Questions tagged [physical-chemistry]

The study of chemical systems using the laws and concepts of physics. This usually requires the techniques of thermodynamics, statistical mechanics and quantum mechanics.

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855 views

Why do gases need to be cooled to liquefy/solidify?

My book states: Gases do not liquify on compression only, although molecules come close to each other and Weak forces operate at a maximum. Why is it that we cannot just keep on compressing gas ...
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2answers
333 views

Equilibrium constant. Can it be reached?

Consider the reaction below: $$\ce{A + B <=> C}$$ Suppose that the equilibrium constant for this reaction is $K = 10$. I then prepare a reaction vessel with volume of $\pu{1 dm^{-3}}$ which ...
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pH and materials selction [closed]

I working on the Navy study guide for their nuclear engineering programs and I am not a Chemist. Thus, I have come here to try and develop a better understanding of the subject matter. Why is pH ...
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1answer
735 views

Gibbs Free Energy and Maximum Work

I am a high school student and my professor mentioned that the Gibbs free energy is the maximum amount of work (or useful work) that a system can do, whereas entropy is a measure of the non-available ...
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1answer
4k views

Internal pressure of ideal gas

$$\left(\frac{\partial U}{\partial V}\right)_{\!T}$$ I know that this partial derivative is equal to zero for an ideal gas, but how do I determine that? Do I need to use the fundamental thermodynamic ...
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3answers
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Is a liquid in a container always in equilibrium with its vapour?

This is essentially a question about the meaning and significance of the term vapour pressure (or vapor pressure if you're American). From what I understand a liquid in a container will have a certain ...
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2answers
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Derivation of mean kinetic energy

I read from a book that average kinetic energy is equal to $3kT/2$ where $k$ is Boltzmann's constant and $T$ is the kelvin temperature. I don't know how the formula was derived. Any help to gain ...
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Can you heat water with additives?

I have been curious about this question for a while. If you want to warm up a large amount of water, is it feasible to do this by adding a substance that has an exothermic reaction with the water? ...
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2answers
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Scientific Reason for Salt Solution Gaining Volume

A little background to this question: my mom placed a glass with salt and enough water to just cover the salt in a room to "absorb negative energy" out of the room at her office. It obviously "worked"....
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235 views

How to obtain the Raman spectrum along every coordinate of a scan in Gaussian?

I am doing a scan calculation using Gaussian09 and adding the Freq=Raman keyword to my input file. My objective is to obtain the ...
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Howcome orbitals become 'core-like' when electrons are removed?

It seems to me that f-orbitals for lanthanide metals are treated as 'core-like' when a certain number of electrons have been removed. Or, as Radiochemistry puts it, The 4f binding energy is so ...
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What decides whether a reaction releases light or heat

I understand the reason that a chemical reaction would create photons (physics is more my strong point), but why would it create heat instead? My only guess is that the light released somehow causes ...
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Predominance of III oxidation state for lanthanides [closed]

For most lanthanide metals, the stable oxidation state is III [*]. The general electronic structure is $$\ce{[Xe] 4f^{0}^{-14} 5s^2 5p^6 5d^{0}^{-1} 6s^2}\ \ [**].$$ Elements that have the d-electron ...
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1answer
371 views

How can the size of atom be the cause of octet expansion?

Lately, I was reading The Lewis Theory of Covalent Bonding by Peter Atkins in Appendix 4 of 'Elements of Physical Chemistry'. There he was talking about expansion of octet . As he wrote: Many ...
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1answer
226 views

What does a molecules color have to do with its bond/orbital energies?

For example, elemental iodine is deep violet. Its sigma bond or perhaps the lone pairs are capable of absorbing all visible light frequencies except violet which is why we see it as that color. ...
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melting and boiling …really equilibrium? [closed]

Why are melting and boiling considered equilibrium processes even though the amount (concentration) of both phases keep changing i.e from solid to liquid and so on?
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2answers
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How does one find the ground-state term symbol for a configuration that is exactly half-filled?

