Is This Question On My AQA Unit 2 Spec?

18. Describe the arrangement of bonds around the carbon atoms in ethane and in ethene, and state the bond angle in each compound. Compare the relative lengths of the carbon-carbon bonds in the two molecules and explain why they are different.
(6 Marks)

It's taken from an old old past paper, my school have these copies of stuff, some of which is relevant and some just isn't, they take questions from the old Unit 3 stuff before it was the EMPA and ISA. But can't work out if it's something I'll need to learn for my Unit 2 Resit tomorrow?

Thanks.

Reply 1

Plato's Trousers

I would say it's the kind of material you should understand for AQA yes. Do you need hekp with this particular question?

Reply 2

k1n60fthew0rld

Original post by Plato's Trousers

I would say it's the kind of material you should understand for AQA yes. Do you need hekp with this particular question?

That would be great if you don't mind? Could you possibly help me with the blanks?:

Within ethene, the atoms bonded to it are all within the same plane, therefore the shape is trigonal planar. The bond angles are 120 degrees. The C=C bond is rigid therefore does not allow rotation of atoms around it, unlike single bonds. This can cause isomerism in larger molecules.

Ethane has bond angles of ????? . It's bonds are not rigid, however are totally saturated.

The C=C double bond in ethene is shorter in length than the C-C in ethane, increasing the strength of the covalent bond. This increases the energy required to break the bond?

Thats the best I can do, and I'm guessing some of that stuff isn't relevant?

Many Thanks :smile:

Reply 3

Plato's Trousers

Original post by k1n60fthew0rld

Correct. Each carbon atom has only three atoms bonded to it and carbon has no lone pairs, so the geometry is trigonal planar, as this is the spatial arrangement that places them furthest apart.

(In fact, the bond angles aren't all exactly 120^o since the three bonds are not the same type. The extra pi-bond between the two carbon atoms tends to repel the C-H bonds slightly, making the H-C-H bond angles very slightly less than 120^o. But I suspect you can ignore this at this level and just say they are all 120).

The C=C bond (a sigma and a pi bond) distance in ethene is shorter than the C-C distance in ethane. It also has a higher bond enthalpy ie it is harder to break. Both of these facts are due to the fact that there are two pairs of electrons contributing to the C-C bonds in ethene, compared to only one pair in ethane.

Original post by k1n60fthew0rld

Ethane has bond angles of ????? . It's bonds are not rigid, however are totally saturated.

The C=C double bond in ethene is shorter in length than the C-C in ethane, increasing the strength of the covalent bond. This increases the energy required to break the bond?

Thats the best I can do, and I'm guessing some of that stuff isn't relevant?

Many Thanks :smile:

the bond angles in ethane are all 109.5^o. This is because all the bonds are single bonds, there are no lone pairs and so they repel each other equally. 109.5^o is the angle in a regular tetrahedron.

I don't think you need to bother with the stuff about isomerism. The question isn't really asking for that. It is concerned with the shape of the molecules, ie their geometry, and the strength of the bonds.

(edited 13 years ago)

Reply 4

REV

all you need to know for AQA Unit 2 is that the C=C doesn't rotate and that it's planar! :smile:

Reply 5

Plato's Trousers

Original post by REV

all you need to know for AQA Unit 2 is that the C=C doesn't rotate and that it's planar! :smile:

You certainly need to know both those things for AQA, I agree. But my point was that the question originally posted is not asking about isomerism, but bond lengths and angles.

Reply 6

REV

Original post by Plato's Trousers

You certainly need to know both those things for AQA, I agree. But my point was that the question originally posted is not asking about isomerism, but bond lengths and angles.

Yeah, no need for bond lengths though :smile:

Although I guess it wouldn't be any harm knowing the bond angles :biggrin:

Reply 7

k1n60fthew0rld

Just checked as well, Sigma and Pi bonds aren't on it, which was something I panicked about because a lot of my notes they give us (these word documents they've reproduced off another school, on a memory stick) are out of date, so we sort of have to pick what's needed.

Thanks very much for your help :smile: