58
votes
Accepted
What is the difference between ∆G and ∆G°?
Short answer
Does it need to be at $25~^\circ\mathrm{C}$?
No. $\Delta_\mathrm{r} G^\circ$ can be defined at any temperature you wish to define it at, since the standard state does not ...
- 71.1k
24
votes
Accepted
Which equilibrium constant is appropriate to use?
As noted in this previous question, the correct definition of the equilibrium constant $K$ depends on activities. If you are interested in the derivation of the equation $\Delta G^\circ = -RT \ln K$ (...
- 71.1k
24
votes
Can single molecules of C and O2 react in isolation, and if so how will momentum be conserved?
$\ce{C + O2}$ is awfully complicated, so let's just pretend you've asked this:
In a single act of the reaction $\ce{H. + H .-> H2}$, how is momentum conserved?
That's a legitimate concern all right....
- 31k
19
votes
Why is entropy favorable?
Thermodynamics. The second law of thermodynamics states that entropy always increases in an isolated system. This is taken as a fundamental postulate---we simply accept this statement as a fact ...
- 5,431
15
votes
Accepted
What is the difference between ΔG and ΔrG?
Not for the faint-hearted: There is an excellent, but very mathsy, article here: J. Chem. Educ. 2014, 91, 386 describing the difference.
The Gibbs free energy change, $\Delta G$
You are quite right ...
- 71.1k
15
votes
Accepted
Why proton concentration is divided by 10⁻⁷?
The textbook is precisely correct.
The equilibrium constant $K$ which the logarithm is taken of is dimensionless, and includes activities or fugacities, and not concentrations and pressures.
In ...
- 37.4k
15
votes
Accepted
Why would there be a non-zero Gibbs energy of mixing for ideal gases?
When we talk about mixing, we usually mean that two components are in different parts of a container before mixing and then they share the entire container after mixing. In other words, the total ...
- 39.1k
14
votes
Accepted
Is there a way to experimentally measure entropy?
The most common way of measuring $\Delta S^\circ$ for a chemical reaction is probably by making a van't Hoff plot. You measure the equilibrium constant $K$ at different temperatures and plot $\ln K$ ...
- 4,799
13
votes
Accepted
Why is entropy favorable?
It appears you're looking for an ELI5-style answer, not an elaborate definition.
Entropy just happens – as long as the universe isn't frozen solid, things will always be moving around, and that ...
- 248
13
votes
Accepted
If change in free energy (G) is positive, how do those reactions still occur?
$\Delta G^\circ_m$ is the difference in molar Gibbs free energy between the reagents and products in their standard states (in the case of $\ce{AgI(s)}$, the standard state for the reagent is the pure ...
- 20.8k
11
votes
Why exactly are standard potentials additive?
Following on Derek's great answer, it is very important to remind that the conventional way we use to add half-cell potentials is a consequence of the conservation of energy. Therefore, we should look ...
- 301
11
votes
True or false: "If a reaction has a large negative value of ∆G, then it will be a fast reaction."
There is the Bell–Evans–Polanyi principle stating that "the difference in activation energy between two reactions of the same family is proportional to the difference of their enthalpy of ...
- 987
10
votes
Why is entropy favorable?
Do not think of entropy as 'disorder' as this is misleading, better is that it is a 'measure of disorder' but this is equally vague. It is better to think of entropy as the number of ways that '...
- 30.1k
9
votes
When is a reaction reversible?
"Reversible" is not binary. Both the forward and backward reactions always occur and the equilibrium system never has zero reactants or zero products. Thus, irreversible reactions are called this ...
- 209
9
votes
What is the difference between ∆G and ∆G°?
I think the answer to these questions can be simplified considerably. My answer would start out with the part of orthocresol's answer which says:
What exactly, then, is $\Delta G^\circ$? The truth ...
- 13.1k
9
votes
Why at constant pressure and temperature Gibbs energy change of a process can be negative?
The relation
$$\mathrm dG = V\,\mathrm dp - S\,\mathrm dT\tag{1}$$
implies that the Gibbs free energy of the system depends only on the two variables $T$ and $p$, e.g., as in the case of a single-...
