Laws of Thermochemistry

Related Resources • Worked Chemistry Problems • Endothermic & Exothermic Reactions • Heats of Formation - Compounds • Heats of Formation - Ions • Physical Chemistry • Stoichiometry

Thermochemical equations are just like other balanced equations except they also specify the heat flow for the reaction. The heat flow is listed to the right of the equation using the symbol DH. The most common units are kilojoules, kJ. Here are two thermochemical equations:

H₂(g) + 1/2 O₂(g) --> H₂O(l); DH = -285.8 kJ

HgO(s) --> Hg(l) + 1/2 O₂(g); DH = +90.7 kJ

When you write thermochemical equations, be sure to keep the following points in mind:

1. Coefficients refer to the number of moles. Thus, for the first equation, -282.8 kJ is the DH when 1 mol of H₂O(l) is formed from 1 mol H₂(g) and 1/2 mol O₂.

2. Enthalpy changes for a phase change, so the enthalpy of a substance depends on whether is it is a solid, liquid, or gas. Be sure to specify the phase of the reactants and products using (s), (l), or (g) and be sure to look up the correct DH from heat of formation tables. The symbol (aq) is used for species in water (aqueous) solution.

3. The enthalpy of a substance depends upon temperature. Ideally, you should specify the temperature at which a reaction is carried out. When you look at a table of heats of formation, notice that the temperature of the DH is given. For homework problems, and unless otherwise specified, temperature is assumed to be 25°C. In the real world, temperature may different and thermochemical calculations can be more difficult.