brightsky's Chem Thread

[Aurelian]

Of what use are formal charges? What does their determination achieve?

The first use which springs to mind is that they are helpful for allowing organic chemists to make sense of certain reaction mechanisms or reaction tendencies by assigning charges to specific atoms in a molecule rather than just saying that the molecule has an overall charge. Sorry if that explanation is a bit abstract, the only concrete examples I can think of right now involve uni stuff... Formal charges also allow us to draw Lewis structures which in turn allow us to apply simple VSEPR theory to predict molecular shapes. There are probably other things as well, but my brain is being a bit slow today

Again, though, this isn't stuff you need to worry about just yet =)

[Mao]

Q1.

I understand that every atomic orbital can be uniquely defined by a set of three quantum numbers. The magnetic quantum number distinguishes orbitals with given n and l, which have different orientations in space. For example, a p-orbital can have three possible orientations in space, each, as the internet reveals, denoted by p_x, p_y and p_z. I'm fine with that, but when you get to orbitals of d-orbitals and f-orbitals, the subscripts become awfully complex. What I want to know is: how were the subscripts determined? Is there some sort of way in which one can derive each subscript from scratch?

Following on from what Aurelian has said (extremely good answer), the subscript are related to the actual formulas of the different orbitals (see: http://en.wikipedia.org/wiki/Cubic_harmonic#Table_of_cubic_harmonics). More often than not though, the subscripts refer to nodes in the wavefunction (where probability is exactly zero)

[Mao]

The first use which springs to mind is that they are helpful for allowing organic chemists to make sense of certain reaction mechanisms or reaction tendencies by assigning charges to specific atoms in a molecule rather than just saying that the molecule has an overall charge. Sorry if that explanation is a bit abstract, the only concrete examples I can think of right now involve uni stuff... Formal charges also allow us to draw Lewis structures which in turn allow us to apply simple VSEPR theory to predict molecular shapes. There are probably other things as well, but my brain is being a bit slow today

Again, though, this isn't stuff you need to worry about just yet =)

A good example is ammonia, NH4+. The standard textbook picture using formal charge would place a +ve charge on the central nitrogen, while all the H would be neutral. In reality, this +ve overall charge is smeared over the entire molecule.

Wikipedia has a surprisingly good tutorial on this: http://en.wikipedia.org/wiki/Formal_charge

[thushan]

A good example is ammonia, NH4+. The standard textbook picture using formal charge would place a +ve charge on the central nitrogen, while all the H would be neutral. In reality, this +ve overall charge is smeared over the entire molecule.

Wikipedia has a surprisingly good tutorial on this: http://en.wikipedia.org/wiki/Formal_charge

I think you meant ammonium, mate

[Mao]

I think you meant ammonium, mate

I blame it on the beer.

[brightsky]

Suppose I was trying to determine the sulfur content of lawn fertiliser via gravimetric analysis. The procedure demands that I add a few drops of HCl into the fertiliser solution before completing all the standard steps. What purpose does the HCl serve? Does it help the sulfate ions dissolve into solution?

Thanks!

[Stick]

This was discussed in the Chemistry 3/4 thread a little while ago, because I had to do the exact same experiment as my first SAC. Basically, carbon dioxide is soluble in water and could precipitate with some of the ions that may be present in solution. Ensuring that the solution is acidified means that carbonic acid is formed, which remains in solution for the duration of the practical.

[brightsky]

This was discussed in the Chemistry 3/4 thread a little while ago, because I had to do the exact same experiment as my first SAC. Basically, carbon dioxide is soluble in water and could precipitate with some of the ions that may be present in solution. Ensuring that the solution is acidified means that carbonic acid is formed, which remains in solution for the duration of the practical.

legend.

[brightsky]

A few queries regarding the definitions of certain words:

1. What exactly is the difference between dissociation and ionisation? Is one a subset of the other?

2. Does 'hydrolysis reaction' refer to ALL reactions which have H2O(l) as a reactant? Or must the reactants combine in a certain way for a reaction to qualify as one?

Thanks!

[REBORN]

2. Some reactions with water need not be hydrolysis. See: hydration.

[|ll|lll|]

A few queries regarding the definitions of certain words:

1. What exactly is the difference between dissociation and ionisation? Is one a subset of the other?

Dissociation is when electrostatic forces between ionic lattices are broken
For example:
                H2O (l)
NaCl (s) -------------> Na⁺ (aq) + Cl^- (aq)

Ionisation is when covalent molecules form hydrogen bonds in water
CH₃COOH (aq) + H₂O (l) <--> CH₃COO^- (aq) + H₃O⁺ (aq)
(Also hydrolysis in this case)

Then there's also dissolution, which is:
                     H2O (l)
C₂H₅OH (l) -------------> C₂H₅OH (aq)

Wouldn't see them as a subset of the other; they (dissociation and ionisation, that is, in regards to your question) are sort of mutually exclusive. So nope, not a subset

[lzxnl]

Slight correction. The third process, dissolution, is when the solute particles form bonds to the solvent particles, which in water and covalent molecules tend to be polar interactions. Dissolution = noun form of dissolve.
Ionisation is when ions form that weren't originally there. HCl ionises because it doesn't originally consist of H+ and Cl- ions.

[Mao]

Slight correction. The third process, dissolution, is when the solute particles form bonds to the solvent particles, which in water and covalent molecules tend to be polar interactions. Dissolution = noun form of dissolve.

Not quite. There are no 'bonds' as such in dissolution. This is when a solute is transferred from its own solid/liquid, and becomes surrounded by solvent molecules.

As a side note, ion dissolution is also a dissociation process. These aren't always mutually exclusive.

[lzxnl]

But...when ethanol dissolves in water, extensive hydrogen bonding occurs between ethanol and water. Are you saying that this isn't dissolving? For a solute to be surrounded by solvent molecules, it has to form stronger bonds with the solvent than with itself, otherwise it won't dissolve.
I didn't say they were mutually exclusive. I said that dissolution is just the noun form of dissolving, which encompasses ion dissolution.

[Mao]

But...when ethanol dissolves in water, extensive hydrogen bonding occurs between ethanol and water. Are you saying that this isn't dissolving? For a solute to be surrounded by solvent molecules, it has to form stronger bonds with the solvent than with itself, otherwise it won't dissolve.

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.