Hi Ultima, I have some Qs from past yr papers and prelim papers
:)
2011 P2 Q4(c)(iii) What is 'inertness'? The suggested answers I
have says that C does not have low-lying vacant d-orbitals to
accept loe pair of electron from water. But I answered that energy
released from weak VDW forces of attraction formed between ccl4 and
h20 is insufficient to compensate energy required to break strong
H-bonds between h20 molecules hence ccl4 is insoluble in h20 and
hence inert.
2011 P2 Q4(e) Since the electrode cell potential for oxidation
of (VO)2+ to (VO2)+ and (VO3)- is the same, can we say it is
oxidised to (VO3)- instead
of (VO2)+? Also, when do
we use (VO3)-?
2012 P2 Q2(c)(iii) My answer was that from reaction 1, position
of equilibrium shifts left at higher temperature as high
temperature favours backwards endothermic reaction hence SO2 is
more stable at high temperature when heated hence SO2 is formed.
The suggested answer says that formation of SO3 is a slow process
with higher Ea and is hence not formed. Is my answer wrong?
2012 P3 Q5(a) can the synthesis pathway be:
1. HCN in trace amount of NaCn, cold at 10-20 degree celsius. 2.
KCN in ethanol, heat under reflux. 3. HCl(aq), heat under
reflux
2014 P2 Q5(d) Will there be formation of white ppt or is
phenylamine originally white?
2015 P2 Q2(c) Is it enough to mention H-bonding and permanent
dipole between methanol and only weak idid between methane or do I
have to compare idid between both methanol and methane as
well?
NYJC 2016 Prelim P2
Q2(a) In step 2, Al(OH)3 is produced but in (b)(i), it was
mentioned that excess NH3 is used. Shouldn't soluble complex
Al(OH)4- be formed when excess NH3 is used?
Q2(b)(iii) Why is CO2 produced?
NYJC 2016 Prelim P3
Q2(a) Should the bond angle be 109.5 degree of slightly less
than that due to electronegative O atoms around S? For such Qs,
sometimes answer keys take into consideration the electronegativity
of atoms in the molecule but sometimes they don't. Do we always
consider during the exam?
Thanks :)
Nope, your answer irrelevant and not acceptable. Inertness refers
to chemically inactive. In this context, inert = resistant to
hydrolysis (a chemical reaction). You're only talking about
solubility, which doesn't involve a chemical reaction.
At acidic pH, dioxovanadium(V) ion dominates. At alkaline pH,
vanadate(V) ion dominates.
Both are correct. When you're not sure which Cambridge wants, be
exam-smart and write both. You'll only be penalized when one of
your points directly contradict a required point, no penalty if the
point is simply irrelevant to the mark scheme.
Yes.
It's not about the color, it's about solubility.
The reason why you're confused, is becoz your school (ie.
Singapore JCs), erroneously and artificially separate the different
types of van der Waals.
The 1st reason and more important reason is, as you've
mentioned, hydrogen bonding exists between methanol molecules only
(and not methane molecules).
The 2nd reason is, the van der Waals forces between methanol
molecules, are stronger than the van der Waals forces between
methane molecules, for 2 reasons (yes, 2 sub-reasons within the 2nd
reason) : the presence of a permanent dipole for the polar methanol
molecule involving an electronegative O atom, as well as the
greater number of electrons present in methanol from the additional
O atom.
But no worries, Cambridge is lenient when it comes to the
(somewhat) erroneous way Singapore JCs teach this, so your answer
should be fine.
No, the pH from hydrolysis of NH3(aq) isn't sufficiently
alkaline to generate the Al(OH)4 - coordination complex.
It isn't. H2 is produced, not CO2.
Again, you've been erroneously brainwashed by your school (ie.
Singapore JCs), which teach wrongly for this subtopic.
The correct reasoning for any observed deviation (eg. in H2S)
from the basic geometries as predicted by period 2 VSEPR (eg. in
H2O), is moreso because S is in period 3 (more diffused orbitals),
hence reducing the degree (pun intended) of hybridization, compared
to period 2 VSEPR geometries.
Only when you're comparing across the same period (ie. period 2
vs period 2), then you look at other factors such as
electronegativity. Note that electronegativity features primarily
as the reason for angular deviation from VSEPR geometry,
specifically when electronegative F atoms are present.
In this case of H2SO4, the deviation is primarily due to greater
electron repulsion between doubly bonded S=O groups and the greater
magnitude of partial negative charge on these O atoms in the
resonance hybrid, compared to moderate electron repulsion between
the OH groups (due to steric strain), compared to least electron
repulsion between the OH and S=O groups (favorable electrostatic
interaction between partial positive H atom of OH groups and
partial negative O atom of S=O groups).
Be that as it may, for A level purposes, be BedokFunland JC
exam-smart, and always write the basic H2 syllabus basic VSEPR
geometry first, with qualification (ie. "assuming identical
geometry as a period 2 central atom"), before writing the
additional Uni level considerations stated above (if you suspect
Cambridge is asking you for it, eg. "The angles actually deviate
from the expected geometry, suggest reasons why", or if you've
the time and you're bored and just feel like showing off your elite
Uni level understanding, then go ahead and add these points in your
A level paper answer script like I do, just for fun).
You're welcome, Ephemeral (and all students), but in future pls
post 1 qn at a time, and wait for my reply, before posting the next
qn, instead of posting 10 qns at 1 shot (which may be more
convenient for u, but more troublesome for me to ans so many qns at
1 shot).
