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Enthalpy of Formation

Chemistry: IB Diploma Higher Level, topic 4.2.3

Enthalpy of Formation

A standard enthalpy of formation Δ𝐻f° is an enthalpy change for a reaction in which exactly 1 mole of a pure substance is formed from free elements in their most stable states under standard state conditions.

Δ𝐻f°

Δ𝐻f°

  • Δ𝐻f° is useful for computing or predicting enthalpy changes for chemical reactions that are impractical or dangerous to carry out.
  • If we have values for the appropriate standard enthalpies of formation, we can determine the enthalpy change for any reaction.
  • We will practice in the next section on Hess’s law.
Example

Example

  • The standard enthalpy of formation of CO2(g) is −393.5 kJ/mol.
  • This is the enthalpy change for the exothermic reaction:
  • C(s)+O2(g)⟶CO2(g)         Δ𝐻f° =Δ𝐻°=−393.5kJ
  • In this reaction:
  • The reactants are at a pressure of 1 atm and 25 °C .
  • Carbon is present as graphite, the most stable form of carbon under these conditions.
  • It ends with one mole of CO2, also at 1 atm and 25 °C.

Test yourself on Enthalpy of Formation

  • Which of the following shows the correct equation for the enthalpy change of formation of the gas NO2?

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