Calculating the enthalpy change of formation of ethaneCalculate ΔHf?[ethane]. The relevant change in standard enthalpy of combustion (ΔHc) values are shown in the table below:

Answer

Step 1:?Write the equation for enthalpy change of formation at the top and add oxygen on both sides

Step 2:?Draw the cycle with the combustion products at the bottom

Step 3:?Draw all arrows in the correct direction

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)

Calculating average C-H bond energyCalculate the average bond energy of the C-H bond using the relevant ΔHf??and?ΔHat??values in the table below:

Answer

Step 1:?Write down the equation for the dissociation of methane at the top

Step 2:?Write down the elements at the bottom

Step 3:?Draw all arrows in the correct direction

Step 4:?Apply Hess’s Law

Step 5:?Since there are 4 C-H bonds in methane:

Remember to take into account the number of moles of each reactant and product.For example, there are two moles of NaHCO3(s) so the ΔHf?value is multiplied by 2.