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AQA Chemistry Unit 2 with Anon_98.

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Reply 80

Anon_98

These are the structures of an aldehyde, ketone and carboxylic acid.

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Reply 81

Anon_98

Alcohols- Classification + reactions - Topic 10.

The alcohol oxidising agent is - K2CR2O7 which is represented as [O] in equations.

Oxidation of primary alcohols to aldehydes

E.g. CH3CH2CH2OH (propan-1-ol) + [O] -----------> CH3CH2CHO(propanal) + H2O

Conditions: Heat oxidant, add excess alcohol, distill off product as it forms.

~

Oxidation of primary alcohols to carboxylic acids

E.g. CH3CH2CH2OH (propan-1-ol) + 2[O] -----------> CH3CH2COOH(propanoic acid) + H2O

Conditions: Excess oxidising agent, reflux.

~

Oxidation of secondary alcohols to ketones

E.g. CH3CH(OH)CH3 (propan-2-ol) + [O] -----------> CH3COCH3(propanone) + H2O

Conditions: Excess oxidising agent, reflux.

~

Oxidation of tertiary alcohols

E.g. 2-methylpropan-2-ol.

Tertiary alcohols are not easily oxidised bc they lack a H atom on the C atom bonded to OH therefore it will not turn orange acidified K2CR2O7 solution green.

Reply 82

Anon_98

Alcohols- Classification + reactions - Topic 10.

Distinguishing between aldehydes and ketones

Right, so there are basically 3 tests which can be used to distinguish between an aldehyde and a ketone.

The first test is K2CR2O7, H2SO4 (orange solution)

Observation after adding to aldehyde: Green solution.
Observation after adding to ketone: No reaction.

The second test is Fehling's solution (blue solution)

Observation after adding to aldehyde: Brick red precipitate.
Observation after adding to ketone: No reaction.

The third + final test is Tollen's reagent (colourless solution)

Observation after adding to aldehyde: Silver mirror.
Observation after adding to ketone: No reaction.

~

In all three reactions, the aldehyde is oxidised to a carboxylic acid on warming.

E.g. CH3CH2CHO + [O] ------------------> CH3CH2COOH

Reply 83

Anon_98

Alcohols- Elimination - Topic 10.

Alkenes from alcohols

E.g. CH3CH2CH2OH --------------------------> CH3CH=CH2 + H2O

Type of reaction: Elimination AKA dehydration.
(Remember the H lost must be on adjacent C atom to C-OH bond)

Conditions: Concentrated H2SO4 (catalyst), 180C

Usefulness: It's a possible route to polymers without using monomers derived from oil.

Reply 84

Anon_98

Analytical techniques- Mass spectrometry - Topic 11

Low resolution (Integer values): Gives the Mr of a compound.

High resolution(> 4 dp values): Gives Mr which can be used to determine molecular formula.

Reply 85

Anon_98

Analytical techniques- Infrared spectroscopy - Topic 11

You need to know that groups in molecules absorb infrared radiation at characteristic frequencies. (The values are all on the back of the periodic table)

There is a region below around 15000cm^-1 known as the fingerprint region.

The finger print region allows the identification of a molecule by matching it with a known sample spectrum.

Also, you can use spectra to identify impurities that may be revealed by absorptions that should not be there.

~

Global warming and infrared

The bonds in CO2, water and methane absorb infrared radiation and retain the heat in the earth's atmosphere and an increase in these gases due to human activity may contribute to global warming.

Reply 86

Anon_98

Well, I have officially covered the entire spec + I know everything I need to.. so I suppose this thread is now, finally done! I'M TAKING A BREAK UNTIL MIDNIGHT... (then I'll do past papers until 4am)

Spoiler

Reply 87

Guls

Original post by Anon_98

Equilibria -The Dynamic nature of Equilibria - Topic 3.

Dynamic

- Both forward and backward reactions are proceeding at equal rates.
- Concentrations of reactants and products remain constant.

the dynamic in dynamic equilibrium actually means that the rate of the forward and backward reactions occur at the same time.

