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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37 views

What will be the form of coulomb and exchange operators for electron m in an n-electron molecule?

I know that the Fock operator for electron $m$ in $n$-electron molecule would be: $$\hat{f}\!(m)u_i(m) = \varepsilon_iu_i(m)$$ $$\hat{f}\!(m) \equiv -\frac 1 2 ∇_m^2 - \sum_α\frac{Z_α}{r_{ma}} + \...
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1answer
170 views

Can anyone help me with this question on vibrational modes of free base porphyrin (C20H14N4)?

4 a. The structure of free-base porphyrin $(\ce{H2P}$, chemical formula: $\ce{C20H14N4})$ is shown below. Given that this molecule belongs to the $D_\mathrm{2h}$ point group (character table given ...
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31 views

Given solubility of sodium carbonate, find mass of hydrate sodium carbonate dissolved?

I am working on a problem from my scholarship exam practice (MEXT). I am not quite sure if I got it right. The question was provided with multiple choices (a) to (f) below: The solubility of sodium ...
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33 views

Reaction Mechanism with a Rate Determining Step

Just a quick question on how to deal with multiple intermediaries in a proposed reaction mechanism. Here is the overall reaction: $$\ce{I-(aq) + OCl-(aq) -> IO-(aq) + Cl-(aq)}$$ Here is the ...
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Solubility in different pressure conditions

I am working on my scholarship exam practice. I believe this exam assumes high school + first year university knowledge. And I'm not quite sure what I did wrong. Could you please have a look? The ...
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13 views

What is the difference between radii of Gyration and hydrodynamic radii of a polymer? [closed]

Why do you consider that the relationship between RH and RG should depend on the quality of the solvent?
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34 views

First order reaction

I am little bit confuse about relating the half life time period of a first order reaction something like $$\ce{2A -> B + C}$$ Here the rate of reaction will be half of the rate at which $\ce{A}$ ...
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35 views

What kind of chemical reaction happens when combining Tahini and Date syrup?

While both Tahini and Date syrup are quite thin, when mixed together they form a paste-like substance that is very delicious, and quite thick. But what kind of chemical reaction leads to such sudden ...
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52 views

Steric factor is greater than one will the collision frequency determined by the arrhenius equation be greater or less than the experimental value? [closed]

In a bimolecular reaction the steric factor p was experimentally determined to be 4.5 Experimentally determined value of frequency factor is higher than that predicted by Arrhenius equation ...
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1answer
40 views

In a weak acid-strong base titration why is [H+] no longer equal to [A-] after addition of the base? (A2 level chemistry)

I understand that before any base is added $\ce{[H+]}$ is equal to $\ce{[A-]}$ but why is $\ce{[H+]}$ no longer equal to $\ce{[A-]}$ when the strong base is added? If all the following reactions ...
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1answer
76 views

Why do we use external pressure when calculating work done by gas? [duplicate]

For irreversible expansion and compression, why is dw= -pextdV but not dw= -pdV ? Many of the texts and related questions on this site I encountered give a same general idea: because work done= ...
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1answer
49 views

Entropy and total differential

In an exercise, I have derived that $$\mathrm dS_\mathrm{m} = \frac{3R}{2T}\mathrm dT + \frac{R}{V}\mathrm dV$$ using the 1st and 2nd law of thermodynamics (1 mol ideal gas). Now, I want to write $\...
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2answers
189 views

How do you calculate the correct charge on F after F2 dissociates?

When you ionize $\ce{F2}$ to $\ce{F2^5+}$, it quickly dissociates. Experiments show that the products of dissociation are $\ce{F^2+}$ and $\ce{F^3+}$. I have tried to model the dissociation of $\ce{...
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1answer
72 views

Calculating the ph of an unknown solution by measuring the precipitate [closed]

You have an unknown metal hydroxide, but no meters or indicators. You have lead (II) nitrate, which is soluble in it, and lead (II) hydroxide that is insoluble in it. You take one liter of the unknown ...
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2answers
64 views

Relation between ΔH and ΔE for the reaction in gaseous phase at constant temperature and pressure

Which of the following statements is correct for the reaction at constant temperature and pressure: $$\ce{CO(g) + \frac{1}{2}O_2(g) -> CO2(g)}$$ $\Delta H = \Delta E$ $\Delta H > \...
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290 views

Why is a LCAO necessary within Hartree Fock?

As I understand it, the electronic Schrödinger equation cannot be solved for polyelectronic systems. To circumvent this problem in the Hartree-Fock method, it is assumed that the polyelectronic ...
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26 views

Negative Voltage for Full Redox Reaction?

