Questions tagged [entropy]

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

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How do I calculate the average initialisation time of a reaction?

I want to know the mean average after which, for example, H2 and O2 react under standard conditions. As there is enough energy in the system to start the reaction, it should just be a matter of time ...
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What is the entropy change in isochoric process

I have studied that entropy increases with increase in temperature and it decreases with increase in pressure but in case of isochoric process both are happening at the same time but still the overall ...
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2 answers
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How exactly is heat supplied in an isothermal process

How exactly is heat supplied in an reversible isothermal process. If the temperature of system and surroundings always remains the same, then how come surrounding is supplying heat? I studied that T ...
2 votes
1 answer
114 views

What processes generate entropy as heat flows across temperature gradient

Suppose, we have a source at high temperature $T_\mathrm h$ and sink at temperature lower temperature $T_\mathrm l$. If $Q_\mathrm h$ amount of heat flow from source to sink, then change in entropy of ...
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1 answer
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Question about entropy generated in isothermal expansion of an ideal gas

So, this has been bugging me for a while. Consider the isothermal expansion of an ideal gas in a container. As the gas is allowed to expand, it does work on the surroundings which causes the ...
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Is delta S of universe always 0 when an ideal gas undergoes an isothermal irreversible process?

Since for an irreversible process, $$dS_\mathrm{Surrounding} =-\frac{\text{dq}_\mathrm{irr,sys} }{T_{\text {surr }}}\tag{1}$$ where $\text{dq}_\mathrm{irr,sys}$ is heat exchange of system and $-dq_\...
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Determine enthalpy and entropy from measurements of Kd at different temperatures?

In biochemical experiments, it is very common to measure IC50 (half inhibitory concentration) eg by adding different amounts of an inhibitor to an enzyme+substrate and measuring the reaction rate or ...
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1 answer
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Can we interpret the number of micro states calculated from molar entropy?

Forgive me if this is not the correct place to ask this. When I encounter a molar entropy, is it possible to rearrange the Boltzmann entropy formula $$S=k\log W$$ and meaningfully interpret the number ...
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How are fundamental equations valid for both reversible and irreversible processes?

We know for an irreversible process, $\mathrm dS\gt\mathrm dq/T$. And if the process is done at constant pressure we can take the equation as $\mathrm dH-T\,\mathrm dS\lt0$. And we defined Gibbs ...
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Can Vdp work be considered irreversible work (e.g., production of entropy)?

Since conduction across a finite temperature difference will produce entropy, will work done due to pressure difference generates entropy because system tends towards mechanical equilibrium which ...
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2 answers
118 views

If entropy is a state function, how can it be calculated for both reversible and irreversible processes?

$$\Delta S = nR\ln( V_2/V_1)$$ The value of ΔS of the system is independent of the path between the initial and final states, so this expression applies whether the change of state occurs reversibly ...
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2 answers
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Find the percentage of dissociation of nitrogen tetroxide given pressure, temperature, enthalpy and entropy

Given $$ \begin{align} \Delta_\mathrm f H^\circ(\ce{N2O4}) &= \pu{9.16 kJ mol^-1} &\quad \Delta_\mathrm f H^\circ(\ce{NO2}) &= \pu{33.18 kJ mol^-1} \\ S^\circ(\ce{N2O4}) &= \pu{304.3 ...
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Why would a reaction be nonspontaneous at higher temperatures?

Typically we think of a higher temperature speeding up the reaction rate and/or supplying the activation energy of a reaction. So why is it the case that some reactions are only spontaneous at lower ...
3 votes
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Total entropy change for reversible reactions and its relationship to equilibrium constant

I'm in year 12 of school doing A2 chemistry learning about entropy and its relationship to reversible equilibria. I learned about the following three ideas from my course book (verbatim), for a ...
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Use the first law of thermodynamics to derive the pressure as a function of T, U, V and N

I am struggling with a thermodynamics question given $$S(U,V,N)=C_VNK_\mathrm b\ln U/U_0+NK_\mathrm b\ln V/V_0$$ (where $U_0$ and $V_0$ are the reference energies and volumes) and the first law of ...
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3 answers
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How is the entropy of mixing related to chemical reactions in solution?

When calculating the Gibbs energy of reaction for reaction mixtures at arbitrary concentrations, you use the following expression: $$\Delta_\mathrm r G = \Delta_\mathrm r G^\circ + RT\ln Q$$ The term $...
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1 answer
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Is the entropy change positive or negative in this reaction?

