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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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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11 views
Electrolysis of water and Gibbs Energy
I'm writing a chemical equilibrium program which minimizes the Gibbs energy. The program accepts as input the total amounts of the elements comprising the system, and a list of possible molecular ...
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3answers
65 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:
$$reduction: \ \ \ce{2H_{3}O^{+} +2e^{-} ->...
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1answer
39 views
Ground state elevation for phase transition
I have a question about the illustration from chem.libretexts.org:
the ground states of the liquid and gas are offset from that of the previous state by the heats of fusion and vaporization, ...
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69 views
Reaction energies of solids from quantum chemistry?
I was reading a paper by Catlow et al. [1], and they mentioned the dissolution of copper into copper ions, which is given by this equation:
$$\ce{Cu2O(s) + 2 h+ <=> Cu+(aq) + 1/2 O2(g)} \tag{...
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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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75 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 ...
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115 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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43 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 $\pu{700 K}$ ...
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1answer
121 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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45 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 ...
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52 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 ...
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1answer
53 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
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1answer
42 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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52 views
Finding the temperature at which the system reaches equilibrium
I’m really stuck on this problem. This involves dissolving Borax:
$$\ce{Na2B4O7•10H2O(s)<=>2Na^+(aq) + B4O7^2-(aq) + 10H2O}$$
$$\begin{align}\Delta H&=\pu{109 kJ/mol} \\ \Delta S&= \pu{...
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590 views
How to explain disagreement between Le Châtelier's principle and the simplified Gibbs free energy equation?
For example, for the dissolution of a salt in water that is exothermic, heating the solution would drive the reaction towards the solid form of the salt according to Le Châtelier's principle.
However,...
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70 views
How to calculate the variation of the standard reaction enthalpy for the CO, CO2, H2O, H2 equilibrium?
For the reaction
$$\ce{CO2 (g) + H2 (g) <=> CO (g) + H2O (g)}$$
the equilibrium constant $K_p$ has a value of $0.534$ at $\pu{960 K}$ and $1.571$ at $\pu{1260 K}$. Calculate the variation of ...
3
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1answer
799 views
Relation between equilibrium constant and entropy change
Like the Van't Hoff equation, which relates change in enthalpy to equilibrium constant, is there a similar equation for the relation between change in entropy and equilibrium constant?
Consider the ...
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2answers
1k views
Reaction quotient and Gibbs free energy at the start of a reaction
According to the equation 33 above, shouldn't be all the reactions be spontaneous initially? Because the reaction quotient Q, is zero at the start of the reaction and logarithm of Q makes it a very ...
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432 views
If change in free energy (G) is positive, how do those reactions still occur?
I was doing a couple of problems for homework:
Calculate $K_\mathrm{sp}$ of $\ce{AgI}$ at $55.0\ \mathrm{^\circ C}$
Calculate $K_\mathrm{b}$ of $\ce{NH3}$ at $36.0\ \mathrm{^\circ C}$
I have to use ...
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48 views
Gibbs energy at a certain temperature
For the reaction
$$\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....
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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
41 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
147 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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39 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
214 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
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1answer
346 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
384 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 $-∆G^\circ/R$ and a $y$-intercept of $0$. I prove it simply:
$$∆G^\circ = -RT\ln K \quad\to\quad \ln K = -\frac{∆G^\circ}{RT}$$...
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1answer
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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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Derivation of Gibbs free energy formula
Okay, so I'm trying to derive an important formula which states that -
∆G(r) = ∆G° + RTln( RQ )
where ∆G(r) is the instantaneous rate of change of Gibbs energy with ...
2
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2answers
48 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 ...
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1answer
182 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 ...
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2answers
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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 ...
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2answers
448 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 ...
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286 views
Conversion of graphite into diamond [duplicate]
The standard state Gibbs free energies of formation of C(graphite) and C(diamond) at $T = \pu{298 K}$ are $\pu{0 kJ mol-1}$ and $\pu{2.9 kJ mol-1}$, respectively.
The conversion of graphite [C(...
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Is there any sort of correlation between melting point and the degrees of freedom for the molecules of a substance?
