Several questions!
Inorganic Chem:
For transition metal complex right, how do you determine the
coordination number. I understand that most of them are 6 but
sometimes it can be 4 or 2, and i get stumped when i see the answer
key. so how do i predict the coordination number?
Physical chem:
Do you mind explaining how do you predict solubility? isit as
simple as polar solvents dissolve polar reagents and non polar
dissolve non polar? refering to TYS 2009 P2 Q2(b) I dont understand
why E values are used D:
The topic of complex ions is actually more complex (pun intended)
and cannot be fully understood at A levels. At A levels, you only
need to know the basic principles and concepts, and you've to
memorize the coordination numbers, geometries and colors of the
commonly encountered coordination complexes.
For A level purposes, 3 simple concepts are relevant here : size
of metal cation (if small, ie. period 2, only 4 ligands are
possible, due to lack of vacant, energetically accessible d
orbitals to accommodate an expanded octet), size of ligands (eg.
water ligands small, so 6 are possible ; chloride ligands large, so
only 4 are possible, to avoid excessive steric strain caused by van
der Waals repulsions), charge on ligands (eg. water ligands
neutral, so 6 are possible ; hydroxide ligands are negatively
charged, so only 4 are possible, to avoid excessive steric strain
caused by anionic repulsions between ligands).
Beyond these A level concepts, University level chemistry such
as the Jahn-Teller
effect, will be required to explain deeper questions on
coordination complexes, eg. why [Cu(NH3)4]2+ has a square planar
geometry instead of tetrahedral geometry.
Solubility is also a complex issue spread over several topics in
the H2 syllabus, eg. chemical bonding, thermodynamics, solubility
equilibria, Group 2, electrochemistry, organic chemistry, etc. The
required answer at A levels depends on the context and H2 Chem
syllabus topic that the question falls under. So there's no simple
way to predict solubility (simple ways are for O levels, A levels
are preparation for Uni level, in which everything becomes
increasingly complex*). For the particular question you mentioned,
falls under electrochem : only the oxidation potentials of Mg, Zn
and Al, but not Cu, are sufficiently positive for their oxidation
into aqueous and soluble metal cations when reacted with H+ from
HCl (aq), to be thermodynamically feasible.
* In primary school, you thought you already knew everything in
the Universe that could be known. In secondary school, you begin to
realize you don't everything, but you still feel confident that you
know almost everything. In JC and poly, you begin to understand
that you don't really know all that much, and that the Universe is
more complex than you thought. In University, you're humbled by the
understanding that the Universe is so infinitely complex that you,
and all sentient beings on Earth, actually know almost nothing
relative to the infinitely vast Universe, but we know just enough
to begin to appreciate and respect its beauty and complexity.