DF 4 

==Energy density (kjkg^-1) === enthalpy change of combustion times moles in kg

Bonds being broken – takes in energy (endothermic)
Bonds being made – releases energy (exothermic)

==Bond enthalpy === the energy needed to break one mole of a bond to give separate atoms all in the gaseous state. The units are kilajoules per mole

• The more energy needed the stronger the bond
• The stronger the bond, the closer the atoms will be to each other = the shorter the bond length
• Bond enthalpy’s are normally measuring using hess’ cycles as they are hard to measure directly
• Bond enthalpy’s are average values from several compounds and they assume all molecules to be in gaseous state
  ==Endothermic === the energy needed to break the bonds is greater than the energy realeased when the new bonds are made
  ==Exothermic === the energy needed to break the bonds is less than the energy released when new bonds are made

==Rules to work out the enthalpy change of a reaction –==

• Count the number of each bond in a balances symbol equation
• Work out the total energy of the bonds breaking
• Work out the total energy of the bonds forming
• Determine the energy change by (bonds breaking – bonds forming)