Enthalpy Versus Internal Energy
The U term can be interpreted as the energy required to create the system, and the pV term as the energy that would be required to "make room" for the system if the pressure of the environment remained constant. When a system, for example, n moles of a gas of volume V at pressure p and temperature T, is created or brought to its present state from absolute zero, energy must be supplied equal to its internal energy U plus pV, where pV is the work done in pushing against the ambient (atmospheric) pressure.
In basic physics and statistical mechanics it may be more interesting to study the internal properties of the system and therefore the internal energy is used. In basic chemistry scientists are typically interested in experiments conducted at atmospheric pressure, and for reaction energy calculations they care about the total energy in such conditions, and therefore typically need to use H. Furthermore the enthalpy is the workhorse of engineering thermodynamics as we will see later.
Read more about this topic: Enthalpy
Famous quotes containing the words internal and/or energy:
“Even if fathers are more benignly helpful, and even if they spend time with us teaching us what they know, rarely do they tell us what they feel. They stand apart emotionally: strong perhaps, maybe caring in a nonverbal, implicit way; but their internal world remains mysterious, unseen, “What are they really like?” we ask ourselves. “What do they feel about us, about the world, about themselves?””
—Augustus Y. Napier (20th century)
“While the State becomes inflated and hypertrophied in order to obtain a firm enough grip upon individuals, but without succeeding, the latter, without mutual relationships, tumble over one another like so many liquid molecules, encountering no central energy to retain, fix and organize them.”
—Emile Durkheim (1858–1917)