5.3 Enthalpy  (Page 6/25)

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Standard enthalpy of formation

A standard enthalpy of formation $\text{Δ}{H}_{\text{f}}^{°}$ 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. These values are especially useful for computing or predicting enthalpy changes for chemical reactions that are impractical or dangerous to carry out, or for processes for which it is difficult to make measurements. If we have values for the appropriate standard enthalpies of formation, we can determine the enthalpy change for any reaction, which we will practice in the next section on Hess’s law.

The standard enthalpy of formation of CO ₂ ( g ) is −393.5 kJ/mol. This is the enthalpy change for the exothermic reaction:

starting with the reactants at a pressure of 1 atm and 25 °C (with the carbon present as graphite, the most stable form of carbon under these conditions) and ending with one mole of CO ₂ , also at 1 atm and 25 °C. For nitrogen dioxide, NO ₂ ( g ), $\text{Δ}{H}_{\text{f}}^{°}$ is 33.2 kJ/mol. This is the enthalpy change for the reaction:

A reaction equation with $\frac{1}{2}$ mole of N ₂ and 1 mole of O ₂ is correct in this case because the standard enthalpy of formation always refers to 1 mole of product, NO ₂ ( g ).

You will find a table of standard enthalpies of formation of many common substances in Appendix G . These values indicate that formation reactions range from highly exothermic (such as −2984 kJ/mol for the formation of P ₄ O ₁₀ ) to strongly endothermic (such as +226.7 kJ/mol for the formation of acetylene, C ₂ H ₂ ). By definition, the standard enthalpy of formation of an element in its most stable form is equal to zero under standard conditions, which is 1 atm for gases and 1 M for solutions.