Energy cycles in reactions

bond enthalpy

• bond breaking is endothermic as it requires the input of energy

  • energy required to break a bond is called bond energy/enthalpy

• bond formation releases energy, so is exothermic

• amnt of energy released when bond is formed has same magnitude as energy taken in when bond is broken but with opposite sign

• bond enthalpy is the enthalpy change for this process:

  • X-Y(g) → X(g) + Y(g)

enthalpy change from bond enthalpies

ΔH^ꝋ = enthalpies for bonds broken - enthalpies for bonds formed

overall enthalpy changes

• if more energy released when new bonds formed than energy taken in to break bonds, it’s exothermic, products more stable than reactants

• if more energy required to break bonds than energy released when new bonds are formed, it’s endothermic, products less stable than reactants

average bond energy

• energy needed to break one mole of bonds in a gaseous molecule averaged over similar compounds

• limitation:

  • bond energies are affected by the environment

  • so energy of a particular bond will vary in different compounds

  • an average measure isn’t very accurate

• to calculate:

  • take bond enthalpy for the whole molecule and divide by number of bonds

hess’s law

enthalpy change for a reaction is independent of the pathway between the initial and final states

enthalpy change of formation

• elements placed at the bottom of hess cycle

• arrows pointing up

enthalpy change of combustion

• combustion products always at bottom of hess cycle

• arrows pointing down

born haber cycles

• enables to calculate lattice enthalpy for ionic compounds

• shows all the steps needed to turn atoms into gaseos ions, and then into ionic lattice

• alternative route begins from enthalpy of formation

drawing born haber cycles

1. start with elements and their state symbols

2. turn these into gaseous atoms

   • this is a bond breaking process, so arrows drawn up

   • this is called enthalpy of atomisation

3. create gaseous ions

   • metal loses electrons, so this is first ionisation energy

     • arrow goes up because endothermic

   • non-metal gains electrons, so this is electron affinity

     • arrow goes down because exothermic

4. complete cycle

   • enthalpy of formation is added at the bottom of the diagram, above/below the elements

     • this can be exothermic or endothermic, so arrow will change direction depending on compound

   • lattice enthalpy is the change from ionic solid to gaseous ions, so arrow will point up

general born haber cycle equation