1.4 Energetics

• enthalpy change

  heat energy change measured under conditions of constant pressure
• standard conditions of enthalpy change

  100 kPa

  298 K
• enthalpy of formation

  e.c when 1 mole of a compound is formed from its elements in their standard states under standard conditions
• enthalpy of combustion

  e.c when 1 mole of a substance is completely burned in oxygen under standard conditions and its standard state
• enthalpy of reaction

  e.c when a reaction occurs in the molar quantities shown in the reaction, ...
• enthalpy of neautralisation

  e.c when an acid + alkali react to produce 1 mole of water
• bond enthalpy

  average energy needed to break or form a covalent bond in gaseous states, over many different molecules
• bond enthalpy = reactants - products
• bonds breaking - endo (need energy)

  bonds forming - exo (release energy)
• Hess' Law


  total enthalpy change for a reaction is independant of the route taken
• e.c of formation = products - reactants
• e.c of combustion = reactants - products
• why may experimental values of enthalpy differ from data book value?

  heat loss

  incomplete combustion