Reply 88

Anon_98

Original post by Guls

the dynamic in dynamic equilibrium actually means that the rate of the forward and backward reactions occur at the same time.

(edited 7 years ago)

Reply 89

Guls

Original post by Anon_98

I know that they occur at the same rates but look at question 1(b) for june 2014
filestore.aqa.org.uk/subjects/AQA-CHEM2-W-MS-JUN14.PDF

Spoiler

Reply 90

Anon_98

Original post by Guls

I know that they occur at the same rates but look at question 1(b) for june 2014
filestore.aqa.org.uk/subjects/AQA-CHEM2-W-MS-JUN14.PDF

Spoiler

Lucky that it's only 1 mark- I would've got that wrong. :s-smilie:

Thanks!

Reply 91

Guls

Original post by Anon_98

Lucky that it's only 1 mark- I would've got that wrong. :s-smilie:

Thanks!

lol, i dropped that mark myself. Your welcome :h:

Reply 92

username1194172

Ah you're a legend anon, are you making a CHEM4 thread too?

Reply 93

Anon_98

Original post by Louisss

Ah you're a legend anon, are you making a CHEM4 thread too?

Not sure I'd say legend but ty aha :blush:

Yes, well - I'm currently doing the Biology unit 4 + I'm planning to make one straight after I've finished with that. :'3

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Reply 94

hopingmedicinae

Stereoisomerism. - Molecules with the same structural formula, but their bonds are arranged differently in space.

E-Z isomerism

- This is a type of stereoisomerism which is exhibited by alkenes.
- They exist due to the restricted rotation about the C=C bond.
- E-Z isomers have different physical properties but they contain the same functional group so they have similar chemical properties.

Sorry if this is wrong, these notes are mega helpful; but don't stereoisomers have same molecular formula; not structural formula?

Reply 95

Anon_98

Original post by hopingmedicinae

Sorry if this is wrong, these notes are mega helpful; but don't stereoisomers have same molecular formula; not structural formula?

I'm glad you're finding them useful!

+ ah no, that's alright but I think you're defining structural isomers which is one of the required definitions for Chem unit 1 (chain, position, functional group stuff) whereas here we're looking at stereoisomerism.(E-Z + optical stuff)

Reply 96

hopingmedicinae

Original post by Anon_98

I actually just looked it up, it can be both! :smile:

Good luck with the exam, just looked up how electrolysis fully works just in case they decide to ask us to outline an experiment or something obscure like that.

Trying to mindmap what other obscurities there could be testing theory like the VDW question in Chem1.

Reply 97

Anon_98

Original post by hopingmedicinae

I actually just looked it up, it can be both! :smile:

Oh great!

+ yeah, after sitting quite a few resit papers this season, I really don't think it'll be as straightforward as last year's.- I don't particularly like this new style, personally think that there'll be a more than usually horrible Hess' Law Cycle or a two-step complicated calorimetry question.. or something like that. Hm..we'll just have to see.

Thank you though + good luck to you too! <3

Reply 98

username1194172

Original post by hopingmedicinae

I actually just looked it up, it can be both! :smile:

Could you share some of these ideas/obscure questions you think may come up?

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Reply 99

hopingmedicinae

Just some notes

Fermentation done in an absence of air b/c ethanol could be oxidized to ethanoic acid
- Uses of polyethene - plastic bags, buckets, bottles
- Polypropene - carpets, rope, stiffer polymer so used in utensils and is recyled

- Aluminium useful because it does not corrode or less corrosion
- In Hess Cycles difficult to measure enthalpy directly because it's difficult to measure
e.g in an experiment where water is being added, it's difficult to know how much water to add for the reaction to complete

- Methanol from CO
CO + 2H2----> CH3CHOH
- Zinc and chromium oxide catalysts
T = 400
Low temperature = good yield slow rate
High pressure = good yield and high rate but costs for pumps to produce the pressure are high

- Recyle unreacted reactants back into reaction is a way for improving yield

- Using alcohols to make alkenes provides a way to polymers without using monomers derived from oil

- bond angles between C-O-H in CH3-CH-OH is 104.5

That's all I can think of