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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37
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3answers
32k views

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 ...
36
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4answers
26k views

Gibbs free energy-minimum or zero?

A reaction proceeds towards the direction of lesser Gibbs free energy (at constant $T$ (temperature) and $P$ (pressure)). So, we could say that Gibbs free energy at equilibrium is minimum. On the ...
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5answers
112k views

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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4answers
17k views

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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1answer
7k views

What does "reaction coordinate" mean? What does it mean when a reaction has TWO reaction coordinates?

I'm familiar with the concept of a reaction coordinate from high school chemistry as some generic conformational parameter that all of the intermediate states of a chemical reaction lie upon. Recently,...
20
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2answers
2k views

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 ...
19
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1answer
507 views

How to treat pressure in computational chemistry?

I have a reaction where a gas at high pressure and a solution with all kinds of species is involved. How do I take pressure correctly into account to get reasonable values for $G$? Do I calculate ...
14
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3answers
5k views

Transition state and free energy

We have products $\ce{A + B}$ combining to form $\ce{C + D}$ through the transition state $\ce{X}$. Are all reactions at least virtually reversible? Is the difference between a reversible reaction ...
14
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3answers
1k 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 ...
14
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1answer
7k views

How are $\sigma$ and $\sigma^\pm$ determined in Hammett plots?

The Hammett plot is commonly invoked in organic chemistry to reason about the plausibility (or implausibility) of various reaction mechanisms. The vertical axis is essentially the logarithm of an ...
12
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4answers
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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 ...
12
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1answer
13k views

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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2answers
5k views

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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4answers
9k views

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/...
11
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2answers
1k views

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 ...
11
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3answers
5k views

What is the difference between ΔG and ΔrG?

Consider the reaction $$\ce{A -> B}$$ The reaction Gibbs free energy, $\Delta_\mathrm{r} G$ is given by the following equation $$\Delta_\mathrm{r} G = \Delta_\mathrm{r} G^\circ + RT \ln Q$$ Now ...
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1answer
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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
10
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2answers
3k views

What kind of equilibrium constant we use for Gibbs free energy and Van't Hoff equation?

We know that the Gibbs free energy is related to the equilibrium constant by the following equation: $$\Delta_\mathrm{r}G^\circ=-RT\ln K$$ We also know the Van't Hoff equation: $$\ln\left(\frac{K_2}{...
9
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2answers
18k views

Derivation of relationship between Gibbs free energy and electrochemical cell potential

Why is $\Delta G = -nFE$? I don't understand what the motivation is behind this definition. Was it derived or just given? The textbook provides no justification for this equation. In fact, much of ...
9
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5answers
57k views

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 ...
9
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1answer
10k views

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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1answer
2k views

Why is Gibbs free energy more useful than internal energy?

$$\mathrm{d}U<T\,\mathrm{d}S-p\,\mathrm{d}V= \left(\frac{\partial U}{\partial S}\right)_{\!V}\,\mathrm{d}S +\left(\frac{\partial U}{\partial V}\right)_{\!S}\,\mathrm{d}V$$ So for a change to occur ...
9
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1answer
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Why is $\Delta G = -T \Delta S_{\mathrm{total}}$ valid only at constant pressure?

According to my book(Elements of Physical Chemistry by Atkins and de Paula, 5th ed.), $\Delta G = - T \Delta S_{\mathrm{total}}$ is valid only for constant pressure and temperature. Enthalpy is ...
9
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1answer
790 views

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}\...
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4answers
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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 ...
8
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2answers
912 views

Why proton concentration is divided by 10⁻⁷?

I am reviewing the book Biochemistry Concepts and Connections by Appling, Cahill, and Mathews and I cannot understand why they divide by the hydrogen concentration by $10^{-7}$. Why not just leave it ...
8
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2answers
179 views

Why is it necessary to consider infinitesimal changes in p,V,T for H,U and G given that they're state functions?

State functions such as $G$ only depend on the state of the system and are not dependent on the "path" that took the system to that state (which would be the case for work, for example, which is not a ...
8
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1answer
20k views

What is the difference between enthalpy of formation and Gibbs free energy of formation?

I'm having trouble (not alone, apparently) understanding the concept of Gibbs free energy, but I'm beginning to. What I still don't understand, however, is how Gibbs free energy of formation differs ...
8
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2answers
19k views

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 ...
8
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2answers
19k views

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 $\...
8
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2answers
554 views

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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3answers
7k views

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 ...
7
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3answers
8k 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 $\...
7
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2answers
3k views

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 ...
7
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1answer
12k views

How to derive the pressure dependency for the Gibbs free energy?

$$\mathrm{d}G=\left(\frac {\partial G}{\partial p}\right)_T+\left(\frac {\partial G}{\partial T}\right)_p$$ The above is an exact differential. Firstly what does this mean physically? (I hear about ...
7
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3answers
9k 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 ...
7
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2answers
17k views

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 $\...
7
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1answer
24k views

Calculating the Gibbs Free Energy of Mixing

This is a Physical Chemistry Question and comes from the Atkins Physical Chemistry book. Suppose that $2.0~\mathrm{mol}$ $\ce{H2}$ at $2.0~\mathrm{atm}$ and $25~^\circ\mathrm{C}$ and $4.0~\mathrm{...
7
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3answers
928 views

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 ...
6
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3answers
12k views

How to derive the Gibbs free energy for an ideal gas?

Determine the Gibbs Free Energy for an ideal gas and show that, this energy, and its derivative can be related to the enthalpy of the system via Gibbs-Helmholtz equation $$H = -T^2 \left( \frac{\...
6
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2answers
1k views

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 ...
6
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1answer
904 views

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 ...
6
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2answers
610 views

Thermodynamics: Relation between different types of energy: My Analogy

I have always been confused between the different kind of energies in thermodynamics so today I thought of finally getting it right. Here's my analogy(which is wrong and I need help in improving it, ...
6
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2answers
188 views

What would be the physical interpretation of the equation dG = Vdp − SdT?

I was trying to understand the derivation of the formula $G=G^\circ+RT\ln (p/p^\circ)$ starting with the following relationships: $$ \begin{align} \mathrm dU&=T\,\mathrm dS-p\,\mathrm dV\tag{1}\\ \...
6
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1answer
2k views

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 ...
6
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1answer
21k views

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!
6
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1answer
113 views

Biomolecular energy functions and entropy

From what I've been reading, it seems that entropy (rather than enthalpy) is the biggest driver of protein folding (especially the burying of hydrophobic residues). However, popular energy functions ...
6
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5answers
959 views

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 -...
6
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1answer
89 views

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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5answers
37k views

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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