Chemistry: Post your doubts here!

[princess Anu]

Are you confused between C and D?

yeah

 

[benny law]

is there any thread that do question-by-question explanation? because i am quite weak at chemistry and when i do these past year i m getting crazy ......

 

[FrostBite92]

Do I need to study the H2 Singapore papers too, for chemistry? It seems significantly harder than the ones we have to do, so I thought it'll make good practice papers. But, does the questions there differ much from our syllabus? Or studying Intl' AS and A level papers would be sufficient?

 

[princess Anu]

when do we consider cis trans separately and when together?

why is ans C not D?

 

[princess Anu]

why not A?

 

[Metanoia]

View attachment 51123 can somebody explain this example
High charge density causes increase in covalent charater OR ionic character? :s

High charge density increases covalent character. The cation would be able to attract the electron cloud of the anion, resulting in a slight sharing of the electrons. Recall that sharing of electrons basically means covalent bonding.

What is the reason for the change in the shape of the graph for BOILING and Melting Points? :/

Start by grouping the elements into types of bonding
metallic (Na,Mg,Al) among metals, more delocalise electrons, higher mp/bp
giant covalent (Si) strongest among the 3 types of bonding.
simple covalent (P, S, Cl, Ar) , among simple molecules, higher Mr, higher mp/bp

 

[Metanoia]

can sumbody explain me q16
http://papers.xtremepapers.com/CIE/Cambridge International A and AS Level/Chemistry (9701)/9701_s07_qp_1.pdf

Are you confused between C and D?

Both C and D are true statements on their own, but we have to decide which one contributes most to the observation that overall,
H2 + I2 --> 2HI is more endothermic than H2 + Cl2 --> 2HCl



The reactions requires breaking of I-I and Cl-Cl bonds, and forming of H-I and H-Cl bonds.
Comparing I-I and Cl-Cl , there is a difference of 242 - 151 = 92 kJ
Comparing H-I and H-Cl , there is a difference of 431 - 299 = 132 kJ

Therefore, it is the difference in strengths of the H-X bonds that makes the most impact.

 

[Metanoia]

Do I need to study the H2 Singapore papers too, for chemistry? It seems significantly harder than the ones we have to do, so I thought it'll make good practice papers. But, does the questions there differ much from our syllabus? Or studying Intl' AS and A level papers would be sufficient?

Even though the topics covered are similar, the questions for Singapore papers are much tougher and demanding than 9701.

It would be best to practice past year papers from Intl' AS and A level instead.

 

[Metanoia]

when do we consider cis trans separately and when together?

why is ans C not D?

Consider all possibilities whenever applicable e.g. cis-cis, cis-trans, trans-cis, trans-trans

Post your diagrams and I'll try to tell you where you went wrong.

 

[princess Anu]

Consider all possibilities whenever applicable e.g. cis-cis, cis-trans, trans-cis, trans-trans

Post your diagrams and I'll try to tell you where you went wrong.

 

[princess Anu]

High charge density increases covalent character. The cation would be able to attract the electron cloud of the anion, resulting in a slight sharing of the electrons. Recall that sharing of electrons basically means covalent bonding.


Start by grouping the elements into types of bonding
metallic (Na,Mg,Al) among metals, more delocalise electrons, higher mp/bp
giant covalent (Si) strongest among the 3 types of bonding.
simple covalent (P, S, Cl, Ar) , among simple molecules, higher Mr, higher mp/bp

I agree, but the example says increase in ionic character :s

 

[princess Anu]

Start by grouping the elements into types of bonding
metallic (Na,Mg,Al) among metals, more delocalise electrons, higher mp/bp
giant covalent (Si) strongest among the 3 types of bonding.
simple covalent (P, S, Cl, Ar) , among simple molecules, higher Mr, higher mp/bp

Right, but the picture i posted had separate graphs for M.P and B.P that were Not exactly same.. so my question is why not

 

[Metanoia]

I agree, but the example says increase in ionic character :s

The book is incorrect.

Right, but the picture i posted had separate graphs for M.P and B.P that were Not exactly same.. so my question is why not

We would not expect the graphs to be totally the same shape, nor have a linear relation. Forces of attraction is depending of size, charges, electrons, 3D packing, etc. there are too many factors to expect totally overlapping graphs for both mp and Bp.
More important is the general trend.

 

[Metanoia]

In this case, the cis-trans is the same as the trans-cis. So your 2nd and 4th are essentially the same molecules.

The often neglected part is the the molecule can rotate (twist) along the C-C single bond.

 

[princess Anu]

In this case, the cis-trans is the same as the trans-cis. So your 2nd and 4th are essentially the same molecules.

The often neglected part is the the molecule can rotate (twist) along the C-C single bond.

View attachment 51212

even if those single bonded carbon atoms are double bonded with other carbon atoms?

 

[Metanoia]

even if those single bonded carbon atoms are double bonded with other carbon atoms?

Yes. The rotation is between the C-C bonds, doesn't matter C atoms are attached else where.
I can't find a video on C=C-C=C, but here is a video of C-C-C-C, where the rotation along the middle bond is shown.

 

[cashew]

http://papers.xtremepapers.com/CIE/...d AS Level/Chemistry (9701)/9701_s09_qp_1.pdf
mcq 5 and 6. I am stuck, can someone explain me the answers?

 

[Metanoia]

http://papers.xtremepapers.com/CIE/Cambridge International A and AS Level/Chemistry (9701)/9701_s09_qp_1.pdf
mcq 5 and 6. I am stuck, can someone explain me the answers?

Hi there, could you share your thoughts on the 2 questions so I know why you are stuck.

 

[cashew]

Hi there, could you share your thoughts on the 2 questions so I know why you are stuck.

hey metonia, I am done with q6 but i was still wondering for question 5, why can't option B and D be the answers?
thanks in advance

 

[cashew]

why C? ._.