brightsky's Chem Thread

[thushan]

Thanks Stick! Nah, I've seen chemical equations wherein a reactant that actually takes place in the reaction is written above the arrow. For example, in the chemical equation describing the reaction between ethene and hydrogen chloride to produce chloroethane, the hydrogen chloride (an active participant in the reaction) is sometimes written above the arrow for reasons which I am trying to find out. I'm starting to think it's simply for compactness of expression.

It's just another way of expressing a reaction - HCl is an actual reagent in your case. It's more compact, and very appropriate for situations where you have a reaction pathway.

[thushan]

2. Can an alcohol undergo a substitution reaction?

Yes, but it's out of the scope of the course. FYI though:

- alcohol to chloroalkane, react with SOCl2 or C2O2Cl2 (OUT OF THE COURSE)

[Aurelian]

1. Is there a special reason why chemists sometimes write one of the reactants above the arrow?

Organic chemists are very lazy =3

[thushan]

Organic chemists are very lazy =3

Organic chemists are awesome.

[teletubbies_95]

After some thinking , would a substitution reaction be something like this

ie. C4H10O + HCl-----> C4H9Cl + H20

or would the HCl be on the reactants?

confused ...
and thanks to brightsky for asking these questions!

[thushan]

After some thinking , would a substitution reaction be something like this

ie. C4H10O + HCl-----> C4H9Cl + H20

or would the HCl be on the reactants?

confused ...
and thanks to brightsky for asking these questions!

This reaction cannot occur. But you have the right idea, we are talking about one functional group being replaced by another.

[brightsky]

Ah okay, thanks thush + Aurelian!

[teletubbies_95]

but in the general equation for alcohol substitution reaction is ROH + HX----> RX +H20, right?
so why is it impossible?

[Mao]

but in the general equation for alcohol substitution reaction is ROH + HX----> RX +H20, right?
so why is it impossible?

It's difficult when uncatalysed, but it is possible. You usually see NaOH substituting X, but not the other way around.

[teletubbies_95]

thanks mao and thush !

[brightsky]

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?

Q2.

What is meant by 'formal charge'?

Thanks!

[Aurelian]

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?

Ultimately the simple answer is just that they "fall out" of the Schrodinger equation. You solve the Schrodinger equation for those particular orbitals, and mapping those solutions will give you the shape of that orbital*; the "subscripts" are then assigned in accordance with the orientations of those 'shapes' with respect to each other using common sense.

So, for example, three of the solutions for d orbitals of any given n value look exactly the same, but are oriented in different 2D planes of space all orthogonal to each other. It therefore makes sense to call these the d_xy, d_xz and d_yz orbitals. The same ideas apply to the more 'complicated' subscripts as well, although I imagine there are more formal rules for their determination...

You really don't need to worry about any of this at all for VCE though!

(*orbitals don't actually have boundaries, so the "shapes" you see are (usually) the region of space within which the electron will be found 90% of the time)

Q2.

What is meant by 'formal charge'?

Thanks!

"Formal charge" is just the charge on an atom in a molecule if you assume that the electrons in any covalent bond are shared equally between both participating atoms (i.e. one each and ignoring differences in electronegativity between the atoms).

[brightsky]

Thanks Aurelian!

I have yet to become acquainted with the the Schrodinger equation. Perhaps I will understand in a few years time...

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

Another question on significant figures. As far as I am aware, the rules for identifying sig figs are as follows:
1. All non-zero digits are significant.
2. All zeros between two non-zero digits are significant.
3. Leading zeros are never significant.
4. Trailing zeros are only significant if there is a decimal place present.

According to the above rules, 400 would have only 1 decimal place. But various sources, include TSFX, seem to disagree. A quick search of Wikipedia yielded the following:

"The significance of trailing zeros in a number not containing a decimal point can be ambiguous. For example, it may not always be clear if a number like 1300 is precise to the nearest unit (and just happens coincidentally to be an exact multiple of a hundred) or if it is only shown to the nearest hundred due to rounding or uncertainty. Various conventions exist to address this issue:
 A bar may be placed over the last significant figure; any trailing zeros following this are insignificant. For example, 1300 has three significant figures (and hence indicates that the number is precise to the nearest ten).
 The last significant figure of a number may be underlined; for example, "2000" has two significant figures.
 A decimal point may be placed after the number; for example "100." indicates specifically that three significant figures are meant.[2]
 In the combination of a number and a unit of measurement the ambiguity can be avoided by choosing a suitable unit prefix. For example, the number of significant figures in a mass specified as 1300 g is ambiguous, while in a mass of 13 h‍g or 1.3 kg it is not."

What is VCAA's stance on this matter? Does the number 400 have 1 or 3 sig figs?

[paulsterio]

What is VCAA's stance on this matter? Does the number 400 have 1 or 3 sig figs?

It can have either, that's why it's universally better to use scientific notation. I advise my physics students to ALWAYS use scientific notation, I guess the same can be said for chem.

If you decide to write using non-scientific notation, then specify the error.

E.g.

is to 1 significant figure

is to 2 significant figures

is to 3 significant figures

[paulsterio]

I should also add, the chances of you getting something like 400 are pretty rare anyways.

The only physical quantity which will give you a value near 400 in chemistry is mL and grams. You won't have 400 moles, amps, coulombs...etc.

In either of those cases, it would make no sense for 400 to be 1 significant figure and I don't think VCAA would have a strong case to argue that it's 2 significant figures. So, in reality, you can assume that it's three. Most analytical stuff is correct to 3 significant figures anyways as titration equipment are usually accurate to three significant figures.

I can't recall a case where this ambiguity has occurred, though if someone has, it'd be interesting to look at.