A overview of enthalpy

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Enthalpy Change

  • Enthalpy Change is the heat energy transfered in a reaction at constant pressure (kJ mol -1 )

Reactions can be either exothermic or endothermic. Exothermic is reactions that gives out heat (energy). Endothermic is reactions that take in heat (energy)

When a reaction is exothermic, ΔH (enthalpy change) is negative

When a reaction is endothermic, ΔH is positive

Reactions are all about breaking/making bonds -

Energy is required when you break bonds, so making bonds is endothermic

Energy is given out when you make bonds, so making bonds is exothermic

Mean bond enthalpies are not exact

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Enthalpy Changes

Enthalpy Change of a reaction = Total energy absorbed - Total energy released

This could also be interpreted as -

Energy needed to break bonds - Energy given out by forming bonds

There is different types of enthalpy changes -

  • Standard enthalpy change of reaction
  • Standard enthalpy change of formation
  • Standard enthalpy change of combustion

Standard enthalpy of formation - The enthalpy change when one mole of a compound is formed from its constituent elements in there standard states under standard conditions

Standard enthalpy of Combustion - The enthalpy change when one mole of a substance is completely burned in oxygen under standard conditions

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