• Enthalpy change
  • The Born-Haber cycle
  • The ionic model
  • Bond enthalpies
  • Free energy change and entropy change
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  • Created by: Emilie
  • Created on: 17-06-15 15:37
Preview of Thermodynamics

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Enthalpy change, H
The enthalpy change is the amount of heat taken in or given out at
constant pressure during any physical or chemical change. The size
of any enthalpy change depends on the pressure and the
temperature as well as on the amount of substance used. Chemists
use agreed standard conditions in order to make useful
comparisons between different sets of data.
The standard state of a substance at a given temperature (usually
298 K) is its pure, most stable, form at 100kPa.
There remains the need to decide both an amount of substance and a temperature in order to
fully specify any enthalpy change, since enthalpy changes can be quoted for any chosen amount of
substance, or for any chosen temperature. Almost always, the standard amount used by chemists
is the mole.
The standard enthalpy of formation is the enthalpy change under standard conditions when one
mole of a compound is formed from its elements with all reactants and products in their standard
Consequently, the standard enthalpy of formation for an element must always be zero. Values of
standard enthalpy of formation can be found in data books.
The ionisation enthalpy is the standard molar enthalpy change for the removal of an electron from
a species in the gas phase to form a positive ion and an electron, both also in the gas phase. For
Na (g) Na+(g) + e-(g) Hi
= +496 kJ mol-1
The cation formed (Na+) may be further oxidised, according to the process:
(g) Na2+(g) + e-(g) Hi
= +4562 kJ mol-1
This is called the second ionisation enthalpy. This is always larger than the first, because the
removal of the second electron from an already positively charged species requires more energy
(to overcome to electrostatic attraction) than the removal of the first from a neutral species.
Enthalpy of atomisation, H at
The enthalpy of atomisation is the standard enthalpy change that accompanies the formation of
one mole of gaseous atoms from the element in its standard state. For an atomic solid, such as an
element, the standard enthalpy of atomisation is simply the standard enthalpy of sublimation of
the solid. In such a case, the enthalpy of atomisation is identical to the enthalpy of sublimation.
Enthalpies of atomisation (sublimation) for a selection of substances are shown in the table.
Sublimation always requires an input of energy (endothermic process), so these enthalpies are all

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H kJ mol
at /
C (graphite) +715
Na (s) +109
K (s) +90
Mg (s) +150
In the case of bond fission, a diatomic molecule will produce two moles of atoms, so the enthalpy
of atomisation is half the bond dissociation enthalpy.…read more

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The enthalpy of lattice formation is the converse of this, i.e. the standard enthalpy change that
accompanies the formation of one mole of a solid ionic lattice from its gaseous ions.
For example:
NaCl(s) Na+ (g) + Cl-(g) HL
= +771 kJ mol-1 (=dissociation)
If lattice dissociation is used as a defining equation, as above, all
lattice enthalpies are positive, since all ionic crystals are energetically
more favoured than their separated gaseous ions.…read more

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H -H
L = ½H
sub - H
diss - H
i- H
ea + f
HL = -109 ­ 121 ­ 494 ­(-364) +(-411)
= -771 kJ mol-1
The ionic model
The ionic model can be used to calculate a theoretically-based value for lattice dissociation
enthalpy. It makes the assumption that ionic (electrostatic) forces are solely responsible for the
stability of ionic crystalline solids.…read more

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Reactions in which there is a release of energy (exothermic processes) and those where there is an
increase in entropy may sometimes conflict. The two are brought together in the relationship:
G = H ­ TS
G must be negative (or zero) for spontaneous change.…read more


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