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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What is the pH of a 5M solution of hydrochloric acid?

What is the $\mathrm{pH}$ of a $\pu{5M}$ solution of hydrochloric acid? So in order to solve this, apparently all you need to do is plug it into the equation: $$\mathrm{pH} = -\log[\ce{H3O+}]$$ ...
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Solvated solids

I have a very basic question concerning solvation. If a solid (solute) was dissolved in a liquid (solvent), is the solute still in the solid state? or does it convert into another state of matter when ...
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Is the first ionization energy in oxygen slightly more than nitrogen?

Why is the first ionization energy in oxygen slightly more than nitrogen? In nitrogen: $\ce{[He] 2s^2 2p^3}$ In oxygen: $\ce{[He] 2s^2 2p^4}$ This tells me that it should be easier to remove an ...
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Gibbs Free Energy and Maximum Work

I am a high school student and my professor mentioned that the Gibbs free energy is the maximum amount of work (or useful work) that a system can do, whereas entropy is a measure of the non-available ...
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Proving RTln($a_i$) equal to non PV work done by system,where $a_i$ is activity

We know that chemical potential is defined as ${\mu}_i={\mu}_i^{standard}+RT\ln(a_i)$. Here $a_i$ is the activity of $i^\text{th}$ component of solution. In case of gases instead of $a_i$ it is $f_i$ (...
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How can we say that one mole of a substance contains avogadro number of particles?

I just want to know, what was the procedure through which the value of one mole of a substance was considered as a an aggregate of $6.023\cdot10^{23}$ particles. Secondly, how do we calculate the ...
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Why does Le Chatelier's principle not apply to pure solids and liquids?

If we had a homogeneous reaction of pure liquids only, would Le Chatelier's principle still apply ? Also, wouldn't increasing the amount of solid in a heterogeneous reaction mixture at equillibrium ...
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When simulating spectral line broadening, which convolution is preferred?

Many computational chemistry packages permit the calculation of vibrational and electronic spectra. These spectra are obtained as a set of discrete eigenvalues however they are often convolved with ...
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Why do some gases have lower value of Z for a particular pressure?

In the above graph,the minima of the curve for methane is more than that of nitrogen. Also, for a given value of pressure, the value of $Z$ for methane is less than that of nitrogen. They seem to meet ...
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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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A misunderstanding about the energy profile of reactions with a catalyst involved

All of us are aware of the importance of the catalysts in bio-chemistry. For a high school learner like me, catalysts ,and therefore, enzymes play a bridge-like role that connect high school bio to ...
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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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Chemical properties of isotopes

I've always heard that different isotopes of the same element have exactly the same chemical properties. But for example, I've read that some compounds which have hydrogen atoms interchanged by ...
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How to determine the empirical formula for an unknown iron oxide?

I have to determine the empirical formula of an oxide of iron which has $69.9\%$ iron and $30.1\%$ dioxygen by mass in it. I have started as follows: For our convenience we take $\pu{100 g}$ of that ...
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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 ...
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Disproportionation of hydrogen peroxide

Hydrogen peroxide is decomposed as follows: $$\ce{2H2O2 -> 2H2O +O2}$$ This is a disproportionation redox reaction of $\ce{H2O2}$ involving the 2 half reactions $$\ce{H2O2 -> O2 + 2H^+ + 2e-}$$ $...
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Anomalous Electronic Configuration of Thorium

The electronic configuration of thorium ($Z=90$) is $5\mathrm f^0 6\mathrm d^2 7\mathrm s^2$. But, according to the aufbau principle, the electrons should first enter the $\mathrm f$ subshell and not ...
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361 views

Just how quantized are energy levels?

One thing which has bothered me for a while about the idea of quantized energy levels is that it's clear just how quantized they are. Let me give an example. If I was looking the energy some ...
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Is there a soluble salt, or compound that when dissolved in water results in a solution with density less than pure water?

Generally water soluble salts tend to 'fit between' the molecules of water such that the volume of the resulting solution does not increase much relative to the volume and added mass of the salt. So ...
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828 views

Is sublimation caused by an environmental difference or an element's composition?

I was having a conversation when the topic of sublimation came up. Prior to the discussion, I had always thought that sublimation was caused by a great pressure/temperature difference, but another ...
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Conservation equation in an enzyme-catalysed reaction

Consider the reaction scheme: $$\ce{S + E ->[k_1] C1} \qquad \ce{C1 ->[k_2] E + P} \qquad \ce{S + C1 <=>[k_3][k_4] C2}$$ where $\ce{S}$ is the substrate, $\ce{E}$ is the enzyme, $\ce{P}$ ...
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For two compounds that are symmetrical, why does the more compact one have a higher melting point?

In this post, I refer to two molecules B (Di-tert-butyl ether) and C (Dibutyl ether). Wikipedia: Di-tert-butyl ether (B) b.p. 107.2 C m.p. - 61 C ...
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Quotable Literature for the Enthalpy of a Proton

A proton obviously has no electronic energy, no vibrational and no rotational degrees of freedom. Therefore I think it is fair to assume, that $$H(\ce{H+})=\frac32\cdot \mathcal{R}\cdot T$$ holds for ...
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Why some materials are transparent, but other ones aren't?

We have two materials; say glass and wood. Light passes through glass but it doesn't pass through wood. Is it caused by atoms and molecules arranged in glass and wood such that they interact ...
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Relation between molar mass and van der Waals force

In the book that I'm reading it is mentioned higher molar mass results in stronger van der Waals force, but didn't continued why. I wonder why is that so? molar mass is actually dependable on the ...
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Why can dissolution reactions (e.g. dissolution of calcium chloride) be exothermic?

