Covid Assignment 6: Transition Metal

Question 1

  1. Vanadium(V) oxide is used as a heterogeneous catalyst in the Contact Process.
  1. Explain what is meant by the terms heterogeneous and catalyst and state, in general terms, how a catalyst works.
  2. State the essential feature of vanadium chemistry which enables vanadium(V) oxide to function as a catalyst and, by means of equations, suggest how it might be involved in the Contact Process.

 

  1. The following method was used to determine the percentage by mass of vanadium in a sample of ammonium vanadate(V).
  • A solution was made up by dissolving 0.160 g of ammonium vanadate(V) in dilute sulfuric acid.The ammonium vanadate(V) formed ions in this solution.When an excess of zinc was added to this solution, the ions were reduced to V2+ ions and the zinc was oxidised to Zn2+ ions.After the unreacted zinc had been removed, the solution was titrated against a 0.0200 mol dm–3 solution of potassium manganate(VII).In the titration, 38.5 cm3 of potassium manganate(VII) solution were required to oxidise all vanadium(II) ions to vanadium(V) ions.Using half-equations, construct an overall equation for the reduction of to V2+ by zinc in acidic solution. Calculate the percentage by mass of vanadium in the sample of ammonium vanadate(V).

Question 2

  1. The reaction between aqueous persulfate ions, S2O82-, and iodide ions, I–(aq), is catalysed by Fe2+(aq) ions.
    1. Suggest why this reaction has a high activation energy.
    2. Write equations to explain the catalytic action of Fe2+(aq)
    3. Suggest why V2+(aq) ions will also act as a catalyst for this reaction but Mg2+(aq) ions will not.

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  1. Outline a mechanism for the reaction between benzene and ethanoyl chloride and explain why AlCl3 acts as a Lewis acid catalyst for this reaction. Predict, with an explanation in each case, the suitability of FeCl3 and of NH4Cl to act as a catalyst for this reaction.

Question 3

  1. The ion C2O42- can act as a bidentate ligand.
  1. Explain the meaning of the term bidentate ligand.
  2. Sketch the structure of the octahedral complex ion formed by Fe3+ ions which contains C2O42- as the only ligand. Include the overall charge on the complex ion.
  1. Explain the meaning of the term chelate effect.
  2. The chloride ion can act as a monodentate ligand.
  1. Deduce the formula of the linear complex formed when an excess of concentrated hydrochloric acid is added to silver chloride.
  2. Explain why metal(II) ions do not usually form octahedral complexes when chloride ions are the only ligands.
  1. (d) The concentration of ions can be determined by titration in acidic solution using a standard solution of potassium manganate(VII). At room temperature, the reaction proceeds very slowly at first but becomes faster after some of the manganate(VII) ions have reacted.
  1. Suggest why this reaction is very slow at first.
  2. This is an example of an autocatalytic reaction. State the meaning of the term autocatalytic and identify the catalyst.
  3. Suggest how this catalyst might be involved in the reaction.

Question 4

  1. State the origin of the colour of transition-metal complexes.
  2. Give three changes to a transition-metal complex which result in a change in colour.

Question 5

  1. State the electron configuration of a Ti(III) ion and that of a Ti(IV) ion. Explain, in terms of electron configurations and electron transitions, why Ti(III) compounds are usually coloured but Ti(IV) compounds are colourless.
  2. Transition metal ions and their complexes can often be identified from their colours. For each of the following, identify a complex ion responsible for the colour of the aqueous solution. Restrict your answers to complexes formed from the elements Cr, Fe, Co and Cu.
  1. A deep blue solution formed in an excess of concentrated aqueous ammonia.
  2. A green solution formed in an excess of aqueous sodium hydroxide.
  3. A yellow–green solution formed in an excess of concentrated hydrochloric acid.

Question 6

When anhydrous iron(III) chloride is added to water the following reactions occur.

FeCl_3 + 6H_2O \rightarrow [Fe(H_2O)_6]^ 3^+ + 3Cl^-

[Fe(H_2O)_6] ^3^+ + H_2O \rightarrow [Fe(H_2O)_5(OH)]^2^+ + H_3O^+

  1. State the type of acidity shown by FeCl3 and by [Fe(H2O)6]3+ in these reactions. Explain your answers.
  2. Explain why the pH of a solution of iron(II) chloride is higher than that of a solution of iron(III) chloride of the same concentration.
  3. Transition metals have variable oxidation states. This is an important factor in their ability to act as heterogeneous and homogeneous catalysts.
  1. Vanadium(V) oxide acts as a heterogeneous catalyst in the Contact Process. Write equations to show the role of vanadium(V) oxide in this process.
  2. In aqueous solution, Fe2+ ions act as a homogeneous catalyst in the reaction between I– and S2O82– ions. Give one reason why the reaction is slow in the absence of a catalyst. Write equations to show how Fe2+ ions act as a catalyst for this reaction.

Question 7

A 0.263 g sample of impure iron, containing an unreactive impurity, was reacted with an excess of hydrochloric acid. All of the iron in the sample reacted, evolving hydrogen gas and forming a solution of iron(II) chloride. The volume of hydrogen evolved was 102 cm3, measured at 298 K and 110 kPa. The percentage, by mass, of iron in the sample can be determined using either the volume of hydrogen produced or by titrating the solution of iron(II) chloride formed against a standard solution of potassium dichromate(VI).

  1. Write an equation for the reaction between iron and hydrochloric acid.
  2. Calculate the number of moles of hydrogen produced in the reaction.
  3. Use your answers to parts (a) and (b) to determine the number of moles of iron and the mass of iron in the original sample.
  4. Calculate the percentage of iron in the original sample.
  5. Write half-equations for the oxidation of Fe2+ and for the reduction of Cr2O72- in acidic solution, and use these to construct an overall equation for the reaction between these two ions.
  6. The number of moles of iron in the sample was determined in part (c). Use this answer to calculate the volume of a 0.0200 mol dm–3 solution of potassium dichromate(VI) which would react exactly with the solution of iron(II) chloride formed in the reaction.
  7. Explain why an incorrect value for the number of moles of iron(II) chloride formed would have been obtained if the original solution had been titrated with potassium manganate(VII).