D17.3 Enthalpy

Chemists ordinarily use a property known as enthalpy (H) to describe energy transfers that accompany chemical and physical processes. Enthalpy is closely related to the total energy of all molecules in a sample of matter. Enthalpy increases when total energy increases; enthalpy decreases when total energy decreases. Enthalpy values for specific substances cannot be measured directly. Only enthalpy changes for chemical or physical processes can be determined.

The enthalpy change (ΔH) for a physical or chemical process that takes place at constant pressure is equal to the heat transfer of energy:

For example, the heat transfer of energy to the surroundings when you operate a Bunsen burner is equal to the enthalpy change of the methane combustion reaction that takes place, because the reaction occurs at the essentially constant pressure of the atmosphere. Chemists usually perform experiments under normal atmospheric conditions, at constant external pressure with q = ΔH, which makes ΔH the most convenient choice for comparing energies of reactants and products of chemical reactions.

Consider the process of phase change we explored earlier. On the molecular level, two argon atoms at 377 pm apart require 1.16 kJ/mol to be pulled apart. On the macroscopic level, a sample of liquid argon (for instance, 100 mL of liquid argon) has an enthalpy of vaporization (ΔH_vaporization) of 6.4 kJ/mol. This is because each argon atom is surrounded by many other argon atoms in the liquid phase, at various distances apart, and all of those LDFs need to be overcome to bring the argon atoms into the gas phase. Therefore, on average, it takes 6.4 kJ/mol of energy to vaporize a sample of liquid argon.

Enthalpy of vaporization varies with the overall strength of intermolecular interactions. For example, propane (CH₃CH₂CH₃) has ΔH_vap = 19.04 kJ/mol, while dimethyl ether (CH₃OCH₃) has a higher ΔH_vap = 21.51 kJ/mol, and ethanol (CH₃CH₂OH) has an even higher ΔH_vap = 38.56 kJ/mol. Therefore, similar to boiling points, we can use enthalpy of vaporization to estimate the strength of intermolecular attractions.