# Standard Heat of Formation

• Define standard heat of formation.
• List some factors affecting the standard heat of formation.

Is the Hope diamond just a very expensive pencil?

The Hope diamond is a very expensive piece of jewelry, currently worth about \$350 million. A pencil 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 (see video below). 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   $(Delta H{_f}^circ)$ 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°C and 101.3 kPa. Therefore, the standard state of an element is its state at 25°C and 101.3 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 is by definition equal to zero. The  $Delta H{_f}^circ=0$ for the diatomic elements, H 2 (g), N 2 (g), O 2 (g), F 2 (g), Cl 2 (g), Br 2 (l), and I 2 (g). The graphite form of solid carbon is its standard state with $Delta H{_f}^circ = 0$ , while diamond is not its standard state. Some standard heats of formation are listed in the Table below.

 Substance ΔHfº (kJ/mol) Substance ΔHfº (kJ/mol) Al2O3(s) -1669.8 H2O2(l) -187.6 BaCl2(s) -860.1 KCl(s) -435.87 Br2(g) 30.91 NH3(g) -46.3 C (s, graphite) 0 NO(g) 90.4 C (s, diamond) 1.90 NO2(g) 33.85 CH4(g) -74.85 NaCl -411.0 C2H5OH(l) -276.98 O3(g) 142.2 CO(g) -110.5 P(s, white) 0 CO2(g) -393.5 P(s, red) -18.4 CaO(s) -635.6 PbO(s) -217.86 CaCO3(s) -1206.9 S(rhombic) 0 HCl(g) -92.3 S(monoclinic) 0.30 CuO(s) -155.2 SO2(g) -296.1 CuSO4(s) -769.86 SO3(g) -395.2 Fe2O3(s) -822.2 H2S(g) -20.15 H2O(g) -241.8 SiO2 -859.3 H2O(l) -285.8 ZnCl2 -415.89

Watch a video of the reaction between sodium metal and chlorine gas

#### Summary

• The standard heat of formation is defined.