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 to calculate the concentration after a certain timespan from the specific reaction constant?

A container at $30~^\circ\pu{C}$ contains $\ce{HNC_{(g)}}$ reacting to form $\ce{HCN_{(g)}}$. Knowing that the specific reaction constant is $4.4\cdot10^{-4}~\pu{s-1}$ and that you initially have $\pu{...
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Rate equation variations

I am having a bit of trouble understanding variations to rate equation problems. What I know: For $\ce{A + B -> AB}$, if the rate is proportional to the concentration of A and B, the rate equation ...
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In a galvanic cell, why do electrons move through the wire from zinc to copper?

I understand that there is aqueous $\ce{Cu^2+}$ in this scenario which pulls on the electrons from the zinc causing the electrons to move through the wire, but how does the electronegativity of $\ce{...
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1answer
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How do we figure out whether reactions are endothermic or exothermic? [closed]

Are the following reactions exothermic or endothermic: $$\ce{H2 + I2 -> 2HI}$$ $$\ce{N2 + O2 -> 2NO}$$ So I know the second reaction is endothermic as we need to break triple bonds between ...
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1answer
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Casimir effect and van der Waals forces

The Casimir effect makes two close metal plates in a vacuum attract. I was pondering and figured out that two close molecules should experience the same effect, as they are close and between molecules ...
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Uncertainity principle of a particle on a ring in $m=0$ state

When a particle is on a ring, we know that the commutator $[L_z,x]=i\hbar y$ and $[L_z,y]=-i\hbar x$. So we can't measure $L_z$ and $x$ & $L_z$ and $y$ simultaneously with infinite accuracy. When $...
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d-Menthol vs dl-menthol: Does an enantiomer and its racemic mixture have different melting points?

I am checking out Wikipedia and it shows a different melting point (m.p.) for l-menthol $(\pu{42 ^\circ C}$ to $\pu{45 ^\circ C})$ vs dl-menthol $(\pu{36 ^\circ C}$ to $\pu{38 ^\circ C}).$ How is ...
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Why isn't the chemistry of a zinc-air battery (meaningfully) hindered by an activation energy barrier?

Virtually every source I've read pertaining to zinc-air batteries explain the chemistry of it either using these formulae or a simplified version of them: $$ \begin{align} &\text{Anode:} &\...
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Activity of a chemical species and compound

While reading a book on Instrumental analysis by Skoog, in appendix part they explain activity of a species. The relationship between activity of a species and it's molar concentration $[X]$ is given ...
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How is the Born-Lande equation modified when the structure is not NaCl?

Often the Born-Lande equation is quoted (alongwith the calculation of the Madelung constant and Born exponent) with reference to rock-salt structure. But what if we take some other crystal, like ...
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1answer
643 views

Why Freundlich adsorption isotherms fails at high pressure conditions?

Experimentally it was determined that extent of gas adsorption varies directly with pressure, and then it directly varies with pressure raised to the power 1/n until saturation pressure Ps is reached. ...
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1answer
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How to apply the Heisenberg Uncertainty Principle in calculations on the movement of bacteria?

In this problem, I know the formula, but I get confused with all the steps and how to use what I got, to get to what I need. A student is examining a bacterium under the microscope. The E. coli ...
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1answer
84 views

On the origin of Gibbs Free Energy

It is intuitive to me that for a spontaneous process involving any system i.e. one occurring without constant human interaction $$ \text{d}S \geq 0 $$ as I can observe increasing energy dispersal in ...
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1answer
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Find the uncertainty in position

The question says: A proton is accelerated to one tenth of the velocity of light. If it's velocity can be measured with a precision $\pm 1\%$. What must be its uncertainty in position? Therefore, $v=...
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Find the color of the solution

The acid-base indicator methyl red has a $K_\mathrm{a}$ = $1 \times 10^{-5}$. Its acidic form is red while its alkaline form is yellow. If methyl red is added in a colorless solution with a $\mathrm{...
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Formulas for the number of spectral lines

While reading through Narendra Avasthi's Problems in Physical Chemistry, I came across two formulas on p. 64 (Scanned page): When electrons de-excite from higher energy level ($n_2$) to lower ...
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Why is the water solubility of CO worse than that of CO2?

