Questions tagged [entropy]
For questions about the thermodynamical state function entropy, commonly understood as a measure of disorder.
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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 ...
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1answer
68 views
Gibbs energy: the term −TdS. 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
74 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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26 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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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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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)$$\Delta G$ is ...
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2answers
91 views
What is wrong in this argument that dG must always be zero?
Under constant temperature and pressure, the change of the Gibbs free energy can be written as
$$\mathrm dG_\textrm{sys} = \mathrm dH_\mathrm{sys} - T\,\mathrm dS_\mathrm{sys}$$
And in the textbook ...
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1answer
53 views
Troubles with Gibbs free energy
The book I'm reading writes that for a system at constant pressure and temperature it holds:
$$dG=-TdS_{tot}$$
where $dG=dH-TdS$.
So if you want to see if a certain process is spontaneus, you just ...
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2answers
68 views
Entropy as an intrinsic property of matter
I would like to ask how we can see entropy as an intrinsic property of molecules, namely as something depending also on their geometry (and not only as gross disorder). Is this what is called the "...
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36 views
Thermochemical data for thiocyanatoiron(III) ion
I am currently writing a lab report about the formation constant of $\ce{FeSCN^2+}.$ All I could find online was a formation constant at $\pu{25 °C}$ $(890),$ which was 70% higher than the ...
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23 views
Estimation of Molar Entropies
There are plenty of compounds out there with unknown standard molar entropies. I know that molar entropy has a strong dependance on the physical phase, molecular weight, and number of atoms. So is it ...
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1answer
47 views
Irreversible Adiabatic Process [closed]
While studying about the entropy change of surroundings in irreversible adiabatic process, I came across a sentence.
For infinite reservoir, all processes are considered reversible
And suddenly ...
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2answers
43 views
Spontaneity of an exothermic chemical reaction
When the process is exothermic (ΔHsystem<0), and the entropy of the system increases (ΔSsystem>0), the sign of ΔGsystem is negative at all temperatures. Thus, the process is always spontaneous.
...
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1answer
189 views
Entropy changes during transfer of heat between two bodies
Consider two metal blocks, one at 500K and another at 300K, they are bought to contact with each other until they reach a common temperature.
Find the entropy change of each object and the total ...
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votes
1answer
200 views
Infinite Increase in Entropy when Energy added to Absolute Zero
My textbook states the following:
If a system were at absolute zero, an additional small amount of heat energy would lead to an infinite increase in entropy. Such a state is impossible. Absolute ...
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122 views
Entropy in organic chemistry
Why do we not talk about entropy when deciding feasibility of an organic reaction?...We always seem to only be interested in stability of product and intermediate(essentially delta H)and draw their ...
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0answers
32 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 ...
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33 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 ...
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1answer
290 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 ...
3
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1answer
795 views
Relation between equilibrium constant and entropy change
Like the Van't Hoff equation, which relates change in enthalpy to equilibrium constant, is there a similar equation for the relation between change in entropy and equilibrium constant?
Consider the ...
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1answer
112 views
Why is the entropy of solidifying solution higher than that of a pure solvent?
According to a source,
Solutions have a greater entropy than pure liquids as the particles in a solution are more separated and solvent molecules separate the solute particles. If the number of ...
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2answers
51 views
Can entropy in a closed system remain constant when you heat a liquid?
This is in regards to an exam question that I did not understand and got wrong.
The question was: When you heat acetone from 200 K to 400 K, what happens to the entropy?
Answers: Increases, ...
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1answer
52 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$$
using the 1st and 2nd law of thermodynamics (1 mol ideal gas).
Now, I want to write $\...
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1answer
214 views
Heat and work of glucose combustion
$$\ce{C6H12O6(s) + 6 O2(g) -> 6 CO2(g) + 6 H2O(g)}$$
When looking at glucose combustion I found that the change in Gibbs free energy is greater than the change in enthalpy. If we assume the process ...
1
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1answer
123 views
Temperature dependency of enthalpy H and entropy S [closed]
In textbooks, I often read that the temperature dependency of the enthalpy of reaction $\Delta_\mathrm{r}H$ and the entropy of reaction $\Delta_\mathrm{r}S$ can be neglected (in a limited $T$-range).
...
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2answers
227 views
Relationship between the ideal gas constant and entropy
So it dawned on me the other day that the ideal gas constant $$R = \pu{8.31 J mol-1 K-1}$$ has the same units as molar entropy. Is there some deeper meaning behind this or is it just a coincidence ...
3
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0answers
127 views
Whether the entropy change of the system is positive or negative
$$\ce{N2(g) + O2(g) → 2NO(g)}$$
My thinking: since gases are combined together, so there randomness or disorder gets decreased. Hence the sign of entropy change of the system should be negative.
...
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votes
2answers
153 views
Feasibility and Entropy
Suppose you've got some reaction that has a positive $\Delta H$, and also has positive $\Delta S_\mathrm{sys}$. Due to the equation
$$\Delta S_\mathrm{tot} = \Delta S_\mathrm{sys} - \frac{\Delta H}{T}$...
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1answer
94 views
How does refrigerator actually work?
In refrigerator, heat energy got extracted from cold sink and dumped into hot source. This is not spontaneous so work must be done. I came across the following while reading a textbook:
For ...
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1answer
462 views
Effect of temperature on equilibrium constant in terms of entropy change
This question was asked in the JEE examination of 2017 :
[A] option is incorrect since the system is losing heat and hence $$ \Delta S = -ve$$
[B] Lets assume we have an endothermic reaction $$\...
