H2 Chemistry help: UltimaOnline
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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? 8)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?
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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.
Originally posted by JC2011: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?
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Hi, thank you for the response. I'd like to know if H2 syllabus includes the reduction of ester by lithium aluminium hydride into 2 alcohols?
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Originally posted by JC2011:
Hi, thank you for the response. I’d like to know if H2 syllabus includes the reduction of ester by lithium aluminium hydride into 2 alcohols?
Not explicitly included, but yes, that's one of the good-to-knows if you are committed to earning a distinction.
I'd recommend high-achieving H2 Chem students attempt the mechanism for the reduction (using LiAlH4 in dry ether, as a source of hydride ion nucleophiles) of an ester electrophile into two alkoxide ions, including the mechanism for the subsequent protonation (eg. via hydrolysis) of the alkoxide ions into alcohols.