Lattice Enthalpy

• Lattice Enthalpy

  • The enthalpy change when 1 mole of a ionic compound is formd from its gaseous ion under standard conditions
• Lattice enthalpy

  • Lattice enthalpy cant be measured directly so we use other known enthalpies and link them together with an enthalpy cycle
  • Known as Born-Haber cycle
• Lattice Enthalpy of Sodium Chloride

  • Na+(g) + Cl- (g) ——-> NaCl (s)
• Lattice Enthalpy.

  • Negative Enthalpy Changes
• Enthalpy Change of Atomisation

  • The enthalpy chnage when 1 mole of gaseous atoms are formed from an element in its standard state under standard conditions
• Enthalpy Change of Atomistaion

  Example

  • Na (s) ——-> Na(g)
  • 1/2Cl2 (g) ——> Cl(g)
• Enthalpy Change of atomisation

  • Positive enthalpy changes
• First Ionisation energy definition

  • The energy requried to remove 1 mole of electron from one mole of gaseous atoms to form 1 mole of gaseous 1+ ions in their gaseous state
• First ionisation energy

  Example

  • Na (g) ——-> Na+(g) + e-
• First Ionisation energy

  • Has a positive enthalpy change
• Electron Affinity

  • The enthalpy change when every atom in 1 mole of gaseous atom acquires an electron to form one mole of gaseous 1- ions
• Electron Affinity Example

  • Cl(g) + e- ——-> Cl-(g)
  • First electron affinity is an exothermic process
  • Second electron affinty is an endothermic process
• • First electron affinity

  O(g) + e- —-> O-(g)

  • 2nd ionsation energy
  • O-(g) + e- ——>O2-(g)
• First electron affinity

  • Negative enthalpy changes
• Enthalpy Change of Formation

  • The enthalpy chnage when 1 mole of a compound is formed from its constituent elements in their standard states under standard conditions
• Enthalpy Change of Fornation

  • Negative enthalpy change
  • H2(g) + 1/2O2(g) —-> H2O(l)
• Enthalpy Change of Solution

  • The enthalpy change that occurs when 1 mole of a compound is completely dissolved in water to form one mole of hydrated ions under standard conditions
  • NaCl(s) ——-> Na+(aq) + Cl-(aq)
• Enthalpy Change of Hydration

  • The Enthalpy change takes place when 1 mole of gaseous ions are dissolved in excess water to form 1 mole of hydrated ions under standard conditions
  • Na(g) + (aq) ——-> Na+(aq)
• Factors affecting lattice enthalpy and Hydration enthalpies 


  • Ionic size
  • Ionic charge
• Effect Of ionic size on lattice enthalpy

  • As you go down the group, positive ions become bigger
  • Attratction between oppositely charged ions decrease
  • Lattice enthalpy becomes less negative
  • Melting point decreases
• Effect of ionic charge on Lattice Enthalpy

  • As ionic charge increases,
  • Attraction between oppositely charged ions increases
  • Lattice enthalpy becomes more negative
  • Melting point will increase
• Effect of ionic size on Hydration enthalpy

  • As the ionic radius increases
  • Attraction between the ion and water molecules decrease
  • Hydration enthalpy becomes less negative
• Effect of ionic charge on the Hydration enthalpy

  • As the ionic charge increases, attraction between the ions and water molecules increases
  • Hydration enthalpy becomes more negative