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Thermodynamics A level Chemistry Notes
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Full AQA Thermodynamics A level Chemistry Notes
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ThermodynamicsEnergy needed to break a bond between two ions is the same amount of energy that is given out when the bond is
formed.
LATTICE FORMATION ENTHALPY – Enthalpy change when 1 mole of a solid ionic compound is formed from its
gaseous ions.
LATTICE DISSOCIATION ENTHALPY – Enthalpy change when 1 mole of a solid ionic compound is completely
dissociated into its gaseous ions.
Lattice enthalpy cannot be measured directly; enthalpies of different processes must first be calculated before it can
be calculated.
ENTHALPY CHANGE OF FORMATION – Enthalpy change when 1 mole of a compound is formed from its elements in
their standard states.
BOND DISSOCIATION ENTHALPY – Enthalpy change when all bonds of the same type in 1 mole of gaseous molecules
are broken.
ENTHALPY CHANGE OF ATOMISATION OF AN ELEMENT – Enthalpy change when 1 mole of gaseous atoms is formed
from an element in its standard state.
ENTHALPY CHANGE OF ATOMISATION OF A COMPOUND – Enthalpy change when 1 mole of a compound in its
standard state is converted into gaseous atoms.
FIRST IONISATION ENERGY – Enthalpy change when 1 mole of gaseous 1+ ions is formed from 1 mole of gaseous
atoms.
FIRST ELECTRON AFFINITY – Enthalpy change when 1 mole of gaseous 1- ions is made from 1 mole of gaseous
atoms.
ENTHALPY CHANGE OF HYDRATION – Enthalpy change when 1 mole of aqueous ions is formed from gaseous ions.
ENTHALPY CHANGE OF SOLUTION – Enthalpy change when 1 mole of an ionic substance dissolves in enough solvent
to form an infinitely dilute solution.
Born-Haber cycles can work out the enthalpy change for a
reaction through using a less direct route. It follows Hess’s law
as the total enthalpy change of a reaction is always the same,
independent of the route taken to reach it.
The perfect ionic model assumes that all ions are spherical and
have their charge evenly distributed around them, as well as having only ionic bonding without any covalent
character. Different ions can be polarised to different extents based on whether the size of the ion is small or large.
For example, a small positive ion will polarise a large negative ion, meaning that there will be some covalent
character.
The value for experimental enthalpy of lattice dissociation may be different to the theoretical value as there may be
some covalent character that the theoretical value does not account for as theoretical assumes that all bonding is
perfectly ionic. If the two values are very different, it means that a
compound will have a lot of covalent character.
Enthalpy change of solution can be calculated using a thermochemical
cycle.
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