brightsky's Chem Thread

[brightsky]

How might one work out the Lewis structure for formaldehyde (H2CO)?

[Mr. Study]

I'll have a crack at this.

Try and remember the amount of 'lines' (With one line being 2 electrons) that Hydrogen, Carbon and Oxygen need to be complete.

Hydrogen needs 1 line (2 electrons, represented as 2 dots), Carbon needs 4 lines (8 Electrons, represented as 16 dots), and Oxygen needs 2 lines (4 Electrons, represented as 8 dots).

From this we can draw: .

For why there are 4 lone electrons at oxygen, I, sadly, cannot explain it very well.

Hopefully that helps out in some way.

For how it's written, you can see that they're all bonded to the Carbon.

EDIT: Thought I'll mention that I used the dots method first, when learning how to draw structures, as it was a way for me to remember how much electrons each element needed to complete. After a hand full of questions, I switched to the lines, as 1 line = 2 electrons and it was quicker.

[brightsky]

I see...thanks for that. Yeah I sorta understand...although how would one determine the bond angles? I wrote 120 degrees, under the impression that both the electron pair geometry and the molecular shape of the molecule is trigonal planar, but the answers say that 'O-C-H bond angles are slightly more than 120 and H-C-H bond angles are slightly less than 120'. Why is this the case?

Also, how would one work out the bond angles for SF4?

Chem is doing my head in...D:

[pi]

Chem is doing my head in...D:

That's because your questions are way outside the course haha! I doubt many assessors know that either

A general few steps to determining the Lewis-dot and bond angle can be found here: http://faculty.northseattle.edu/tfurutani/che101_2008/shape.pdf but I think the case you have what might be an exception.

My guess is because the atoms attached to the central carbon are not identical, hence one cannot assume the perfect trigonal planar 120deg angles. A similar problem exists with cyclohexane where the bond angles between the C-C-C isn't exactly 120deg (from memory...).

*tags Mao*

[jasoN-]

The double bond in formaldehyde (C=O) exerts more repulsion on the C-H bonds, that's why the bond angle is slightly more than 120º.
The example with SF4, is beyond the scope of what you learn in VCE, you definitely don't need to know how to work out the bond angles for that compound.
A good read: http://www.chemguide.co.uk/atoms/bonding/shapes.html

[brightsky]

I see. Just to clarify, is the difference between electron pair geometry and molecular shape that the former takes the lone pairs into consideration while the latter disregards them?

And is there a way of 'working out' the bond angles for more complicated molecules using maths or do we have to refer to the table every time?

Also, what is meant by the term radical? (The exact wording of the question is: 'Which of these species (NH3, H2CO, SF4, NO2) is a radical?')

[jasoN-]

Yep you're right about the electron pair geometry / molecular shape
There probably is a way to work out the bond angles of complicated molecules, but you won't need to know how to do that. I probably won't learn it in uni, maybe ask the chemistry majors

The term radical just means that there's an unpaired electron in the species -> makes it highly reactive.
An easy way to work out which one is a radical is to count the valence electrons in each species:
NH3: 5 + 3 = 8
H2CO: 2 + 4 + 6 = 12
SF4: 6 + 7 + 7 + 7 + 7 = 34
NO2: 5 + 6 + 6 = 17
Odd no. electrons = a radical
Hence NO2 is a radical.
Not sure where you get these questions from, but I don't think they are covered in depth if any in the VCE chemistry course.

You would probably hear about radicals in mass spectrometry, where the molecular species is bombarded by electrons, producing radical cations.

[brightsky]

Ah I see, thanks jason!

Also another question, what would the Lewis structure of ozone look like?

[pi]

Just googing it (not what I expected it to look like):


I'll leave any explanation for Jason (or others) though haha

[HenryP]

Basically with Ozone, you can draw more than 1 equivalent lewis structures. You would call those resonance structures.

[jasoN-]

^ Yep vegemite has provided the resonance structures.
I'll give you a step guide to draw out lewis structures

1) Calculate the no. of valence electrons
* Note, don't forget to take into account the charge, i.e. For every + charge in the molecule, there's 1 less valence electrons, vice versa with -ve charges.
So in this example we have O3
No. valence electrons = 6*3 = 18 electrons

2) Draw central atom with other atoms connected by single bonds
*Usually the central atom would be the least electronegative atom.
In this case, it would just be any oxygen in the middle.

3) Complete octets of outer atoms

4) Add up electrons used
So far we have used 16 electrons (each bond counts for 2 electrons as well)

5) All electrons used up?
If yes, move to 6
If no, use the remainder of your electrons to complete the octet of central atom
So we haven't used up all our electrons, we have 18 available and used up 16, add 2 electrons to the central atom.

6) Move electrons from outer atoms to complete octet of central atom by converting them to bonds

Don't forget to check your charges and that it makes logical sense (+ and - cancel out to 0)

So as we see there are 2 resonance structures. In this case they are equivalent contributing structures to the hybrid which I've shown below

Basically what resonance structures tell you is that 50% of the time it will assume the structure on the left, and the other 50% of the time it assumes the structure on the right.
The hybrid resembles this and says that the overall charge of each outer oxygen will be -1/2, and the central atom still retains its full positive charge.
The dotted + solid bonds resemble the bond O-O is halfway between a single bond and a double bond (so 1.5 bonds basically).

Just to note, you do not need to know how to draw resonance hybrid, or the mechanisms (the arrows). This will never be examined in VCE chemistry, but it's just for interest sake.

[brightsky]

noob question:

so latent heat measures the energy required to change the state of a substance at its melting (fusion) or boiling (vaporisation) temperatures. for water, the values given are, for latent heat of fusion, 6 kJ mol and, for latent heat of vaporisation, 44 kJ mol. but according to another formula associated with heat capacity, Energy = SPC * mass * change in temperature.

my question is, according to the formula above, the latent heat values of a substance, say water, should be both 0 kJ/mol right, since there is no change in temperature from, for water, 0 C to 0C and 100 C to 100 C? what exactly is meant by saying changing the state at melting/boiling temperature. i would have thought that once the temperature of water passes a certain threshold, say 100 C, it would automatically turn to gas from a liquid. why do we need to put more energy in?

thanks.

[brightsky]

bump

and also, can anyone explain to me how a photon can have a frequency? i've read up on how light can behave like a particle and a wave, but i'm finding this idea hard to conceptualise. for example, i think of a photon as being like a small ball that moves around in space. how can a small ball have a frequency...does it sort of move around in a wave-like motion, or?

thanks in advance.

[brightsky]

For the following question, I got the right answer, but according to the solutions, I didn't express the answer to the correct amount of sig figs. My question is, how are we to determine to how many sig figs we're supposed to write our answer when the question itself does not explicitly state it?

Magnesium reacts with hydrochloric acid according to the equation: Mg(s)+2HCl(aq) --> MgCl2(aq) + H2(g)
If 10.0 g of Mg reacts completely, calculate the mass of Mg chloride that forms. (I wrote 39.17g - expressing it to 4 s.f. for no apparent reason - but the answer says it should be 39.2g.)

Thanks in advance!

[pi]

10.0g has 3 sig figs, hence, your answer must be to three sig figs as that's the lowest (and only) figure given in the stem of the question