Transition Elements (OCR A Level Chemistry A): Exam Questions

This question is about the transition metal copper.

i) Give the full electronic configuration of a Cu atom and a Cu²⁺

ii) State four characteristic features of the chemistry of copper and its compounds.

The chloride ion is a monodentate ligand. When concentrated hydrochloric acid is added to [Cu(H₂O)₆]²⁺(aq) ions the water ligands are replaced.

i) Explain what is meant by the term monodentate ligand.

ii) Write an equation to represent the ligand substitution reaction.

iii) Draw a diagram to show the structure of the complex ion formed and name the shape it takes.

iv) State the change in coordination number. 

[Co(NH₃)₄Cl₂]⁺ is an example of an octahedral complex which exhibits cis-trans isomerism.

i) Deduce the oxidation state of cobalt in this complex and explain your answer.

ii) Draw and label each isomer of the complex.

Bidentate ligands form two coordinate bonds to a central metal atom or ion in a complex.  1,2-diaminoethane is an example of a compound which is a bidentate ligand. 

When added to [Cu(H₂O)₆]²⁺ all of the other ligands are replaced and two optical isomers are formed.

i) Write an equation for the ligand substitution reaction that is taking place when [Cu(H₂O)₆]²⁺ reacts with 1,2-diaminoethane.

ii) Draw the displayed structures of the two isomers produced.

12.2 g of hydrated iron(II) sulfate, FeSO₄.xH₂O were dissolved in acidic solution and made up to a volume of 500 cm³.  A 25.0 cm³ sample of this solution was titrated against 0.0200 mol dm^-3 potassium manganate(VII) solution.  21.95 cm³ of this solution were required.

i) Give an equation for the reaction between iron(II) ions and manganate ions.

ii) State the colour change that occurs at the end-point.

iii) Calculate the value of x in FeSO₄.xH₂O.

Complexes of [Pt(NH₃)₂Cl₂] and [Ni(H₂NCH₂CH₂NH₂)₃]²⁺ exhibit different forms of stereoisomerism.

i) State the type of stereoisomerism shown by each of the complexes.

ii) Draw the isomers of each complex.

iii) Give a use of an isomer of [Pt(NH₃)2Cl₂].

iv) Explain why [Ni(H₂NCH₂CH₂NH₂)₃]²⁺  has a 2+ charge overall.

Vanadium (V) oxide is the catalyst used in the contact process as shown by the reactions:

SO₂ + V₂O₅ → SO₃ + V₂O₄
V₂O₄ + ½O₂ → V₂O₅

i) Using the equations, write an overall equation for the reaction.

ii) Explain using the equations why V₂O₅ is a catalyst.

iii) Explain why V₂O₅ is able to act as a catalyst in this reaction.

When iron(II) compounds dissolve in water they form [Fe(H₂O)₆]²⁺, the hexaaquairon(II) complex.

i) State the full electronic configuration of an iron(II) ion.

ii) Predict the shape, and the bond angle of the hexaaquairon(II) complex.

Ligand and coordination number are common terms related to transition metals and their complexes.

i) Explain the meaning of the terms ligand and coordination number. 

ii) When ammonia (NH₃) is added to [Co(H₂O)₆]²⁺ a ligand substitution reaction takes place and a new complex is formed, [Co(NH₃)₆]²⁺. Give an equation for this reaction.

The complex [Ni(H₂O)₆]²⁺ reacts with 1,2-diaminoethane according to the equation:

[Ni(H₂O)₆]²⁺ + 3H₂NCH₂CH₂NH₂ → [Ni(H₂NCH₂CH₂NH₂)₃]²⁺ + 6H₂O

State and explain the change in coordination number during this reaction.

People with anaemia are often advised to take iron tablets which contain hydrated iron(II) sulfate, FeSO₄.6H₂O.  The composition of these tablets can be analysed by titration with acidified potassium manganate(VII) solution.  

6 g of tablets were crushed, dissolved in deionised water and made up to a volume of 250 cm³ in a volumetric flask. 25.0 cm³ samples of the iron tablet solution were titrated with 0.0200 mol dm^-3 potassium manganate(VII) solution and the average titre value was 22.4 cm³.

Use this information to calculate the percentage of FeSO₄.6H₂O in the tablets.

