Questions tagged [free-energy]
Also known as Gibbs energy, it is the enthalpy minus the product of thermodynamic temperature and entropy.
242
questions
3
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1answer
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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
65 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?
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30 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
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0answers
33 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
83 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:
$$
\begin{array}{lcc}
\hline
\text{Compound} & \...
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How to use anodic and cathodic overpotentials to calculate electrolytic energy efficiency?
I am trying to calculate the energy efficiency for $\ce{CO2}$ reduction to formate, coupled to water oxidation based on literature data.
$$\ce{CO2 + 2e- + 2H+ -> HCOOH}$$
$$\ce{H2O -> 2H+ + 1/...
-4
votes
1answer
43 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
145 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:
$$
\begin{align}
\mathrm dU&=T\,\mathrm dS-p\,\mathrm dV\tag{1}\\
\...
3
votes
2answers
42 views
Isobaric entropy relation from isothermal Gibbs free energy relation
I have been given a problem to derive a relation for change in entropy from change in Gibbs free energy:
Starting with the isothermal equation
$$G(p_2) = G(p_1) + nRT\ln\frac{p_2}{p_1}\label{eqn:1}\...
1
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1answer
70 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
49 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
201 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
18 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
137 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
47 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
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0answers
58 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 ...
0
votes
2answers
57 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 $$\ce{k_r=\frac{\kappa k_BT}{h}e^\frac{- \Delta G^\ddagger}{RT}}$$
where $\ce{\Delta G^\ddagger}$ is the activation ...
2
votes
1answer
107 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
66 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
80 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
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1answer
179 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....
12
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 ...
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2answers
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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
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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
63 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
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0answers
47 views
How to derive thermodynamic activity?
From wikipedia thermodynamic activity is given as
$$ a = \exp(\frac{\mu - \mu_{0}} {RT})$$
which becomes,
$$ RT\ln(a) = \mu - \mu_{0}\tag{1}$$
And i have to do a calculation where activity is given as
...
0
votes
1answer
109 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
59 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
106 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
94 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
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0answers
40 views
How to find entropy of chemical network reaction?
I have the following system of reactions:
$$
\begin{aligned}
\ce{X_1 + Y_1 &<=>[$k_{21}$][$k_{12}$] X_2} \\
\ce{X_2 &<=>[$k_{32}$][$k_{23}$] X_3} \\
\ce{X_3 &<=&...
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0answers
33 views
Gibb's energy change for system
Heat liberated by a given amount of an ideal gas is $\pu{1200 cal}$ at $\pu{300K}$. What is the Gibbs free energy change for the gas in this process?
My attempt: I know that for a reversible process ...
4
votes
2answers
106 views
Deriving the equation for reaction spontaneity
I'm trying to make sense of the derivation of the equation $$\Delta G = \Delta G^0_r + RT\ln(Q)$$
While searching for a derivation of this equation that made sense to me I came across a publication ...
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0answers
262 views
Calculating Gibbs free energy for the reaction of burning methanol
Calculate $Δ_\mathrm{r}G^\circ$ for the following reaction:
$$\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
55 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
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0answers
63 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
106 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
330 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
46 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 ...
0
votes
1answer
49 views
How is the change in Gibb's free energy a condition for spontaneity for reactions not happening under constant pressure conditions?
According to what I have studied, the change in Gibb's free energy is only a condition for spontaneity for processes happening at constant pressure and temperature conditions.
Consider a reversible ...
0
votes
2answers
60 views
T∆S implies that heat flows which way?
From Schroeder's Introduction to Thermal Physics.
Parts $(a)$ and $(b)$ are quite simple, and we find that $ \Delta H =-2808\ \mathrm{kJ}$ and $\Delta G =-2879\ \mathrm{kJ}$, so by $\Delta G = \Delta ...
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1answer
63 views
How come I get different Gibbs free energy for the same reduced equation?
I want to calculate the Gibbs free energy of this reaction which can be:
$\ce{H2(g) + 1/2O2(g) -> H2O(g)}$
or
$\ce{2H2(g) + O2(g) -> 2H2O(g)}$
I am using the entropy values (J/molK): H2(g) ...
1
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0answers
36 views
Is Gibbs energy minimized for processes at constant temperature are pressure only?
I've had this doubt for quite a while, This link
https://chem.libretexts.org/Bookshelves/Physical_and_Theoretical_Chemistry_Textbook_Maps/Map%3A_Physical_Chemistry_for_the_Biosciences_(Chang)/04%...
2
votes
4answers
4k views
Are all exothermic reactions spontaneous?
$\ce{GaAs (s)}$ is used in solar cells deployed on satellites found in the orbit of Earth. The best way to make these materials is through a technique called "molecular beam epitaxy", in ...
3
votes
1answer
95 views
Gibbs energy: the term −TdS. Where does the reversible heat go?
When we have an exothermic reaction as well as a decrease of the entropy in the system after the reaction, why does the decrease in the entropy of the system diminishes the amount of useful work we ...
0
votes
3answers
179 views
Why is this reaction spontaneous in standard conditions?
To study if this reaction is spontaneous I proceed in this way:
$$\ce{2H_{3}O^{+} +Fe ->H_{2} +Fe^{2+} + 2H_{2}O}$$
First I write the two semireaction:
$$reduction: \ \ \ce{2H_{3}O^{+} +2e^{-} ->...
1
vote
1answer
41 views
Ground state elevation for phase transition
I have a question about the illustration from chem.libretexts.org:
the ground states of the liquid and gas are offset from that of the previous state by the heats of fusion and vaporization, ...
0
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0answers
70 views
Reaction energies of solids from quantum chemistry?
I was reading a paper by Catlow et al. [1], and they mentioned the dissolution of copper into copper ions, which is given by this equation:
$$\ce{Cu2O(s) + 2 h+ <=> Cu+(aq) + 1/2 O2(g)} \tag{...
1
vote
2answers
190 views
What is wrong in this argument that dG must always be zero?
Under constant temperature and pressure, the change of the Gibbs free energy can be written as
$$\mathrm dG_\textrm{sys} = \mathrm dH_\mathrm{sys} - T\,\mathrm dS_\mathrm{sys}$$
And in the textbook ...
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0answers
124 views
Gibbs free energy of phosphorus pentachloride decomposition reaction
The equilibrium constant at $\pu{227 °C}$ for the equation
$$\ce{PCl5(g) <=> PCl3(g) + Cl2(g)}$$
is $K_p = \pu{4.50E3 bar}.$ Calculate the value of $Δ_\mathrm{rxn}G^\circ$ at $\pu{227 ...
