Chemistry: Post your doubts here!

[qwertypoiu]

http://studyguide.pk/Past Papers/CIE/International A And AS Level/9701 - Chemistry/9701_s08_qp_2.pdf
In this same paper part aii, why can't we write the structural formula like CO2H CO2H? why do we have to writeHO2CCO2H? I always get confused while writting structural formulas! SOMEONE?

If you're referring to this:

Then we must write this as:
HO2CCO2H

Or as:
HOOCCOOH

We cannot however, put:
CO2HCO2H,
Since the carbons are bonded together, and in this you cannot tell that the carbons are bonding, rather it seems as though the carbon is bonded to hydrogen!

 

[shaeer999]

So, if it's an alcohol then it'll be C2(OH)H2 with double bond between C=C?

yes, dehydration reaction of an alcohol! So a double bond comes in

 

[immie.rose]

Please help me with this, anyone?

 

[Sarosh Jameel]

What is formed when HO2CCH=CHCO2H is reacted with hot conc KMNO4 ?

 

[immie.rose]

What is formed when HO2CCH=CHCO2H is reacted with hot conc KMNO4 ?

2 molecules of HOOCCOOH. The C=C bond breaks with both the C's involved in the bond being oxidised to form COOH.

 

[The Sarcastic Retard]

Please help me with this, anyone?
View attachment 52585

I hope this is helpful : http://www.docbrown.info/page07/equilibria6c.htm

 

[Studydayandnight]

Those lone pairs tho

You always seem to be panicking in your questions
A is with an alcohol so the malic acids Carboxy groups will react with them to form a diester
B is with a carboxylic acid so it'll react with the single OH present to form an ester.


Just keep these 2 things in mind and draw the structural formula like your simply making an ester from an acid and an alcohol.

Some of these questions arent that much of a problem,so i think you should at least revise the syllabus of organic a few times,and then attempt the past papers,no use just doing them without a clear concept.

Thanks! I did do that, but my structure was kinda messed up. :/

There are just sooooo many reactions and equations to remember. :'( I always forget one thing or the other. -_-

Thanks again though.

 

[Xylferion]

Here's the mark scheme: http://freeexampapers.com/A-Level/Chemistry/CIE/2012-Jun/9701_s12_ms_22.pdf

My doubt: How do I figure out how many lone pairs are present?

Look at the groups in which Nitrogen and Sulfur reside.

Nitrogen is in group 5 and Sulfur in group 6.
That means Nitrogen has 5 electrons in its outer-shell, and Sulfur, 6.

The number of bonds an atom can form is,
No of bonds = Full valence shell - Number of valence electrons

For example, nitrogen already has 5 electrons, it needs 3 more to achieve a noble gas structure. So it will form 3 bonds.
The above formula works the same way,
No of bonds in N = 8 - 5 = 3

So with that in mind, Nitrogen can form 3 bonds and Sulfur can form 2.

The next thing to note, is that each bond will have two electrons.
Since nitrogen needs to form 3 bonds, it will need 6 electrons.
Sulfur needs to form 2 bonds, so it will need 4 electrons.

Out of the 8 electrons that can exist in the outer-shell,

-Nitrogen needs 6, which leaves 2 electrons not participating in any bond.
-Sulfur needs 4, which leaves 4 not participating in a bond.

A lone pair = 2 electrons not participating in a bond.

Nitrogen therefore has 1 lone pair and Sulfur has 2.

Hope that helped you!

 

[Studydayandnight]

Look at the groups in which Nitrogen and Sulfur reside.

Nitrogen is in group 5 and Sulfur in group 6.
That means Nitrogen has 5 electrons in its outer-shell, and Sulfur, 6.

The number of bonds an atom can form is,
No of bonds = Full valence shell - Number of valence electrons

For example, nitrogen already has 5 electrons, it needs 3 more to achieve a noble gas structure. So it will form 3 bonds.
The above formula works the same way,
No of bonds in N = 8 - 5 = 3

So with that in mind, Nitrogen can form 3 bonds and Sulfur can form 2.

The next thing to note, is that each bond will have two electrons.
Since nitrogen needs to form 3 bonds, it will need 6 electrons.
Sulfur needs to form 2 bonds, so it will need 4 electrons.

Out of the 8 electrons that can exist in the outer-shell,

-Nitrogen needs 6, which leaves 2 electrons not participating in any bond.
-Sulfur needs 4, which leaves 4 not participating in a bond.

A lone pair = 2 electrons not participating in a bond.

Nitrogen therefore has 1 lone pair and Sulfur has 2.

Hope that helped you!

That really did help! Thank you!!

 

[Turki AbdulAziz]

Q.P: http://freeexampapers.com/A-Level/Chemistry/CIE/2013-Nov/9701_w13_qp_21.pdf
M.S : http://freeexampapers.com/A-Level/Chemistry/CIE/2013-Nov/9701_w13_qp_21.pdf

-In question 2b(ii) Do the isomers of the 3 Cyclic compounds represent cis-trans isomerism? If so, then how does cis-trans isomerism arise in cylclic compounds because I know that it occurs across double bonds of alkenes.

