CIE A Level Chemistry复习笔记5.1.1 Lattice Energy & Enthalpy Change of Atomisation

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 ΔHatꝋ is 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

5.1-Chemical-Energetics-Worked-example_Writing-equations-for-the-standard-enthalpy-change-of-atomisation

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

5.1-Chemical-Energetics-Worked-example_Writing-equations-for-the-lattice-energy

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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