Questions tagged [free-energy]
Also known as Gibbs energy, it is the enthalpy minus the product of thermodynamic temperature and entropy.
305
questions
-2
votes
1
answer
48
views
How come ΔG∘ is related to the equilibrium constant?
As I understand it, standard state is when the pressure of a gas is 1 bar and quantity 1 mol.
Now the following relation holds:
$$\Delta G^\circ=-RT \ln(K)$$
Suppose I declare a new standard state to ...
0
votes
0
answers
39
views
Gibbs Free Energy, Temperature and Equilibrium [duplicate]
I was recently taught one of the Gibbs Free Energy formulas, where the change in Gibbs Free Energy is related to the change in enthalpy and entropy
∆G = ∆H - T∆S,
and that the system is at equilibrium ...
- 11
-1
votes
1
answer
33
views
If the electron affinity for a neutral element is positive (i.e. endothermic process), is the anion more stable than the neutral atom (at STP)?
This might be a bad question, but I have to ask because I'm finding conflicting answers online. Let's say that I have the following reaction: $$O + e^-\rightarrow O^-$$ While I understand that the ...
0
votes
0
answers
51
views
Change in Gibbs energy for lithium redox reaction
I was reading an electrochemistry book by Thomas F. Fuller, in this book the thermodynamic values for the lithium redox reaction at standard conditions are the following:
Reaction
$\Delta G^o$ [kJ/...
- 289
4
votes
1
answer
104
views
Gibbs energy of an imperfect crystal
Our materials professor likes to do Kahoot quizzes and I am confused about why the following statement is true:
A defect in a crystal gives it higher energy.
I know that an imperfect crystal has a ...
- 211
3
votes
1
answer
101
views
Calculation of Reaction Extent as a Function of Pressure and Temperature: Issue with Activity-Based Approach
I'm working on modeling reactions involving combinations of hydrogen (H), nitrogen (N), and strontium (Sr) elements. I've obtained the temperature-dependent Gibbs free energy of formation, enthalpy, ...
- 31
0
votes
0
answers
40
views
Confused about free energy equation and entropy change
I'm confused about two equations and how they relate to each other. These are $\Delta S = \Delta Q/T$ and $\Delta G = \Delta H - T\Delta S$.
To derive the Gibbs' free energy change for the universe ...
- 11
0
votes
1
answer
148
views
How does cell potential change with temperature for positive enthalpy and negative entropy?
A recent United States National Chemistry Olympiad question asked:
A certain voltaic cell has a standard cell potential that increases with increasing temperature. Which best explains this ...
- 337
1
vote
0
answers
59
views
Define a rate-determining step and an energy barrier for a multi-step reaction profile
I am currently making reaction profile by means of DFT calculations. I am able to correctly draw a whole reaction profile.
However, I am struggling to find what exactly is the energy barrier of a ...
- 398
0
votes
3
answers
127
views
Definition of Gibbs Free Energy and its interpretation [closed]
RELATIONSHIP BETWEEN $\Delta G$ & $W_\mathrm{non-PV}$
$dU = dq + dw_\mathrm{py} + dw_\mathrm{non-PV}$
for reversible process at constant T & P
$dU+pdV-TdS = dw_\mathrm{non-PV}$
$dH-TdS = dw_\...
- 37
1
vote
0
answers
85
views
Why doesn't acid dissociation always have K>1?
Acid dissociation is usually exothermic, and it has an increase in entropy, so the change in Gibbs free energy should be negative. So why isn't always $K_\mathrm{a}>1$ in accordance with $\Delta G^...
- 337
2
votes
1
answer
207
views
Change in entropy with micelle formation
Micelles are formed only above the Kraft temperature.
We know ∆G = ∆H - T∆S (Gibbs energy relation).
For micelle formation to be spontaneous, ∆G has to be negative.
This implies ∆H - T∆S has to be ...
- 171
-2
votes
1
answer
126
views
Inconsistency in between Nernst Equation and Gibbs Free Energy Equation [closed]
Part 1 - Derivation of the Gibbs Free Energy Equation: [copied from this]
Using the fundamental equations for the state function (and its natural variables):
\begin{align}
\mathrm{d}G &= -S\...
0
votes
0
answers
53
views
Spontaneity of irreversible reactions
I saw this reaction in my textbook:
$\ce{NH4Cl(s) -> NH3(g) + HCl(g)}$ given $\Delta{H}_{r} = \pu{176 kJ/mol}$ and $\Delta{S} = \pu{0.285 kJ/(mol \cdot K)}$
On a worksheet my teacher gave me, I saw ...
- 9
0
votes
1
answer
118
views
What is the difference between standard Gibbs energy and Gibbs energy of formation?
Good day guys,
I am reading a book on electrochemical engineering, I came across some definitions of Gibbs energy and I am a bit confused as the book does not go into a lot of detail.
The book first ...
