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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Why does the equilibrium constant change on scaling the stoichiometric coefficients of a reaction?

I read in my textbook that if we multiply a chemical reaction by some factor(let's say $b$) its new equilibrium constant becomes $K^b$.But I don't understand why this happens.. What is the difference ...
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Rule of mutual exclusion: is the converse true?

There is a "famous" rule in spectroscopy,1 that goes this way: If a compound is centrosymmetric, then its normal vibrational modes cannot be simultaneously IR and Raman active. and this is simple ...
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Why should a system try to reverse any change that has been done to it?

I understood Le Chatelier's principle perfectly but my question is: Why should a system try to reverse any change that has been done to it? I understand equilibrium constant, mole concept and ...
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Differences between chemical physics and physical chemistry?

What makes these two seemingly identical topics separate? What does each field more focus on, like do chemical physics researchers study more the atomic/molecular interactions while the physical ...
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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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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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Why are the total energies of transition states so commonly corrected for zero-point vibrational energy?

So often I see total energies of transition states corrected for zero-point vibrational energy which always confuses me. Zero point energy is the lowest energy that a ground state minimum energy ...
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Why are sigma bonds stronger than pi bonds?

Why are sigma bonds stronger than pi bonds?
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Why do some gases have lower value of Z for a particular pressure?

In the above graph,the minima of the curve for methane is more than that of nitrogen. Also, for a given value of pressure, the value of $Z$ for methane is less than that of nitrogen. They seem to meet ...
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Has IUPAC been inaccurate in their 1994 definition of kinetic stationary state?

Summary of IUPAC definition $\def\d{\mathrm{d}}$In IUPAC recommendations from 1994[1, 1166–1167], the authors discuss the process $$\ce{A <-->[$k_1$][$k_{-1}$] X\\ X + C ->[$k_2$] D}$$ A ...
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How is enthalpy change equal to heat supplied at constant volume?

We know that: $$\begin{align} H &= U + pV \\ \Delta H &= \Delta U + \Delta (pV) \end{align}$$ Now, If $p$ is constant, $$\begin{align} \Delta H &= \Delta U + p\Delta V \\ &= q_p + ...
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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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Are there any (simple) molecules with very different absorption and emission dipole directions?

When a single molecule absorbs or emits light, it does so perpendicular to the direction of the respective transition dipole. In principle, the directions of the dipoles for absorption and emission ...
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What properties a compound should have to be called an “ice”?

I wonder what compounds are usually called "ice" and what properties do determine it? I can bring some examples of ice and not. Not ice: $\ce{H2S}$ cellulose sugar graphite lithium solid mercury ...
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Reaction mechanism in fluoride adsorption to aluminum oxide

Despite a fair amount of research (excluding non-open access journals, to which I have not got access), I cannot seem to find an explanation of the process that takes place when $\ce{F^-}$ adsorbs to ...
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How different concentration of HCl on magnesium affect the enthalpy change

I had conducted an experiment where I reacted a $0.3\ \mathrm{g}$ strip of magnesium with $100\ \mathrm{mL}$ of varying concentrations of $\ce{HCl}$: $1\ \mathrm{mol/L}$, $2\ \mathrm{mol/L}$ and $3\ \...
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Ellingham Diagram

My textbook says the following about the Ellingham diagram: Each plot is a straight line except when some change in phase ($\ce{s -> liq}$ or $\ce{liq -> g}$) takes place. The temperature at ...
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In rovibrational spectra, when exactly is the Q branch forbidden?

I'm aware that the Q branch ($\Delta j = 0$) is usually not observed in rotational-vibrational spectra of diatomic molecules in the gas state, however, I've heard different things about when exactly ...
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Why doesn't a table sublimate, while ice does?

A table does not sublimate, and nor does a spoon. Ice does, however. What is the fundamental difference?
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What makes substances explosive

Can someone explain clearly to me why some things are explosive?
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What is the point of using molality when we have molarity?

Molality and molarity are both concentration terms. Given molarity's popularity molality seems rather antiquated. Why is molality still discused in modern chemistry classes?
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Is there a reason for the mathematical form of the equilibrium constant? [duplicate]

Why are the two molarities multiplied and not added, and why is each raised to the power of the coefficient rather than multiplied by it? What is the reasoning behind this form? Was it simply ...
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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 the height of an hcp lattice?

