Chemistry Questions

[thebillsfan]

Can anyone explain (briefly) how oxidation-reduction titrations work?

Trends in the density of naturally occurring elements: do elements get dense as you go down the group or go across a period?

I read somewhere that for most salts, crystallization is exothermic. Why? Is thought solution FORMATION (opposite of crystalliation) was exothermic. How can they both be exothermic?'

Does someone have a strict definition of exponential increase? By that I mean, does a substance have to increase by the same percentage during each time interval? (i hope that question makes sense...i'll try and reword if it doesnt)

sort of an orgo question, but why are alcohols less acidic than water? they both have negatively-charged oxygens as conjugate bases.

If the MCAT says room temperature, can I consider that to be 25 degrees Celsius?

Related to physics: heat and absorption. If I have something that absorbs well (a blackbody), it also emits well, correct? That doesn't mean that it's absorbing as much as its emitting at any given time, but if the temperature of the environment was much much hotter than the body, it would be absorbing at some max rate x, while if the temperature of the environment was much much colder, it would be emitting at some maximum rate x. Is that accurate?

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[sleepy425]

sort of an orgo question, but why are alcohols less acidic than water? they both have negatively-charged oxygens as conjugate bases.

They aren't less acidic than water. The pKa of water is 15.7, and the pKa of a standard aliphatic alcohol ranges from 15-16. So they're about the same acidity. Furthermore, depending on what is attached to the alcohol, the alcohol could be much more acidic than water (for example, phenol has a pKa of around 9 or so).

 

[G1SG2]

They aren't less acidic than water. The pKa of water is 15.7, and the pKa of a standard aliphatic alcohol ranges from 15-16. So they're about the same acidity. Furthermore, depending on what is attached to the alcohol, the alcohol could be much more acidic than water (for example, phenol has a pKa of around 9 or so).

Alcohols are weaker acids than water because they're conjugate base is destabilized by the electron donating R group of the alkoxide ion.

 

[sleepy425]

Alcohols are weaker acids than water because they're conjugate base is destabilized by the electron donating R group of the alkoxide ion.

it depends on what the R group is. In most cases for aliphatic alcohols, the pKas of the alcohols and the water are about the same. You're right that alkyl R groups increase the basicity of the alkoxide ion, but only slightly (the exact pKa of ethanol is 15.9). Benzyl alcohol, which is a common compound, has a pKa of 15. Since alcohols encompass a wide variety of compounds, it is incorrect to generalize that they are weaker acids than water. It depends entirely on the R group of the alcohol. As I mentioned, in general, most alcohols have pKas between 15-16, which, for all intents and purposes is the same as the pKa of water (15.7). The most well known deviations from the 15-16 range are phenols, which are several units lower due to delocalization into the aromatic system.