3.2.1 Enthalpy changes

• Standard enthalpy of combustion

  is the change in enthalpy that occurs when one mole of a substance is completely burned in excess oxygen under standard conditions, all reactants and products being in their standard states
• Enthalpy change is the heat energy change at constant pressure
• Standard enthalpy of formation

  enthalpy change when one mole of a compound is formed from its constituent elements under standard conditions, with all reactants and products being in their standard states
• Endothermic

  enthalpy change - positive
• Exothermic

  enthalpy change - negative
• Endothermic

  • absorbs energy from surroundings
  • products higher in energy than reactants
• Exothermic

  • releases energy to surroundings
  • products lower in energy than reactants
• Combustion reactions enthalpy change always negative
• combustion reaction

  reactant burned - 1 mole
• formation

  product - 1 mole
• Neutralisation

  add masses of liquids
• Neutralisation

  water - 1 mole
• standard conditions

  • 298 K temperature
  • 100 kPa pressure
  • 1 mol/dm3 concentration
• enthalpy change of formation 0 for all ELEMENTS
• hess cycles - combustion

  arrows DOWN
• hess cycles - formation

  arrows UP
• Mean bond enthalpy: the energy required to break a covalent bond in a molecule averaged over a range of compounds.
• enthalpy change = bonds BROKEN - bonds MADE
• explain why values from mean bond enthalpy calcs differ from those by Hess's law

  less accurate because the mean bond energies are not exact
• Calorimetric method

  A) J

  B) g

  C) K

  
• enthalpy change = q/mols
• neutralisation

  enthalpy change - negative

  temp increase
• displacement or solution

  use mass of solution
• Hess's Law


  heat energy change is independent of route taken
• average bond enthalpy is the enthalpy change when one mole of bonds are broken in gaseous molecules
• energy profile diagrams

  A) exothermic

  B) endothermic

  
• energy profile

  A) endothermic

  B) reactants

  C) products

  D) activation energy

  E) enthalpy change

  
• energy profile

  A) exothermic

  B) reactants

  C) products

  D) enthalpy change

  E) activation energy

  
• activation energy 


  minimum energy which particles need to collide to start a reaction
• enthalpy change of reaction

  enthalpy change when the number of moles of reactants as specified in the balanced equation react together
• enthalpy change of neutralisation

  enthalpy change when solutions of an acid and an alkali react together under standard conditions to produce 1 mole of water
• if temp increases - reaction exothermic
• bond dissociation enthalpy 


  energy needed per mole to break attractive forces in bonds, only in gaseous compounds
• bond breaking

  • absorbs energy
  • endothermic
• bond making

  • releases energy
  • exothermic
• errors in calorimetry

  • energy losses from calorimeter
  • incomplete combustion
  • incomplete transfer of energy
  • evaporation of fuel after weighing
  • measurements not carried out under standard conditions