Enthalpy (
\Delta
H) is equal to heat (q) at constant pressure. Under conditions where pressure
is constant and our system does not undergo changes in volume, enthalpy is
equal to internal energy. Enthalpy is easily measured experimentally (we will
study calorimetry at the end of the term) and is a common quantity you will
see. When heat
exits
a system (loses heat), we define that as
exothermic
. When heat
enters
a system (gains heat), we
define the process as
endothermic
. Let's look at an example you are
familiar with. When your body sweats, the sweat will evaporate from your skin.
Thinking of the sweat as the system, you have to put energy in for evaporation
to take place. As a result, heat was transferred from the surroundings (your
body) and you feel cooler! In a similar way, if you were to place your arm
over a pot of boiling water, the water vapor (gas) would condense on your
skin. This process would cause heat to exit the system (water molecules) and
release heat to the surroundings (you!) likely causing a burn (ouch!).