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last SG · 2015-08-19T22:54:00+00:00

Topic : atomic radius Hi! This is from singapore cambridge nov a level exam year 2010,Paper 1 qn 1Qn: which element has the largest atomic radius?BrKKrScSo...

Flying grenade SG

Yes, might retake, and also for the pursuit of knowledge, and considering taking chemistry major in uni

UltimaOnline SG

2009 paper 3 qn 4d

Seems like all the amino acids can have ionic interactions+vdw interaction+hydrogen bonds with the given amino acid residues??

Help!!

What are the factors affecting hydrophobicity?

List out specifically all the possibilities of which amino acid R group has which interactions with which amino acid R group, and I'll tell you if you're correct or wrong.

Non-polar = hydrophobic. Go google yourself for generic conceptual qns like these.

Flying grenade SG

For di) amino acid residue,

Seems like there is a nh3+ group at ph7, and a N-H group

So since all val,tyr, aspartic acid has coo- at ph7, hence can interact with it via ionic interactions ?

Err, no h bond involved becos at ph 7 all cooh group deprotonate to form coo-?

Dii) amino acid residue

Have a benzene side chain group and a N-H side chain group of interest

Hence only hydrophobic methyl group on val can interact with it? Err, no compound can interact with the N-H here? Idk ))):

Flying grenade SG

What is the distinction between equilibrium shifts (to a side ) and lies (to a side) ??

Is it that for *shift* ,

Eqm alr shifted?

While *lies* to a side is the eqm is more inclined towards that side?

When to use which term ?? ):

UltimaOnline SG

For di) amino acid residue,

Seems like there is a nh3+ group at ph7, and a N-H group

So since all val,tyr, aspartic acid has coo- at ph7, hence can interact with it via ionic interactions ?

Err, no h bond involved becos at ph 7 all cooh group deprotonate to form coo-?

Dii) amino acid residue

Have a benzene side chain group and a N-H side chain group of interest

Hence only hydrophobic methyl group on val can interact with it? Err, no compound can interact with the N-H here? Idk ))):

Oi, do you know the difference between amino acid residue and amino acid??? This question is about R group interactions lah!

UltimaOnline SG

What is the distinction between equilibrium shifts (to a side ) and lies (to a side) ??

Is it that for *shift* ,

Eqm alr shifted?

While *lies* to a side is the eqm is more inclined towards that side?

When to use which term ?? ):

If you didn't change conditions, and Qc < Kc, then you say position of equilibrium LIES to the RHS. If you didn't change conditions, and Qc > Kc, then you say position of equilibrium LIES to the LHS.

If at first Qc = Kc, ie. system is at equilibrium, but then you changed experimental conditions, then you say position of equilibrium SHIFTED to either RHS or LHS.

If you increased or deceased the moles or molarities or partial pressures of reactants or products, then it's Qc which changes, not Kc.

If you or increased or decreased temperature, then it's Kc which changes, not Qc.

If Qc now < Kc, then you say position of equilibrium has SHIFTED to the RHS.
If Qc now > Kc, then you say position of equilibrium has SHIFTED to the LHS.

All these your own JC never teach you meh? That proves that you (yo JC students reading this) should come for my BedokFunland JC tuition. *hmmph*

Flying grenade SG

why and how* the 'sparingly soluble' metal hydroxide solid precipitate, eg. Cu(OH)2(s) dissolves when adding in excess a solution containing a suitable ligand, eg. NH3(aq), in terms of shifting of positions of equilibria across 2 (or more) reaction equations.

Eqn 1 : NH3(aq) + H2O(l) ---> NH4+(aq) + OH-(aq)

Eqn 2 : Cu2+(aq) + 2OH-(aq) ---> Cu(OH)2(s)

Eqn 3 : Cu2+(aq) + 4NH3(aq) ---> [Cu(NH3)4]2+(aq)

Attempted to apply this for

Agcl (s) + 2nh3 (aq) -> [Ag (NH3)2]+ + cl-

I got stuck , pls help :

1) NH3 (aq) +H2O -> NH4+ + OH-

2) Ag+ +Cl- -> Agcl (s)

3)Agcl (s) + 2nh3 (aq) -> [Ag (NH3)2]+ + cl-

Is this correct?

