Module Five [[Thermodynamics]] Born Haber Cycles

Enthalpy Change is the amount of heat taken in (or given out) during any change (physical or chemical) under conditions of constant pressure.

Standard Pressure: 100kPa
Standard Temperature: 298k

Enthalpy of formation:
enthalpy change involved in the production of one mole of a compound from its elements under standard conditions.

Enthalpy of Atomisation:
Also known as Sublimation. The enthalpy change when one mole of gaseous atoms is formed from an element in its standard state.
Na(s) ---> Na(g)

First Ionisation Enthalpy:
The energy required to form one mole of gaseous unipositve ions from one mole of gaseous ion (removing an electron)
Na(g) ---> Na+(g) + e-

Bond Dissociation Enthalpy:
energy required for the breaking of a covalent bond in a molecule.

Electron Affinity Enthalpy:
enthalpy change when one mole of a gaseous atom each gain an electron to form a mole of gaseous negative ions.
Cl(g) + e- ---> Cl-(g)

Lattice Enthalpy:
enthalpy change when one mole of an ionic solid is formed from its gaseous ions.
Na+(g) + Cl-(g) ---> NaCl(g)

Enthalpy of Solution:
heat energy change at constant pressure when one mole of a compound dissolves completely in water.
NaCl(s) --(water)--> Na+(aq) + Cl-(aq)

Enthalpy of Hydration:
enthalpy change when one mole of a isolated gaseous ions is dissolved in water forming one mole of gaseous ions.
X+-(g) -----> X+-(aq)

Born Haber Cycles

* The endothermic reactions absorb energy and the arrows point downwards (-ve enthalpys)

* The endothermic reactions release energy and the arrows point upwards (+ve enthalpys)

* The enthalpy of any element in standard state is 0, as absolute energies cannot be measured.

* The sum of the enthalpy change around the cycle must equal 0.

Born Haver cycles do not need to be drawn out each time, as some questions do not require them to be drawn. Often, energy cycles or Hesses law can be used to work out a certain enthalpy change.

AQA Text Book Summary:

* Show theorectical energy changes associated with formation of ionic compund

* Can be used to calculate lattice energies

* Two largest enthalpies are ionisation and lattice and it is the balance of these that determine whether or not an ionic compound can be formed

* Lattice enthalpy is defined in 2 ways: one mole of ionic solid formed from its gaseous ions (endothermic) [or] one mike if ionic solid is separated into its gaseous ions (endothermic)

* Enthalpy of solution of ionic compound can be calculated from lattice enthalpy and enthalpies of hydration.