Born-Haber cycles

The lattice enthalpy

• the enthalpy change when one mole of solid ionic lattice is formed from its gaseous ions under standard conditions

The standard enthalpy of atomisation

• the enthalpy required to form one mole of gaseous atoms from an element under standard conditions

First electron affinity

• the enthalpy change when one mole of gaseous atoms is converted into one mole of gaseous 1- anions

The standard enthalpy of formation

• the enthalpy change when one mole of a compound is formed from its elements in their normal physical states under standard conditiosn

The standard enthalpy of reaction

• the enthalpy change when the molar quantities of reagents shown in a given equation react according to that equation under standard conditions

The standard enthalpy of combustion

• the enthalpy change when one mole of a substance is completely burned in air or oxygen under standard conditions

The standard enthalpy of neutralisation

• the enthalpy change when solutions of an acid and alkali react together under standard conditions to produce 1 mole of water

The 1st ionisation energy

• the energy needed to convert one mole of gaseous atoms into one mole of gaseous 1+ cations

The enthalpy of solution

• the enthalpy change when one mole of a solute is dissolved in enough water for there to be no further heat change on further dilution

The enthalpy of hydration

• the enthalpy change when one mole of gaseous ions are hydrated with enough water for there to be no further heat change on further dilution

Lattice energies of ionic compounds

• the charges on the ions - the greater the charges on the ions the stronger the electrostatic force of attraction between them and therefore the greater the magnitude of the lattice enthalpy

• The sizes of the ions - the smaller the ions, the shorter the distance between the ions, and therefore the stronger the electrostatic attraction between the ions and the greater the magnitude of the lattice enthalpy

Theoretical vs experimental value of lattice energy

• theoretical energy is the computational methods that use the model of perfectly spherical ions

• Experimental value is the reality because the cation is small so it polarises the large anion - the electron density around the anion is distorted as the cation draws the electron density towards itself therefore the bonds in compound are ionic with some degree of covalent character

• The experimental lattice energy is always going to be more exothermic than the theoretical because the bonds have some covalent character as well as the strength of ionic bonds

Predict whether the lattice energy of MgO is more or less exothermic than the lattice energy of MgS

• the Mg2+ is the same in each compound

• the anions have the same charge but 02- is smaller than S2-

• the electrostatic forces of attraction in MgO will be stronger, as the ions will be closer together

• the lattice energy for MgO will therefore be more exothermic

The magnitude of the lattice energy depends on

The charges on the ions:

• the greater the charges on the ions, the stronger the electrostatic force of attraction between them, and therefore the greater the magnitude of the lattice enthalpy

The sizes of the ions:

• the smaller the ions, the shorter the distance between the ions, and therefore the stronger the electrostatic attraction between the ions, and therefore the greater the magnitude of the lattice enthalpy