Dissolution reactions are normally endothermic, and crystallization reactions are normally exothermic, but there are some exceptions -- such as the dissolution of calcium chloride. Why is this so?
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Is sublimation a property of all substances?

I wonder if sublimation can be done only on certain substances like iodine, or it can be done on all substances. In other words, can sublimation be done, for example, on water? If it cannot, then why?...
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Is the E in Arrhenius equation the change of enthalpy?

I'd like to know whether the activation energy in Arrhenius equation is the enthalpy difference between the transition state and the reactant. If it's not enthalpy, is it Gibbs energy or something ...
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Application of the Arrhenius equation to a parallel reaction

I'm struggling with this homework question: "For a parallel reaction A goes to B with rate constant $k_1$ and A goes to C with rate constant $k_2$, you determine that the activation energies are 71.9 ...
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1answer
540 views

Pauli-forbidden term symbols for atomic carbon

Carbon has a $\mathrm{p^2}$ configuration, and within the Russell-Saunders coupling scheme, we have $$\begin{align} s_1 = s_2 &= \frac{1}{2} & S &= 1,0 \\ l_1 = l_2 &= 1 & L &=...
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Justify the value of the product of the uncertainty in position and momentum

The Heisenberg uncertainty principle states that: $$\Delta x\Delta p_x\ge\frac12\hbar$$ But more generally for operators $A$ and $B$: $$\Delta A\Delta B=\frac12|\langle[A,B]\rangle|$$ However, $[x,p_x]...
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Depression of freezing point [closed]

Recently, I learnt that by adding a non-volatile solute to a solvent, the vapour pressure of the solution is decreased and consequently its boiling point. Facts and definitions freezing point: "The ...
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What are the units that measure free energy?

Is it $\pu{kJ/mol}$? I have done problems and I always end up with these units, but is this correct?
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When water boils at its boiling point how do we call it as an equilibrium?

On boiling, water becomes a vapor, liquid volume decreases and vapour volume increases. Since volumes are not constant, how do we call it as equilibrium?
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How does one find the ground-state term symbol for a configuration that is exactly half-filled?

For instance, the ground-state configuration of N atom is a $p^3$ configuration of all parallel spins and one electron in each $2p$ orbital, which has: Total spin angular momentum, $S = 3 * \frac{1}{...
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What is the exact definition of state functions? How to determine state functions?

How do you know if something is a state function? I know Internal Energy (U) is a state function since it is the sum of energies, which is constant at a given time. However I don't know for sure if $...
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Chlorofluorocarbons rising

Chlorofluorocarbons are heavier than atmospheric gases. Then how are chlorofluorocarbons produced at the surface able to reach the ozone with such a heavy mass ?
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Is there any formula that can be used to find loss of mass due to binding forces in atomic and sub atomic particles?

Atomic weight of Br-79 is 79.641 if you add the masses of protons and neutrons. However, in periodic table, it is less than the value given here. How is the difference arrived at for all the elements ...
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Computing the concentration of a reactant given formation of products

At high temperatures nitrogen dioxide decomposes into nitrogen oxide and oxygen gas according to $$\ce{2 NO_2 -> 2 NO + O_2}$$ I have a question now. Let's say at a certain temperature I'm given ...
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Why is absolute zero unattainable?

We were dealing with the Third Law of Thermodynamics in class, and my teacher mentioned something that we found quite fascinating: It is physically impossible to attain a temperature of zero ...
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Is there an energy cost associated with flipping the spin of an electron?

THE STORY: A common example used to illustrate the limitations of restricted Hartree-Fock (RHF) theory is the H$_2$ dissociation energy ($D_e$) curves. RHF enforces electrons to be paired into spin ...
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Why are the total energies of transition states so commonly corrected for zero-point vibrational energy?

So often I see total energies of transition states corrected for zero-point vibrational energy which always confuses me. Zero point energy is the lowest energy that a ground state minimum energy ...
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What is antisymmetric exchange? What is J-strain? Where does it come from?

I'm reading a paper1 by Sanakis, et al. that characterises the magnetic coupling in the $\ce{Fe3S4}$ clusters present in bacterial ferredoxin II and beef heart aconitase as arising through something ...
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Why does a mixture of siloxene and cerium(IV) sulfate luminesce?

I performed an experiment where siloxene and cerium(IV) sulphate were mixed together: when I did so the mixture produced an orange-yellow glow. Why does it glow? What is it about the two chemicals in ...
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Is there a least reactive chemical species?

As a total chemistry layman I enjoyed reading "Why doesn't $\ce{H2O}$ burn?", but it prompted another question in my mind. One of the answers there was that $\ce{H2O}$ can burn in the presence of a ...
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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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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 \...
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What is melting? Which bonds do we break to melt something?

To melt diamond, we have to break the covalent bonds, which we can consider 'intermolecular' because it is one giant molecule. To melt Methane, we have to break the van der Waals (intermolecular) ...
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Why are equations of state for a non-ideal gas so elusive?

The ideal gas equation (daresay "law") is a fascinating combination of the work of dozens of scientists over a long period of time. I encountered Van der Waals interpretation for non-ideal gases ...
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Why do some chemical reactions require many steps?

I posted the following question in Physics SE and was advised to transfer it to Chemistry SE. I studied physics in college ten years ago and I recently started to learn biochemistry. I enjoy finding ...