COTW-13

A student carries out a titration three times and obtains the following volumes: 3.0 ± 0.1 cm³, 3.2 ± 0.1 cm³, and 3.2 ± 0.1 cm³.  What is the average volume?

• 3.1 ± 0.1 cm³
• 3.13 ± 0.1 cm³
• 3.1 ± 0.3 cm³
• 3.13 ± 0.3 cm³

Which statement is correct for the following reaction?
2ClO₃^-(aq) + SO₂(aq) + H⁺(aq) --> 2ClO₂(g) + HSO₄^-(aq)

• ClO₃^- is the oxidizing agent and it undergoes reduction.
• ClO₃^- is the reducing agent and it undergoes oxidation.
• SO₂ is the oxidizing agent and it undergoes oxidation.
• SO₂ is the reducing agent and it undergoes reduction.

Which species are produced at each electrode during the electrolysis of molten lead(II) bromide, PbBr₂(l)?

• Br^-(l) at the negative electrode; Pb²⁺(l) at the positive electrode.
• Pb²⁺(l) at the negative electrode; Br^-(l) at the positive electrode.
• Br₂(g) at the negative electrode; Pb(l) at the positive electrode.
• Pb(l) at the negative electrode; Br₂(g) at positive electrode.

Consider the following standard electrode potentials.
Sn²⁺(aq) + 2e^- <--> Sn(s) E^Θ = -0.14 V
H⁺(aq) + e^- <--> ½H₂(g) E^Θ = 0.00 V
Fe³⁺(aq) + e^- <--> Fe²⁺(aq) E^Θ = +0.77 V
Which species will reduce H⁺(aq) to H₂(g) under standard conditions?

• Fe²⁺(aq)
• Sn²⁺(aq)
• Sn(s)
• Fe³⁺(aq)

A number of molten metal chlorides are electrolysed, using the same current for the same length of time.  Which metal will be produced in the greatest amount, in mol?

• Mg
• Al
• K
• Ca

THIS IS A FREE RESPONSE, NOT MULTIPLE CHOICE! So answer all the questions.

Iron rusts in the presence of oxygen and water. Rusting is a redox process involving several steps that produces hydrated iron(III) oxide, Fe₂O₃·nH₂O, as the final product. The half-equations involved for the first step of rusting are given below:

Half-equation 1: Fe(s) → Fe²⁺(aq) + 2e^-

Half-equation 2: O₂(aq) + 4e^- + 2H₂O(l) → 4OH^-(aq)

a i) Identify whether half-equation 1 represents oxidation or reduction, giving a reason for your answer.

a ii) Identify the oxidation number of each atom in the three species in half-equation 2 (see Diagram A for set up.)

a iii) Deduce the overall redox equation for the first step of rusting by combining half-equations 1 and 2.

a iv) Identify the reducing agent in the redox equation in part (iii).

b) The oxygen in half-equation 2 is atmospheric oxygen that is found in dissolved water in very small concentrations. Explain, in therms of intermolecular forces, why oxygen is not very solubale in water.

c) Transition metals and their compounds often catalyse reactions. The catalyzed decomposition of hydrogen peroxide by CuO is an example. State two other examples of catalyzed reactions giving the transition metal or its compound acting as catalyst.

d) State the relationship between the electron arrangement of an element and its group and period in the periodic table.

e i) State a chemical equation for the partial dissociation of water into ions, including state symbols.

e ii) The dissociation of water into ions is reversible. State the expression for the ionic product constant of water.

e iii) The ionic product constant of water was measured at three different temperatures as shown in Chart E. Deduce whether the ionization of water is exothermic or endothermic, giving your reason.

e iv) Using the data in Chart E, determine the pH of water at 373K, correct to two decimal places.

f i) An aqueous solution of sodium chloride is electrolysed using inert electrodes. Explain which product is obtained at the positive electrode (anode) if the concentration of sodium chloride is high.

f ii) State the half-equations occurring at the electrodes during the electrolysis of the concentrated aqueous solution of sodium chloride.

g) Describe how electrolysis can be used to electroplate a bracelet with a layer of silver metal. Include the choice of electrodes and electrolyte needed in your description.

• I have answered ALL questions a - g.

• I did NOT answer all questions a - g.