Calculating ΔHr?from ΔHf?using Hess’s Law energy cycles

• The products can be directly formed from the elements =?ΔH2

OR

• The products can be indirectly formed from the elements =?ΔH1?+ ΔHr

The enthalpy change from elements to products (direct route) is equal to the enthalpy change of elements forming reactants and then products (indirect route)

• Equation

ΔH2?= ΔH1?+ ΔHr

Therefore,

ΔHr?= ΔH2?– ΔH1

Answer

Step 1:?Write the balanced equation at the top

Step 2:?Draw the cycle with the elements at the bottom

Step 3:?Draw in all arrows, making sure they go in the correct directions. Write the standard enthalpy of formations

Step 4:?Apply Hess’s Law

Calculating average bond energies using Hess's cycles

• Bond energies cannot be found directly so enthalpy cycles are used to find the?average bond energy
• This can be done using enthalpy changes of?atomisation?and?combustion?or?formation
• The?enthalpy change of?atomisation?(ΔHat??) is the enthalpy change when?one mole of gaseous?atoms is?formed?from its elements under standard conditions.
  • Eg.?ΔHat??[H2] relates to the equation:

   

? H2(g) → H(g)