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19 votes
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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 ...
Ivan Neretin's user avatar
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15 votes
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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 ...
Ariana's user avatar
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14 votes
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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 ...
thomij's user avatar
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14 votes
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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 ...
William R. Ebenezer's 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 ...
orthocresol's user avatar
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11 votes
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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 ...
theorist's user avatar
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11 votes
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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 ...
Safdar Faisal's user avatar
10 votes
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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 ...
andselisk's user avatar
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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{\...
Nicolau Saker Neto's user avatar
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 ...
Nicolau Saker Neto's 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 ...
andselisk's user avatar
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9 votes
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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, ...
Poutnik's user avatar
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8 votes

C-C bond dissociation energy

The terminal bonds are stronger than the internal bonds. In simple hydrocarbons where strain or electronegative substituents are absent, the bond strengths will correlate with the stability of the ...
ron's user avatar
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7 votes
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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{...
Linear Christmas's user avatar
7 votes
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Is the change in enthalpy (ΔH) for dissolution of urea in water positive or negative?

The sign of Q depends on the perspective. The water temperature decreased because it "lost" heat. The process of dissolving urea required energy, it "gained" energy. If I give you a penny, should that ...
Karsten's user avatar
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7 votes
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Heat involved in isenthalpic process

After further reflection I realized an initial comment I made was flawed*. To quote the Wikipedia: Isenthalpic processes on an ideal gas follow isotherms, since $\mathrm dh = 0 = c_p\,\mathrm dT$. ...
Buck Thorn's user avatar
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7 votes
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Rubbing alcohol gets warm after diluting with water

Ethanol does the same. It is the consequence of deviation from the Raoult law. The other consequences are existence of azeotropes ( affects distillation ) and mixing volume deviations. If you mix 1 L ...
Poutnik's user avatar
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7 votes

Does entropy contribute work?

Suppose you have an ideal gas in a container in mechanical and thermal equilibrium with its surroundings (same $T$ and $p$—say, 298 K and 1 atm). It can't do any work on the surroundings. Now ...
theorist's user avatar
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7 votes
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Atomization enthalpy trends. The case of Phosphorus

I don't think I can give a canonical answer, but here are some quick thoughts: Breaking intermolecular interactions almost always requires significantly less energy than breaking covalent or ionic ...
Geoff Hutchison's user avatar
7 votes
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Enthalpy change of a solution of 1-pentanol in 1-butanol

It is more about careful reading of the task info than about knowledge of chemistry. Both molecules are almost identical, it may be hard to guess the enthalpy sign. But whatever sign the mixing ...
Poutnik's user avatar
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6 votes
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Enthalpy change defined at constant pressure only?

Is change in enthalpy defined only at constant pressure? No. The definition of $\Delta H$ is $\Delta H = \Delta U + \Delta (PV)$ Simple as that. I know Q-W=del(U) And Q at constant pressure ...
Chet Miller's user avatar
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6 votes

The dissolution of ammonium chloride

When discussing this question, one needs to think about all the processes involved (or at least those that account for most of what happens). In general, any process will occur spontaneously, if $\...
Jan's user avatar
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6 votes
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Why are condensation reactions endergonic?

One of the most simple biosynthetic reactions that generates ‘oligomers’ from ‘monomers’ (between inverted commas because that description is not fully correct here) is the ester formation used to ...
Jan's user avatar
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6 votes

Enthalpy definitions. What are their main differences?

Most of the chemistry happens in open containers. Open containers have constant pressure. At constant pressure work is done to carry out the reaction and also to "push" the air up (called as ...
Dipesh Somvanshi's user avatar
6 votes

Entropy and specific heat capacity

It is not true that heat exchanged at constant pressure is always reversible. But, if you want to determine the change in entropy from thermodynamic equilibrium state 1 at $(T_1,P)$ to state 2 at $(...
Chet Miller's user avatar
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6 votes
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Which of the following species has an enthalpy change of formation = 0 kJ/mol?

The correct answer is indeed c. $\ce{Na(s)}$ Since for the heat of formation to be zero, the molecule should be the must abundant chemical formula of the element. $\ce{Na}$ is a metal and is ...
fireball.1's user avatar
6 votes
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Would ionic compounds dissolve in methanol?

You don't need to do compare the said enthalpies. Maybe the process of dissolution of $\ce{KI}$ in $\ce{CH3OH}$ is energetically favorable, maybe it isn't (the latter seems more likely to me); that's ...
Ivan Neretin's user avatar
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6 votes

Why is creating water exothermic?

You’re mistakenly conflating the bond dissociation energies between the diatomic components and the product molecule. If the bond dissociation energy of water is larger than the bond dissociation ...
Bruce Kirkpatrick's user avatar
6 votes

Why enthalpy is defined at a constant pressure?

How are absolute enthalpy and enthalpy change defined? Enthalpy $H$ is defined as in your question. The most basic definition of enthalpy change when going from state 1 to state 2 is the enthalpy of ...
Karsten's user avatar
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