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Apr 19, 20221 min read

Definitions: Enthalpy changes

For the following, let AB represent a compound


* ‘The energy change when…’


Lattice Enthalpy

*… 1 mole of an ionic compound is formed from its gaseous ions under STP (298K, 100kPa).

Always exothermic

Standard Enthalpy Change of Formation

*… 1 mole of compound is formed from its constituent elements in their standard state.

Usually exothermic.

Standard Enthalpy Change of Atomisation

*… 1 mole of gaseous atoms are formed from the element in its standard state.

Always endothermic (bonds are broken)

1st Ionisation Energy

*… 1 mole of gaseous 1+ ions is formed from gaseous atoms

Always endothermic (an electron overcomes the attraction from the nucleus)

2nd Ionisation Energy

*… mole of gaseous 2+ ions forms from 1 mole gaseous 1+ ions

Always endothermic (electron overcomes nuclear attraction)

1st Electron Affinity

*… 1 mole of gaseous 1- ions forms from gaseous atoms

Exothermic (electron attracted to the outer shell by the nucleus)

2nd Electron Affinity

*… 1 mole of gaseous 2- ions forms from 1 mole gaseous 1- ions

Endothermic (arriving electron is repelled)

Enthalpy Change of Solution

*… 1 mole of a compound is dissolved in water under standard conditions

Can be endothermic or exothermic

Enthalpy Change of Hydration

*… 1 mole of aqueous ions are formed from their gaseous ions under standard conditions

Exothermic process


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