
minimal amount of energy needed to
start the reaction (i.e. minimum amount
of energy to break bonds in the
reactants).
ENERGY PROFILE DIAGRAM OF ENDOTHERMIC
REACTIONS
The diagram below shows an energy profile diagram for an endothermic
reaction
In an endothermic reaction the
products have more energy than the
reactants, so energy is taken in
Also in an endothermic reaction more
bonds are broken than are made, so
overall energy is taken in
EXAMPLES OF EXO AND ENDOTHERMIC REACTIONS
DEMO 1 – THERMAL DECOMPOSITION OF
LIMESTONE
DEMO 2 – HEATING HYDRATED COPPER II SULPHATE CRYSTALS
QUANTITIES
 Energy is always measured in kJmol-1 in Chemistry
 In general we always talk about 1 mole of a fuel burning
 So when writing balanced equations such as:
CH4(g) + 2O2(g)  CO2(g) + 2H2O(l)
DH = -890kJmol-1
This means that when ONE mole of methane reacts with 2 MOLES of oxygen then
890kJ of energy is given out
OR
C2H6(g) + 3½O2(g)  2CO2(g) + 3H2O(l)
(NOT 2C2H6(g) + 7O2(g)  4CO2(g) + 6H2O(l))
DH = -86kJmol-1
So in this case the value of -86kJmol-1 is the energy given off by 1 MOLE of ethane
NOT for two moles of ethane
REMEMBER FROM GCSE THAT
 If DH is negative in value then the reaction is exothermic
 If DH is positive in value then the reaction is endothermic
So,
C(s) + O2(g)  CO2(g) DH = -394kJmol-1
(This means that 394kJ of energy is given out for 1 mole of carbon
burnt
NH4NO3(s) + (aq)  NH4NO3(aq) DH = +26kJmol-1
(This means that 26kJ of energy is absorbed by 1 mole of NH4NO3
solid)
So why do we bother to work out the energy given out for
different fuels:
ANSWER:
It allow us to compare the efficiency of different fuels.
WORKING OUT ENERGY OF COMBUSTION OF FUELS IN kJg-1
 Chemists generally refer to the energy given out when a fuel burns in kJmol-1
because this compares the same number of molecules of each fuel.
 For use as fuels it is sometimes better to convert the units from kJmol-1 to kJg-1
(OR the energy density) of a fuel
NOTICE: That petrol stores significantly more energy than either
ethanol or methanol. This is important when considering vehicles
fuelled by either of these alcohols.
Also at first sight we might think the hydrogen energy density is
amazing, but that’s not the only thing to consider. All the other
three fuels are liquids whereas hydrogen is a gas.
Although hydrogen stores more energy per gram, a gram of
hydrogen takes a lot of space because of the low density of gases.
How to store the hydrogen efficiently is a challenge for designers.
QUESTIONS