Questions tagged [free-energy]
Also known as Gibbs energy, it is the enthalpy minus the product of thermodynamic temperature and entropy.
217
questions
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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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0answers
55 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 ...
-1
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0answers
18 views
Calculate free energy of frames in an NVT simulation
I have a liquid within an NVT simulation and I want to calculate the Gibb's Free Energy of the frames in the run. I am not sure what software or formula to look towards.
The system is water ( auto-...
2
votes
1answer
88 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
64 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), ...
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0answers
25 views
A Thermodynamics question involving the molar solubility and Ksp of CaCO3
This thermodynamics question is from my textbook, I am not sure how to solve it correctly and I do not have the answer.
The first thing I did was to calculate the number of moles from the Ca(OH)2 ...
0
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0answers
37 views
How to find entropy of chemical network reaction?
I have the following system of reactions:
$$
\begin{aligned}
\ce{X_1 + Y_1 &<=>[$k_{21}$][$k_{12}$] X_2} \\
\ce{X_2 &<=>[$k_{32}$][$k_{23}$] X_3} \\
\ce{X_3 &<=&...
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0answers
30 views
Gibb's energy change for system
Heat liberated by a given amount of an ideal gas is $\pu{1200 cal}$ at $\pu{300K}$. What is the Gibbs free energy change for the gas in this process?
My attempt: I know that for a reversible process ...
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2answers
81 views
Deriving the equation for reaction spontaneity
I'm trying to make sense of the derivation of the equation $$\Delta G = \Delta G^0_r + RT\ln(Q)$$
While searching for a derivation of this equation that made sense to me I came across a publication ...
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0answers
49 views
Calculating Gibbs free energy for the reaction of burning methanol
Calculate $Ī_\mathrm{r}G^\circ$ for the following reaction:
$$\ce{2 CH3OH(l) + 3 O2(g) -> 2 CO2(g) + 4 H2O(l)}$$
using $Ī_\mathrm{r}H^\circ$ and $Ī_\mathrm{r}S^\circ$ at $\pu{298.13 K}$ $(\pu{25 Ā°C}...
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1answer
39 views
GibbsāHelmholtz equation
The GibbsāHelmholtz 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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0answers
56 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
48 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
82 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 ...
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1answer
40 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
32 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
55 views
TāS 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
21 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
32 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%...
3
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1answer
79 views
Gibbs energy: the term āTdS. 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
76 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
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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, ...
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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:
$$\ce{Cu2O(s) + 2 h+ <=> Cu+(aq) + 1/2 O2(g)} \tag{...
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2answers
154 views
What is wrong in this argument that dG must always be zero?
Under constant temperature and pressure, the change of the Gibbs free energy can be written as
$$\mathrm dG_\textrm{sys} = \mathrm dH_\mathrm{sys} - T\,\mathrm dS_\mathrm{sys}$$
And in the textbook ...
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0answers
85 views
Gibbs free energy of phosphorus pentachloride decomposition reaction
The equilibrium constant at $\pu{227 Ā°C}$ for the equation
$$\ce{PCl5(g) <=> PCl3(g) + Cl2(g)}$$
is $K_p = \pu{4.50E3 bar}.$ Calculate the value of $Ī_\mathrm{rxn}G^\circ$ at $\pu{227 ...
4
votes
2answers
144 views
Clarification on the Clausius inequality
In Atkins' Physical Chemistry the criteria for spontaneity using Gibbs energy was calculated using the Clausius inequality:
$$\mathrm dS ā„ \frac{\mathrm dq}{T},$$
and at constant pressure, $\mathrm ...
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0answers
44 views
Calculating thermodynamic quantities for hydrogenation of iron nitride
HW#6.3 If the reaction
$$\ce{Fe2N(s) + 3/2 H2(g) <=> 2 Fe(s) + NH3(g)}$$
comes to an equilibrium at a total pressure of $\pu{1 bar},$ analysis of the gas shows that at $\pu{700 K}$ and $\pu{800 ...
3
votes
1answer
140 views
Relationship between Gibbs free energy and potential energy (related to reaction coordinate diagrams)
For a rate law, the rate constant $k$ is given by the equation
$$\ln k = \ln A - \frac{E_\mathrm{a}}{RT}\tag{1}$$
If $k_\mathrm{fwd}$ is the forward rate constant and $k_\mathrm{rev}$ is the reverse ...
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0answers
48 views
Bond dissociation energy calculation
I am looking for methods to determine the dissociation free energy in a precise way, ideally from a simulations. My hope is to get the most precise values possible that are specific to the structure ...
4
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0answers
61 views
Calculating ideal chemical work using extent of reaction and chemical potential [closed]
I have a electrochemical reactor assumed isothermal and isobaric with 4 reactions and I am trying to calculate the "ideal chemical work' exerted by each reaction using the extent of reactions for each ...
2
votes
1answer
70 views
Is Gibbs Free Energy of formation equal to Gibbs Free Energy of transition state?
Is activation energy equivalent to Gibbs Free Energy of transition state as related by Eyring equation?
$$E_a=\Delta ^\ddagger G \, \, ?$$
Is Gibbs Free Energy of transition state defined by the ...
3
votes
1answer
53 views
How to explain Gibbs free energy is a pressure-dependent state function?