For instance, the ground-state configuration of N atom is a $p^3$ configuration of all parallel spins and one electron in each $2p$ orbital, which has: Total spin angular momentum, $S = 3 * \frac{1}{...
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IR spectra and hydrogen bonds

Normally we see IR signals that correspond to the vibration of covalent bonds. Can a (strong) hydrogen bond (or a vibration thereof, to be precise) correspond to an IR signal as well? How about other ...
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1answer
974 views

If a gas always occupies the volume of its container, will its volume always be 22.4L at STP?

According to the Avogadro's law, an equal volume of gases have a equal number of moles (with constant pressure and temperature), therefore STP molar volume is $\pu{22.414 dm3 mol-1}$. Think this: ...
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4answers
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Why boiling can't take place in closed containers

I am being told that boiling can't take place in closed containers. Can someone please explain why this is so.
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729 views

Do catalysts shift equilibrium constant towards 1?

I want to be able to understand shifts in equilibrium from the maxwell boltzmann distribution. One thing I cannot get my head around is the effect of catalysts on the equilibrium position - supposedly ...
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1answer
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How many hydrogen bonds are formed by water and by HF?

How many hydrogen bonds are possible for $\ce{H2O}$, given that oxygen has two lone pairs? Is it 4 or 2? Related: why is it that HF forms only 1 hydrogen bond, given that HF has three lone pairs?
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What are the units of Kc and Kp?

They are both equilibrium constants as far as I know. Kc is in terms of molarity and Kp is in terms of pressure. Also both of them are ratios of respective quantities [ ratio of molarity(s) in Kc and ...
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Is there an energy cost associated with flipping the spin of an electron?

THE STORY: A common example used to illustrate the limitations of restricted Hartree-Fock (RHF) theory is the H$_2$ dissociation energy ($D_e$) curves. RHF enforces electrons to be paired into spin ...
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What is the mathematical basis behind the Jahn-Teller effect?

Both first-order and second-order Jahn-Teller distortions play a very important role in chemistry. It is often said that the Jahn-Teller effect is based on symmetry arguments, and hence nothing can ...
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Derivation of van 't Hoff equation for temperature dependence of equilibrium constant

While I was reading about the usefulness of the quantity $\Delta H$, I found that it can be used to calculate the how the equilibrium constant varies with temperature. How can this be done? Does it ...
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1answer
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How do electronegativity and lone pairs affect bond angles?

How do bond angles vary in molecules with a lone pair and central atom of different electronegativity, but in the same period so that electronegativity matters more than orbital size? Let's assume ...
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Why don't gases escape Earth's atmosphere?

Some gases are lighter than others and rise. Why don't they continue going up, leave the atmosphere, and then enter outer space?
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Are there any known chemical properties of tritium water that make it unusually different from protium water?

I suppose the first question supporting the main question is, has tritium water ever been synthesized in sufficient quantity to test chemical properties? If so, and apart from the obvious radioactive ...
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6answers
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Does the Maxwell-Boltzmann distribution apply to gases only?

The Maxwell-Boltzmann distribution can be used to determine the fraction of particles with sufficient energy to react. I know that the curve applies to gaseous reactants and would like to know whether ...
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Relation between constant-pressure and constant-volume heat capacities: Cp - Cv = nR

For an ideal gas, we have $$C_p - C_V = nR$$ where $C_p$ is heat capacity at constant pressure, $C_V$ is heat capacity at constant volume, $n$ is amount of substance, and $R=N_\mathrm A\cdot k_\...
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How can two electrons lie together in an orbital?

Two electron of opposite spin can lie in a single orbital.. But what about the electron-electron repulsion. Okay! I got that the nuclear charge rather the large Z-effective overcome this repulsion by ...
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1answer
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Why is Kc not affected by change in pressure?

