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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How can we say that Helium (He) is liquid at “absolute zero”?

According to Wikipedia, Helium can never be solidified at $\pu{1 atm}$ ($\pu{101325 Pa}$) pressure, not even at absolute zero. A minimum of $\pu{25 atm}$ pressure is required to make Helium solid. But ...
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1answer
24 views

Why is black-body radiation curve smooth without a sharp cutoff?

Planck's law is able to predict a graph that is consistent with experimental observation: In essence, unlike Rayleigh-Jeans law that assumes equipartition theorem to hold (that each mode of motion ...
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Finding percent contamination of sulfur dioxide in air by redox reaction with iodine

I would really like some help with this problem. My work is at the bottom. I've tried multiple ways and have gotten the same answer for both, but for some reason the computer program won't accept the ...
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How is Pi antibonding orbital more polar than Pi bonding orbital?

While reading about the solvent effect on electronic transition in ESR spectroscopy, I found it written somewhere that the pi anti-bonding orbitals are more polar compared to pi bonding orbitals. Can ...
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1answer
25 views

Temperature relation for isomerism reactions

When we have an isotherm isomerism reaction $\ce{A(g) -> A^*(g)}$ with $\ce{ΔH^0 = 0}$ is the ratio of preassures $\ce{P_A^* / P_A}$ independent from temperature?
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What is the principal reaction

I was reading this chapter about ionic equilibrium in my textbook when I stumbled upon this: Knowing the ionisation constant, $K_\mathrm a$, of an acid and its initial concentration, c, it is ...
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Percent Composition and Empirical Formulas [closed]

Question: One day, you find a wonderful money-making opportunity on the internet. Someone is willing to sell you a mineral called axinite for $\pu{$ 9.00/kg}$. He furnished you with directions on how ...
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1answer
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What is a Herzberg?

Whatman lists the flow rates for their filters in a unit called a 'Herzberg': What is this? I've been searching around and I did find information about Herzberg flow rate testers, for example this ...
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1answer
40 views

Boiling point elevation with multiple solutes

I've encountered a problem where glucose and $\ce{NaCl}$ are mixed with water and $\Delta T$ is to be calculated. I've calculated $\Delta T$ for both glucose ($i = 1$) and $\ce{NaCl}$ ($i = 2$) ...
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1answer
32 views

What is the effect of putting salt on water during electrolysis [closed]

I want to decompose $\ce{H}$ ion and $\ce{O}$ ion by putting carbon electrode in the water, but it look so slow. What if I put sodium chloride? Would putting $\ce{NaCl}$ have effect on output gas($\ce{...
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1answer
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Why active mass of solids and pure liquids taken as unity

There is a similar question already asked here and answers provided say that activity of solid is taken as unity because their density dosn't changes... but I still have some doubts in it. Suppose we ...
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USNCO Preparation Guidance Request [closed]

Recently, I have been exploring the chemistry olympiad (USNCO). I reviewed their website in detail, but I could not find much information on training materials and learning path. Please let me know if ...
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3answers
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Entropy paradox and Confusion with Gibbs energy

Define $G=H-TS_\text{sys}$. Then, \begin{align} \mathrm dG &=\mathrm dH-S_\text{sys}\,\mathrm dT-T\,\mathrm dS_\text{sys}\\[3pt] &=-T\left(\frac{-\mathrm dH}T+\mathrm dS_\text{sys}+S_\text{sys}...
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Can a reaction have negative external pressure? [closed]

A certain process is carried out at constant pressure, at a temperature of 27 ° C. The heat released by the system is 5 kJ, while 10 kJ work is performed on the system. if the volume variation in this ...
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The minima of minimum boiling azeotropes

This is a very basic doubt, so I'll be really brief about it. The discussion I raise is particularly about minimum boiling azeotropes but it's understood that whatever I ask is about the general ...
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Applying Bohr's model for Helium atom

Consider the following atomic model for Helium. Helium contains 2 electrons revolving in circular orbit around the nucleus,in such a way that there is minimum possible repulsion between them.Calculate ...
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How does ammonium nitrate explode on its own?

