OCR A2 CHEMISTRY F324 and F325- 14th and 22nd June 2016- OFFICIAL THREAD

Original post by ranz

someone pls clarifyyy! if a zwitteriom contained 2 nh2 and 2 cooh would they all protonate? or just one each?

Posted from TSR Mobile

just one each because the definition of a zwitterion is the diploar ionic form of an amino acid that is formed by the donation of a H ion from the carboxyl group to the amino group. As both charges are present there is no overall charge.

Both = 2

Reply 1288

El Chapo

What is a simple repeat unit?

Reply 1289

tcameron

19

Original post by itsConnor_

if in pH below isoelectric point (pH at which amino acid exists as a zwitterion), all COOH are COOH and all NH2 are NH3+
if in pH above isoelectric point, all COOH are COO- and all NH2 are NH2

Hope this makes any sense

the isolectric point is when it exists as a zwitterion, not below the isoelectric point

Reply 1290

embem10

1

Sorry I know people have asked a couple of times in this thread for the 2015 F324 mark scheme but I can't find it, please can someone post it again?

Reply 1291

marioman

http://stsmith.co.uk/data/documents/F324_MS_June15.pdf

Original post by embem10

Sorry I know people have asked a couple of times in this thread for the 2015 F324 mark scheme but I can't find it, please can someone post it again?

Reply 1292

http://stsmith.co.uk/data/documents/F324_MS_June15.pdf

This is what the mark scheme says on Zwitter ions for this question i was confused too so emailed my teacher, and this was her reply but I am still confused:
The NH2 group that is part of the R group is not affected (ionised) at the zwitterion pH, this is because the pH is adjusted to take this group into account. E.g the pH of this amino acid will be above pH 7 as it has a basic group.The NH2 group will be ionised if the amino acid is placed into a solution with a pH less than the zwitterion i.e. in acidic conditionsHope that helpsMiss P

(edited 7 years ago)

Reply 1293

embem10

1

Original post by marioman

You're a saint, thank you so much! <3

Reply 1294

annaj97

When drawing optical isomers in the tetrahedral shape, is there a standard place each group should be? Eg. on amino acids is the COOH drawn on the wedge and the NH2 on the dash? etc.

Reply 1295

Sahil_

Original post by annaj97

When drawing optical isomers in the tetrahedral shape, is there a standard place each group should be? Eg. on amino acids is the COOH drawn on the wedge and the NH2 on the dash? etc.

Nope, you can draw it in any orientation

Reply 1296

Original post by annaj97

When drawing optical isomers in the tetrahedral shape, is there a standard place each group should be? Eg. on amino acids is the COOH drawn on the wedge and the NH2 on the dash? etc.

I tend to draw the R group as the wedge as in its in the middle and the NH2 and COOH on either side with the single H vertically upwards, that way it quite clearly and a-amino acid, but i imagine you are not punished either way :smile:

Reply 1297

Original post by Magicalgeofray

This is what the mark scheme says on Zwitter ions for this question i was confused too so emailed my teacher, and this was her reply but I am still confused:

Help on this please as I very confused :frown:

Reply 1298

tcameron

19

Original post by Magicalgeofray

This is what the mark scheme says on Zwitter ions for this question i was confused too so emailed my teacher, and this was her reply but I am still confused:

Zwitterion is DIPOLAR ion not tripolar or quartpolar so only 2 groups should have a charge
Only when you add and acid or alkali do the other R groups then gain a charge as it is no longer in the dipolar ion (zwitterion) form

Reply 1299