- 289
0
votes
0
answers
46
views
Justifying that Standard Entropy of a Reaction is a Constant of Temperature
One of the parts in a question I'm trying to solve for an assignment goes like this:
A key issue in the Hydrogen Economy is Hydrogen Storage. This can be restored if a substance $X$ can bind to $H_2$....
- 11
-5
votes
2
answers
62
views
Why 40J energy was wasted in case of a non spontaneous process by the system? [closed]
If there is a system which while undergoing a process produced 30J energy and wasted 40J energy therefore the useful energy was -10J and this explains the process was non spontaneous because ∆G is ...
2
votes
1
answer
166
views
Does it make any sense to graph 1/T versus ln(Kc)?
I calculated $K_c$ experimentally for the reaction $$\ce{CO2 + H2O -> H2CO3}$$ at different temperatures. Because $\Delta G =-RT\ln(K)$, I thought that graphing 1/T against $\ln(K_c)$ would give me ...
0
votes
1
answer
174
views
Criterion for Spontaneity for Closed Systems at Constant Volume and Pressure
For a closed system at constant temperature and volume, the criterion for spontaneity is $dA < 0$. However, for a system at constant composition, the total differential of $A$ is given by
$$dA = -...
- 281
5
votes
1
answer
425
views
Total Differential of Chemical Potential
I have seen many sources (for example this libretext) cite that the total differential of chemical potential is
$$d\mu = V_m dP - S_m dT$$
where $V_m$ is the molar volume and $S_m$ is the molar ...
- 281
1
vote
1
answer
86
views
Why is melting spontaneous when the liquid phase is less stable and should have more free energy than the solid phase?
Imagine a 1kg block of ice that has completely melted at room temperature. If melting is a spontaneous reaction, then wouldn't that mean the products (1kg of water) would have less Gibbs free energy ...
0
votes
1
answer
154
views
What meaning does Gibbs energy acquire when applied to solutions?
I have been picturing functions like enthalpy and Gibbs free energy as mathematical tools, which were defined in such a way as to be useful for some specific cases. I understood enthalpy as a useful ...
- 87
1
vote
2
answers
177
views
Understanding the Role of Gibbs Free energy and Enthalpy as the energy required to drive a reaction
I was looking at the enthalpy change for water-splitting reaction:
$$ \Delta H^o_R = [\Delta H^0_{H_2(g)} +\frac{1}{2}\Delta H^0_{O_2(g)}]-\Delta H^0_{H_2O(l)} = \pu{285.83 kJ/mol}$$
According to ...
- 289
0
votes
0
answers
84
views
Finding ΔH and ΔS given Gibbs free energy at two temperatures
Question: Gibbs free energy of a reaction at $\ce{300 K}$ and $\pu{310 K}$ are $\pu{-28 kcal}$ and $\pu{–28.5 kcal}$
respectively. Determine $\Delta H$ and $\Delta S$ at $\pu{300 K}$.
I tried ...
0
votes
1
answer
527
views
Gibbs free energy versus reaction progress plots
I am trying to make sense of these two graphs. They appear to have similar axes, yet display different curves.
Please let me know if this understanding is correct:
In the top image (x-axis = "...
- 11
-2
votes
1
answer
165
views
Chemical Equilibrium and Sponteneity
Consider the reaction $$\ce{N2O4 <=> 2 NO2}.$$ The forward direction of this reaction is non-spontaneous and under standard conditions $\Delta G^\circ = \pu{4.76 kJ/mol}$. Suppose we begin with $...
- 281
1
vote
0
answers
92
views
Enthalpy of adsorption and adsorption rate
What I want to know is about the adsorption enthalpy change by the change of surface coverage and the adsorption rate. In general situations, adsorption would be exothermic. So in the Gibbs free ...
- 11
3
votes
0
answers
114
views
Is the change ΔG for an ideal gas unknown?
When one is introduced to the first law of thermodynamics, one learns that enthalpy is only a function of temperature, and calculating its changes is straightforward
\begin{equation}
\Delta H^\pu{...
- 3,704
1
vote
1
answer
213
views
Can ΔG = ΔG° + RT ln Q be used to calculate K at different temperatures?
Specifically, can $\Delta G=\Delta G^{\circ}+RT \ln Q$ be used to calculate reaction quotients if we choose T to be a different temperature than the one provided by ΔG°?
For example, suppose that we ...
- 111
11
votes
2
answers
1k
views
Why would there be a non-zero Gibbs energy of mixing for ideal gases?
When two ideal gases $\ce{A}$ and $\ce{B}$ mix, they don't interact. The chemical potential of each gas is independent of the other, e.g. for $\ce{A}$:
$$\mu_\ce{A} = \mu_\ce{A}^\circ + R T \ln \frac{...
- 41.3k
0
votes
2
answers
447
views
∆G or ∆G° to predict spontaneity
I have a doubt about using Gibbs free energy to predict the spontaneity of a reaction.
