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
4k views

which chemical element has the lowest boiling point? [closed]

which chemical element has the lowest boiling point? Is it Helium(He) or is it Hydrogen(H)? Hydrogen is lighter than Helium.
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Standard electrode potential of a galvanic half cell is zero at equilibrium at standard conditions?

At standard conditions(at 1atm pressure and unit activity(1molal or 1molar concentration)of all dissolved compounds),the electrode potential is equal to the standard electrode potential by the Nernst ...
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Why do frying pans stick less with heat?

I know this is simple, compared to most questions asked here, but I notice this several times a week and I'm curious. I notice if I break an egg and drop it into the frying pan, and the pan is still ...
6
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1answer
487 views

What does under-stoichiometric or stoichiometric mean?

When people talk about oxides like $\ce{SiO2}$ or $\ce{Al2O3}$, they use expressions like stoichiometric or under-stoichiometric. I understand that this refers to the relative composition of materials,...
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3answers
303 views

How does caesium’s density change with temperature?

According to Wikipedia caesium’s density is $1.90\ \mathrm{g/cm^3}$ at $\Theta =20\ \mathrm{^\circ C}$. How does this change when $T$ changes? E.g. will it expand when melting?
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How to find the molecular weight of a given gas based on its density?

The density of a gas $\ce{X}$ is $10$ times that of hydrogen. In that case, what is the molecular weight of gas $\ce{X}$? Well what I've done up to now is this: The molecular mass of hydrogen is $M(\...
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163 views

In acidic solution, what does ClO₃⁻ reduce into?

In acidic solution, what does $\ce{ClO3-}$ reduce into and, most importantly, why? It's $$\ce{6e- +ClO_{3}^- +6H^+ -> Cl^- + 3 H_2O}.$$ Now, with the gift of foresight, why was the equation for ...
2
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1answer
587 views

Solid hydrophilic transparant materials that don't dissolve in water

I am looking for hydrophilic transparant materials which are solid at room temperature and that don't dissolve in water. I need these for an experiment in which I condense droplets on a plate made out ...
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566 views

Measuring Thermodynamic Stability

We can predict whether a reaction can occur spontaneously by calculating the change in Gibbs' free energy. I was just wondering whether there is any general measure of the thermodynamic stability of ...
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Strength of intramolecular vs intermolecular hydrogen bonds

Why are intramolecular hydrogen bonds weaker than intermolecular hydrogen bonds?
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Carbon atoms in triple bonds [closed]

I was trying to know what is the probability for the carbon atoms to rotate .I know that in single bond it is 0 % and I think it is 100 % in double bond but how about the triple bond ?
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1answer
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How do I calculate the number of moles in the mixture?

5.5 g of a mixture of $\ce{FeSO4.7H2O}$ and $\ce{Fe2(SO4)3.9H2O}$ requires $5.4 \:\mathrm{mL}$ of $0.1 \:\mathrm{N}$ $\ce{KMnO4}$ solution for complete oxidation. Calculate the number of moles of ...
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1answer
149 views

$K_{eq} $ vs $K_c$

I was wondering if there was a difference between $K_{eq}$ and $K_c$. I think they both refer to the Equilibrium Constant. If I'm wrong, could you please tell me the difference between the two? Thanks!...
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1answer
2k views

Boiling and Melting point of Lithium (or any other element that does not occure in free state)

The melting and boiling point of Lithium as compared to other members of the Alkali group metals is high. But, Lithium does not occur in the free state. So, what does it mean that it has high ...
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296 views

Why does oxygen not like to be a double anion?

The electron affinity of a neutral oxygen atom is −142 kJ (it releases this energy). The electron affinity for the now double anion $\ce{O^2-}$ is 710 kJ (work must be done on the atom). My question ...
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Why can't we use fusion to make ourselves rich?

