4.13: Standard Heat of Formation

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The Hope diamond, a form of carbon, is a very expensive piece of jewelry—currently worth about \$350,000,000. A graphite pencil, comprised partly of carbon, can be purchased for less than a dollar. Both items contain carbon, but there is a big difference in how that carbon is organized. The diamond was formed under very different reaction conditions than the graphite, so it has a different heat of formation.

Standard Heat of Formation

A relatively straightforward chemical reaction is one in which elements are combined to form a compound. Sodium and chlorine react to form sodium chloride. Hydrogen and oxygen combine to form water. Like other reactions, these are accompanied by either the absorption or release of heat. The standard heat of formation $$\left( \Delta H^\text{o}_\text{f} \right)$$ is the enthalpy change associated with the formation of one mole of a compound from its elements in their standard states. The standard conditions for thermochemistry are $$25^\text{o} \text{C}$$ and $$101.3 \: \text{kPa}$$. Therefore, the standard state of an element is its state at $$25^\text{o} \text{C}$$ and $$101.3 \: \text{kPa}$$. For example, iron is a solid, bromine is a liquid, and oxygen is a gas under those conditions. The standard heat of formation of an element in its standard state by definition is equal to zero. The $$\Delta H^\text{o}_\text{f} = 0$$ for the diatomic elements, $$\ce{H_2} \left( g \right)$$, $$\ce{N_2} \left( g \right)$$, $$\ce{O_2} \left( g \right)$$, $$\ce{F_2} \left( g \right)$$, $$\ce{Cl_2} \left( g \right)$$, $$\ce{Br_2} \left( g \right)$$, and $$\ce{I_2} \left( g \right)$$. The graphite form of solid carbon is in its standard state with $$\Delta H^\text{o}_\text{f} = 0$$, while diamond is not its standard state. Some standard heats of formation are listed in the table below.

Standard Heats of Formation of Selected Substances
Table $$\PageIndex{1}$$: Standard Heats of Formation of Selected Substances
Substance $$\Delta H^\text{o}_\text{f}$$ $$\left( \text{kJ/mol} \right)$$ Substance $$\Delta H^\text{o}_\text{f}$$ $$\left( \text{kJ/mol} \right)$$
$$\ce{Al_2O_3} \left( s \right)$$ -1669.8 $$\ce{H_2O_2} \left( l \right)$$ -187.6
$$\ce{BaCl_2} \left( s \right)$$ -860.1 $$\ce{KCl} \left( s \right)$$ -435.87
$$\ce{Br_2} \left( g \right)$$ 30.91 $$\ce{NH_3} \left( g \right)$$ -46.3
$$\ce{C} \left( s, graphite \right)$$ 0 $$\ce{NO} \left( g \right)$$ 90.4
$$\ce{C} \left( s, diamond \right)$$ 1.90 $$\ce{NO_2} \left( g \right)$$ 33.85
$$\ce{CH_4} \left( g \right)$$ -74.85 $$\ce{NaCl} \left( s \right)$$ -411.0
$$\ce{C_2H_5OH} \left( l \right)$$ -276.98 $$\ce{O_3} \left( g \right)$$ 142.2
$$\ce{CO} \left( g \right)$$ -110.5 $$\ce{P} \left( s, white \right)$$ 0
$$\ce{CO_2} \left( g \right)$$ -393.5 $$\ce{P} \left( s, red \right)$$ -18.4
$$\ce{CaO} \left( s \right)$$ -635.6 $$\ce{PbO} \left( s \right)$$ -217.86
$$\ce{CaCO_3} \left( s \right)$$ -1206.9 $$\ce{S} \left( rhombic \right)$$ 0
$$\ce{HCl} \left( g \right)$$ -92.3 $$\ce{S} \left( monoclinic \right)$$ 0.30
$$\ce{CuO} \left( s \right)$$ -155.2 $$\ce{SO_2} \left( g \right)$$ -296.1
$$\ce{CuSO_4} \left( s \right)$$ -769.86 $$\ce{SO_3} \left( g \right)$$ -395.2
$$\ce{Fe_2O_3} \left( s \right)$$ -822.2 $$\ce{H_2S} \left( s \right)$$ -20.15
$$\ce{H_2O} \left( g \right)$$ -241.8 $$\ce{SiO_2} \left( s \right)$$ -859.3
$$\ce{H_2O} \left( l \right)$$ -285.8 $$\ce{ZnCl_2} \left( s \right)$$ -415.89

Summary

• The standard heat of formation $$\left( \Delta H^\text{o}_\text{f} \right)$$ is the enthalpy change associated with the formation of one mole of a compound from its elements in their standard states.
• The standard conditions for thermochemistry are $$25^\text{o} \text{C}$$ and $$101.3 \: \text{kPa}$$.

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