# Tag Info

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 ...
• 31.3k
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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 ...
• 415
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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 ...
• 10.5k
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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 ...
• 2,925

### 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 ...
• 71.5k
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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 ...
• 12.2k
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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 ...
• 7,040
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• 7,060
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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 ...
• 40.8k
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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 $\mathrm dh = 0 = c_p\,\mathrm dT$. ...
• 22.5k
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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 ...
• 42.5k

### 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 ...
• 12.2k
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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 ...
• 27.9k
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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 ...
• 42.5k
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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 ...
• 13.5k

• 13.5k
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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 ...
• 460
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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 ...
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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 ...