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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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24
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1answer
4k views

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
716 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 ...
7
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4answers
331 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 ...
7
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4answers
915 views

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 ...
5
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1answer
209 views

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....
4
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4answers
740 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 ...
18
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1answer
578 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 ...
14
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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 ...
5
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3answers
1k views

How are nuclei stable?

We all know that the density of the nucleus is very high. Nuclei are made up of protons and neutrons, and while protons have the same charge, they are closely packed in a nucleus. How does the ...
4
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2answers
3k 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 ...
0
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2answers
642 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 = \...
10
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2answers
19k views

Why is active mass of a pure solid or liquid always taken as unity?

Active mass is defined as the molar concentration ie. number of Gram-moles per litre. My book then wrote Active mass of pure solid/liquid is always 1 . The book reasoned that Molar ...
30
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2answers
51k views

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): $\...
15
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4answers
98k views

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 ...
28
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2answers
6k views

Can magnetic fields affect a chemical reaction?

This question was asked recently in an interview and I just said that "Yeah, like if the reaction involves ions or paramagnetic species". But the interviewer went on and asked me to elaborate on HOW ...
22
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4answers
41k views

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 ...
13
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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 ...
6
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1answer
13k views

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 ...
16
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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 ...
8
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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 ...
14
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3answers
4k views

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 ...
12
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4answers
4k views

What makes substances explosive

Can someone explain clearly to me why some things are explosive?
11
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2answers
5k views

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 ...
10
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3answers
2k views

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 ...
10
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3answers
3k views

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 ...
9
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1answer
411 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 ...
9
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4answers
81k 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 ...
8
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2answers
46k 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?
4
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1answer
387 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
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.
12
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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?
10
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2answers
18k views

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
11k views

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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2answers
1k views

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 ...
5
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1answer
817 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 ...
14
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2answers
22k views

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})...
9
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1answer
6k views

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 ...
7
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1answer
2k views

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
votes
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}]}{...
5
votes
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 ...
5
votes
1answer
17k views

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, ...
5
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1answer
1k views

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 ...
4
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1answer
3k views

Stability of ortho- and para-hydrogen

can the stability of ortho-hydrogen greater than para-hydrogen can be stated as this? The intrinsic energy of ortho is greater than para-hydrogen and also with rise in temperature %of ortho form ...
4
votes
1answer
674 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 ...
4
votes
3answers
582 views

What are dipole moments in a molecule supposed to act upon?

In a $\ce{CO_2}$ molecule, a total of four electron pairs are shared between the carbon and oxygen atoms, such that 2 pairs are shared between the carbon atom and each oxygen atom. Oxygen has a ...
3
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3answers
1k views

pH and materials selction [closed]

I working on the Navy study guide for their nuclear engineering programs and I am not a Chemist. Thus, I have come here to try and develop a better understanding of the subject matter. Why is pH ...
2
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2answers
4k views

Derivation of mean kinetic energy

I read from a book that average kinetic energy is equal to $3kT/2$ where $k$ is Boltzmann's constant and $T$ is the kelvin temperature. I don't know how the formula was derived. Any help to gain ...
2
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1answer
3k views

Internal pressure of ideal gas

$$\left(\frac{\partial U}{\partial V}\right)_{\!T}$$ I know that this partial derivative is equal to zero for an ideal gas, but how do I determine that? Do I need to use the fundamental thermodynamic ...
2
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3answers
5k views

Is a liquid in a container always in equilibrium with its vapour?

This is essentially a question about the meaning and significance of the term vapour pressure (or vapor pressure if you're American). From what I understand a liquid in a container will have a certain ...
11
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3answers
8k views

Can you heat water with additives?

I have been curious about this question for a while. If you want to warm up a large amount of water, is it feasible to do this by adding a substance that has an exothermic reaction with the water? ...