# Tag Info

### Is the S–O bond strength in sulfur trioxide or sulfite anion larger?

Due to symmetry constraints ($D_\mathrm{3h}$) in $\ce{SO3}$ there 6 electrons in $\pi$ type orbitals. In a wider sense of the term this molecule is Y-aromatic, but the HOMO actually represents two ...
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### Why is the standard enthalpy of formation of black phosphorus not zero?

Reproducibility is more important than stability. To obtain black phosphorus, you have to heat your sample under high pressure for quite a while, and even then it may still contain a significant ...
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### Enthalpy definitions. What are their main differences?

Explanation of notation: $H$ is the enthalpy of the system. $\Delta$ means change of, so $\Delta H$ means change of the enthalpy This symbol means standard condition, standard condition is defined as ...
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### Why is the standard enthalpy of formation of diamond not zero?

You are on the right track - diamond is not the thermodynamically stable carbon phase at STP. Taking two figures from A.T. Dinsdale, 'SGTE Data for Pure Elements', CALPHAD 15(4) 317-425 (1991) one ...
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### Why are there different values for enthalpy of combustion, depending on the calculation method?

Average bond enthalpies (note the word "average") are calculated in a different way from formation enthalpies. Formation enthalpies are well-defined and precise, meaning that two different people ...
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### Are precipitations exothermic and/or endothermic?

No, absolutely not. Precipitation reactions can be either endothermic and exothermic. Table 1. Thermodynamic data of precipitation for some salts \begin{array}{cccccc} \hline \text{Salt} & \Delta ...
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### Why are there different values for enthalpy of combustion, depending on the calculation method?

Because calculation with bond energies are notoriously imprecise. Those bond energies that you use do not reflect the exact C-C bond energy in ethanol, for example. It is merely an "average" C-C bond ...
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### Will heating diatomic oxygen enough break the O=O bonds?

Yes, adding at least 495 kJ/mol of kinetic energy one way or another (thermally, photochemically by irradiation with photons of that energy, sonication, etc.) will cause $\ce{O2}$ to dissociate into ...
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### Enthalpy of formation and reaction equations

As far as I can tell, your reasoning is sound, though the question is actually not written spectacularly well. By "enthalpy of the reaction" it seems to be implicitly assuming that the stoichiometric ...
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### Why is the standard enthalpy of formation of elements in their native forms zero?

It is purely a matter of definition. A standard enthalpy of formation describes the change in enthalpy during the formation of 1 mol of a target compound by reacting the (pure) elements it consists of,...
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### 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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### Why does spontaneity of a reaction depend on temperature?

The issue here is in your definition of entropy. According to your definition: $$\mathrm dS = \frac{\text{đ}q}{T}.\tag{1}$$ However, there is a small difference. As you may know, the entropy of a ...
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### What kind of chemicals would exist at the heat death of the universe?

There is a lot of uncertainty regarding the far future of our Universe, but it seems that chemistry as we know it will be gone long before the end. Both free and bound protons (and neutrons) are ...
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### How is it possible for a substance to have a high heat of vaporization but a low boiling point?

This seeming contradiction can be reconciled by examining the thermodynamic quantities involved. First, per Wikipedia, the enthalpy of vaporization is "the enthalpy change required to transform a ...
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### Is a carbon-fluorine bond stronger than a carbon-chlorine bond?

For free radical reactions, the most important parameter in assessing bond strength is bond dissociation enthalpy (BDE). Typical values for $\ce {C-F}$ bonds are around $\mathrm {100\ kcal/mol}$, ...
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### 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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### How to calculate an enthalpy change for this reaction

To preface, the reason that this is not a combustion reaction is because this is a redox reaction that occurs at room temperature. In fact, chloroform is noncombustible. Though at first glance, this ...
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### Is it possible to freeze water by dissolving a salt?

It is possible, in a significantly different way that you envision. Freezing point depression via the cryoscopic constant is an example of a colligative property, which holds only for relatively ...
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### Calculating the strength of an ionic bond that contains poly-atomic ions

With enough effort, Born–Haber cycle can be extended to polyatomic ionic solids, however it's practically never done in practice due to the lack of experimental data or because it's impossible to ...
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### Infinite Increase in Entropy when Energy added to Absolute Zero

The textbook is referring to the entropy change of the system. While the textbook is correct that absolute zero can never be attained, its statement that the entropy change is infinite is wrong. The ...
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### Why is delta-H negative here?

The bond enthalpy is conventionally the enthalpy change of bond breaking, not of bond forming, in the contrary to formation enthalpy of compounds. This way they are all positive. The consequence is, ...
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### Why is the enthalpy of vaporization greater than the enthalpy of fusion?

Enthalpies of phase changes are fundamentally connected to the electrostatic potential energies between molecules. The first thing you need to know is: There is an attractive force between all ...
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### Comparing formula for enthalpy change with bond dissociation energy and formation enthalpy

Yes. The two methods are equivalent. You can try this with the dimerization of $\ce{NO_2}$ into $\ce{N_2O_4}$. In this reaction we form one $\ce{N-N}$ bond. Every other bond stays the same. \$\ce{...
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### 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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### 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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