Questions tagged [entropy]
For questions about the thermodynamical state function entropy, commonly understood as a measure of disorder.
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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 ...
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1answer
42 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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1answer
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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
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3answers
68 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 ...
7
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1answer
172 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 ...
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2answers
55 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 ...
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1answer
63 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 ...
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1answer
87 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: ...
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2answers
45 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}=...
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1answer
51 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 ...
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2answers
325 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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1answer
49 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 ...
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1answer
84 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 ...
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1answer
53 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 ...
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1answer
485 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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1answer
50 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 ...
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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}=...
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How to find entropy of chemical network reaction?
I have the following system of reactions:
$$
\begin{aligned}
\ce{X_1 + Y_1 &<=>[$k_{21}$][$k_{12}$] X_2} \\
\ce{X_2 &<=>[$k_{32}$][$k_{23}$] X_3} \\
\ce{X_3 &<=&...
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1answer
36 views
Is entropy of surroundings a state function?
It is a pretty common well known fact that the entropy of a system is a state function ie. it doesn't depend upon the path taken by the process as long as the initial and final states are the same.
I ...
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1answer
75 views
What's the minimum amount of energy needed to remove CO2 from the atmosphere?
I've been learning about CDR (Carbon Dioxide Removal) techniques to combat climate change. These all seem to involve extracting $\ce{CO2}$ from the atmosphere and converting to a mineral, which is ...
3
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1answer
91 views
How does the entropic force arise within the hydrophobic effect?
I know that the hydrophobic effect is usually explained by the entropic effect originating from the disruption of hydrogen bonds between molecules of water and the nonpolar substance. The hydrogen ...
4
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0answers
49 views
Is entropy maximum in equilibrium?
Usually, in the context of non-equilibrium thermodynamics, it is said that entropy achieves a maximum in equilibrium, so the Taylor series expansion of entropy around the equilibrium state as a ...
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1answer
78 views
A Curious (definitely wrong) result of Absolute Entropy [closed]
The Gibbs free energy is defined as
$$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, $...
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0answers
60 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 ...
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1answer
64 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 ...
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1answer
56 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 ...
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3answers
773 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
75 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
57 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 ...
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2answers
125 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
358 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\ ...
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1answer
203 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 ...
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2answers
55 views
TāS 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 ...
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1answer
51 views
Is constant pressure heat capacity = entropy during adiabatic reversible expansion of perfect gas?
For any process, āG = āH - ā(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
āG = āH - SāT
The ...
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1answer
84 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 ĀŗC ...
4
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2answers
421 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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1answer
60 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)}$$
My question: Why is this reaction entropically favorable?
Clearly there are more ways to arrange ...
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2answers
1k views
Can someone intuitively explain the reason for the units of entropy (J/K )? [duplicate]
If entropy is just a measure of the "randomness" or "chaos" of a system, then why does it have specific units. What is the reason behind the units of entropy being J/K?
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Discussion about Warren-Cowely Parameter
The Warren-Cowely Parameter (WCP) for short range solution order is defined as:
$$\mathrm{WCP}_{ij} = 1 - \frac{Z_{ij}}{x_j \cdot Z_i}$$
where $Z_{ij}$ is the partial Coordination Number (CN) of ...
2
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2answers
64 views
Can you approximate the change in entropy of a reaction?
I found a problem in which I am given a hypothetical reaction in which from 2 liquids (reactants) I get 4 moles of gas and 1 mole of a solid substance (products):
$$\ce{A (l) + B (l) -> 4C (g) + ...
2
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1answer
59 views
Density of water increases from 273 to 277 K due to increase in entropy?
In a set of questions that I was solving, the answer to one of the questions asking why the density of liquid water increases from 0 to 4 degrees C had the above answer: due to an increase in entropy. ...
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0answers
29 views
Why don't we use standard entropies of formation?
In almost any appendix, the standard enthalpies and standard free energies of formation are listed. Why is the standard entropy of formation not relevant? Why do we use the standard molar entropy ...
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1answer
170 views
What is the precise definition of standard molar entropy?
I understand what the standard molar entropy is, and how to use it in calculations, but I'm interested in understanding exactly how it's defined and measured.
My recollection from a long time ago is ...
3
votes
1answer
81 views
Gibbs energy: the term āTdS. Where does the reversible heat go?
When we have an exothermic reaction as well as a decrease of the entropy in the system after the reaction, why does the decrease in the entropy of the system diminishes the amount of useful work we ...
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1answer
114 views
Entropy as a state function independent of the path
When entropy is a state function and the entropy change for a reversible process is
$$\mathrm dS_\mathrm{rev} = \frac{\delta Q_\mathrm{rev}}{T} = 0,$$
how can an irreversible process be ...
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0answers
74 views
Adiabatic compression of an ideal monatomic gas. What would the change in entropy be?
Two moles of an ideal monatomic gas initially at 300K and a pressure of 2 atm is compressed adiabatically using a constant external pressure $P_{ext}$ until equilibrium is reached. The temperature of ...
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2answers
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Clarification required on whether to use $C_v$ or $C_p$ in this question on finding entropy change
$1 \text{ mol}$ of an ideal monoatomic gas at $300 \text{ K}$ is subjected to a reversible isentropic compression process until final temperature reaches $600 \text{ K}$. If initial pressure is $1 \...
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1answer
129 views
Gibbs free energy for an ideal gas
You know that for a generic system which during a process exchanges heat with a reservoir at a constant temperature $T_{a}$ it holds:
$$\Delta G=\Delta H-T_{a}\Delta S$$
Where:
$\star)$$\Delta G$ is ...
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2answers
163 views
What is wrong in this argument that dG must always be zero?
Under constant temperature and pressure, the change of the Gibbs free energy can be written as
$$\mathrm dG_\textrm{sys} = \mathrm dH_\mathrm{sys} - T\,\mathrm dS_\mathrm{sys}$$
And in the textbook ...
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1answer
57 views
Troubles with Gibbs free energy
The book I'm reading writes that for a system at constant pressure and temperature it holds:
$$dG=-TdS_{tot}$$
where $dG=dH-TdS$.
So if you want to see if a certain process is spontaneus, you just ...
