Understanding the laws of thermodynamics is paramount to understanding why chemical reactions proceed or not. Unfortunately, many times they seem detached or irrelevant when learning to predict reactions, but they are always there in the background. In a chemists view, thermodynamics is what makes the world go round!
Perhaps it is easier to say that thermodynamics is the accounting system of science and the currency is energy. Nothing happens that can’t be balanced into a ledger and unlike spending money, it will never end up in the red.
In a past post, I explored the three questions posed in chemistry:
What product(s) can be made?
How much can be made
How fast can it be made?
These questions are based on the details of particular components coming together. Thermodynamics is concerned only with the big picture or final tally of energy, and is not usually interested in the details but of course assumes that you are a master of the details so that you can understand the big picture.
This of course leads to a never ending loop, how can the big picture be understood without the details but how do the details make any sense if I can’t see the big picture!!!
Any science starts out laying out the details by presenting theories that make sense of individual parts of the whole. This often frustrates the student because they can’t see the why or appreciate how it fits into the whole. After all, most of us would love to run before we walk.
The 2 questions of Thermodynamics
Ultimately there are only two questions that matter regardless of how they are approached:
What could happen?
Will it happen?
the rest are just details. The final answer is always given in terms of energy.
Based on thermodynamic data, different tools have evolved to predict this final energy but present it in a more visual or workable format. Tools like, solubility rules, oxidation/reduction, electron mechanisms, etc., help work through the details leading to the final energy and therefore are more easily explained/demonstrated.
What are the laws or rules of thermodynamics?
The first law is often called the law of conservation, the second deals with spontaneity and order/disorder and the third deals with absolute zero conditions. The first law of thermodynamics, simply stated, is that nothing can be created nor destroyed, merely changed from one form to another. This applies to mass, energy, charge, basically everything. It is used to identify any changes that are possible in terms of energy (those that conserve energy) and answers ‘what could happen?’.
It is the first law explored when learning basic chemistry and is usually introduced by balancing basic equations, followed by mass/stoichiometry problems and predicting reactions. The second law deals with whether a reaction or process is spontaneous. Energy is required to maintain order so the second law is often expressed as the increase of disorder or entropy of a system. It answers the question, ‘will it happen?’.
Once the basic concepts of reactions and prediction have been explored, the second law is introduced followed by the use of thermochemical equations to mathematically determine whether a reaction will occur based on the overall energy. The third law explores what would happen to entropy when solids are taken to absolute zero and is used more as proof for the first two laws.
Ultimately, any question posed in chemistry, if it is to truly be understood, must be expressed in thermodynamic terms. There are a lot of tools available to make that transition easier.
The motivated student will follow the path through to its conclusion and be rewarded with the ultimate power of answering definitively, what could happen and will it!