It is shown that ∆G = ∆G° + RT ln (Q).
That said, in order to predict which direction the reaction spontaneously ...
- 127
2
votes
0
answers
70
views
Gibbs Free Energy - Maximum Work
I've been having a lot of trouble trying to truly understand Gibbs free energy from a practical perspective. I have no background in physical chemistry, but I think I have a firm grasp on many of the ...
- 36
2
votes
2
answers
114
views
Is there an algebraic form for the textbook reaction coordinate curves?
So here is a curve often used in textbooks to illustrate a reaction coordinate.
It's a nice short-hand device to learn the relationship between kinetic and thermodynamic parameters that control the ...
- 731
6
votes
2
answers
252
views
Interpretation of Helmholtz energy and Gibbs energy
I know that this question has many other variations on this site, but I'm trying to see if I understood Helmholtz and Gibbs energy properly or not. The material I'm reading from is Physical chemistry ...
- 305
0
votes
1
answer
74
views
Why does the reaction of dissolution stop at an equilibrium point? [closed]
The formula for Gibbs free energy is $\Delta G=\Delta H-T\Delta S$.
If Gibbs free energy is negative, the reaction is spontaneous. This also applies to dissolution reactions.
However, we know that ...
- 19
0
votes
0
answers
51
views
Enthalpy and Bond Energy Sign Convention
I read that a negative enthalpy of formation means an exothermic reaction. Does that mean this reaction is true? The enthalpy of formation for $\ce{H_2O}$ is -285.8 kJ/mol.
$$\ce{2H2(g) + O2(g) -> ...
- 101
0
votes
0
answers
97
views
Why is the Gibbs free energy of titanium(IV) chloride reduction to titanium metal positive?
I use the Ellingham diagram to find the Gibbs free energy at the equilibrium state to find the partial pressure of chlorine. I have to use the Van 't Hoff reaction isotherm and the partial pressure of ...
- 21
0
votes
1
answer
430
views
Gibbs Free Energy of Ideal Gas in Carnot Cycle
Let's just say we have 2 moles of monoatomic (maybe helium) ideal gas that is doing a Carnot Cycle with reservoir temperature of 300 and 750 K.
Now, here's a simple question, what is the Gibb Free ...
- 283
3
votes
1
answer
1k
views
Is the entropy change positive or negative in this reaction?
Suppose I have a reversible reaction
\begin{align}
\ce{A(g) + B(g) <=> C(g)}\\
\end{align}
with equilibrium constant K.
Its ΔΗ is positive throughout.
Now, suppose I start with 1 mole ...
-2
votes
3
answers
110
views
In regards to the Gibbs free energy equation, it seems that the two terms oppose one another, Enthalpy and -TS. Can you explain this?
So for example, if H is endothermic, then the enthalpy term is positive, meaning energy is absorbed by system overall, meaning net bonds broken, meaning the equation leans towards nonspontaneity. If ...
- 1
0
votes
0
answers
64
views
Thermodynamics derivatives: Does $(\partial A/\partial B) = (\partial B/\partial A)^{-1}$?
If I need to find, for instance:
$$\mathcal{T} := \left(\frac{\partial U}{\partial S}\right)_{V, N}$$
knowing:
$$\left(\frac{\partial S}{\partial U}\right)_{V, N} = \frac{1}{T}$$
Can I say that $\...
- 332
0
votes
1
answer
958
views
Standard Gibbs free energy when all the reactants and products are at standard condition
I have read in my textbook that for a reaction $\Delta G=\Delta G^o-RT\ln Q $, where $\Delta G^o$ is the Gibbs free energy change when the initial concentration of products and reactants are unity. ...
- 159
5
votes
0
answers
475
views
Is the formation of nitrogen dioxide spontaneous?
I was studying Thermodynamics from my Chemistry textbook (page 181-182, section 6.6 (a)).
I have a question regarding the following text from my book.
Now let us examine the following reactions:
$$\...
- 125
0
votes
0
answers
18
views
Is the maximum work done by voltaic cell per mole of reaction? [duplicate]
From the equation $w_{elec,rev}=-nF\Delta E$, and n = the number of moles of electrons transferred in the overall redox reaction, does that make the work per mole of the redox reaction? I am confused ...
2
votes
1
answer
720
views
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 ...
- 23
2
votes
0
answers
203
views
Gibbs energy of electron transfer
The reaction Gibbs free energy of a cell is
$\Delta_\mathrm{r}G^\circ = -nFE^\circ, \tag{1}$
and the Gibbs energy of photoinduced electron transfer according to IUPAC is
$$\Delta_\mathrm{ET}G^\circ = ...
user78790
11
votes
2
answers
3k
views
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 ...
- 459
2
votes
2
answers
248
views
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):
...
- 265
0
votes
0
answers
75
views
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 ...
3
votes
1
answer
3k
views
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 ...
- 133