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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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 $...
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1answer
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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 ...
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1answer
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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 ...
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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 ...
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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 ...
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1answer
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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 ...
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1answer
48 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 ...
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1answer
59 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 ...
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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....
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2answers
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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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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 ...
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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:
$\Delta G^0 = \Delta H^0 - T \Delta S^0$
$\Delta G^0 = - RT \ln K$
...
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1answer
59 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 ...
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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
...
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1answer
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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 ...
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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 ...
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1answer
101 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 $F$ ...
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1answer
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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), ...
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40 views
How to find entropy of chemical network reaction?
I have the following system of reactions:
$$
\begin{aligned}
\ce{X_1 + Y_1 &<=>[$k_{21}$][$k_{12}$] X_2} \\
\ce{X_2 &<=>[$k_{32}$][$k_{23}$] X_3} \\
\ce{X_3 &<=&...
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32 views
Gibb's energy change for system
Heat liberated by a given amount of an ideal gas is $\pu{1200 cal}$ at $\pu{300K}$. What is the Gibbs free energy change for the gas in this process?
My attempt: I know that for a reversible process ...
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2answers
94 views
Deriving the equation for reaction spontaneity
I'm trying to make sense of the derivation of the equation $$\Delta G = \Delta G^0_r + RT\ln(Q)$$
While searching for a derivation of this equation that made sense to me I came across a publication ...
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118 views
Calculating Gibbs free energy for the reaction of burning methanol
Calculate $Δ_\mathrm{r}G^\circ$ for the following reaction:
$$\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}...
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1answer
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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 ...
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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 ...
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1answer
66 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 ...
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2answers
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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 ...
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1answer
43 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 ...
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1answer
47 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 ...
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2answers
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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
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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) ...
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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%...
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4answers
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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 ...
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1answer
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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 ...
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3answers
109 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^{-} ->...
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1answer
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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, ...
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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:
$$\ce{Cu2O(s) + 2 h+ <=> Cu+(aq) + 1/2 O2(g)} \tag{...
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2answers
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What is wrong in this argument that dG must always be zero?
Under constant temperature and pressure, the change of the Gibbs free energy can be written as
$$\mathrm dG_\textrm{sys} = \mathrm dH_\mathrm{sys} - T\,\mathrm dS_\mathrm{sys}$$
And in the textbook ...
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Gibbs free energy of phosphorus pentachloride decomposition reaction
The equilibrium constant at $\pu{227 °C}$ for the equation
$$\ce{PCl5(g) <=> PCl3(g) + Cl2(g)}$$
is $K_p = \pu{4.50E3 bar}.$ Calculate the value of $Δ_\mathrm{rxn}G^\circ$ at $\pu{227 ...
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2answers
171 views
Clarification on the Clausius inequality
In Atkins' Physical Chemistry the criteria for spontaneity using Gibbs energy was calculated using the Clausius inequality:
$$\mathrm dS ≥ \frac{\mathrm dq}{T},$$
and at constant pressure, $\mathrm ...
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Calculating thermodynamic quantities for hydrogenation of iron nitride
HW#6.3 If the reaction
$$\ce{Fe2N(s) + 3/2 H2(g) <=> 2 Fe(s) + NH3(g)}$$
comes to an equilibrium at a total pressure of $\pu{1 bar},$ analysis of the gas shows that at $\pu{700 K}$ and $\pu{800 ...
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1answer
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Relationship between Gibbs free energy and potential energy (related to reaction coordinate diagrams)
For a rate law, the rate constant $k$ is given by the equation
$$\ln k = \ln A - \frac{E_\mathrm{a}}{RT}\tag{1}$$
If $k_\mathrm{fwd}$ is the forward rate constant and $k_\mathrm{rev}$ is the reverse ...
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Bond dissociation energy calculation
I am looking for methods to determine the dissociation free energy in a precise way, ideally from a simulations. My hope is to get the most precise values possible that are specific to the structure ...
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Calculating ideal chemical work using extent of reaction and chemical potential [closed]
I have a electrochemical reactor assumed isothermal and isobaric with 4 reactions and I am trying to calculate the "ideal chemical work' exerted by each reaction using the extent of reactions for each ...
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1answer
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Is Gibbs Free Energy of formation equal to Gibbs Free Energy of transition state?
Is activation energy equivalent to Gibbs Free Energy of transition state as related by Eyring equation?
$$E_a=\Delta ^\ddagger G \, \, ?$$
Is Gibbs Free Energy of transition state defined by the ...
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1answer
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How to explain Gibbs free energy is a pressure-dependent state function?
I'm aware that through deriving Gibbs free energy to infinitesimal changes, we could get the formula: $\mathrm dG = V\,\mathrm dp - S\,\mathrm dT$, giving that Gibbs free energy is pressure-dependent.
...
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3answers
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How to explain disagreement between Le Châtelier's principle and the simplified Gibbs free energy equation?
For example, for the dissolution of a salt in water that is exothermic, heating the solution would drive the reaction towards the solid form of the salt according to Le Châtelier's principle.
However,...
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1answer
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Relation between equilibrium constant and entropy change
Like the Van't Hoff equation, which relates change in enthalpy to equilibrium constant, is there a similar equation for the relation between change in entropy and equilibrium constant?
Consider the ...
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2answers
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Reaction quotient and Gibbs free energy at the start of a reaction
According to the equation 33 above, shouldn't be all the reactions be spontaneous initially? Because the reaction quotient Q, is zero at the start of the reaction and logarithm of Q makes it a very ...
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3answers
555 views
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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0answers
49 views
Gibbs energy at a certain temperature
For the reaction
$$\ce{1/2 N2(g) + 3/2 H2(g) -> NH3(g)}$$
$ΔG$ at $\pu{298.15 K}$ is $\pu{-3980 cal},$ and the change in enthalpy is given by the expression:
$$ΔH = -9.19 - 7.12T + 3.182\cdot 10^{-...