UltimaOnline SG

Attempted to apply this for

Agcl (s) + 2nh3 (aq) -> [Ag (NH3)2]+ + cl-

I got stuck , pls help :

1) NH3 (aq) +H2O -> NH4+ + OH-

2) Ag+ +Cl- -> Agcl (s)

3)Agcl (s) + 2nh3 (aq) -> [Ag (NH3)2]+ + cl-

Is this correct?

Ag+(aq) + 2 NH3(aq) ---> [Ag(NH3)2]+(aq)

Flying grenade SG

Yes i will spread the word of the majestic chem guru in the east to my friends, promise

Still unsure about the concept of Qc..

Qsp i know but what is Qc?

UltimaOnline SG

Yes i will spread the word of the majestic chem guru in the east to my friends, promise

Still unsure about the concept of Qc..

Qsp i know but what is Qc?

Eh, "chem guru" is the title used by Maverick Puah lah. He gives tuition at Bishan, while I give tuition at Bedok / Marine Parade. Just tell ur friends (in the eastern part of Singapore) to google "BedokFunland JC".

Qc has the same formula as Kc, but Qc is at initial and can take on any mathematical value, while Kc is at equilibrium, and has a fixed constant mathematical value for any reaction at a given temperature.

Flying grenade SG

Yea this the ionic eqn

Ag+(aq) + 2 NH3(aq) ---> [Ag(NH3)2]+(aq)

When ammonia solution is added to copper(II) sulfate solution, the hydrolysis of ammonia (Eqn 1) generates OH-(aq), and when Qsp (aka Ionic Product) > Ksp (aka Solubility Product), position of equilibrium in Eqn 2 shifts to the RHS, generating Cu(OH)2(s) precipitate. When excess ammonia solution is added, the position of equilibrium in Eqn 3 shifts to the RHS, resulting in a decrease in molarity of Cu2+(aq), which causes the position of equilibrium in Eqn 2 to shift to the LHS, which causes the copper(II) hydroxide precipitate to dissolve.

I attempted to rephrase this to suit agcl, but couldn't, cos there's no oh- and deleted the chunk i typed

Seems like theres a gap cos no oh- in eqn 2

How?

Flying grenade SG

Okay will do so to instruct them to search up your website

Seems like youre advocating mainly people in the east to sign up for yr tui

Perhaps thats unnecessary, those who see the treasure of knowledge : 40+ pages chem information on this forum and many more on ur website that you have shared for public use, think those that seek enlightenment wouldn't mind the travelling task

UltimaOnline SG

-_-"

Eqn 1 - Ag+(aq) + Cl-(aq) --> AgCl(s)
Eqn 2 - Ag+(aq) + 2 NH3(aq) ---> [Ag(NH3)2]+(aq)

When you add excess NH3(aq), position of equilibrium for Eqn2 shifts to RHS, hence position of equilibrium for Eqn1 shifts to LHS, hence AgCl(s) ppt dissolves.

With Br-, you need concentrated NH3(aq) because the Ksp for AgBr is much smaller, ie. less soluble. With I-, the Ksp for AgI is so damn small, that no amount of concentrated NH3(aq) or even NH3(l) can dissolve the AgI(s) ppt.

Flying grenade SG

For di) amino acid residue,

Seems like there is a nh3+ group at ph7, and a N-H group

So since all val,tyr, aspartic acid has coo- at ph7, hence can interact with it via ionic interactions ?

Err, no h bond involved becos at ph 7 all cooh group deprotonate to form coo-?

