Q1. You never realized, all these years, that
the so-called "displacement" reaction is actually redox?!?!? Cl2 is
reduced to Cl-, while oxidizing Br- to Br2!!!
Q2. Acid anhydrides are electrophilic. That's something
which you should be able to figure out by analyzing its structure.
Guys like girls, nucleophiles like electrophiles. Only
nucleophiles have the 'balls' (the 'balls' usually refer to a
lone pair, but can refer to a pi bond in the case of
alkene nucleophile and benzene ring nucleophile) to attack
electrophiles. A nucleophile such as phenol, can
attack acid anhyrdide electrophiles in a similar fashion to
attacking acyl chloride electrophiles, to generate a phenate
ester.
Q3. Enthalpy of neutralization is defined as the
enthalpy change
per mole of water generated. So multiply by the
coefficient of water in the equation.
Q4. Not required for H2 syllabus. If Cambridge wants you to
apply it directly, the question will provide the formula.
Q5. If you keep adding excess strong base, then the pH will
continue to rise, approaching (but never reaching) the pH of the
strong base used. The OH- ion is a stronger base compared to the
basic salt generated, which means if you're asked to calculate the
pH at a point beyond the equivalence point (equivalence point =
stoichiometric neutralization between acid and base, or oxidizing
agent and reducing agent; end-point = point when a suitable
indicator changes colour, it only approximates equivalence point;
but for 'A' levels both terms may be used interchangeably), just
consider the molarity of the stronger OH- ion from the excess NaOH
added in, and ignore the contribution of OH- from the hydrolysis of
the salt, because as predicted by Le Chatelier's principle, the
hydrolysis of the salt is suppresssed by addition of extra OH-
ions.
Q6. Yellow. Fe3+ is a clear-cut case. The case of Cr3+ is more
probematic. For 'A' levels, you can just state the colour of the
hydrated (ie. hexa-aquated) Cr3+ as green, even though strictly
speaking and in theory, if all 6 ligands are water, the colour
of (hexa-aquated) Cr3+ is pale violet. Problem is, in a
percentage of any given sample, the counter anion (eg. Cl-, NO3-,
SO4 2-, etc) will always substitute one or more of the water
ligands, generating the stronger colour green, which will mask away
the weaker pale violet colour. For more info on complex ions and
their colours, check out Jim Clarke's website, and Rod Beavon's
website.
Q7. Do not use the term
"acylation" on its own, like some JCs badly teach. Because the term
when used alone is ambiguous : it could refer to either
Friedal-Crafts acylation, which involves first the reaction between
a suitable Lewis acid catalyst such as AlCl3 and the acyl halide,
followed by electrophilic aromatic
substitution (recommended you always include the
word "aromatic" to remind yourself that the benzene ring is the
nucleophile in these reactions, but for 'A' levels Cambridge won't
penalize you if you leave out the word "aromatic" in the term)
where the benzene ring is the nucleophile. Or the term "acylation"
could refer to nucleophilic acyl substitution, the
mechanism for which is addition-elimination, and is
the reaction responsible for generating esters, generating
amides, hydrolyzing esters, hydrolyzing amides, and the reaction of
nucleophiles with acid anhydride electrophiles, amongst other
reactions.
If Cambridge asks you "what type of reaction is this? acid +
alcohol ---> ester + water", the exam-smart candidate will
give all 4 of these correct answers, to ensure you secure
the full marks : esterification (because an ester is
generated), condensation (because a water molecule or small alcohol
is eliminated to join two molecules together intermolecularly, or
two parts of the same molecule together if intramolecularly),
nucleophilic acyl substitution (because it is nucleophilic
substitution occuring on acyl group rather than an aliphatic
group), and addition-elimination (which is the mechanism for
nucleophilic acyl substitution, just as SN1 and SN2 are the
mechanisms for nucleophilic aliphatic substitution).
To answer your question : with a Lewis acid catalyst, the
reaction proceeds at room temperature, no heating is necessary.
Q8. Do not blindly follow
Markovnikov's rule. It may fail sometimes (eg. in addition of
HX onto 3,3,3-trifluroprop-1-ene). Rather, understand why
it usually works (ie. comparative stability of the
two alternative possible carbocation intermediates
during the mechanism, which you may be asked to draw out in the 'A'
level exam) and you can work out when it may fail.
Q9. Hydrolysis can be acidic (ie. proton-catalyzed) or alkaline
(ie. base-promoted). Since esterification (using alcohol and
carboxylic acids, instead of acyl halides) employs acidic
conditions, therefore acidic hydrolysis of esters are also
reversible (ie. it's actually the same reaction under the same
conditions! it's only a matter of initial molarities of LHS vs RHS;
ie. Qc vs Kc : at initial conditions, does position of equilibrium
lie to the left or right?). Accordingly, alkaline hydrolysis of
esters is not reversible.
Hi, as i’m doing my revision, I realised that I am unsure of the
qns below. Thanks.
1) Redox
I understand that Cl2 and Br ion can undergo displacement
reactions, but can they undergo redox as well?
2) Organic Compounds
Are we required to learn all the reactions of acid anhydrides in H2
chemistry? I only know that it reacts with alcohols &
amines.
3) Chemical Energetics
Why do we need to multiply the stoichiometric coefficient of the
limiting reagent for enthalpy change of reaction and not the
enthalpy change of neutralisation?
4) Reaction Kinetics
I understand that the Arrhenius equation means that a lower Ka
would result in a larger rate constant, k, hence faster reaction.
However, how do we apply this equation to a kinetics question?
5) Ionic Equilibra
In a titration curve between a weak acid (CH3COOH) and Strong base
(NaOH), I understand that salt hydrolysis occurs at the
equivalence/end point. However, what happens after end point?
6) Transition Elements
I’m unsure of the real colour of [Fe(H2O)]3+, is it yellow or very
pale violet?
7) Does the process of acylation (Electrophilic substitution)
reaction requires heating? other than anhydrous AlCL3?
Does Electrophilic addition of Br2 (aq) (E.A.) follow
the markovnikov’s rule?
9) Why is the hydrolysis if ester an irreversible rxn and
esterification a reversible rxn?