Redox II (Edexcel A Level Chemistry): Exam Questions

This question is about the Ag⁺(aq)|Ag(s) half-cell.

A student was asked to plan an experiment to measure the standard electrode potential of the Ag⁺ (aq)|Ag(s) half-cell.

i) State the conditions of temperature and pressure under which standard electrode potentials are measured.

(1)

ii) The student drew the diagram shown.

Identify three mistakes in this diagram and the modifications that should be made to correct them.

(3)

The standard electrode potential, , of the Ag⁺ (aq) | Ag (s) half-cell is +0.80 V.

The effect of changing the concentration of the ions on the value of the electrode potential, E, in this half-cell is calculated using the equation.

where T is the temperature in kelvin and R is the gas constant.

The electrode potential of a Ag⁺ (aq) | Ag (s) half-cell was measured at 20 °C and found to be +0.72 V.

Calculate the concentration of silver ions, in mol dm^-3, in this half-cell.

Tablets containing potassium manganate(VII), KMnO₄ , are dissolved in water forming an antiseptic solution to treat skin conditions. The manufacturers claim that each tablet contains 400 mg of KMnO₄ . To check the claim, the titration procedure outlined was carried out.

• Five tablets were dissolved in distilled water to make 100.0 cm³ of solution.

• Some of the KMnO₄ solution was used to fill a burette.

• 25.0 cm³ of sodium ethanedioate solution, Na₂C₂O₄ (aq), of concentration 0.200 mol dm⁻³, was added to a conical flask and warmed.

• Sulfuric acid, of concentration 2 mol dm⁻³ , was also added to the conical flask.

• The KMnO₄ solution was added to the flask from the burette, until the end-point.

The equation for the reaction between MnO₄⁻ ions from the KMnO₄ and C₂O₄²⁻ ions from the sodium ethanedioate solution is shown.

16H⁺ (aq) + 2MnO₄⁻ (aq) + 5C₂O₄²⁻ (aq) → 2Mn²⁺ (aq) + 10CO₂ (g) + 8H₂O (l)

Give the colour change at the end-point of the titration.

i) Complete the dot-and-cross diagram for the ethanedioate ion. Show the outer electrons only.

(2)

ii) Determine the oxidation number of carbon in the ethanedioate ion, C₂O₄²⁻.

(1)

Give the reason why sulfuric acid was also added to the conical flask.

This redox reaction could be used in an electrochemical cell.

The cell half-equations are

2CO₂(g) + 2e⁻⇌ C₂O₄²⁻ (aq)

8H⁺(aq) + MnO₄⁻(aq) + 5e⁻ ⇌ Mn²⁺(aq) + 4H₂O(l)

Write a cell diagram for this cell using the conventional representation.

The results of the titration are shown.

i) Complete the table.

(2)

ii) The equation for the reaction between MnO₄^- ions from the KMnO₄ and C₂O₄²⁻ ions from the sodium ethanedioate solution is shown.

16H⁺(aq) + 2MnO₄⁻(aq) + 5C₂O₄²⁻(aq) → 2Mn²⁺(aq) + 10CO₂(g) + 8H₂O(l)

Use this equation and your mean titre from (e)(i) to calculate the mass, in mg, of KMnO₄ in one tablet.

Give your answer to an appropriate number of significant figures.

(5)

iii) A textbook suggested the conical flask should be heated during the titration, as the reaction between the MnO₄⁻ ions and the C₂O₄²⁻ ions is slow.

Use these electrode potentials and your knowledge of homogeneous catalysis to deduce why the heating is very important at the start of the titration, but less important as the titration proceeds. Justify your answer. You may include equations in your justification.

(4)

Yellow gold is used to make jewellery. It is an alloy of copper, gold and silver. The purity of gold is measured in carats. The higher the carat, the higher the percentage of gold in the alloy. Pure gold is 24 carat. A sample of yellow gold is analysed using the steps below.

Write the equation for the reaction of copper with concentrated nitric acid, using the half-equations given in Step 1. State symbols are not required.

State the indicator used and its colour change at the end-point in the titration in Step 6.

The table shows the percentage by mass of gold in four different carats of yellow gold.

Determine, using the experimental data, the carat of the sample of yellow gold that was analysed.