Hi Ultima,
Some questions here which I hope I can seek your advice :)
1. With reference to 2014 TYS Paper 2, why is FeS2 considered a
"complex" with a yellow / gold colour even though it is supposed to
be ionic (Fe2+ and (S2)2- ions)?
2. Also with reference to the trend of boiling points of the
Transition Metals, why does chromium have a higher boiling point of
2672 than that of Mn of 2061 despite it having less 4s electrons
that Mn? Shouldn't having more electrons in Mn result in stronger
metallic bonding?
3. Generally, what is the safest / most efficient bet when
Cambridge asks for the "structural formula" of a compound? What
about "displayed formula"? In such circumstances, should we draw 1)
condensed 2) all bonds 3) skeletal? Or is a hybrid even accepted by
Cambridge?
4. Regarding the usage of "in paper" and "out of paper" wedges in
illustrating Walden's Inversion in Sn2 Mechanism, do the
orientations of the wedges (how they are placed before and after
reaction i.e. "do they flip or stay the same") matter and does
Cambridge see it as a marking point?
5. In Sn2 Mechanism, in the transition state, is it a must to label
the nucleophile and leaving group as delta minus and the attacked
carbon atom as delta plus? Are there any circumstances where we do
not label as such?
6. I understand that one can use "arrow pushing" to illustrate
"inorganic mechanisms" such as how SO2 becomes H2SO3 and how CO2
becomes H2CO3 when attacked by H2O. In seeking a better
understanding of other similar reactions, how can we also use
"arrow pushing" to illustrate how giant covalent SiO2 becomes ionic
Sio3 2- when reacted with concentrated NaOH?
7. I understand that we often use terms like pi / π
electron cloud / ring system / aromatic ring system
interchangeably. I would like to ask if all of these terms are
accepted by Cambridge and if not, which would be the safest bet to
use in elaboration in Paper 2 and 3 (for instance when illustrating
partial double bond character in chlorobenzene)
8. With regards to the graph of rate of reaction against
concentration of substrate, we often say that at low concentrations
of substrate, the order of rxn is 1 wrt substrate. When at high
concentrations of substrate, the order of rxn is 0 wrt substrate
due to catalyst saturation. However, what if even without the
substrate, the order of reaction wrt to the reactant (our
substrate) is non-zero. How would that change how the graph looks
like at low and high reactant (substrate) concentrations in the
presence of catalyst?
9. In illustrating complex ions, say CuCl4 2-, which would be the
preferable way of writing in equations: 1) [CuCl4(H2O)2] 2- or 2)
[CuCl4-]2-. If it is the former, should we be writing precipitates
that we commonly know of (like CuOH2) as [Cu(OH)2(H2O)4] from then
on? Also, will we be penalised when we state the molecular geometry
of the complex based on our answer in 1) and 2)? i.e. when we write
1), we would then proceed to write "octahedral" but when we write
2), we would tend to write "square planar" later on if the question
requires.
10. In solving "calculate pH" questions given the intial acid/base
concentration and the ka / kb values respectively, what would be
the most correct statement to use/write to assume that [acid/base
intial] =approx= [acid/base eqm]?
11. In questions where Cambridge asks why compounds like Al2O3
react with acid and base, will it suffice to just say high charge
density = acidic, and ionic oxide = basic? Must we go in depth to
talk about how Al3+ deprotonates coordinating H2O molecules to make
Al2O3 acidic and how O2- is a strong base and thus this makes Al2O3
basic?
Thx alot! :)
Q1. Unlike O levels, now at A levels you begin to appreciate that
Chemistry, a microcosm of real-life, is not so clear-cut
black-&-white, but many fascinating shades over the entire
spectrum. While it's still not wrong to consider FeS2 as ionic,
it's also not wrong for Cambridge to call it a complex, because
there's an area of overlap between the 2, for which FeS2 falls in.
Firstly, unlike O, S atoms have a significantly more polarizable
electron charge clouds (due to larger ionic radius and shielding
effect), and hence the bonds for FeS2 are actually closer to dative
covalent than ionic. Secondly, in the crystal lattice structure of
FeS2, each S atom (of S2 2- disulfide anions) donates 3 bond pairs
unto each Fe2+ cation, concordantly each S2 2- dilsulfide anion
donates 6 bond pairs unto each Fe2+ cation, while each Fe2+ cation
accepts dative bonds from, and is thus (distorted octahedrally)
coordinated with, 6 S atoms (of S2 2- disulfide anions), giving
rise to the stoichiometry of 1 Fe2+ : 1 S2 2-, ie. FeS2. Cambridge
chose to call it a coordination complex, simply out of convenience
in the context of the (coordination complex topical) questions they
chose to ask the A level candidate.