As we know on fusing two hydrogen atoms we can convert it to helium and by breaking them we get two $\ce{H}$ atoms. So why can't we convert mercury to gold by removing 1 proton per atom?
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What reactions will their reaction rates increase with time?

My textbook states that for most reactions, the reaction rate decreases with time. I wonder what reactions will have increasing reaction rates with time. I know that adding a catalyst will increase ...
3
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1answer
341 views

Unsure how to find the activity, once I found the activity coefficient

Ok, this is a simple problem, but I can't see how to get the answer. I calculated the activity coefficient, $\gamma_\pm$ = 0.389, and I need to find the activity $a_\pm$ for a 0.0120m solution of $\ce{...
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2answers
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Bonding and anti-bonding orbitals in the light of time-dependent Schrödinger equation?

In organic chemistry, people draw 2p orbitals like this: and then they explain how the orbitals combine to non-bonding (π*) or bonding (π) molecular orbitals, like this: depending on whether the ...
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1answer
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The colour of fireworks

When the electrons that are excited by the energy of burning drop back into lower energy levels they emit light at the wavelength of colour. When metals are burnt, metal oxides(some coloured) are ...
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1answer
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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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Why are these molecular orbitals invalid for hexatriene?

Here are the MOs (more accurately, the breakdown of the MOs) for conjugated hexatriene: Now, when first asked to draw the MOs myself, I drew this one for $\psi_3$: It still has two nodes, and is ...
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2answers
640 views

Effect of light on ionic compounds

If I had an ionic molecule which needed $\pu{4 eV}$ to break the ionic bond, a $\pu{7 eV}$ photon is shot at it. If it is absorbed by the molecule and breaks, where does the rest of the energy go?
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1answer
121 views

Is it possible to form bubbles of ionized hydrogen and ionized oxygen under the right pressure/temperature conditions during electrolysis of water?

When we run a current through water (pure or not), $\ce{O_2}$ forms at the anode while $\ce{H_2}$ forms at the cathode. If I understand correctly, as water is already slightly ionized, $\ce{H}$'s snap ...
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1answer
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How can oxygen atoms suddenly appear on the product side of an equation?

The question states the following equation: $$ \ce{C(s) + 2H2(g) -> CO2(g) + 2H2O}$$ And then asks you to calculate the enthalpy change for it, given enthalpies of combustion. The stated values ...
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1answer
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How to determine which gas is more soluble

I have a chemistry homework problem that asks whether $Br_2$ or $O_2$ is more soluble in water in room temperature. However, I can't seem to find a way to figure it out, short of looking it up. Is ...
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1answer
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What is the chemical composition of a Rubbermaid Roughneck container and is it feasible for me to make my own molds?

I'm looking for more information on the composition of a Rubbermaid Roughneck container. The description says "Roughneck containers are made with a durable polyethylene material for a lifetime of ...
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1answer
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What commonly availible salts are strong dessicants?

I'm looking into building an absorption chiller as a decidedly non-professional DIY project. For safety reasons and because temps. > 0°C are sufficient, I won't be using ammonia but some saline ...
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1answer
940 views

What is the nature of chemical energy? [closed]

Is chemical energy basically stored energy? I'm not sure how to define "chemical energy" while I know many physical formulas for energy e.g. potential, movement and other. Is chemical energy an ...
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Why shouldn't Uranium come in contact with water?

In the documentary about Chernobyl, it was mentioned that Uranium should not come in contact with water, otherwise an explosion occurs. What is the reason for that? What kind of reaction makes it ...
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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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Balancing and combining the half equations in redox

I can understand redox equations of the following form, I break these down into half equations and combine them. $$\ce{MnO4^- + C2O4^{2-} -> Mn^{2+} + CO2}$$ Where I have issue is with the ...
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1answer
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Why does the boiling and freezing-point of non-polar molecules rise with increasing molecular mass?

