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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17
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2answers
2k views

What do these labels for molecular electronic states mean?

What do these symbols mean in excited states? $${}^2\!A_2,{}^2\!B_1,{}^4\!A''$$ I am confused with these representations, found in the abstract of this paper. I think it is kind of a representation ...
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3answers
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Why weren't the electrons attracted to doubly charged Helium particles in the Rutherford experiment?

I have a chapter and in it there was a topic on the Rutherford experiment. It is written that doubly charged helium particles or alpha particles were thrown towards a very thin gold foil and some of ...
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2answers
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Estimation of the bond angle of water

We know from experimental data that $\ce{H-O-H}$ bond angle in water is approximately 104.5 degrees. If its two lone pairs were bonds (which is unfortunately impossible) also $\ce{O-H}$ bonds and a ...
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2answers
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Strength of intramolecular vs intermolecular hydrogen bonds

Why are intramolecular hydrogen bonds weaker than intermolecular hydrogen bonds?
10
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3answers
424 views

Non-radiative transitions or what features of rhodamine result in it being so highly fluorescent?

Rhodamine is a fluorescent dye. What features of the molecule result in it being so highly fluorescent ? I assume the extended conjugated system with a stabilized first excited state is necessary, ...
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2answers
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Comparing formula for enthalpy change with bond dissociation energy and formation enthalpy

I learnt that given a reaction: $$\ce{A -> B}$$ the enthalpy change is given by: $$\Delta H = \left( \begin{array}{c} \text{total enthalpy of}\\ \text{bonds broken}\end{array}\right)-\left( \begin{...
6
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1answer
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Calculating pH in titration of monoprotic weak acid with strong base

$25.0\:\mathrm{ml}$ of $0.10\:\mathrm{M}$ $\ce{CH3COOH}$ is titrated with $0.10\:\mathrm{M}$ $\ce{NaOH}$. What is the $\mathrm{pH}$ after the addition of $30.0\:\mathrm{ml}$ $\ce{NaOH}$? In broad ...
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What is known about liquid carbon?

The wiki tells me that if you heat carbon at atmospheric pressure it eventually turns directly into a gas without being liquid first. At what pressure can you make liquid carbon? Has anyone actually ...
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1answer
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How can there be decimal subscripts in a molecular formula?

While learning about how batteries work I have encountered the following notation for a Li-ion cathode: $\ce{Li_{0.5}FePO4}$.[1] According to Wikipedia, the subscript number in a reaction equation ...
17
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2answers
915 views

Do bare protons exist, even transiently, in aqueous solution?

Generally $\ce{H^+}$ ions in water remain in the form of hydronium ions ($\ce{H3O+}$), and not as "free" aqueous protons, as far as I've been taught. My question: is this always the case, even when ...
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1answer
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What is Sodium Chloride like in gas state?

Since sodium chloride is sodium and chlorine atoms bonded as a lattice and there are no discrete molecules, doesn't that mean in gas state, sodium chloride is simply sodium and chlorine atoms separate ...
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On Heisenberg's uncertainty principle

Why can't we find the exact position and velocity of a particle? And why is it that if the uncertainty in position is very, very large then the velocity can be determined, and vice versa? Please ...
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4answers
371 views

What is the simplest approach to balance a complex reaction equation?

Consider the redox reaction: $$\begin{multline} \ce{ a K4Fe(CN)6 + b Ce(NO3)4 + c KOH ->\\ d Ce(OH)3 + e Fe(OH)3 + f H2O + g K2CO3 + h KNO3} \end{multline}$$ The book's answer for the ...
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1answer
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Prove that a 10-Degree Temperature Increase Doubles the Rate Constant (k), when the Activation Energy is Approximately 50 kJ/mol

I read that increasing the temperature by $10~^\circ\mathrm C$ will double the rate constant ($k$), when the activation energy for the reaction is relatively close to $50~\mathrm{kJ/mol}$. However, ...
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1answer
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What is meant by 'electrons of like/unlike rotation'?

Explaining the reasons behind electron repulsion, Radiochemistry asserts that, Interelectronic repulsion in related not just to electron pairing, but also to [the] angular momentum of the electrons....
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1answer
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How does radiocarbon dating work?

I'd like to better understand radiocarbon dating, however, I do not know what resources I must use in this regard. Could someone walk me through the process of radiocarbon dating with a few examples? (...
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4answers
911 views

How do things glow in the dark?

What makes something glow in the dark? The only explanation that I can come up with for what makes things glow in the dark is that there is probably a chemical reaction slowly releasing the light that ...
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1answer
263 views

Le Chatelier's principle and van 't Hoff equation

When temperature is increased, then according to Le Chatelier's principle the reaction will move in the direction where energy is absorbed. I can understand this, but there is also another fact that ...
18
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1answer
600 views

Analytical solution of the Schrödinger Equation for AB^+ systems

According to Wikipedia, there are not many problems for which the Schrödinger equation can be solved analytically. [1] The $\ce{H2+}$ ion is probably the most complex molecule that can be treated this ...
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5answers
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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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2answers
4k views

Why are activities of solids and liquids taken to be unity?

First, I should mention that the question was already asked here: Why is active mass of a pure solid or liquid always taken as unity? However, the treatment there is qualitative and I do not really ...
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836 views

Why is the rate of a reaction proportional to the concentrations of reactants raised to their stoichiometric coefficients?

Consider a gaseous state elementary reaction $$\ce{aA(g) + bB(g)} \overset{k_\mathrm{f}}{\underset{k_{\mathrm{b}}}{\ce{<=>}}}\ce{ cC(g) + dD(g)}$$ I know that for this reaction, $$\Delta G = \...
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What is the difference between ΔG and ΔrG?

