Questions tagged [free-energy]

Also known as Gibbs energy, it is the enthalpy minus the product of thermodynamic temperature and entropy.

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Dependency of Standard Gibbs free energy (using electrode potential) on stoichiometric moles of electron exchanged

I have a question regarding the definition of the number of moles $n$ when using the equation- $$\Delta G=-nF\Delta E$$ I understand that $n$ is the number of moles transferred in the overall redox ...
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reconciling free energy equations

I am struggling with a very basic concept and I think the issue lies in the definition of standard states. When calculating standard state free energy change via Gibbs equation $$\Delta G^ \circ = \...
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Gibbs Free Energy and Entropy

I understand Enthalpy, the exchange of energy between products and reactants. But what is Gibbs Free Energy and Entropy? I know that Gibbs free Energy is the difference between the change in Enthalpy -...
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Energy required to break salt bridges in peptides and proteins

Is there a way to calculate the energy required to break a salt bridge in a peptide or on the exposed surface of a protein in aqueous solution? Is there any useful literature on this topic? I'm ...
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468 views

Chemical potential and it's implications in an open system

Take the fundamental equation for the Gibbs free energy of a system: $$dG=VdP-SdT+\sum_i\mu_idn_i$$ Thus, the chemical potential can be defined in terms of the Gibbs free energy as follows: $$\mu_i=...
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Utility of Delta G

What is the main utility of delta G? As far as I could understand, if we do not know whether the reaction we are dealing with occurs in nature or not, we would not be able to tell in which direction ...
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Why are Kc and Kp interchangeable in the free energy-equilibrium equation? [duplicate]

In this equation that relates Standard free energy and the equilibrium constant: $\Delta Gº=-RT \ln K_{eq}$ My textbook (and my teacher) say both $K_p$ (constant related to pressure) and $K_c$ (...
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Active mass,Pressure and concentration

The concentration is connected to the activity via $$a(\ce{A})= \gamma_{c,\ce{A}}\cdot{}\frac{c(\ce{A})}{c^\circ},$$ where the standard concentration is $c^\circ = 1\:\mathrm{mol/L}$. At reasonable ...
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Which step below is rate determining

What is the rate determining step in the following energy profile? To clarify, the reaction is: A -> B -> C The energy of A is greater than B which in turn is also greater than C. The intermediate A-...
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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 $\...
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What is the difference between ∆G and ∆G°?

In Brady's Molecular Nature of Matter, I read that $\Delta_\mathrm{r} G^{\circ}$ is $\Delta_\mathrm{r} G$ at $25~^\circ\mathrm{C}$. But later, it gives a value for $\Delta_\mathrm{r} G^\circ$ at ...
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Equilibrium constant. Can it be reached?

Consider the reaction below: $$\ce{A + B <=> C}$$ Suppose that the equilibrium constant for this reaction is $K = 10$. I then prepare a reaction vessel with volume of $\pu{1 dm^{-3}}$ which ...
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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 ...
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Change in Gibbs free energy of vaporization reaction

In my chemistry book, I see the equation $$\Delta G_{rxn} = \Delta G_{rxn}^0 + RT \ln(Q)$$ where $\Delta G_{rxn}^0$ is the change in Gibbs free energy under standard conditions (1 atm pressure in ...
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Does Gibbs free energy of formation always equal zero for elements in standard state?

I was looking through the appendix in my chemistry textbook when I noticed that every compound that had a $\Delta H_f$ of zero also had a $\Delta G_f$ of zero. So of course the compounds that have a $\...
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Gibbs free energy in chemical reaction

I know this kind of question has been asked many times but I think this is a bit different. We have $G = H - TS$. Here I am talking about chemical reactions at constant pressure and temperature. We ...
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Which equilibrium constant is appropriate to use?

I have learnt that the standard free energy change is related to the equilibrium constant of a reaction by, $$\Delta G^\circ = -RT \ln K$$ Here, does $K$ refer to $K_p$ or $K_c$? Also, please give ...
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Predicting the spontaneity of non-isothermal reactions

We use free energy equations -- Helmholtz or Gibbs -- to predict whether or not a reaction is spontaneous. These equations depend on constant temperature. This forum post describes a scenario where ...
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Difference between chemical potential and gibbs free energy

I know that the chemical potential is the molar Gibbs free energy and that it is an intensive variable and that the gibbs free energy is extensive. However, they have identical units: $\mathrm{J~mol^{-...
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What's the difference between ΔG° and ΔG°'?

In biochemistry contexts, one often sees $\Delta G^{\circ\prime}$ ("delta G naught prime"), rather than the normal standard free energy change $\Delta G^{\circ}$ ("delta G naught"). What's the ...
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Is the Gibbs standard free energy always constant?

I am a biochemistry student and we are learning about thermodynamics. Is the Gibbs standard free energy for a reaction always constant? The equation below suggests that it changes with temperature: $$...
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How are partial molar Gibbs excess functions correctly defined?

I think I've found a mistake in my thermodynamics textbook, Chemical Thermodynamics for Process Simulation, but as thermodynamics is hard and I'm a relative novice, I wanted to check here. The ...
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Why does equilibrium exist?

I've read an article that equilibrium exists when $G$ (Gibbs free energy) becomes minimum. If formation of products decreases Gibbs free energy, why won't the reaction proceed to completion so that ...
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How is change in entropy of universe related to the energy released by system that can be used to do work?

What does change in entropy ($\Delta S$) represent ? How does change in entropy of a universe ($\Delta S_{univ}$) give us information about energy available to do work. I would like to know the ...
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Why does the Gibbs free energy only correspond to non-expansion work?