-And I really could not understand question 4(f).

It would be realy nice if someone could help.
And Thanks in Advance!

 

[NIM]

Need help!!!

 

[Xylferion]

Need help!!!

There's two axes, solvent 1 and solvent 2.

If the points move across the x axis, they have separated from solvent 2.
If the points move across the y axis, they have been separated from solvent 1.

It's asking for the "two" amino acids, that did not separate from solvent 1. Which means these two points are on the same line.


They did separate from solvent 2 though.

 

[Xylferion]

Q.P: http://freeexampapers.com/A-Level/Chemistry/CIE/2013-Nov/9701_w13_qp_21.pdf
M.S : http://freeexampapers.com/A-Level/Chemistry/CIE/2013-Nov/9701_w13_qp_21.pdf

-In question 2b(ii) Do the isomers of the 3 Cyclic compounds represent cis-trans isomerism? If so, then how does cis-trans isomerism arise in cylclic compounds because I know that it occurs across double bonds of alkenes.

-And I really could not understand question 4(f).

It would be realy nice if someone could help.
And Thanks in Advance!

Cis/Trans isomerism is basically just geometrical isomerism. It does not only apply to alkenes only, but to cycloalkanes too. They both have the formula CnH2n.

Try to think of the cycloalkane, as a ring placed flat on a surface. There are two planes, the top and the bottom one. So when you substitute an atom in place of the hydrogen, it really matters whether the atom is on the top or bottom plane.

For cyclopropane, since you're adding 2 chlorine atoms, there's two possible options. The halogen atoms are on the same carbon, or on two different carbons. Once you've finished drawing the structure for the atoms being on the same carbon, you have to consider the other situation.

When they are on two different carbons, they can both be on the same plane, or one on each. Similar to alkenes, when they're on the same plane, they're cis, and on different planes, trans.

----------------------------

For 4(f), you're going to be using your answer to (e).

If you look at the formula of your isomer, it's C4H4O4. In (f), it's C4H2O3.
Water has been removed, since two hydrogen atoms and one oxygen atom have been removed from the formula.

Now for this to happen, the OH group at the end of both -COOH groups, need to be close to each other, in order for this to happen under the presence of heat.
If you use your trans isomer, the -CO(OH) endings are far apart when compared to cis, so that's why trans does not react at the that temperature. ( 110 degrees )

The way it works is, the OH groups being close to other, one of the carbons ejects its OH group completely. With the OH- ion now with a lone pair, it attracts the H on the other -COOH group.

You end up with a molecule as such,

-COCH=CHCOO-

One carbons missing a bond and on the other end, oxygen is missing a bond. So the two end up sharing a bond, forming the cyclic compound.

-------> -------->

Hope all of that made sense!

 

[NIM]

There's two axes, solvent 1 and solvent 2.

If the points move across the x axis, they have separated from solvent 2.
If the points move across the y axis, they have been separated from solvent 1.

It's asking for the "two" amino acids, that did not separate from solvent 1. Which means these two points are on the same line.
View attachment 52614

They did separate from solvent 2 though.

Can u plz tell me the third part as well!!
Thnx a lot for d help

 

[Xylferion]

Can u plz tell me the third part as well!!
Thnx a lot for d help

X will be the most soluble spot in both solvents, since it's at the maximum values on both axes.

To find the most soluble amino acid besides this, you need to consider the spots that went the farthest on their axes as well as any spots that are on the straight line.

The spots that went the maximum are, (3,5) and (6,1), the one that's the highest up the straight line is (5,4).

If you multiply these numbers, you can find out which of them travelled the farthest.

3x5 = 15
6x1 = 6
5x4 = 20,

Hence the spot at (5,4) is most soluble in both solvents.

Note:- I do not take A2 chemistry, so I don't really know if there's an actual way you're meant to go about this. This is simply what made the most sense to me in terms of the way graphs work generally. ( This isn't a graph, but I considered it to be one )

 

[DESTROYER1198]

please help... why does the nucleus has more attraction in the cations than their atoms and why does the nucleus have less attraction in anions than their atoms

 

[ahmedish]

So

In excess NH3, a complex cation is formed [Ag(NH3)2]+

this works as tollens reagent right?

 

[ahmedish]

please help... why does the nucleus has more attraction in the cations than their atoms and why does the nucleus have less attraction in anions than their atoms

+ve ions are smaller as one electron shell is lost and the nuclear charge increases as it has to attract less electrons so ion is smaller.
-ve ions are larger as more electrons added which due to repulsion with already existing atoms expand to minimise repulsion.

 

[My Name]

http://onlineexamhelp.com/wp-content/uploads/2013/10/9701_s08_qp_2.pdf
question 3 part c iii

 

[asadalam]

http://onlineexamhelp.com/wp-content/uploads/2013/10/9701_s08_qp_2.pdf
question 3 part c iii

if kc increases with increase in temp that means forward is endo,as yield of right hand side will increase,hence increasing KC as KC is proportional to yield of forward reaction.