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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