F325 Lattice Enthalpy & Born-Haber Cycles

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  • Created on: 08-03-14 21:46
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16/12/2013 14:17
1.Enthalpy changes and ionic bonding
When metals react with non-metals, the ions produced form ionic crystals. These ionic
crystals are giant lattices consisting of positive and negative ions packed in a regular
Standard enthalpy changes
The standard conditions are:
Temperature of 25C (298K)
Pressure of 1 atmosphere (100kPa)
Reactants in their standard states
Concentration of solutions 1 moldm-3
Standard enthalpy change of formation of a compound is the enthalpy change
when one mole of the compound forms from its elements under standard conditions
with the compound and the elements in their standard states.
Enthalpy change of formation of sodium chloride can be written as:
Na(s) + 0.5Cl2(g) -> Na+Cl-(s) Hf=-411kJmol-1
Equations should always include state symbols. In addition the standard state for
carbon is graphite and not diamond, because graphite is energetically more stable
than diamond at 25C and 1 atm.
Standard enthalpy change of atomisation of an element is the energy needed to
produce one mole of gaseous atoms of the element under standard conditions.
Na(s) -> Na(g) Hat[Na(s)]=+107kJmol-1
0.5Cl(g) -> Cl(g) Hat[Cl(g)]=+122kJmol-1

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The first ionisation energy of an element is the energy needed to remove one
electron from each atom in one mole of gaseous atoms of the element to form one mole
of gaseous +1 ions under standard conditions
Na(g) --> Na+(g) + e- HI.1[Na(g)]=+496kJmol-1
Successive ionisation energies for the same element measures the energy needed to
remove a second, third, fourth electron and so on.…read more

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A Born-Haber cycle identifies all the enthalpy changes that contribute to the standard
enthalpy change of formation of a compound. These changes involve:
The enthalpy changes required to create free gaseous ions by atomising and
then ionizing the elements
The energy given out (the lattice enthalpy) when the ions come together to
form a crystal lattice.
Standard enthalpy change of formation of sodium oxide has been written downwards
on the cycle, showing that it is exothermic.…read more

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Enthalpy changes during dissolving
An ionic compound such as sodium chloride does not dissolve in non-polar solvents
like hexane, but it will dissolve in a polar solvent like water. When one mole of
sodium chloride dissolves in water to produce a solution of concentration 1 moldm-3
under standard conditions, there is and enthalpy change of +3.8 kJmol-1. This enthalpy
change is described as the standard enthalpy change of solution.
NaCl(s) + aq -> Na+ (aq) + Cl- (aq) H= +3.…read more

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Furthermore, both processes become more exothermic as the charges on the ions
increase because the charge density of an ion increases and therefore its attraction for
any opposite charge increases.
The effect of ionic radius on enthalpy change of hydration and lattice enthalpy
As the radius of an ion increases, its charge density decreases. This results in a
weaker attraction for oppositely charged ions and for the + and - charges on polar
molecules such as water.…read more


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