Physical Chemistry – Overview of Thermodynamics
Song: the Laws of Thermodynamics by Flanders and Swann:
The challenge of teaching thermodynamics to physical chemistry life science students is to have them understand the relationships between the macroscopic properties involving heat, work, energy and entropy. After dispelling the myth that energy is stored in chemical bonds; after introducing the concept of temperature, and contrasting it to heat capacity; and after doing some introductory examples, I give an overview of the Laws of thermodynamics. The purpose is to encapsulate the ideas in simple terms in an effort to dispel angst.
In one example I use bond energies to calculate the energy per mole of sucrose and TNT (the explosive trinitrotoluene). Most students expect that TNT has more energy, but it turns out the two have about the same. So why is TNT an explosive (actually a detonation)? TNT burns rapidly and involves a huge volume change. It is the rate of reaction (chemical kinetics) and the rapid volume change that causes the explosive damage. Then I can move to the thermodynamics overview.
In fact these ideas are readily accepted by the students but that is not the issue. The issue is to translate these into useful mathematical representations so they can be applied to their particular area of interest. Since basically all the equations of thermodynamics can be concisely can be placed on a page, the problem is those equations take some time to appreciate.
From Chapter 6 of Laidler, Meiser and Sanctuary, Physical Chemistry.
I do NOT show students this at first. (click to enlarge)
“Thermodynamics”, I say, “is a macroscopic theory that gives no numbers. All it gives is the relationship between quantities we can measure under different conditions. The numbers come from comparison with experimental data.”
Of course this is the problem: too many conditions. I refrain from talking about reversible and irreversible at this stage and contrast two common cases:
- Bench top experiment at constant temperature and pressure: a reaction occurs in a lab on a table. Hence the temperature is room temperature and the pressure is the atmospheric pressure.
- Bomb calorimetric experiment at constant temperature and volume: a reaction occurs in an apparatus that has constant volume because the volume is fixed. Since the apparatus is immersed in a heat bath, the temperature is constant.
I think this makes sense to them: I show pictures and animations.
Now I say that thermo is concise and is summarized in four laws:
- The Zeroth Law of thermodynamics: thermal equilibrium.
- The First Law of thermodynamics: conservation of energy
- The Second Law of thermodynamics: heat won’t flow uphill
- The Third Law of thermodynamics: at absolute zero of temperature, the entropy of a perfect crystal is zero.
The Zeroth Law of thermodynamics:
When two bodies are in thermal contact their temperature is the same and no heat flows between them.
Well that is easy.
The First Law is conservation of energy,
Now I mention the system, which is the engine in this case, and the surroundings, a hot heat source and the cold outdoors. Heat flows into the engine from the hot reservoir (we pay for that heat), and the engine produces some work and the rest is ejected into the cold reservoir.
This introduces the sign of energy. When energy goes into the engine it is positive and when it comes out the signs are negative, so the energy balance is
qh = qc +w
The First Law concept is also easy to grasp.
The Second Law says heat won’t flow uphill
Whereas the First Law is about energy, the Second Law is about entropy. Entropy is a measure of the randomness of a system. It is a substance as tangible as energy.
Examples of entropy of substances
The Third Law says that at zero K a perfectly ordered crystal has entropy of zero.
When we measure the energy of a substance, we take the difference between the start and finish of a process. This means there is no absolute energy. In contrast entropy does have an absolute value. If a substance is cooled to absolute zero of temperature, (-273 C) then there is no motion and so a crystal has only one state. That is, if the position of one atom in the crystal is known, then the position of all the others is known. A system with one state only has zero entropy.
The Second and Third Laws involve entropy, and are more challenging conceptually than the Zeroth and First Laws.
I state that this overview encapsulates what we will study for two months.