# 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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### 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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### 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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### Is there a reason for the mathematical form of the equilibrium constant? [duplicate]

Why are the two molarities multiplied and not added, and why is each raised to the power of the coefficient rather than multiplied by it? What is the reasoning behind this form? Was it simply ...
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### Derivation of relationship between equilibrium constant and Gibbs free energy change

Why is $\Delta G=\Delta G^o+RT\ln Q?$ It feels like all online sources were written for introductory Chemistry students! Where do I find a rigorous proof of this identity? Greatly appreciate it!
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### 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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### 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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### 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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### Difference between exothermic and exergonic

In High School I learned that an exothermic reactions releases energy, while an endothermic reaction needs energy to occur. Now I learned that there is a separate, somewhat similar classification ...
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### If change in free energy (G) is positive, how do those reactions still occur?

I was doing a couple of problems for homework: Calculate $K_\mathrm{sp}$ of $\ce{AgI}$ at $55.0\ \mathrm{^\circ C}$ Calculate $K_\mathrm{b}$ of $\ce{NH3}$ at $36.0\ \mathrm{^\circ C}$ I have to use ...
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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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### How is hydration free energy of a solute measured experimentally?

I am reading a paper (Shirts, M. R.; and Pitera, J. W. and Swope, W. C. and Pande, V. S. J. Chem. Phys. 2003, 119, 5740-5761). Near the beginning of the paper (page 5745), the authors state: ...
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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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### How to derive the relation between gibbs energy and equilibrium constant?

I want to understand the derivation between gibbs energy and equillibrium constant $$\Delta G=\Delta G^o+RT\ln Q?$$ I have seen a similar post on CSE Derivation of relationship between equilibrium ...
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### Proving RTln($a_i$) equal to non PV work done by system,where $a_i$ is activity

We know that chemical potential is defined as ${\mu}_i={\mu}_i^{standard}+RT\ln(a_i)$. Here $a_i$ is the activity of $i^\text{th}$ component of solution. In case of gases instead of $a_i$ it is $f_i$ (...
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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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### What are the units that measure free energy?

Is it $\pu{kJ/mol}$? I have done problems and I always end up with these units, but is this correct?
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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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### 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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### Gibbs free energy=potential free energy? (Transition state theory)

Online I found the plot Gibbs free energy vs. reaction coordinate: https://en.wikipedia.org/wiki/Transition_state_theory and on many books I found the plot potential energy vs. reaction coordinate (...
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### 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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### Can single molecules of C and O2 react in isolation, and if so how will momentum be conserved?

I am trying to figure out how is it possible to reconcile the reality of exothermic reactions, which means that kinetic energy is transferred to (heats) the surrounding matter, with the principle of ...
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### Rate-limiting in transition state theory

Suppose an initial state $A$ can transition to either state $C$ or state $D$. Suppose further that both of these two processes are rate-limited by a transition to the same intermediate state $B$, as ...
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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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### How can -G/T and -A/T be thought of as 'disguised' entropies?

I understand that entropy is defined as $\mathrm{d}S=\mathrm{d}q/T$ for a reversible change but I fail to see why $-G/T$ and $-A/T$ can be thought of as 'disguised' entropies. This is a question on my ...