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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Why can one set the Gibbs free energy change to 0 when calculating the standard free energy change?

I am trying to understand the following equation: $$\Delta G = \Delta G^{\circ} + RT \ln\left(\frac{[C][D]}{[A][B]}\right)$$ for a reversible reaction with reactants A and B and products C and D. The ...
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Finding reaction equilibrium constant from Gibbs free energy and enthalpy

For the reaction $$\ce{3 Fe2O3(s) <=> 2 Fe3O4(s) + 1/2 O2(g)}$$ $\Delta_\mathrm{r}H^\circ(\pu{298 K}) = \pu{55.5 kcal mol^-1}$ and $\Delta_\mathrm{r}G^\circ(\pu{298 K}) = \pu{46.5 kcal mol^-1}.$ ...
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Gibbs energy of electron transfer

the reaction Gibbs energy of a cell: $\Delta_rG^o=-nFE^o$ and the Gibbs energy of photoinduced electron transfer: $\Delta_{ET}G^o=N_A\{e[E^o(D^{+.}/D)-E^o(A/A^{-.})]+\omega(D^{+.}A^{-.})-\omega(DA)\}-\...
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True or false: "If a reaction has a large negative value of ∆G, then it will be a fast reaction."

I recently took a Gen. Chem. 2 exam that contained this question. I answered false, as I knew that thermodynamics and kinetics are separate matters. My professor said the answer is true. I pressed him ...
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Loss of entropy and solvation energy in proteins

I am reading a chapter on protein stability. One section outlines the role of salt bridges or ion pairs in the stability of a protein. The excerpt goes as follows (italization added for emphasis): ...
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How to find the temperature for when Kc = 1

The formulas I can use are $\Delta$G = H - T$\Delta$S, and $\Delta$G = $\Delta$G(s.t.d) + RTlnK I understand that when Kc is 1, the reaction is barely spontaneous and thus I can use the first formula ...
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Heat capacity at constant volume and Gibbs free energy

I want to know if it is possible to derive heat capacities, in this case, in constant volume from another thermodynamic Potential which is not the Helmholtz free energy $F$. I am aware of the ...
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Does this coupled reaction actually happen (decomposition of calcium carbonate)

I found the following example of a coupled reaction to drive the decomposition of calcium carbonate. I get the calculation part of it, that the changes in Gibbs energy sum to a negative amount. But ...
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107 views

Why is the Gibbs free energy of a reversible reaction not always 0? [duplicate]

From my understanding, reversible proceses are those where the expression: $$\Delta S_\mathrm{total} = \Delta S_\mathrm{system} + \Delta S_\mathrm{surroundings} = 0$$ is true for all for the entire ...
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At equilibrium, which one of the following is always Zero? [duplicate]

At equilibrium, which one of the following is always zero? (A)∆H (B)∆S(Total) (C)∆S(System) (D)∆G° I know that at equilibrium ∆G=0, and was facing difficulty in connecting the above mentioned ...
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How switching values of $R$, $K_p$ and $K_c$ does not alter the value of change in standard Gibbs energy? [duplicate]

$\Delta_\text r G=RT\ln \frac{Q}{K}$ If we switch $K_p$ from $K_c$, accordingly $Q_p$ will change to $Q_c$. Therefore, value of $\Delta_\text r G$ remains the same. $ΔG^\circ=−RT\ln K=ΔH^\circ−TΔS^\...
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Equilibrium chemical potential if the population is not conserved?

The chemical potential, $\mu$, is equal to the free energy $F$, per photon. $$F = \mu = u-Ts$$ The energy per photon is $u=h\nu$ and $s$ is the entropy per photon. Say we have a process where an ...
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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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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 ...
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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 ...
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80 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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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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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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198 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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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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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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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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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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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 $$k_\mathrm{r} = \frac{\kappa k_\mathrm{B}T}{h}\exp\left\{\frac{- \Delta G^\ddagger}{RT}\right\},$$ where $\Delta G^\...
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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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How does DeepHF (a CRISPR sgRNA design tool) compute binding free energy?

I am looking at the code of DeepHF[1] that computes $\Delta G_\text{binding}$, and I see that it breaks a sequence into overlapping dimers and compute a weighted sum according to a dictionary with 16 ...
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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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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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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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219 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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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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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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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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