Lattice Energy & Enthalpy Change of Atomisation (Cambridge (CIE) A Level Chemistry): Revision Note
Exam code: 9701
Lattice Energy & Enthalpy Change of Atomisation
Enthalpy change (ΔH) refers to the amount of heat energy transferred during a chemical reaction, at a constant pressure
Lattice energy
The lattice energy (ΔHlattꝋ) is:
The enthalpy change when 1 mole of an ionic compound is formed from its gaseous ions (under standard conditions)
The ΔHlattꝋ is always exothermic, as when ions are combined to form an ionic solid lattice there is an extremely large release of energy
The enthalpy change will always have a negative value
Because of the huge release in energy when the gaseous ions combine, the value will be a very large negative value
The large negative value of ΔHlattꝋ suggests that the ionic compound is much more stable than its gaseous ions
This is due to the strong electrostatic forces of attraction between the oppositely charged ions in the solid lattice
Since there are no electrostatic forces of attraction between the ions in the gas phase, the gaseous ions are less stable than the ions in the ionic lattice
The more exothermic the value is, the stronger the ionic bonds within the lattice are
The ΔHlattꝋ of an ionic compound cannot be determined directly by one single experiment
Multiple experimental values and an energy cycle are used to find the ΔHlattꝋ of ionic compounds
The lattice energy (ΔHlattꝋ) of an ionic compound can be written as an equation
E.g. magnesium chloride is an ionic compound formed from magnesium (Mg2+) and chloride (Cl-) ions:
Mg2+ (g) + 2Cl- (g) → MgCl2 (s)
Worked Example
Write the equations which represent the lattice energy of:
1. Magnesium oxide
2. Lithium chloride
Answer 1:
Mg2+ (g) + O2– (g) → MgO (s)
Answer 2:
Li+ (g) + Cl– (g) → LiCl (s)
Enthalpy change of atomisation
The standard enthalpy change of atomisation (ΔHatꝋ) is:
The enthalpy change when 1 mole of gaseous atoms is formed from its element under standard conditions
The ΔHatꝋ is always endothermic as energy is always required to break any bonds between the atoms in the element into its gaseous atoms
Since this is always an endothermic process, the enthalpy change will always have a positive value
Equations can be written to show the standard enthalpy change of atomisation (ΔHatꝋ) for elements
E.g. The standard enthalpy change of atomisation for sodium is the energy required to form 1 mole of gaseous sodium atoms:
Na (s) → Na (g) ΔHatꝋ = +107 kJ mol -1
Worked Example
Write the equations for the standard enthalpy change of atomisation, (ΔHatꝋ) for:
1. Potassium
2. Mercury
Answer 1:
Potassium in its elemental form is a solid, therefore the standard enthalpy change of atomisation is the energy required to form 1 mole of K (g) from K (s)
K (s) → K (g)
Answer 2:
Mercury in its elemental form is a liquid, so the standard enthalpy change of atomisation of mercury is the energy required to form 1 mole of Hg (g) from Hg (l)
Hg (l) → Hg (g)
Examiner Tips and Tricks
Standard conditions in this syllabus are a temperature of 298 K and a pressure of 101 kPa
Make sure the correct state symbols are stated when writing these equations – it is crucial that you use these correctly throughout this entire topic
Unlock more, it's free!
Did this page help you?