Thermochemical Equations

Thermochemical equations specify heat energy changes and states of the reactants and products.

Writing Conventions

1. The heat evolved or absorbed in a reaction is affected by the physical state of the reacting substances. Therefore, gaseous, liquid and solid states are represented by putting symbols (g), (l), and (s) along side the chemical formula respectively. For example, to represent burning of methane in oxygen,

CH₄ (g) + 2O₂ (g) → CO₂ (g) + 2H₂O (l) + heat

2. In case of elements which exhibit allotropy, the name of allotropic modification is mentioned. For example, C (graphite), C (diamond), etc.

3. The substances in aqueous solutions are specified using the symbol (aq). For example NaCl (aq) stands for an aqueous solution of sodium chloride.

4. Thermochemical equations may be balanced even by using fractional coefficients, if so required. The coefficients of the substances of the chemical equation indicate the number of moles of each substance involved in the reaction and the ΔH values given correspond to these quantities of substances.

5. In case the coefficients are multiplied or divided by a factor, ΔH value must also be multiplied or divided by the same factor. In such cases, the ΔH value will depend upon the coefficients.

H₂ (g) + ½O₂ (g) → H₂O (g) ; ΔH = –242 kJ

If coefficients are multiplied by 2

2H₂ (g) + O₂ (g) → 2H₂O (g) ; ΔH = –484 kJ