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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2answers
573 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 ...
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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, ...
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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 ...
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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 ...
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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. ...
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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 ...
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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 ...
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Standard Gibb's Free energy change, standard state and equilibrium

I just want to confirm if I am right in thinking that: statement: When a reaction in standard state is in equilibrium, then equilibrium constant K=1 Reason: since in a equilibrium, $Q=K$ and $ΔG=0$. ...
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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
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1answer
145 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 ...
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73 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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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 ...
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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 ...
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1answer
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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/...
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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 ...
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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}\\ \...
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2answers
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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}\...
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1answer
108 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 ...
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1answer
59 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 ...
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425 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 ...
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1answer
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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
184 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 ...
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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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1answer
150 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^\...
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1answer
154 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 ...
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1answer
71 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
89 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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1answer
281 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....
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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
66 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
158 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 ...
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60 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 ...
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1answer
112 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
105 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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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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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
116 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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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
64 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 ...
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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
127 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
521 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
53 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
52 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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63 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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