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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Do all gases occupy same volume at equal temperature and pressure conditions?

Why does one mole of all gases occupy 22.4 L at standard temperature and pressure? According to my thinking, it may be due to equal diffusion of all particles when at a particular temperature and ...
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Most probable point for finding an electron in the 1s orbital of a Hydrogen atom

My professor says that the most probable point for finding an electron in a 1s orbital of a hydrogen atom is at its origin. He explains this by citing the fact that the square of the wave function ...
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How to calculate an enthalpy change for this reaction

I've got an assignment to solve problem number 4 in this worksheet, that is to calculate change in enthalpy for reaction: $$\ce{CHCl3 + O -> COCl2 + HCl}$$ How do I solve it? Why is oxygen $\ce{O}$...
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Why does oxygen not condense onto open dewers of liquid nitrogen?

I had a discussion with my supervisor today after I was a touch careless pumping down a flask with a liquid nitrogen cold trap attached. I checked, and there are reports of explosions after running ...
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Is the Self Consistent Field an actual field?

I know this is really stupid. But bear with me. In everything I have read so far, the Self Consistent Field (SCF) is always used to describe a method or a process, namely, that of iteratively ...
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Why is CO practically nonpolar?

This question was in my book. According to me CO should be polar as it should have a dipole moment. But I found that the $\sigma$-electron drift from C to O is almost nullified by the $\pi$-electron ...
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Is there an easy way to remember charges on ions?

I have a chemistry test coming up and I might need to know the charge that goes with the different ions like $\ce{SO4}$ has $-2$, $\ce{NO2}$ is $-1$ and $\ce{PO4}$ is $-3$. Is there an easy way to ...
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Can orbitals in a molecule be occupied via excitation like they can in atomic orbitals?

I am facing a conundrum in the way I am viewing molecule orbitals due to the existence of excited states. What I wanted to know is, since atomic orbitals can exist in excited states (i.e. you get ...
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Overall effective activation energy for parallel reactions

Today, I was solving a question from Chemical Kinetics. Consider the following parallel, first order reactions: $$\begin{align} \ce{A -> P} \tag{1} \\ \ce{A -> Q} \tag{2} \end{align}$$ with ...
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Whats the difference between entropy and the (dis)order of a system?

Entropy is often verbally described as the order/disorder of the thermodynamic system. However, I've been told that this description is a vague "hand-waving" attempt at describing what entropy is. For ...
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Why are these molecular orbitals invalid for hexatriene?

Here are the MOs (more accurately, the breakdown of the MOs) for conjugated hexatriene: Now, when first asked to draw the MOs myself, I drew this one for $\psi_3$: It still has two nodes, and is ...
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What's the difference between precipitate and turbidity?

In many tests for various radicals, a "<color> precipitate" or "<color> turbidity" is mentioned. For example, lead ...
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Is there a physical explanation why benzene's pi molecular orbitals in a Frost circle are degenerate but pyridine's are not?

From here, the Frost Circles for both benzene and pyridine: The $\pi$-MOs formed by 6 $\pi$-AOs of an aromatic ring are: Listed with respect to the ordering in the above image, these MOs in benzene'...
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683 views

Working with wave number equations, use trial-and-error?

I have a wave number question which I have partially solved: What transition in $\ce{He+}$ ion shall have the same wavenumber as the first line in Balmer series of $\ce{H}$ atom? (a) $7\to5$ (b) $...
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What's the relationship between Van der Waals forces and hydrophobic interactions?

A question on a past exam for a course I'm studying for asks: What's the relation between Van der Waals forces and hydrophobic interactions? From what I understand, Van der Waals forces are ...
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894 views

What is the significance of the kink in the [A] vs T graph for a 2nd order reaction?

The 2nd order graph shows a bit of a kink, whereas the 1st order graph is smooth. What is the significance of this kink, and why does it appear in only the 2nd order graph. What is the significance ...
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Why isn't neptunium used in nuclear reactors in nuclear power plants?

Why isn't neptunium used in nuclear reactors in nuclear power plants? Uranium is, and plutonium is. But neptunium isn't and it is in the middle of them. Is it like it is too hard to make it do ...
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How does dimethylsulfoxide serve as a cryoprotectant?

In cell culture, dimethylsulfoxide (DMSO) is added to prevent the formation of ice crystals which may lyse the cells. Exactly how does dimethylsulfoxide act as a cryoprotectant?
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270 views

Why is the expectation value of Hermitian conjugate operators $RR^\dagger$ always real and non-negative?

I've been reading through a derivation of the wavefunctions and energy levels for the quantum harmonic oscillator. It defines $$\hat R^\pm=\frac{1}{\sqrt{2}}[\hat p \pm \mathrm{i}\omega \hat q]$$ in ...
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Non-radiative transitions or what features of rhodamine result in it being so highly fluorescent?

Rhodamine is a fluorescent dye. What features of the molecule result in it being so highly fluorescent ? I assume the extended conjugated system with a stabilized first excited state is necessary, ...
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Why is active mass of a pure solid or liquid always taken as unity?

Active mass is defined as the molar concentration ie. number of Gram-moles per litre. My book then wrote Active mass of pure solid/liquid is always 1 . The book reasoned that Molar ...
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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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Law of mass action & Collision Theory

I don't understand why the stoichiometric coefficients of reactants and products are expotentialized in law of mass action. So I checked on Wikipedia. And this popped out: In the 1879 paper[9] the ...
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Gas chromatography/mass spectrometry: Can I use peak ratios and molecular mass to derive quantity?

Can derive the quantity of a substance from a GC/MS report if I know: the ratio to another substance in the data, the quantity of that second substance, and the molecular mass of both substances. ...
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Boltzmann Distribution in Molecular Dynamics Simulation?