In the section about fuels, my textbook mentions that the boiling and freezing-points of alkanes (which are non-polar) increase with an increase in molecular mass due to London dispersion forces. ...
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1answer
831 views

Kinetic energy of electrons emitted in Photoelectron Spectroscopy

In multiple sources (example), I found the information that the kinetic energy $E_k$ measured using (Ultraviolet / X-ray) Photoelectron Spectroscopy is given by: $E_k = h\nu - B.E._F - \phi_{spec}$ ...
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5answers
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Why does radium have a higher first ionisation energy than barium?

I'm wondering why radium appears to buck the general trend that first ionisation energies decrease as you move down a group in the periodic table: barium (the group 2 element preceding it) has a first ...
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1answer
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Why is the relative atomic mass of carbon not exactly 12?

Relative atomic masses of atoms of all chemical elements are numbers without units, being the value of proportion compared to $\frac{1}{12}^\text{th}$ the mass of the carbon atom. But the relative ...
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2answers
921 views

How to derive partial gas equation?

If the following gasses are to mix, what is the partial pressure of neon? The correct answer is 0.1. I cannot figure out how my professor derived this equation. From the ideal gas law, all I ...
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2answers
361 views

Classical reason for good heat conductance in diamond?

My teacher in my physics course attributed this effect to phonons, more here, but I am skeptical about this argument, it feels like he is overlooking the whole question -- what about with Silica that ...
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1answer
914 views

Does sublimation happen only on the surface or throughout the body?

Like evaporation happens only on the surface of a liquid, and boiling/vapourisation takes place throughout the body, how does sublimation occur? Why?
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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 $...
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1answer
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Can we create a galvanic cell with only half a cell connected to the ground? [closed]

I have a question about electrical potential in galvanic cells ,suppose that we have only half a cell wich means that we have only one Ox/Red couple (one metal and the other as ionic solution ...)...
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5answers
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Why don't the electrons move through the electrolyte (instead of the circuit) in a galvanic cell?

I was learning about galvanic cells and I had a problem understanding why electrons do not travel through the electrolyte solutions themselves, instead preferring to travel through metals. Can ...
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1answer
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How do I calculate partial pressure here?

I need some help with partial pressures of reactions for a fuel cell project. The overall reaction for a fuel cell is $$\ce{H_{2} +\frac{1}{2}O_{2}->H_{2}O}$$ Which means that the mole fraction ...
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4answers
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What exactly is happening when sodium, potassium, or other alkali metal explodes in water?

There are lots of videos on YouTube showing sodium, potassium, etc. exploding when dropped into water (this, for example). I understand that when an alkali metal is exposed to water, a violent ...
6
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1answer
242 views

Reduction and re-oxidation of TiO₂

I'm having a hard time finding data about an experiment that I know has been done a bunch of times: I have a sheet of single crystal rutile $\ce{TiO_{2}}$ (110) that I want to reduce to various $\ce{...
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3answers
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Electronegativity of zinc vs copper in galvanic cell

I am reading up on how galvanic cell works and I realised that the flow of electrons is from Zn to Cu. But Zn is more electronegative compared to Cu according to periodic table trends I read it ...
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2answers
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Why does gas particle velocity affect rate of effusion?

I understand why smaller particles have more velocity, but I don't understand what velocity has to do with rate of effusion: My reasoning is thus: Pressure is the number of impacts of particles in a ...
8
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1answer
159 views

Why does hydrogen form on such a long time scale?

If we were trying to figure out the time scale for a gas-phase reaction between two hydrogen atoms in a molecular cloud (which has density $~\pu{10^4cm^-3}$), apparently the reaction would happen on a ...
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Which materials/compounds expand in response to cold temperatures?

Which materials or compounds expand in response to cold temperatures? This material or compound should expand at a slow (but constant rate), from about 0–20 months when in the following temperature ...
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1answer
137 views

What is in silico energy optimization?

While doing computational chemistry for both organic and inorganic compounds, an energy optimization of that compound is done before starting a study. For example, if we do drug docking studies using ...