# Usage of the word “entropy”

The physicist and renown television personality described the 2nd law of thermal dynamics this way:

"The second law of thermal dynamics is essentially a death warrant. The amount of disorder in the universe is increasing -- meaning that things break down, things corrode, things decay."

Then I tried looking at some definitions of the 2nd law of thermal dynamics, which is essentially entropy (right?) and found this rather intuitive explanation:

When energy changes from one form to another form, or matter moves freely, entropy (disorder) in a closed system increases.

Question: If this concept applies to the universe, as articulated by Kaku, then by association/inheritance, can we use the term entropy to explain phenomenon here on earth?

Example: The US military has many airplane hangars in the desert because there is less moisture in the air and the parts corrode slower and require less maintenance.

Is it then fair to say that:

The entropy in the desert is lower than, say, at the beach (all else held equal)?

Now of course, corrosion and entropy may not be synonymous with each other, but again, by inheritance of the axiomatic properties of entropy, is that not a fair statement? Or will physicists object to that?

• 1. Entropy is not just a disorder. Entropy is the amount of information. The view of entropy as just a disorder is misguided. The universe evolves in the direction of the increase in the amount of information to describe it. 2. The entropy increase in a closed system is required for the entropy decrease in an open system. This is how life and intelligence evolved on Earth. Clearly not just a death warrant. 3. Don't be discouraged. The rate of corrosion in a desert is lower, so the disorder is lower. The overall entropy there may be irrelevant, but there is a rational seed in your conclusion. – safesphere May 16 '18 at 16:19

Physicists are going to object.

First, the process of increasing entropy is not the same as entropy itself; entropy is just a characteristic of the system. Statements such as "Entropy made this process occur," for example, invite misconception. It would be more accurate to say "The tendency for entropy maximization in the universe made this process occur." Again, the state function of entropy is not the same as the tendency for entropy maximization.

(In fact, if chunks of the desert and the beach contain the same volume of the same sand and the desert is hotter than the beach, then the desert system would have a higher, not lower, entropy because entropy for a simple system at equilibrium increases with temperature.)

Second, the speed of corrosion is a kinetics problem, not a thermodynamics one. Corrosion is mediated by how fast water vapor diffuses into the area, among other parameters, which has little to do with entropy (except inasmuch as entropy maximization of the universe drives every spontaneous process).

If I could change your system slightly from corrosion speed to oxidation (i.e., rusting) vs. reduction (i.e., turning ore into a pure metal), we could discuss one way that entropy and entropy maximization affect us worldwide on a daily basis. This example is widely discussed in textbooks and is useful for firming up your understanding of these concepts.

Although it's true that entropy always tends to increase in a closed system, things are different in our common surroundings of an open system with constant pressure and temperature. Here, the quantity $H-TS$ tends to decrease, where $H$ (termed the enthalpy) is largely a measure of bond strength, $T$ is the temperature, and $S$ is the entropy. If bonding isn't involved, then a decrease in $-TS$ essentially says that we're back to maximizing entropy.

The quantity $H-TS$ is negative for all spontaneous processes at constant temperature and pressure. Rust is a more stable compound than iron; its $H$ is lower, and so at lower temperature such as room temperature, rusting proceeds spontaneously. However, a consequence to rusting is that oxygen is turned from a gas into a solid (iron oxide), which causes a decrease in entropy. As a result, at higher temperatures and in an oxygen-scarce environment, $H-TS$ is negative only if the reaction runs the other way: rust turning into iron. (In other words, entropy maximization has won out due to the presence of the $T$ coefficient in the $H-TS$; the spontaneity of the forward or backward reaction can be visualized using an Ellingham diagram.) This is one of the ways that civilization obtains metal from ore.

So this is a quick example of a description of entropy and entropy-driven spontaneity that physicists (and engineers) would be on board with.

• excellent explanation! I feel like I'm on the right track now. I'm still trying to understand your comments about open and closed systems and how that applies to Earth. Particularly, H meaning chemical bond strength? Or when you said H is pressure, as in atmospheric pressure? – Arash Howaida May 16 '18 at 15:36
• $H$ is the enthalpy, not the pressure. Can you point to where I said or implied it was the pressure? I'm not seeing it. – Chemomechanics May 16 '18 at 15:45
• Yea, maybe you didn't say it. I think I connected the dots that way for some reason. "Although it's true that entropy always tends to increase in a closed system, things are different in our common surroundings of an open system with constant pressure and temperature. " Since you mentioned entropy and temperature (2 variables in the equation) I thought (mistakenly) that pressure was referring to the only other variable not mentioned yet: H. My mistake. So basically, in an open system, like Earth's ecosystems, entropy won't be doomed to increase? at least in the short term? – Arash Howaida May 16 '18 at 15:49
• Correct; any time a hot object cools down, for example, its entropy decreases. This is no problem because the same amount of entropy is transferred to the cooler environment and because some additional entropy is generated during this irreversible process. – Chemomechanics May 16 '18 at 15:53
• Cool. I'm having a hard time googling $H - TS$ due to the syntax stripping of punctuation symbols, what is that relationship called? Is that the process of entropy maximization written formally? And last question, if we cannot measure $H$ directly, but instead measure changes in $H$, could we find out which ecosystem (beach, desert) had a higher $H$? – Arash Howaida May 16 '18 at 16:13