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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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1answer
1k views

How does WD-40 remove rust?

One of the often-touted uses of WD-40 is that it "dissolves rust". The official website states that WD-40 "breaks down the bonds between metal and rust". I can't understand how this would work, since ...
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2answers
4k views

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
172 views

Is there a consensus on whether amber can seal in air bubbles for millions of years?

I read about deriving the oxygen content in the atmosphere in the Cretaceous by analyzing the air trapped in bubbles in amber from that period, but I have also found papers disputing the study's ...
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1answer
317 views

Conflict Between Organic and Physical Chemists--explaining the intensities of IR absorptions

I am currently a Chemistry undergrad and I love to fact check everything my O Chem professor says against my research advisor, a physical/computational chemist. Well, my O Chem professor, when ...
15
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1answer
466 views

How does composition of air affect vapor saturation limit?

Suppose we hold temperature, pressure, and gas composition (say, a standard atmosphere) constant. Then we know that if we allow the water content of the air to vary it can range between 0 and some ...
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4answers
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Is activation energy temperature-independent?

I know that activation energy for a reaction is the extra energy given to the reactants to reach the threshold energy so that they can collide and react. But then, why is it said that the activation ...
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3answers
3k views

Is it possible to boil a liquid by just mixing many immiscible liquids together?

In open air, when vapour pressure reaches 1 atm, boiling takes place. I read that if we add two immiscible liquids together, the total vapour pressure of the 'mixture' is close to $p = p^*_A + p^*_b$,...
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5answers
28k views

What is a pure substance?

I found as I was preparing to teach "Introduction to thermodynamics" that there are varying definitions or examples of pure substances. For instance the text book I have to use "Thermodynamics an ...
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5answers
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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 ...
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1answer
2k views

Why is the carbon dioxide we exhale transparent?

Why is carbon dioxide that comes from dry ice opaque and smoky while the carbon dioxide that we exhale transparent?
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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{...
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Transition state and free energy

We have products $\ce{A + B}$ combining to form $\ce{C + D}$ through the transition state $\ce{X}$. Are all reactions at least virtually reversible? Is the difference between a reversible reaction ...
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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:...
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2answers
709 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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1answer
8k views

How can a group be both a good nucleophile and a good leaving group?

I'm reading about $\:\mathrm{S_N1}$ and $\:\mathrm{S_N2}$ reaction mechanisms, and I have a few questions. My book has a couple of tables, one lists a bunch of substances grouped as good, moderate ...
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1answer
296 views

Help with derivation on why antibonding orbitals are more antibonding

I have read How can antibonding orbitals be more antibonding than bonding orbitals are bonding?. But I am intersted in a specific derivation. During lecture my professor stated that the overlap $S_{ij}...
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1answer
175 views

Aurora borealis - less favored energy transitions higher in atmosphere?

This question describes how some energy transitions become more prominent as you higher in the atmosphere. Short summary: Some electron transitions are "forbidden" by quantum mechanical selection ...
14
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1answer
413 views

Dipole-quadrupole polarizability term for induced dipole

For describing an induced dipole, I have usually seen the following equation, $$ P_{i} = \alpha_{ij}E_{j} + \frac{1}{2}\beta_{ijk}E_{j}E_{k} $$ where $P_{i}$ is the $i^{\text{th}}$ component of the ...
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2answers
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How to find the number of atomic microstates for a given electronic configuration?

Given some electron configuration, I know the following formula can be used to determine the number of microstates: $$\text{# mircostates} = \frac{(\text{# electron positions})!}{((\text{# electrons})...
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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. ...
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4answers
867 views

Room-conditions supercritical fluids?

Are there any reasonably obtainable supercritical fluids that I could, say, run my hand through? Wikipedia makes it sound like there are plenty of room-temperature ones, but no room-pressure fluids.
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Francium has isotopes so how can its atomic mass be a whole number?

Why do francium, radon, radium, and actinium have whole numbers for average atomic mass even though they have isotopes? But then elements like beryllium, fluorine, and sodium are listed as having no ...
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776 views

NMR chemical shift range of different elements

A typical $\ce{^1H}$ NMR runs from approximately 0 to 10 ppm, give or take a bit. $\ce{^13C}$ NMR runs from 0 to 200. And $\ce{^59Co}$ NMR runs from -5000 to 15000 ppm! There seems to be some ...
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3answers
715 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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2answers
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Does the pre-exponential factor (Arrhenius constant) depend on temperature?

I was taught that the the pre-exponential factor $A$ in the Arrhenius equation \eqref{arrhenius} does not increase with temperature. $$k = A\cdot\mathrm \exp\left(-\frac{E_\mathrm a}{RT}\right)\tag1\...
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2answers
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Rule of mutual exclusion: is the converse true?

