AQA Chemistry A2 Unit 4

We've been told nothing about lactic acid. You could check the specification. I will just leave it because I have not seen anything about it and I think it's unlikely to come up.

For the structure of benzene you'll need to know:

> The simplest arene is benzene. It has the molecular formula C6H6. Its basic structure is six C atoms in a hexagonal ring, with one H atom bonded to each C atom. Each C atom is bonded to two other C atoms and one H atom by single covalent σ-bonds. This leaves one unused electron on each C atom in a p orbital, perpendicular to the plane of the ring.

> The Six p electrons are delocalised in a ring structure above and below the plane of carbon atoms.

> The six electrons in the pi bonds are delocalised and spread out over the whole ring. Delocalised means not attached to a particular atom.

> Delocalisation and evidence for delocalisation i.e. enthalpies of hydrogenation. Lone pairs of groups delocalise into the benzene ring. Know the effects of this on acidity and reactions.

> Benzene is planar, each (sigma) bond in the ring has the same length.

> Length of bonds in benzene are longer than double bonds and shorter than single bonds.

> Benzene ring is negative and attracts electrophilles.


That's all that comes to my mind right now! Good luck! (:

Thank you so much! The only other thing I have been told is that the electrons are free to move in the pi system (explaining teh delocalisation) and this is why it is more stable). Also are required to know that graph that shows the different enthalpies of hydrogenation? Where you compare benzene with the triene?

Also why is cyclohexatriene theoretical? Is there are reason why it doesn't exist?

My pleasure!

You're not expected to know any graphs or data for benzene. If any questions come up about it they will provide the information. Just know how to prove that benzene and cyclohexatriene have different enthalpies, which I do below.

It's theoretical because actual experiments have shown that benzene has a different enthalpy to what we'd expect. To break one double bond requires an enthalpy of -120kJ/mol. For cyclohexatriene (which has three double bonds) we'd expect 3 x -120 = -360kJ/mol. However in the real world benzene has an enthalpy of -208kJ/mol. Suggesting that some other force is in play - delocalisation (also known as resonance) energy.

The original enthalpy of cyclohexatriene was based on what we know from previous experiments - 3 x (C=C enthalpy) so is purely theoretical (just like using mean bond enthalpies in AS) where as an actual experiment with benzene shows different values (due to other forces). So cyclohexatriene doesn't technically exist because we don't get it's expected values through experiments, we've just calculated it.

Any more questions on any unit, I'd be happy to answer! :P

Okay, thank you, that makes a lot of sense.
The one last question I wanted to ask is what kind of questions could they ask about chromatography? Because not many past papers seem to have questions on it so I think they might ask us questions on it.

Idk if you're doing the exam this year but if you are, how are you revising for it? I'm just doing past papers and then going over where I lost the marks and doing more notes and completing questions on those topics. I'm not really sure what else to do?

I am also doing the exam this year and I've been doing pretty much the same as you, just past paper questions especially on areas I dislike. Also I've treated the organic reactions/routes and ligand ion colours as maps which allow me to recall them better, rather than trying to memorize multiple combinations. My main focus right now is on OCR Psychology because unfortunately that is just writing fast and remembering loads of random numbers! xD

Here's what you need to know for chromatography:

> Chromatography is an analytical technique that separates components in a mixture between a mobile phase and a stationary phase.

> Separation by column chromatography depends on the balance between solubility in the moving phase and retention in the stationary phase.

> A solid stationary phase separates by adsorption,A liquid stationary phase separates by relative solubility.

> The mobile phase may be a liquid or a gas.The stationary phase may be a solid (as in thin layer chromatography, TLC) or either a liquid or solid on a solid support (as in gas chromatography, GC).

> If the stationary phase was polar and the moving phase was non- polar e.g. Hexane. Then non-polar compounds would pass through the column more quickly than polar compounds as they would have a greater solubility in the non-polar moving phase.(Think about intermolecular forces).

> In gas-liquid chromatography GC the mobile phase is an inert gas such as nitrogen, helium, argon.The Stationary phase is a liquid on an inert solid.

> Gas-liquid chromatography can be used to separate mixtures of volatile liquids.

> The time taken for a particular compound to onto a solid.travel from the injection of the sample to where it leaves the column to the detector is known a sits retention time. This can be used to identify a substance.

> Some compounds have similar retention times so will not be distinguished.

> Basic gas-liquid chromatography will tell us how many components there are in the mixture by the number of peaks. It will also tell us the abundance of each substance. The area under each peak will be proportional to the abundance of that component.

> It is also possible for gas-liquid chromatography machine to be connected to a mass spectrometer, IR or NMR machine, enabling all the components in a mixture to be identified.

> Most commonly a mass spectrometer is combined with GC to generate a mass spectra which can be analysed or compared with a spectral database by computer for positive identification of each component in the mixture.

> GC-MS is used in analysis, in forensics, environmental analysis, airport security and space probes.


Of course you can also check the specification because that covers everything that can possibly be asked. If you have twitter, feel free to tweet me any quick questions @akaDirewolf cause my responses here will be kinda slow haha! xD Best of luck for your exams!

that was so helpful mate thanks!

what book are you using to revise, like where did you get all this information from

Np!

