Redox Equilibria (Edexcel International A Level Chemistry): Exam Questions

This question is about manganese compounds. Some data are given below.

i) Write the ionic equation for the disproportionation of manganate(VI) ions, , in acidic conditions, using relevant half-equations from the table. State symbols are not required.

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ii) Calculate for the disproportionation of manganate(VI) ions in acidic conditions, stating whether or not the reaction is thermodynamically feasible.

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iii) Using the standard electrode potentials in the table, assess the thermodynamic feasibility of preparing manganate(VI) by reacting manganate(VII) and manganese(IV) oxide in alkaline conditions.

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Steel is an alloy of iron and carbon. A group of students determined the iron content of a sample of steel wire by a titration method.

A known mass of the wire was dissolved in dilute sulfuric acid and the resulting solution made up to 250.0 cm³ with more dilute sulfuric acid and mixed thoroughly.

Fe + H₂SO₄ → FeSO₄ + H₂

25.0 cm³ samples of the resulting solution were titrated with 0.0195 mol dm⁻³ potassium manganate(VII) solution.

i) State the colour change at the end-point of the titration.

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ii) One student used 1.53 g of the wire (weighed directly on the balance pan) and obtained a mean titre of 27.35 cm³ .

Using half-equations 3 and 5 from the table, calculate the percentage of iron in the steel wire. Give your answer to three significant figures.

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iii) A second student carried out the same experiment but used distilled water to make up the solution in the volumetric flask.
A brown suspension formed during the titration. Explain how, if at all, the titre value would be affected by this student’s error.

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The uncertainties of the apparatus used in the experiment in (b) are shown.

i) Complete the table.

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ii) A third student obtained a value of 95.863% for the proportion of iron in the wire. State whether or not this student has given their answer to an appropriate number of significant figures. Justify your answer in terms of the total percentage uncertainty of the experiment.

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This question is about titanium(III) chloride, TiCl₃ .

Titanium(III) chloride is used as a catalyst in the production of poly(propene).

State the property of transition metals, such as titanium, that makes their compounds effective catalysts.

When dissolved in tetrahydrofuran (THF), titanium(III) chloride forms a blue solution containing complex Y.

i) THF acts as a monodentate ligand in complex Y. State the meaning of the terms monodentate and ligand.

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ii) Complex Z is a stereoisomer of complex Y. Complete the diagram to show the arrangement of the ligands in complex Z.

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A student determines the change in oxidation number of nitrogen when a solution of magnesium nitrate, Mg(NO₃)₂ , is titrated with aqueous titanium(III) chloride.

Procedure

During the titration, Ti³⁺ ions are oxidised to TiO²⁺ ions

Ti³⁺ + H₂O → TiO²⁺ + 2H⁺+ e^–

Alizarin indicator is green in the presence of aqueous Ti³⁺ and yellow in the presence of aqueous TiO²⁺. The end‐point of the titration is reached on the addition of 20.70 cm³ of aqueous titanium(III) chloride.

i) State the colour change that would be observed at the end‐point of the titration.

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From .............................. to .............................. 

ii) Use the results to determine the final oxidation state of nitrogen in the titration. You must show your working.

[M_r value: Mg(NO₃)₂∙6H₂O = 256.3]

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iii) Write the overall ionic equation for the titration reaction using your answer to (c)(ii) and the relevant half‐equations from the list below.

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TiO²⁺ + 2H⁺+ e^–  ⇌  Ti³⁺ + H₂O   E^θ = +0.10V

NO₃^– + 2H⁺+ e^–    ⇌  NO₂ + H₂O   E^θ = +0.80V

NO₃^–  + 3H⁺+ 2e^–  ⇌ HNO₂ + H₂O   E^θ  = +0.94V

NO₃^– + 4H⁺+ 3e^–  ⇌  NO + 2H₂O   E^θ = +0.96V

iv) Use the electrode potential data provided to calculate E^θ_cell for the overall titration reaction.

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v) Suggest why the contents of the conical flask are heated.

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vi) The student’s teacher said, “As TiCl₃ is blue and TiO²⁺ ions are colourless in aqueous solution, the titration can be carried out without an alizarin indicator.”

Assess the teacher’s statement. In your answer you should

• identify, by name or formula, a coloured complex ion expected to be present when TiCl₃ dissolves in water

• explain how the colour of this complex ion arises

• suggest why the titration may be more accurate with an alizarin indicator.
  

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