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Questions tagged [entropy]

For questions about the thermodynamical state function entropy, commonly understood as a measure of disorder.

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Formula of entropy change

In my 12th standard book, the formula for entropy change is given as $\Delta S = \frac{q_\text{reversible}}{T}$. What is the importance of absorbing heat reversibly and not irreversibly? What does ...
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On adding sugar to water, does the entropy increase or decrease?

This question is a little confusing. According to my teacher, the entropy should decrease, but I think that sugar is crystalline first and later it dissolves, so the randomness increases and so the ...
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Why does like dissolve like?

Polar solvents love polar solutes to be dissolved in it and non polar with non polar. Often said as like dissolves like. Okay, polar loving polar can be understood with help of the facts: same polar ...
Pranjal Rana's user avatar
14 votes
3 answers
2k views

Why can't a reaction go to completion?

What is the reason for why an exergonic reaction would still have some remaining reactants (or an endergonic reaction to have any products)? The explanation given on this page (see Chemical reactions ...
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Entropy change in an adiabatic expansion

Question One mole of ideal gas initially at a pressure of 1 atmosphere and $T = 298\ \mathrm K$, is expanded into a volume 50 % larger adiabatically. In this adiabatic process, no work is done on ...
RobChem's user avatar
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Entropy - "Wiggle"?

The title is not a reference to a Jason Derulo song. In any case: 1) How is change in entropy measured, experimentally? I've Googled this for a bit and I've found all sorts of mathematical ...
Dissenter's user avatar
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Is helium-4 a liquid at zero temperature and zero pressure?

As far as I know, for all substances other than helium, if you extrapolate the solid-gas line on the phase diagram, it passes through the origin. That is, no matter how low the temperature is, you can ...
Brian's user avatar
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Why is entropy favorable?

I cannot seem to grasp the logic behind it. We say that more entropy (or more disordered system) is favorable over less entropy. But why? Why is randomness preferred over proper arrangement of atoms/...
completely newbie's user avatar
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Does deuterated water slow down the overall metabolism of a cell?

Would deuterated water, being heavier, slow down the metabolic rate of the cell and subsequently the aging process? edit: lets say I wanted to observe a cellular event, like the formation of the ...
Confusedbyeverything's user avatar
11 votes
3 answers
12k views

Is there a way to experimentally measure entropy?

I've been doing thermodynamic chemistry, and recently focusing on Gibbs Free Energy. Whilst doing calculations using, $$\Delta G = \Delta H - T \Delta S$$ I have been given a table of values for $\...
Saul McShane's user avatar
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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 ...
DonielF's user avatar
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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?
Flinn Bella's user avatar
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Can the idea of entropy be extended to orbitals?

The forward reaction for: $\ce{HF<->H^+ +F^-}$ is entropically favourable; but energetically unfavorable: as there is too much electron density for $\ce{F^-}$ ion to cope up. That's a reason why ...
Mockingbird's user avatar
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Whats the difference between entropy and the (dis)order of a system?

Entropy is often verbally described as the order/disorder of the thermodynamic system. However, I've been told that this description is a vague "hand-waving" attempt at describing what entropy is. For ...
RobChem's user avatar
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What does the Clausius Inequality mean in thermodynamics? (entropy)

$$\delta S\ge\frac{\delta q}T$$ I don't understand how a quantity (the change in entropy in this case) can be greater than the very thing that defines it ($\mathrm{\frac{\delta q}{T}}$). Also what ...
RobChem's user avatar
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Why does everything go in the direction of positive entropy change?

