Enthalpy (H) is the heat flow in or out of a system at constant pressure (i.e., carried out in open containers at or near atmospheric pressure):

where E = energy, P = pressure, and V = volume. Enthalpy depends on the amount of substance present.

Change in enthalpy (DH) is measured because the enthalpy of a substance is impossible to determine:

What is the relationship between the change in energy and enthalpy? Energy transfer occurs as heat when little or no PDV work gets done. This is the case in three scenarios:

	Reactions that do not involve gases (done at constant pressure with little or no volume change)
	Reactions in which the number of moles of gas does not change (when Dn = 0 then DV = 0)
	Reaction in which the volume (i.e., moles) changes but the work is negligible compared to the heat