AQA AS Chemistry Unit 2: Energetics

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Chemistry Unit 2: Energetics

Endothermic/Exothermic Reactions

  • If overall energy is given out it is EXOTHERMIC. Energy is given out when forming bonds

E.g. Neutralising and acid with an alkali and burning fuels

  • If overall energy is taken in it is ENDOTHERMIC. Energy is put in to break bonds

E.g. The break down of limestone

  • Energy given out or taken in is measured in kilojoules per mole (kJmol-1)

Enthalpy Change and Bond Enthalpies

Enthalpy Change: Heat energy change measured at constant pressure

Activation Energy: Minimum energy required for a reaction to occur

  • In exothermic reactions the system loses energy to the surroundings and the energy of the products is lower than the reactants

  • In endothermic reactions the system removes energy fro the surroundings, energy in products is higher than the reactants

Bond enthalpy: Energy attributed to every bond (measured in constant pressure)

Mean Bond Enthalpy: The average of many


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