I read about entropy from different sources. Still don’t get why (universal) entropy change has to be greater than zero for irreversible process, i.e. $\mathrm{d}S_\mathrm{Univ}>0$. Is it a result ...
ayadev's user avatar
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How can enthalpy change of a system be negative while entropy change is positive?

$$\Delta G = \Delta H_\text{system}-T\Delta S_\text{system}$$ $$ \begin{array}{ccc} \hline \text{Sign of}~ΔH & \text{Sign of}~ΔS & \text{Spontaneity} \\ \hline + & + & \text{The ...
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Spontaneity of an exothermic reaction when temperature is decreased

How is the spontaneity of an exothermic reaction changed when temperature is decreased? Assuming the entropy is negative. Using Gibbs free energy equation, $\Delta G=\Delta H-T\Delta S$, $\Delta G$ ...
mongy910's user avatar
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Entropy of Activation and Temperature Dependence

I am trying to understand the extrapolation of enthalpy $\Delta H^{\ddagger}$ and entropy of activation $\Delta S^{\ddagger}$ from the Eyring equation. It's typically cast as: $$\ln\left(\frac{k}{T}\...
captainalright's user avatar
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deltaS of a decomposition reaction

Suppose we had the following reaction: $$\ce{Polymer ($n$~units)_{(aq)} -> Polymer($a$~units)_{(aq)} + Polymer($n-a$~ units)_{(aq)}}$$ Without knowing the specific monomer unit of the polymer, ...
Breaking Bioinformatics's user avatar
9 votes
1 answer
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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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4 answers
13k views

When is a reaction reversible?

When speaking in thermodynamic terms, when is a reaction reversible? My attempt: A reaction is reversible when $\Delta\text{S} < 0$, because according to the second law of thermodynamics, "in ...
phi2k's user avatar
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8 votes
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535 views

Does entropy contribute work?

I've seen that Gibbs free energy is defined as the maximum amount of work that a system can do. From this I gather, this is how much energy it will release - right? But, I've also learnt that the heat ...
John Hon's user avatar
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Why does increasing the temperature of a *solid* increase its entropy?

I understand why increasing the temperature of a fluid would increase its entropy, as the particles are free to move, and therefore an increase in kinetic energy would allow the particles to move more,...
DividedByZero's user avatar
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2k views

What kind of chemicals would exist at the heat death of the universe?

As the universe reaches the maximum entropy, presumably only the most stable chemicals would exist. Assume that all the elements haven't turn into iron yet, what kind of chemicals would dominate at ...
user9294's user avatar
8 votes
5 answers
12k views

How can a nonspontaneous reaction occur?

$$\frac{-\Delta G}{T}=\Delta S_{universe}$$ (This equation applies under isobaric and isothermal conditions.) I understand that if $\Delta G$ is positive, the reaction is nonspontaneous, and adding ...
lightweaver's user avatar
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What Causes there to be a Maximum Amount of Salt which can be Dissolved in Water?

When you add salt to water and stir, after a while the salt will no longer dissolve. Why is this?
zordman's user avatar
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What is the difference between ΔG and ΔG "with an o on top"? [duplicate]

What is the difference between $\Delta G$ and $\Delta G^\circ$? I couldn't figure it out. Ex: $\Delta G^\circ = -RT\ln K$ $\Delta G = \Delta G^\circ + RT \ln Q$ What is the difference, is it that $\...
Asker123's user avatar
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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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Where to find data for Gibbs energy, enthalpy, and entropy?

Where can you find the enthalpy and Gibbs free energy of formation and entropy of compounds either as a data file or web API? Technically, I only need two since I can figure out the other one from two ...
Christopher King's user avatar
7 votes
4 answers
5k views

Why does entropy increase when the difference in temperatures is decreased?

From the Wikipedia article about Entropy (Source view via The Internet Archive): In irreversible heat transfer, heat energy is irreversibly transferred from the higher temperature system to the lower ...
hb20007's user avatar
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1 answer
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Misconception? "That gas is heavier so it sinks."

I've heard people say many times that one gas is heavier than the other so the heavier gas sinks. Now this must certainly be true to a certain extent (I think of gas used in WWI that sunk into ...
jheindel's user avatar
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7 votes
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The entropy change during micellization

I was reviewing the thermodynamics of micelle formation from the Wikipedia page and this book. I understood most of the arguments but micellization is supposed to be an entropy driven process with a ...
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1 answer
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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 ...
Liam's user avatar
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4 answers
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Can one predict the entropy change for aqueous phase reactions without calculation?

