Official AQA AS Chemistry Unit 1 - 23Rd May 2013

I thought alkanes had the formula Cn H2n+2?

I thought alkanes had the formula Cn H2n+2?

Just did my last past paper and got 56! Hopefully can replicate in exam! GOOD LUCK EVERYONE

Well done, that's great! Good luck tomorrow!

You too! Can't wait till it's over! (And then start revising for maths on friday )

This also confused me at the time, too. Since Cyclobutane is technically an isomer of butene, I guess the reason it is cyclobutane is because there is no carbon carbon double bond

(LINEAR) ALKANES have the molecular formular CnH2n+2 (not always linear, but not 1st carbon and last carbon are seperate)
CYCLOALKANES have the molecular formular CnH2n (functional group isomers of alkenes)

Ugh yes, I guess you have C2?
Remind me to go back in time and tell my past self not to do 3 Sciences and Maths!

Ah, that's why.

Isomers are definitely my weakest topic.

Ah, that's why.

Isomers are definitely my weakest topic.

I agree, I only fully learnt how to find out how many isomers a molecule has today!

Haha! lol

I had all the unit 1 sciences and 2 general studies.
And then I have got all the unit 2 sciences on the 3rd, 4th and 5th of June! Its crazy!
What subjects do you do?
I do the sciences and German.

How do you do that?

Unfortunately it's just by drawing out the displayed formula I tried to find a pattern to find out how many isomers, but apparently the pattern exists but requires an army of computers to calculate!

Luckily however, in the past papers I've only encountered I think was isomers for an alkane (of which there is only chain isomers) which are really easy to find out!

Could you show me an example and draw it out and post it on here please, would help a lot

This is basically a matter of learning some numbers. You will be presented with a molecule and you need to determine two things. The number of bonded pairs and the number of lone pairs on the centeral atom. Here's the learn and churn bit:

If there are 2 bonded pairs on the central atom then the angle is 180˚
If there are 3 bonded pairs on the central atom then the angle is 120˚
If there are 4 bonded pairs on the central atom then the angle is 109.5˚
If there are 6 bonded pairs on the central atom then the angle is 90˚

Now, lone pairs act the same as bonded pairs except the angle is reduced by 2.5˚. For example:

4 pairs on the central atom but one is a lone pair reduce the normal angle of 109.5 to 107˚ because the lone pair reduces the angle by 2.5˚

Thats all you need to know except the names of all of them which can be found easily in your textbook or the internet somewhere. The key to it is just drawing a good diagram and clearly stating the number of lone and bonded pairs..