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3answers
232 views
Estimating the absolute entropy of water at room temperature
I thought it would be fun to get an idea of how enormously large $W$ in $S = k\ln W$ is. I'd specifically like to estimate this for a mole of water at $\pu{1 atm}$ and $\pu{298 K}$. To do this, I ...
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2answers
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Entropy change of isothermal irreversible expansion of ideal gas [duplicate]
Two mole of an ideal gas is subjected to isothermal expansion from $\pu{2 atm}$ to $\pu{1 atm}$ at $\pu{300 K}$. Calculate entropy change of the system, surrounding and total, if the process is ...
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1answer
57 views
When a piston cylinder system that evolves isothermally and quasistatically is irreversible?
Suppose we have two states: $\ce{A}$ and $\ce{B}$.
Considering a quasi-static isothermal process of an ideal gas, (and another condition, that is $T = T_\mathrm{surr}$) that join those states.
Then ...
2
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2answers
302 views
Hydrogen vs neon in terms of entropy
I know that both molecular size and complexity increase overall entropy, but between the two I'm not sure which increases it more. Would 1 mole of hydrogen have the greater entropy because it is two ...
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2answers
133 views
dSsur, dS and dStotal relation
By definition,
dS=dqrev/T.
Based on my understanding, the subscript rev means that the dq is heat exchanged in a reversible pathway from a state i to another state f, regardless of the actual ...
2
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1answer
75 views
How is “the surrounding” and its temperature defined to calculate entropy change during a reaction?
Lets say supercooled liquid water at $263\ \mathrm K$ isobarically changes to solid ice at the same temperature. I wish to calculate the change in entropy of the surroundings and I happen to know the $...
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23 views
In dS≥dQ/T, material equilibrium means that there is not material changes at all or that a material change can be reversible and at equilibrium?
I am reading Levine's Physical Chemistry, I know how to arrive to that equation, but I have questions.
First, $\mathrm{d}S=\mathrm{d}Q/T$ for material equilibrium, but if there is a heat flow I ...
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1answer
603 views
Why are catabolic reactions exergonic and anabolic reactions endergonic?
Catabolism breaks apart large molecules and anabolism creates large molecules. Catabolism is supposed to be exergonic, while anabolism is supposed to be energonic. Id on't really understand this.
By ...
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How does the concentration of a gas affect the entropy of a system in a chemical equilibrium?
The question asks about factors that increase the entropy of the system. The mark-scheme states that "decreasing the concentration of a gas without changing the volume of the container" is the answer. ...
0
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1answer
278 views
Thermodynamic stability and Inertness [duplicate]
The following is from my teacher's notes:
The reaction (where the reaction mixture is thermodynamically unstable but kinetically inert) tends to go as indicated by a positive total entropy change, ...
2
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1answer
42 views
In chemistry what do they mean by disorder when in the context of entropy?
Entropy is often the amount of disorder is present in a system, but when they refer it to the degree of disorder in a system what do they mean. It just seems the term is quite vague and not very ...
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2answers
445 views
Why does ATP has a lower entropy than ADP
I was confused about why ATP has a lower entropy than ADP when ATP has 1 more phosphate group. According to https://chem.libretexts.org/Textbook_Maps/Biological_Chemistry/Metabolism/ATP%2F%2FADP#...
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1answer
49 views
An equal amount of hydrogen gas is dissolved in a 1L vessel and a 2L vessel, both filled with water. Which vessel has higher entropy? [closed]
This question came up in our chemistry class and we could not determine which would have the higher entropy. Clearly, Vessel 2 has a larger volume, but Vessel 1 has a higher concentration of $H_2$. ...
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0answers
328 views
Molar entropy of vaporization
If a substance has a molar heat of vaporization of $\pu{3.05\times10^4J/mol}$ and a normal boiling temperature of $\pu{80.0^\circ C}$, what is the value of its molar entropy of vaporization $\Delta S_\...
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Why don't these Entropy equations contradict each other?
My textbook says that $\delta S_{rev}= \frac{q_{rev}}{T}$. It also says that $\delta S_{surr}= \frac{-q_{rev}}{T}$. Logically, this should mean that the change in entropy in both the surroundings and ...
3
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3answers
289 views
How can residual entropies be reconciled with the Third Law?
The third law of thermodynamics states that the absolute entropy of a pure crystalline substance is $0$ at $0\,\mathrm{K}$. In the analysis of chemical reactions this absolute scale is sued to ...
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1answer
34 views
Reaction direction change based on phase of reactants and products
I am curious specifically how the entropy of a reaction will change if the phases of the reactants and products at different conditions change.
For instance, a reaction
$$\ce{O2(g) + C(s) -> CO2(...
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1answer
196 views
Where can I find the standard molar entropy for ammonium bicarbonate?
I'd like to know the standard molar entropy S0solid for ammonium bicarbonate to use in an example in class, to calculate ΔS0 for the reaction, NH4HCO3(s) → NH3(g) + H2O(g) + CO2(g).
Already checked: ...
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1answer
124 views
Entropy change of evaporating NaCl [closed]
What is $\Delta S$ for this process?
$$\ce{NaCl(s) -> NaCl(g)}$$
(A reference that I could quote would be ideal.)
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1answer
136 views
What exactly is entropy? [closed]
I have looked at other questions, videos and articles but I kept reading disorganization, messiness or energy distribution (probability) or a thermodynamical equilibrium and stuff. What do we mean by ...