Why is $K_c$ not affected by change in pressure? I know the mathematical explanation, but I don't really understand the reason when only looking at $K_c$. The explanation I know is with the reference ...
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What is the differences between partial pressure and vapour pressure?

Was looking at Henry's law and Raoult's law constants and there seemto be lots of equations involved. Henry's law involves partial pressure and the latter involves the vapor pressure. Wondering what ...
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1answer
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Quadrupole moment of a molecule

What is a quadrupole moment of a molecule and how does it arise? How is it measured for a particular molecule? I've read the Wikipedia article on quadrupoles and understand that it has to do with the ...
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2answers
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When I heat up a balloon, does the air inside increase in pressure as well as volume?

When I heat up a balloon, does the air inside the balloon increase in pressure as well as volume? I thought pressure and volume were inversely proportional? Or does pressure and volume increase as ...
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How to derive the Gibbs-Helmholtz equation? [closed]

Can someone please explain to me how to derive the Gibbs-Helmholtz relationship from $G = H - TS$?
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5answers
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Evidence of orbitals?

How do we know that there are different types of orbitals? For example, what evidence is there for the existence of $\mathrm{p}$ orbitals instead of there being multiple $\mathrm{s}$ orbitals (for ...
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5answers
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Why is SiO2 a solid while CO2 is a gas?

I was under the impression that chemistry almost exclusively involves valence electrons because there isn't enough energy to strip off electrons located closer to the nucleus. If that is true, and ...
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3answers
417 views

Entropy - “Wiggle”?

The title is not a reference to a Jason Derulo song. In any case: 1) How is change in entropy measured, experimentally? I've Googled this for a bit and I've found all sorts of mathematical ...
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Why does like dissolve like?

Polar solvents love polar solutes to be dissolved in it and non polar with non polar. Often said as like dissolves like. Okay, polar loving polar can be understood with help of the facts: same polar ...
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Why does water volume decrease when salt is added? [duplicate]

Why does water volume decrease when salt is added? Our teacher asks us in the class but I don't find any strong reason.
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1answer
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how are the frequencies at a local maximum of PES like?

On the potential energy surface, if you find a local maximum and calculate its frequencies in Gaussian or something like that, will you get all negative frequencies or all positive frequencies? I know ...
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Connection of term symbols with specific microstates for atomic carbon

I'm currently studying atomic term symbols. I wanted to try it on a simple atomic carbon with the electron configuration $1s^22s^22p^2$. I know, that only open-shell electrons are involved in the ...
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1answer
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Boundary lines in phase diagrams and the lever rule

My first question is: What exactly happens on a point situated on a line? Or on a point such as E in this figure? Does point E mean that the $\delta , \gamma + \delta , \delta + \epsilon , \gamma + \...
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Energy-efficiency aside, what are the chemical constraints on CO₂ capture and methanation?

Synthesising $\ce {CH4}$ from air and water (in a non-biological process) has been proposed as one form of energy storage. What are the chemical constraints at play here? That is to say, what sort of ...
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2answers
944 views

NMR chemical shift range of different elements

A typical $\ce{^1H}$ NMR runs from approximately 0 to 10 ppm, give or take a bit. $\ce{^13C}$ NMR runs from 0 to 200. And $\ce{^59Co}$ NMR runs from -5000 to 15000 ppm! There seems to be some ...
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Le Chatelier's principle: Are there any exceptions?

The way Le Chatelier's principle is presented in most introductory chemistry books (high-school) is as though it's an indisputable law of the physical world (in the sense that we're never shown an ...
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What is the difference between Molecularity and Order Of Reaction?

In the field of Chemical Kinetics, terms like Molecularity and Order Of Reaction are often used. I am a bit confused between these two terms. Can someone explain to me about these two terms and their ...
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Does the dipole moment increase or decrease by increasing the bond length?

It is established the dipole moment is a result of multiplication of the magnitude of charges (Q) and the distance between them (r). What I understand is that when an electron and a proton get closer (...

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