Dii) amino acid residue

Have a benzene side chain group and a N-H side chain group of interest

Hence only hydrophobic methyl group on val can interact with it? Err, no compound can interact with the N-H here? Idk ))):

I still dont get it. Why only coo- group of aspartic acid can interact with the amino acid di) residue given by the qn

Val and tyr aren't polypeptide chains, their discrete amino acids

Aren't the groups on val and tyr considered R groups?

UltimaOnline SG

I still dont get it. Why only coo- group of aspartic acid can interact with the amino acid di) residue given by the qn

Val and tyr aren't polypeptide chains, their discrete amino acids

Aren't the groups on val and tyr considered R groups?

Aspartic acid's R group is a COO- group. Valine's R group is an alkyl group. Tyrosine's R group is a phenol group. The question isn't asking for interactions between discrete amino acids. It's asking for interactions between R groups of these amino acid residues within a polypeptide chain. Don't lose your marks carelessly like this, it'll cost you at least 1 grade (with steep bell-curves).

Flying grenade SG

I still dont get it. Why only coo- group of aspartic acid can interact with the amino acid di) residue given by the qn

Val and tyr aren't polypeptide chains, their discrete amino acids

Aren't the groups on val and tyr considered R groups?

Update

Is my understanding correct, and can help improve my explanation?

Sian, think question interpretation might also be a problem

Is it because :in this qn, the amino acids are part of a polypeptide chain, e.g. val-tyr-asp- etc etc

Then, both val and tyr's nh2 and cooh is used up in amide bond linkage

Only aspartic acid has the extra cooh group, hence can interact with the qn?

But then wouldnt it be Hbond interactions?

I thought of another :

Is it because ph 7, so means its in water? ( thats why the acid and basic group will deprotonate and protonate??)

Thats why will have ionic interactions instead of h bonding?

But if its in water, then hydrolysis would occur, then wouldnt the nh2 and cooh group on val and tyr able to interact? ?

Help!

Flying grenade SG

If ph7 in water, will it cause the polypeptide chain to undergo hydrolysis and cause it to break into small portions of amino acid residues?

UltimaOnline SG

Update

Is my understanding correct, and can help improve my explanation?

Sian, think question interpretation might also be a problem

Is it because :in this qn, the amino acids are part of a polypeptide chain, e.g. val-tyr-asp- etc etc

Then, both val and tyr's nh2 and cooh is used up in amide bond linkage

Only aspartic acid has the extra cooh group, hence can interact with the qn?

But then wouldnt it be Hbond interactions?

I thought of another :

Is it because ph 7, so means its in water? ( thats why the acid and basic group will deprotonate and protonate??)

Thats why will have ionic interactions instead of h bonding?

But if its in water, then hydrolysis would occur, then wouldnt the nh2 and cooh group on val and tyr able to interact? ?

Help!

1st of all, hydrolysis only occurs partially, it's an acid-base *equilibrium* remember?

2ndly, technically (Singapore JCs don't teach this well so it's not surprising JC students don't really understand this), while COOH and NH2 groups are only capable of (both donating and accepting) H bonds, COO- and NH3+ groups are actually capable of participating in both H bonds (COO- can only accept H bonds, while NH3+ can only donate H bonds) as well as ionic bonds, depending on who the other party is.

And yes, Cambridge expects you to automatically first deprotonate COOH groups at pH7, and protonate NH2 groups at pH7.

Flying grenade SG

Can tyrosine's phenol group interact with nh3+?

If cannot, is it because phenol doesn't ionise in water?

UltimaOnline SG

If ph7 in water, will it cause the polypeptide chain to undergo hydrolysis and cause it to break into small portions of amino acid residues?

Oi!!! Wah biangz eh, if at pH7 and room temperature, proteins spontaneously "break into small portions of amino acid residues", then you won't be able to remain in 1 piece typing this post with your phone liao (you won't even be able to hold your phone), you'll disintegrate instantly into a sludge. Sludge man!

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