- 723
9
votes
Why would there be a non-zero Gibbs energy of mixing for ideal gases?
With your reply you seem to have answered part of your question. I would like to add a bit about the other part. I will have to do it mathematically first, there is no other shortcut, but then we will ...
- 2,246
8
votes
Accepted
Why can't a reaction go to completion?
You alluded to the answer when you mention activation energy. Kinetically the equilibrium constant is $K_e = k_f/k_b$ where $k_f$ and $k_b$ are the forward are reverse reaction rate constants in the ...
- 30.1k
8
votes
How to estimate the temperature needed to overcome an activation energy barrier?
You could convert the rate constant($k$) to half-life($t_{1/2}$) which would give you an idea of the time scale required for the reaction to finish at a certain temperature.
The equation to obtain ...
- 5,647
8
votes
How to estimate the temperature needed to overcome an activation energy barrier?
From the Eyring equation, we can simply calculate the $k$ value for it.
\begin{align}
k &= \frac{k_\mathrm{b} T}{h}\exp\left(\frac{-\Delta G^\ddagger}{RT}\right)\\
k_\mathrm{b} &= \pu{1.38E-9 ...
- 2,037
7
votes
Is there a reason for the mathematical form of the equilibrium constant?
I will make "the proof" mentioned by Silvio Levy. Let's consider a chemical reaction, written as:
$$\sum_{i} \nu_i \ce{A}_i \longrightarrow 0$$
where $\nu_i$ are the stoichiometric coefficients of ...
- 4,167
7
votes
Accepted
Why is $\Delta G = -T \Delta S_{\mathrm{total}}$ valid only at constant pressure?
You have actually assumed in your equations that both pressure and temperature are constant. Firstly, we have that
$$\mathrm{d}S_\text{total} = \mathrm{d}S_\text{system} + \mathrm{d}S_\text{...
- 7,021
7
votes
Accepted
Pressure at which graphite and diamond are in equilibrium
For each phase $i$ (graphite or diamond) you can show that
$$\mathrm{d}\mu _i = V_i \mathrm{d}p - S_i \mathrm{d}T$$
or after integration
$$ \mu_i = \mu_i^\circ+\int_{p^\circ}^{p} V_{i} dp $$
at ...
- 20.8k
7
votes
Gibbs Free Energy and Maximum Work
In order for a process to happen, it has to increase the combined entropy of the system in which it happens and of the surrounding (2nd law of thermodynamics). As we will see in a bit, the more work ...
- 39.1k
7
votes
Is temperature double-counted in the Gibbs free energy equation?
One reason we write equations this way is because all of the parameters on the right-hand-side can be measured or computed independently.
Consider an analogous (but more intuitive) equation, the ...
- 20.8k
7
votes
Accepted
Total Differential of Chemical Potential
$$dG=VdP-SdT-\sum{\mu_in_i}$$So, $$\frac{\partial G}{\partial P}=V$$and $$\frac{\partial G}{\partial n_i}=\mu_i$$So, $$\frac{\partial^2 G}{\partial n_i \partial P}=\frac{\partial^2 G}{\partial P\...
- 13.1k
6
votes
What is the difference between ∆G and ∆G°?
I see it in different way. You are right in most textbooks $\Delta G^0$ means free energy change at $25~\mathrm{^\circ C}$ but in this case you have to think in a different way. I see it as ...
- 2,467
6
votes
Is there a way to experimentally measure entropy?
The entropy change between two thermodynamic equilibrium states of a system can definitely be directly measured experimentally. To do so, one needs to devise (dream up) a reversible path between the ...
- 13.1k
6
votes
Accepted
How can a nonspontaneous reaction occur?
When you add energy from somewhere, let's say a battery, you are creating a greater amount of entropy there (in the battery or whatever). That's all there is to it. The Second Law is not violated.
The ...
- 71.1k
6
votes
How to derive the Gibbs free energy for an ideal gas?
The fundamental relation for an Ideal Gas, in the entropy representation is
$$S(U,V,N) =
NS_0 + NR\ln\left[ \left( \frac{U}{U_0} \right)^c
\left( \frac{V}{V_0} \right)
...
- 8,465
Only top scored, non community-wiki answers of a minimum length are eligible