Stpm Inorganic Chemistry: Aluminium

1.

Which of the following properties does not apply to modern ceramics?

High melting point

Application in engineering

Corrode readily

Very hard

Modern ceramics are known for their high melting point, application in engineering, and being very hard. However, they do not corrode readily. This is because ceramics are typically made from inorganic materials that are resistant to chemical reactions and corrosion. They are stable and durable, making them suitable for various applications in industries such as aerospace, automotive, and electronics.

Explanation

Modern ceramics are known for their high melting point, application in engineering, and being very hard. However, they do not corrode readily. This is because ceramics are typically made from inorganic materials that are resistant to chemical reactions and corrosion. They are stable and durable, making them suitable for various applications in industries such as aerospace, automotive, and electronics.

2. The reaction between aluminium foil and hydrochloric acid is slow in the beginning. After a few minutes, the rate increases. The reason is

One of the products act as a catalyst

The layer of oxide on the surface aluminium slows down the reaction at the beginning

The hydrolysis of aluminium ions decreases the rate of reaction

The chloride ion has a small size and hence difficult to be polarized by the aluminium ion

The slow reaction is the reaction between the oxide layer and hydrochloric acid. Once the oxide layer is used up and the pure aluminium is exposed a vigorous reaction between aluminium and hydrochloric acid will take place.

Explanation

The slow reaction is the reaction between the oxide layer and hydrochloric acid. Once the oxide layer is used up and the pure aluminium is exposed a vigorous reaction between aluminium and hydrochloric acid will take place.

3. Which statement is not true concerning the aluminium ion, Al³⁺?

The Al3+ ion has a high charge density and developes covalent character in its ionic compounds

In water, the Al3+ ions exist as a complex ion [Al(H2O)6]3+.

An aqueous solution of Al3+ ions has a pH = 7.

If hydroxide ions are added to a solution of an aluminium salt, a white precipitate is formed.

An aqueous solution of Al3+ is acidic. Hence the pH value will be lesser than 7

Explanation

An aqueous solution of Al3+ is acidic. Hence the pH value will be lesser than 7

4. Which of the following statements is true of dimer Al₂Cl₆?

It is an ionic compound.

Aluminium atom undergoes sp2 hybridisation.

It is a white solid which melts when heated.

There are two coordinate covalent bonds in the molecule.

The correct answer is "There are two coordinate covalent bonds in the molecule." This is because dimer Al2Cl6 consists of two aluminum atoms bonded to each other through two coordinate covalent bonds. In a coordinate covalent bond, one atom donates a pair of electrons to the other atom, forming a bond. In this case, the chlorine atoms act as Lewis bases, donating their lone pairs of electrons to the aluminum atoms, which act as Lewis acids. Therefore, the statement is true.

Explanation

The correct answer is "There are two coordinate covalent bonds in the molecule." This is because dimer Al2Cl6 consists of two aluminum atoms bonded to each other through two coordinate covalent bonds. In a coordinate covalent bond, one atom donates a pair of electrons to the other atom, forming a bond. In this case, the chlorine atoms act as Lewis bases, donating their lone pairs of electrons to the aluminum atoms, which act as Lewis acids. Therefore, the statement is true.

5. Which of the following is true?

Both aluminium fluoride and aluminium chloride are covalent compounds

Both aluminium fluoride and aluminium chloride are ionic compounds

Aluminium fluoride is an ionic compound while aluminium chloride is an covalent compound

Aluminium fluoride is an covalent compound while aluminium chloride is an ionic compound

Aluminium fluoride is an ionic compound because it is formed by the transfer of electrons from aluminium to fluorine, resulting in the formation of positively charged aluminium ions and negatively charged fluoride ions. On the other hand, aluminium chloride is a covalent compound because it is formed by the sharing of electrons between aluminium and chlorine atoms.

