In thermodynamics we often need to distinguish between the system and surroundings; frequently people become confused about what the system is, what the surroundings are. Then, we start to talk about all the different types of systems! Arrrgh! Why should it matter, and who cares?

Well, as students of chemistry, it is important to care, so that when we talk about changes in thermodynamic quantities, we know what is happening. Learning the basics of thermodynamics in chemistry will lead to increased understanding of how these quantities are important in other fields, like biology, physiology, and exercise science. Finally, having a good understanding of thermodynamics helps you spot crackpot ideas, and write coherent letters to the editor (more on those later).

First, some definitions:

The Universe – everything.

The System – the part of the universe you are interested in studying. This might be a chemical reaction in a beaker, a candle flame, a cell membrane, or a cyclist.

The Surroundings – everything that isn’t part of the system.

Mathematically:

Universe = System + Surroundings

You may have already started to learn the laws of thermodynamics. The first law, for example, states that the energy of the universe is conserved. In other words, the universe has a constant amount of energy. Mathematically:

The total energy of the universe depends on the system and surroundings:

which means that

Energy can move between the system and surroundings, but what goes out of the system has to go into the surroundings and vice versa. This is where systems become important. The three that are used in thermodynamics are:

• Open systems – both matter (mass) and energy can move into and out of an open system. The earth, for example, is an open system. Energy comes in from the sun, and is re-radiated back to space. Meteors strike the Earth, and cause parts of the Earth to be ejected into space. We send satellites into interstellar space. These are difficult systems to study scientifically because there are too many variables to track.
• Closed systems – matter can’t be transferred from a closed system, but energy can. We study these frequently, and a great example is the ubiquitous “coffee cup” calorimeter used in many general chemistry laboratories. These are great ways to see how a system (some chemical reaction) causes a change in temperature in the surroundings (the solvent, often water) that is contained in the cup.
• Isolated systems – neither matter or energy can be transferred from these systems to the surroundings.

Recently I talked with one of my colleagues about a textbook that claims the flame of a candle is an open system, on the basis that the combustion reaction continues to pull oxygen from the surroundings (i.e. mass transfer) and that the candle radiates heat to the surroundings. I think this is an ok example of an open system, although I wouldn’t want to try to find out how much heat is released to the surroundings this way since I can’t really control the experiment well enough.

If I wanted to determine the heat content in a given candle, I’d rather do a bomb calorimetry experiment. In such an experiment, I could put a piece of the candle in the bomb (see picture below), seal the bomb, fill it with a high pressure of oxygen gas, place it in a water bath, ignite the contents of the bomb and measure the change in temperature of the surroundings. In this case, the candle would be “burned” in a closed system, where I could determine the change in energy of the combustion reaction with reasonable precision.

Ok, so about using thermodynamics to catch people who make strange claims. One argument that many people make to counter Darwin’s theory of evolution is that the second law of thermodynamics means that systems spontaneously move toward greater disorder, so that the evolution of something as complex as a human from less complex life forms is not possible.

People who make this claim do so on a limited understanding of thermodynamics. Isolated systems will tend toward greater disorder spontaneously. Open systems won’t. The Earth is an open system, with abundant energy input from the Sun, and mass input from the solar system. In this case, greater disorder is not the spontaneous pathway.

Oh, by the way, I really disagree with the idea of entropy being a measure of how messy your room is or is not. But that’s another post.