brightsky's Chem Thread

[brightsky]

Thanks for all the responses!

(1):

This is how I've always learned it:

There are three types of substances that can dissolve in water:
(i) Polar covalent molecules that can H-bond with water (e.g. ethanol)
(ii) Polar covalent molecules that can ionise in water (e.g. HCl)
(iii) Most ionic compounds (e.g. NaCl)

Substances that fall under the third category dissociate in water; dissociation occurs when a soluble ionic solid dissolves in water, releasing ions that were already present in the beginning.

Substances that fall under the second category ionise in water. Ionisation occurs when a polar covalent molecule reacts with water, forming ions that were not present in the first place.

Substances that fall under the first category are simply surrounded by water molecules, which act as ligands.

Is this correct? Or is this too childish a model? Mao, you mentioned that ion dissolution is also considered as a dissociation process, which suggests that the admittedly quite narrow definitions I proposed above are inaccurate...


(2):

I'm still after a formal definition of hydrolysis. Also, are the phrases 'acid hydrolysis' and 'base hydrolysis' well-defined in the scientific community?

[Mao]

> (i) Polar covalent molecules that can H-bond with water (e.g. ethanol)
> Substances that fall under the first category are simply surrounded by water molecules, which act as ligands.

Definitely not ligands. Ligands tend to be more 'permanent' fixtures, and form chemical bonds with another ion/molecule. Water molecules in the hydration shell of an uncharged, polar molecule only remain for a few picoseconds.

> (ii) Polar covalent molecules that can ionise in water (e.g. HCl)
> Substances that fall under the second category ionise in water. Ionisation occurs when a polar covalent molecule reacts with water, forming ions that were not present in the first place.

Weak acids and bases can ionise in water, but only a small percentage of molecules would ionise. The remaining unionised molecules can remain insoluble/inmiscible.

> (iii) Most ionic compounds (e.g. NaCl)
> Substances that fall under the third category dissociate in water; dissociation occurs when a soluble ionic solid dissolves in water, releasing ions that were already present in the beginning.

Many ionic compounds are not soluble, or sparingly soluble.

The model you have given is good enough for VCE Chemistry. I have outlined some cases where they might fail, but it's highly unlikely if VCAA would question you about them. As far as VCE is concerned, it's fine.

We must however keep in mind that this does not reflect what actually happens. We cannot make sweeping assumptions such as "solutes must make hydrogen bonds with water", or indeed any other black-and-white statements. This rife misunderstanding is a problem of VCAA, VCE teachers as well as several branches of chemistry and biochemistry that don't accurately treat intermolecular forces and thermodynamics.

[lzxnl]

There is no scientific evidence for such 'extensive hydrogen bonding'. Other effects, some involving entropy (a concept not taught at the VCE level), strongly influence dissolution. The truth is intermolecular bonding is a transient attraction between molecules, they aren't really 'bonds'.

In simpler language, A dissolves in B not because A can bond to B. The reason why A would dissolve in B is complex, it is an active area of research, and we don't really understand it completely yet.

Yes, my bad. I completely forgot that dissolution is a thermodynamic process and that entropy is really the deciding factor.

We must however keep in mind that this does not reflect what actually happens. We cannot make sweeping assumptions such as "solutes must make hydrogen bonds with water", or indeed any other black-and-white statements. This rife misunderstanding is a problem of VCAA, VCE teachers as well as several branches of chemistry and biochemistry that don't accurately treat intermolecular forces and thermodynamics.

I like how you detest the VCE system so much

[brightsky]

Is there a systematic way of approaching questions which ask you do determine the effect of a particular error on the final result that will always 'work'? Sometimes I get lost in the logic of it all, and think that a particular error would result in a higher result than the actual when in fact it would result in a lower.

Cheers.

[Patches]

I usually draw a simple diagram of the experiment, so I can see the effect on each stage sequentially. Same goes for the repeated dilution questions that come up a lot in spectroscopy. Probably an obvious technique but I can't see how else you could do it.

[brightsky]

1. How accurate do we need to be in determining the concentration of say cadmium in a urine sample from the calibration graph? It seems unreasonable to expect us to be able to discern the difference between 380 mg/L and 385 mg/L, for instance.

2. Can an acid-base indicator ever be a weak base, at VCE level?

3. How do we attend to sig figs in spectroscopy questions? For instance, does the concentration you read off the calibration graph affect the number of sig figs in which you express your final answer? 380 mg/L for instance would be counted as 3 sig figs? How about when converting between units, e.g. from mg/L to ppb or something? Do we retain the number of sig figs in the original expression?

