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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18
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6answers
4k views

Does the Maxwell-Boltzmann distribution apply to gases only?

The Maxwell-Boltzmann distribution can be used to determine the fraction of particles with sufficient energy to react. I know that the curve applies to gaseous reactants and would like to know whether ...
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2answers
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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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1answer
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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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989 views

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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5answers
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Why do some chemical reactions require many steps?

I posted the following question in Physics SE and was advised to transfer it to Chemistry SE. I studied physics in college ten years ago and I recently started to learn biochemistry. I enjoy finding ...
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2answers
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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 ...
16
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4answers
371 views

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 ...
13
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4answers
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Why are sigma bonds stronger than pi bonds?

Why are sigma bonds stronger than pi bonds?
11
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3answers
43k views

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.
11
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3answers
418 views

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 ...
7
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2answers
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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 ...
6
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2answers
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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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1answer
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Why is Kc not affected by change in pressure?

Why is $K_c$ not affected by change in pressure? I know the mathematical explanation, but I don't really understand the reason when only looking at $K_c$. The explanation I know is with the reference ...
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1answer
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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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2answers
451 views

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 ...
15
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3answers
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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 ...
14
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3answers
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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{...
13
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3answers
745 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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9answers
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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 ...
8
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4answers
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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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1answer
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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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1answer
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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 ...
3
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1answer
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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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3answers
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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 ...
23
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1answer
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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 ...
17
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4answers
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Why is the electron-nucleus attraction modelled with only electrostatic interactions?

While reading about the structure of an atom, I've encountered (even in some renowned books) the statement that electrons and nuclei are attracted due to electrostatic, or Coulombic, attractions. ...
15
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1answer
391 views

How does conductivity work for non-redoxed ions?

Related (very similar, but here I want a mechanism) https://physics.stackexchange.com/q/21827/7433 By the Kohlrausch law, all ions contribute to the conductivity of an electrolyte. Now, as I ...
14
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2answers
737 views

Is zero-point vibrational energy an intensive or extensive property?

The zero-point vibrational energy (ZPVE) is the non-zero amount of non-electronic energy that molecules have even at zero kelvin and is purely quantum mechanical in nature. For a molecule with $c$ ...
14
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3answers
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Derivation of Maxwell–Boltzmann statistics

I have 2 questions regarding the derivation of the formula which calculates the probability of molecules having a particular amount of kinetic energy $E_x$ in a system of $N$ molecules. It states that:...
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4answers
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Why does liquid water form when we exhale on a mirror?

In the descriptions below, I always assume external pressure to be constant at 1 atm, the condition where daily observations are made. 1) When I exhale on a mirror, liquid water forms on the mirror. ...
12
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2answers
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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 ...
12
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2answers
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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 ...
11
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3answers
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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 ...
10
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1answer
857 views

When can I use concentrations instead of activities in equilibrium calculations?

To calculate an equilibrium constant, it is said that you should actually use activities $a_i$ instead of concentrations $c_i$. But it is also said that within a certain range, you can use ...
10
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2answers
377 views

How can enzyme/substrate reactions that adhere (largely) to quantum theory also require 'Newtonian' consideration of gravity?

I'd just like to ask for a little clarification here due to confusion from interdisciplinary studies. I'm currently reading the 1976 paper related to the recent 2013 Nobel Prize for Chemistry, by 2 ...
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0answers
607 views

Mixing two containers of each water and ethanol to varying degrees of purity

In the following question, the water/wine mixing problem was recast in chemical terms. However, partial molar volumes can make this a much more challenging problem than it appears on the surface. ...
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1answer
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How is it possible for a substance to have a high heat of vaporization but a low boiling point?

The final paragraph of Dissenter's question here is worthy of standing alone: [H]ow does one square a high heat of vaporization with a low boiling point? If it takes a lot of energy to vaporize ...
8
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1answer
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Simultaneous equilibria

Rephrasing: The following reactions are a small example of my big system $$(1)\ \ce{A + B <=> AB}$$ $$(2)\ \ce{AB -> CB}$$ $$(3)\ \ce{A -> C}$$ $$(4)\ \ce{C + B <=> CB}$$ with $...
8
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1answer
209 views

Is there any empirical upper limit for order of reaction?

I learned from one of my teachers that empirically reaction with order greater that 3 are never found. But, I am suspicious of the truth value of his claim. To add to my suspicion there is this paper ...
7
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1answer
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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 ...
6
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3answers
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How can sodium chloride melt ice or keep it frozen?

In European countries, they use $\ce{NaCl}$ or $\ce{KCl}$ to melt ice during the winter season. In Asian Countries, they use $\ce{NaCl}$ to keep the ice without melting, for example in ice cream and ...
5
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2answers
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Whose concentration is more 1 molar or 1 molal?

I was reading about concentration terms like molarity, molality, etc. and then I came across a question: which concentration is more: 1M(molar) of solute x dissolved in water vs. 1m(molal) of solute ...
5
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1answer
497 views

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....
4
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1answer
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Computing accurate vibrational and rotational contributions to the free energy of transition states and loosely bound complexes

Recently I have been dealing with a lot of transition states and relatively loosely bound ion-dipole complexes and I have some trouble figuring out how to make sure that the rotational and vibrational ...
4
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1answer
818 views

Difference in work done by a gas in reversible and irreversible isothermal expansion

Why there is difference in work done by gas in isothermal expansion when gas expands through reversible and irreversible process?
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2answers
339 views

Interaction between energy and entropy in highly elastic polymers?

Consider the interaction of energy and entropy in the highly elastic materials of an ideal polymeric network. Now Gibb's free energy cannot be used directly $\Delta H = \Delta G + T \Delta S$ where $\...
3
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3answers
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Effect of impurities on melting point

This is a 10th-grade question. The melting point of a substance decreases with increase in presence of impurities in it. The melting point of ice decreases from 0 °C to -22 °C on mixing salt in it ...
3
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2answers
717 views

Why does a dynamic equilibrium exist?

We know that at equilibrium Gibbs free energy is minimum. We also know that at equilibrium both forward and reverse reactions occur simultaneously, and we also know that for a reaction to be ...
3
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1answer
939 views

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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2answers
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How to calculate the equilibrium constant from the density of a nitrogen dioxide and dinitrogen tetroxide mixture?

The density of an equilibrium mixture of $\ce{N2O4}$ and $\ce{NO2}$ at $\pu{1 atm}$ and $\pu{373.5 K}$ is $\pu{2.0 g/L}$. Calculate $K_c$ for the dissociation reaction I did this: Let initial amount ...