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Chemistry · Ch 4 — Chemical Thermodynamics

Standard enthalpy of formation (ΔfH0)

4.10.5

Standard enthalpy of formation (ΔfH0)

Consider

H2(g)+12 O2(g)⟶H2O(l),ΔrH0=−286 kJ\mathrm{H_2(g)} + \frac{1}{2}\,\mathrm{O_2(g)} \longrightarrow \mathrm{H_2O}(l), \quad \Delta_r H^0 = -286\ \mathrm{kJ}

For this reaction, where one mole of liquid water in its standard state is formed from H2_2 and O2_2 gases in their standard states, the enthalpy change of the reaction is the standard enthalpy of formation of water: ΔfH\Delta_f H of water is -286 kJ mol−1^{-1}.

The standard enthalpy of formation of a compound is the enthalpy change that accompanies a reaction in which one mole of pure compound in its standard state is formed from its elements in their standard states.

The formation of one mole of CH4_4 in its standard state from the elements carbon and hydrogen in their standard states is represented by

C(graphite)+2H2(g)⟶CH4(g),ΔrH0=−74.8 kJ\mathrm{C_{(graphite)}} + 2\mathrm{H_2(g)} \longrightarrow \mathrm{CH_4(g)}, \quad \Delta_r H^0 = -74.8\ \mathrm{kJ}

or ΔfH0 (CH4)=−74.8\Delta_f H^0\,(\mathrm{CH_4}) = -74.8 kJ mol−1^{-1}.

Note

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