Lattice Energy & Enthalpy Change of Atomisation (Cambridge International A-Level Chemistry)

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(ΔH_latt^ꝋ) is:

The enthalpy change when 1 mole of an ionic compound is formed from its gaseous ions (under standard conditions)

• The ΔH_latt^ꝋ 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 ΔH_latt^ꝋ 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 ΔH_latt^ꝋ of an ionic compound cannot be determined directly by one single experiment

• Multiple experimental values and an energy cycle are used to find the ΔH_latt^ꝋ of ionic compounds

• The lattice energy (ΔH_latt^ꝋ) 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) + O^2– (g) → MgO (s)

Answer 2:

• Li⁺ (g) + Cl^– (g) → LiCl (s)

Enthalpy change of atomisation

• The standard enthalpy change of atomisation(ΔH_at^ꝋ) is:

The enthalpy change when 1 mole of gaseous atoms is formed from its element under standard conditions

• The ΔH_at^ꝋ 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 (ΔH_at^ꝋ) 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) ΔH _at^ꝋ = +107 kJ mol -1

Worked Example Write the equations for the standard enthalpy change of atomisation, (ΔH_at^ꝋ) 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)

Exam Hint 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