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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11
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3answers
41k 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
385 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 ...
8
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4answers
3k views

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 ...
7
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2answers
4k views

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 ...
5
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2answers
13k views

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 ...
19
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2answers
3k views

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 ...
18
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2answers
446 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 ...
17
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5answers
8k views

Evidence of orbitals?

How do we know that there are different types of orbitals? For example, what evidence is there for the existence of p orbitals instead of there being multiple s orbitals (for example, why isn't the ...
16
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4answers
359 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 ...
15
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3answers
12k views

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
8k views

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
701 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 ...
13
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4answers
75k views

Why are sigma bonds stronger than pi bonds?

Why are sigma bonds stronger than pi bonds?
11
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9answers
1k views

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 ...
7
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1answer
54k views

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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2answers
25k views

Why is Kc not affected by change in pressure?

Why is $K_\mathrm{c}$ not affected by change in pressure? I know the mathematical explanation, but I don't really understand the reason when only looking at $K_\mathrm{c}$. The explanation I know is ...
3
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1answer
4k views

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 ...
2
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1answer
18k views

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 ...
25
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3answers
20k views

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
19k views

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
2k views

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
388 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
690 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
1k views

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:...
13
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4answers
3k views

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
4k views

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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2answers
3k views

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
2k views

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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2answers
368 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 ...
9
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1answer
1k views

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 ...
9
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0answers
590 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. ...
8
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1answer
169 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 ...
8
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1answer
2k views

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 $...
7
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1answer
6k views

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
12k views

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
12k views

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
479 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
1k views

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 ...
3
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2answers
333 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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1answer
837 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 ...
3
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2answers
656 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
1k views

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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2answers
3k views

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 ...
0
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1answer
7k views

When Z>1 are repulsive forces dominant or are attractive forces dominant?

According to PVreal = ZRT If z >1 repulsive forces are dominant or are attractive forces? Different teachers are telling contradictory answers and after referring a few books, I have become more ...
14
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5answers
19k views

Why is Charcoal such an excellent adsorbent?

Charcoal (also activated charcoal) is known to adsorb a huge variety of substances including a variety of paints, dyes and many different kinds of ions. Moreover, the amount of adsorption at normal ...
13
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1answer
8k views

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 ...
12
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2answers
3k views

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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4answers
14k views

Maximum temperature attainable by thermite reaction

In the thermite reaction $$\ce{2Al(s) + Fe2O3(s) -> 2Fe(l) + Al2O3(s)}$$ what is the maximum temperature attainable by the products? Assume reactants at $25\ \mathrm{^\circ C}$ and $1\ \mathrm{...
10
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1answer
262 views

Why are complex basis functions not used in electronic structure calculations?

I've noticed while reading about Hartree-Fock in Modern Quantum Chemistry (Szabo and Ostlund) that most basis sets use only real basis functions. What I'm wondering is why aren't complex basis ...
10
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1answer
759 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 ...