Original image from Andres Rueda

Sci-fi authors often deal with a really big problem: how to obtain all the energy needed by spaceships to move? How can people obtain longer lives, or immense supernatural powers? The answers are different: from the Warp Drives to the Midi-chlorians, to the Melange, to countless other means!
Of course, the main problem is that sci-fi technology needs a lot of energy, and how can we obtain such amounts of energy without creating a reliable, immense (and fake) source?
A bigger problem is: what is what we call “energy”? What should this word mean?
Energy is one of these “magic” words, which everyone uses, and which almost no one would try and explain. Why so? Under a certain point of view, the correct answer is: “I have no idea how to describe energy using one definition”.
With the name “energy” a good number of seemingly different concepts can be flagged: kinetic energy, for example, is the energy of a mass when in motion, while thermal energy characterizes a mass which temperature is above 0 K; potential gravitational energy belongs to all masses under the attraction of another mass, but electric potential energy refers to a system able to move electrons.
They clearly are very difficult to understand as a whole, using a simple definition; however, the reason for all of them being labeled as “energies” is the same: all of these quantities describe the possibility or the fact that the studied system is able to produce work or heat. Work and heat, on the other hand, are much simpler to define and understand!
So, an energy can be regarded as a “reservoir” from which the system can take what it needs to perform  work or to release heat.
Some years ago, students asked me for the main difference between a chemist and a physicist. My answer then was something like “the latter studies new things, the former creates new things”. After these years, I think I would  answer differently: “the latter is interested in work, the former in heat”.
The First Law of Thermodynamics (studied in Chapter 2 of the Physical chemistry textbook) is just a way to say that each and every energy transfer can happen via work or via heat. However, usually chemical reactions are carried out without production of work, and so the only transfer occurs via heat (produced or absorbed).
For this reason, in Paragraph 2.4 of the textbook,  a long explanation of concepts is needed to understand the relevance of the state function enthalpy, which simply means “heat produced by a system operating at fixed pressure”. And, since no work comes out or in, we can read the heat transferred as due to some energy “stored” in our molecules.
What kind of energy can our molecules hold? Well, their bonds, of course! So, enthalpy can give us information about how strong a chemical bond is, as well as how difficult it is to break it. And the enthalpy of a phase change can thus give us information even about intermolecular forces… not bad for something I had problem defining, right?
Image credit: Andres Rueda