Chemistry - Energetics

Standard enthalpy change of formation

Enthalpy change when one mole of a substance is formed from its constituent elements with all reactants and products in their standard states under standard conditions (298K, 100kPa)

Standard enthalpy change of combustion

Enthalpy change when one mole of a substance undergoes complete combustion under standard conditions

Standard enthalpy change of neutralisation

Enthalpy change when one mole of water is formed in a reaction between an acid and an alkali under standard conditions (298K, 100kPa)

Standard enthalpy change of solution

Enthalpy change when one mole of an ionic solid dissolves in an amount of water large enough that the dissolved ions are well separated and don’t interact

Standard enthalpy change of hydration

Enthalpy change when one mole of gaseous ions become hydrated (dissolved in water)

Lattice enthalpy of formation

Enthalpy change when one mole of a an ionic solid is formed from its constituent ions in the gas phase

Lattice enthalpy of dissociation

Enthalpy change when one mole of and ionic solid is broken into its constituent ions in the gas phase

Enthalpy of atomisation

Enthalpy change when one mole of gaseous atoms is produced from an element in its standard state

1st electron affinity

The energy released when each atom in a mole of gaseous atoms gains an electron to form one mole of gaseous 1- ions

2nd electron affinity

The energy required for each atom in a mole of gaseous 1- ions to gain an electron to form one mole of gaseous 2- ions

1st ionisation energy

The energy required to remove one electron from each atom in a mole of gaseous atoms to produce one mole of gaseous 1+ ions and one mole of electrons

Nth ionisation energy

The energy required to remove one electron from each ion in a mole of gaseous (N-1)+ ions to produce one mole of N+ gaseous ions and one mole of electrons

How can Δ_LEFH˚ be found

Using a Born-Haber cycle (experimental) or theoretical calculation

Which Δ_LEFH˚ is considered the real value?

experimental (theoretical makes modelling assumptions)

What does the difference between experimental and theoretical Δ_LEFH˚ values tell you?

The greater the difference the greater the covalent character of an ionic compound

What are the features of ionic compounds with covalent character?

Low solubility in water, melting points lower than expected, electrical conductivity lower than expected

Hess’s Law

The enthalpy change for a reaction is independent of the route taken