140

There was a story in my days about a physical chemist who was asked to explain some effect, illustrated by a poster on the wall. He did that, after which someone noticed that the poster was hanging upside down, so the effect appeared reversed in sign. Undaunted, the guy immediately explained it the other way around, just as convincingly as he did the first ...


66

Absolute zero is a tricky concept, particularly once you start getting precise about it. Thermodynamics and quantum mechanics is a tricky business! I'll try to avoid the precise parts, and see if I can give you an answer which is more intuitive than a pile of equations. The first question is what does it mean to "attain a temperature of absolute zero." ...


38

Rankine is commonly used in the aerospace industry in the United States. Rankine is to Fahrenheit what Kelvin is for Celsius. So when people in the United States were creating programs and using equations that needed an absolute temperature, they used Rankine before Celsius became dominate for scientific calculations. The reason people still sometimes use ...


33

Dry ice (solid carbon dioxide) sublimes at −78 °C. Dry ice and acetone are a common cold bath for chemical reactions. The melting point of acetone is -95 °C so the bath never gets cold enough to freeze the acetone. The bubbling of the carbon dioxide gas as the dry ice sublimes keeps the cold bath well stirred.


33

Interesting question! A few things first: As the ice melts, it cools the water around it. Technically, the ice cube melts because the water cools down. This may sound ridiculous at first, but you must consider the fact that the ice melts because it has drawn "heat" (energy) from its surroundings. The "surroundings" being the air and water that surround ...


30

First, I think I should make it clear that when water boils, the bonds in the water molecule linking the hydrogen and oxygen atom are not broken. During boiling, the intermolecular bonds in water are the ones that get broken, that is the bonds that link the water molecules together. At room temperature, there is evaporation (I wouldn't call it excitation). ...


26

In the actual theories of physics the highest temperature which has a physical meaning is the Plank's temperature. $$T_\mathrm{P} = \frac{m_\mathrm{P} c^2}{k} = \sqrt{\frac{\hslash c^5}{G k^2}} \approx \pu{1.4e32 K}$$ For the moment no theory predict higher temperature because of the limit of our theories. There is a Wikipedia article about absolute hot ...


20

Yes. But it is a bit of a joke of a sort. In any system with energy levels, the lower energy levels never have a lower population than the higher ones. In fact there is an equilibrium arrangement of the populations of such levels characterized by the temperature. The higher the temperature, the more evenly populated the levels are. There is even a ...


18

Most of us in the world use the Celsius scale to measure temperature for day-to-day purposes. The Kelvin scale has been designed in such a way, it is not only an absolute temperature scale, but also 1°C change is equal to a 1K change. This makes conversion from Celsius to Kelvin pretty easy, involving just the addition or subtration of a certain constant (in ...


17

I'm not a transition-state physical chemist, but I think a good approach to this problem is transition-state theory, specifically the Eyring equation: $$k = \kappa \frac{k_b T}{h} e^{\frac{-\Delta G^{\ddagger}}{RT}}$$ This equation tries to predict the rate constant $k$ from an assumed pseudo-equilibrium between the transition-state and the starting ...


16

I didn't know that balloons expanded during the fly because of thermodynamics, and I didn't know how high they can fly, but a rapid search tells that a partially unfilled regular balloon can fly until an altitude of around $\pu{25 km}$. Now, $\pu{25 km}$ means that it reaches the first part of the stratosphere, with temperatures of $\pu{-60 ^\circ C}$, that ...


15

No, the paper will not burn without oxygen being present. Paper is made primarily of cellulose which is a polymer of glucose. If you heat paper in a vacuum the cellulose simply decomposes to $\ce{H2O}$, $\ce{CO2}$, $\ce{CO}$ and carbon. As the paper decomposes it will "char" or turn brown to black as the cellulose polymer degrades. Here is a link to an ...


15

Why do all gases occupy 22.4 L [per mol] at STP? The question is based on a false premise. Only ideal gases are guaranteed to occupy 22.4 L/mol at STP. There are many gases that are not ideal. So going by this, all gases should occupy same $x$ L at some other temperature-pressure conditions. Is this true? No, again this would be true only for ideal ...


15

As others have pointed out, it is purely kinetics, but you may still wonder, why. For a reaction to actually occur (in both directions) and thus for an equilibrium to be reached, you need to overcome the activation energy. In the case of the Haber-Bosch process, this involves breaking the highly stable $\ce{N#N}$ triple bond. Even with the catalysts used, ...


15

Leaving quantum mechanics aside (it gives me a headache) the second law of thermodynamics prevents absolute zero from being reached in practice. To cool something down, its heat must be transferred to something cooler than it. Since nothing can be cooler than absolute zero, one cannot cool something to absolute zero. One can sidle right up close to $0\ \...


