Download THERMOCHEMISTRY ENERGETICS/ENTHALPY

THERMOCHEMISTRY
ENERGETICS/ENTHALPY
Introduction
All reactions require energy to break bonds in the reactants, and all reactions give off
energy when new bonds are made to form the products. The difference in the energy
required to break the bonds and to make the new bonds can tell you whether the
reaction is endothermic or exothermic.
It is possible to work out the change in energy – the enthalpy change – of a reaction if
you know the products and the reactants since Hess’s law states that whichever route a
reaction takes, the enthalpy change will always be the same.
How do you calculate the enthalpy change of a reaction?
Prior knowledge form GCSE
Before you start, remind yourself of what you already know from GCSE:
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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 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
(NOT
C2H6(g) + 3½O2(g)  2CO2(g) + 3H2O(l)
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 importan
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 nt 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