A hexagonal closed packing (hcp) unit cell has an ABAB type of packing. For calculating the packing fraction we require the volume of the unit cell. Volume of hcp lattice = (Base area) $\cdot$ (...
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Does gravity affect the trajectory of gas particles?

We're studying the kinetic theory of gases in school, and one of the points that was brought up was that: "Gases consist of particles in constant, random motion." How is it possible for gas particles ...
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Why does ZnO turn yellow when it is hot?

Why does ZnO's color become yellow when it is hot? I have read that it is because of physical properties, but I need a more elaborate explanation.
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Destabilization of antibonding MO vs stabilization of bonding MO

My textbook writes that The net energy stabilization due to the occupation of a bonding MO is equal to the net energy destabilization due to the occupation of the corresponding antibonding MO. ...
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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 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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Difference between Reaction Rate and Rate Law?

Lets say you have the reaction $$\ce{$a$A + $b$B -> $c$C + $d$D}$$ The rate of the reaction is given by the change in initial concentration over the change in time. $$-\frac{\Delta \left[\ce A \...
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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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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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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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A misunderstanding about the energy profile of reactions with a catalyst involved

All of us are aware of the importance of the catalysts in bio-chemistry. For a high school learner like me, catalysts ,and therefore, enzymes play a bridge-like role that connect high school bio to ...
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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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What should the Rehm-Weller equation look like?

The Rehm-Weller equation, which defines the energy change for photoinduced electron transfer according to Lakowicz "Principles of Fluorescence Spectroscopy" [1, p. 337] should look like: $$\...
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What is the opposite of a surfactant?

Surfactants decrease surface tension between two fluids. What do you call a substance that increases the surface tension between two fluids? How is this effect acheived?
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How does absorption spectroscopy work?

So a beam of white-light photons passes through a gas and various photons, having exactly the same energy as the gas's molecular energy transition state deltas, get absorbed. We then get a ...
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What are the major factors that influence LogP

The partition coefficient is a very useful metric in characterizing compounds particularly drugs. What chemical properties influence that partitioning? Are chemical moieties additive in their LogP? ...
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Why does small value of van der Waals gas constant “b” ensure easier liquefication?

Larger value of b signifies that the size of the molecules is larger and hence the forces acting should become larger due to increased surface area.This should ensure easier liquefication. But my book ...
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Explaining Ionization to kids

I have a quest to explain a physical, chemical theory to kids of age 8-12. The topic is "Ionization". We all understand from early experiments of Physics and Chemistry (the Millikan oil drop ...
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Verifying data acquired from a molecular dynamics simulation

This question is about verifying a system simulated with MD. I am currently building a model of a two phase gas-liquid system, where I initially have $\ce{N2}$, $\ce{CO2}$, $\ce{Ar}$, $\ce{He}$ and $\...
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How to calculate partition coefficients?

Below is a transcript of this doubtful question: A weak monobasic organic acid $\ce{HA}$ is soluble in both water and $\ce{CHCl3}$. $\pu{500.0 cm3}$ of a solution of $\ce{HA}$ in $\ce{CHCl3}$, ...
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Calculating Triplet-Triplet Spin-Orbit Coupling

According to the Q-Chem manual [1], the singlet-triplet spin-orbit couplings between a singlet excited state $I$ and a triplet excited state state $J$ is: $$\langle\Phi_\text{singlet}^I|\hat{\mathrm H}...
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How do I excel in Chemistry? [closed]

I wanted to ask an online group of experts/community about how I could improve in chemistry. I study in 10th grade currently and don't get very good grades. Most of the time its a B or C. I have to ...
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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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Why do 3d orbitals have lesser energy than 4s orbitals in transition metals? [duplicate]

This is quoted from Jim Clark's Chemguide For reasons which are too complicated to go into at this level, once you get to scandium, the energy of the 3d orbitals becomes slightly less than that of ...
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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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Why does the C–C bond have extremely weak absorptions?

My book, Pavia's Introduction to Spectroscopy, tells me this about the infrared spectrum of alkanes: $\ce{C-C}$ : Stretch not interpretatively useful; many weak peaks. This is very interesting. ...
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Why exactly does precipitation occur?

In a solution, we have ions floating around but when we have a precipitate, they're arranged as they would be in a solid. This conversion should cause a decrement in the entropy of the system and ...

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