Q2. For a variety of reasons too complex to be dealt with at A
levels, different metals across the d block have different metallic
crystal lattice packing structures, with significantly closer
packing for Cr compared to Mn. This is evidenced by the fact that
in spite of the significantly higher molar mass of Mn over Cr, they
have similar g/cm3 densities. Closer packing allows for stronger
metallic bonds, which explains why Cr has a higher melting/boiling
point and hardness compared to Mn.
Q3. When "structural formula" is asked, Cambridge accepts either
displayed / full, skeletal, condensed, or a hybrid of all 3.
Students should use whichever they're most comfortable with. More
competent students will find skeletal structure to be advantageous,
both in terms of being less time-consuming, as well as being
clearer (for both the student and Cambridge marker).
Q4. Cambridge requires A level students to draw the following
diagram :
https://en.wikipedia.org/wiki/SN2_reaction#Reaction_mechanism
Q5. Yes it is required : all formal charges must always be shown,
and partial charges must be shown when relevant. Although the
positive (partial or occasionally formal) charge on the
electrophilic C atom should be illustrated in the reactant rather
than transition state. Understand the meaning of transition state,
and apply chemistry sense to adjust accordingly for different cases
or exceptions, eg. if the incoming nucleophilic atom has a negative
formal charge, it'll have a partial negative charge in the
transition state to reflect its process of donating its lone pair
and losing its negative formal charge, eg. hydroxide ion ; if the
incoming nucleophilic atom has a no formal charge and only a
partial negative charge, it'll have a partial positive charge in
the transition state to reflect its process of donating its lone
pair and gaining a positive formal charge, eg. ammonia molecule.
And likewise for the leaving group.
Q6. As far as helping you to understand the process and to
elucidate the product is concerned, you can proceed with the
mechanism by visualizing SiO2 as a simple covalent molecule rather
than a giant covalent lattice. Just be aware that because the
reactant exists as a giant lattice, so will (at least some of) the
products (even as you draw it as simple molecular for the sake of
simplification). After which, you can proceed to write out the
overall balanced equation (which will be all that Cambridge would
require from you in the A levels).
Q7. Yes, they are mostly all acceptable, and different JCs will use
different terms and phrasing. A safe bet is to follow the phrasing
of CS Toh, Chan Kim Seng & Jeanne Tan, and George Chong in
their A level books. If you're still worried about this point, go
look up the Cambridge Mark Schemes to learn exactly how they mark
(as I've said many times before, Cambridge is more reasonable and
less anal than JC teachers in marking students' answers).
Q8. You would have to mathematically adjust the graph of course,
but which is beyond the A level Chem syllabus and will not be
asked.
Q9. [CuCl4-]2- is [CuCl4-]2-, totally different from [CuCl4(H2O)2]
2-. The former is tetrahedral and the correct formula, while the
latter is octahedral and the wrong formula. Other than the common
coordination complexes from the A level H2 syllabus, for uncommon
coordination complexes the Cambridge question will give you
sufficient data for you to elucidate the correct formula and
accordingly, the correct geometry. You should write the formulae of
precipitates as Cu(OH2)(s) and not as [Cu(OH)2(H2O)4](s). There is
usually error carried forward for such questions, but not
always.
Q10. For most pH calculation questions, there is no need for a
specific declaration of estimation (in spite of what your anal JC
teacher might insist), unless of course, specifically required by
the question, in which case any clear explanatory phrasing will
suffice, eg. "Because the Ka value is very small *and* the
initial molarity is fairly large, hence the change in molarity x,
may be considered negligible relative to the initial molarity, and
concordantly it will be mathematically valid to 3 significant
figures to approximate the equilibrium molarities back to the
initial molarities".
Q11. No details of the underlying mechanism to explain the
acid-base nature of metal vs non-metal oxides are required in the
basic A level H2 syllabus (but of course, it's entirely possible
for distinction-type questions, for Cambridge might go with
"Suggest a mechanism to explain..." etc). For the basic A level H2
syllabus, students need only state that "metal oxides, being ionic
oxides, are basic oxides; non-metal oxides, being covalent oxides,
are acidic oxides ; oxides with a high degree of both covalent and
ionic character are amphoteric", and of course, the balanced
equation must always be written for such questions, eg. Al2O3
reacting with acids, and Al2O3 reacting with alkalis.
No prob :)
