Hess's Law

• Hess' Law


  The enthalpy change of a reaction is independent of the route taken.
• If the data is enthalpy of formation then:

  enthalpy of reaction = products - reactants
• If data is enthalpy of combustion then:

  enthalpy of reaction = reactants - products
• Enthalpy of formation


  Enthalpy needed for 1 mol of product to be formed from its pure elements under standard conditions
• Enthalpy of combustion 


  Enthalpy needed to burn 1 mole of fuel in pure oxygen under standard conditions
• Standard conditions are at room temperature (298K) and atmospheric pressure (100kPa)
• Q = m c ΔT
• ΔH = -Q / n