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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How do things glow in the dark?

What makes something glow in the dark? The only explanation that I can come up with for what makes things glow in the dark is that there is probably a chemical reaction slowly releasing the light that ...
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Stability of ortho- and para-hydrogen

can the stability of ortho-hydrogen greater than para-hydrogen can be stated as this? The intrinsic energy of ortho is greater than para-hydrogen and also with rise in temperature %of ortho form ...
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Understanding Beta Decay

Reading Wikipedia on "Beta Decay" they list the equation of Carbon-14 decaying into N-14. see equation https://en.wikipedia.org/wiki/Beta_decay I presume the 14 refers to total protons and neutrons. ...
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Why don't the electrons move through the electrolyte (instead of the circuit) in a galvanic cell?

I was learning about galvanic cells and I had a problem understanding why electrons do not travel through the electrolyte solutions themselves, instead preferring to travel through metals. Can ...
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Confusion about direction of dipole arrow in alpha-helices and other molecules

I understand that molecular dipoles are electric dipoles. And electric dipole moment vectors point from the negative to the positive charge. In class we learned to draw these special molecular dipole ...
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When can a molecule be considered freely rotating at room temperature?

This question sparked from a long discussion in chat about the nature of $\ce{H2O2}$ and whether that molecule can be considered to rotate around the $\ce{O-O}$ axis (and hence display axial chirality)...
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Why does water evaporate spontaneously at room temperature despite ΔG > 0?

Standard Gibbs free energy of formation of liquid water at $\pu{298 K}$ is $\pu{−237.17 kJ mol-1}$ and that of water vapour is $\pu{−228.57 kJ mol-1}$ therefore, $$\ce{H2O (l) -> H2O (g)}\qquad\...
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How does a lone pair of a central atom affect the dipole moment?

Dipole moment is the degree of polarity, i.e. the seperation of positive and negative charges. But I am not getting the intuition why and how lone pairs affect the polarity and dipole moment. I cannot ...
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Can nonpolar molecules exhibit dipole-dipole forces?

Dipole-dipole forces occur when the positive part of a polar molecule is attracted to the negative part of a polar molecule. In a nonpolar molecule, there may still be polar bonds, it's just that the ...
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Why does the Gibbs free energy only correspond to non-expansion work?

It has been defined as the energy available for work other than expansion work. Why can't it be used for expansion work
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Why don't equivalent hydrogens cause splitting in NMR?

When doing NMR spectroscopy, it is an observed fact that equivalent hydrogens do not split one another. Why don't equivalent hydrogens split each other's signals? For example, why is the NMR spectrum ...
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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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Why is NaCl3 possible?

There. And there. Almost a year ago, a group of scientists claimed to have reached compounds of $\ce{Na}$ and $\ce{Cl}$ with weird stochiometries ($\ce{NaCl3, Na3Cl, NaCl7, Na3Cl2}$ and $\ce{Na2Cl}$). ...
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How do equilibrium shifts affect solids?

Say we have the following reversible reaction: $$\ce{NaOH{(s) }<=> Na^+{(aq) }$+ $OH^{-}{(aq)}} +10.6 \mathrm{kcal}$$ If we add $\ce{OH^-}$ and equilibrium shifts to the left, does that affect ...
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What exactly is temperature?

I've read at many places that temperature is the average kinetic energy of particles present in an object. I just don't intuitively get how kinetic energy is connected with temperature. And how is ...
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What do these labels for molecular electronic states mean?

What do these symbols mean in excited states? $${}^2\!A_2,{}^2\!B_1,{}^4\!A''$$ I am confused with these representations, found in the abstract of this paper. I think it is kind of a representation ...
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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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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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Why weren't the electrons attracted to doubly charged Helium particles in the Rutherford experiment?

I have a chapter and in it there was a topic on the Rutherford experiment. It is written that doubly charged helium particles or alpha particles were thrown towards a very thin gold foil and some of ...
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Strength of intramolecular vs intermolecular hydrogen bonds

Why are intramolecular hydrogen bonds weaker than intermolecular hydrogen bonds?
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Le Chatelier's principle with pure solids and liquids

In the following reaction: $$\ce{A(s) <=> B(g) + C(g)}$$ since adding a pure solid is not supposed to shift the equilibrium in any way, does that mean that I can essentially remove all of the ...
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Estimation of the bond angle of water

We know from experimental data that $\ce{H-O-H}$ bond angle in water is approximately 104.5 degrees. If its two lone pairs were bonds (which is unfortunately impossible) also $\ce{O-H}$ bonds and a ...
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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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Comparing formula for enthalpy change with bond dissociation energy and formation enthalpy

I learnt that given a reaction: $$\ce{A -> B}$$ the enthalpy change is given by: $$\Delta H = \left( \begin{array}{c} \text{total enthalpy of}\\ \text{bonds broken}\end{array}\right)-\left( \begin{...
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What is the simplest approach to balance a complex reaction equation?