Suppose I had a solid whose molecules did not have rotational freedom. But if I were able to make it so that the molecules had rotational freedom ceteris paribus, is there a way to tell how the ...
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2answers
179 views
What is the Gibbs free energy difference between conformational states
In statistical mechanics, for a system of $N$ particles $x_1, \ldots, x_N$ in three dimensions, the Gibbs free energy is defined in terms of the Hamiltonian $H$ as
$$ G = -k_\mathrm B T \log \int_{\...
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2answers
857 views
Why proton concentration is divided by 10⁻⁷?
I am reviewing the book Biochemistry Concepts and Connections by Appling, Cahill, and Mathews and I cannot understand why they divide by the hydrogen concentration by $10^{-7}$. Why not just leave it ...
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1answer
394 views
Find the minimum temperature at which the reaction will become spontaneous [closed]
This question came in my entrance exam and I think 273 K should be the answer because the reaction is already spontaneous. I just want to know if I've attempted it correctly.
For a certain ...
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1answer
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Pressure at which graphite and diamond are in equilibrium
I came across this problem :
The standard state Gibbs free energies of formation of C(graphite) and C(diamond) at $T = \pu{298 K}$ are $\pu{0 kJ mol-1}$ and $\pu{2.9 kJ mol-1}$, respectively.
...
1
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1answer
151 views
Calculate Gibbs energy of dinitrogen pentoxide using its vapor pressure and sublimation
I am having trouble with the following problem:
Dinitrogen pentoxide, $\ce{N2O5}$, is a solid with a high vapor pressure. Its vapor pressure at $\pu{7.5 °C}$ is $\pu{100 mmHg}$, and the solid ...
3
votes
1answer
212 views
Free energy of solvation and Henry's law constant
I am trying to calculate the free energy of solvation of $\ce{CO2}$ from its Henry's law constant. As given on Wikipedia, the dimensionless Henry's law constant is $0.83$. If I try to calculate $\...
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1answer
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How to calculate the standard Gibbs energy at equilibrium?
Given that $K_c = 1.7 \times 10^{-13}$, calculate $\Delta G^{\circ}$ for this equilibrium mixture at $\pu{298 K}$.
$$\ce{N2O (g) + \frac{1}{2}O2 (g) <=> 2 NO (g)}$$
I've calculated:
$$
\...
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Why does standard reduction potential get multiplied by the change in oxidation state and not by the number of electrons transferred?
The whole basis of Frost diagrams is to have a graphical representation of the Gibbs free energy of formation ($\Delta{G}_{f}$) of different oxidation states of an element using the proportional ...
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2answers
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How to derive the relation between gibbs energy and equilibrium constant?
I want to understand the derivation between gibbs energy and equillibrium constant $$\Delta G=\Delta G^o+RT\ln Q?$$
I have seen a similar post on CSE Derivation of relationship between equilibrium ...
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1answer
155 views
Connection between Gibbs Energy of Formation and Gibbs Energy of Mixing
Recently I found the following formula, which I cannot verify to be correct. Since I found this formula in several publications I assume that it is correct but I don't know why.
First there is the ...
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202 views
Gibbs Free Energy: Exothermic, low temperature & reduced system entropy - such reaction is spontaneous or not?
According to $\ce{\Delta G^\mathrm{o}= \Delta H^\mathrm{o} -T\Delta S^\mathrm{o}}$, if a reaction
a) is exothermic,
b) reduces system entropy, and
c) the temperature is low,
then it is spontaneous....
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1answer
105 views
Is the stoichiometric coefficient equivalent to the mole fraction?
I have a small problem to understand (or distinguish between) two concepts of free mixing enthalpy.
In several textbooks, I found the following formula for the molar free mixing enthalpy (binary ...
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Calculating the free energy of separation in AMBER
I am currently busy with my master's research project and I'm trying to calculate the free energy of separation of two cellobiose molecules in ionic liquid, that is, calculate the free energy of ...
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2answers
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Proof of the chemical potential formula
My textbook as well as Wikipedia both state the formula $\mu_A = \mu_A^0 + RT\ln\{A\}$ which gives the chemical potential $\mu_A$ of a chemical species $A$ as function of the standard chemical ...