There is a "famous" rule in spectroscopy,1 that goes this way: If a compound is centrosymmetric, then its normal vibrational modes cannot be simultaneously IR and Raman active. and this is simple ...
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3answers
21k views

Can nonpolar molecules exhibit dipole-dipole forces?

Dipole-dipole forces occur when the positive part of a polar molecule is attracted to the negative part of a polar molecule. In a nonpolar molecule, there may still be polar bonds, it's just that the ...
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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
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 ...
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4answers
60k views

General rules for deciding volatility

Given an organic compound, is there any way to decide if it is volatile (or compare volatility--everything is volatile in the end)? Volatility is due to the tendency to evaporate. The conclusion I ...
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4answers
76k views

Why are sigma bonds stronger than pi bonds?

Why are sigma bonds stronger than pi bonds?
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1answer
351 views

Entropy - “Wiggle”?

The title is not a reference to a Jason Derulo song. In any case: 1) How is change in entropy measured, experimentally? I've Googled this for a bit and I've found all sorts of mathematical ...
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236 views

Has IUPAC been inaccurate in their 1994 definition of kinetic stationary state?

Summary of IUPAC definition $\def\d{\mathrm{d}}$In IUPAC recommendations from 1994[1, 1166–1167], the authors discuss the process $$\ce{A <-->[$k_1$][$k_{-1}$] X\\ X + C ->[$k_2$] D}$$ A ...
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1answer
820 views

How is enthalpy change equal to heat supplied at constant volume?

We know that: $$\begin{align} H &= U + pV \\ \Delta H &= \Delta U + \Delta (pV) \end{align}$$ Now, If $p$ is constant, $$\begin{align} \Delta H &= \Delta U + p\Delta V \\ &= q_p + ...
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2answers
10k views

What causes the lowering of vapour pressure in volatile/nonvolatile solvent mixtures?

"Based on Figure 13.18, you might think that the reason volatile solvent molecules in a solution are less likely to escape to the gas phase, compared to the pure solvent, is that the solute molecules ...
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1answer
181 views

Are there any (simple) molecules with very different absorption and emission dipole directions?

When a single molecule absorbs or emits light, it does so perpendicular to the direction of the respective transition dipole. In principle, the directions of the dipoles for absorption and emission ...
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1answer
304 views

What properties a compound should have to be called an “ice”?

I wonder what compounds are usually called "ice" and what properties do determine it? I can bring some examples of ice and not. Not ice: $\ce{H2S}$ cellulose sugar graphite lithium solid mercury ...
13
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1answer
790 views

Reaction mechanism in fluoride adsorption to aluminum oxide

Despite a fair amount of research (excluding non-open access journals, to which I have not got access), I cannot seem to find an explanation of the process that takes place when $\ce{F^-}$ adsorbs to ...
13
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1answer
4k views

How different concentration of HCl on magnesium affect the enthalpy change

I had conducted an experiment where I reacted a $0.3\ \mathrm{g}$ strip of magnesium with $100\ \mathrm{mL}$ of varying concentrations of $\ce{HCl}$: $1\ \mathrm{mol/L}$, $2\ \mathrm{mol/L}$ and $3\ \...
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2answers
806 views

Ellingham Diagram

My textbook says the following about the Ellingham diagram: Each plot is a straight line except when some change in phase ($\ce{s -> liq}$ or $\ce{liq -> g}$) takes place. The temperature at ...
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1answer
2k views

In rovibrational spectra, when exactly is the Q branch forbidden?

I'm aware that the Q branch ($\Delta j = 0$) is usually not observed in rotational-vibrational spectra of diatomic molecules in the gas state, however, I've heard different things about when exactly ...
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3answers
4k views

Why doesn't a table sublimate, while ice does?

A table does not sublimate, and nor does a spoon. Ice does, however. What is the fundamental difference?
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What makes substances explosive

Can someone explain clearly to me why some things are explosive?
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What is the point of using molality when we have molarity?

Molality and molarity are both concentration terms. Given molarity's popularity molality seems rather antiquated. Why is molality still discused in modern chemistry classes?
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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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4answers
1k views

Reaction molecularity and order

Question: A reaction involving two different reactants can never be a unimolecular reaction bimolecular reaction second order reaction first order reaction The answer as per my ...
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1answer
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Destabilization of antibonding MO vs stabilization of bonding MO

My textbook writes that The net energy stabilization due to the occupation of a bonding MO is equal to the net energy destabilization due to the occupation of the corresponding antibonding MO. ...
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2answers
5k 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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24k views

Why does ZnO turn yellow when it is hot?

Why does ZnO's color become yellow when it is hot? I have read that it is because of physical properties, but I need a more elaborate explanation.
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24k views

Difference between Reaction Rate and Rate Law?

Lets say you have the reaction $$\ce{$a$A + $b$B -> $c$C + $d$D}$$ The rate of the reaction is given by the change in initial concentration over the change in time. $$-\frac{\Delta \left[\ce A \...