The Boltzmann distribution is often used to describe the population of energy states at different temperatures, i.e. something like $$\dfrac{g_i}{g_j} = \mathrm{exp}\left(-(E_i-E_j)\times \frac{1}{...
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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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Kolmogorov backward equations in active biological systems

I am trying to understand the following equations from a paper by Wang et al. [1, p. 5 at the SI]: Michaelis–Menten Representation of the Kinesin Cycle We assumed that there is a strong ...
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Can the van der Waals coefficients be negative in the van der Waals equation for real gases?

A gas described by van der Waals equation has the pressure that is lower than the pressure exerted by the same gas behaving ideally. True or false? My approach $$P(\text{ideal}) = P(\text{real}) + P(...
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Miller Indices: How to deal with some weird(-ish) cases?

I've recently been doing a spot of self-learning (Crystallography), and some of the examples provided in the Wikipedia article for Miller Indices have stumped me. I can't for the world figure out how ...
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263 views

What is the actual reason behind PCl₅ existing as an ionic solid?

Why does $\ce{PCl_5}$ exists in the solid state as $\ce{[PCl_6]^{-}[PCl_4]^{+}}$? What is the problem with it that it doesn't exist in its original form in solid state? Like why isn't it just $\ce{...
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Why can't light pass through a gold foil, but alpha particles can?

Rutherfords experiment showed that most of the alpha particles passed through a thin gold foil undeflected. So why can't light pass through the foil, which is supposedly thinner than the heavy alpha ...
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Why does water volume decrease when salt is added?

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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How can enzyme/substrate reactions that adhere (largely) to quantum theory also require 'Newtonian' consideration of gravity?

I'd just like to ask for a little clarification here due to confusion from interdisciplinary studies. I'm currently reading the 1976 paper related to the recent 2013 Nobel Prize for Chemistry, by 2 ...
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What is melting? Which bonds do we break to melt something?

To melt diamond, we have to break the covalent bonds, which we can consider 'intermolecular' because it is one giant molecule. To melt Methane, we have to break the van der Waals (intermolecular) ...
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When can I use concentrations instead of activities in equilibrium calculations?

To calculate an equilibrium constant, it is said that you should actually use activities $a_i$ instead of concentrations $c_i$. But it is also said that within a certain range, you can use ...
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Oxidation of guanine with singlet oxygen

I'm studying the oxydation of guanine using singlet oxygen and one step of the reaction is not very clear to me. My problem is with the first step: it is a 4+2 Diels Alder cycloaddition, and Fukui's ...
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862 views

Disproportionation of plutonium species - Latimer's diagram

Why (according to provided answers) $\ce{Pu(IV)}$ disproportionates into adjacent species whilst $\ce{Pu(V)}$ does not in acid aqueous solution. $$\ce{\underset{(+6)}{Pu}O2^2+ ->[\pu{+1.02 V}] \...
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Calculating Commutator of Differential Angular Momentum

I have tried for hours to calculate the commutator of angular momentum in the differential form, but I cannot get the correct answer. This is my first experience with actually checking if two ...
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What are the applications of quantum field theory to chemistry?

I have a physics background and I learned how quantum field theory is applicable to condensed matter physics, and since some parts of condensed matter physics overlap with chemistry I was wondering if ...
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Why are complex basis functions not used in electronic structure calculations?

I've noticed while reading about Hartree-Fock in Modern Quantum Chemistry (Szabo and Ostlund) that most basis sets use only real basis functions. What I'm wondering is why aren't complex basis ...
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What are the decay products of HTO?

Tritium, symbol $\ce{T}$, is hydrogen with a mass of $3\ \mathrm{amu}$. It is radioactive and undergoes $\beta$ decay. Which of the following could be the only products after a quantity of $\ce{HTO}$ ...
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What is the underlying chemistry behind the triboelectric effect?

I remember briefly studying the triboelectric effect in Physics but as I now approach the domain of Chemistry, I'd like to know what processes are involved (at the molecular level) when the the ...
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Which graph shows Chemical Equilibrium?

So my teacher started the explanation of Chemical Equilibrium saying.. The concentration does not change with time. And, gave us certain graphs asking which ones show chemical equilibrium. I cannot ...
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How to determine which gas is more soluble

I have a chemistry homework problem that asks whether $Br_2$ or $O_2$ is more soluble in water in room temperature. However, I can't seem to find a way to figure it out, short of looking it up. Is ...
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What is 'autogenous pressure'?

Compound 1 was hydrothermally synthesized under autogenous pressure[1] I've seen this term used a few times, but with no real explanation of what it means forthcoming. I suspect it refers to the ...
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Boltzmann factor vs. graph of Maxwell–Boltzmann distribution

In discussions of the Arrhenius equation describing how rates change with temperature, there is often a reference to collision theory. In the Arrhenius equation, $$ k = A \exp\left(-\frac{E_\mathrm ...
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Optical activity of starch

I found this beautiful picture of starch granules in this German Wikipedia article about optical activity. Unfortunately, I have no idea what am I looking at. What are these crosses and why are they ...
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Why do solutions become saturated?

Why can a solvent dissolve only a particular amount of solute? If we add more solute to the solution, the number of solute particles in contact with water increases. So rate of dissolution should ...
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Conservation of angular momentum in rotational spectroscopy

In spectroscopy, transitions between rotational energy levels obey the selection rule $\Delta J = \pm1$. The rule follows from the fact that the photon absorbed has an angular momentum of $\pm \hbar$, ...
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How to determine the equivalent weight of carbon dioxide correctly?

I just started learning about equivalent weight and encountered this solved example. Problem Statement: $4.215~\mathrm{g}$ of a metallic carbonate was heated in a hard glass tube and the $\ce{...