Blacktoast94 asked :
some questions to ask, pls help thank u!
1. "down the group II, the elements dissolve in water with
increasing ease." why is this so? is it because of lattice
energy?
2. when they say "higher lattice energy," i always can't tell if
they mean higher as in the absolute value or higher in terms of the
negative value. does it mean easier or harder to dissolve? ._.
3. how to explain fully if they ask why a ring structure
restricts the formation of cis-trans isomers?
4. explain fully why the hydrogen radical cannot be
formed?
5. the enthalpy change of combustion for alkanes gets more and
more negative as the carbon chain length increases. what is the
explanation for that?
6. just to clarify, is it true that ethanolic KOH/NAOH can not
only do elimination, but also can do hydrolysis?
7. explain why in SN2, the optical isomerism is reversed.
8. also, is K or Kr larger in atomic radius? across the period
the radius is supposed to get smaller, but my lecture notes says
because van der waal's radii is measured for the group 0 elements,
therefore it has the largest atomic radius. the answer from the
book however, is K has the larger atomic radius.
Q1.
For solubility of group II metals itself, look at oxidation
potentials in Data Booklet. Bear in mind that by Hess Law,
oxidation enthalpy = ionization enthalpy + hydration
enthalpy.
For solubility of group II hydroxides versus carbonates &
sulfates, read :
http://www.chemguide.co.uk/inorganic/group2/problems.html
Q2.
By default, Cambridge will always refer to the
magnitude of the lattice energy. If this was Paper
3 or Paper 2, then you should be exam-smart and use the full terms
"endothermic lattice dissociation energy" versus "exothermic
lattice formation energy" (similarly for "endothermic bond
dissociation energy" versus "exothermic bond formation energy" when
describing covalent bond energies).
Q3.
For small rings, only the so-called cis-isomer exists, because the
trans-isomer will break the ring. For larger rings, the
trans-isomer will exist, but may cause some degree of angle
strain.
Q4.
The hydrogen radical can indeed be formed, just that it's much less
stable than most other radicals, and is therefore not encountered
in the H2 syllabus, as other radicals (eg. alkyl, halogen, etc)
would be preferentially formed instead during initiation and
propagation steps.
Q5.
Because we're not talking about more exothermic per C atom, but
more exothermic per hydrocarbon molecule. The more C atoms present
in a single molecule, the more bonds are broken (in reactants) but
also formed (in products), and since combustion has proven to
beexothermic, the magnitude of exothermic energy transfer always
exceeds the magnitude of endothermic energy transfer, with this
difference in magnitude increasing with an increasing number of C
atoms per molecule.
Q6.
Both are possible. In practice, all 4 reactions, SN1 vs SN2 vs E1
vs E2 are always in competition with each other. It's only in the
simplified 'A' level syllabus that you simply state the major
product when ethanol solvent is used, is the elimination product
rather than substitution product. But in a tricky Cambridge
question, you may still have to consider other (minor) possible
products.
Q7.
(If this was a Paper 3 or Paper 2 question, then)
Draw the diagram of the SN2 mechanism to clearly illustrate
the inversion of
configuration which occus during SN2 reaction, to secure full
marks for such a question.
Q8.
In a sense, it's true that you cannot make a fair comparison
because Group 0 aka Group VIII noble gases only have van der Waals
radius, while Group I metals only have atomic and ionic radius. But
for simplified 'A' level purposes, just state that you expect the
Kr atom to have a smaller atomic radius than K, because across the
period, there is increasing effective nuclear charge.