Consider the redox reaction: $$\begin{multline} \ce{ a K4Fe(CN)6 + b Ce(NO3)4 + c KOH ->\\ d Ce(OH)3 + e Fe(OH)3 + f H2O + g K2CO3 + h KNO3} \end{multline}$$ The book's answer for the ...
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Calculating pH in titration of monoprotic weak acid with strong base

$25.0\:\mathrm{ml}$ of $0.10\:\mathrm{M}$ $\ce{CH3COOH}$ is titrated with $0.10\:\mathrm{M}$ $\ce{NaOH}$. What is the $\mathrm{pH}$ after the addition of $30.0\:\mathrm{ml}$ $\ce{NaOH}$? In broad ...
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How can there be decimal subscripts in a molecular formula?

While learning about how batteries work I have encountered the following notation for a Li-ion cathode: $\ce{Li_{0.5}FePO4}$.[1] According to Wikipedia, the subscript number in a reaction equation ...
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What is the edge of a diamond like?

The chemical structure of a diamond is defined as an endless lattice in which each carbon atom is covalently bonded to four other carbon atoms situated at the four ends of a tetrahedron. But of ...
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Analytical solution of the Schrödinger Equation for AB^+ systems

According to Wikipedia, there are not many problems for which the Schrödinger equation can be solved analytically. [1] The $\ce{H2+}$ ion is probably the most complex molecule that can be treated this ...
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Law of mass action

If you have the forward reaction $$\ce{2X ->[K] P}$$ which of the following systems of differential equations would model the reaction's kinetics? $$\begin{array}{rl} \dfrac{\mathrm{d}[\ce{X}]}{...
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On Heisenberg's uncertainty principle

Why can't we find the exact position and velocity of a particle? And why is it that if the uncertainty in position is very, very large then the velocity can be determined, and vice versa? Please ...
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Prove that a 10-Degree Temperature Increase Doubles the Rate Constant (k), when the Activation Energy is Approximately 50 kJ/mol

I read that increasing the temperature by $10~^\circ\mathrm C$ will double the rate constant ($k$), when the activation energy for the reaction is relatively close to $50~\mathrm{kJ/mol}$. However, ...
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What is meant by 'electrons of like/unlike rotation'?

Explaining the reasons behind electron repulsion, Radiochemistry asserts that, Interelectronic repulsion in related not just to electron pairing, but also to [the] angular momentum of the electrons....
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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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How are nuclei stable?

We all know that the density of the nucleus is very high. Nuclei are made up of protons and neutrons, and while protons have the same charge, they are closely packed in a nucleus. How does the ...
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How does radiocarbon dating work?

I'd like to better understand radiocarbon dating, however, I do not know what resources I must use in this regard. Could someone walk me through the process of radiocarbon dating with a few examples? (...
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What is a pure substance?

I found as I was preparing to teach "Introduction to thermodynamics" that there are varying definitions or examples of pure substances. For instance the text book I have to use "Thermodynamics an ...
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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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Which materials/compounds expand in response to cold temperatures?

Which materials or compounds expand in response to cold temperatures? This material or compound should expand at a slow (but constant rate), from about 0–20 months when in the following temperature ...
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Why is the rate of a reaction proportional to the concentrations of reactants raised to their stoichiometric coefficients?

Consider a gaseous state elementary reaction $$\ce{aA(g) + bB(g)} \overset{k_\mathrm{f}}{\underset{k_{\mathrm{b}}}{\ce{<=>}}}\ce{ cC(g) + dD(g)}$$ I know that for this reaction, $$\Delta G = \...
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What is the difference between ΔG and ΔrG?

Consider the reaction $$\ce{A -> B}$$ The reaction Gibbs free energy, $\Delta_\mathrm{r} G$ is given by the following equation $$\Delta_\mathrm{r} G = \Delta_\mathrm{r} G^\circ + RT \ln Q$$ Now ...
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What is the unit of pH?

I'm making some graphs and I have to label the axes. I want to be extra careful and put the units in even though the meaning of $\text{pH}$ is well known. But I have a problem (though a simple one): $\...
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What are the magnetic quantum numbers for the three real p orbitals?

The form of the p orbitals that we are familiar with are the $\mathrm{p}_x$, $\mathrm{p}_y$, and $\mathrm{p}_z$ orbitals: (source: ChemTube 3D) I also know that the p subshells have the quantum ...
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During phase change in matter, why doesn't the temperature change?

I was working on something in school and came across the question: Why does the temperature not change much during a phase change? I'm really not sure why this happens in matter and I couldn't ...
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what kind of suspension liquid should be used with ferrofluid (so it does not stain the glass)

Please take a look at the following video: I am working on a new project, and I need to find whats the best liquid to hold ferrofluid inside the glass (or maybe even plastic) container, so that ...
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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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Justification for Freezing Point Depression & Boiling Point Elevation in Solutions?

I was wondering if the following justification for freezing point depression and boiling point elevation are conceptually correct. The reason why I ask this question is because I have been self ...
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Transition state and free energy

We have products $\ce{A + B}$ combining to form $\ce{C + D}$ through the transition state $\ce{X}$. Are all reactions at least virtually reversible? Is the difference between a reversible reaction ...
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What makes substances explosive

Can someone explain clearly to me why some things are explosive?
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Why calcium chloride is used to melt ice over sodium chloride

I'm not quite sure I understand this. My question is: Calcium chloride is a salt used widely to melt ice on sidewalks and roads. Explain why one mole of $\ce{CaCl2}$ would be more effective than ...

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