Questions tagged [free-energy]
Also known as Gibbs energy, it is the enthalpy minus the product of thermodynamic temperature and entropy.
251
questions
0
votes
1answer
94 views
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 ...
0
votes
0answers
29 views
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 ...
0
votes
1answer
60 views
How switching values of $R$, $K_p$ and $K_c$ does not alter the value of change in standard Gibbs energy? [duplicate]
$\Delta_\text r G=RT\ln \frac{Q}{K}$
If we switch $K_p$ from $K_c$, accordingly $Q_p$ will change to $Q_c$. Therefore, value of $\Delta_\text r G$ remains the same.
$ΔG^\circ=−RT\ln K=ΔH^\circ−TΔS^\...
0
votes
2answers
80 views
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 ...
4
votes
2answers
684 views
How to estimate the temperature needed to overcome an activation energy barrier?
Trying to elucidate a reaction pathway, I calculated the energies for different transition states possible for that reaction (computational chem. DFT). I found activation free energies ranging from 29 ...
3
votes
0answers
45 views
Gibbs free energy of mixing in a control volume
I'm trying to model the amount of energy lost due to mixing of a $\mathrm{HCl}$ solution using the Gibbs free energy of mixing. However, I really like a second opinion on the model and the results, ...
5
votes
0answers
80 views
Is a change in delta-delta-G typically notated as delta-delta-delta-G?
I'm currently writing a research paper, and need to refer to a difference between $\Delta{\Delta}G$s. The naive way to notate this would be $\Delta{\Delta}{\Delta}G$, but that looks cumbersome and ...
0
votes
0answers
32 views
Equilibrium and endergonic reaction
If a reaction A <-> B has a value of dG°>0, then I know that A -> B is endergonic while A <- B is exergonic.
Now if I start with 100% B I could see how equilibrium is reached, B reacts ...
-1
votes
1answer
72 views
Cause of reversible reactions in terms of Gibbs free energy
Heading
I recently learned about the standard Gibbs free energy change of reaction, ΔG=ΔH-TΔS, and how its sign indicates whether the conversion of (ALL) reactants and products is spontaneous or not.
...
1
vote
0answers
71 views
Maximum non-PV work and reversibility at constant temperature and pressure
When we talk about systems at constant temperature and pressure, maximum amount of non-PV work can be extracted if process is carried reversibly and in that case it is equal to change in Gibbs energy ...
4
votes
0answers
41 views
Why is the reverse aldol cleavage of fructose 1,6-bisphosphate highly endergonic under standard conditions?
I have another question similar to this one. I just don't have intuition for why reactions are thermodynamically favorable or unfavorable and I'd like to build it. This time I'd like to ask about ...
0
votes
0answers
18 views
Relationship between Enthalpy and Gibbs energy [duplicate]
I am a little stuck understanding the difference between Gibbs free energy and enthalpy. Both are measured in Joules and hence some sort of energy.
In a chemical reaction where C --> A + B
If A and ...
3
votes
1answer
247 views
Why is the formation of a peptide bond not spontaneous?
Why is the formation of a peptide bond from a carboxylic acid and amine not spontaneous?
I'm not looking for the free energy calculation; I have it from several sources. I'm just asking for an ...
-1
votes
1answer
78 views
Ordered and random energy [closed]
One of the books refer to potential energy being ordered whereas heat energy being random. Could someone explain what is ordered and random energy?
0
votes
0answers
38 views
Entropy and Gibbs Free Energy of Chelate Complexes
Of [Cd(NH2CH3)4]2+ and [Cd(en)2]2+ the stability of the latter is higher than the former attributing to chelation in the en complex .This is evident from the fact that the Gibbs Free Energy change for ...
2
votes
0answers
37 views
Is it possible to measure the entropy of a chemical reaction through calorimetry?
My broad question is how do you measure entropy change? I was doing a bit of digging into how thermodynamic tables are developed at I got this from NIST. Basically it seems like you need to indirectly ...
1
vote
1answer
112 views
Gibbs free energy for the evaporation of water
Problem
Find the value of $\Delta G$ for the evaporation of water at $\pu{100 °C}$ and $\pu{1 atm}.$
Answer
Question
I have the following data:
$$
\begin{array}{lcc}
\hline
\text{Compound} & \...
0
votes
0answers
17 views
How to use anodic and cathodic overpotentials to calculate electrolytic energy efficiency?
I am trying to calculate the energy efficiency for $\ce{CO2}$ reduction to formate, coupled to water oxidation based on literature data.
$$\ce{CO2 + 2e- + 2H+ -> HCOOH}$$
$$\ce{H2O -> 2H+ + 1/...
