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Enthalpy change

AS

Energetics

Physical

AQA Content

define standard enthalpy of combustion (∆cHƟ)
define standard enthalpy of formation (∆fHƟ).

Specification Notes

Reactions can be endothermic or exothermic.
Enthalpy change (∆H) is the heat energy change measured under conditions of constant pressure.
Standard enthalpy changes refer to standard conditions, ie 100 kPa and a stated temperature (eg ∆H298Ɵ).

Notes

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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)


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