52 votes
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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 ...
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30 votes
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Why does water evaporate spontaneously at room temperature despite ΔG > 0?

Standard Gibbs free energy of formation of liquid water at 298 K is −237.17 kJ/mol and that of water vapour is −228.57 kJ/mol. Therefore, $$\ce{H2O(l)->H2O(g)}~~\Delta G=8.43~\mathrm{kJ/mol}$$ ...
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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....
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20 votes
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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 $\Delta G^\circ = -RT \ln K$ (...
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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 ...
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16 votes
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What is non-expansion work?

Pressure-Volume work is a type of mechanical work . There are also non-mechanical forms of work where in pressure and volume terms are not involved . The most famous example is electrical work . The ...
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  • 768
15 votes
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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 ...
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13 votes

Is there a reason for the mathematical form of the equilibrium constant?

Equilibrium is also the state in which the forward and reverse reactions are proceeding at equal rate. Consider a hypothetical reaction $$\ce{aA + bB} \overset{k_1}{\underset{k_{-1}}{\ce{<=>}}}...
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13 votes
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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 $\ln K$ ...
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  • 4,759
13 votes
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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 ...
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  • 248
13 votes
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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 ...
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12 votes

Why does water evaporate spontaneously at room temperature despite ΔG > 0?

There are two considerations about why water evaporates but only one of them has to do with the Gibbs free energy of vaporisation. As the first answer correctly states, there is an equilibrium (...
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12 votes

Is the Gibbs standard free energy always constant?

In short, no, the standard Gibbs free energy change is not constant; it is a function of temperature. The same is true for practically all other standard-state quantities. This gets a little ...
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  • 780
11 votes

How can enthalpy change of a system be negative while entropy change is positive?

For example, suppose you have a solid block of TNT. It explodes and releases much energy. $\Delta H$ is negative. Gaseous products like nitrogen, carbon dioxide and water vapor are formed. The ...
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10 votes
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Why is Gibbs free energy more useful than internal energy?

Thanks for the edit - I see now what you meant. The short answer for why we "need" Gibbs energy is that the internal energy for a spontaneous process at constant temperature $T$ and pressure $p$ does ...
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10 votes
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What's the difference between ΔG° and ΔG°'?

The prime usually denotes a standard free energy that corresponds to an apparent equilibrium constant where the concentration (or activity) of one or more constituents is held constant. For example, ...
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10 votes
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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 ...
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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 '...
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10 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 ...
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9 votes

Where to find data for Gibbs energy, enthalpy, and entropy?

NIST webbook does have a lot of data, though they are not in any kind of an API form as far as I know. http://webbook.nist.gov/chemistry/
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9 votes

Why does water evaporate spontaneously at room temperature despite ΔG > 0?

$\Delta G = -RT \ln K$ Which means there for any given process an equilibrium constant can be deduced. Given the products are 'gaseous' they will escape the system, and further drive the equilibrium....
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  • 6,458
9 votes

Why does the Gibbs free energy only correspond to non-expansion work?

The association of the Gibbs free energy with “additional”, or “non-expansion” work, is simply a mathematical result of its definition: $G = H - TS = U + pV - TS$. From the First Law, for a closed ...
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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 ...
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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 ...
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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-...
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  • 713
8 votes

Is there a reason for the mathematical form of the equilibrium constant?

The answer is both. The formula for the equilibrium constant was first observed experimentally (that is, it's empirical) for many many reactions. Then it was justified theoretically in terms of what ...
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  • 2,677
8 votes

What is the difference between enthalpy of formation and Gibbs free energy of formation?

Gibbs free energy is not more "precise." The two are different but complementary. Enthalpy and Gibbs Free Energy indicate different things. Enthalpy can tell you about the relative stabilities of the ...
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8 votes

Where to find data for Gibbs energy, enthalpy, and entropy?

NIST is the best place to turn for lots of data. However, more easily parsed, smaller datasets are available in a couple of other locations. The CHNOSz package in ...
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  • 20.2k
8 votes

Why is the melting of ice a spontaneous reaction?

First of all, what exactly is spontaneous reaction? In very easy language, reaction that occurs in a given set of conditions without intervention is called spontaneous reaction. Now let us ...
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8 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 ...
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