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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10
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3answers
7k views

How can the equilibrium shift, while Kc remains constant?

Consider the following reversible reaction. $$\ce{Cr2O7^2-(aq) + H2O(l) <=> 2 CrO4^2-(aq) + 2 H+(aq)}$$ What will happen to the position of equilibrium and the value of $K_c$ when more ...
2
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1answer
32 views

How to explain Gibbs free energy is a pressure-dependent state function?

I'm aware that through deriving Gibbs free energy to infinitesimal changes, we could get the formula: $\mathrm dG = V\,\mathrm dp - S\,\mathrm dT$, giving that Gibbs free energy is pressure-dependent. ...
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0answers
33 views

Arrhenius equation using concentration

I've been conducting some experiments to determine the temperature dependence of a reaction. To do this, I've been conducting the experiments at different temperatures and generated a graph of $\ln{[\...
2
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1answer
33 views

Is there any book where specific heat capacities are given?

I want to find the value of the specific heat capacity of aluminium nitrate nonahydrate, in order to calculate mixing effects in a chemical reaction. I am looking for a source, such as a book or link ...
2
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0answers
53 views

Are there any gases that can not be turned into liquid, or liquids that can not be frozen solid?

My son points out that some solids can not be melted (because they combust, for example). He wonders if there are any liquids that can not be frozen, or gases that can not be liquified.
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2answers
853 views

Comparing formula for enthalpy change with bond dissociation energy and formation enthalpy

I learnt that given a reaction: $$\ce{A -> B}$$ the enthalpy change is given by: $$\Delta H = \left( \begin{array}{c} \text{total enthalpy of}\\ \text{bonds broken}\end{array}\right)-\left( \begin{...
10
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2answers
373 views

Kolmogorov backward equations in active biological systems

I am trying to understand the following equations from a paper by Wang et al. [1, p. 5 at the SI]: Michaelis–Menten Representation of the Kinesin Cycle We assumed that there is a strong ...
3
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2answers
123 views

Why is there a z² label for d orbitals, but no x² and y² labels with corresponding shapes?

I know that the $\mathrm{d}_{z^2}$ orbitals are named as a result from the conversion of the angular component of the hydrogen-like wave function in terms of Cartesian coordinates, but how come we ...
2
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1answer
79 views

Conductance of the mixture of two solutions with given resistance values [duplicate]

If $R_1$ and $R_2$ are the resistances of two solutions of equal volume what is the conductance of the mixture in the same conductivity cell? In general, I grasped neither the approach, nor the ...
4
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2answers
87 views

Exothermic: enthalpy or heat?

I am used to thinking of exothermic reactions as being defined as having negative $ΔH.$ However, I also am used to hearing/thinking of exothermic reactions as "releasing heat". Since change in ...
24
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5answers
14k views

Why does radium have a higher first ionisation energy than barium?

I'm wondering why radium appears to buck the general trend that first ionisation energies decrease as you move down a group in the periodic table: barium (the group 2 element preceding it) has a first ...
2
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0answers
48 views

How to calculate the value of heat absorption/ heat release during water electrolysis?

The overall reactions with the enthalpies of water electrolysis at room temperature can be represented by the equation below: $$\ce{2H2O + \pu{237.2 kJ mol-1} + \pu{48.6 kJ mol-1} → 2 H2 + O2}$$ ...
5
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3answers
124 views

Why doesn't beryllium, an elemental solid, obey Dulong and Petit's law?

Beryllium has a molar heat capacity of $\pu{17.7 J mol-1 K-1},$ which is quite far from the $\pu{25 J mol-1 K-1}$ as predicted by Dulong and Petit's law. What is the reason for this discrepancy?
1
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1answer
61 views

What would be a stable way to hold lithium hydride powder?

I currently have 100 grams of lithium hydride powder that needs to be held in a solid form. Initially, I was considering using cement, however, the moisture require converts the lithium hydride to ...
5
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1answer
62 views

What drives vibrational cooling in an excited stated?

As we excite a molecule from its ground state, $S_{0,v=0}$, to some excited state in a higher vibrational state, i.e. $S_{1,v'=3}$, what drives vibrational cooling within that excited state manifold (...
7
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1answer
9k views

Is the Aufbau principle violated in the diagram (b)?

