What does the term enthalpy really mean and what is its physical interpretation? What does the wikipedia definition of enthalpy mean, which says that $pV$ work in the enthalpy definition is the energy required to place the system in its surroundings? And why is the heat of a reaction measured in enthalpies and not internal energy?
[Physics] What exactly is enthalpy
thermodynamics
Related Solutions
In an isothermal process, as you say, the temperature of the system doesn't change. If your question is "why?" the answer is simple: by definition.
If your question is "how?", then there are two possible answers. The first is it doesn't matter, whatever process you come up with, which makes the temperature constant, will be called isothermal.
A more useful answer, however is that typically this applies to systems which are in contact with a large heat reservoir. So, the "condition" that you mention is given by the zeroth law of thermodynamics, which imposes thermal equilibrium of systems in contact with each other. To ensure this happens during the whole process, the process has to be slow enough to guarantee the equilibrium is reached at all times.
A brilliant analogy by Daniel Schroeder:
To summon a rabbit the magician must "build" it with all the energy it consists of. He must provide its internal energy $U$.
But first he must push away all the air that is in the way. This requires some work, $W=pV$. In total, the energy he must spend is $U+pV$. Let's call that enthalpy $H$:
$$H=U+pV.$$
- But the surroundings might help him out a bit. The warm air might provide some energy, while he is working on the summoning, by adding heat $Q=TS$. The only energy he actually has to spend himself is therefore $U+pV-TS$. Let's call this the free energy needed, or Gibbs free energy $G$:
$$G=H-TS.$$
Best Answer
Enthalpy is all energy associated with something at its current position; it is internal energy and the work (found as $pV$) required to place it where ever it is.
$$H=U+pV$$
(Thanks, @user115350, for link to this illustration.)
It is a useful parameter when you for example compare different parts in a pipe system. A system with flow through pipes is e.g. What we have in the back of our fridge, or in a power plant etc. Enthalpy takes into account not only the energy stored in each part/point of the fluid in this moment, but also the energy required to "place" it there.
If there are no pumps or turbines etc. along the way, the enthalpy is therefore constant within a pipe even if the pipe narrows in and pressure and volume change.
However, in many textbooks I've read the reason for defining this parameter is out of convenience, since internal energy plus $pV$ is a very commonly used expression in energy systems. But I personally like to think of it in the above way for a intuitive picture.