Question

1. (a) co₂ and sio₂ are both covalent compounds. explain briefly why co₂ is a gas whereas sio₂ is a solid under ordinary conditions. 2 marks (b) an element x reacts with bromine to form an ionic compound xbrᵣ. (i) state with reasons the physical state of xbrᵣ at room temperature. (ii) state with reasons whether x is a metal or a non - metal. (iii) predict two properties of xbrᵣ other than its physical state. (iv) state the charge of an ion of x. 7 marks (c) (i) state the law of definite proportions. (ii) what volume of stock hcl with percentage purity of 36% and relative density 1.18 would be required to prepare 2.0 dm³ of 0.25 mol dm⁻³ hcl. m(hcl)=36.5 g mol⁻¹ 8 marks (d) iron was added to dilute tetraoxosulphate (vi) acid, the mixture was warmed gently and a gas was given off. the iron dissolved and a pale green solution was formed when more iron was added, the reaction eventually stopped and some iron was left. (i) write the formula of two ions present in dilute tetraoxosulphate (vi) acid. (ii) state the physical state of iron to be warmed gently. (iii) what would be observed to indicate that a gas was being given off. (iv) name the gas. (v) explain why the reaction stopped. (vi) what is the name of the salt that gives the solution its green colour? (vii) state a method that could be used to separate the excess iron from the green solution. 8 marks 3. (a) (i) state hesss law of constant heat summation. (ii) sulphur (vi) oxide is formed according to the following reaction: 2so₂(g) + o₂(g) ⇌ 2so₃(g) δh⁰=-1900 kj mol⁻¹ state and explain the effect of increase in pressure on the: (α) equilibrium position of the reaction; (β) equilibrium constant of the reaction. (iii) what would be the effect of an increase in temperature on the equilibrium position of the reaction? 10 marks (b) (i) explain why the production of aluminium may be considered as an environmentally friendly process but electrolysis of sodium chloride is not. (ii) name two major factors which would favour the siting of an aluminium smelter in a country. 6 marks

Explanation:

Step1: Explain physical state of CO₂ and SiO₂

CO₂ is a simple molecular compound with weak intermolecular forces (London - dispersion forces). Little energy is needed to separate molecules, so it is a gas. SiO₂ has a giant covalent structure with strong covalent bonds throughout the lattice. Large amount of energy is required to break these bonds, so it is a solid.

Step2: (b)(i) Physical state of XBrₓ

Ionic compounds like XBrₓ have strong ionic bonds. At room - temperature, XBrₓ is a solid because these strong bonds hold the ions in a fixed lattice position.

Step3: (b)(ii) Metal or non - metal

X is a metal. In an ionic compound, metals lose electrons to form cations and non - metals gain electrons to form anions. Here, X forms a cation in XBrₓ by losing electrons to bromine.

Step4: (b)(iii) Predict property of X

Metals are good conductors of heat and electricity in the solid and molten states due to the presence of delocalized electrons.

Step5: (b)(iv) Charge of X ion

X has a positive charge. In an ionic compound with bromine (which forms Br⁻), X loses electrons to form a cation.

Step6: (c)(i) Law of definite proportions

The law of definite proportions states that a chemical compound always contains exactly the same proportion of elements by mass.

Step7: (c)(ii) Calculate volume of stock HCl

First, find the molarity of the stock HCl.
The mass of HCl in 1 dm³ of the stock solution:
The density of the stock solution is 1.18 g/cm³ = 1180 g/dm³.
The mass of HCl in 1 dm³ of the solution is \(m = 1180\ g/dm^{3}\times0.36\).
The molar mass of HCl is \(M = 36.5\ g/mol\).
The molarity of the stock solution \(c_{1}=\frac{1180\times0.36}{36.5}\ mol/dm^{3}\approx11.6\ mol/dm^{3}\).
Using the dilution formula \(c_{1}V_{1}=c_{2}V_{2}\), where \(c_{1}\) is the molarity of the stock solution, \(V_{1}\) is the volume of the stock solution, \(c_{2}\) is the molarity of the diluted solution and \(V_{2}\) is the volume of the diluted solution.
We know \(c_{1}\approx11.6\ mol/dm^{3}\), \(c_{2} = 0.25\ mol/dm^{3}\) and \(V_{2}=2.0\ dm^{3}\).
\(V_{1}=\frac{c_{2}V_{2}}{c_{1}}=\frac{0.25\ mol/dm^{3}\times2.0\ dm^{3}}{11.6\ mol/dm^{3}}\approx0.043\ dm^{3}=43\ cm^{3}\).