I have seen several sources that claim aqueous solutions always have more entropy than pure liquids or solids, however, some aqueous ions have negative standard entropy. This opens the possibility of ...
Mike O's user avatar
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1 answer
853 views

Entropy change when indistinguishable particles suddenly become distinguishable

We have that $$ S = \frac{U}{T} + k\ln Q $$ Using that for indistinguishable particles in the canonical ensemble $$ \ln Q_{\mathrm{indis}} = N\ln \frac{q_{\mathrm{tr}}e}{N} + N\ln q_{\mathrm{rot}} ...
Yoda's user avatar
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7 votes
5 answers
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Dependence of change in entropy on temperature

The thermodynamic definition of entropy is expressed as $$dS = \frac{dq_{rev}}{T}$$ I understand that it depends on amount of heat transferred because heat transfer can be understood as the degree of ...
Apoorv's user avatar
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7 votes
2 answers
618 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}$...
Hal Gee's user avatar
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How to calculate the entropy change of benzene when it changes from vapour into liquid?

Given information: The boiling point of benzene at atmospheric pressure is $353~\mathrm{K}$ the enthalpy of vaporization of benzene is $30.8~\mathrm{kJ~mol^{−1}}$ at this temperature. ...
justbehappy's user avatar
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7 votes
0 answers
178 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....
Furrier Transform's user avatar
6 votes
2 answers
2k 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 ...
Manit Agarwal's user avatar
6 votes
3 answers
609 views

Trying to Understand Entropy

I'm currently looking at Principles of Chemistry by Atkins, and I have a few questions involving entropy. These questions probably arise from my poor understanding of reversible/irreversible processes....
Tae Hyung Kim's user avatar
6 votes
2 answers
2k views

Na(s) vs NaCl(s) Entropy

I would think the crystalline, ionic solid would have the lower entropy, however, the tablature indicates otherwise. Why does $\ce{NaCl(s)}$ have a greater entropy than $\ce{Na(s)}$?
123chemask's user avatar
6 votes
1 answer
1k 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\ ...
Sarah Brown's user avatar
6 votes
1 answer
3k views

Derive expression for internal energy of mixing and entropy of mixing using statistical thermodynamics

I want to derive an expression for the internal energy of mixing, $\Delta_\mathrm{mix}U$, and entropy of mixing, $\Delta_\mathrm{mix}S$. The framework for this is the Lattice Theory of Ideal Solutions....
Yoda's user avatar
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6 votes
2 answers
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Entropy of surroundings and heat

Apparently the entropy change of a system can be found by $$\mathrm dS=\frac{\mathrm\delta q_\mathrm{rev}}T$$ I am slightly confused about this because I have seen this being used when the temperature ...
Meep's user avatar
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6 votes
2 answers
758 views

Calculating Entropy

Atkin's Physical Chemistry Book; Chapter 3; question 3.3(a): Question Calculate $\Delta S$ when the state of $\mathrm{3~mol}$ of perfect gas atoms, for which $C_{p,m}$ = $\frac{5}{2}R$, is changed ...
Nanoputian's user avatar
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6 votes
1 answer
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Entropy and specific heat capacity

I have seen the equation $S(T_2)=S(T_1)+C_p\ln(T_2/T_1)$ where $C_p$ is the molar heat capacity at a constant pressure. I understand that this assumes that the temperature range is sufficiently small ...
Meep's user avatar
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6 votes
3 answers
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Intuition on Gibbs Free Energy

In many places, I read that Gibbs Free Energy is called "free energy" because some of the enthalpy that comes from the chemical reaction becomes "waste heat" due to a change in the entropy of the ...
Yunfei Ma's user avatar
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6 votes
1 answer
2k views

What is an entropic trap?

Can someone explain the concept of entropic traps, or provide good sources to look up? The term is not present in any chemistry books I own, and I cannot find online sources explaining it.
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