Questions tagged [free-energy]
Also known as Gibbs energy, it is the enthalpy minus the product of thermodynamic temperature and entropy.
266
questions
-2
votes
3
answers
71
views
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 ...
- 1
0
votes
0
answers
61
views
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 $\...
- 312
0
votes
1
answer
77
views
Standard Gibbs free energy when all the reactants and products are at standard condition
I have read in my textbook that for a reaction $\Delta G=\Delta G^o-RT\ln Q $, where $\Delta G^o$ is the Gibbs free energy change when the initial concentration of products and reactants are unity. ...
- 159
5
votes
0
answers
93
views
Is the formation of nitrogen dioxide spontaneous?
I was studying Thermodynamics from my Chemistry textbook (page 181-182, section 6.6 (a)).
I have a question regarding the following text from my book.
Now let us examine the following reactions:
$$\...
- 65
0
votes
0
answers
18
views
Is the maximum work done by voltaic cell per mole of reaction? [duplicate]
From the equation $w_{elec,rev}=-nF\Delta E$, and n = the number of moles of electrons transferred in the overall redox reaction, does that make the work per mole of the redox reaction? I am confused ...
1
vote
1
answer
89
views
Why can one set the Gibbs free energy change to 0 when calculating the standard free energy change?
I am trying to understand the following equation:
$$\Delta G = \Delta G^{\circ} + RT \ln\left(\frac{[C][D]}{[A][B]}\right)$$
for a reversible reaction with reactants A and B and products C and D.
The ...
- 13
1
vote
0
answers
32
views
Gibbs energy of electron transfer
the reaction Gibbs energy of a cell:
$\Delta_rG^o=-nFE^o$
and the Gibbs energy of photoinduced electron transfer:
$\Delta_{ET}G^o=N_A\{e[E^o(D^{+.}/D)-E^o(A/A^{-.})]+\omega(D^{+.}A^{-.})-\omega(DA)\}-\...
11
votes
2
answers
732
views
True or false: "If a reaction has a large negative value of ∆G, then it will be a fast reaction."
I recently took a Gen. Chem. 2 exam that contained this question. I answered false, as I knew that thermodynamics and kinetics are separate matters. My professor said the answer is true. I pressed him ...
- 429
2
votes
2
answers
63
views
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):
...
- 247
0
votes
0
answers
36
views
How to find the temperature for when Kc = 1
The formulas I can use are $\Delta$G = H - T$\Delta$S, and $\Delta$G = $\Delta$G(s.t.d) + RTlnK
I understand that when Kc is 1, the reaction is barely spontaneous and thus I can use the first formula ...
2
votes
1
answer
194
views
Heat capacity at constant volume and Gibbs free energy
I want to know if it is possible to derive heat capacities, in this case, in constant volume from another thermodynamic Potential which is not the Helmholtz free energy $F$. I am aware of the ...
- 123
3
votes
1
answer
83
views
Does this coupled reaction actually happen (decomposition of calcium carbonate)
I found the following example of a coupled reaction to drive the decomposition of calcium carbonate. I get the calculation part of it, that the changes in Gibbs energy sum to a negative amount. But ...
- 39
0
votes
1
answer
139
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$$
is true for all for the entire ...
- 19
0
votes
0
answers
38
views
At equilibrium, which one of the following is always Zero? [duplicate]
At equilibrium, which one of the following is always zero?
(A)∆H
(B)∆S(Total)
(C)∆S(System)
(D)∆G°
I know that at equilibrium ∆G=0, and was facing difficulty in connecting the above mentioned ...
0
votes
1
answer
65
views
How switching values of $R$, $K_p$ and $K_c$ does not alter the value of change in standard Gibbs energy? [duplicate]
$\Delta_\text r G=RT\ln \frac{Q}{K}$
If we switch $K_p$ from $K_c$, accordingly $Q_p$ will change to $Q_c$. Therefore, value of $\Delta_\text r G$ remains the same.
$ΔG^\circ=−RT\ln K=ΔH^\circ−TΔS^\...
- 1,147
0
votes
2
answers
101
views
Equilibrium chemical potential if the population is not conserved?
The chemical potential, $\mu$, is equal to the free energy $F$, per photon.
$$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 ...
- 534
4
votes
2
answers
771
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
0
answers
60
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, ...
- 131
5
votes
0
answers
88
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 ...
- 61
0
votes
0
answers
44
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
-1
votes
1
answer
131
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.
...
- 534
1
vote
0
answers
113
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 ...
- 739
4
votes
0
answers
57
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 ...
- 239
0
votes
0
answers
22
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
1
answer
553
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 ...
- 239
-1
votes
1
answer
95
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?
- 25
0
votes
0
answers
50
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 ...
- 657
2
votes
0
answers
57
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 ...
- 289
1
vote
1
answer
150
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} & \...
- 11
0
votes
0
answers
24
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.
$$\ce{CO2 + 2e- + 2H+ -> HCOOH}$$
$$\ce{H2O -> 2H+ + 1/...
- 157
-4
votes
1
answer
330
views
Reactions with a positive Delta H and a negative Delta S are said to be nonspontaneous at ALL temperatures. But what if the temperature IS low enough? [closed]
What I mean is, that if you use G = H-TS and make the temperature low enough, couldn't you make any reaction spontaneous? For example, you have a Delta H of 85 kJ and a Delta S of -66 J/K. I've been ...
