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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13
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4answers
1k views

Room-conditions supercritical fluids?

Are there any reasonably obtainable supercritical fluids that I could, say, run my hand through? Wikipedia makes it sound like there are plenty of room-temperature ones, but no room-pressure fluids.
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1answer
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Does aromaticity require a co-planar pi system?

It's well established that angular twisting can decrease the degree of delocalization across a $\pi$ system (e.g. biphenyl). Moreover, there's considerable debate about aromaticity in spherical ...
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1answer
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Why is osmium the densest known element?

Why is osmium so dense despite there being heavier elements after it in the periodic table?
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1answer
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How exactly is activation energy defined?

In a common interpretation of the Arrhenius rate equation $$k = A\exp\left(-\frac{E_\mathrm a}{RT}\right),$$ the activation energy $E_\mathrm a$ is understood to represent the difference in the ...
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1answer
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Applying the Le Chatelier principle

I am having issues understanding the consequence(s) of a decrease in pressure on a chemical reaction. I have understood that when we increase the pressure, the side having more number of moles wants ...
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2answers
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Is there a simple way to get the circular dichroism of a molecule from its structure?

Are there any heuristics to get the relative absorbtion of left and right circularly polarized light by a molecule from its molecular structure? Is it even possible to predict which polarization is ...
14
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2answers
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How to find the number of atomic microstates for a given electronic configuration?

Given some electron configuration, I know the following formula can be used to determine the number of microstates: $$\text{# mircostates} = \frac{(\text{# electron positions})!}{((\text{# electrons})...
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2answers
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How does the partial pressure of gas C change with the change in volume of the container?

Suppose I have the equilibrium in a closed container: $$\ce{3A(s) + 4B(g) -> 4C(g) + D(s)}$$ What happens to the partial pressure of C if the volume of the container is halved? I was pondering ...
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1answer
931 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 ...
4
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3answers
720 views

What are dipole moments in a molecule supposed to act upon?

In a $\ce{CO_2}$ molecule, a total of four electron pairs are shared between the carbon and oxygen atoms, such that 2 pairs are shared between the carbon atom and each oxygen atom. Oxygen has a ...
4
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2answers
350 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 ...
4
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2answers
450 views

Y-axis of the reaction co-ordinate graph

In the graph of the reaction co-ordinate, is the $y$-axis that is used $∆H,$ $∆U$ or $∆G$?
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1answer
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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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1answer
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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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3answers
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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 ...
2
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2answers
7k views

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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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
213 views

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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1answer
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Why can dissolution reactions (e.g. dissolution of calcium chloride) be exothermic?

Dissolution reactions are normally endothermic, and crystallization reactions are normally exothermic, but there are some exceptions -- such as the dissolution of calcium chloride. Why is this so?
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2answers
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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 ...
4
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0answers
337 views

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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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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0answers
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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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2answers
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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 ...
3
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1answer
786 views

Pauli-forbidden term symbols for atomic carbon

Carbon has a $\mathrm{p^2}$ configuration, and within the Russell-Saunders coupling scheme, we have $$\begin{align} s_1 = s_2 &= \frac{1}{2} & S &= 1,0 \\ l_1 = l_2 &= 1 & L &=...
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1answer
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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 ...
2
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1answer
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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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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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3answers
455 views

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?
2
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1answer
250 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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2answers
762 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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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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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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1answer
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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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1answer
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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 ...
12
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1answer
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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 ...
12
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3answers
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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_\...
6
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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 ...
6
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2answers
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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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3answers
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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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1answer
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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 ...
18
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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 ...
16
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2answers
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What causes the lowering of vapour pressure in volatile/nonvolatile solvent mixtures?

"Based on Figure 13.18, you might think that the reason volatile solvent molecules in a solution are less likely to escape to the gas phase, compared to the pure solvent, is that the solute molecules ...
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3answers
46k views

Why is the specific heat of water high?

How is the specific heat of water so high? I was reading the hyperphysics article on it but could not understand it properly.
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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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2answers
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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 ...
4
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2answers
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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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Why do some chemical reactions require many steps?

I posted the following question in Physics SE and was advised to transfer it to Chemistry SE. I studied physics in college ten years ago and I recently started to learn biochemistry. I enjoy finding ...
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7answers
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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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