Sunday, November 20, 2016

An Introduction to Thermodynamics and the Gibbs Free Energy Law

I recently learned about the amazing science behind a part of thermodynamics, the bases behind the both the what and why of chemistry. Thermodynamics is one of two principles that defines an experiment, with the other being kinetics. Thermodynaics deals with whether a reaction will occur. Kinetics determines the rate at which a reaction occurs. These principles shouldnt be confused, because althiugh they seem interlocked by definition, they are actually independent of each other. This is why I have decided to keep this article as an intro purely to thermodynamics and principles like Gibbs free energy.

I already told you that the basic definition of thermodynamics is whether a reaction  can occur based on the entropy and enthalpy of the the reactants. Now it's time I start throwing some definitions at you so get ready. Entropy is the so called "order" or "randomness" of the system of the reactant. For example, imagine the molecules inside a cloud of smoke. The molecules in the system are going to be moving very fast and there is a large potential for different possibilities or configurations as they fly past each other. On the other hand, a solid block of ice will have little movement on a molecular scale, mostly vibrating in place based on its known properties as a solid. Entropy can be estimated based on logic, but it is quantitativelt measured in J/K based on the SI system.

A visual to understand  entropy. As the temperature increases, a trend can be noticed of large entropy increases during state changes.

The other important principle to calculate the thermodynamics of a system in a reaction is enthalpy defined as the total heat content if a system. This is measured by the internal energy plus the product pressure and volume. This isnt as easily imagined as entropy, but the easiest way to imagine it is simply state changes. The enthalpy of an ice cube is lower than that of water vapor because the gaseous state of water has greater internal energy. The unit of measurement for this is simply Joules by the SI system.

The graph above models enthalpy and expands upon what I said above. The products of a reaction typically have a lower enthalpy than the reactants. However, sometimes an initial activation energy to kick off this reaction, which is the hump in the graph.

Now for the exciting stuff. Thermodynamics is the key to calculate the Gibbs free energy of a reaction, which determines whether a reaction can happen. In laments terms, a system always wants to obtain a lower energy state, with exceptions of course. So if we had a way to calculate the internal energy in a reaction and find whether it is increasing or decreasing, we use the gibbs free energy equation.


H is the symbol to reference enthalpy. S is the symbol for entropy. T is the symbol for temperature. G is the symbol for Gibbs free energy. If the change is enthalpy minus the change in entropy multiplied by the temperature, measired in K, is less than 0, we know the reaction can occur This is to represent that the energy is lower in the new system, shown by the negative, than before.

Demonstrates the effect of entropy and enthalpy on a reaction. The change in enthalpy(H) must be greater than the change in entropy(S) times the temperature for the change in Gibbs free energy to be negative.

Although this is a very simplified explanation of thermodynamics, it is where Ive decided to start my journey. The laws and science of thermodynamics is immense, and it takes years of study to even get a grasp on it. I have to start at some point though, so I've decided to begin with the gibbs free energy law. I will post the lab I did, the supercooling of water, to understand these certain principles further. and some interesting issues I found to have initiallt with it. It will be exciting, I promise! The theory behind kinetics will come after that but it may be a bit delayed, because I would like to conduct a lab to understand the equations better before I try to write about it.

 Stay tuned!
Vinay Konuru

Link to wikepedia articles:

Thermodynamics equations:

Gibbs Free Energy:



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