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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When water boils at its boiling point how do we call it as an equilibrium?

On boiling, water becomes a vapor, liquid volume decreases and vapour volume increases. Since volumes are not constant, how do we call it as equilibrium?
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Why does a buffer work best at the pH closest to its pKa?

I think I'm having trouble understanding exactly how the $\mathrm{p}K_\mathrm{a}$ relates here on a conceptual level. For example I know that carbonic acid works somewhat well as a buffer at $\mathrm{...
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Justify the value of the product of the uncertainty in position and momentum

The Heisenberg uncertainty principle states that: $$\Delta x\Delta p_x\ge\frac12\hbar$$ But more generally for operators $A$ and $B$: $$\Delta A\Delta B=\frac12|\langle[A,B]\rangle|$$ However, $[x,p_x]...
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Using Arrhenius equation to calculate the ratio of two reaction rates

For two reactions, $$\begin{align} \ce{A ->[$k_1$] B} \tag{1} \\ \ce{A ->[$k_2$] C} \tag{2} \end{align}$$ you have determined that the activation energies are $\pu{71.9 kJ/mol}$ for $(...
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Is sublimation a property of all substances?

I wonder if sublimation can be done only on certain substances like iodine, or it can be done on all substances. In other words, can sublimation be done, for example, on water? If it cannot, then why?...
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749 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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What are the units that measure free energy?

Is it $\pu{kJ/mol}$? I have done problems and I always end up with these units, but is this correct?
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1answer
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What is the exact definition of state functions? How to determine state functions?

How do you know if something is a state function? I know Internal Energy (U) is a state function since it is the sum of energies, which is constant at a given time. However I don't know for sure if $...
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Depression of freezing point [closed]

Recently, I learnt that by adding a non-volatile solute to a solvent, the vapour pressure of the solution is decreased and consequently its boiling point. Facts and definitions freezing point: "The ...
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1answer
120 views

Chlorofluorocarbons rising

Chlorofluorocarbons are heavier than atmospheric gases. Then how are chlorofluorocarbons produced at the surface able to reach the ozone with such a heavy mass ?
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Determine bond length via rotational spectroscopy

Estimate the bond length of 12C16O (pure rotational spectrum) given J''=3 (15.356 cm-1) and J'=9 (38.356) I understand that the first step is the calculate the rotational constant. So to do this I ...
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Internal Energy vs Enthalpy

I struggle to see the difference between internal energy and enthalpy, or at least the need for differentiation between the two. I understand that enthalpy, supposedly unlike internal energy, accounts ...
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1answer
973 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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Is there any formula that can be used to find loss of mass due to binding forces in atomic and sub atomic particles?

Atomic weight of Br-79 is 79.641 if you add the masses of protons and neutrons. However, in periodic table, it is less than the value given here. How is the difference arrived at for all the elements ...
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1answer
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Computing the concentration of a reactant given formation of products

At high temperatures nitrogen dioxide decomposes into nitrogen oxide and oxygen gas according to $$\ce{2 NO_2 -> 2 NO + O_2}$$ I have a question now. Let's say at a certain temperature I'm given ...
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Why is absolute zero unattainable?

We were dealing with the Third Law of Thermodynamics in class, and my teacher mentioned something that we found quite fascinating: It is physically impossible to attain a temperature of zero ...
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Molecular Imaging - Any surprises to be had?

Molecular imaging, using STM and AFM technologies, appear so far to visually prove most of what we already know about chemical structures, such as VSEPR theory. For instance, here are the ideal ...
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Solutions of Group 1 and Group 2 metals in Ammonia

When we add Group-1 and Group-2 metals to liquid ammonia, they dissolve to form metal cations and solvated electrons. $$\ce{M + NH3(liq) -> M+ + e-}$$ Now, when the G-1 solutions evaporate, we get ...
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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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What is antisymmetric exchange? What is J-strain? Where does it come from?

I'm reading a paper1 by Sanakis, et al. that characterises the magnetic coupling in the $\ce{Fe3S4}$ clusters present in bacterial ferredoxin II and beef heart aconitase as arising through something ...
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Why does a mixture of siloxene and cerium(IV) sulfate luminesce?

I performed an experiment where siloxene and cerium(IV) sulphate were mixed together: when I did so the mixture produced an orange-yellow glow. Why does it glow? What is it about the two chemicals in ...
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Is there a least reactive chemical species?

As a total chemistry layman I enjoyed reading "Why doesn't $\ce{H2O}$ burn?", but it prompted another question in my mind. One of the answers there was that $\ce{H2O}$ can burn in the presence of a ...
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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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Why are sigma bonds stronger than pi bonds?

Why are sigma bonds stronger than pi bonds?
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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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What is melting? Which bonds do we break to melt something?

To melt diamond, we have to break the covalent bonds, which we can consider 'intermolecular' because it is one giant molecule. To melt Methane, we have to break the van der Waals (intermolecular) ...
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Why are equations of state for a non-ideal gas so elusive?

The ideal gas equation (daresay "law") is a fascinating combination of the work of dozens of scientists over a long period of time. I encountered Van der Waals interpretation for non-ideal gases ...
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Why was Avogadro's number chosen to be the value that it is? [closed]

I understand that the mole is an extremely useful way of measuring and understanding atomic quantities. I did find some history of the number in its wikipedia article, but I did not find an answer to ...
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What is a rigorous definition of gas volume, and how is the Van der Waals equation derived?

