Energetics

• ΔH = Σbonds broken - Σbonds made
• Exothermic = negative enthalpy

  Endothermic = positive enthalpy
• Standard enthalpy conditions

  100kpa, 298k
• define enthalpy of formation/ΔHfθ

  Enthalpy change when one mole of a substance is formed from it’s pure elements under standard conditions
• define standard enthalpy of combustion/ΔHcθ

  Enthalpy change when one mole of a substance is burned completely in oxygen under standard conditions.
• Define mean bond enthalpy

  the amount of energy required to break one mole of a specific covalent bond in the gas phase
• Define hess’s law 


  Overall enthalpy change for a reaction is independent of the route a reaction takes.
• Vector method

  
• Calculation method

  
• $Q=$$mc\Delta t$


  Q = energy released (j)

  m = mass of water (g)

  c = specific heat capacity / constant (4.18)

  Δt = temperature change (°c/k)
• Example of Q=mcΔt

  
• enthalpy change

  • heat change at constant pressure