I tried my best to ask SciFinder, Google, and Google Scholar the wrong questions ... I found many paper on the fitting of the solubility to some equations to be able to calculate the amount of $\ce{CO}...
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1answer
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Is constant pressure heat capacity = entropy during adiabatic reversible expansion of perfect gas?

For any process, ∆G = ∆H - ∆(TS). Apply this on reversible adiabatic expansion of perfect gas, since the entropy of system does not change during this process, S is constant so ∆G = ∆H - S∆T The ...
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How does the entropic force arise within the hydrophobic effect?

I know that the hydrophobic effect is usually explained by the entropic effect originating from the disruption of hydrogen bonds between molecules of water and the nonpolar substance. The hydrogen ...
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3answers
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Finding is the concentration of H⁺ in a benzoate buffer solution after adding HCl

Q: A buffer with a $\mathrm{pH}$ of 4.44 contains $\pu{0.21 M}$ of sodium benzoate and $\pu{0.12 M}$ of benzoic acid. What is the concentration of $\ce{[H+]}$ in the solution after the addition of $\...
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109 views

Enthalpy of reaction between ammonia and oxygen

I have to ask this here because I asked literally everybody I know and I still disagree with them. Using the information in the table below, calculate the heat of released from the following ...
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Deriving The Triple Point Temperature and Pressure as the following expression

Using the usual notations of $\Delta H_\text{fus}$, $\Delta H_\text{vap}$, $\Delta H_\text{sub}$, as well as ($T_\mathrm l,p_\mathrm l$) and ($T_\mathrm s,p_\mathrm s$), which corresponds to the ...
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How does the charge imbalance affect the cell reaction when there is no salt bridge in a galvanic cell?

As mentioned in the answers to this question Why is it important to use a salt bridge in a voltaic cell? Can a wire be used? a salt bridge is used to keep the two half cells neutral. i understand how ...
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What is fugacity?

When one attachs a manometer to car tires or carbon dioxide fire extinguishers, what is being measured: the pressure or the fugacity? For ideal gases both are the same, but not for real gases. As ...
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Gibbs function for pure substances

I know that the Gibbs function for pure substances is given by $$\mathrm dG = V\,\mathrm dp - S\,\mathrm dT.\tag{1}$$ However, I also saw the same equation in the form $$ΔG = VΔp - SΔT,\tag{2}$$ ...
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How is a reaction with more products than reactants spontaneous?

If I have a reaction that has $\Delta G^\circ = 0,$ where there are more products than reactants (for example, $\ce{A <=> 2B + 2C}),$ then if I change the concentration of all the reactants and ...
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Gibb's energy change for system

Heat liberated by a given amount of an ideal gas is $\pu{1200 cal}$ at $\pu{300K}$. What is the Gibbs free energy change for the gas in this process? My attempt: I know that for a reversible process ...
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2answers
64 views

Is this statement for the First Law of Thermodynamics correct?

My Biochemistry book states the First law of Thermodynamics like this: "The First Law of Thermodynamics states that in a closed system, energy is conserved". My book defines Closed System as a system ...
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1answer
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Effect of solutes on surface tension

What happens to the surface tension of a solvent, say, water when we mix a solute in it? For the sake of concrete discussion, the examples that I encountered in a test were sodium chloride $(\ce{NaCl})...
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1answer
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Doubt regarding Raoult’s Law definition

Raoult’s law states that: The partial pressure of any volatile constituent of a solution at a constant temperature is equal to the vapour pressure of pure constituent multiplied by the mole ...
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Definition of “solution” [closed]

In my textbook “solutions” are defined as follows: Homogenous mixtures of two or more substances are known as solutions. Should the two substances always be non-reacting? The definition has no ...
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1answer
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Associated occupation numbers of frozen natural orbitals (FNOs) from MP2 method

The paper Frozen Virtual Natural Orbitals for Coupled-Cluster Linear-Response Theory gives the following (simplified) description of frozen natural orbitals (FNOs). In the MP2 method, the unrelaxed ...
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3answers
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The meaning of permanent dipole moment

I was asked in one question that which of all given molecules have a permanent dipole moment? And $\ce{XeF6}$ was given as a correct answer quoting that it is a distorted octahedral, and hence the ...
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1answer
366 views

How to find the potential of a mixed Iron/ Cer solution?

