Questions tagged [entropy]
For questions about the thermodynamical state function entropy, commonly understood as a measure of disorder.
268
questions
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0answers
21 views
Find delta G, delta H, and Delta S for the following reaction at 25 degrees celcius [closed]
Calculate the ΔGo, ΔHo, and the ΔSo for the following reaction at 25 degrees C.
4 Fe (s) + 3 O2 (g) --> 2 Fe2O3 (s)
Substance ΔGo ΔHo ΔSo
(kJ/K mol) (kJ/K mol) (J/K mol)
Fe (s)...
2
votes
1answer
57 views
Entropy change during irreversible process
Entropy change is defined as $\Delta S = \int \frac{\delta Q_{rev}}{T} $, where $Q_{rev}$ is heat transferred through a reversible process. I’m a little confused how this works, so I have a couple of ...
7
votes
0answers
64 views
Why does entropy increase in reactions that make more molecules?
I used to understand this (more molecules ⇒ more degrees of freedom), but do no longer, because the total number of atoms is conserved, so translational d.o.f. should just be turned into internal d.o....
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votes
0answers
17 views
How can we tell if a substitution reaction will be thermodynamically favoured
My understadning was like in the case of a cyclohexane we would see a thermodynamically favoured reaction if an axial substituent became equitorial in the case of a inverted Sn2 reaction but I would ...
0
votes
1answer
43 views
How to calculate ∆rGº from entropy and ∆fGº in different temperatures? [closed]
"The total oxidation of glucose occurs according to the following chemical equation:
C6H12O6 (s) + 6O2 (g) -> 6CO2 (g) + 6H2O (l)
The following table gives us the free energies of standard ...
-1
votes
1answer
39 views
Change of entropy of a thermal reservoir [closed]
How to calculate/derive the change in entropy of a heat reservoir as function of its internal energy? If the constant temperature of heat reservoir is T, and
$$ S(U_0)=S_0,$$
what is $$ S(U)? $$
3
votes
0answers
90 views
(Probabilistic) entropy associated with many equally probable microstates
This is a pretty naive question. The probabilistic definition of entropy is (for a mole of particles)
$$-R\sum_i p_i\ln p_i $$ in which $p_i$ is the probability of a given microstate and $\sum p_i =1, ...
2
votes
0answers
48 views
Why does the plot of volume and temperature have a kink in a second order phase transition?
The $(b)$ part of the figure shows how thermodynamic variables change in a second order phase transformation.
We observe there is a kink in the plot of volume with temperature.
Why is that so?
The ...
3
votes
0answers
30 views
Total Entropy of the Atkinson Cycle
I was researching for a question that asked me to find a ratio of temperatures, and I came across this, which would help me greatly:
Since, most of the temperature changes occur during the isochoric (...
2
votes
1answer
37 views
What is the mechanism of endothermic dissolution?
My real question is in fact "How can dissolution happen if it's endothermic?" However, there
have been many questions regarding this exact topic and the answers about Gibbs free energy
...
1
vote
0answers
25 views
What is the fundamental difference between temperature and energy? [duplicate]
We know that temperature is proportional to the average kinetic energy of a system. We also know that temperature is not the same as energy because the temperature is intensive while the temperature ...
1
vote
0answers
48 views
Entropy of a van der Waals' gas
I would like to ask how I could derive the entropy change $dS$ for a van der Waals' gas.
I think I would need to use the total differential of entropy and then use the Maxwell relation shown below.
...
0
votes
0answers
34 views
Question about reading NIST datasheets & Gibbs energy of reaction
I'm peeping the NIST thermo tables for $\text{H}$ and $\text{H}_2$, and I'm perplexed.
Seems to me that using
$$G=[\Delta H^\circ_f+H-H^\circ(T_r)]-T\cdot S^\circ$$
(well, actually correcting the ...
1
vote
1answer
59 views
Helmholtz Free Energy Minimization Proof
In the proof that Helmholtz free energy is minimized at equilibrium, we use the fact that $$\mathrm dS_\text{total}=\mathrm dS_\text{system}+\mathrm dS_\text{bath}\geq 0$$
But we also use the fact ...
