Quick chemistry discrepancies

[DeathandTaxes]

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I've been keeping a log of mistakes in my practice tests and have come across a few concepts that seemed to contradict each other:

Enantiomers:

In one problem, we were expected to know that enantiomers can have different physical properties besides polarizing light differently (such as boiling point), and thus certain enantiomeric mixtures could be separated by distillation. If they had different boiling points, it could mean that one configuration of a molecule was able to bond to itself and pack more greatly than the other (from TBR)

Yet just recently there was a problem that assumed that enantiomers are supposed to have similar boiling points (from TPR) Is the answer that physical properties of enantiomers vary with circumstances?

Specific Heat:

In Kaplan, it was stated in an explanation that adding NaCl to water would lower the specific heat because the ions would disrupt hydrogen bonding. In TBR, it explained that this would increase specific heat because the ion-water bonding required more energy to break. This is consistent with the concept of boiling point elevation.

TBR makes more intuitive sense to me. Kaplan must be in the wrong here, right?

 

[Czarcasm]

I've been keeping a log of mistakes in my practice tests and have come across a few concepts that seemed to contradict each other:

Enantiomers:

In one problem, we were expected to know that enantiomers can have different physical properties besides polarizing light differently (such as boiling point), and thus certain enantiomeric mixtures could be separated by distillation. If they had different boiling points, it could mean that one configuration of a molecule was able to bond to itself and pack more greatly than the other (from TBR)

Yet just recently there was a problem that assumed that enantiomers are supposed to have similar boiling points (from TPR) Is the answer that physical properties of enantiomers vary with circumstances?

Specific Heat:

In Kaplan, it was stated in an explanation that adding NaCl to water would lower the specific heat because the ions would disrupt hydrogen bonding. In TBR, it explained that this would increase specific heat because the ion-water bonding required more energy to break. This is consistent with the concept of boiling point elevation.

TBR makes more intuitive sense to me. Kaplan must be in the wrong here, right?

TPR is correct, although TBR may be correct as well and here's why: As TPR explained, entantiomers have identical physical and chemical properties (which makes it particularly difficult to separate a mixture of both enatiomeric species). I believe what TBR is referring to is the actual separation process: One way we exploit this in lab is by indirectly using a second chiral compound which can bind to each enantiomer. In doing so, we create two new compounds both with differing physical properties, which we can then take advantage of in order to separate them. This is in contrast to a pair of diastereomers which have already have different stereochemistry (in the absence of introducing a chiral compound) and therefore exhibit different physical properties. Generally enantiomers exist in nature as a mixture and this process of separation is pretty expensive but it's necessary in some instances for certain drugs (where one is chemically useful, and the other entantiomer is hazardous or toxic).

As far as the discrepancy with salt and specific heat, that's not something that's generally presented. It seems more like passage information, so I would not stress over it. As long as you understand the basic relationships of specific heat, what it is, and how to calculate it, you should be fine. Anything more would be explained in a passage. (However, I do recall someone addressing the discrepancy a few weeks ago, with many sources citing support against TBR).

 

[Teleologist]

In Kaplan, it was stated in an explanation that adding NaCl to water would lower the specific heat because the ions would disrupt hydrogen bonding. In TBR, it explained that this would increase specific heat because the ion-water bonding required more energy to break. This is consistent with the concept of boiling point elevation.

TBR makes more intuitive sense to me. Kaplan must be in the wrong here, right?

Both are plausible in that hydrogen bonding is what accounts for the specific heat of water, and that ion-dipole interactions can also contribute a fair bit of specific heat to water solution.

Remember temperature is the average kinetic energy of the molecules in a substance and that ion-dipole interactions can reduce this kinetic motion (as well as hydrogen bonding). Think about it - if you had to run 100 meters while holding hands with two other people, then you'd probably be quite a bit slower than running the 100 meters by yourself.

Now, the magnitude of reduction of kinetic energy for H-bonding vs. ion-dipole interactions, I don't know.