Given the following half-reactions: \begin{align} \ce{Ce^{4+} + e^{−} &-> Ce^{3+}} & E° &= \pu{1.72 V}\\ \ce{Fe^{3+} + e^{−} &-> Fe^{2+}} & E° &= \pu{0.771 V}\\...
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Gravimetric analysis

In a gravimetric determination of phosphorus an aqueous solution of dihydrogen phosphate ion is treated with the mixture of ammonium and magnesium ions to precipitate magnesium Ammonium Phosphate. ...
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1answer
363 views

What does m± represent in the equation for mean activity of a salt solution?

I need to find the activity $a_\pm$ for a $\pu{0.0120 M}$ solution of $\ce{Na3PO4}$, and have calculated the activity coefficient, $\gamma_\pm = 0.389$. I know that $$a_\pm = \frac{m_\pm}{m_0}\gamma_\...
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My question is about solubility (solvation/hydration) [closed]

I am a high school student and I am reading this chapter of chemistry named Chemical Bonding. In many books it is written that in hydration, water molecules surround the dissociated ions of ionic ...
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What are the limitations of the Born-Lande' equation?

The Born-Lande' equation is used to theoretically calculate the lattice energy, $\Delta U$, of ionic compounds. It is often cited as such in literature, $$\Delta U = -\frac{k_Az_1z_2Me^2}{4 \pi \...
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How can Ellingham diagrams be linked to Gibbs energies of formation?

My textbook1 quotes the following as the limitation of Ellingham diagram: The interpretation of $\Delta G_\mathrm{f}^\circ$(standard Gibbs free energy of formation) is based on $K$ ($\Delta G_\...
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1answer
366 views

Amphoteric amino acid titration

Glycine $(\ce{NH2CH2COOH})$ is an amino acid with two $\mathrm{p}K_a$ values $(\mathrm{p}K_{a,1}=2.0, \ \ce{COOH};\ \mathrm{p}K_{a,2}=10.0, \ \ce{NH2})$. (a) If $\pu{0.01 mol}$ of this amino acid is ...
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Determining cation-to-anion radius ratio for “ideal” Anti-fluorite structure

My book under the heading of Anti-fluorite structure makes the following remark, about the radius ratio of an ideal Anti-fluorite: The ideal antifluorite would have a cation-to-anion radius ratio ...
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2answers
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How does overvoltage affect the products of electrolysis?

When you electrolyse an aqueous solution of $\ce{NaCl},$ the product formed at cathode is $\ce{H2}$ gas (preferential discharging), but the product formed at anode is $\ce{Cl2}$ gas. According to ...
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1answer
53 views

Is an amorphous solid a single phase? [closed]

Amorphous solid possess short-range order but no long-range order (e.g. amorphous silica, the short-range order is originated from the tetrahedral bonding between Si and O). Is it possible to have ...
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Critical separation at which molecule breaks

I have the following question. The potential energy of two atoms, a distance $r$ apart, is: $$U = -Ar^{-2} + Br^{-10}$$ Given that the atoms form a stable molecule at a separation of $\pu{0.3 nm}$ ...
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1answer
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How can I model cellular membranes in terms of topology?

I'm reading from Principles of Physical Biochemistry by Kensal E van Holde (1998-01-06) in order to review some biochemistry and work my way toward a more mathematical approach to understanding ...
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135 views

Does hot water freezes faster than cold water? [closed]

When we put cold water in the fridge it takes time to freeze but sometimes hot water is faster in freezing.Why?
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121 views

Strength of bases and acids

Is it possible that strong acids like $\ce{H2SO4}$ behave as weaker ones like $\ce{CH3COOH}$ when the concentration is really high and vice versa for weak acid? And the same question for bases. By ...
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1answer
442 views

Lambert-Beer's law coefficient

While the Beer's law equation is $A=a \cdot b \cdot c$, according to my book, $a$ is a coefficient which is: a characteristic property of the analyte in a specific solvent, and its values will ...
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1answer
54 views

On mechanism behind Marcus theory

Regarding the mechanism behind Marcus theory, I was wondering if reaction occurs through tunneling when it does not have sufficient activation energy? Wouldn't expecting reactions to occur this way be ...
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81 views

Why does a first order reaction only depend on the concentration of a single reactant?

I am new to kinetics so please explain it from basic, If there is a reaction - $$\ce{A + B + C -> D}$$ In first order reaction rate is given by- $\mathrm R = k[\mathrm A]$ or $\mathrm R = k[\...

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