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dEnthalpy
Standard Enthalpy of Formation ΔHf ; The enthalpy change when one mole of a compound is
formed from its constituent elements under standard conditions with all reactants and products in
their standard states
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Enthalpy of Lattice formation ΔHL; The standard enthalpy change which accompanies the formation
of one mole of solid ionic lattice formed from its separated gaseous ions
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Ionic
bonding is the electrostatic attraction between two oppositely charged ions
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Lattice dissociation is a bond- breaking process and is endothermic, ΔH>0
Lattice formation is a bond making process and is exothermic, ΔH<0

The Born Haber Cycle
The Born Haber cycle links the enthalpy of formation of an ionic solid with its lattice enthalpy
through a series of separate steps
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ΔHf = Σ(Atomisation enthalpies) + Σ(Ionisation enthalpies) + ΔHL
Atomisation
Converting constituent elements (metal + non- metal) in standard states into gas phase
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The non- metal will usually be a diatomic gas so covalent bonds will have to be broken
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Also atomisation produces gaseous atoms from the element in its standard state, while dissociation
produces gaseous atoms from gaseous molecules
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Electron Affinity ΔHea ; The standard molar enthalpy change when an electron is added to an
isolated atom in the gas phase
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Two negative
ion (oxygen, sulphur) then the electron addition is in two steps
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Ionisation
1) Formation of positive ions in gaseous phase
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The process is endothermic as it involves removing an electron
2) Formation of negative ions in gaseous phase
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This process is
exothermic as non- metal ion wants extra electron to achieve full outer shell
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Enthalpy of Combustion, ΔHc; The enthalpy change when one mole of a compound burns
completely in oxygen under standard conditions with all products and reactants in their standard
states
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Enthalpy of Hydration and Solution
Heat energy may be given out (Exothermic) or taken in (Endothermic) when an ionic solid dissolves
in water
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Enthalpy of Solution, ΔHSol; Standard enthalpy change for the process in which one mole of an ionic
solid dissolves in an amount of water large enough to ensure that the dissolved ions are well
separated and do not interact with each other
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Holding oppositely
charged ions together in the ionic lattice
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NaCl (s) >>>> Na+ (g) + Cl- (g)
ΔHL= +771 kJ mol-1
Second step, energy released as gaseous ions become hydrated
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Na+ (g) >>>> Na+ (aq)
ΔHHyd= -406 kJ mol-1
Likewise the negative ions attract the positive ends of the water molecules as they become
surrounded by the water molecules
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If ΔHL is very large
solid will probably not dissolve
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If a reaction is spontaneous then the reverse will
require an input of energy
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Spontaneous reactions seem to be driven by a favourable change in enthalpy
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The enthalpy change is
negative from higher to a lower value
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Entropy can be thought of as disorder
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Processes which increase in entropy are inherently more likely to occur than those leading to an
increase in disorder
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Simple molecules tend to have lower
entropy values than more complicated molecules or structures
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The most favourable reaction will be when ΔH<0 and ΔS>0
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May require activation energy or
the reaction may take an infinite amount of time
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g
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A reaction may be feasible at one temperature, not another
To find the temperature at which a reaction becomes feasible which is where ΔG= 0 so;
ΔH=TΔS
Reversible Reactions
In some reactions the value of ΔG is only just positive, so the reaction may go part way and
equilibrium is set up so that there is always some reactant present
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Exothermic, ΔH (–ve)
Endothermic, ΔH (+ve)

ΔS Positive
Feasible at all temperatures
Feasible at high temperatures

ΔS Negative
Feasible at low temperatures
Not feasible at any temperatures

Enthalpy Changes, ΔH and Physical Changes
Melting (Fusion)
ΔS>0
This process has a positive entropy change as highly ordered solid becomes a less ordered liquid
ΔGFus = ΔHFus - TΔSFus
Boiling (Vapourisation)
ΔS>0
This process has positive entropy as a ordered liquid becomes a more chaotic gas
ΔGvap = ΔHVap- TΔSVap

Bond Enthalpy
Reactant >>>> Product
 Bonds are broken and there is an input of energy from the input surroundings (Endothermic)
 Bonds are formed and there is an output of energy to the surroundings (Exothermic)
The Enthalpy Change for the reaction is the net balance between energy in and energy out
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Bond Dissociation Energy, ΔHDiss,; Enthalpy change when one mole of covalent bonds of the same
type is broken in gaseous molecules under standard conditions
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This
is because the energy needed to break each of the covalent bonds in the polyatomic molecules is
different depending on the environment for which the bond is in
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g
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For such covalent bonds a mean bond enthalpy is used which are not entirely accurate but
can be used to predict the enthalpy changes
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In a pure ionic bond each ion is separate, spherical and the electron densities are evenly distributed
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The electron distribution around the negative ion
becomes distorted to produce a region of electron density between the two ions
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The negative ion is said to be polarised by the
positive and the bond is described as ‘ionic with covalent character’
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Has 13 protons in the nucleus
attracting only 10 electrons
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Lattice Enthalpies
Lattice enthalpy values calculated using a Born Haber cycle are experimental values because
the values used in the series of enthalpy changes have been found by the experiment
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Theoretical values can be calculated even for ionic
compounds which do not exist
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The electrostatic force of attraction between two ions of opposite
charge will be proportional to the magnitude of the charge on each ion and will be inversely
proportional to the distance between the ions
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For example theoretical lattice enthalpy values can be calculated for MgCl and MgCl3
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This may tell us why these lattices do not form
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The positive ion may attract part of the
negative charge of the ion towards itself
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For the halides of Alkali metals, the theoretical values are similar to the experimental;
 Simple ionic model is suitable and acceptable
 Ions are spherical with uniformly distributed charge
 Ionic crystals have strong character
For silver halides, large discrepancies exist between the values
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This is because the bonding is partially covalent and the
electron density between the nuclei of the bonded ions is higher and extra bonding exists
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Summary
 Good agreement means that the crystals are essentially ionic in nature
 Poor agreement means that considerable covalent character exists in the ionic crystal, the
ionic bond is polariesd



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