171
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Accepted
Why is absolute zero unattainable?
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 ...
- 31k
78
votes
Why is absolute zero unattainable?
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 ...
- 1,569
39
votes
Why do we still need to know about the Rankine temperature scale?
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 ...
- 491
38
votes
Accepted
Why is −78 °C a magic temperature for organic reactions?
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 ...
- 22.3k
34
votes
Accepted
A glass of water with ice-cubes in it. Where's the water the coldest; at the top or bottom?
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 ...
- 18.6k
30
votes
Accepted
There's an absolute zero, is there an "absolute hot"?
In the actual theories of physics the highest temperature which has a physical meaning is the Planck's temperature.
$$T_\mathrm{P} = \frac{m_\mathrm{P} c^2}{k} = \sqrt{\frac{\hslash c^5}{G k^2}} \...
- 4,167
21
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What is the difference between "vapour" and "gas"?
Vapor is a much older word alluding to dampness and it was not coined by scientists. It is in use since the 1300s. The actual meaning of meaning of vapor is "Matter in the form of a steamy or ...
- 38.9k
20
votes
Is there an uncertainty associated with the value 0 K for absolute zero?
As another answer explains, absolute zero is defined in the Kelvin temperature scale as precisely $\pu{0 K}$. But this is not the entire story, as all measurements have an associated uncertainty (all ...
- 20.8k
18
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Accepted
When can a molecule be considered freely rotating at room temperature?
I'm not a transition-state physical chemist, but I think a good approach to this problem is transition-state theory, specifically the Eyring equation:
$$k = \kappa \frac{k_b T}{h} e^{\frac{-\Delta G^{...
- 21.8k
18
votes
Accepted
Why is the Haber process carried out at such high temperatures?
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 ...
- 67.3k
18
votes
Why is absolute zero unattainable?
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 ...
- 556
18
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Why do we still need to know about the Rankine temperature scale?
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°...
15
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What is the difference between "vapour" and "gas"?
The normal use distinguishes "vapour" from permanent gas
At normal lab conditions there is a (fairly obvious) distinction between things that could exist as liquids and things where no ...
- 35.4k
14
votes
There's an absolute zero, is there an "absolute hot"?
It depends on what you mean by ceiling. Are we talking about a 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 ...
- 17.3k
14
votes
What exactly is temperature?
Temperature vs kinetic energy
[OP:] I've read at many places that temperature is the average kinetic energy of particles present in an object.
Temperature has to do with the average kinetic energy ...
- 39k
14
votes
Does motor oil get more viscous when heated & what is the meaning of the SAE motor oil code?
The numbers in the code have nothing to do with viscosity index. However, your understanding of 'W' for winter is correct. The real code for your car is actually SAE 5W-30 where SAE stands for the ...
- 39.5k
13
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There's an absolute zero, is there an "absolute hot"?
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 ...
13
votes
What exactly is temperature?
Heat is the transfer of energy to or from the body in forms other than matter flow or work (organized energy transfer, such as pushing).
Temperature is only a well-defined property for a collective ...
- 1,540
12
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Why is the Haber process carried out at such high temperatures?
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 ...
- 71.1k
12
votes
Why is absolute zero unattainable?
The usual answer is that it actually is possible because even though the ground state of spacetime itself has non-zero energy, it is not kinetic energy, which is the very definition of temperature.
...
- 221
12
votes
Accepted
Prove that a 10-Degree Temperature Increase Doubles the Rate Constant (k), when the Activation Energy is Approximately 50 kJ/mol
Your method and your mathematics seem perfectly fine, and your calculated result is correct. You are also quite correct that the result will change depending on the value of $E_\mathrm a$ that you ...
- 17.3k
12
votes
A glass of water with ice-cubes in it. Where's the water the coldest; at the top or bottom?
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, ...
- 22.3k
12
votes
Why do we still need to know about the Rankine temperature scale?
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. ...
- 221
11
votes
Accepted
How is Negative Temperature Hotter than Infinite Temperature?
The other answer given is correct, but for completeness, I will give the thermodynamic, rather than statistical mechanics, explanation.
Thermodynamics gives us that $$\frac{\partial E}{\partial S}=T$$...
- 12.9k
11
votes
Accepted
At what temperature (in kelvin) are most of the elements on the periodic table liquids?
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 ...
- 31k
11
votes
Accepted
Are complex ions thermochromic?
Indeed, this effect is observed in certain select cases. The effect of spin-spin cross over has been mentioned in this regard. There can be some other effects at play, namely: phase transitions, ...
- 8,465
11
votes
What is the difference between "vapour" and "gas"?
I'm surprised the OED has such a strict definition for gas. I could not find a strict definition in the IUPAC color books (certainly not in the gold book). Presumably these words are in such common ...
- 20.8k
Only top scored, non community-wiki answers of a minimum length are eligible