Enthalpy Definitions

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Enthalpy

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The amount of heat energy possessed by a chemical substance. It is stored in the chemical bonds as potential energy. When substances react, the difference in enthalpy between the reactants and the products (at constant pressure) can be measured.

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Standard enthalpy change of combustion

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The enthalpy change when one mole of a substance undergoes complete combustion under standard conditions (with no change in volume).

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Standard Enthalpy change of formation

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The enthalpy change when one mole of a substance is formed under standard conditions from its elements in their standard state.

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Standard Enthalpy change of neutralisation

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

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Enthalpy change of solution

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The enthalpy change of solution is the enthalpy change when 1 mole of an ionic substance dissolves in water to give a solution of infinite dilution.

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Bond dissociation enthalpy

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The enthalpy change when one mole of bonds in one mole of gaseous molecules is broken

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Standard Enthalpy of atomisation

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The standard atomisation enthalpy is the enthalpy change when 1 mole of gaseous atoms is formed from the element in its standard state. Enthalpy change of atomisation is always positive.

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Lattice enthalpy

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The lattice (dissociation) enthalpy is the enthalpy change needed to convert 1 mole of solid crystal into its scattered gaseous ions. Lattice dissociation enthalpies are always positive.

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Ionisation energy

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is the enthalpy change (energy required) to remove one mole of electrons from one mole of gaseous atoms to produce 1 mole of gaseous ions each with a charge of 1+.

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second ionisation energy

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is the enthalpy change (energy required) to remove one mole of electrons from one mole of gaseous 1+ ions to produce 1 mole of gaseous ions each with a charge of 2+.

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electron affinity

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is the enthalpy change (energy released) when 1 mole of gaseous atoms gain an electron to form 1 mole of gaseous 1- ions.

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second electron affinity

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is the enthalpy change (energy required) when 1 mole of gaseous 1- ions gain an electron to form 1 mole of gaseous 2- ions.