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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Estimating the Comparative Volatility of Compounds with a Common Ion

Reading through different related topics here on Stackexchange I've come to accept the notion that it isn't straightforward to predict volatility from the sheer atomic composition of a compound. I'm ...
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241 views

Difference between ΔQ and dQ

What is the difference between $$\Delta Q$$ and $$\mathrm dQ$$ in thermodynamics? I think one is used when the change is very small and the other with bigger changes, but I am not sure.
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Half life of first order reaction in case of variable volume

I have a doubt about the meaning of half-life of a first-order reaction. Earlier I used to think that it is the time in which concentration of reactant reduces to half of its initial but recently I ...
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4answers
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What exactly is happening when sodium, potassium, or other alkali metal explodes in water?

There are lots of videos on YouTube showing sodium, potassium, etc. exploding when dropped into water (this, for example). I understand that when an alkali metal is exposed to water, a violent ...
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Does the methyl radical form a stable solid at low temperatures?

As the title indicates: can $\ce{CH_3^.}$ form a stable solid at cryogenic temperatures? Of course, bulk methyl would dimerize very quickly at room temperature, but I'm interested in whether it can ...
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337 views

Enthalpy change of gas expansion against constant external pressure

A certain mass of gas is expanded from $\pu{1 L}$ at $\pu{10 atm}$ to $\pu{4L}$ at $\pu{5 atm}$ against a constant external pressure of $\pu{1 atm}$. If initial temperature of gas is $\pu{300 K}$ and ...
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Methylene “anti” Jahn-Teller effect

Yesterday a Reddit user posted a page from Morrison's Organic Chemistry in which it is said that singlet methylene is less stable than triplet methylene. Another user asked basically the same I'm ...
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Calculating energy density and carbon footprint given heats of formation

This is the problem that I'm attempting to solve: Calculate for the energy density and carbon footprint of a reformate manufactured via continuous catalytic regenerative reformer (Benzene, C6H6 – 17%;...
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You have a solution of 1 mol of a solute A in 250gms of water..what will be the temperature required to freeze the mixture completely? [closed]

If we try to calculate the freezing point of mixture using formula substitution.-∆Tf=Kf*m -∆Tf=1.86×1/.25, ∆Tf comes around 7.44℃..so Freezing point is now -7.44℃... Now if someone cools this mixture ...
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1answer
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Rigorous reason behind internal energy change being zero while mixing

A container is divided in two parts: one part contains oxygen gas $(n_1$ moles, at temperature $T_1)$ and the other part contains helium gas $(n_2$ moles, at temperature $T_2).$ The partition ...
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Why is Avagadro's number=6.023×10^23 [duplicate]

I've wondered how Avagadro was able to draw out the Avagdro number as 6.023×10^23. How did he figure out that number?
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Pressure exerted by a particle of ideal gas

There is a step I fail to grasp in the proof that for a monatomic ideal gas, $U=\frac{3}{2}PV$, where $U$ denotes the internal energy of the gas. The proof involves considering a particle with mass $m$...
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Existence of Hartree–Fock limit

I'm reading McQuarrie's Physical chemistry textbook [1]. On p. 281, the author explains: If we use a more flexible trial function of the form in which $\psi(\mathbf{r_1},\mathbf{r_2})$ is a product ...
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1answer
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Finding root-mean-square speed of neon using kinetic energy

Find the root-mean-square speed of $\ce{Ne}$ atoms at the temperature at which their kinetic energy is $\pu{6.24 kJ mol-1}.$ I tried using the kinetic energy formula $$\mathrm{KE} = \frac{mv^2}{2},$$ ...
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physical chemistry [closed]

Ethanoic acid and ethanol react reversibly to form ethyl ethanoate and water. (It doesn't matter if you have never come across this before.) In a closed system, a dynamic equilibrium is set up....
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What roles do neutrons play in an atom?

