Thermo Speak

When you dive into the study of thermodynamics you need to get the lingo down - pretty much like any discipline. Plus the rule book. So lets get the ball rolling here.

1st Law of Thermodynamics

Energy can neither be created nor destroyed - only changed in form.

Or you could say the energy of the universe is constant... or energy is always conserved. We will adhere to this law.

2nd Law of Thermodynamics

Any spontaneous* change is accompanied by an increase in the entropy of the universe.

So entropy (symbol S) is a measure of the dispersal of energy. The driving force for all spontaneous processes is that energy is dispersing some way, somehow. Contrast that with energy concentrating or pulling itself together into a smaller and smaller space. That never happens - concentration of energy all by itself with no help from an outside force. This is the essence of the 2nd Law. The BEST way for us to obey the second law is to realize that it tends to be in full force when your products are in a lower energy state than your reactants. This is another way of saying that exothermic processes (\(-\Delta H\)) tend to be more spontaneous than endothermic processes (\(+\Delta H\)).

* a spontaneous change in terms of thermodynamics, is any process than tends to occur on its own without any external forces or aide of another entity. Many spontaneous changes are easy to spot with your common sense gene. An object knocked off the table will fall to the floor (gravity). Water flows downhill. Hitting a piggy bank with a hammer smashes it to pieces. Note how none of those examples ever spontaneously go in the reverse direction.

System, Surroundings, and the Universe

Diagram showing open, closed, and isolated systems.

The system is what you are studying or focusing on. In a chemistry class, the system is typically all the reactants and products of a chemical reaction. The surroundings are everything else around that is not the system. Although the immediate surroundings are the most important, ALL the surroundings is in the definition. The universe is just like you've been taught - it is everything we know to exist from here to infinity and beyond. So do the math and you get

universe = system + surroundings

What is critically important here is that you realize #1 - the universe has constant energy (1st Law), and #2 is that all energy is exchanged between the system and the surroundings. If one goes up, the other goes down and vice versa. We will mostly be concerned with heat flow. So any heat loss by the system is a heat gain by the surroundings. The quantities will always be identical but opposite in mathematical signs.

Open, Closed, and Isolated Systems

The diagram below shows the three types of systems that we study. The differences are in what is exchanged between the system and the surroundings. An open system allows both matter and energy to freely flow from system to surroundings and vice versa. A closed system prevents any matter from transferring but does allow energy to flow across the boundary. Finally, an isolated system allows neither matter nor energy to exchange. On that last topic - scientists refer to the universe as an isolated system, nothing gets in, and nothing gets out.

The most important system type for us as we try to measure the energy in/out of chemical reactions is the closed system. This is what calorimetry is based on. The heat leaving the system is "caught" by the surroundings and we measure it. We then logically reason that the amount that entered the surroundings is equal to the amount that left the system.

system surroundings and universe diagram

Endothermic / Exothermic

You'll see this again but here it is up front.

  • An endothermic process is one in which heat is absorbed by the system (heat enters).
  • An exothermic process is one in which heat is released by the system (heat exits).

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