Consider the reaction $$\ce{A -> B}$$ The reaction Gibbs free energy, $\Delta_\mathrm{r} G$ is given by the following equation $$\Delta_\mathrm{r} G = \Delta_\mathrm{r} G^\circ + RT \ln Q$$ Now ...
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What is the unit of pH?

I'm making some graphs and I have to label the axes. I want to be extra careful and put the units in even though the meaning of $\text{pH}$ is well known. But I have a problem (though a simple one): $\...
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4answers
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During phase change in matter, why doesn't the temperature change?

I was working on something in school and came across the question: Why does the temperature not change much during a phase change? I'm really not sure why this happens in matter and I couldn't ...
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what kind of suspension liquid should be used with ferrofluid (so it does not stain the glass)

Please take a look at the following video: I am working on a new project, and I need to find whats the best liquid to hold ferrofluid inside the glass (or maybe even plastic) container, so that ...
16
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3answers
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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 ...
6
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1answer
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What are the magnetic quantum numbers for the three real p orbitals?

The form of the p orbitals that we are familiar with are the $\mathrm{p}_x$, $\mathrm{p}_y$, and $\mathrm{p}_z$ orbitals: (source: ChemTube 3D) I also know that the p subshells have the quantum ...
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1answer
13k views

How do equilibrium shifts affect solids?

Say we have the following reversible reaction: $$\ce{NaOH{(s) }<=> Na^+{(aq) }$+ $OH^{-}{(aq)}} +10.6 \mathrm{kcal}$$ If we add $\ce{OH^-}$ and equilibrium shifts to the left, does that affect ...
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2answers
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Justification for Freezing Point Depression & Boiling Point Elevation in Solutions?

I was wondering if the following justification for freezing point depression and boiling point elevation are conceptually correct. The reason why I ask this question is because I have been self ...
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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 ...
14
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3answers
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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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4answers
4k views

What makes substances explosive

Can someone explain clearly to me why some things are explosive?
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4answers
86k views

Why calcium chloride is used to melt ice over sodium chloride

I'm not quite sure I understand this. My question is: Calcium chloride is a salt used widely to melt ice on sidewalks and roads. Explain why one mole of $\ce{CaCl2}$ would be more effective than ...
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3answers
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Law of mass action & Collision Theory

I don't understand why the stoichiometric coefficients of reactants and products are expotentialized in law of mass action. So I checked on Wikipedia. And this popped out: In the 1879 paper[9] the ...
9
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1answer
470 views

How to obtain product ratio from energy differences via Boltzmann statistics

I am currently calculating some reactions and I would like to express the ratio of the products in per cents. I am using the Boltzmann statistic in the following way: The probability $p_j$ of a ...
8
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2answers
48k views

Calculating charge on amino acid from pKa

I have been given $\mathrm pK_\mathrm a$ values of an amino group, a carboxyl group and a side chain of cysteine. How can I find the ionic charge on it at different $\mathrm{pH}$ values?
5
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1answer
526 views

Le Chatelier's principle with pure solids and liquids

In the following reaction: $$\ce{A(s) <=> B(g) + C(g)}$$ since adding a pure solid is not supposed to shift the equilibrium in any way, does that mean that I can essentially remove all of the ...
13
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4answers
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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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3answers
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What is the difference between Molecularity and Order Of Reaction?

In the field of Chemical Kinetics, terms like Molecularity and Order Of Reaction are often used. I am a bit confused between these two terms. Can someone explain to me about these two terms and their ...
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1answer
5k views

Why is osmium the densest known element?

Why is osmium so dense despite there being heavier elements after it in the periodic table?
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2answers
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What's the relationship between Van der Waals forces and hydrophobic interactions?

A question on a past exam for a course I'm studying for asks: What's the relation between Van der Waals forces and hydrophobic interactions? From what I understand, Van der Waals forces are ...
8
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1answer
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Why does the equilibrium constant depend on the temperature but not on pressure and concentration?

According to my textbook (and intuitively) certain changes when the aforementioned 3 variables are altered occur in accordance with Le Chatelier's Principle. However, what I don't understand is what ...
6
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1answer
989 views

Applying the Le Chatelier principle

I am having issues understanding the consequence(s) of a decrease in pressure on a chemical reaction. I have understood that when we increase the pressure, the side having more number of moles wants ...
20
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2answers
5k views

What is the edge of a diamond like?

The chemical structure of a diamond is defined as an endless lattice in which each carbon atom is covalently bonded to four other carbon atoms situated at the four ends of a tetrahedron. But of ...
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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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1answer
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How exactly is activation energy defined?

In a common interpretation of the Arrhenius rate equation $$k = A\exp\left(-\frac{E_\mathrm a}{RT}\right),$$ the activation energy $E_\mathrm a$ is understood to represent the difference in the ...
7
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1answer
1k views

Law of mass action

If you have the forward reaction $$\ce{2X ->[K] P}$$ which of the following systems of differential equations would model the reaction's kinetics? $$\begin{array}{rl} \dfrac{\mathrm{d}[\ce{X}]}{...
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2answers
4k views

How does the partial pressure of gas C change with the change in volume of the container?

Suppose I have the equilibrium in a closed container: $$\ce{3A(s) + 4B(g) -> 4C(g) + D(s)}$$ What happens to the partial pressure of C if the volume of the container is halved? I was pondering ...
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1answer
859 views

Why do gases need to be cooled to liquefy/solidify?

My book states: Gases do not liquify on compression only, although molecules come close to each other and Weak forces operate at a maximum. Why is it that we cannot just keep on compressing gas ...

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