I'm aware that through deriving Gibbs free energy to infinitesimal changes, we could get the formula: $\mathrm dG = V\,\mathrm dp - S\,\mathrm dT$, giving that Gibbs free energy is pressure-dependent.
...
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3answers
790 views
How to explain disagreement between Le Châtelier's principle and the simplified Gibbs free energy equation?
For example, for the dissolution of a salt in water that is exothermic, heating the solution would drive the reaction towards the solid form of the salt according to Le ChĆ¢telier's principle.
However,...
3
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1answer
2k views
Relation between equilibrium constant and entropy change
Like the Van't Hoff equation, which relates change in enthalpy to equilibrium constant, is there a similar equation for the relation between change in entropy and equilibrium constant?
Consider the ...
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2answers
3k views
Reaction quotient and Gibbs free energy at the start of a reaction
According to the equation 33 above, shouldn't be all the reactions be spontaneous initially? Because the reaction quotient Q, is zero at the start of the reaction and logarithm of Q makes it a very ...
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3answers
507 views
If change in free energy (G) is positive, how do those reactions still occur?
I was doing a couple of problems for homework:
Calculate $K_\mathrm{sp}$ of $\ce{AgI}$ at $55.0\ \mathrm{^\circ C}$
Calculate $K_\mathrm{b}$ of $\ce{NH3}$ at $36.0\ \mathrm{^\circ C}$
I have to use ...
0
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0answers
48 views
Gibbs energy at a certain temperature
For the reaction
$$\ce{1/2 N2(g) + 3/2 H2(g) -> NH3(g)}$$
$ĪG$ at $\pu{298.15 K}$ is $\pu{-3980 cal},$ and the change in enthalpy is given by the expression:
$$ĪH = -9.19 - 7.12T + 3.182\cdot 10^{-...
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2answers
71 views
Is temperature double-counted in the Gibbs free energy equation?
Complete amateur here. When looking at one form of the equation deriving the change in Gibbs free energy for a given chemical reaction, the terms change in enthalpy, and the negative product of ...
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1answer
46 views
Change in heat of photosynthesis
Consider a plant that uses the energy it receives from the sun to do work (to undergo photosynthesis) to produce glucose. The minimum amount of work this plant can do to produce 1 mole of glucose is ...
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1answer
196 views
Gibbs Free Energy Notation
What's the difference between $G$, $\Delta G$, $\Delta G^\circ$, and $\Delta_\mathrm r G^\circ$?
I've seen the first two used interchangably, and seen $G$ specifically referred to as change in Gibbs ...
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0answers
43 views
Derivation of relationship betwee standard Gibbs energy of reaction and equilibrium constant? [closed]
How does one derive $\Delta_\mathrm r G^\circ = -RT\ln(K)$? I don't see the connection mathematically or conceptually; how does Gibbs at all relate to the equilibrium constant of a reaction?
This was ...
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1answer
380 views
Heat and work of glucose combustion
$$\ce{C6H12O6(s) + 6 O2(g) -> 6 CO2(g) + 6 H2O(g)}$$
When looking at glucose combustion I found that the change in Gibbs free energy is greater than the change in enthalpy. If we assume the process ...
3
votes
1answer
734 views
Gibbs Free Energy and Maximum Work
I am a high school student and my professor mentioned that the Gibbs free energy is the maximum amount of work (or useful work) that a system can do, whereas entropy is a measure of the non-available ...
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2answers
444 views
Why is my conclusion inconsistent with the van't Hoff equation?
Let's say I hypothesize that a graph of $\ln K$ vs. $1/T$ has a slope of $-āG^\circ/R$ and a $y$-intercept of $0$. I prove it simply:
$$āG^\circ = -RT\ln K \quad\to\quad \ln K = -\frac{āG^\circ}{RT}$$...
2
votes
1answer
224 views
Change in Gibbs free energy vs. change in enthalpy
When learning about other thermodynamic quantities, like $\Delta H$, I was taught that this represents the energy absorbed or released by a system after the reaction has gone 100% to completion. For ...
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2answers
448 views
Derivation of Gibbs free energy formula
Okay, so I'm trying to derive an important formula which states that -
āG(r) = āGĀ° + RTln( RQ )
where āG(r) is the instantaneous rate of change of Gibbs energy with ...
2
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2answers
66 views
Where does Free Energy go in a Redox Reaction?
This doubt came to my mind while answering the following question:
Why does a voltaic cell not operate unless the two separate compartments are connected by an external circuit?
The answer seems ...
3
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1answer
188 views
Gibbs free energy of formation vs. Gibbs free energy of reaction?
Consider the following reaction:
$$\ce{A(s) + B(s) <=> AB(s)}.$$
I know there might be kinetic limitations but I am only interested in thermodynamic relations.
In general, the Gibbs free ...
4
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2answers
2k views
Gibbs Free energy of a reversible reaction
From what I understand, it sounds like the Gibbs Free Energy change of a reversible reaction at equilibrium is zero. However, since I know that Gibbs Free energy change depends on temperature, does ...
2
votes
2answers
707 views
Gibbs free energy in standard state vs. equilibrium
I have a problem with the definition of the standard Gibbs energy and its connection to the equilibrium constants.
I think, that I've basically understood what the different equation mean but there ...
