AQA Chemistry Unit 2: 7 Energetics

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7.1 Endo/Exothermic Reactions

  • Thermochemistry is the study of heat changes due to chemical reactions
  • Energy is put in to break bonds
  • Energy is given out when forming bonds
  • If overall energy is given out it is exothermic
  • If overall energy is taken in it is endothermic
  • Exothermic example: neutralising an acid with an alkali
  • Endothermic example: the breakdown of limestone


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

Burning Fuels

  • These are very exothermic reactions
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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 Enthalpies

  • Bond enthalpy: Energy attributed to every bond (measured in constant pressure)
  • Different bonds have different enthalpies
  • Mean Bond Enthalpy: The average of many bond enthalpy values for the same bond taken from different environments
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Definitions of Enthalpy Changes

Standard Enthalpy Change: Enthalpy change measured at standard pressure (100kPa) and standard temperature (298K)

Standard States: Elements and compounds are said to be in standard states if they are in their normal stable state at 298K and 100kPa

Standard Enthalpy Change of Formation: Enthalpy change when 1 mole of compound produced from its elements. Under standard conditions and all reactants and products in standard states

Standard Enthalpy Change of Combustion: Enthalpy change when 1 mole of substance (element/compound) is burnt completely in oxygen. Under standard conditions and all reactants and products in standard states

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  • Determines heat absorbed, evolved or transferred
  • 1st Law of Thermodynamics: Energy cannot be created or destroyed only converted
  • Heat energy (q) required to change the temperature of a substance is given by:

q=m X c X /\T

  • m: Mass of substance whose temperature is being changed
  • /\T is temperature change
  • c is the specific heat capacity

Assumptions Made

  • All the heat energy produced goes to increasing the temperature
  • None is lost to the surroundings
  • None is transferred in phenomena
  • Heat capacity for water is always used, which is good for when fuel is alight and used to heat know mass of water, okay when 2 solutions are mixed and the temperature of the mixture is monitored, less good using concentrated solutions 
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Calorimetry Cont

Molar Enthalpy Change =Heat Energy change per Mole

This is q/n ( Heat energy divided by mole)

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Hess' Law

Definition: Enthalpy change of a reaction depends only on the initial and final states of the reaction and is independent of the route by which it may occur

Mean Bond Enthalpy Calculations

  • Draw out the displayed formula of reactants and products
  • Calculate the enthalpy change to break bonds
  • Calculate the enthalpy change to form bonds

Strategy for Enthalpy Calculations

  • Write the equation for the unknown enthalpy change across the top of the thermochemical cycle
  • Write the elements or combustion products on the bottom
  • Make sure arrows go in direction of enthalpy change
  • Look out for multiples and negative negatives (form a positive)
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