Reactivity 1.2 – Energy cycles in reactions

R1.2.4 – Calculating enthalpy changes

📌 Using enthalpies of combustion

ΔH°= ∑(ΔHc° reactants) − ∑(ΔHc°products)

  • This equation is easy to use when given the enthalpies of combustion to find the enthalpy change of combustion
  • When given the enthalpies of combustion and asked to find the enthalpy change of formation the following steps are used
  1. Write down the equation corresponding to the enthalpy of formation of the compound
  2. Use the formula booklet to determine the enthalpies of combustion for each compound
  3. Multiply the enthalpy by the number of moles in a balanced reaction
  4. Use the formula given above

EXAMPLE :

Calculate the enthalpy of formation of glucose using the enthalpies given in section 14 of the data booklet

  1. 6C + 6H2 + 3O2 → (1)C6H12O2
  2. [6(-394) + 6(-286) + 3(0)] – [1(-2803)] = -1277 kJ mol-1

Therefore, the enthalpy of formation of one mole of glucose is -1277 kJ mol-1 (using the enthalpy of combustion)

📌 Using enthalpies of formation

ΔH°= ∑(ΔHf° products) − ∑(ΔHf°reactants)

  • Similarly to the example given above, the enthalpy of formation can also be used to determine the enthalpy of combustion of a
  • When given the enthalpies of combustion and asked to find the enthalpy change of formation the following steps are used
  1. Write down the equation corresponding to the combustion of the compound
  2. Use the formula booklet to determine the enthalpies of formation for each compound
  3. Multiply the enthalpy by the number of moles in a balanced reaction
  4. Use the formula given above

EXAMPLE :

Calculate the enthalpy of combustion of butane using the enthalpies given in section 13 of the data booklet

  1. 2C4H10 + 13O2→ 8CO2 + 10H2O
  2. [8(-394) + 10(-286)] – [2(−126) + 12(0)] = -5760 kJ mol-1

Therefore, the enthalpy of combustion of butane is -5760 kJ mol-1(using the enthalpy of formation)