An atom typically consists of electrons, protons, and neutrons. Electrons are negatively charged and participate in bonding to stabilize the atom. Conversely, protons are positively charged and ...
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HSAB theory details [closed]

I have read a bit about HSAB theory and its applications, but I couldn't find sources for details about the theory and especially the direct explanation for the reason of that phenomena.So, what books ...
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26 views

Primary kinetic isotope effect

Why does the transition state in both protonated and deuterated molecules have the same energy? My textbook assumes that during transitional state the isotopic bond is half-broken or on its way to ...
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Mulliken Spin Density

I would like to know if a Mulliken population analysis to calculate spin densities is in general a valid choice. I see that it is made use of, for example here1. So up to-date researches apparently ...
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24 views

Enthalpy of Atomisation [closed]

Is there any difference between the enthalpy of atomisation of H2 molecule and enthalpy of atomisation of H..Why or Why not ?? Because The Wikipedia definition says that for a molecule it is defined ...
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Correct representation of Lewis structure for sulphate ion

sulphate ion has tetrahedral geometry as predicted by VSEPR theory; the four S-O bonds are indistinguishable and have the same bond order, so the Lewis structure is better represented avoiding S=O ...
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1answer
132 views

Why is platinum denser than gold?

The atomic masses of gold and platinum are 196.96657 u and 195.084 u respectively, meaning that (on average) an individual gold atom is heavier than an individual platinum atom. At the same time, the ...
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How does iceskating work?

I was willing to understand a little bit better the physics behind iceskating. Messy thoughts From New Scientist (1964) (probably not the most up-to-date reference $\ddot \smile$), I found a diagram ...
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When, if ever, can standard enthalphy be subsituted for standard chemical potiential?

I'm reading through a combustion modeling paper by Gordon & McBride which is a bit scant on some of the algebra & calculus used to derive their equations. My advisor and I have derived nearly ...
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3answers
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Why isn't the American nickel magnetic?

I never thought that modern American nickels actually contained nickel anymore. However, according to this wiki article, the coins actually do contain 25% nickel, the rest being copper. And yet, no US ...
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Electroplating of Copper [closed]

I know that oxygen gets liberated at the anode in the electrolysis of aqCuSO₄ solution in the presence of inert platinum electrodes as the oxidation potential of H₂O is higher than that of SO₄²⁻,...
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1answer
78 views

Apparent paradox regarding spontaneity

Consider the following analysis ( rev and irrev respectively refer to a reversible and an irreversible path, between the same initial and final states): $dU_{rev}=dq_{rev}+dw_{rev}$,and,$dU_{irrev}=...
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206 views

Decay scheme of sodium-24

Sodium-24 decays into an excited nucleus of magnesium-24. Magnesium reaches nuclear ground level via two consecutive gamma emissions. What are the half-lives (time scales) for these two processes? I ...
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1answer
78 views

Colour due to transmission and reflection

It makes sense to me that when looking through a sample (observer | sample | light), it should appear as the opposite of the light absorbed, but it does not make sense to me to expect the same when ...
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Doubt of solid state [closed]

A compound is formed by two elements Y and Z. Y ions in 2/3 of octahedral holes and 1/3 octahedral holes are vacant in closest packed array of Z ions. The formula of the compound is. (Options: Y2Z3, ...
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7answers
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Positive or Negative Anode/Cathode in Electrolytic/Galvanic Cell

In a galvanic (voltaic) cell, the anode is considered negative and the cathode is considered positive. This seems reasonable as the anode is the source of electrons and cathode is where the electrons ...
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2answers
64 views

Reversibiliy and equilibrium

My textbook states "A system is in thermodynamic equilibrium if it is in Thermal, Mechanical, and chemical equilibrium simultaneously". The Wikipedia article on "reversible processes&...
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Stoichiometric defects in silver bromide

Stoichiometric defects (Schottky and Frenkel defects) in ionic crystals are formed due to irregularities in the ionic arrangement which maintain the stoichiometry and electroneutrality of the solid. ...
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1answer
569 views

Salt Hydrolysis

Referring to this, when "hydrolysis of a salt" is its reaction with water: How is it that both the cation and anion of the salt are reacting in an equal amount with water? To state this specifically ...
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1answer
666 views

Can silicon dioxide melt at 20 °C?

