Reactivity 1.2 – Energy cycles in reactions

R1.2.3 – Standard enthalpy changes of combustion and formation (HL only)

📌 Combustion

  • The standard enthalpy of combustion is defined as the enthalpy change occurring when one mole of a substance burns completely
  • The symbol for standard enthalpy of combustion is  ΔHc° 
  • The thermochemical equation of complete combustion will always involve oxygen as a reactant and carbon dioxide and water as the only products
  • The equation used for these calculations is given below

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

🧠 formulas and values for calcualtions are given in sections 1 and 14 of the data booklet

📌 Formation

  • The standard enthalpy of formation is defined as the enthalpy change occurring when one mole of a substance forms from it’s constituent elements in standard states
  • The symbol for standard enthalpy of formation is ΔHf°
  • Enthalpy of formation is used to determine the enthalpy change in both hypothetical and real reactions
  • The equation used for these calculations is given below

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

⭐️ remember that the products of an equation involving the enthalpy of formations MUST form only one mole of the product. This can cause fractional coefficients in the reactants.