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votes
1answer
49 views
conceptual doubt on change in entropy [closed]
we have learnt in thermo that delta S is zero in reversible process
Now, when a solid melts reversibly is delta s zero since reversible or positive since the particles are more energetic in liquid ...
2
votes
3answers
83 views
entropy calculation in an irreversible process
I found out that if we want to find the entropy change in an irreversible process let's say $\text{state A} \to \text{state B}$ , we find or make an reversible path for the same process and find the ...
8
votes
1answer
186 views
My Misconception of Entropy
It was recently brought to my attention that my understanding of entropy is wonky at best.
In my experience, entropy was introduced (superficially at best) during general chemistry/foundations of ...
0
votes
2answers
64 views
How can you calculate the tendency of one compound to become another?
Excuse my inexperience with chemistry here. I've been poring over papers with terms like "Standard Gibbs free energy of formation", "Hess's Law", "standard enthalpy of ...
2
votes
1answer
146 views
entropy of vaporization vs entropy of fusion
Is the entropy of vaporization greater than entropy of fusion, for elements? And in which case they can be same, if it so happens?
I have asked this here because I couldn't find any satisfactory ...
1
vote
1answer
183 views
Adiabatic expansion of gas
If a real gas is adiabatically expanded against constant pressure, then which of the following will definitely increase?
Compressibility factor (Z) or Entropy (S)
Correct answer: Entropy
My answer: ...
0
votes
2answers
47 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}=...
0
votes
1answer
52 views
Extrema in free energy of a solution?
I am a high school student reading about the second law of thermodynamics, and one of the equations given was the free energy change in dilution.
$$\Delta G_{\text{dil}} = -RT \log(C_1/C_2)$$
This ...
6
votes
2answers
346 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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votes
1answer
57 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 ...
0
votes
1answer
88 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 ...
1
vote
1answer
81 views
Entropy increase of both system and surroundings [closed]
In "why chemical reactions happen" the author (James Keeler) says that "a process which leads to an increase in the entropy of the system can be ... exothermic." Could someone ...
4
votes
1answer
491 views
Why does spontaneity of a reaction depend on temperature?
As we know, $$\Delta G=\Delta H - T\,\Delta S.$$
Both in my textbook and on the internet, it was given that temperature was a determining factor in the spontaneity of a reaction. For example, when $\...
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votes
1answer
51 views
Change in Entropy and Spontaneity [closed]
I was studying thermodynamics and got stuck with some questions. I am pointing them down.
I have studied that $\Delta S_{\text{total}}$
accompanied by a spontaneous process is greater than 0. But is ...
1
vote
1answer
81 views
Apparent paradox regarding spontaneity
Consider the following analysis ( rev and irrev respectively refer to a reversible and an irreversible path, between the same initial and final states):
$dU_{rev}=dq_{rev}+dw_{rev}$,and,$dU_{irrev}=...
0
votes
0answers
40 views
How to find entropy of chemical network reaction?
I have the following system of reactions:
$$
\begin{aligned}
\ce{X_1 + Y_1 &<=>[$k_{21}$][$k_{12}$] X_2} \\
\ce{X_2 &<=>[$k_{32}$][$k_{23}$] X_3} \\
\ce{X_3 &<=&...
0
votes
1answer
61 views
Is entropy of surroundings a state function?
It is a pretty common well known fact that the entropy of a system is a state function ie. it doesn't depend upon the path taken by the process as long as the initial and final states are the same.
I ...
3
votes
1answer
77 views
What's the minimum amount of energy needed to remove CO2 from the atmosphere?
I've been learning about CDR (Carbon Dioxide Removal) techniques to combat climate change. These all seem to involve extracting $\ce{CO2}$ from the atmosphere and converting to a mineral, which is ...
3
votes
1answer
95 views
How does the entropic force arise within the hydrophobic effect?
I know that the hydrophobic effect is usually explained by the entropic effect originating from the disruption of hydrogen bonds between molecules of water and the nonpolar substance. The hydrogen ...
4
votes
0answers
53 views
Is entropy maximum in equilibrium?
Usually, in the context of non-equilibrium thermodynamics, it is said that entropy achieves a maximum in equilibrium, so the Taylor series expansion of entropy around the equilibrium state as a ...