I have a question about voltages including redox reactions. I know that half equations may have negative voltages when choosing what is undergoing reduction, and what is undergoing oxidation in a ...
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1answer
79 views

Comparing strength of the C-H, N-H, and O-H bonds

$$ \begin{align} \ce{CH4(g) + Cl(g) &-> CH3(g) + HCl(g)} &\quad ΔH^\circ &= \pu{-14 kJ mol-1}_\mathrm{rxn}\\ \ce{NH4(g) + Cl(g) &-> NH2(g) + HCl(g)} &\quad ΔH^\circ &= \...
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1answer
39 views

Calculating pH of weak base and strong acid: Why is K(a) not required?

Question: 35mL of 1.5M $\ce{HCN}$, a weak acid(K(a)= $6.2*10^{-10}$) is mixed with 25mL of 2.5M $\ce{KOH}$. Calculate the pH of final solution. The solution given in my textbook is as follows:- $\ce{...
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1answer
28 views

How does energy and speed affect sub-atomic particles (protons, neutrons, electrons) in an electric field [closed]

(Source: https://gyazo.com/a4d5beed3272a7ca793dd8e2f7e8be3c) I am having trouble understanding the problem above. Why does it matter if the particles are held at constant energy or speed? How does ...
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1answer
51 views

SN1, SN2 and enantiomeric excess

When (R)-2-bromobutane is heated with water, the $\mathrm{S_N1}$ substitution proceeds twice as fast as $\mathrm{S_N2}$. Calculate the appropriate enantiomeric excess and the specific rotation of the ...
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1answer
59 views

Electrolytes are ionic compounds that conduct electricity [closed]

Electrolytes are ionic compounds that conduct electricity, or the flow of charged particles, when dissolved in water. This occurs via the separated ions acting as those charged particles, or charge ...
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3answers
49 views

Which of the following options are correct? [closed]

The equilibrium constant $K$ for the reaction $\ce{2HI <=> H2 + I2}$ at room temperature is $2.85$ and at $\pu{698 K}$ is $0.014$. This implies that: a) $\ce{HI}$ is exothermic compound b)...
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Rate of cathodic reaction vs Tafel slope

One of my reference engineering books says that for the cathodic protection process (in seawater): Al $\rightarrow$ Al$^{3+}$ + $3e^-$ (anode, setting the protection potential at about -900 mV SCE) ...
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23 views

Saturation dissolved oxygen level at pressure

The saturation level of dissolved oxygen in contact with a moist atmosphere is $$c = 0.21(p_\mathrm{atm} - p_\mathrm{vap}) K_\mathrm{H} M$$ where the $p_\mathrm{atm}$ and $p_\mathrm{vap}$ are the ...
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1answer
79 views

Negative Molar Specific Heat of a Gas

It is known that the specific heat of a gas is process dependent. So it must be theoretically possible to have a negative value for a gas according to the following equation (for polytropic process, $...
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25 views

Why different Asbestos types are hydrophobic and hydrophilic

I was doing research into the structure of asbestos to see that white asbestos chrysotile is hydrophilic where as both brown and blue asbestos is hydrophobic. Could you please explain why this is?
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1answer
60 views

I think elements can be broken down, but their definition says otherwise, why? [closed]

Elements are made up of quarks, right? and physicists have smashed atoms into one another to get quarks, right? Then if the first two statements are true then elements can be broken down into simpler ...
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1answer
34 views

Rate expression for the final product [closed]

Let's say we have: $$ \begin{align} \ce{A &-> B}&\quad&\text{(fast equilibrium)}\label{rxn:1}\tag{1}\\ \ce{2 B + C &-> D}&\quad&\text{(slow)}\label{rxn:2}\tag{2}\\ \...
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Calculate the energy in the form of heat (in kJ) required to convert 325 grams of liquid water at 20.0 °C to steam at 115 °C

Calculate the energy in the form of heat (in kJ) required to convert 325 g of liquid water at 20.0 °C to steam at 115 °C. Assume that no energy in the form of heat is transferred to the environment. (...
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1answer
432 views

When to use 1/Ka vs Kb [closed]

I'm really unsure what the difference is. We're using these when calculating $K$ of combined reactions. In general, I am also confused what $K$ is in comparison to $K_\mathrm{a}$ and $K_\mathrm{b}$. ...
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2answers
121 views

Find the pH of an amphiprotic salt dissolved in water at room temperature

Let's say I have a polyprotic acid $\ce{H2A}$ with the following properties: $$ \begin{align} \ce{H2A &<=> HA- + H+} & K_1 &= 1\cdot 10^{-1}\\ \ce{HA- &<=> A^2-+ H+} & ...
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69 views

