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

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Why does the equation here have to involve OH- ions??

The CO2 that is bubbled reacts with the water, H2O, that is on the left of the equilibrium equation, to form carbonic acid, H2CO3,

which dissociates to form H+ and HCO3-,

the H+ reacts with OH- on the right of the equilibrium equation, to form H2O,

So the position of equilibrium shifts to the right to replace the OH-,

sorry if this was already answered or didn't make sense

Reply 181

Professor L

Original post by mechanism

Can't polyamides be made from a carboxylic acid and an amine? Whereas polypeptides are made from lots of amino acids specifically, not lots of carboxylic acids and amines.

Yep

Reply 182

bioloves

2015 f321 f322 papers anyone ???

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Reply 183

mechanism

Can someone please explain how to construct the redox equation June 2014 Q8? Strugglingggggg

Reply 184

ranz

Original post by Kamara7

Yep

yes but u can also make amide bonds with amino acids according to the book thats why im confused

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Reply 185

itsConnor_

Start from the left to get the more 'important' peaks, and explain the splitting pattern of each (e.g. quartet=3 hydrogens on adjacent carbon) and try to link to hydrogen environment using the data sheet. You'll get quite a few marks from this even if you can't get the structure :smile:

Reply 186

mechanism

Original post by ranz

yes but u can also make amide bonds with amino acids according to the book thats why im confused

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an amide link is just the bond between an amine group, -NH2, and the carboxylic acid -COOH. So both polypeptides and polyamides have amide links, they are just formed in different ways.

Reply 187

ranz

Original post by mechanism

an amide link is just the bond between an amine group, -NH2, and the carboxylic acid -COOH. So both polypeptides and polyamides have amide links, they are just formed in different ways.

but theyre both formed by condesation reaction isnt it?

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Reply 188

rory58824

If you are unsure about the group, try writing all the information about every other peak, and then see what is remaining. It's a process of elimination in some way. See if that works out for you :smile:

Reply 189

zirak46

YOU KNOW THIS

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Reply 190

zirak46

Hello peeps,
In this question, why do I have to times the [OH-] by two??

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Reply 191

ReeceM1

Original post by zirak46

Hello peeps,
In this question, why do I have to times the [OH-] by two??

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There are two moles of OH- dissociated per mole of Ca(OH)2

Ca(OH)2 ---> Ca^2+ + 2OH-

Reply 192

zirak46

Original post by ReeceM1

There are two moles of OH- dissociated per mole of Ca(OH)2

Ca(OH)2 ---> Ca^2+ + 2OH-

Whaaaaat, i never knew that was a rule.

Is that the same if its h2so4? Meaning theres two moles of H?
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Reply 193

ReeceM1

Original post by zirak46

Whaaaaat, i never knew that was a rule.

Is that the same if its h2so4? Meaning theres two moles of H?
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It will always tell you in the question how many moles of H+ will dissociate in H2SO4,
But normally only 1 H+ does and with our spec I've only ever seen pH questions with monobasic acids.
In reality the first proton dissociates but the HSO4^- that remains actually acts as a weak acid:

(edited 7 years ago)

Reply 194

zirak46

Original post by ReeceM1

Ahhh nice but if theres ever Mg (OH)2 then itll be 2 moles of OH- ions?

Reply 195

ReeceM1

Original post by zirak46

Ahhh nice but if theres ever Mg (OH)2 then itll be 2 moles of OH- ions?

Yepp

Reply 196

username986184

Original post by mechanism

Can someone please explain how to construct the redox equation June 2014 Q8? Strugglingggggg

Have you managed this now?

Reply 197

mechanism

Original post by ranz

but theyre both formed by condesation reaction isnt it?

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Yes, H2O is released

Reply 198

mechanism

Original post by TeachChemistry

Have you managed this now?

Yes, thank you. Was not on my game yesterday haha.

Reply 199

itsConnor_

I struggle with it myself, but you would usually get some kind of an incling, e.g. a molecular formula (which may indicate no nitrogens present or a benzene ring at 7ppm)