Lattice energy

• As with bond enthalpy,?lattice energy?(ΔHlatt?) can be expressed as a formation or dissociation process
• As a formation process, it is the enthalpy change when?1 mole?of an?ionic compound?is formed from its?gaseous ions?(under standard conditions)
• The ΔHlatt??is therefore?exothermic,?as when ions are?combined?to form an?ionic solid lattice?there is an extremely large release of energy
  • Since this is an exothermic process, the enthalpy change will 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, sodium chloride is an ionic compound formed from sodium (Na+) and chloride (Cl-) ions
  • Since the lattice energy is the enthalpy change when 1 mole of sodium chloride is formed from gaseous sodium and chloride ions, the equation for this process is:

  Na+(g) + Cl-(g) → NaCl (s)? ΔHlatt??= -776 kJ mol?-1

Answer

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?(ΔHat?) is the?enthalpy change?when?1 mole?of gaseous atoms?is formed from its?element?under standard conditions
• The ΔHat??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??= +108 kJ mol?-1

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)

Electron Affinity

• The?electron affinity?(ΔHea) of an element is the energy change when?one mole?of electrons is gained by?one mole?of gaseous atoms of an element to form?one mole?of gaseous ions under standard conditions
• For example, the first electron affinity of chlorine is:

Cl (g)+ e–?→ Cl-?(g) ? ? ? ? ?ΔHea??= -364 kJ mol-1

• The?first electron affinity?is always?exothermic?as energy is released when electrons are attracted to the atoms
• However, the?second electron affinity?of an element can be?endothermic?as illustrated by oxygen:

O–?(g) + e–?→ O2-?(g) ? ? ? ? ?ΔHea??= +844 kJ mol-1

• This is because a large force of?repulsion?must be overcome between the negatively charged ion and the second electron requiring a large input of energy