For revision I use 'chemrevise' notes, because normal textbooks tend to have lots of rubbish and additional information which you will never be tested on. These notes however are based directly off the specification, so only cover things that can come up and are to the point!

> https://chemrevise.org/revision-guides/

Make sure you scroll down and select the ones for the previous spec, not the ones at the top, as they are for the new chemistry specification.

Best of luck for the exams my friend! (:

I am also doing the exam this year and I've been doing pretty much the same as you, just past paper questions especially on areas I dislike. Also I've treated the organic reactions/routes and ligand ion colours as maps which allow me to recall them better, rather than trying to memorize multiple combinations. My main focus right now is on OCR Psychology because unfortunately that is just writing fast and remembering loads of random numbers! xD

Here's what you need to know for chromatography:

> Chromatography is an analytical technique that separates components in a mixture between a mobile phase and a stationary phase.

> Separation by column chromatography depends on the balance between solubility in the moving phase and retention in the stationary phase.

> A solid stationary phase separates by adsorption,A liquid stationary phase separates by relative solubility.

> The mobile phase may be a liquid or a gas.The stationary phase may be a solid (as in thin layer chromatography, TLC) or either a liquid or solid on a solid support (as in gas chromatography, GC).

> If the stationary phase was polar and the moving phase was non- polar e.g. Hexane. Then non-polar compounds would pass through the column more quickly than polar compounds as they would have a greater solubility in the non-polar moving phase.(Think about intermolecular forces).

> In gas-liquid chromatography GC the mobile phase is an inert gas such as nitrogen, helium, argon.The Stationary phase is a liquid on an inert solid.

> Gas-liquid chromatography can be used to separate mixtures of volatile liquids.

> The time taken for a particular compound to onto a solid.travel from the injection of the sample to where it leaves the column to the detector is known a sits retention time. This can be used to identify a substance.

> Some compounds have similar retention times so will not be distinguished.

> Basic gas-liquid chromatography will tell us how many components there are in the mixture by the number of peaks. It will also tell us the abundance of each substance. The area under each peak will be proportional to the abundance of that component.

> It is also possible for gas-liquid chromatography machine to be connected to a mass spectrometer, IR or NMR machine, enabling all the components in a mixture to be identified.

> Most commonly a mass spectrometer is combined with GC to generate a mass spectra which can be analysed or compared with a spectral database by computer for positive identification of each component in the mixture.

> GC-MS is used in analysis, in forensics, environmental analysis, airport security and space probes.


Of course you can also check the specification because that covers everything that can possibly be asked. If you have twitter, feel free to tweet me any quick questions @akaDirewolf cause my responses here will be kinda slow haha! xD Best of luck for your exams!

Thank you! We were never properly taught chromatography so I never understood it. I just wanted to ask, I have had a look on the spec and there doesn't seem to be a mention about understanding how nmr works like with it being 13C rather than 12C. Do you know if we need to know it?

Also, I'm getting more worried about this exam because Unit 2 yesterday was just so weird and also the A2 EMPA paper this year was a joke

was chem 2 hard? was it like normal papers or?

trying to prepare for this chem 4 mock, and wanna have some sort of idea of how they'll ask questions

Thank you! We were never properly taught chromatography so I never understood it. I just wanted to ask, I have had a look on the spec and there doesn't seem to be a mention about understanding how nmr works like with it being 13C rather than 12C. Do you know if we need to know it?

Also, I'm getting more worried about this exam because Unit 2 yesterday was just so weird and also the A2 EMPA paper this year was a joke

I'm sure I have seen lactic acid in the spec- I think they call it 2-hydroxypropanoic acid- and I think its under the Nomenclature & isomerism topic.
But basically for that all you need to know is how to draw it as its two optical isomers.
Also seen a question in a past paper about its synthesis which again is in the CGP AQA CHEM A2 textbook if you have that.

Would deffo check it out, I'm certain its come up before and it is in the textbook.

Also having a look at last years paper, they asked about benzene itself and used its bond length to ask questions on hydrogenation enthalpies of two other cyclic alkenes. So you had to apply the rules about delocalisation and bond length - check last years paper out for deffo!

Hope this helps.
Good luck to everyone

Haha agreed! The unit 2 was horrible! Perhaps because I didn't do much revision lol! xD We've been told nothing about how NMR works, so I'd give it a pass as long as you know what it does, you should be fine!

Yeah, I think it's mostly just about knowing how to draw out an optical isomer. But I'll have a look for the question about it's synthesis.

I had last year's paper as a mock back in Jan so I saw the whole thing about benzene. It was really harsh of them tbh, in the spec there isn't really anywhere it mentions that you need to understand about delocalisation and bond length outside the context of benzene. I'm guessing they won't put anything like that in the paper this year because everyone knows about it now.

Good luck to you as well!

When it asks to work out the number of peaks in HNMR spec, thats for the number of hyrogens on the adjacent carbon right? So how do you know if they're talking about on the left or the right?
I hope this makes sense!

i dont think youll get asked about a compound that has carbons with hydrogen on either side of the carbon with hydrogen we're looking at.

ygm?