Edexcel Chemistry Unit 5 June 19 2012

Lol i'll ask my question here - i dont get question 11 for unit 5 june 2010. Also, why do zwitterions have a high melting temperature?

I haven't started my revision yet so can't help you with the past paper question, but Zwitterions have a high melting temperature because they can form intermolecular ionic bonds which are stronger than other interactions such as hydrogen bonding I believe.

they form hydrogen bonds...

they form hydrogen bonds...

The amino acids are crystalline solids with surprisingly high melting points. It is difficult to pin the melting points down exactly because the amino acids tend to decompose before they melt. Decomposition and melting tend to be in the 200 - 300°C range.

For the size of the molecules, this is very high. Something unusual must be happening.

If you look again at the general structure of an amino acid, you will see that it has both a basic amine group and an acidic carboxylic acid group.



There is an internal transfer of a hydrogen ion from the -COOH group to the -NH2 group to leave an ion with both a negative charge and a positive charge.

This is called a zwitterion.



A zwitterion is a compound with no overall electrical charge, but which contains separate parts which are positively and negatively charged.

This is the form that amino acids exist in even in the solid state. Instead of the weaker hydrogen bonds and other intermolecular forces that you might have expected, you actually have much stronger ionic attractions between one ion and its neighbours.

These ionic attractions take more energy to break and so the amino acids have high melting points for the size of the molecules

No they don't. Strong electrostatic attractions (ionic bonding) between the COO- and NH3+ of different zwitterions.

The longer version of what I said.

learn somethin new everyday.

the shape of proteins is caused by h bonds tho.

Thank you Revent for making a new thread.
Can't stand the unit 4 discussion; need to see unit 5 only

Oh btw, june 11 section C was hard!

The amino acids are crystalline solids with surprisingly high melting points. It is difficult to pin the melting points down exactly because the amino acids tend to decompose before they melt. Decomposition and melting tend to be in the 200 - 300°C range.

For the size of the molecules, this is very high. Something unusual must be happening.

If you look again at the general structure of an amino acid, you will see that it has both a basic amine group and an acidic carboxylic acid group.



There is an internal transfer of a hydrogen ion from the -COOH group to the -NH2 group to leave an ion with both a negative charge and a positive charge.

This is called a zwitterion.



A zwitterion is a compound with no overall electrical charge, but which contains separate parts which are positively and negatively charged.

This is the form that amino acids exist in even in the solid state. Instead of the weaker hydrogen bonds and other intermolecular forces that you might have expected, you actually have much stronger ionic attractions between one ion and its neighbours.

These ionic attractions take more energy to break and so the amino acids have high melting points for the size of the molecules

And you said you haven't started revision yet....

I thought it was okay, however, most people will have got low marks on it because they wouldn't have studied breathalysers in depth.

Google

I found the June 10 and Jan11 section C really difficult.
Anyone else agree?! :/ Going to fail..

Not so much jun10 but jan 11 caused some problems such as the last Q and the dry oil Q