23 votes

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 ...
user avatar
19 votes
Accepted

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 ...
user avatar
  • 29.6k
14 votes
Accepted

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 ...
user avatar
  • 405
13 votes

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 ...
user avatar
  • 7,591
13 votes
Accepted

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 ...
user avatar
  • 10k
13 votes
Accepted

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 ...
user avatar
12 votes

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 ...
user avatar
  • 65.6k
11 votes
Accepted

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 ...
user avatar
  • 81.6k
11 votes

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 ...
user avatar
11 votes

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,...
user avatar
  • 17.2k
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 ...
user avatar
  • 38.8k
11 votes
Accepted

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 ...
user avatar
10 votes

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 ...
user avatar
10 votes
Accepted

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 ...
user avatar
  • 16.8k
10 votes
Accepted

What is the bond energy of H-O? Wikipedia disagrees with itself

According to the 97th edition of the CRC Handbook of Chemistry and Physics, table 1 Bond Dissociation Energies in Diatomic Molecules [1, p. 9-76]: $$D^\circ_{298}(\ce{H-O}) = \pu{(429.74\pm 0.03) kJ ...
user avatar
  • 34.7k
10 votes

What is an example of an exothermic reaction with negative ∆S?

A very simple reaction which is readily accessible for study is the dimerisation of nitrogen dioxide: $$\ce{2 NO2(g) -> N2O4 (g)}$$ This process is has a standard reaction enthalpy change of $\rm{\...
user avatar
9 votes

at constant volume why the change of enthalpy doesn't equal the change of heat

At constant volume, the pressure is not necessarily constant. The pressure will usually increase when the system is heated. This makes the $PV$ term not constant. For this reason, you get: $$\Delta H ...
user avatar
  • 248
9 votes
Accepted

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 $\mathrm {100\ kcal/mol}$, ...
user avatar
  • 2,480
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/
user avatar
  • 1,711
9 votes

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 ...
user avatar
  • 23.2k
9 votes

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 ...
user avatar
9 votes

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 ...
user avatar
  • 34.7k
9 votes
Accepted

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 ...
user avatar
  • 11.5k
9 votes
Accepted

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, ...
user avatar
  • 27.2k
8 votes

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 ...
user avatar
  • 10k
8 votes
Accepted

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{...
user avatar
  • 17.7k
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 ...
user avatar
  • 17.7k
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 ...
user avatar
  • 20.2k
8 votes
Accepted

Can negative (abnormally) lattice enthalpies during the formation of ionic compounds exist?

Short answer: Not for "normal" compounds. First off, let's consider what the lattice enthalpy means. As you've written it, these lattice enthalpy is part of the Born-Haber cycle of formation of an ...
user avatar

Only top scored, non community-wiki answers of a minimum length are eligible