Degree of Hardness Numerical

To remove the temporary hardness from $5\ \mathrm L$ of $\ce{H2O}$ $5.6\ \mathrm{mg}$ of lime is required. Calculate the degree of hardness due to $\ce{Ca(HCO3)2}$ present in water. My attempt: $$ \...
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1answer
152 views

Replacing Salt Bridge with Platinum Sheet

What would happen, if in a galvanic cell, the salt bridge is replaced by an inert platinum sheet? Would charges accumulate on the sheet? Would the cell still work, and produce less emf than earlier, ...
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51 views

Regarding Standard Electrode Potentials

If the standard hydrogen electrode (SHE) is set arbitrarily to have a potential that is equal to 0 V, and has an absolute real potential. Then, in electrochemistry, how are the electrode potentials of ...
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2answers
416 views

When does Hartree-Fock fundamentally fail?

The Hartree-Fock approximation (HFA) works by assuming each electron sees the effective electric field from all the other electrons as some self-consistent field. The HFA is known to give pretty ...
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Free induction decay in NMR (as stated in Pavia)

There is a statement in a very popular textbook of spectroscopy by Pavia (Chapter 3, Intro. to Spectroscopy), it says "The observed FID is actually an interference signal between the radiofrequency ...
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2answers
96 views

Molarity Numerical on Water Gas Shift Reaction

For the water gas shift reaction below, $K_c = 3.491$ at a certain temperature. What are the equilibrium concentrations of all the components of the reaction if $\pu{0.3815 mol}$ of $\ce{CO}$ and $\ce{...
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1answer
34 views

Meaning of polarization pulse in microwave (rotational) spectroscopy

In the recent rotational spectroscopy literature, "microwave polarization pulse" is often used in conjuction with pulsed Fourier transform microwave spectroscopy (Google Scholar). Sometimes the pulse ...
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1answer
54 views

Why is the graph of fluorescence vs. wavelength continuous?

If the emission of a photon from an excited state to the ground state occurs with a precise energy, then why is it that wavelengths that differ from lambda max have values on a fluorescence vs. ...
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2answers
535 views

Can the van der Waals coefficients be negative in the van der Waals equation for real gases?

A gas described by van der Waals equation has the pressure that is lower than the pressure exerted by the same gas behaving ideally. True or false? My approach $$P(\text{ideal}) = P(\text{real}) + P(...
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1answer
71 views

Derive expression for heat flow from oil bath to metal sphere

A spherical ball of iron is put into a hot bath of oil. The iron ball is initially at a low temperature and heat transfer from the oil to the iron will increase the temperature of the iron ball. ...
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Isobaric-isothermal ensemble with degeneracy

I've been studying statistical mechanics for a while, the topic is very far from what I've learned till now so my questions may be superficial. When trying to see what my entropy is in an isobaric-...
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1answer
63 views

Relationship between rate of reaction and pressure

Given the equation $$\ce{A(g) + B(g) → C(g) + D(g)}$$ with reaction rate given by $$v = k[\ce{A}]^2$$ How can I prove that reaction rate goes up with pressure? I tried substituting $k$ for $Ae^{-...
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43 views

How will the equilibrium concentration change on adding inert gas at constant pressure? [duplicate]

The given reaction is $$\ce{AB2(s) → A(g) + B2(g)}$$ Now on adding inert gas at constant pressure equilibrium should shift on direction of more gaseous moles. But the answer says that equilibrium ...
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36 views

How to obtain spectroscopic constants Omega_e, Alpha_e, B_e, D_e and D_0 from centrifugal constants?

I need to compute a spectroscopic constants of $N_2^+$ states and I would like to do it using LEVEL16 software, due to its high interpolation flexibility. But it seems, that LEVEL16 can only compute ...
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1answer
70 views

Questions in Equilibrium [closed]

I have two inquiries concerning equilibrium: Does $K_c = 5.0$ indicate that the equilibrium mixture contains both products and reactants at an approximately equal amount? I know that $K_c \approx 1.0$...
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1answer
203 views

Electrolysis of water: Which equations to use? (IB Chem)

There is a list of standard electrode potentials at 298 K from the p. 23 of IB Data Booklet 2016. Which of the following equations (forward/backward reactions), from the two possible ones involving ...
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1answer
70 views

Proving a relation for fugacity of component i in term of independent variables (T,V)

In Molecular Thermodynamics of fluid phase equilibria by Prausnitz et al. [1] the authors recommend to use the following equation which gives the fugacity of component $i$ in terms of independent ...
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Why do old black and white photos fade?

I understand why photos based on organic dyes would fade (discussed previously). But photos based on metallic silver fade as well such as like the one shown below. As a metal (a fairly stable one at ...