1
vote
1answer
78 views
A Curious (definitely wrong) result of Absolute Entropy [closed]
The Gibbs free energy is defined as
$$G=H-TS$$
Differentiating both sides
$$dG=dH-TdS-SdT$$
For an adiabatic reversible expansion of an ideal gas, we would have $dG=0$, and $dS=0$.
Thus we can write, $...
0
votes
0answers
103 views
Why doesn't Delta S total = 0 for this reversible process?
My textbook (Engel Physical Chemistry 3rd Edition) says:
For any irreversible process in an isolated system, there is a unique direction of spontaneous change: $\Delta S > 0$ for the spontaneous ...
1
vote
1answer
121 views
Are phase transitions reversible when performed at supercooled temperatures?
For the state transition of supercooled water to ice, is the process reversible? I wonder this because I want to know if I can use entropy change of the universe is 0. If it isn't reversible, how do ...
0
votes
1answer
94 views
Entropy change of surroundings and system
The entropy change of the surroundings can be calculated by the equation $dS_{sur}=\frac{dq}{T_{sur}}$ regardless of the path (irreversible or reversible).
The argument is that because the ...
10
votes
3answers
872 views
Why do the first and second laws of thermodynamics not contradict each other?
We're learning about entropy right now in general chemistry, and I'm trying to understand something.
From the fact that entropy can be directly compared with enthalpy, free energy, electromotive ...
4
votes
2answers
79 views
Why do things want to lower their energy? [closed]
Like if we consider a container of gas molecules as a system, Why does it want to lower its energy?
I have read that always more spontaneous events are favored. So Gibb's Free energy should have a ...
3
votes
1answer
66 views
Assumption behind Gibbs energy and maximum work
I'm somewhat confused by this derivation (in Schroeder's Book of Thermal Physics) of the fact that in a closed system, the change in the Gibbs Energy is always less than the Non-Expansion work done on ...
2
votes
2answers
200 views
Entropy and Equilibrium Constant: Chemical Thermodynamics
For a reaction taking place in a container in equilibrium with its surroundings, the effect of temperature on its equilibrium constant $K$ in terms of change in entropy is described by:
(A) With the ...
4
votes
1answer
412 views
Why is the Standard Entropy of a Proton Zero?
It was my assumption that the standard entropy was the absolute entropy of a substance at standard state; however, my textbook states various standard entropies ranging from $-144.77\ \mathrm{J/(mol\ ...
0
votes
1answer
360 views
Entropy of Irreversible Isothermal Compression
If I want to find the entropy change of universe in isothermal irreversible compression, I use 2 approaches:
I use q/T. Which gives me positive entropy change for surroundings positive and negative ...
0
votes
2answers
56 views
T∆S implies that heat flows which way?
From Schroeder's Introduction to Thermal Physics.
Parts $(a)$ and $(b)$ are quite simple, and we find that $ \Delta H =-2808\ \mathrm{kJ}$ and $\Delta G =-2879\ \mathrm{kJ}$, so by $\Delta G = \Delta ...
1
vote
1answer
61 views
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 ...
0
votes
1answer
122 views
Change in Entropy and Enthalpy found over a temperature range and asked to compute the change in Gibb's Energy
I was given the following question by my tutor:
"The variation of enthalpy ($\Delta H$) and entropy ($\Delta S$) in a reaction carried out at
constant pressure in the temperature range between 300 ºC ...
4
votes
2answers
433 views
What is the entropy of mixing a gas into the whole atmosphere?
Suppose I release 1 mole of pure argon into the atmosphere. What is the entropy of mixing? The equations I have found seem to only apply to the case of a fixed volume. Should I use an estimate of the ...
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votes
1answer
62 views
Why does the combustion of graphite have a positive Delta S?
The following reaction is exothermic and entropically favorable:
$$\ce{C (s) + O2 (g) -> CO2 (g)}$$
My question: Why is this reaction entropically favorable?
Clearly there are more ways to arrange ...
10
votes
2answers
1k views
Can someone intuitively explain the reason for the units of entropy (J/K )? [duplicate]
If entropy is just a measure of the "randomness" or "chaos" of a system, then why does it have specific units. What is the reason behind the units of entropy being J/K?
