1. Enthalpy changes (definition)

Enthalpy change

The heat energy transferred in a reaction at a constant pressure.

Standard pressure

100kPa

Is delta h - or + for exothermic reactions

-, because heat energy is given out, so the chemicals lose energy.

Is delta h - or + for endothermic reactions

+, because heat is taken in, so the chemicals gain energy.

Lattice formation enthalpy

the enthalpy change when 1 mole of a solid ionic compound is formed from it;s gaseous ions.

Latice dissociation enthalpy

the enthalpy change when 1 mole of a solid ionic compound is completely dissociated into it’s gaseous ions.

enthalpy change of formation

the enthalpy change when 1 mole of a compound is formed from its elements with all reactants and products in their standard states under standard conditions.

bond dissociation enthalpy

the enthalpy change when all the bonds of the same type in 1mole of gaseous molecules are broken.

enthalpy change of atomisation of an element

the enthalpy change when 1 mole of gaseous atoms is formed from an element in its standard state.

enthalpy change of atomisation of a compound

the enthalpy change when 1 mole of a compound in its standard state is converted to gaseous atoms.

first ionisation energy

the enthalpy change when 1 mole of gaseous 1+ ions is formed from 1 mole of gaseous atoms.

second ionisation energy

the enthalpy change when 1 mole of gaseous 2+ ions is formed from 1 mole of gaseous 1+ ions.

First electron affinity

the enthalpy change when 1 mole of gaseous -1 ions is made from 1 mole of gaseous atoms

second electron affinity

the enthalpy change when 1 mole of gaseous -2 ions is made from 1 mole of gaseous -1 ions.

enthalpy change of hydration

the enthalpy change when 1 mole of aqueous ions is formed from gaseous ions.

enthalpy change of solution

enthalpy change when 1 mole of an ionic substance dissolves in enough solvent to form an infinitely dilute solution.

enthalpy

a measure of the heat content of a substance

bond enthalpy

the enthalpy change to break one mole of covalent bonds in the gas phase.

mean bond enthalpy

the enthalpy change to break one mole of covalent bonds in the gas phase averaged over several compounds.

Substances must be in the gas phase before bonds are broken, so the enthalpy change to go to the gas state is needed for solids and liquids.

Substances must be in the gas phase before bonds are broken, so the enthalpy change to go to the gas state is needed for solids and liquids.

Enthalpy of vapourisation

the enthalpy change to convert a liquid to a gas.