• Enthalpy change?(ΔH) 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?(ΔH_at^?) 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 ΔH_at^??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 (ΔH_at^?) 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) ΔH_at^??= +107 kJ mol^?-1

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?(ΔH_latt^?) is the enthalpy change when?1 mole?of an?ionic compound?is formed from its?gaseous ions?(under standard conditions)
• The ΔH_latt^??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 ΔH_latt^??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^??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^??of ionic compounds
• The lattice energy (ΔH_latt^?) 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

Answer

Answer 1:?Mg²⁺(g) + O^2-(g) → MgO(s)

Answer 2: Li⁺(g) + Cl^-(g) → LiCl(s)