Explanation

Aluminium fluoride is an ionic compound because it is formed by the transfer of electrons from aluminium to fluorine, resulting in the formation of positively charged aluminium ions and negatively charged fluoride ions. On the other hand, aluminium chloride is a covalent compound because it is formed by the sharing of electrons between aluminium and chlorine atoms.

6. Which of the following statements about aluminium is not true?

It is extracted by electrolysis.

Its chloride is covalent.

It is soluble in concentrated nitric acid

It is soluble in sodium hydroxide solution

With concentrated nitric acid, the aluminium metal is being oxidised causing the thickening of the aluminium oxide layer.

Explanation

With concentrated nitric acid, the aluminium metal is being oxidised causing the thickening of the aluminium oxide layer.

7. When sodium sulphide is added to an aqueous solution of aluminium chloride, the gas evolved is

Cl2

CO2

H2S

HCl

2[Al(H2O)6]3+ (aq) + 3S2- (aq) → 2Al(OH)3.3H2O (s) + 3H2S(g)

Explanation

2[Al(H2O)6]3+ (aq) + 3S2- (aq) → 2Al(OH)3.3H2O (s) + 3H2S(g)

8. Why is the aqueous solution of an aluminium salt acidic?

[Al(H2O)6]3+ ion receives a proton in aqueous solution.

[Al(H2O)6]3+ ion donates a proton in aqueous solution.

[Al(H2O)6]3+ ion disproportionate and produces H+ (aq) ions.

Acidic [Al(H2O)6OH]2+ ions are produced in solution.

In the the salt solutions the complex aluminium ion behave accordingly:
[Al(H2O)6]3+ (aq) + H2O (l) ↔ [Al(H2O)5(OH)]2+ + H3O+ (aq)

Explanation

In the the salt solutions the complex aluminium ion behave accordingly:
[Al(H2O)6]3+ (aq) + H2O (l) ↔ [Al(H2O)5(OH)]2+ + H3O+ (aq)

9. In the extraction of aluminium from bauxite, a current of 40 000 A is used. The function of the high current is to

Prevent the discharge of Na+ from the dissociation of cryolite

Increase the yield of aluminium

Keep the temperature of the electrolysis cell at 900 oC

Increase the rate of reaction

The high current is used to keep the temperature of the electrolysis cell at 900 oC. This is because the extraction of aluminium from bauxite is an endothermic process, meaning it requires heat to proceed. By passing a high current through the cell, resistance in the cell generates heat, which helps to maintain the desired temperature of 900 oC.

Explanation

The high current is used to keep the temperature of the electrolysis cell at 900 oC. This is because the extraction of aluminium from bauxite is an endothermic process, meaning it requires heat to proceed. By passing a high current through the cell, resistance in the cell generates heat, which helps to maintain the desired temperature of 900 oC.

10. Which equation represents the reaction between aqueous aluminium and potassium hydroxide?

Al (s) + 4OH- (aq) → [Al(OH)4]- (aq)

Al (s) + 6OH- (aq) → [Al(OH)6]3- (aq)

Al (s) + 4H2O (l) → [Al(H2O)4]- (aq)

Aluminium reacts with aqueous potassium hydroxide to form the aluminate ion and hydrogen gas

Explanation

Aluminium reacts with aqueous potassium hydroxide to form the aluminate ion and hydrogen gas

11. A piece of aluminium foil is treated with aqueous mercury(II) chloride to expose a clean surface of the metal. The foil is rinsed and left to stand in water. Bubbles of gas are given off. The final products include

Aluminium oxide and hydrogen

Aluminium chloride and hydrogen

Aluminium hydroxide and hydrogen

Aluminium hydroxide and oxygen

Normally, a layer of aluminium oxide is found on the surface of aluminium metal.. This oxide layer is removed by mercury(II) chloride. Aluminium will react with water to form a white precipitate of aluminium hydroxide and hydrogen gas.

Explanation

Normally, a layer of aluminium oxide is found on the surface of aluminium metal.. This oxide layer is removed by mercury(II) chloride. Aluminium will react with water to form a white precipitate of aluminium hydroxide and hydrogen gas.