-4
votes
1answer
158 views
Reactions with a positive Delta H and a negative Delta S are said to be nonspontaneous at ALL temperatures. But what if the temperature IS low enough? [closed]
What I mean is, that if you use G = H-TS and make the temperature low enough, couldn't you make any reaction spontaneous? For example, you have a Delta H of 85 kJ and a Delta S of -66 J/K. I've been ...
6
votes
2answers
192 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:
$$
\begin{align}
\mathrm dU&=T\,\mathrm dS-p\,\mathrm dV\tag{1}\\
\...
3
votes
2answers
50 views
Isobaric entropy relation from isothermal Gibbs free energy relation
I have been given a problem to derive a relation for change in entropy from change in Gibbs free energy:
Starting with the isothermal equation
$$G(p_2) = G(p_1) + nRT\ln\frac{p_2}{p_1}\label{eqn:1}\...
1
vote
1answer
156 views
Can endergonic reactions be driven by heat?
I am having trouble conceptualizing endergonic reactions and their difference from endothermic reactions. An endergonic reaction is one that has a positive Gibbs free energy change, such that energy ...
3
votes
1answer
67 views
Derivation and visualization of cell potential dependency on temperature and entropy
Working from the thermodynamic identity
$$\Delta{S} = -\left(\frac{\partial{\Delta{G}}}{\partial{T}}\right)_p,$$
is it allowed to integrate both sides to obtain an expression derived from the Nernst ...
3
votes
2answers
589 views
Change in enthalpy and entropy when sugar dissolves in water
We know that dissolution of sugar in water is a spontaneous process.
So, change in Gibbs free energy $(ΔG)$ must be negative for the overall process:
$$ΔG = ΔH - TΔS < 0$$
Hence either the enthalpy ...
-1
votes
1answer
23 views
Finding Gibbs energy at different temperatures just given Gibbs energy at one temperature
How would it be found the Gibbs energy at a certain temperature, if they just give you another Gibbs energy at a temperature? For example: Given $\Delta G = -230 Kcal/mol$ at $773K$ for the reaction $...
-1
votes
1answer
204 views
Are all exothermic reactions necessarily spontaneous? [closed]
I was told that the statement "all exothermic reactions have negative free energy change" is false.
But I learnt Gibbs free energy as the "energy stored inside a system capable of doing ...
0
votes
1answer
51 views
How to calculate ∆rGº from entropy and ∆fGº in different temperatures? [closed]
"The total oxidation of glucose occurs according to the following chemical equation:
C6H12O6 (s) + 6O2 (g) -> 6CO2 (g) + 6H2O (l)
The following table gives us the free energies of standard ...
2
votes
0answers
71 views
Why does the plot of volume and temperature have a kink in a second order phase transition?
The $(b)$ part of the figure shows how thermodynamic variables change in a second order phase transformation.
We observe there is a kink in the plot of volume with temperature.
Why is that so?
The ...
3
votes
1answer
172 views
Can rate constant depend upon the concentration of reactants (or other species involved in the reaction)?
Transition state theory tells us that the rate constant of an elementary step is $$k_\mathrm{r} = \frac{\kappa k_\mathrm{B}T}{h}\exp\left\{\frac{- \Delta G^\ddagger}{RT}\right\},$$
where $\Delta G^\...
2
votes
1answer
203 views
Exergonic vs Endergonic Reactions
Problem
Problem 3, concept check 8.2 from Campbell Biology [1, p. 150]:
Some partygoers wear glow-in-the-dark necklaces that start glowing once they are “activated” by snapping the necklace. This ...
5
votes
1answer
73 views
How does DeepHF (CRISPR sgRNA design tool) compute $\Delta G_{\text{binding}}$?
I am looking at the code of DeepHF that computes $\Delta G_{\text{binding}}$, and I see that it breaks a sequence into overlapping 2-mers and compute a weighted sum according to a dictionary with 16 ...
1
vote
1answer
93 views
Helmholtz Free Energy Minimization Proof
In the proof that Helmholtz free energy is minimized at equilibrium, we use the fact that $$\mathrm dS_\text{total}=\mathrm dS_\text{system}+\mathrm dS_\text{bath}\geq 0$$
But we also use the fact ...
0
votes
1answer
354 views
About spontaneous combustion of methane
I heard that spontaneous reaction happens if $\Delta G=\Delta H-T\Delta S$ is negative.
For combustion of methane, according to Chemguide:
$\Delta H=\pu{-891.1 kJ K^{-1} mol^{-1}}$
$\Delta S=\pu{-0....
11
votes
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 ...
1
vote
2answers
60 views
How does work from other sources affect Gibbs free energy of the system?