Step8: (d)(i) Ions in tetraoxosulphate(VI) acid

The formula of the two ions present in dilute tetraoxosulphate(VI) acid (\(H_{2}SO_{4}\)) are \(H^{+}\) and \(SO_{4}^{2 - }\).

Step9: (d)(ii) Physical state of tetraoxosulphate(VI) acid

Dilute tetraoxosulphate(VI) acid is a liquid.

Step10: (d)(iii) Indication of gas evolution

Bubbles are observed to indicate that a gas is being given off.

Step11: (d)(iv) Name of the gas

The gas is hydrogen (\(H_{2}\)). The reaction between iron and dilute \(H_{2}SO_{4}\) is \(Fe + H_{2}SO_{4}=FeSO_{4}+H_{2}\uparrow\).

Step12: (d)(v) Reaction stopping

The reaction stopped because all the acid was used up. There was no more \(H^{+}\) ions to react with the iron.

Step13: (d)(vi) Salt for green colour

The salt that gives the solution its green colour is iron(II) sulphate (\(FeSO_{4}\)).

Step14: (d)(vii) Separation method

Filtration can be used to separate the excess iron from the green solution. The solid iron can be retained on the filter paper while the solution passes through.

Step15: (3)(a)(i) Hess's law

Hess's law of constant Heat Summation states that the enthalpy change for a chemical reaction is the same whether the reaction takes place in one step or in a series of steps, provided the initial and final conditions are the same.

Step16: (3)(a)(ii)(α) Effect of pressure on equilibrium position

The reaction \(2SO_{2(g)}+O_{2(g)} ightleftharpoons2SO_{3(g)}\) has a decrease in the number of moles of gas from the left - hand side (3 moles) to the right - hand side (2 moles). An increase in pressure will shift the equilibrium position to the right (towards the side with fewer moles of gas) to reduce the pressure according to Le Chatelier's principle.

Step17: (3)(a)(ii)(β) Effect of pressure on equilibrium constant

The equilibrium constant \(K_{c}\) is only affected by temperature. An increase in pressure does not affect the value of the equilibrium constant because the ratio of the concentrations of the products and reactants at equilibrium, which defines \(K_{c}\), is not changed by pressure (as long as temperature is constant).

Step18: (3)(a)(iii) Effect of temperature

The reaction is exothermic (\(\Delta H^{\theta}=- 1900\ kJ/mol^{-1}\)). An increase in temperature will shift the equilibrium position to the left according to Le Chatelier's principle. The system will try to absorb the added heat by favouring the endothermic direction (the reverse reaction).

Step19: (3)(b)(i) Aluminium production and electrolysis of NaCl

The production of aluminium can be considered environmentally friendly because bauxite (the main ore of aluminium) is abundant and the process can be made more energy - efficient. Also, aluminium is highly recyclable. Electrolysis of sodium chloride is not environmentally friendly because it produces chlorine gas, which is a toxic gas. Also, it requires a large amount of energy.

Step20: (3)(b)(ii) Factors for aluminium smelter

Two major factors for siting an aluminium smelter are:
1. Availability of cheap electricity: The electrolysis process for aluminium production is highly energy - intensive.
2. Proximity to bauxite source: To reduce transportation costs of the raw material.

Answer:

(a) CO₂ is a gas due to weak intermolecular forces in simple - molecular structure; SiO₂ is a solid due to strong covalent bonds in giant covalent structure.
(b)(i) Solid, due to strong ionic bonds.
(ii) X is a metal, as it forms a cation in ionic compound.
(iii) Good conductor of heat and electricity in solid and molten states.
(iv) Positive charge.
(c)(i) A chemical compound always contains exactly the same proportion of elements by mass.
(ii) \(43\ cm^{3}\)
(d)(i) \(H^{+}\) and \(SO_{4}^{2 - }\)
(ii) Liquid
(iii) Bubbles
(iv) Hydrogen
(v) All the acid was used up.
(vi) Iron(II) sulphate
(vii) Filtration
(3)(a)(i) Enthalpy change for a reaction is same whether in one step or series of steps, given same initial and final conditions.
(ii)(α) Shifts to the right.
(β) No effect.
(iii) Shifts to the left.
(3)(b)(i) Aluminium production: abundant ore, recyclable; NaCl electrolysis: toxic gas production, high energy use.
(ii) Availability of cheap electricity; Proximity to bauxite source.