Hello! Thank you for stopping by to learn about the third law of thermodynamics. I really appreciate you taking the time to learn about chemistry which is such an important foundation of science.
Why you should understand the third law of thermodynamics: Though the third law may seem silly, it's important to learn about the third law because it helps better understand the complete picture of thermodynamics. The implications of the third law incorporate some important thermodynamics ideas which will help you better understand all of thermodynamics.
Key Ideas
Defining the 3rd Law of Thermodynamics
What does the 3rd Law of Thermodynamics mean?
Defining the 3rd Law of Thermodynamics
The third law of thermodynamics states that the entropy of a perfect crystal is zero at absolute zero. While this law seems very simple, there are some terms we should break down to really understand what the third law means.
Entropy: I have a post solely dedicated to Entropy so if you're having trouble understanding it or would like a more complete picture please go here. Entropy is conventionally thought of as "randomness" in a chemical system or in life. However, entropy is more closely linked to probability and the options an atom has to be in a location in space with particular energy at a specific time.
Perfect crystal: This part of the definition seems very silly, however, to understand the definition you need to know what a perfect crystal is. A perfect crystal is solid with perfectly ordered atoms. This means the atoms within the crystal are neatly packed together with no impurities (other atoms besides what the crystal is made of).
Zero entropy: If entropy is the option for an atom to be in different locations, then zero entropy means that there are no other options for the atoms to be in except where they are currently located. Normally, atoms shift and vibrate slightly in a solid. However, with zero entropy, the atoms are not shifting, vibrating, or changing energy.
Absolute zero: Absolute zero is a topic that scientists have been arguing about for some time. Absolute zero is the temperature at which molecules no longer vibrate or move because the temperature around them is too cold. Absolute zero gives the atoms no energy and doesn't allow them to transfer energy between each other. Though it has never been truly reached in the lab, scientists have gotten very close to reaching absolute zero.
Now that we know what entropy, a perfect crystal, zero entropy, and absolute zero are; let's put it all together to make a more complete and understandable definition of the third law of thermodynamics.
The third law of thermodynamics states that: The randomness of the atoms in a perfectly ordered substance is zero at an extremely cold temperature.
I don't know about you, but that definition makes a lot more sense to me.
What does the 3rd Law of Thermodynamics mean?
There are some very complicated and difficult to understand implications related to the third law of thermodynamics that I couldn't explain if I tried. However, there are two implications that I do understand and can explain.
1. Because absolute zero is unachievable, the creation of a perfect crystal is unachievable.
2. As substances get colder, their atoms move less and they contain less energy.
Because absolute zero is unachievable, the creation of a perfect crystal is unachievable.
This is the classic, chicken versus egg conundrum. Absolute zero doesn't exist in nature and scientists can't create temperatures that low in a lab. This means that we can't get crystals cold enough to observe perfection. Ok, so then why not create a perfect crystal to look at the structure of a crystal at absolute zero? Because we can't create a perfect crystal without absolute zero. By definition, one of these conditions can't exist without the other.
As substances get colder, their atoms move less and they contain less energy.
Now for the second implication which in my opinion is much more relevant to understanding the third law of thermodynamics. When you remove heat or heat energy (because decreasing the temperature is actually considered "removing heat". AKA there is no cold, only the absence of heat) the vibrations and energy of the individual atoms decrease. If the energy of each atom decreases, then the energy of the overall substance decreases. If this concept is confusing, I would recommend checking out my post/video on enthalpy.
Summary
The third law of thermodynamics states that the entropy of a perfect crystal is zero at absolute zero.
The third law of thermodynamics states that: The randomness of the atoms in a perfectly ordered substance is zero at an extremely cold temperature.
Because absolute zero is unachievable, the creation of a perfect crystal is unachievable.
By definition, a perfect crystal and absolute zero can't exist without the other.
As substances get colder, their atoms move less and they contain less energy.
I hope you've found this post on the 3rd Law of Thermodynamics helpful. If you have any questions feel free to comment on this post, email me, or comment on the video I've linked below.
Saren
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