- 11
6
votes
2
answers
267
views
What would be the physical interpretation of the equation dG = Vdp − SdT?
I was trying to understand the derivation of the formula $G=G^\circ+RT\ln (p/p^\circ)$ starting with the following relationships:
$$
\begin{align}
\mathrm dU&=T\,\mathrm dS-p\,\mathrm dV\tag{1}\\
\...
- 71
3
votes
2
answers
77
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
$$G(p_2) = G(p_1) + nRT\ln\frac{p_2}{p_1}\label{eqn:1}\...
- 143
1
vote
1
answer
296
views
Can endergonic reactions be driven by heat?
I am having trouble conceptualizing endergonic reactions and their difference from endothermic reactions. An endergonic reaction is one that has a positive Gibbs free energy change, such that energy ...
- 357
3
votes
1
answer
92
views
Derivation and visualization of cell potential dependency on temperature and entropy
Working from the thermodynamic identity
$$\Delta{S} = -\left(\frac{\partial{\Delta{G}}}{\partial{T}}\right)_p,$$
is it allowed to integrate both sides to obtain an expression derived from the Nernst ...
user85551
3
votes
2
answers
1k
views
Change in enthalpy and entropy when sugar dissolves in water
We know that dissolution of sugar in water is a spontaneous process.
So, change in Gibbs free energy $(ΔG)$ must be negative for the overall process:
$$ΔG = ΔH - TΔS < 0$$
Hence either the enthalpy ...
- 159
-1
votes
1
answer
56
views
Finding Gibbs energy at different temperatures just given Gibbs energy at one temperature
How would it be found the Gibbs energy at a certain temperature, if they just give you another Gibbs energy at a temperature? For example: Given $\Delta G = -230 Kcal/mol$ at $773K$ for the reaction $...
- 1
-1
votes
1
answer
273
views
Are all exothermic reactions necessarily spontaneous? [closed]
I was told that the statement "all exothermic reactions have negative free energy change" is false.
But I learnt Gibbs free energy as the "energy stored inside a system capable of doing ...
- 39
0
votes
1
answer
70
views
How to calculate ∆rGº from entropy and ∆fGº in different temperatures? [closed]
"The total oxidation of glucose occurs according to the following chemical equation:
C6H12O6 (s) + 6O2 (g) -> 6CO2 (g) + 6H2O (l)
The following table gives us the free energies of standard ...
2
votes
0
answers
109
views
Why does the plot of volume and temperature have a kink in a second order phase transition?
The $(b)$ part of the figure shows how thermodynamic variables change in a second order phase transformation.
We observe there is a kink in the plot of volume with temperature.
Why is that so?
The ...
- 287
3
votes
1
answer
269
views
Can rate constant depend upon the concentration of reactants (or other species involved in the reaction)?
Transition state theory tells us that the rate constant of an elementary step is $$k_\mathrm{r} = \frac{\kappa k_\mathrm{B}T}{h}\exp\left\{\frac{- \Delta G^\ddagger}{RT}\right\},$$
where $\Delta G^\...
- 5,249
2
votes
1
answer
416
views
Exergonic vs Endergonic Reactions
Problem
Problem 3, concept check 8.2 from Campbell Biology [1, p. 150]:
Some partygoers wear glow-in-the-dark necklaces that start glowing once they are “activated” by snapping the necklace. This ...
6
votes
1
answer
128
views
How does DeepHF (a CRISPR sgRNA design tool) compute binding free energy?
I am looking at the code of DeepHF[1] that computes $\Delta G_\text{binding}$, and I see that it breaks a sequence into overlapping dimers and compute a weighted sum according to a dictionary with 16 ...
- 904
1
vote
1
answer
120
views
Helmholtz Free Energy Minimization Proof
In the proof that Helmholtz free energy is minimized at equilibrium, we use the fact that $$\mathrm dS_\text{total}=\mathrm dS_\text{system}+\mathrm dS_\text{bath}\geq 0$$
But we also use the fact ...
- 13
0
votes
1
answer
649
views
About spontaneous combustion of methane
I heard that spontaneous reaction happens if $\Delta G=\Delta H-T\Delta S$ is negative.
For combustion of methane, according to Chemguide:
$\Delta H=\pu{-891.1 kJ K^{-1} mol^{-1}}$
$\Delta S=\pu{-0....
11
votes
2
answers
1k
views
Can single molecules of C and O2 react in isolation, and if so how will momentum be conserved?
I am trying to figure out how is it possible to reconcile the reality of exothermic reactions, which means that kinetic energy is transferred to (heats) the surrounding matter, with the principle of ...
- 147
1
vote
2
answers
73
views
How does work from other sources affect Gibbs free energy of the system?
In most physical chemistry textbooks, Gibbs free energy is given like this:
$$\Delta_\mathrm rG = \Delta_\mathrm rG^\circ + RT\ln Q\tag{1}$$
and Gibbs free energy is part of this three-way ...
- 323
1
vote
2
answers
3k
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:
$\Delta G^0 = \Delta H^0 - T \Delta S^0$
$\Delta G^0 = - RT \ln K$
...
- 131
1
vote
1
answer
75
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 ...
- 1,558
0
votes
1
answer
307
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 ...