It has been defined as the energy available for work other than expansion work. Why can't it be used for expansion work
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Pourbaix diagram – writing reactions

How would you go about writing the reaction between water and chromium metal under standard acid and base conditions? I know you would have to look at the vertical lines at pH 0 and 14 but I don’t ...
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How to find the standard enthalpy and entropy for the dissolution of silver chloride?

A maximum of $1.89~\mathrm{mg}$ of $\ce{AgCl}$ will dissolve in $1.00~\mathrm{L}$ of solution at $25.00~^\circ\mathrm{C}$. A maximum of $5.26~\mathrm{mg}$ of $\ce{AgCl}$ will dissolve in $1.00~\...
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What are ω and ν in this expression?

In the picture below a proportionality equation is shown: $$\omega \propto \nu \exp\left(\frac{-\Delta G_m}{RT}\right)\propto \dots$$ While $\Delta G_m$ is the Gibbs free energy, $R$ is the gas ...
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Why exactly are standard potentials additive?

I don't really study chemistry so while my question may be very obvious, its not obvious to me. If we take an electrochemical reaction like $$\ce{2Fe^2+ + Au^3+ -> 2Fe^3+ + Au+}$$ we can find its ...
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Why is the melting of ice a spontaneous reaction?

So here's my question: we know that melting of ice is an spontaneous reaction but is it spontaneous? We have to supply energy in the form heat for the reaction to take place.
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How to determine the value for n in the Gibbs free energy and redox potential equation?

I am having a problem with this equation for redox potentials $$\Delta G = -nFE_\mathrm{cell}$$ In this equation I never am totally sure about what the value of $n$ should be, for example for the ...
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What is the precise definition of maximum work in the context of electrochemistry and thermodynamics?

I'd like clarification of the meaning of $W_\text{max}$ in the following equations (and thus in the combined result in the third equation): \begin{align} W_\text{max} &= \Delta G\\ W_\text{max} &...
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How to calculate the temperature at which a reaction becomes spontaneous?

At $25~^\circ\mathrm{C}$, $298~\mathrm{K}$, the reduction of copper(I) oxide, $ \Delta H = 58.1~\mathrm{kJ}$, $\Delta S = 165~\mathrm{J/K}$, is nonspontaneous, $ \Delta G = 8.9~\mathrm{kJ}$. ...
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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 $\...
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Truly Understanding the Second Law of Thermodynamics and Entropy

There are some seemingly contradictory things I’d like cleared up: Second Law states: $\mathrm dS_\text{univ} = \mathrm dS_\text{sys} + \mathrm dS_\text{surr}$, and $\mathrm dS_\text{univ}$ is always ...
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Why does water evaporate spontaneously at room temperature despite ΔG > 0?

Standard Gibbs free energy of formation of liquid water at $\pu{298 K}$ is $\pu{−237.17 kJ mol-1}$ and that of water vapour is $\pu{−228.57 kJ mol-1}$ therefore, $$\ce{H2O (l) -> H2O (g)}\qquad\...
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Spontaneous Reaction and Gibbs Free Energy

I know how to derive the expression for the equilibrium constant starting from $\mathrm dG=0$, $$\ln(K)=-\frac{\Delta G ^\circ}{RT}$$ However, if we are not at equilibrium, $\mathrm dG$ is not ...
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If Gibbs energy of activation is positive, how is the change to the transition state possible?

The Gibbs energy increases from the reactants to the transition state, but according to the Second Law of Thermodynamics, entropy of the universe can only increase. Why is the transition state ...
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What is non-expansion work?

I learnt that Gibbs free energy is the maximum amount of non-expansion work. But the phrase non expansion work confuses me. Work is defined as pressure times change in volume. If there is no expansion,...
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Heat Capacity temperature dependence and Gibbs energy

I got this problem in school as a tool to get ready for International Chemistry Olympiad, but I have some problems with this challenge. So, the problem is to calculate as precise as possible the Gibbs ...
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Relative energies between reactants and products

I've been asked to draw a relative energy sketch of the reactants and the two products. The reaction being examined was adding copper acetate monohydrate to glycine in two different fashions to create ...
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Are all naturally occuring reactions exothermic?

If a reaction is endothermic, then the reaction must take energy from the environment to occur (as far as I know), and if this is the case, then to me the reaction seems energetically unfavorable, as ...
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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 ...
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Given the Pair Correlation Functions for a Multi-component Fluid, is There a Simple way to Obtain the Helmholtz Free Energy?

I think it may not be possible. Given all the pair correlation functions, it seems possible to obtain the internal energy (assuming different species interact through pair-potentials (i.e. ...
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Correlation between the equilibrium constant and delta G

I understand this up until the last two steps. How does $\ln(K_\mathrm{eq})$ become $2{,}303\log(K_\mathrm{eq})$ and how does $-RT\cdot2{,}303$ become $-1{,}42$?
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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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Gibbs Free Energy

My teacher said that The decrease in Gibbs free energy is the maximum non-mechanical work that can be obtained when the process occurs reversibly at constant temperature and pressure But for a ...
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Can a reaction with positive Gibbs free energy change ever occur?

If you have an endothermic reaction with a negative entropy change, is it still possible to induce the reaction in some way despite the fact that the Gibbs free energy change is positive or all ...
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Gibbs free energy or reaction enthalphy

If I conduct a reaction say, $$\ce{CaO(s) + H2O(l) \rightarrow Ca(OH)2 (aq)}$$ let us say , I want to measure the energy generated from the reaction.Will this energy I calculate equal the change in ...
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Gibbs Free Energy : What is it trying to say actually?

"Gibbs Energy is the useful work that can be extracted from the heat of a reaction or a thermodynamic process." I understand how it predicts the feasibility of a chemical reaction , considering the ...