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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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58 views

Expansion work by a gas when pressure is not explicitly constant?

How is it possible to calculate the work done by a gas when pressure is not explicitly stated to be constant? The question states: Calculate $\Delta H$, $Q$, and $W$ when $1\ \mathrm{mol}$ of He ...
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95 views

How to without calculations understand the liquid-liquid extraction efficiency?

I'm asking myself this question for a very long time and I haven't found a simple an good explanation yet for the fact that it's better (if you want to clean them) to wash your hands two times with ...
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1answer
70 views

Does a molecule need to be placed symmetrically in the unit cell?

I was watching an OCW by MIT (3.091X) on solid state. My previous notion of lattice point and unit cell got shattered after this example. Where are the unit cell and lattice points in this picture? ...
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1answer
678 views

Why do different gases have the same molar volume?

Molar volume: The molar volume, symbol $V_\mathrm m$ is the volume occupied by one mole of a substance (chemical element or chemical compound) at a given temperature and pressure. At S.T.P ...
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122 views

should charge balance be based on activities?

We have a fluid with $\pu{0.01 M}~\ce{HCl}$ and $\pu{0.02 M}~\ce{K2HPO4}$, so if we assume activity coefficients are $1$ throughout we can find pH as $6.800012$ and confirm that at that pH there is ...
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76 views

Maximum Concentration of Benzene possible

A question for my PhysOrg course states: Assume you are performing a reaction in which benzene is a reagent. What is the maximum concentration of benzene possible? Hint: what facts do you need to ...
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1answer
164 views

Why do the 4f and 5d orbitals have similar energies?

I've come to know this phenomena when learning about the exceptions to the aufbau principle. But there is no explanation of why 4f and 5d have similar energies (as I've read in some websites).
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119 views

PCl3 vs SCl2 boiling point

Why does sulfur dichloride have a lower boiling point than phosphorus trichloride? Is it to do with the higher number of electrons causing stronger id-id interactions in PCl3, or do we need to compare ...
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1answer
81 views

How many equal Xe–O bond length are present in (XeO_6)-3? [closed]

I tried to solve this problem by drawing a structure but the structure did not give me any answer and I was not able to draw the proper structure please help me out.
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1answer
43 views

Rate of a reaction [duplicate]

We know that the rate of a reaction $\ce{aA + bB -> cC + dD}$, the rate of the forward reaction is given by $r_\mathrm f = k_\mathrm f[\ce A]^p[\ce B]^q$ where $ a\neq p$ and $ b\neq q$ according ...
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20 views

Molarity of a substance in a solution when more fraction is given

Mole fraction of ethanol in a solution with water is $0.040$. What will be the molarity of ethanol in water? Density of water can be assumed to be $1\ \mathrm{g/mL}$. This is how I tackled the ...
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1answer
103 views

How are the LCAO ansatz and Slater determinant ansatz for the wavefunction connected?

Say we have a system of 2 electrons, an ansatz for the wavefunction is then a Slater determinant consisting of two orbitals. $$ \left|\Psi_{Slater}\right>= \frac{1}{\sqrt{2!}}\left(\left|\phi_1\...
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3answers
261 views

What is Nesbet's theorem?

In a talk by Frank Neese there was a slide with title "Nesbet's theorem" and an equation indicating something like "if your wavefunction has the double excitation amplitudes correct, then you can get ...
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2answers
46 views

Can there be any substance whose number of electrons and protons are different at neutral state

I want to ask that if there any substance that has the different number of electrons and protons at its neutral state
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1answer
33 views

States of matter.Liquid

In a book a read that a simple defination of liquid i.e."it is a material that assumes the shape of a container without filling it completely" but i cannot able to understand "without filling it ...
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1answer
196 views

What is 'power law' kinetics?

I've come across a lot of instances where 'power law' description of the kinetics was mentioned, but I cannot think of any integrated rate law that follows something like: $$A(t)=bt^n+c$$ I'm pretty ...
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1answer
79 views

How do we know that we have isolated a substance?

