New answers tagged physical-chemistry
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Other answers have covered it adequately. But there's one other way in which Avogadro's constant is not completely arbitrary. It's an order-of-magnitude measure of the quantity of particles that can sustain sentience on an Earth-type planet. 12 grams of carbon-12 is a sample of matter that's roughly on the same scale as us humans.
I'm serious here. How many ...
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I think you are close to the right idea, but let me try to clarify some points.
The full Schrodinger equation for a molecule should depend on the $3N$ nuclear coordinates, as well as the $3n$ electronic coordinates, where $N$ and $n$ are the number of nuclei and electrons respectively. This is challenging both due to the number of coordinates and in ...
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This equation holds for a reaction involving ideal gases at constant T. Under these conditions, $\Delta (pV) = \Delta n RT$. Assume now that a gas phase reaction occurs at constant pressure and temperature. Then $\Delta (pV) = p\Delta V$. This is equal to the negative of the pressure-volume work done by the system during the reaction since $w = -p\Delta V$. ...
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Well. A gas is at constant pressure if the pressure does not change in the container with the time. This pressure may be 1 atm, but it may have any other value, provided this value does not change during your experiment or your measurement.
A gas at constant volume is a gas inside a cloud vessel, where the temperature or itse composition may change. This ...
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Why was Avogadro's number chosen to be the value that it is?
Your question implies that you already know that it was a choice rather than something derived from first principle. There are some numbers that are derived from first principles. In math, $\pi$ and $e$ are not a choice, but can be derived from their properties. In physics, the fine structure ...
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Initially the Avogadro was defined as the number of atoms contained in 1 g Hydrogen. Later on, it was understood that Hydrogen can contain various amounts of Deuterium. So that, instead of referring to the fluctuating H atom, it was decided that the Avogadro would be related to the weight of another atom. There had been long discussions about the choice of ...
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From the practical historical perspective chemists and physicists needed a number to use as a conversion factor from Daltons to grams to perform stoichiometric reactions.
In the beginning, Dalton proposed to use the mass of the Hydrogen and assign it to a value of 1 Dalton without knowing how many grams correspond to 1 Dalton. Scales use grams, not daltons ...
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Nowadays you have to differentiate between pH measurements with non-glass sensors and with glass sensor. In the typical pH meter with a glass sensor, there could be many things that could go wrong resulting in biased results. In general, some electrodes may be sensitive to interfering ions such as $\ce{Ag+}$. So some potential could be generated also by ...
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A possible real-world example of directional force use may be apparent from the commercial application of so-called magnetizers (see discussion here).
I once experimented with a small inexpensive one and it appears to accelerate select reactions.
A commercial example for reducing the amount of chlorinating agent required in public swimming pools in ...
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Let's imagine triplet $\ce{He}$, hence one electron in the 1s. The other electron could not be in the 1s, too (it would have to have opposite spin, thus, singlet $\ce{He}$). It is in either the 2s or one of the 2p orbitals.$^{1}$ Among other things, the Coulomb interaction with the 1s orbital/electron will affect their orbital energies. Given that the 1s and ...
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[EDIT] I start with the following definition:
Dissociation, in chemistry, the breaking up of a compound into simpler constituents that are usually capable of recombining under other conditions. In electrolytic, or ionic, dissociation, the addition of a solvent or of energy in the form of heat causes molecules or crystals of the substance to break up into ...
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Here are four examples of substances dissolving:
$$\ce{C6H12O6(s) <=> C6H12O6(aq)}\tag{1}$$
$$\ce{CH3COOH(l)<=> CH3COOH(aq) <=> CH3COO-(aq) + H+(aq)}\tag{2}$$
$$\ce{NaCl(s) <=> Na+(aq) + Cl-(aq)}\tag{3}$$
$$\ce{SO3(g) <=> SO3(aq);\ \ SO3(aq) + H2O(l) <=> HSO4-(aq) + H+(aq)}\tag{4}$$
Because NaCl is an ionic solid, ...