In humans, oxygen from the lungs dissolves in the blood and enters red blood cells where it forms a complex with haemoglobin.  A molecule of haemoglobin consists of four subunits, each containing an iron-haem complex, as shown below.

i) Explain the role of iron(II) in haemoglobin.

ii) With reference to iron(II) in haemoglobin, explain why breathing carbon monoxide can result in death.

Describe the reactions of aqueous copper(II) ions with:

• aqueous sodium hydroxide
• excess aqueous ammonia
• hydrochloric acid.

In your answer you should link observations with equations

Hydrogen peroxide, H₂O₂, is used as an oxidising agent in the oxidation of Cr(OH)₆^3- to form CrO₄^2-.

CrO₄^2- is formed from Cr³⁺ ions.

i) Explain how Cr(OH)₆^3- be formed from the hexaqua chromium(III) ion. Your answer should include reagents required.

ii) Write the balanced, full equation for the reaction in part (i)

The half equations for the redox reaction between H₂O₂ and Cr(OH)₆^3- are:

H₂O₂ + 2e^- → 2OH^-

2Cr(OH)₆^3- + 4OH^- → 2CrO₄^2- +8H₂O + 6e^-

i) Write the balanced, full equation for this redox reaction.

ii) Describe the colour change that is observed in this reaction.

iii) Once chromate, CrO₄^2- is formed, suggest a method to convert it to dichromate, Cr₂O₇^2-.

Tollens’ reagent is used to distinguish between aldehydes and ketones.  It can also be referred to as the diamminesilver(I) complex.

i) Write the formula of the complex.

ii) Draw the displayed structure of the complex.

iii) Name the shape the complex takes and give a value for the bond angle.

Reactions of aqueous copper ions are shown in Figure.

i) In reaction 1, the aqueous ammonia is added dropwise to the aqueous copper ions. State the colour and formula of Precipitate A.

ii) Give an equation for its formation.

Use Figure of part (a) to answer the following questions: 

i) State the colour and the formula of Solution B.

ii) Give an equation for its formation.

A sample of iron(III) chloride is dissolved in water and a pale yellow solution is formed.

i) Write the formula of the iron complex formed when iron(III) chloride is dissolved in water.

ii) State what is observed when aqueous sodium hydroxide is added to the solution containing the iron complex.

Give an equation, including state symbols, for the reaction.

Ferric oxalate, also known as iron(III) oxalate, is a chemical compound composed of ferric ions and oxalate ligands. The anhydrous material is pale yellow; however, it may be hydrated to form several hydrates.  Oxalate ions can also be termed ethanedioate ions, C₂O₄^2-, and are bidentate ligands.

i) Write an equation to show the formation of iron(III) oxalate from the reaction of aqueous iron(III) ions and excess ethanedioate ions.

ii) State an observation if aqueous sodium hydroxide solution is added to the iron(III) oxalate solution.

This question is about transition metals and transition metal complexes. 

When chromium(III) sulfate dissolves in water, a violet solution forms.

i) State the bond angles found in the resulting complex ion.

ii) A small quantity of an unknown chemical is added resulting in the formation of a green precipitate. The extra addition of this chemical results in a purple solution.

Explain the initial reaction in terms of Brønsted–Lowry Acids & Base Theory.

Compound I is a complex with the empirical formula CoN₄H₁₂Cl₃.

The formula of compound I contains one chloride ion and a complex ion J, which has two stereoisomers.

Draw and label the three-dimensional structures of the two stereoisomers of complex ion J. Include the charge of the complex ion in your diagrams.

Chromium (III) picolinate, shown in Figure 2, is a neutral complex that can be prepared from the weak acid, picolinic acid.

Figure 2

Chromium(III) picolinate is used in tablets as a nutritional supplement for chromium. 

i) Draw the structure of the ligand in chromium(III) picolinate.

ii) A typical tablet of chromium(III) picolinate contains 200 μg of chromium.
Calculate the mass, in g, of chromium (III) picolinate in a typical tablet.
Give your answer to three significant figures.

By considering the relevant half equations, write balanced equations for the following chemical reaction.

Acidified potassium manganate(VI) reacting with a lemon-yellow solution of iron ethanedioate, FeC₂O₄ to produce a pale green solution and bubbles of carbon dioxide.

This question is about chromium and cobalt complexes.

