Tutorials

The Laws of Thermodynamics

thermodynamics figure

Core Concepts

There are three fundamental laws of thermodynamics, which deal with the movement and transfer of energy. Let’s learn about them in this tutorial.

What is a System?

Before delving into the three laws of thermodynamics, it is important to understand the concept of a system and surroundings.

Thermodynamics becomes much easier when a clear boundary is drawn. Anything inside the boundary is called the “system,” and anything outside the boundary is called the “surroundings.” Once the boundary diagram is drawn, the movement and transfer of energy can be characterized by the flow across system boundaries.

The term “universe” is all-encompassing. In other words, refers to both the system and the surroundings.

The First Law of Thermodynamics

The first law is the conservation of energy, summarized by: energy cannot be created or destroyed. In other words, total energy of the universe must remain constant.

∆Uuniverse = 0

However, it is important to note that energy takes different forms. Another way of restating the first law, is to say that the change in energy is equal to the heat flow across the system (Q) plus the work done on the system or by the system (W).

∆Esystem = Q + W

δE = δQ + δW in differential form

So, heat and work are are two processes which can change the internal energy of a system. If heat flows into the system, Q is positive. This means that heat is gained by the system, and the same magnitude of heat is lost by the surroundings (endothermic reaction). If heat flows out of the system, Q is negative. This means that heat lost by the system is equal in magnitude to the heat gained by the surroundings (exothermic reaction).

The same idea is for work. If the surroundings does work on the system, then work is positive. If the system does work on its surroundings then work is negative. Either way, total energy is conserved. This is summarized below:

The Second Law of Thermodynamics

The second of the law of thermodynamics deals with entropy and, to an extent, limits the first law. According to the second law, entropy of a spontaneous process must increase, and the the entropy of the universe must always increase. This is because, achieving maximum entropy means that a system is at equilibrium. All systems are always trying to reach equilibrium, and increase their Gibbs Free Energy.

∆Suniverse > 0

Processes can allow entropy of a system to decrease, but in that case, entropy of the surroundings will increase. This does not break the second law.

∆Suniverse = ∆Ssystem + ∆Ssurroundings

Spontaneous Processes

A spontaneous process is one that occurs without any input. According to the second law of thermodynamics, in a spontaneous process, entropy must increase. You can understand entropy as either reaching equilibrium, or as increasing disorder of a system.

An example of a spontaneous process is heat moving from a hot to a cold body. Heat will naturally move from hot to cold without any external input because the overall system is trying to reach an even temperature.

As heat leaves the hot system system, its own entropy decreases, and as it enters the cold system, its entropy increases. This is a spontaneous process.

On the other hand, a non-spontaneous process is one where entropy decreases.

Note that spontaneity does not refer to speed! It refers to a process that naturally occurs without interference. A spontaneous process can actual have a very slow reaction rate.

The Third Law of Thermodynamics

The third law of thermodynamics holds that the entropy of a system nears a constant value as its temperature approaches absolute zero. This law generally applies to a pure material in a perfectly crystalline structure, because its minimal energy allows to approach zero entropy.

Further Reading

Hess’s Law Equation
Bond Enthalpy & Bond Energy
The Chemical Elements

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