Chemistry: Post your doubts here!

[qwertypoiu]

in NMR, is it true that protons in diff chemical environment have a different energy gap between their two states (i.e parallel and antiparallel) ?
Does the energy gap depend on how much shielded they are?

Yes, protons in different chemical environments will have different energy gaps between their low energy state and the higher "flipped" state.
You can read more about NMR here.

 

[qwertypoiu]

I meant to ask whether acidic amino acids ( those having a carboxylic group as their R group) OR basic amino acids (those having a basic group e.g nh3 as their R group ) can form zwitter ions?

According to Wikipedia a zwitterion is a neutral molecule that has both positive and negative charges within it.
So I think if you have an amino acid that has an R group with COOH or NH2, it should still be able to form zwitterions at a certain level of pH (known as isoelectric point). When pH changes, the COOH might turn to COO- or NH2 might turn to NH3+ so it will give the amino acid an overall charge (which you can test for by performing electrophoresis on it). When the amino acid has an overall charge it is no longer considered to be a zwitterion. Note that this effect happens even if the R group doesn't contain COOH or NH2 (eg even if R = H).
You can find out more about zwitterion here.

The isoelectric point of an amino acid with neutral side chain is usually around pH= 6
If the side chain is acidic (eg R = COOH), then a lower pH is required for zwitterions to form so that the COO- can be neutralized to COOH. Usually isoelectric point is pH = 3
If the side chain is basic, then the isoelectric point is higher (like pH=8). This is so that H+ is removed from NH3+ of side chain.
At isoelectric point, the zwitterion form is dominant. I got this from here.

Sorry for the long answer, basically yes they can form zwitterions.

 

[Saad the Paki]

View attachment 60050
ANSWER IS B HOW

We usually don't write the O2 in the cycle since its pretty much useless to the calculation it's ok even if you write it.

 

[holoholo]

http://papers.xtremepapers.com/CIE/... AS Level/Chemistry (9701)/9701_s15_qp_52.pdf

How do draw in question 1 b(i) ? Much appreciated for the answer

 

[The Sarcastic Retard]

Step 1 is slowest: So reaction rate depends on concentration of H2O2 and I-, both one mole, so a=1, b=1, c=0 (no dependence on H+)

Step 2 is slowest: reaction rate is dependent on IO- and H+. Now we have the H+ in the rate equation, so we can set c=1, but where is IO-?? How do we take that to account? Why isn't it even in the rate reaction? Because it is an INTERMEDIARY!.
You must always account for intermediaries in slow steps by tracing it back and seeing what produced it.
So our reaction depends on IO-, and IO- is generated by step1, so IO- depends on H2O2 and I-.
So we depend on three things: H2O2, I-, and H+. Each is 1 mole. So a=1, b=1, c=1.

Step 3 is slowest. Same method as above.
Dependent on HOI, H+ and I-.
HOI was made by IO- and H+, so lets replace the sentence above:
IO-, H+, H+, I-

Replace IO- by what produced it: (H2O2 and I-):

H2O2, I-, H+, H+

So you can see there are two moles of H+ being dependent upon, and two moles of I-, so b=2, c=2, and a =1.

But according to rate equaton if a=1,b=1 and c=1 that means three molecule i.e. H2O2,I- and H+ collide at the same time.
and according to above explanation first IO- is generated and then IO- collide with H+, which denies the fact that three molecules collide at the same time.
Can u please explain this fact..

 

[qwertypoiu]

But according to rate equaton if a=1,b=1 and c=1 that means three molecule i.e. H2O2,I- and H+ collide at the same time.
and according to above explanation first IO- is generated and then IO- collide with H+, which denies the fact that three molecules collide at the same time.
Can u please explain this fact..

IO- was generated as a result of the two other reactants (H2O2 and I-). So is it just like as if the three molecules were colliding together.

 

[princess Anu]

According to Wikipedia a zwitterion is a neutral molecule that has both positive and negative charges within it.
So I think if you have an amino acid that has an R group with COOH or NH2, it should still be able to form zwitterions at a certain level of pH (known as isoelectric point). When pH changes, the COOH might turn to COO- or NH2 might turn to NH3+ so it will give the amino acid an overall charge (which you can test for by performing electrophoresis on it). When the amino acid has an overall charge it is no longer considered to be a zwitterion. Note that this effect happens even if the R group doesn't contain COOH or NH2 (eg even if R = H).
You can find out more about zwitterion here.

