ATAR's y11 Chemistry Qs!

[Russ]

I don't think there's any need for the posts to be deleted but I've had to clean up after you two before (in the Psych board) and I don't want to have to do it a third time.

Keep this thread on topic and answering questions, if you want to discuss the difficulty of the syllabus then do it elsewhere.

[iNerd]

'Shapes of molecules'

How are you meant to know how to draw them? The book says to maximise repulsion and place them as far away as possible - there's so many places I could put them..

For eg - H₂S.

I drew it:

H ------- S ------- H          Linear

But it's like V-shaped with a lone pair at the top, one in the bottom right and two single covalent bonds with two hydrogen atoms.

[nacho]

'Shapes of molecules'

How are you meant to know how to draw them? The book says to maximise repulsion and place them as far away as possible - there's so many places I could put them..

For eg - H₂S.

I drew it:

H ------- S ------- H          Linear

But it's like V-shaped with a lone pair at the top, one in the bottom right and two single covalent bonds with two hydrogen atoms.

here:

http://www.dynamicscience.com.au/tester/solutions/chemistry/molecules/molecularshapes.htm

If you don't get it, seriously do not worry, it's something I've been unable to pick up and it's completely unecessary for year 12 chem.

Edit:
Something more straight to the point:

http://www.dynamicscience.com.au/tester/solutions/chemistry/molecules/molecularshapes5.htm

[iNerd]

It's a relief that I don't need it for y12 but I still want to know how (i don't get it.... )

[pi]

If you don't get it, seriously do not worry, it's something I've been unable to pick up and it's completely unecessary for year 12 chem.

But you also don't want to be drawing things funnily in organic chem. Its good to know shapes.

[iNerd]

If you don't get it, seriously do not worry, it's something I've been unable to pick up and it's completely unecessary for year 12 chem.

But you also don't want to be drawing things funnily in organic chem. Its good to know shapes.

So can you explain it please?

Moderator action: removed real name, sorry for the inconvenience

[pi]

If you don't get it, seriously do not worry, it's something I've been unable to pick up and it's completely unecessary for year 12 chem.

But you also don't want to be drawing things funnily in organic chem. Its good to know shapes.

So can you explain it please?

Its not something you can easily explain. You've actually got to go out of your way to research (internet, uni chem books) and practice (uni chem books maybe?). Maybe even go beyond the course just to get it. Of course I can't draw everything, but I can draw most of the chemicals commonly used in yr12 if needs be.

Moderator action: removed real name, sorry for the inconvenience

[nacho]

If you don't get it, seriously do not worry, it's something I've been unable to pick up and it's completely unecessary for year 12 chem.

But you also don't want to be drawing things funnily in organic chem. Its good to know shapes.

I think he doesn't understand the spacing concepts. Otherwise, in year 12, everything drawn at right angles is acceptable (i.e w/o spacing concepts)

Moderator action: removed real name, sorry for the inconvenience

[Souljette_93]

The reason why it is a v-shape is because of it's lone pair. Just like water, it has a lone pair which forces the atoms to be in a tetrahedral shape.

[Mao]

It's called VSEPR theory. In H2S, the valence electrons form 4 groups: two H-S bond and two lone pairs (total of 8 valence electrons about S). The four groups of electrons repel each other, thus the optimal geometry is a tetrahedron with S at the center. However, since lone-pair electrons are 'invisible', we only see two vertices of the tetrahedron, thus it is 'bent'.

in CO2, the valence electrons form 2 groups: two C=O bonds (total of 8 valence electrons about C). The two groups repel to a linear shape.

in CH2O (formaldehyde), the valence electrons form 3 groups: C=O bond (4 electrons), two C-H bonds (Total of 8 valence electrons about C), the three groups repel to the optimal geometry of a planar triagle.

etc.

[iNerd]

Thanks guys! (esp. Mao)

So in y12 if I did CH₄ as right-angle to each other (wrong shape) would I lose marks?

[Mao]

Thanks guys! (esp. Mao)

So in y12 if I did CH₄ as right-angle to each other (wrong shape) would I lose marks?

No. You are not taught about stereochemistry, so you are not expected to know the correct 3D structure. The convention for organic molecule is to draw zig-zags, but right angles are fine.