Edexcel Chemistry A2 Unit 5 ~ Wednesday 19th June 2013 (Now Closed)

The sodium carbonate will clearly remove SO2 and HBr since they're acids.
Then it hints that sodium sulphate is a drying agent by telling you that it is anhydrous and so it's not going to affect anything but the water.
This leaves you making an educated guess that Bromine would be removed by the sodium carbonate (like this )so the answer must be D.
As far as I know we don't have to actually learn or remember that sodium sulphate is a drying agent or that sodium carbonate removes bromine, you just have to use the hints they give you to try and work it out.

I see, in that case, I'd recommend ploughing through ChemGuide, there is a lot of useful information on there which I have personally found helpful.

Anyone doing unit 4 or can help? I have a few questions.

1. I don't understand drawing titration curves. I get where to start from and and where to end, depending on the acid and base etc. but how do you know what volume the equivalence point vertical section is supposed to be at and how many pH units it's supposed to go vertical for?

2. With buffer solutions is it the anion of the salt and the acid compound that's in excess for acid buffers and the salt cation and the base compound in basic buffers?

3. What criteria does the indicator have to fulfill in order to be suitable, I thought it was the pH range including the starting pH and changing colour just before the end pH?

Thanks

Hey guys. I have no idea which practical apparatus we need to know how to draw? Apart from reflux and distilling. Every now and again something comes up and I have no idea. I asked my teacher for a complete list and she dodged my question. Help please!

Thanks

I think the most likely ones are:
- Normal distillation
- Steam distillation
- Reflux

But who know's, they could do something obscure like get us to draw the apparatus for preparing chromium (II) ethanoate.
Potentially though they could ask us to draw any diagram since the start of unit 1 though which included things like diagrams for apparatus to collect gas and stuff.
I think though you basically just need to remember:
- Never completely seal the apparatus
- Don't leave gaps between joins unless they're meant to be there
- Water goes in at the bottom of a condenser and comes out at the top
- Label everything including reactants and products
- If you put a thermometer in, remember the bell of the thermometer needs to be at the 'junction' in the still head.

Someone's preparing to get an A*...

EDDIIT!! : does anyone also know why E cell doesnt tell us if a reaction will take place ?
thanks

I haven't done it for a while but from what I remember and my notes from unit 4:
1. The pH vertical units should be:
SB & SA → 4 - 10
WB & SA → 3 - 7.5
SB & WA → 6.5 - 11
The equivalence point is where equal proportions of acid and base have been added

2. I think so yes, an acid buffer has a large reservoir of acid and conjugate base whereas a base buffer has a large reservoir of base and conjugate acid.

3. A suitable indicator needs to change colour in the vertical region of the graph. (i.e. a range for colour change that the equivalence point lies in).

So if there's 20cm^3 of acid then the equivalence point is when 20cm^3 of alkali has been added? Thanks again

Because the E cell value only tells us if the reaction is feasible; in reality, the rate may be too slow for us to consider it to be happening under standard conditions

Hi,

Would anyone mind helping me out with a unit 4 question? It's Q 11a) here:

http://www.edexcel.com/migrationdocuments/QP%20GCE%20Curriculum%202000/June%202011%20-%20QP/6CH04_01_que_20110615.pdf

The mark scheme says it's C, but I don't understand why it couldn't be either A or C. Doesn't 2,4 DNP test for C=O. So would it not test positive with propanoic acid and negative with propan-1-ol?

Hi,

Would anyone mind helping me out with a unit 4 question? It's Q 11a) here:

http://www.edexcel.com/migrationdocuments/QP%20GCE%20Curriculum%202000/June%202011%20-%20QP/6CH04_01_que_20110615.pdf

The mark scheme says it's C, but I don't understand why it couldn't be either A or C. Doesn't 2,4 DNP test for C=O. So would it not test positive with propanoic acid and negative with propan-1-ol?

It only works for aldehydes and ketones. Can't remember the reason- sorry! Not sure we need to know why though, but it's definitely only for aldehydes and ketones

The lone pair of electrons from the 2 oxygen atoms are delocalised, so the electrons are shared between the OOH bit and so it doesn't really count as a C=O group.

The lone pair of electrons from the 2 oxygen atoms are delocalised, so the electrons are shared between the OOH bit and so it doesn't really count as a C=O group.

Brady's reagent will only give a positive result with an unmodified carbonyl group which means it has to be a ketone or aldehyde.
Sodium carbonate is always the test for a carboxylic acid group though. It fizzes lots as carbon dioxide is formed because it acts as an acid.