In The Historical Background of Chemistry by Henry M. Leicester page 131, the author says: Carl Wilhelm Scheele (1742-1786) [...] isolated many organic compounds, including hydrogen cyanide, lactic,...
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78 views

Equilibrium for very small amounts of reactants

Is there any difference in the concept of equilibrium when it comes down to dealing with extremely small amounts of reactants? Say we have $$\ce{A + B <=> C + D}$$ and $K_c$ is $100,000$. If ...
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84 views

How does the equilibrium shift when concentration of reactant and product are increased simulatenously

Before answering my question, take a look at this example $$\ce{2NOCl <=> 2NO + Cl2}$$ Let's suppose I have 2 mol/l $\ce{NOCl}$, 2 mol/l $\ce{NO}$, and 1 mol/l $\ce{Cl2}$ in a closed system. ...
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1answer
139 views

Why do electrons in sigma bonds have zero orbital angular momentum about the internuclear axis?

As per the title, why do electrons in σ-bonds have zero orbital angular momentum about the internuclear axis? As far as I know, the outermost electrons participate in bonding. Suppose that the 3p ...
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1answer
70 views

Understanding energy level splitting of an octahedral complex from spin-orbit coupling, 1st and 2nd order zeeman effects [closed]

While studying notes from a couple years ago, I encountered a diagram that I don't have further information on but would like to understand. While I believe I managed to find sources to guide me ...
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1answer
183 views

Excluded Volume in van Der Waals equation

I was just reading over the EOS topic when where I stumbled upon excluded volume. Although the pictorial representation of the hard sphere model clearly shows that the excluded volume is 4 times the ...
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1answer
45 views

Reaction kinetics - Organic chemistry

On an exam in organic chemistry I was asked why the reaction B proceeds faster than reaction A. I answered that: in reaction B, I- is a better leaving group than Cl-, the reason being it is a weaker ...
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1answer
155 views

Properties of azeotropes

Consider two liquids with vapour pressures in pure form being ${P^o}_A$ and ${P^o}_B$. ${P^o}_B >{P^o}_A$ which implies liquid B is more volatile. If the liquids formed an ideal solution, then a ...
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1answer
157 views

Balancing using oxidation number method

Balance the following equation using oxidation number method: $\ce{PbS + H2O2 -> PbSO4 + H2O}$ Attempt: I have written down the oxidation states of all atoms and we can observe reduction of ...
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2answers
509 views

Does the rate constant depend on pressure?

I have read in some books that when pressure increases, the number of effective collisions increases. Hence, $k$ depends on pressure. Is it true? If so, then why doesn't $k$ depends on concentration? ...
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81 views

Rotational barrier of biphenyl in ground and excited electronic states

I'm studying how the energy of biphenyl varies with its dihedral angle. I know how to describe the dependence quantitatively, but still, I don't know how to explain why the rotational barrier of ...
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0answers
43 views

Order of a complex reaction [duplicate]

Is the order of a complex reaction determined by the slowest step always? Is it so that order of a reaction is determined by rate determining step and since in a complex mechanism, we write elementary ...
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0answers
68 views

Oscillator strength of a transition

I am trying to compute the oscillator strength in a molecule. Therefore I've got its Energy level values plus both wave functions $\Psi_i$ and $\Psi_j$ for the transition I want to calculate the ...
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0answers
24 views

Where does hydrogen evolution occur when aluminium amalgam is placed in water - aluminium or mercury?

Let's say a drop mercury is placed on clean aluminium sheet; mercury amalgam is formed. The solubility of aluminium in mercury and vise versa at room temperature is fairly low. If the entire sheet is ...
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1answer
32 views

Will a container made of ice hold an alcoholic beverage that is below freezing in an environment that is also below freezing? [closed]

This very specific question that came after saw I a travel episode where they served alcoholic beverages in ice glasses in an ice bar. I wondered if the drink contents would stay in the glass (of ice) ...
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2answers
903 views

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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63 views

Molecular Geometry [duplicate]

Why is it that for a molecule with trigonal bipyramidal geometry, the lone pair is located in the equatorial region whereas for a molecular with an octahedral geometry, the lone pair is located in the ...
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1answer
373 views

Graphite: Thermal and electrical conductivity

I recently encounter this question: Compare, while providing an explanation, the thermal and electrical conductivity of graphite to that of diamond. Provided answer: Graphite conducts electricity(...
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86 views