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This is a simple logical and semantic problem, which is not a problem at all. Your professor is right and wrong- both at the same time. He is creating a classification which does not exist and which is meaningless.
Look at the word origin of electrolyte:
Etymology from OED: < electro- comb. form + ancient Greek λυτός that may be
dissolved, soluble (...
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As a quick fix: To access the IR spectral data of $\ce{HCl}$, I opted for a seach by (Hill) formula on NIST's web page, which yielded three entries here. For $\ce{HCl}$ in particular (here) I opted for the first one here, indeed offering the spectrum as image file or as JCAMP-DX.
As in the comment by @EdV, this JCAMP-DX an ASCII file indeed. Note JDXview ...
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Assume that the initial temperature is room temperature and neglect the initial amount of acetonitrile in the gas phase. Calculate the equilibrium vapor pressure of acetonitrile at 140 C and compare it with the partial pressure you would calculate from the ideal gas law if all the acetonitrile had evaporated (so that its volume is 100 cc). If the latter is ...
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You notion (1) is just wrong.
With notion (2) You have the right idea. You'd need to make some assumptions to solve the problem. So state your assumption and solve the problem from there.
I had a wonderful high school teacher who was a stickler for answers to include any assumptions. At the time it was painful, but in retrospect it was wonderful ...
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A is wrong. AgBr is one of the least soluble compounds of Ag. Why don't you have a look in the table of solubility products to be convinced. Further more, AgBr will never react with H2O to produce a strong acid like HBr and a base like AgOH. The reaction goes the other way round. Strong acids react with hydroxydes to produce a salt and water some.
C is ...
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(This might get more/better answers on Physics Stack Exchange)
At the high temperatures of the early Big Bang, you have individual protons and electrons, not hydrogen atoms and molecules. This is because the $kT$ thermal energy is much, much more than the binding energy of the hydrogen atom.
In that case, the moving individual charges create a continuous ...
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An indicator does not affect a particular reaction. But in your solution, you have two successive reactions when adding HCl to this solution : First $$\ce{CO_3^{2-} + H^+ -> HCO_3^-}$$ and a given indicator must be added to determine the end of this reaction. If you don't, all you see is a colorless solution being transformed into another colorless ...
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First, not all gases can be liquefied at room temperature by increasing pressure. If the gas is above the critical temperature, it cannot be liquefied by any increase in pressure; it becomes a supercritical fluid. Supercritical fluids have some of the properties of a gas (e.g. diffusing through fine openings), ans some of liquids (e.g. dissolving solids and ...
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As one increases the pressure, the volume occupied by the gas decreases. Like you said, the temperature should increase with increasing pressure. Equivalently you could say that the thermal energy of the molecules of the gas has increased which manifests itself in increased collisions and vibrations of the constituent molecules. The decrement in volume ...
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Here is a simple example that may help from application of the Rault's Law (see this video), where colligative properties of a solution mixes apply (linearly or with deviations) to the relative molar concentration of solute compositions and not the identity of the solute.
First, take a known amount of CaCl2 and place it into a known volume of water, but do ...
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Suppose a solution $0.01$ M of sugar or any moderately big organic compound. If $N$ is the Avogadro number, $1$ Liter of this solution contains $0.01·N$ molecules, and this is about $6·10^{21}$ molecules. The osmotic pressure will be $ p = cRT = 0.01 RT$, because $c$ is the concentration of individual particules in the solution, here molecules.
Now suppose ...
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Roughly (in the ideal world), colligative properties depend on the number concentration of particles, not on what those particles are. A particle can be a micelle or vesicle or other colloidal particle such as a polymer, or a nonassociated solute molecule. For instance, a protein is a polymer usually consisting of a linear chain of aminoacids. It is ...
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The $\sigma_\mathrm v$ mirror planes in the $C_\mathrm{3v}$ point group are themselves related by symmetry: note that they can be interchanged via rotation by 120 degrees about the preexisting $C_3$ axis. To be technically precise, they belong to the same conjugacy class, in the sense that applying $C_3$, then one mirror plane, and then the inverse of $C_3$ ...
