BR In Class Spectrophotometer passage

[TheMightyBoosh]

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19. Which is the MOST likely explanation for why two samples of hydrogen gas show different spectra (in terms of line and band wavelengths)?

A. The two samples have different concentrations
B. The two samples have different isotopic composition
C. The two samples were exposed to different incident energy supplies
D. The two samples are at different temperatures

The correct answer is B. The solution doesn't really explain why isotopes would exhibit different emission patterns. My thinking was that isotopes should exhibit the same spectra because a difference in the number of neutrons in the nucleus shouldn't really affect the energies associated with electrons.

Anyone know why different isotopes exhibit different spectra?

 

[BerkReviewTeach]

This is one of those questions where eliminating three wrong answers goes a long way.

Even if the two samples have different concentrations, it won't matter in terms of what absorbances are observed. The intensity will change, but not the wavelength. Adios A

Even if the two samples were exposed to different incident energy supplies, they still will observe the same frequencies of light, provided the incident light has those frequencies. Ciao C

Even if the two samples are at different temperatures, the energy transitions won't be affected, so we shouldn't see any change in the wavelengths absorbed. Depart D

If the two samples have different isotopic composition, then there will be some H-H mixed with D-H and D-D. Because deuterium is twice as heavy as hydrogen, the D-D bond vibrates differently than the H-H bond, so it requires different amounts of energy to excite the electron vibrationally. You'd get different absorbances in the IR region (where bond vibrations are observed). B for best!

It's not the best answer, but it's enough to choose B as being better than A, C, and D.

 

[whiteshadodw]

19. Which is the MOST likely explanation for why two samples of hydrogen gas show different spectra (in terms of line and band wavelengths)?

A. The two samples have different concentrations
B. The two samples have different isotopic composition
C. The two samples were exposed to different incident energy supplies
D. The two samples are at different temperatures

The correct answer is B. The solution doesn't really explain why isotopes would exhibit different emission patterns. My thinking was that isotopes should exhibit the same spectra because a difference in the number of neutrons in the nucleus shouldn't really affect the energies associated with electrons.

Anyone know why different isotopes exhibit different spectra?

This is one of those questions where you have to think DEEP. it tells you two different hydrogen samples. so how could they differ? how can any two elements differ from each other? they could have different concentrations, but they'd have the same spectra, just different intensities. they could also have different isotope levels. this is where its easy to get tripped up. if you think exclusively about the bohr model, then hydrogen and deuterium don't differ that much. however, once you start considering things like NMR and IR, it becomes much clearer. you can't get a signal from deuterium in NMR. in fact, this piece of information becomes helpful in ochem too, because you can track hydrogens by deuterating a carbon and watching for the signal to disappear.

 

[BerkReviewTeach]

Whiteshadodw!!! Great approach from a test-taking perspective. This is exactly what people should be getting from questions like this. If you can't solve it in a traditional fashion, then you need to use logical and POE. I've ready many of your posts and must say I'm impressed by your approach. You will do great on the MCAT (assuming you haven't already taken it and done great).

As a point of interest, you actually can get an NMR signal from deuterium. It just happens to be at a different energy than 1H. 2H NMR is useful for tracking isotope gain in macromolecules (it's useful in solid state NMR). In 1H NMR we have a small range of energies and only 1H fits in that window, so 2H seems invisible. But at a different range we could see 2H. It's like saying carbonyls don't show in the 2900 to 3600 cm-1 range. We can observe oxidation of an alcohol by watching the hydroxyl peak disappear without ever considering the appearance of the carbonyl signal well below our 2900 cm-1 limit.

 

[TheMightyBoosh]

Thanks BerkeleyReviewTeach and whiteshadodw, you guys are always helpful.

So due to the fact that emission spectra is a combination of vibrational, rotational, and electronic energy transitions, deuterium (having different vibrational energies) would exhibit a different visible pattern than hydrogen.

And as for the scenario where two identical samples are exposed to different incident energies, you assume that the energies will be large enough to encompass the range of frequencies that will be emitted through electron transitions in the visible spectrum. Am I correct?