All Questions
Tagged with theoretical-chemistry thermodynamics
27
questions
6
votes
1
answer
237
views
Non-Arrhenius temperature dependence of bimolecular reaction rates at very high temperatures
Once I have read that in some cases bimolecular reactions can exhibit a maximum as a function of temperature due to the short lifetime of the activated complex at very high temperatures.
At low ...
- 163
2
votes
0
answers
84
views
Diffusion vs mobility and the Einstein relation
This former question is about the mobility constant $M>0$ in the Cahn-Hilliard equation. To determine the value of such a mobility for a simulation study, Kim and Sanders (2020)
use the formula
$$
...
- 193
1
vote
0
answers
55
views
Why probability for an atom is fraction for population?
In the canonical ensemble the probability of finding an atom (or molecule) in its $i$-th energy state is given by the Boltzmann factor.
This probability is interpreted as the number of microstates ...
- 766
8
votes
2
answers
588
views
Practical use of the partition function in molecular simulations
In the second chapter of Understanding Molecular Simulation, Frenkel and Smit derive an equation for the partition function and the thermal average of the generic observable A, stating that these ...
3
votes
0
answers
21
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Correlating specific heat (Cp) of a substance with other substance on the basis of similar physio-chemical properties
I am modelling $\ce{H2O2}$ decomposition in ANSYS Chemkin-Pro and I have difficulty finding $C_p$ (specific heat at constant pressure) for liquid $\ce{H2O2}$ for various temperature ranges.
My ...
- 31
2
votes
0
answers
32
views
expression of apparent reaction quotient
question
When computing the Gibbs energy change of a reaction using Legendre-transformed $\Delta_fG'$ values for a given $\mathrm{pH}$, the formula of the $\Delta_rG'$ of the reaction is:
$\Delta_rG' ...
- 306
1
vote
0
answers
44
views
What makes determining entropy changes theoretically so hard compared to enthalpy?
It is really hard to determine the entropy change of a reaction accurately because of subtle solvent effects and a lack of standard entropies. On the other hand, enthalpy is relatively easy and ...
- 732
1
vote
2
answers
227
views
Concept of enthalpy at a given temperature
In thermodynamics, we always quote a fixed temperature, whenever we mention enthalpy of a reaction. For instance, one can determine the enthalpy of combustion of methane at 25 °C. Now almost all ...
- 41.6k
6
votes
2
answers
327
views
Tunneling in chemical reactions
We know that quantum tunneling is the reason behind several natural phenomenon like alpha decay and thermonuclear fusion inside the stars. How can it influence chemical reactions by tunnelling a ...
6
votes
1
answer
197
views
Understanding elementary rate laws from a probability stand point
I am learning about chemical kinetics and dynamics and as I understand for a general chemical reaction
$$\ce{aA + bB -> cC + dD} $$ whose reaction rate, r, can be described by an elementary rate ...
- 61
11
votes
2
answers
2k
views
How does an oscillating reaction work?
I watched a Belousov--Zhabotinsky reaction video showing an orange solution that goes to clear and then back to orange (and so forth.)
The reaction goes through a number of cycles before it will no ...
- 6,801
7
votes
1
answer
2k
views
Calculating internal energy of methane
I've recently found out my calculated value of internal energy of methane largely deviates from the ab initio output (at $\pu{1000 K}$ $0.058~\text{Hartree} = \pu{152.3 kJ mol-1}$). I used HF method, ...
- 303
2
votes
0
answers
99
views
Why is it impossible to liquefy a gas with only pressure? [duplicate]
Why is it impossible to turn it to liquid by using only pressure after it reaches critical temperature? What happens in the molecular level? If we put enough pressure, the molecules are supposed to ...
- 307
1
vote
1
answer
123
views
Mathematical properties of free Gibbs energy
I'm wandering, if some mathematical properties of the free Gibbs energy (also called thermodynamic potential) are well known.
For instance in chemistry, if $n_i^\alpha$ denotes the number of moles of ...
- 133
3
votes
1
answer
201
views
Thermodynamics in Solution (solvation) from quantum chemistry
The gist of the question is "how do I get a heat of formation in solution" - but let me give it some context to be more specific.
I am interested in calculating the thermodynamic values of molecules ...
- 565
4
votes
1
answer
208
views
Plasma creation and condensation
Every chemical has a decomposition temperature. My understanding is that above that temperature molecular bonds are broken. And if we raise the heat high enough then all molecular bonds will break ...
- 3,340
18
votes
1
answer
732
views
What is the relation between surface tension and initial molecular harmonic?
Initial harmonic is measured as Hartree/Bohr^2 ( like IHarmonic=n in Gaussian Software ). As I am from physics background I am used to look at features from their dimension point of view:
Actually ...
- 2,141
2
votes
2
answers
947
views
Does the third law of thermodynamics apply to all compounds and elements?
According to wikipedia, the third law of thermodynamics is "The entropy of a perfect crystal, at absolute zero (zero kelvins), is exactly equal to zero.".
Then, theoretically, could all substances, ...
- 1,411
16
votes
1
answer
13k
views
Why is there sulfur in black powder / gun powder?
Black powder is composed of charcoal, sulfur and a nitrate salt. The charcoal and sulfur serve as fuel and the nitrate is the oxidizing agent.
However, wouldn't it be cheaper to leave out the sulfur, ...
- 10.9k
12
votes
1
answer
3k
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How to make approximation of rotational partition function of diatomic linear molecules?
Using the rigid rotor approximation to the level energies, and such other appropriate assumptions, we can approximate rotational partition function, $Q_{\mathrm{rot}}$, of linear molecules as follows:
...
- 181
8
votes
1
answer
2k
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Why is N₂ stable but HCN and C₂H₂ unstable?
Compounds with triple bonds generally seem to be unstable. $\ce{HCN}$ and $\ce{C2H2}$ are high-energy, relatively short-lived molecules that will readily polymerise or react with other organic ...
- 1,661
4
votes
1
answer
998
views
How to estimate the dielectric constant of a liquid?
Are there any approximate relationships between the (static) dielectric constant of a liquid and properties of the individual molecules?
For example, suppose I know the polarizability and dipole ...
- 917
2
votes
1
answer
82
views
Polymers with no molecular mass distribution?
What sort of ideal conditions are required - theoretically - to conduct polymerization reactions to make all the macromolecules end up with exactly the same molecular mass, just like the products in ...
- 517
114
votes
1
answer
6k
views
Is there a general consensus on the causes of the alpha-effect?
There have been various explanations posited for the α-effect. The α-effect refers to a phenomenon wherein nucleophiles with lone pairs on atoms adjacent (i.e., in the α- position) to the atom bearing ...
- 11.9k
3
votes
1
answer
2k
views
Electrode potentials at interfaces?
My questions relates to the fundamental concept of electrochemistry, more specifically the electrode potentials.
1) First, why is there a potential difference at the interface of two phases? ...
- 6,795
6
votes
2
answers
814
views
Measuring Thermodynamic Stability
We can predict whether a reaction can occur spontaneously by calculating the change in Gibbs' free energy.
I was just wondering whether there is any general measure of the thermodynamic stability of ...
- 2,135
3
votes
1
answer
577
views
Influence of solubility on the complexation constants at different pH values
Given some small chemical compounds whose solubility varies with pH, and in the case of compounds that can form complexes, I have seen that the complexation constant varies with pH.
I think there ...
- 285