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I agree with Andrew that it depends on whether you define spontaneity based on $\Delta G^\circ <0$ or $\Delta G < 0$. Typically, freshman chemistry books use the former.
However, I've never liked equating spontaneity with the sign of $\Delta G^\circ$, prefering to instead use the sign of $\Delta G^\circ$ as an indicator of whether reactants or ...
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Most azo-based water-soluble thermal initiators can also be excited by UV light and therefore used as a photoinitiator in radical polymerizations, for example, V-50 and V-501. They are inexpensive and available from many commercial sources but they do not usually have very high quantum yield. Depending on your specific application scenario, you can also ...
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I think it is best to start with a real-world example of a spontaneous reaction as cited in electrochemistry, namely, as occurring in an electrochemical (or battery) cell. In particular, comments from Wikipedia:
A spontaneous electrochemical reaction (change in Gibbs free energy less than zero) can be used to generate an electric current in ...
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Careful, if you have samples that contain transition metals and/or their salts, in the presence of ammonia fumes, air (a source of oxygen), water (or even moisture) and an electrolyte (any soluble salts), you may have some electrochemistry afoot as well. In particular, for example, is the leaching of copper ores (removing the copper ions) with NH3/air with a ...
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In general it is necessary to consider any entropy changes in determining whether a system is at equilibrium or if a spontaneous change will occur.
As there must be an increase in entropy in actual processes then $dS_{system}+dS_{surr}=dS_{irrev} \ge 0$. By using the first law with the last expression and after several steps, we find that in an ...
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It sounds like your confusion arises from not making a distinction between $\Delta G$ and $\Delta G^\circ$ when describing a reaction as spontaneous or not.
The $\Delta G^\circ$ is the free energy change for the reaction at the defined "standard" conditions of 1 M solute concentrations and/or 1 bar gas partial pressures of both the reactants and products. ...
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This question shows that you have probably not really understood what the free enthalpy (or Gibbs energy, or free energy) is. I will try to explain it qualitatively without too much thermodynamics. Let's go !
The origin of the Gibbs energy is coming from Gibbs' reflexions on the spontaneity of chemical reactions. He was trying to find a potential energy ...
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If the container is open, as shown below, then you have a total mess.
Obviously eventually all the ammonia and water will evaporate into the room eventually. There is absolutely no way to tell how much ammonia gas would be with the volume of the container at any given time, and the ammonia concentration within the container wouldn't ever be homogeneous.
...
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For an electrochemical reaction, you count the atoms / ions by mol, and use the coulomb as a counting unit of charge. For a more intuitive explaination of the $n$ factor in the Faraday equation, try this analogy:
The summer olympics include swimming in a pool with lanes $50\,\pu{m}$ long. Among the typical competitions are runs about $50$, and $100\,\pu{m}$...
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This is a fun question. Given that you are looking for kJ/g rather than kJ/mol, it seemed most fruitful to explore the possible compounds that can be formed from the lightest-weight elements.
So: I was unable to find any compounds to beat those in the lists provided by Jonathan and Matthew for exothermic heats of formation. But, considering compounds ...
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One can also say that any equilibrium constant $K$ is related to the change of free enthalpy ${\Delta G°}$ of the particular reaction :
$${\Delta G° = RT lnK}$$ And this ${\Delta G°}$ does not depend on any other compound present in the solution.
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In thermodynamics, the basis for a definition of temperature is provided by the $0^{\text{th}}$ Law: two bodies independently in thermal equilibrium with a third body are in thermal equilibrium with one another. Thermal equilibrium allows the definition of temperature: two bodies in thermal equilibrium are said to be at the same "temperature".
The $0^{\...