In most physical chemistry textbooks, Gibbs free energy is given like this:
$$\Delta_\mathrm rG = \Delta_\mathrm rG^\circ + RT\ln Q\tag{1}$$
and Gibbs free energy is part of this three-way ...
1
vote
2answers
2k views
What is the difference between free energy and standard free energy
As I understood $\Delta G^0$ is just $\Delta G$ but at standard conditions i.e. $\pu{1 bar}$ and $\pu{298 K}$. But formulas like:
$\Delta G^0 = \Delta H^0 - T \Delta S^0$
$\Delta G^0 = - RT \ln K$
...
1
vote
1answer
66 views
Quantum calculation of free energy change in neat conditions
I am slightly baffled by a seemingly simple situation: I want to calculate the free energy of association between a molecule $\ce{A}$ and the solvent $\ce{B}$ using quantum methods (e.g. DFT).
From ...
0
votes
1answer
175 views
Sign of change in enthalpy and change in entropy
Why is it wrong to assert that the change in entropy and the change in enthalpy must always have the same sign?
What makes me think that they must have the same sign is the fact that every reaction ...
2
votes
0answers
61 views
Relationship between reaction quotient and potential energy [closed]
I am trying to calculate the Gibbs free energy in an NVT simulation. Although
$$\Delta G = -RT\ln Q,$$
in my simulation I can't calculate
$$Q = \frac{[\text{Products}]}{[\text{Reactants}]}$$
because I ...
2
votes
1answer
114 views
Expression for Gibb's free energy change for a cell reaction
According to Chemistry Libretexts, the Gibbs free energy change of a cell reaction is related to the cell voltage as
$$ \Delta G = -nFE,$$
where $n$ is the number of moles of electrons passed and $F$ ...
2
votes
1answer
122 views
Calculation of free energy from potential and kinetic energy
I am having trouble understanding how (and even whether it's possible) to get free energy from kinetic and potential energy.
I have run molecular dynamics simulations (NPT ensemble, CP2K software), ...
0
votes
0answers
41 views
How to find entropy of chemical network reaction?
I have the following system of reactions:
$$
\begin{aligned}
\ce{X_1 + Y_1 &<=>[$k_{21}$][$k_{12}$] X_2} \\
\ce{X_2 &<=>[$k_{32}$][$k_{23}$] X_3} \\
\ce{X_3 &<=&...
0
votes
0answers
34 views
Gibb's energy change for system
Heat liberated by a given amount of an ideal gas is $\pu{1200 cal}$ at $\pu{300K}$. What is the Gibbs free energy change for the gas in this process?
My attempt: I know that for a reversible process ...
4
votes
2answers
124 views
Deriving the equation for reaction spontaneity
I'm trying to make sense of the derivation of the equation $$\Delta G = \Delta G^0_r + RT\ln(Q)$$
While searching for a derivation of this equation that made sense to me I came across a publication ...
0
votes
0answers
412 views
Calculating Gibbs free energy for the reaction of burning methanol
Calculate $Δ_\mathrm{r}G^\circ$ for the following reaction:
$$\ce{2 CH3OH(l) + 3 O2(g) -> 2 CO2(g) + 4 H2O(l)}$$
using $Δ_\mathrm{r}H^\circ$ and $Δ_\mathrm{r}S^\circ$ at $\pu{298.13 K}$ $(\pu{25 °C}...
1
vote
1answer
67 views
Gibbs–Helmholtz equation
The Gibbs–Helmholtz equation was developed in the 1870's, while Nernst's heat theorem was developed in the early $20^{th}$ century (according to google/wikipedia). Nernst's heat theorem tells us that ...
5
votes
0answers
68 views
Is it possible to estimate the abundance of conformers from J values?
I have a system of two disubstituted cyclohexanes and I want to estimate the abundance/ratio of the two based on the J values and if possible, roughly estimate the difference in free energy.
To get ...
3
votes
1answer
136 views
Assumption behind Gibbs energy and maximum work
I'm somewhat confused by this derivation (in Schroeder's Book of Thermal Physics) of the fact that in a closed system, the change in the Gibbs Energy is always less than the Non-Expansion work done on ...
1
vote
2answers
611 views
How is a reaction with more products than reactants spontaneous?
If I have a reaction that has $\Delta G^\circ = 0,$ where there are more products than reactants (for example, $\ce{A <=> 2B + 2C}),$ then if I change the concentration of all the reactants and ...
2
votes
1answer
62 views
Rationalising an apparent inconsistency in the determination of reaction spontaneity
It is well-known that a reaction is spontaneous if $\Delta G < 0$. This requirement is derived from thermodynamic considerations, as done here. However, there seems to be two different ways of ...