The isoelectric point of an amino acid with neutral side chain is usually around pH= 6
If the side chain is acidic (eg R = COOH), then a lower pH is required for zwitterions to form so that the COO- can be neutralized to COOH. Usually isoelectric point is pH = 3
If the side chain is basic, then the isoelectric point is higher (like pH=8). This is so that H+ is removed from NH3+ of side chain.
At isoelectric point, the zwitterion form is dominant. I got this from here.

Sorry for the long answer, basically yes they can form zwitterions.

Sorry to argue but I still don't get it
look, every amino acid has a amine group, a carboxylic group, a H and a R group
If an amino acid is like glycine with R group neither acidic nor basic the net charge can be O at a certain ph known as the isolectric point as you said
and at this point it exists as a zwitter ion
but what if the R group is not non polar.. in that case won't the additional acidic or basic group ( the R group) confer a net charge as positive or -ve on the amino acid almost always? and so it may not ever form a zwitter ion? :s

 

[qwertypoiu]

Sorry to argue but I still don't get it
look, every amino acid has a amine group, a carboxylic group, a H and a R group
If an amino acid is like glycine with R group neither acidic nor basic the net charge can be O at a certain ph known as the isolectric point as you said
and at this point it exists as a zwitter ion
but what if the R group is not non polar.. in that case won't the additional acidic or basic group ( the R group) confer a net charge as positive or -ve on the amino acid almost always? and so it may not ever form a zwitter ion? :s

If this R group exists in the form COOH or NH2 then it has no charge to contribute. If it exists as NH3+ or COO- then it will contribute a charge so then the amino acid won't be considered a zwitterion.

 

[princess Anu]

Yes, protons in different chemical environments will have different energy gaps between their low energy state and the higher "flipped" state.
You can read more about NMR here.

How does the energy required for promotion of proton from low energy to high energy state depend upon;

• The number/spin states of adjacent protons or protons attached to adjacent atoms ??
  
  J12/42/q7 Marking scheme mentions this point

 

[princess Anu]

If this R group exists in the form COOH or NH2 then it has no charge to contribute. If it exists as NH3+ or COO- then it will contribute a charge so then the amino acid won't be considered a zwitterion.

OH YEAH!
Jazak Allah

))

 

[Hamody]

Plz does anyone know a chemistry revision guide ? I would really appreciate it

 

[My Name]

Plz does anyone know a chemistry revision guide ? I would really appreciate it

Here
This thread does not have the latest version but anyway.

 

[qwertypoiu]

How does the energy required for promotion of proton from low energy to high energy state depend upon;

• The number/spin states of adjacent protons or protons attached to adjacent atoms ??
  
  J12/42/q7 Marking scheme mentions this point

I'll be honest I went through the analytical chemistry portion very fast so I didn't bother with the scientific explanation of how the proton is affected by protons on the adjacent carbon atom (why or how exactly the splitting occurs)
But I think it isn't required in the syllabus. We just need to know how to interpret the splittings. You can read about it here.
If you're interested in the scientific (and I suspect mathematical) explanations of how the triplets occur and in their particular ratios (eg 1:3:3:1) maybe someone else can help.

 

[The Sarcastic Retard]

Paper 5 is the biggest trouble which will not let me end with nice grades. Help me. I usually end to get 80 in paper 4 and 15 in paper 5

 

[awesomaholic101]

Paper 5 is the biggest trouble which will not let me end with nice grades. Help me. I usually end to get 80 in paper 4 and 15 in paper 5

Paper 5 is the easy one. You don't even have to study that much. I guess you just need practice and basic logic.

 

[The Sarcastic Retard]

Paper 5 is the easy one. You don't even have to study that much. I guess you just need practice and basic logic.

What is the minimum and maximum score u got in it?

 

[shahzaib9291]

part 3 and 4

 

[The Sarcastic Retard]

part 3 and 4View attachment 60077

 

[awesomaholic101]

What is the minimum and maximum score u got in it?

I had only two tests in p5 ... both of which were my mocks.
Scored 24 in one and 19 in the other.
But it's true that sometimes we mess up easily in p5 ... that problem is there.

 

[The Sarcastic Retard]

I had only two tests in p5 ... both of which were my mocks.
Scored 24 in one and 19 in the other.
But it's true that sometimes we mess up easily in p5 ... that problem is there.

Can u link that papers here.
True.