Suppose I have a reversible reaction \begin{align} \ce{A(g) + B(g) <=> C(g)}\\ \end{align} with equilibrium constant K. Its ΔΗ is positive throughout. Now, suppose I start with 1 mole ...
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2 answers
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Spontaneity criterion for phase transformations and chemical reactions [closed]

Why is the Gibbs free energy (G) considered a spontaneity criterion for phase transformations and chemical reactions? Why are other thermodynamic parameters such as enthalpy (H), entropy (S), and ...
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1 answer
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Why is rapid expansion/compression reversible?

I am looking over the Otto Cycle on this MIT website and it says at one point "the processes from 1 to 2 and from 3 to 4 are isentropic" in reference to the expansion and compression of the ...
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What does it mean that the standard molar entropy value is the amount of energy that a substance must have to exist at a certain temperature?

What does it mean that the standard molar entropy value is the amount of energy that a substance must have to exist at a certain temperature? According to this reference: http://khimiya.org/volume15/...
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Is there an equivalent term for favourable and non-favourable entropy?

For Gibbs energy, we have 'exergonic' or 'endergonic' For Enthalpy, we have 'exothermic' or 'endothermic' It seems logical there should be an equivalent pair of terms for entropy, but I can't seem to ...
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Effect of degree of freedom on entropy change during a reaction

In the reaction $$\ce{C_{(s)} + O2_{(g)} -> CO2_{(g)}} $$ one mole gaseous reactant gives one mole gaseous product, so there is no significant change in entropy. It can be verified by line of $\ce{...
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In regards to the Gibbs free energy equation, it seems that the two terms oppose one another, Enthalpy and -TS. Can you explain this?

So for example, if H is endothermic, then the enthalpy term is positive, meaning energy is absorbed by system overall, meaning net bonds broken, meaning the equation leans towards nonspontaneity. If ...
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Thermodynamics derivatives: Does $(\partial A/\partial B) = (\partial B/\partial A)^{-1}$?

If I need to find, for instance: $$\mathcal{T} := \left(\frac{\partial U}{\partial S}\right)_{V, N}$$ knowing: $$\left(\frac{\partial S}{\partial U}\right)_{V, N} = \frac{1}{T}$$ Can I say that $\...
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How does Entropy influence the racemization of an optically active compound?

We were doing the topic "Isomerism" in our Organic Chem class and there's a question in our worksheet whose answer I haven't really understood, the question is as follows: The racemization ...
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Can entropy of universe remain constant in a process where P and T is constant?

We know $$\Delta S(\text{univ})=\Delta S(\text{sys})+\Delta S(\text{surr})$$ at constant $P$, $T$. $$\Delta S(\text{sys})=\Delta H(\text{sys})/T$$ $$\Delta S(\text{surr})=\Delta H(\text{surr})/T$$ As $...
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Prigogine vs. Bronsted and the minimum entropy production principle

I apologize for the length of this question that was asked here prigogine-bronsted but got no reply; anyhow, Bronsted's name is much better known among chemists than among physicists... Prigogine's ...
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What is heat bath here?

At the beginning $\pu{1 mol}$ of air is compressed to $\pu{3 atm}$ at a of temperature $\pu{523 K}$. After a random process, the pressure of the air equates to $\pu{1 bar}$ at a temperature of $\pu{...
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1 answer
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If the reaction is spontaneous, is it the ΔS surroundings that is always positive or ΔS universe? [closed]

If the reaction is spontaneous, is it the ΔS surroundings that are always positive or ΔS universe? I tried researching this on Google. However, the answers that I have found vary with different ...
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Can reverse micelle's exist outside solution?

As in reverse micelle's, the polar liquid remains inside the micelle while the exterior is one of lower dielectric constant; if the liquid were to be carefully removed, would the micelle structure ...
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1 answer
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Question regarding Entropy [closed]

I had some doubts regarding the concept of entropy. My first doubt was Entropy is defined as $ dS = dQ _{rev}/ T $ yet my textbook says that entropy is a state function. Isn't it the definition of a ...
2 votes
2 answers
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Loss of entropy and solvation energy in proteins

I am reading a chapter on protein stability. One section outlines the role of salt bridges or ion pairs in the stability of a protein. The excerpt goes as follows (italization added for emphasis): ...
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5 answers
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What is an example of an exothermic reaction with negative ∆S?

