Practice Question

1. Aluminium metal is widely used in countries worldwide. Aluminium has low density and strength that makes it ideal for construction of aircraft, lightweight vehicles, and ladders.

(a) [Chemical Bonding] Suggest with reference to the bonding why aluminium has higher melting point than sodium. [2]

(b) [Chemical Bonding] Aluminium compounds also have many uses in industries. Aluminium chloride, often in the form of derivatives, is a common component in antiperspirants, at low concentrations.
Draw the structure that aluminium chloride exists in vapour state. [1]
State the angle of Cl-Al-Cl in the above structure. [1]

(c) [Chemical Bonding] Aluminium chloride has a low melting point of 192.4 °C. Explain this phenomenon with reference to its bonding. [2]

(d) [Chemical Bonding] Draw the Lewis structure of the product when aluminium chloride reacts with nitrogen trifluoride, NF3 in a 1:1 mole ratio.[2]

(e) [Arenes] Friedal-Craft alkylation occurs when benzene reacts with chloroethane to form ethylbenzene with aluminium chloride acting as a catalyst.

C6H5CH2CH3 + CH3CH2Cl --> C6H5(CH2CH3)(CH2CH3)

Suggest the first step of the mechanism involving chloroethane and aluminium chloride. [2]

(f) [Hydroxy Compounds] The reaction of phenol with chlorine does not require aluminium chloride as a catalyst. Explain, with reference to phenol, the reason why aluminium chloride is not needed. [2]

(g) [Periodicity] Aluminium chloride and iron(III) chloride in solution are acidic. Suggest with a chemical equation why iron(III) ions in solution are acidic. [2]

(h) [Periodicity] Write the chemical equations with brief explanations when aluminium chloride and phosphorus trichloride, PCl3, react with water. [3]

(i) [Periodicity] Explain the reaction(s) and state the observations between aluminium nitrate and sodium carbonate solutions. [3]
[Total : 20 marks]

Answer :

2(a) Aluminium atom has three valence electrons, which has more valence electrons compared to one sodium atom. Hence, the metallic bonding formed by aluminium cations with the delocalised electrons is stronger than the metallic bonding in sodium. Hence, more energy is required to overcome the metallic bonding in aluminium than sodium. Aluminium has a higher melting point than sodium.
(b) Since the structure around Al is tetrahedral, the angle of Cl-Al-Cl is 109.5o.
(c) Aluminium chloride is a simple molecular substance. Little energy is required to overcome the Van der Waals’ forces of attraction between the molecules. Hence, it has a low melting point.
(e) CH3CH2Cl + AlCl3 <==> CH3CH2+ + AlCl4-
(f) The lone pair on oxygen in phenol is able to delocalise into the delocalised pi orbitals of benzene ring of phenol, making the phenyl ring electron rich. The hydroxyl group on phenol makes it an activating group. This makes it easy for electrophilic substitution to occur, and hence a Lewis Acid such as aluminium chloride is not needed.
(g) [Fe(H2O)6]3+ ions exist in solution. The O-H bond is able to be broken as it is polarised by the high charge density of Fe3+. This results in a proton being released for each complex ion. Hence, iron(III) complex ions are acidic in water. [Fe(H2O)6]3+ + H2O <==> [Fe(H2O)5(OH)]2+ + H3O+
(i) Brief answer : Aluminium carbonate is not formed. Instead, aluminium hydroxide is formed, which is a white precipitate. ....

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Note : Cambridge has used some university questions from university textbooks in the recent years' exam papers.

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Analysis of 2012 Cambridge Singapore A level Chem Paper 3

2012 Cambridge-Singapore Chem Papers are difficult, surprising but still doeable.

Paper 3

1(a)        Candidates need to illustrate the reaction of aluminium oxide with aqueous sodium hydroxide to form a complex. Candidates need to link the partial covalent character in the ionic bonds of aluminium oxide to the amphoteric character of aluminium oxide.

(b)(i) Candidates need to learn what are dative bonds in chemical bonding.

(b)(iv) Students need to be careful to convert cm3 to m3, which is the SI unit.
Cambridge examiners seem to favor the chapter of chemical equilibrium in recent years.

