Applied Science AQA/Enthalpy

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Topic Title[edit | edit source]

Setting Applied context[edit | edit source]

The knowledge of enthalpy changes and the applications of Hess’s Law are important in many areas of scientific research and industry. Biotechnologists develop new fuels such as biodiesel and must compare their energy values with other types of fuel. Development scientists working in the food industry analyse new products for their calorific value and these will also be checked by chemical analysts working for Trading Standards. Chemical engineers in industry will apply Hess’s Law to enable the calculation of enthalpies of reaction in order to determine and better understand the likely effect of reaction conditions on yields.

Syllabus Content What you need to do
•    energy profiles for exothermic and endothermic reactions

•    activation energy using an energy profile

•    types of enthalpy changes from equations (limited to combustion, neutralisation, formation and mean bond enthalpies)

•    units for molar enthalpy change (kJ mol –1)

•    enthalpy changes as the heat energy change (at constant pressure)

•    enthalpy of formation, enthalpy of combustion and enthalpy of reaction, as represented by ∆ f H, ∆cH and ∆rxH

•    the term ‘mean bond enthalpy’

•    calculating enthalpy changes based on Hess’s Law cycles

•    calculating enthalpy changes based on mean bond enthalpies and why these values are only approximate

•    determining practically the molar enthalpy of combustion of a liquid fuel (eg ethanol)

•    determining practically the molar enthalpy of neutralisation for a simple acid–base reaction,


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