CIE A Level Chemistry

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First teaching 2020

Last exams 2024

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5.1.1 Lattice Energy & Enthalpy Change of Atomisation

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Lattice Energy & Enthalpy Change of Atomisation

  • Enthalpy change H) refers to the amount of heat energy transferred during a chemical reaction, at a constant pressure

Enthalpy change of atomisation

  • The standard enthalpy change of atomisation Hat) is the enthalpy change when 1 mole of gaseous atoms is formed from its element under standard conditions
    • Standard conditions in this syllabus are a temperature of 298 K and a pressure of 101 kPa

  • The ΔHatis always endothermic as energy is always required to break any bonds between the atoms in the element, to break 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 (ΔHat) for elements
  • For example, sodium in its elemental form is a solid
  • The standard enthalpy change of atomisation for sodium is the energy required to form 1 mole of gaseous sodium atoms:

Na(s) → Na(g) ΔHat = +107 kJ mol -1

Worked example: Writing equations for the standard enthalpy change of atomisation

Chemical Energetics - Worked example_Writing equations for the standard enthalpy change of atomisation, downloadable AS & A Level Chemistry revision notes

Answer

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)

Lattice energy

  • The lattice energy Hlatt) is the enthalpy change when 1 mole of an ionic compound is formed from its gaseous ions (under standard conditions)
  • The ΔHlatt is always exothermic, as when ions are combined to form an ionic solid lattice there is an extremely large release of energy
    • Since this is always an exothermic process, 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 ΔHlatt 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 ΔHlatt of an ionic compound cannot be determined directly by one single experiment
  • Multiple experimental values and an energy cycle are used to find the ΔHlatt of ionic compounds
  • The lattice energy (ΔHlatt) of an ionic compound can be written as an equation
    • For example, magnesium chloride is an ionic compound formed from magnesium (Mg2+) and chloride (Cl-) ions
    • Since the lattice energy is the enthalpy change when 1 mole of magnesium chloride is formed from gaseous magnesium and chloride ions, the equation for this process is:

Mg2+(g) + 2Cl-(g) → MgCl2(s)

Worked Example: Writing equations for lattice energy

Chemical Energetics - Worked example_Writing equations for the lattice energy, downloadable AS & A Level Chemistry revision notes

Answer

Answer 1: Mg2+(g) + O2-(g) → MgO(s)

Answer 2: Li+(g) + Cl-(g) → LiCl(s)

Exam Tip

Make sure the correct state symbols are stated when writing these equations – it is crucial that you use these correctly throughout this entire topic.

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Author: Francesca

Fran studied for a BSc in Chemistry with Forensic Science, and since graduating taught A level Chemistry in the UK for over 11 years. She studied for an MBA in Senior Leadership, and has held a number of roles during her time in Education, including Head of Chemistry, Head of Science and most recently as an Assistant Headteacher. In this role, she used her passion for education to drive improvement and success for staff and students across a number of subjects in addition to Science, supporting them to achieve their full potential. Fran has co-written Science textbooks, delivered CPD for teachers, and worked as an examiner for a number of UK exam boards.