I thought ammonium nitrate was an oxidizer that needed to be mixed with fuel to form a high explosive (e.g., ANFO). But apparently there have been accidental explosions involving just the "...
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1answer
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Thermodynamics of chemical reactions “at constant pressure”

Chemical reactions occur at constant temperature and pressure. Consider a gaseous, equilibrium reaction: $\ce{2NO2(g) <=> N2O4(g)}$. Most questions/textbooks formulate such questions by stating:...
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Rate law expressions [closed]

Question For the reaction, $$\ce{2NO(g) + 2H2(g) -> N2(g) + 2H2O(g)}$$ The rate expression can be written in the following ways: $\frac{\mathrm{d}[\ce{N2]}}{\mathrm{dt}} = \mathrm{k_1}[\ce{NO}][\...
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2answers
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Analysing reversible processes

A reversible process is a process in which the system is in mechanical and Thermal equilibrium with its surroundings. Consider a simple thermodynamic system: a gas in a vessel, with a piston (free to ...
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2answers
39 views

Thermal equilibrium in reversible processes

In whatever resources I have consulted to study reversible thermodynamic processes, a common statement seems to be : The system and surroundings (if the boundary allows) are in mechanical ($P_{sys}=...
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1answer
40 views

Conductivity of molten or aqueous salts [closed]

If there is a 1M aqueous solution of 1 liter NaCl (aq), and the molar conductivities of the solution and the chloride ions were found to be $x$ and $y$ respectively, is it accurate to say that the ...
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2answers
63 views

rate of reaction and RDS

An article on Khan-Academy, states the following: The overall reaction rate is determined by the rates of the steps up to (and including) the rate-determining step Here, it says that rate is ...
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Illustrating moles with sand grains, Are there more moles of SiO2 in a grain than moles of sand on the earth

I have been trying to understand the scale of atoms, thinking about sand grains as an example. With approximately $7.5\times10^{18}$ grains of sand on the earth that is $1.25\times10^{-5}$ moles of ...
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1answer
58 views

A theoretical chemistry question about elements and their presence over time [closed]

I'm not a chemist myself, but I can't think of a better place to get some insight about elements and chemistry than from people who live and breathe it here on stackexchange. This is not any kind of ...
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Relation between Gibbs free energy and the order of a reaction

I was reading a biophysics book and at the chapter for membranes it was talking about the melting of the lipid bilayer. Now I don't know much about this topic but this phrase strikes me as kind of odd:...
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Number of moles of acidified potassium dichromate required to liberate 6 moles of iodine from an aqueous solution of iodate ions

I came across this question in a recent exam I had taken: How many moles of acidified $\ce{K2Cr2O7}$ is required to liberate 6 moles of $\ce{I2}$ from an aqueous solution of $\ce{I-}$ ? So this is ...
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1answer
42 views

Using Beer-Lambert law to calculate light intensities

Supposing the mean absorption coefficient of seawater in the visible region is 0.8 m$^{-1}$, calculate the depth at which a diver will experience half the surface light intensity. I know that you have ...
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1answer
31 views

Formation of Goethite

Probably a simple question: When you're mixing potassium hydroxide ($\ce{KOH}$) with iron nitrate ($\ce{(FeNO3)3}$) to form ferrihydrate, does the temperature have to be below $\pu{60 ^\circ C}$?
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1answer
23 views

How is the temperature in which vapor pressure is equal to atmospheric pressure the boiling point of the liquid? [duplicate]

I was just learning about vapor pressure and got very confused. Vapor pressure was defined as the pressure exerted by a vapor in thermodynamic equilibrium with its condensed phases at a given ...
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2answers
317 views

Why is the molar entropy of hexane lower than that of butane?

Entropy is dependent on the complexity of the molecule. Due to this, I expected entropy to increase as the complexity of the molecule increased. According to p 918 of Atkins' Physical Chemistry (9th ...
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28 views

Software for modelling decomposition reactions

What are some good softwares that can model a decomposition reactions? I have a reaction of H2O2 decomposing on metal exchange hexaaluminate catalyst. The catalyst is present in a packed bed thruster. ...
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Fate of compounds after radioactive decay [duplicate]

Let us say there is a $\ce{CO2}$ molecule containing $\ce{^{14}C}$ isotope. $\ce{^{14}C}$ undergoes decay to form $\ce{^{14}N}$. What happens to the molecule?Does it form $\ce{NO2}$? How are the ...
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1answer
36 views

Molality of solution to be used when finding water separated from solution as ice

Question $\pu{1 kg}$ of a solution of cane sugar is cooled and maintained at $\pu{-4 ^\circ C}$. How much ice will separate out if molality of solution is $0.75$? $K_\mathrm{f} \ \text{(water)} = 1....
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1answer
47 views

Feasibility of a Reaction [closed]

I understand for a reaction to be feasible, $\Delta$H needs to be negative and $\Delta$S positive. But in terms of Gibbs free Energy Change, without considering kinectic factors, If $\Delta$H is ...
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3answers
252 views

How to simulate a Lorentzian broadened spectrum in MATLAB?