4. Suppose you are trying to determine the sodium content of a sample using AAS. You would use a sodium hollow cathode lamp. But suppose the sample also contained some potassium. Would there be any overlap with regard to absorbance at certain wavelengths? Each element has a unique set of allowed electronic energies. Does this mean that potassium would not absorb any of the wavelengths emitted by the sodium hollow cathode lamp?

Thanks!

[lzxnl]

1. From the calibration graph, if you can only read to two sig figs, use two sig figs. My teacher said to do that in class.

2. Yes; the conjugate base of a weak acid is a weak base, so you could use the conjugate base of a weak acid indicator as an indicator as well that works in reverse. In fact, when an indicator is in highly acidic solution, we see more of the acidic form of the indicator and when an indicator is in highly basic solution, we see its base form. It does not really matter whether the original indicator was an acid or base; they both function in similar ways.

3. I would say that for each graph, there are only so many significant figures that can be read off the graph. Just use those.

4. I think we can assume that there's no overlap there. In AAS, each metal can only absorb a discrete number of frequencies as metal atoms can only absorb a photon that has the same energy as the energy difference between two electronic energy levels, so you can safely assume that it is impossibly improbably that two different metals have pairs of orbitals that have exactly the same energy difference, given the different numbers of electrons (so repulsions) and protons in the nucleus (attraction to the nucleus).

[brightsky]

just to confirm, the name of the compound CH3(CH2)2C(OH)2CH(NH2)CH2CH2CH3 is 5-aminooctan-4,4-diol, right? and as a general rule, with regards to numbering, do you choose the direction which gives the lowest locant for the carbon with the hydroxyl groups, as opposed to that which gives the lowest locant for the carbon with the amino group, because hydroxyl has a higher priority? for instance, 4-aminooctan-5,5-diol would be wrong, as far as iupac naming is concerned?

[lzxnl]

yes to all of those

[brightsky]

yes to all of those

thanks! also, in calculations like 2.0/18.0 * 0.214 = ... do we retain 2 sig figs or 3 sig figs? the periodic table provided by vcaa quotes molar masses to 1 dp. do these molar masses affect significance?

[Mao]

2.0/18.0 * 0.214 --> 2 s.f.

(2.0 + 18.0)/18.0 * 0.214 --> 3 s.f.

(0.2 + 2.0 + 18.0)/18.0 * 0.214 --> 3 s.f.

The subtle point is that for the elements with only 2 s.f. molecular mass in VCAA's periodic table, these are usually used in calculation of m.w. of much larger molecules. S.f. rules for addition means you will usually end up with 3 s.f. for most problems.

[ignore my grammar, I ceebs]

[brightsky]

This might sound stupid, but is there a difference between a spectrometer and spectrophotometer? Why is one used in preference to the other?

[Mao]

This might sound stupid, but is there a difference between a spectrometer and spectrophotometer? Why is one used in preference to the other?

They are the same thing. Tomaeto tomato.

EDIT:
Okay, they are not exactly the same thing. A spectrophotometer is an instrument that deals with a photospectrum (i.e. light and radiation), while a spectrometer is an instrument that deals with any spectrum (e.g. mass spectrometer measures the spectrum/distribution of masses, audio spectrometer measures the spectrum/distribution of audio frequencies).

For practical applications, they are the same thing.

[brightsky]

Thanks Mao!

Also, is it true that individuals bands on an emission spectrum are not characteristic? If so, why? I thought only sodium absorbed radiation of wavelength 589.0 nm (corresponding exactly to the energy difference between say the 3s orbital and 3p orbital), for instance, and no other element absorbed at that wavelength.

I understand, however, for obvious reasons, that the set of bands corresponding to the emission spectrum is characteristic of a particular element.

Any insight appreciated.

EDIT: Also, how are absorption spectra obtained? Do you simply pass the transmitted beam through a prism (after the beam has gone through like a solution of containing sodium ions) and resolve the light into a spectrum, as is the case with emission spectra? Also, what are spectrographs, and how do they differ from prisms?

[lzxnl]

Individual bands correspond to energy differences between pairs of orbitals in different atoms. By themselves, they may happen to coincide for different elements; after all, a 2s to 2p jump for one element may well be a 4s to 5s jump for another.

Using the Rydberg equation for the hydrogen atom, we can see that the higher energy levels get closer and closer in energy; energy differences decrease. Therefore, it is entirely possible that two elements absorb/emit light of a similar frequency, if one allows the two similar frequencies to be due to different electronic transitions.