Can $\ce{SiO2}$ melt at $\pu{20 ^\circ C}$? I have searched the web for $\ce{SiO2}$ phase diagram, but it seems to me that almost all the graph I can find have the temperature axis where the minimum ...
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1answer
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Henderson-Hasselbalch Equation when pKw is not standard [closed]

The henderson-hasselbalch equation is $\mathrm{pH} = \mathrm{p}K_\text{a} +\log\frac{[\ce{A-}]}{[\ce{HA}]}$. How does it change when $\mathrm{p}K_\text{w}$ is not standard?
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Calculating dissociation constant at different pKw values

What are the standard approaches to calculating how dissociation constants for acids and bases, $\mathrm{p}K_\mathrm{a}$ and $\mathrm{p}K_\mathrm{b}$, change as the ionic product of water, $\mathrm{p}...
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1answer
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Vapour Pressure of solutions and mol fractions

From a solution (ideal) which contains the components A and B ,the vapours over the solution at a temperature T are collected and condensed into liquid and again brought to the same temperature T. ...
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1answer
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I am having trouble with this question on net ionic equation [closed]

Convert into molecular equation: The reaction between potassium dichromate, ferrous sulphate and sulphuric acid:- Cr2O7^2- + 14H^+ + 6Fe^2+ ---> 6Fe^3+ + 2 Cr^3+...
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1answer
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Is reverse osmosis a fancy term for filtration?

In Reverse Osmosis (RO) systems, the water to be purified is pushed against a semipermeable membrane and comes cleaner the other side, the path of water is the reverse of regular osmosis, hence the ...
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How does electrolytic conduction depends on the size of ions & their solvation?

The textbook says, The smaller the effective size of the ion after solvation, the greater will be its conducting power." But, on the Internet, it is explained as follows: Radius decreases, ...
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50 views

Positive work in the Carnot cycle

In the Carnot cycle the overall work done on the piston is positive. I am having trouble with the real meaning of that. Does positive work mean the piston is moved up and has, over the course of the ...
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1answer
60 views

When to use chromatographic separation vs vacuum-assisted rectification distillation?

I'm trying to understand a bit more about industrial scale separation processes. One particular element I'm interested in is the question as to when chromatography (HPLC or Simulated Moving Bed) may ...
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1answer
38 views

Multiday lab courses in classic chemistry [closed]

This time of the year I usually get some extra income I can use on interesting courses. Usually rescue or sport courses. However, I also have a master degree in chemistry, and would like to spend some ...
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1answer
59 views

What is the number of electrons being transferred in electrolysis of aluminum oxide? [closed]

$$\ce{ 2/3 Al2O3 -> 4/3 Al +O2}$$ As we know, to find the Gibbs free energy, we need to use this formula: $$ G= -nFE^\circ$$ What would $n$ be in this case? After breaking it down into the half ...
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1answer
36 views

Benzene - MO states [closed]

When constructing the different combined molecular orbital combinations (from the original 6pi orbitals) we normally say that p orbitals "combine" to produce the different molecular orbitals ...
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57 views

What is the differnce between energy and enthalpy?

I cannot understand the difference between thwe two although the internet suggests me there is a difference. Even asked my chemistry teacher but he could not give an explanation.
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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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2answers
901 views

Why is the rate of a reaction always positive and why? [closed]

Why rate of reaction can never be negative? And why rate is an experimental derived expression?
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102 views

Why does de Broglie wavelength lambda = c / frequency?

For a chemistry problem, I want the velocity of the de Broglie wave to agree with one of these values: A) $c^2/v$ B) $h\nu/mc$ C) $mc^2/h$ D) $v\lambda$ We know that $$\lambda = \frac{h} {mv}$$ But ...

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