In which of the following diagrams is the Aufbau principle violated? Ignoring the Hund principle, the Aufbau principle states that electrons have to first fill the lowest energy levels. Now the ...
0
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0answers
29 views

Calculate change in entropy when both temperature and volume change

Calculate the change in entropy when $\pu{23.5 g}$ of oxygen gas is heated from $\pu{240 K}$ to $\pu{360 K}$ in such a way that its pressure increases from $\pu{2.00 kPa}$ to $\pu{8.00 kPa}.$ Assume ...
45
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4answers
3k views

Unit of the equilibrium constant: contradiction of Bridgman's theorem?

The following equation is standard in thermodynamics: $$ \Delta G^\circ=-RT\log(K) $$ where $K$ is the equilibrium constant. In dimensional analysis, Bridgman's theorem tells us that the argument ...
3
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3answers
5k views

Does temperature affect the %oxygen in air by volume? (even though oxygen density varies inversely with temperature according to P/RT=n/V)

I want to investigate how temperature affect the concentration of oxygen in air as it will affect how much oxygen we inhale per breathe. Oxygen by percent composition in air is 20.95%. However, the ...
5
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2answers
174 views

Derivation of time probability distributions for single molecule kinetics

If we have a protein $\ce{A}$ that can bind to protein $\ce{B}$: $$\ce{A + B <=>[$k_\mathrm{fwd,1}$][$k_\mathrm{rev,1}$] AB}$$ and the bound complex $\ce{AB}$ can be converted into $\ce{C}$: $...
5
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1answer
120 views

Finding identities of diatomics from Spectroscopy and Mass Spec Data

Let's say I have a sample of 2 (assumingly different) diatomics A and B. Through spectroscopy I found the data below: For molecule A: $$\begin{align} B&\approx2.17690\ \mathrm{cm^{−1}}\\ D&\...
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2answers
550 views

Excluded Volume in van Der Waals equation

I was just reading over the EOS topic when where I stumbled upon excluded volume. Although the pictorial representation of the hard sphere model clearly shows that the excluded volume is 4 times the ...
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0answers
17 views

Changing conditions of gaseous mixture

A gaseous mixture of $\ce{NO2}$ and $\ce{N2O4}$ is sealed in 4 identical vessels with movable pistons. When the conditions are modified as described in 1) to 4), in which condition does the molar ...
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2answers
1k views

What are the degrees of freedom (from Gibbs phase rule) for an azeotropic mixture?

For an azeotropic binary mixture, the number of phases is 2 and number of components are 2. So, according to Gibbs phase rule, the number of degrees of freedom should be 2. But, the right answer is 1. ...
0
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1answer
120 views

Entropy change in non-ideal solutions

We know that some solutions show positive deviations and some show negative deviation from Raoult's law.The former is endothermic and the latter is exothermic. And again, the volume change show ...
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0answers
154 views

Which of the following statements is true if HI is formed in the following reaction?

Which of the following statements is true if HI is formed in the following reaction? $$\ce{H2(g) + I2(g) -> 2HI(g)}$$ a. There are no collisions between the reactants in the reaction. ...
-1
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1answer
47 views

Why electrons move to Cu in galvanic cells?

I'm student who learning about chemistry, and I got problem about understanding why electorns move to Cu in galvanic cells. Let's think the reason for electrons going to Cu is that the standard ...
6
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1answer
4k views

What mass of water is present as a liquid when equilibrium is established?

$1.00~\mathrm{g}$ of water is introduced into a $5.00~\mathrm{L}$ flask at $50~^\circ\mathrm{C}$. What mass of water is present as a liquid when equilibrium is established? Also you know $92.5~\mathrm{...
1
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1answer
47 views

Decrease in molar conductivity with increase in concentration

Why does molar conductivity $Λ$ for strong electrolytes decrease with increase in concentration? I mean, more concentration would mean higher number of solute particles present in the solution, and as ...
1
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1answer
53 views

What does it mean for chemical reaction network to be in equilibrium?

What does being in "equilibrium" mean for a set of chemical reactions? If we have some number $n$ of $\ce{A}$ molecules and $m$ of $\ce{B},$ and the following reaction where $\ce{A}$ and $\ce{B}$ can ...
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0answers
29 views

Reaction between copper and silver(I) nitrate in aqueous medium

What is the reaction between $\ce{Cu}$ and $\ce{AgNO3}$ taking place in aqueous medium? The answer given was: "Only" oxidation of $\ce{Cu}.$ My concern is: how is only oxidation of $\ce{Cu}$ ...
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2answers
440 views

Density of saturated n-octane in air at 300 K?

I am trying to calculate (if possible) the gas density of air saturated with n-octane at 300 K. Any suggestion of how to do this, or better, where I can look up the data?
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0answers
25 views

How do you remove lumps in reagents without contaminating them?