Chromium is in the d-block of the Periodic Table.

Hydrated chromium(II) ions react with ethanedioate ions, C₂O₄^2-, to form geometric isomers.

[Cr(H₂O)₆]²⁺ +2C₂O₄^2- ⇌ [Cr(C₂O₄)₂(H₂O)₂]^2- + 4H₂O

i) Draw a structure of the cis [Cr(C₂O₄)₂(H₂O)₂]^2– ion, showing all of the bonds.

ii) Explain, in terms of entropy, why this reaction is feasible.

A complex of cobalt has the following composition by mass:

Co, 22.15%; N, 31.28%; H, 6.79%; Cl, 39.78%

i) Calculate the empirical formula of this complex.

ii) The formula of this cobalt complex can be expressed in the form [Co(L)_m]^x+(Cl⁻)_n

Suggest the chemical formula of [Co(L)_m]^x+.

State and draw the number of stereoisomers that [Co(H₂NCH₂CH₂NH₂)₂Cl₂]⁺ can form.

This question is about tests for transition metals and catalytic complexes.

Fe²⁺ ions can be used to test for NO₃⁻.

In this test, aqueous iron(II) sulfate is added to a solution containing NO₃⁻ ions, followed by the slow addition of concentrated sulfuric acid. The sulfuric acid forms a layer below the aqueous solution.

In the presence of NO₃⁻ ions, a brown ring forms between the two layers.

Two reactions take place.

Reaction 1:

In the acid conditions, Fe²⁺ ions reduce NO₃⁻ ions to NO. Fe²⁺ ions are oxidised to Fe³⁺ ions and water also forms.

Reaction 2:

A ligand substitution reaction of [Fe(H₂O)₆]²⁺ takes place in which one NO ligand exchanges with one water ligand. A deep brown complex ion forms as the brown ring.

i) Construct equations for these two reactions.

ii) State the type of reaction that occurs in reaction 1 and reaction 2.

Phosphine, PH₃, has a similar structure to ammonia.

Explain how phosphine can act as a ligand.

Levulinic acid is a chemical used to make pharmaceuticals which can be made from renewable resources. The hydrogenation of levulinic acid offers a green and sustainable route to a variety of chemicals used in perfumes. A catalyst for this hydrogenation is dihydridocarbonyltris(triphenylphosphine)ruthenium(Z), [RuH₂(CO)(PPh₃)₃], where Z is the oxidation state of ruthenium. PPh₃ is a common abbreviation for P(C₆H₅)₃ which has a similar structure to phosphine.

i) State the value of Z.

ii) Deduce the shape of the catalyst complex.

Draw the stereoisomers of the catalyst [RuH₂(CO)(PPh₃)₃] and determine whether each stereoisomer has an enantiomer.

Reinecke’s salt is formed by gradually adding ammonium dichromate(VI) to molten ammonium thiocyanate.

Reinecke’s salt has the formula NH₄[Cr(SCN)_x(NH₃)_y] and the following composition by mass: 

Calculate the values of x and y in the above formula. 

Calculate the oxidation number of chromium in the complex.

Suggest a shape for the complex anion.

Draw two possible structures for the anion and state the type of isomerism it exhibits.

Carboplatin and oxaliplatin are both platinum(II) diamines, similar to cisplatin, which are widely used in chemotherapy to treat a wide variety of cancers. However, their use is often plagued with severe toxicity and the development of resistance, which leads to disease progression. Recently, oxoplatin, iproplatin, ormaplatin and satraplatin are Pt complexes that have been used clinically or in clinical trials. 

The four complexes, as shown in Figure 1 below, all have the same geometry and oxidation number for the Pt central atom

Figure 1

Write the oxidation state of Pt and the geometry of the complexes.

[2]

Oxaplatin is an isomer of [Pt(NH₃)₂Cl₂(OH)₂] complex. Draw all stereoisomers and indicate the chiral one(s).

Platinum complexes can be considered prodrugs that are intracellularly activated to kill cancer cells.

Cis- platin can be produced by the reduction of the prodrug cis, trans, cis-[PtCl₂(OCOCH₃)₂(NH₃)₂], which has a similar structure to satraplatin.

Draw the structure of cis, trans, cis-[PtCl₂(OCOCH₃)₂(NH₃)₂].