Hückel method for biphenyl

I am trying to calculate the energy levels of biphenyl (see picture below) using Hückel method. Since this is my first time using Hückel I am a little confused. Biphenyl has 12 carbon atoms, ...
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1answer
96 views

Distinguish between two possible configurations for angular momentum in carbon atom

Consider the possible values of $S$ and $L$ for carbon configuration $1s^2 2s^2 2p^2$ and the corresponding rapresentations with arrows indicating the spins (consider only $S=0,L=0$ and $S=0,L=2$, as ...
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187 views

Why is the melting point of hydrogen fluoride lower than that of ammonia?

The melting point of hydrogen fluoride is -83.6°C, as compared to that of ammonia, which is -77.73°C. How does this make any sense? Both hydrogen fluoride and ammonia show hydrogen bonding, which "...
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1answer
94 views

In Bohr's model what is angular momentum in the lowest orbital?

Angular momentum is given by: $n\frac{h}{2\pi}$ where $n=1,2,3...$ And angular momentum is also given by: $\sqrt{l(l+1)}\frac{h}{2\pi}$ By the first equation angular momentum should be $\frac{h}{2\...
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1answer
425 views

Relative molecular mass of an unknown gas in a mixture

A gaseous mixture contains oxygen and another unknown gas in the molar ratio of 4:1 diffuses through a porous plug into 245 seconds, under similar conditions same volume of oxygen takes 220 seconds to ...
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1answer
58 views

Is the stoichiometric coefficient equivalent to the mole fraction?

I have a small problem to understand (or distinguish between) two concepts of free mixing enthalpy. In several textbooks, I found the following formula for the molar free mixing enthalpy (binary ...
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71 views

distillation of aqueous CAN solution for the purpose of a reversible and strong endothermic reaction in cool packs

I know calcium ammonium nitrate (CAN) is used for cold packs. By bending a small metal plate the salt gets dissolved in water (separated pack) and this process is endothermic. I would like to heat ...
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130 views

The nature of Chemical Bonds [closed]

It was nearly a year ago when I appeared for my first interview through the KVPY (a research oriented scholarship exam) when one of the questions the panel asked me was : What is a chemical bond? ...
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1answer
116 views

Do molecules change shape when they become vibrationally excited?

In quantum mechanics we learn the the classical picture of a vibrational excitation is wrong. The atoms will 'vibrate' only while the molecule is in a coherent superposition of two vibrational states, ...
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1answer
95 views

“Proof” that the chemical potential is less than zero for irreversible material changes

From Levine's Physical Chemistry 6th edition: In a closed system, $\mathrm dG = -S\,\mathrm dT + V\,\mathrm dP$ not only applies to constant composition but also reversible changes in composition (...
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1answer
70 views

How can prepare ppb solution of sodium [closed]

How can prepare 1000ppb solution of sodium from sodium chloride salt in one litre.
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177 views

Coordination number of spheres (all identical) in hexagonal close packing (HCP)

I was revising solid state chapter when I came across this question What is the maximum coordination number of an atom in a hcp crystal structure of an element? The word 'maximum' drew my ...
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How to calculate a molecule's oscillator strength?

I was wondering how to calculate a molecule's (e.g. biphenyl) oscillator strength using a very simplified model. If I want to calculate a molecule's excited states a good start is something like a ...
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4answers
117 views

A simple equilibrium problem

Take the following simple equilibrium: $$\ce{CH3COOH + H2O <=> H3O+ + CH3COO-}$$ Ethanoic acid will act as a typical Brønsted-Lowry acid and donate a proton to water, forming a conjugate acid-...
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76 views

Calculation Radial distribution function using Potential

I am trying to understand an article where it is describing the accuracy of a potential function obtained via a NN (neural network). To test the ability of the NN potential to describe also ...
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1answer
88 views

Why are the concentrations in the equilibrium constant multiplied? [duplicate]

$$\alpha\,\mathrm{A}+\beta\,\mathrm{B}+\cdots\rightleftharpoons \rho\,\mathrm{R}+\sigma\,\mathrm{S}+\cdots$$ The equilibrium constant for this reaction can be calculated using the following formula: $...