14

The Celcius scale was originally based on the freezing and boiling points of water, so 0 °C was chosen as the freezing point until 1954. But now, the size of one degree on both Celcius and Kelvin scales is defined as 1/273.16 of the difference between absolute zero and the triple point of VSMOW (for various reasons relating to the actual measurement), but ...


14

The answer "the distance between molecules increase" is incomplete if not plain wrong. Temperature is an effect of energy present. Basically, it's an effect of little movements and vibrations of molecules and atoms due to their energy. In an crystal, the energy of the molecules is so low, that they don't vibrate and move enough to break the structure. The ...


13

Depends on what you mean by ceiling. Are we talking practical or theoretical limit? At a high enough energy, the stress-energy tensor will be large enough that you're going to make a black hole. I'm not sure we understand the astrophysics well enough to know what this will look like in the limits you refer to. Also, at some temperature, you're going to ...


12

First, let me remark to “the size of a molecule” is not particularly well-defined. I assume in the following that you are interested about bond lengths (which are averages over time of distances between bonded atoms), as seems to be the case in your example. In the simplest case, you can consider an isolated diatomic molecule (think: gas phase N2), and we ...


12

Thermodynamic functions are strictly defined only for macroscopic systems (systems that have an essentially infinite number of atoms). You can't apply them to individual atoms because that would be confusing large-scale averages with individual microscopic values. Here's an analogy: the average speed of cars on a stretch of highway might be 55 mph, but it'...


12

The convection to produce uniformity depends on a number of nebulous factors: How much ice? How tall is the glass? Diameter of the glass? Is the "glass" really a glass or paper cup, styrofoam cup, or perhaps a metal cup? Initial temperature of the water. Mass of water to mass of ice. The gist is this. Lakes don't freeze solid in the winter. Without ...


12

I am currently studying mechanical engineering in the US, and I have used Rankine. It is used similarly to Kelvin. For example, in my thermodynamics class we used it to analyze various heat engines. Tables are available with properties of gases and steam using such units as BTU/R. I can tell you that it is somewhat of a pain to use, because it is often ...


11

Suppose you have some reaction: $$ \ce{A <=> B} $$ and the equilibrium constant for the reaction is $K$ and the Gibbs free energy change is $\Delta G$. The equilibrium constant is: $$ K = \frac{[\ce{B}]}{[\ce{A}]} $$ so increasing value of $K$ shifts the equilibrium towards the right, i.e. more $\ce{B}$, and reducing the value shifts it to the left,...


11

The other answer given is correct, but for completeness, I will give the thermodynamic, rather than statistical mechanics, explanation. Thermodynamics gives us that $$\frac{\partial E}{\partial S}=T$$ This means that when entropy--as a function of energy--increases, the temperature must also increase. On a macroscopic level, this makes perfect sense ...


11

Now that's a great question indeed! Evidently, at 0K all elements except helium are solids, at 10000K they are all gases, so someplace in between the number of liquids must reach a maximum; what and where might that be? Well, there is no formula or theorem that says liquid hydrogen must boil at 20K, nor is there such a thing for any other element, so this ...


11

Depends on what you mean by "temperature". In statistical mechanics, a system of interacting parts is in thermal equilibrium if the probability of finding a given part in a state with energy $E$ is proportional to $e^{aE}$ for some constant $a$ that is the same for all of the parts. Usually, $a$ is negative, and this becomes a Boltzmann distribution if we ...


10

I think the best way to think of equilibria intuitively is in terms of rates of reaction. At equilibrium, the forward and the reverse reactions are happening at the same rate. If you increase the temperature, what happens to the rates of the forward and reverse reactions? Using the Arrhenius equation $$k = Ae^{-E_\mathrm{a}/(RT)}$$ you can see that as ...


10

Temperature is more of a relative notion--heat flows from higher temperature to lower temperature, but that's about it. The zero in a temperature scale is meaningless; it is an arbitrary convention. Usually, we measure temperature in Celsius, Fahrenheit, Rankine, etc (note that Kelvin measures temperature as well--but it also measures thermodynamic ...


10

The change in internal energy $U$ is $$\Delta U=Q+W$$ where $Q$ is amount of heat transferred to the system and $W$ is work done on the system. Since the process is adiabatic, no heat is transferred into or out of the system, i.e. $Q=0$ and thus $$\Delta U=W$$ The reversible expansion is performed continuously at equilibrium by means of infinitesimal ...


10

The answer mainly has to do with kinetic considerations, as aml points out. I want to point out another thing. In a typical industrial setting, you don't just mix the $\ce{N2}$ with the $\ce{H2}$ at a certain $T$ and $p$, collect the ammonia, and throw the unused reactants away. That would be horribly inefficient. Greenwood & Earnshaw, Chemistry of the ...


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