(c)           I mentioned Friedal Craft acylation and alkylation in my post last year (Tips for 2012 JC2 students) , and it came out in exams. Compound B is formed by the electrophilic attack of the acyl group (now as a carbocation) on the benzene ring. This is an intra-molecular reaction which I mentioned in my post.

Cambridge   examiners seem to favor the chapter of carbonyl compounds in 2012 papers. Students need to realize that compound A undergoes a positive iodoform test while compound B does not. 

Students need to be creative and quick to realize that and link carbonyl compounds chapter with aromatic compounds chapter.

2(a)(i)     The states of the reagents in the diagram are important. Note that students cannot use an ammeter. A voltmeter has to be used.

(a)(ii)      note that measurements need to be taken and not just mere observations are needed. Students need to be clear which species is oxidized and which reagent will increase/decrease in concentration. The question is phrased vaguely.

 (b)(ii)     Students need to be aware that a complex ion (chapter of transition metals) is formed between cyanide ions and iron(III) ions. Students need to choose the correct reduction half-equations (electrochemistry chapter) from the data booklet.

2(c)        Students need to link the relative reactivities of the RX compounds to the bond energies of C-Cl and C-I groups.

2(d)(ii)    There is a side-product of E, which is an ester. The question is not set very well and is ambiguous. From E to F, Williamson synthesis of ether is actually implied.

2(e)(ii)    This part is tricky. For the hydrolysis of the amide groups on Phenobarbital, carbon dioxide and ammonia may be obtained as there is a CO(NH2)2 group involved, which is urea.

3(b)        Students need to be strong in drawing dot and cross diagrams. Students need to know how to draw the dot and cross diagrams of carbonate, nitrate, sulfate, sulfite, phosphate ions.

There is a corresponding ionic compound in solution : [PH5]+[PH3]-
This year, Cambridge examiners tested extensively on mechanisms. Students need to be strong in organic chemistry mechanisms, as this helps in this question, though it is on inorganic nitrogen compounds.

Students need to know Arrow pushing in mechanisms, which I mentioned in last year’s post. This is only taught extensively in universities.

3(b)(iii) Surprisingly, this mechanism involves hemolytic fission into radicals. This question is very tough and is a great surprise. Thankfully, hints are provided. Students need to know how to draw the correct arrow hook in arrow pushing and to illustrate the dative bonds, unpaired electron(s).

(c)(ii)      Students need to explain where did NO2+ come about by referring to it in the earlier parts. Students need to know their mechanisms in organic chemistry very well.

(e)          Examiners favor entropy section in thermochemistry very much.

4(a)        This is a great surprise as examiners seldom test on atomic structure. Students need to know what are isotopes, and alpha particles (I mentioned this last year) and the behavior of the particles in an electric field.

(c)(ii)      Students also have to explain why phosphorus has a higher melting point than chlorine.

(d)(i)       The question is vaguely phrased.

(e)(i)       Mr Chong has seen a similar molecular structure in supramolecular chemistry (postgraduate level), eg. Crown ethers with a sodium ion in between the structure.

5(a)        This part is very tricky. Students need to lengthen the carbon chain of the original molecule by 2 carbons in 2 different ways. Students need to know the conditions and reagents and of course the type of reaction to go about the synthesis.

(b)(iii)     This part is very tricky. Students need to know that in K, there is hydrogen bonding to stabilize the mono-anion. Similar questions have come out in 2010-2011 papers, just that 2012 papers involves hydrogen bonding too. I have mentioned this in last year’s post.

(c)           Compound L is an acid anhydride, which is out of the syllabus. I mentioned this in last year’s post. From L to M, students can speculate on the reaction.
If K and L are not drawn properly, students may lose a lot of marks in this latter part.
Elucidiation questions are usually tough and this question is even tougher because the chemistry of acid anhydrides are out of syllabus.

(d)          Is a little tricky. Students have to guess that from N to P, decarboxylation of the acid occurs.
Overall, the paper is quite tough. The standards of the papers have stabilized from years 2010-2012. The examiners have launched quite a number of surprises this year. However, some trends can be seen where similar questions have appeared in previous year’s papers. In 2012, examiners really focused on mechanisms, which students need to learn well.

As I cannot reproduce the exam papers, students need to purchase the Ten Year Series from bookshops.
FBP ten year series book by Ms Angeline Tan contain quite a number of misprints. Buyers beware.