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# Enthalpy

“Delta H represents change in enthalpy,
A chemic’lly convenient quantity.
Vessel’s open to air?
Find the Delta T there,
And thus heat transferred in pressure’s constancy.”

The April 16 limerick likely comes the closest of these April 2019 poems to a lecture explanation; I cover all these points “in prose” when I teach chapters on thermochemistry (the chemical bookkeeping of the quantities via which reactions absorb or release heat energy).

“Delta H represents change in enthalpy.”
The Greek letter delta represents a change in a type of function called a state function.  We can subtract the initial value (state) of a function from the final value (state) of that function to obtain the change in that function; altitude is a simple example.  The letter H represents the state function of enthalpy.

“A chemically convenient quantity.”
When I introduce enthalpy to my students, it is with a direct acknowledgement that many functions (as well as symbols and units more generally, as discussed in Andy Weir’s excellent book The Martian) throughout history were derived to provide convenient shorthands for a group of chemists working at a given time.  Generations later, students are tasked with making sense of this polyglot science!

Here, enthalpy is the heat energy transferred at constant pressure. Most chemistry lab work is done at constant (atmospheric) pressure; heat energy is particularly convenient to monitor, via temperature changes. Enthalpy is itself a state function, while heat energy is a path function (involving different types of calculations).  Equating the two under constant-pressure conditions opens the door to many useful concepts and calculations.

“Vessel’s open to air?/  Find the Delta T there./
And thus heat transferred in pressure’s constancy.”
Are we running an experiment in glassware (also called a “reaction vessel”) that is open to the air?   We almost always are, when in a chemistry lab, and so the reaction in question is at constant pressure.

We can infer the change in heat energy for the reaction from the temperature change (Delta T) in the solution in which it occurs… and thus the change in enthalpy, our “convenient quantity.”