I think desiccating the powdered reagent would help remove lumps (at least in preventing lump neoformation) but would desiccation alone collapse all lumps in a powder (say KCl as it lumps easily), or ...
9
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1answer
30k views

How to find the molecular formula for a hydrocarbon compound from known combustion products?

What is the molecular formula of a compound containing only carbon and hydrogen if combustion of $1.05~\mathrm{g}$ of the compound produces $3.30~\mathrm{g}~\ce{CO2}$ and $1.35~\mathrm{g}~\ce{H2O}$ ...
8
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1answer
4k views

Why can a sigma bond rotate?

My book says Because it (the sigma bond) lies along the line joining the two carbon atoms, the sigma bond can rotate about this axis. But that doesn't make sense to me. So what if the sigma bond ...
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0answers
76 views

A tricky question in Eudiometry/Gas analysis

In a eudiometer $\pu{14 ml}$ of $\ce{CH4}$ and $\pu{38.5 ml}$ of $\ce{O2}$ was introduced. After explosion and cooling to initial temperature ($\pu{23°C}$) the volume of eudiometer was found to be $\...
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0answers
24 views
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1answer
25 views

Dosing leaves with Zinc Nitrate for pollution experiment - Need specific concentration

I am trying to assess the pollution tolerance of an insect using Zn as a proxy. I have a supply of Zinc Nitrate Hexahydrate that I want to use to contaminate dried leaves (the insect's food). The LD50 ...
8
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1answer
1k views

What does “lit.” mean in boiling point or melting point specification?

For example, this Xanthene 99% specification from Sigma Aldrich states: \begin{array} \\\hline \text{bp} & 310-312 ~^\circ\mathrm{C~ (lit.)} \\\hline \text{mp} & 101-102 ~^\circ\mathrm{C~ (...
1
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1answer
32 views

What molecules are produced in carbon arc lamp and how do they produce light?

I am curious about the chemistry of the arc lamp (an indirectly for the incandescent bulbs) thus I would like to ask the following. It is a common understanding that the electrons pushed inside an ...
22
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2answers
12k views

Why is distillation not a viable way to separate ammonia from water?

Since the steam pressure of ammonia is higher than that of water, I would expect distillation to be a reasonable way of separating a mixture of both. However, in industrial applications known to me ...
0
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2answers
69 views

What is the physical meaning of fractional oxidation numbers of elements in compound

Many times I encounter oxidation numbers like -1 in the peroxide for oxygen, -1/2 in superoxide, and in some other compounds some elements have 8/3 or such like that...what is the physical meaning of ...
2
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1answer
77 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 ...
6
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1answer
6k views

Why is the standard enthalpy of formation of elements in their native forms zero?

Why is the standard enthalpy of formation of $\ce{O2}$ and other reference forms of elements equal to zero? Well, from what I know, bonding releases energy. Also, does anybody know a source for ...
0
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1answer
208 views

Why does the standard enthalpy of formation diverge so far from the standard Gibbs free energy of formation for some substances?

If you look at a Table of Thermodynamic Values for chemical substances, most substances have very close values for their standard enthalpy of formation ($\Delta H_{\mathrm{f}}^\circ$) and their ...
0
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2answers
860 views

How much water evaporates in a vacuum before freezing?

I placed a container with 1000 grams (1 kg) of water at room temperature (20 degrees Celsius) inside a vacuum at 0.03 atm pressure. The water boiled for a few seconds then froze solid. I need to ...
1
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1answer
379 views

When calculating enthalpy changes of reaction using, why is only the amount of substance of one reactant used?

When calculating the thermal energy released during an acid-base neutralisation using the equation $Q = mc\,\Delta T$, and then using $\Delta H = - Q/n$ to calculate the enthalpy change of ...
6
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1answer
91 views

Equilibrium constant for a solid-solid equilibrium

Suppose there are two equations: $$ \begin{align} \ce{A(s) &<=> B(g) + C(s)} &\quad &K_1 \tag{R1}\\ \ce{B(g) &<=> D(s) + E(s)} &\quad &K_2 \tag{R2} \end{align}$$ ...
1
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1answer
62 views

Why do power requirement figures for water electrolysis on the web seem to be wrong?

I need to calculate the power needed to electrolyse 1 mole of water for a project. We will either use Polymer Electrolyte Membrane Electrolysis or Alkaline Water Electrolysis, they operate on 1.75V-2....
0
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1answer
51 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 $\...