I'm trying to simulate the 2nu2 band of HCN in MATLAB. I generate a stick spectrum from the data I have (taken from the HITRAN database) and then use conv() to ...
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1answer
60 views

Calculation of rupture force in a covalent bond

Is there any way to theoretically or mathematically calculate the rupture force of a covalent bond? One of the articles I saw said that it is the maximum derivative of the potential energy curve. Is ...
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2answers
64 views

Effect on mass when decreasing pressure in equilibrium reaction

For example, if we have the reaction: $$\ce{A + B <=> C}$$ And the volume is decreased, the pressure would increase. Hence, according to Le chatelier's principle, the system will partially ...
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1answer
73 views

Calculating the viscosity of water using particles

There are two particles with radius 0.212 μm. The particles are in a thin liquid film and the microscope can only detect movement in the $x$- and $y$-axis. The temperature is 25 °C. $$ \begin{array}{c|...
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1answer
38 views

Stoichiometric calculation of composition of a compound

I have a question from a high school review package. I don't need the answer, but one thing that is bugging me is that, I feel as if I do not understand the question at all. I don't know what it wants ...
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2answers
193 views

Comparing equilibrium constants between acetone and benzaldehyde in nucleophilic addition reaction

My teacher told me that the $K_\mathrm{eq}$ of acetone is greater than that of benzaldehyde in nucleophilic addition reaction. But I am not able to understand how is this possible. Acetone has a +I ...
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1answer
82 views

How is the formula of mean activity coefficient derived?

The mean activity coefficient is defiend as follows: $$\gamma_\pm = (\gamma_+\gamma_-)^{1/2}.\tag{1}$$ If Debye-Hückel equation $$-\log\gamma_i = 0.5z_i^2\mu^{1/2}\tag{2}$$ is used, then the mean ...
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1answer
39 views

Equilibrium constant for heterogeneous equilibria having aqueous as well as gaseous reactants

Suppose we have a heterogenous equilibrium : $$\ce{A(aq) +B(aq) <=> C(g) +D(aq)}$$ Which equilibrium constant is used here? Both pressure and concentration terms are there. So, according to me, ...
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1answer
82 views

Sign of change in enthalpy and change in entropy

Why is it wrong to assert that the change in entropy and the change in enthalpy must always have the same sign? What makes me think that they must have the same sign is the fact that every reaction ...
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0answers
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Reducing pressure and calculation of concentrations at equilibrium

For the equilibrium $$\ce{PCl5(g) <=> PCl3(g) + Cl2(g)}$$ $$ K_c = \pu{4.16E-2 mol dm^{-3}}~\text{at}~\pu{250 °C}$$ $$[\ce{PCl3}] = [\ce{Cl2}] = \pu{0.204 mol dm^{-3}}$$ If the pressure of ...
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2answers
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The relation between q and T when the volume is constant [closed]

$$ \begin{align} q_V &= ΔU \tag{1}\\ q_p &= ΔH \tag{2}\\ C_V &= \frac{\mathrm dq_V}{\mathrm dT} = \left(\frac{\partial U}{\partial T}\right)_V \tag{3}\\ C_p &= \frac{\mathrm dq_p}{\...
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1answer
130 views

Converting fractional coordinates into cartesian coordinates for crystallography

So let's say for a given molecule I have the volume of a unit cell, the shape of the unit cell (monoclinic, cubic, etc), the cell angles alpha, beta, and gamma, the cell lengths a,b, and c, and a list ...
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2answers
73 views

Calculation of pH in acid base reaction

This problem(from ionic equiibrium) states: We are given $\pu{40 mL}$ of $\pu{0.1 M}$ $\ce{HCl}$ and $\pu{10 mL}$ of $\pu{0.45 M}$ $\ce{NaOH}$ in two separate beakers. These two solutions are mixed. ...
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2answers
176 views

Disproportionation of silver(I) in aqueous solution

Use the following equations to predict whether or not $\ce{Ag+}$ ions will disproportionate in solution: $$ \begin{align} \ce{Ag+(aq) + e- &-> Ag} &\qquad E^\circ = \pu{+0.80 V}\\ \ce{Ag^2+(...
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1answer
43 views

Finding the ratio of rate constants [closed]

A group of mountain climbers set up a camp at a $\pu{3 km}$ altitude and experience a barometric pressure of $\pu{0.69 atm}$. They discover that pure water boils at $\pu{90 ^\circ C}$ and that it ...

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