22 Enthalpy and entropy

Lattice enthalpy

• Solid ionic compounds tend to be v stable - arises from the strength of the ionic bonds.

  • Ionic bonding: the electrostatic attraction between oppositely charged ions in the ionic lattice structure

  • This creates a substantial energy barrier that must be overcome to break down the lattice, reflected in the high mpts of many ionic compounds.

• Lattice enthalpy, ΔLEH^θ: a measure of the strength of ionic bonding in a giant ionic lattice.

• It's the enthalpy change that accompanies the formation of one mole of an ionic compound from its gaseous ions under standard conditions

• Lattice enthalpy involves ionic bond formation from separate gaseous ions. It's an exothermic change.

The Born-Haber cycle

• Lattice enthalpy can't be measured directly and must be calculated indirectly using known energy changes in an energy cycle.

Key enthalpy changes

• Standard enthalpy change of formation ΔfH^θ,: the enthalpy change that takes places when one mole of a compound is formed from its elements under standard conditions, with all reactants and products in their standard states.

• Standard enthalpy change of atomisation, ΔatH^θ: the enthalpy change that takes place for the formation of one mole of gaseous atoms from the element in its standard state under standard conditions.

  • Always an endothermic process because bonds are broken

• The first ionisation energy, ΔIEH^θ: the enthalpy change required to remove one electron from each atom in one mole of gaseous atoms to form one mole of gaseous 1+ ions.

  • Endothermic, energy required to overcome the attraction between a negative e- and a positive nucleus.

• The first electron affinity, ΔEAH^θ: the enthalpy change that takes place when one electron is added to each atom in one mole of gaseous atoms to form one mole of gaseous 1- ions.

  • Exothermic, electron being added is attracted in towards the nucleus

Successive electron affinities

• The second EA is endothermic

• A second electron is being gained by a negative ion, which repels the electron away.

• This means energy must be put in to force the negatively charged electron onto the negative ion.

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