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Temperature is related to kinetic energy, but it can't be simply equated to the average kinetic energy of the system. As I wrote in response to another answer, different systems can have different average kinetic energies/particle, but the same temperature. E.g., at the same temperature the avg. kinetic/energy particle of a diatomic gas is greater than ...
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Temperature is the average kinetic energy of the particles making up a system. That's it, and it is correct. Any other definition, and there are many of this page, are either equivalent or incorrect. What's the problem?
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I agree in part with Mithoron and MaxW's comments: these are different molecules, not even sharing a mutual atom (e.g. C-O vs C-C), so direct comparison is restricted.
However, Pauling's concept of electronegativity does explain why, in general, the heteronuclear A-B bond is stronger than the average of the homonuclear A-A and B-B bonds. the difference in ...
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The equilibrium reaction for the auto-dissociation of water is:
$$2\text{ H}_2 \text{O}(l) \leftarrow \rightarrow \text{H}_3\text{O}^+(aq) + \text{OH}^-(aq)$$
The associated equilibrium constant $K_w$ is:
$$K_w=[\text{H}_3\text{O}^+]\times [\text{OH}^-] \approx 10^{-14}$$
(Strictly speaking the expression is:
$$\frac{[\text{H}_3\text{O}^+]\times [\text{OH}^-...
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Temperature vs kinetic energy
[OP:] I've read at many places that temperature is the average kinetic energy of particles present in an object.
Temperature has to do with the average kinetic energy of particles, but to say the two concepts are the same is incorrect. What is correct is that if the particles in two mono-atomic gas samples have the same ...
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Heat is the transfer of energy to or from the body in forms other than matter flow or work (organized energy transfer, such as pushing).
Temperature is only a well-defined property for a collective body (you wouldn't be able to tell me the temperature of a single atom, for example). Like you said, it's the property of matter describing the amount of kinetic ...
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Using the following relations:
$\Delta{G}^⦵ = \Delta{H}^⦵ -T\Delta{S}^⦵$
$K_P = e^{\frac{-\Delta{G}^⦵}{RT}}$
How would you find the temperature at which $K_P = 1$?
Given what $\Delta{G}^⦵$ must equal, how could you find T?
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I've found a answer for my question from this question.
The way to compute the conductivity of electrolyte with multiple ion types is given from the work Pawlowicz, Rich, ( 2008), Calculating the conductivity of natural waters, Limnol. Oceanogr. Methods, 6, doi:10.4319/lom.2008.6.489.
For general case consider the system, which consist of $N_+$ number of ...
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What is so acidic...and basic...?
Nothing.
This derives from the historical perception that silica in geological systems and in melts was in the form of silicic acid ($\ce{H2SiO4}$) and the alkali and alkali earth elements were considered as bases.
We now know that in high temperature silicate liquids (that eventually solidifies into slag, or rocks, or ...
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There is a description of Marcus theory here How does the inverted Marcus region explain chemiluminescence? . The quantum nature can be added into the theory as necessary, it leads to a asymmetrical plot of rate constant vs free energy with the inverted region (large -$\Delta G$) decaying away more slowly than the normal region. The equilibrium is between ...
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Here are two common ways of measuring the entropy change in a reaction:
Measure the equilibrium constant $K$ at multiple temperatures. This gives you the Gibbs energy (via $\Delta_r G^\circ = - R T \ln{K}$) and, via the van't Hoff relationship, the enthalpy. You can calculate the entropy from those two.
Use microcalorimetry in a titrating mode. You get the ...
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As an interesting bit of history, Boltzmann was the first one to describe entropy as a "measure of disorder" of a system. It's worth noting that he didn't know this was an oversimplification. In reality, entropy is best described as a measure of the number of ways that energy can be distributed in energy levels between or within particles.
From this, it's ...
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There will be leaks around the edge of the paper. If the size difference is not too large (a few mm at most) you can remedy the situation as follows: (1) turn on the vacuum with only the filter paper in the funnel, (2) take a spatula and run the tip of the spoon around the edge of the filter paper, creasing it into the corner between the flat bottom and the ...
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