AQA A Level Chemistry复习笔记1.6.6 Applications of Hess’s Law

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 (ΔHatꝋ ) is the enthalpy change when one mole of gaseous atoms is formed from its elements under standard conditions.
  • Eg. ΔHatꝋ [H2] relates to the equation:

   

½ H2(g) → H(g)

Calculating average C-H bond energyCalculate the average bond energy of the C-H bond using the relevant ΔHfꝋ and ΔHatꝋ 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.