In high school we learn that when a reaction has both negative $\Delta{}S$ (entropy change) and negative $\Delta{}H$ (enthalpy change) it occurs spontaneously at lower temperatures, but becomes non-...
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Why is the Gibbs free energy of a reversible reaction not always 0? [duplicate]

From my understanding, reversible proceses are those where the expression: $$\Delta S_\mathrm{total} = \Delta S_\mathrm{system} + \Delta S_\mathrm{surroundings} = 0$$ is true for all for the entire ...
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At equilibrium, which one of the following is always Zero? [duplicate]

At equilibrium, which one of the following is always zero? (A)∆H (B)∆S(Total) (C)∆S(System) (D)∆G° I know that at equilibrium ∆G=0, and was facing difficulty in connecting the above mentioned ...
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2 answers
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How to derive the relation between entropy and heat described by the equation dS=dq/T?

I know that entropy is measure of disorder, and a book (Intro to Thermal Physics by David J. Schroeder) states that $\mathrm dS=\mathrm dq/T$, where $S$ is entropy, $q$ is heat flow, and $T$ is ...
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Equilibrium chemical potential if the population is not conserved?

The chemical potential, $\mu$, is equal to the free energy $F$, per photon. $$F = \mu = u-Ts$$ The energy per photon is $u=h\nu$ and $s$ is the entropy per photon. Say we have a process where an ...
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Isn't the change in Gibbs free energy for a reversible process zero?

In problem 6, part 6.5 of the 2020 IChO (PDF), a thermodynamic cycle is given for “one mole of monoatomic perfect gas”: $\mathrm{A \to B}$; isothermal reversible expansion receiving $\pu{250 J}$ by ...
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Justifying assumptions about method to find equilibrium pressure for co-existence of graphite and diamond

The standard state Gibbs free energies of formation of $\ce{C(graphite)}$ and $\ce{C(diamond)}$ at $T = \pu{298 K}$ are $\pu{0 kJ mol-1}$ and $\pu{2.9 kJ mol-1}$, respectively. The conversion of ...
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What does the change in Gibbs Free Energy refer to?

Usually change is indicated as the difference between some initial and final state. However, for ΔG there seem to be multiple definitions and I am a bit confused. For example, at equilibrium, ΔG = 0 ...
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Change in entropy in reversible and irreversible processes

The change in entropy for a reversible process is given as- $\Delta$S = $\frac{q_{rev}}{T}$ Where qrev is the heat supplied isothermally and reversibly. The change in entropy for a irreversible ...
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Where is my non-expansive work in combustion?

Consider if I burn some octane. The enthalpy released is $5430 \;\text{kJ/mol}$. The Gibbs free energy released is $5537 \;\text{kJ/mol}$. From my understanding, enthalpy is the heat change in the ...
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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 ...
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2 votes
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Raising melting point of water at STP (using disolved impurities)

Typically, water experiences melting point depression when it is impure. The more impure it is, the lower the melting point, up to a point. I understand you could raise the melting point by increasing ...
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Determining Boiling Point and Melting Point by Graph

This is the plot of $\Delta G^{^\circ}$ vs $T$ for $\ce{Pb-> PbO}$ I understand that the points where there is a sudden change in the slope of the graph, there is a phase change but exactly what ...
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Is there a parameter for degree of ‘mixedness’

So I was making coffee this morning and when mixing some milk into the black coffee, I wondered if there was some parameter or value in chemistry which describes how well mixed two liquids (or gases) ...
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What is 'non-expansion work' and how does ΔG represent the maximum non-expansion work that can be extracted in a reversible process? [duplicate]

I have just learned about Gibbs free energy and how its value indicates the change in entropy of the universe and can thus be used to find the spontaneity of a reaction. However, I found this other ...
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2 answers
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Are spontaneous exothermic reactions spontaneous due to more stable products or to an increase in the entropy of the surroundings? [closed]

The lattice formation of an ionic compound from its infinitely separated gaseous ions is spontaneous under standard conditions, but I cannot figure out the 'driving force' behind this. One way to look ...
2 votes
1 answer
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Entropy change during electrolysis of water

Consider the electrolysis of water reaction: $$\ce{H_2O -> H_2 + 1/2O_2}$$ At room temperature and atmospheric pressure, the change in enthalpy of this reaction is $\Delta H = \pu{+286 kJ mol-1}$. ...
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Entropy change and heat transfer in non-ideal heat engines

Please help me, a mathematician, to make sense of entropy. I know it's a topic with about a million questions already, so I understand if this gets ignored. Yet for the life of me, I cannot make sense ...
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