I am confused about the justification for the corrections to the ideal gas law in the Van der Waals equation: $$p=\frac{nRT}{V-nb}-a\left(\frac{n}{V}\right)^2$$ I understand that the equation ...
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Hierarchy of electronic wavefunctions

The previous question contained too much unnecessary information and was edited. I am wondering about the "hierarchy" of wavefunctions. If one can combine atomic orbitals (AO) into molecular orbitals ...
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Why is the van der Waals coefficient b equal to four times the volume of the particle?

In the van der Waals equation of state $$\left[p + a\left(\frac{n}{V}\right)^2\right](V-nb) = nRT$$ the coefficient $b$ is supposed to represent the volume occupied by the particles. Why then is it ...
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Free chemical database to search for physical properties?

I am looking for an Open-access tool that can search "backwards", eg. give me all the structures corresponding to a given heat of formation, critical pressure, etc. It would be even better if the ...
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Polarising power

I don't understand this statement written in my textbook about the polarising power of cations. "If two cations have the same size and charge, then the one with pseudo noble gas configuration ( with ...
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Which one, Mulliken charge distribution and NBO, is more reliable?

Sometimes the Mulliken and NBO turn out to be so different that I can't decide which one I can trust. I've heard that Mulliken is inaccurate, but is NBO always accurate? And should I use Gaussian or ...
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Reason for negative Joule Thomson coefficient of Helium and Hydrogen at NTP conditions

Recently, while reading my textbook I came to know that Helium, Hydrogen and Neon are the only gases which have negative Joule Thomson coefficient at NTP conditions, i.e heating effect is observed ...
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Why is the boiling point of water and ammonia so different?

Boiling point of water is 100 degrees Celsius, while boiling point of ammonia is minus 33 degrees Celsius, which makes 133 degrees difference. Now when we discuss value of boiling point, we also say ...
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Can molecules diffuse upstream when emptying a cup of tea?

I was drinking some tea, and I randomly decided to pour some into the toilet in an unbroken stream; can particles travel "upstream" quickly enough to reach the mug of tea? How fast can diffusion occur ...
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Why exactly does molten NaCl explode, when it is poured into water?

Why does molten $\ce{NaCl}$ explode, when it is poured into water? $\ce{NaCl}$ has a high melting point, $1074\ \mathrm{K}$ ($801~\mathrm{^\circ C}$). $\ce{NaCl}$ has a molar mass of $58.44\ \mathrm{g/...
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854 views

Effects of atmospheric gases on colour of aurora

Recently I moved to the north of Norway, so I've seen quite a few auroras. Because the colour is mainly green, I was sure this must be due to electron transitions in $\ce{N2}$ (because the atmosphere ...
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Do Electrons Really 'Spin'?

With regard to the 'Electron Spin Number', lots of websites mention that electrons don't really spin and that the electron spin number has nothing to do with any physical spinning. However, my ...
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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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Does a diatomic gas have one or two vibrational degrees of freedom?

Many sources claim that the vibrational degrees of freedom for a diatomic gas is one, but there are also a few which claim it to be two. Which is correct, and why is there even any ambiguity to begin ...
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What is the mass of 1 atom of carbon?

I know that relative atomic mass of $\ce{^{12}C}$ is $12~\mathrm{u}$. Therefore mass of $1~\mathrm{mol}~\ce{C} = 12~\mathrm{g}$ \begin{align} \text{mass of }6.022\cdot 10^{23} \text{ C atoms} &= ...
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Difference between ideal & ideal-dilute solution

These two terms has been frequently used in Physical Chemistry by Atkins. He describes Raoult's Law using ideal solution while Henry's law using ideal-dilute solution. I thought, at first they were ...
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Why does increasing the pressure cause ice to melt more?

This question was given under chemical equilibrium. Why does increasing the pressure on this system cause ice to melt more? $$\ce{H2O(s) <=> H2O(l)}$$ I'm pretty sure this has to be ...
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How do electrons travel through nodes

I understand this is a basic question, but I'm having such a hard time wrapping my head around it. I'm trying to avoid thinking about it as an actual "particle" but as a wave, but that confuses me too....
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Is this a possible explanation as to why sigma bonds are stronger than pi bonds?

Sigma bonds form due to the axial overlap of two orbitals whereas pi bonds form due to their lateral overlapping. Since the former is more effective, we can say that sigma is stronger than pi. We mean ...
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Is dipole moment a vector?

Isn't it a misconception to assume that dipole moments are similar to vectors in their behavior? My reasons are as follows: Let us take an example: Methane ($\ce{CH4}$) Clearly, the hydrogen atoms ...
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Why does ice cream make soda fizz?

I've noticed that adding a chunk of ice cream to soda makes the soda fizz slightly near the soda-ice cream interface. I thought it was a physical effect due to the temperature, but adding ice has no ...
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Why does sunlight cause colors to fade?

If you leave something outside, its colors seem to inevitably fade or bleach due to exposure. Is this due to UV absorption? What sort of mechanism causes this - is it that man-made dyes deform on a ...

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