Transition Metals (AQA A Level Chemistry)

This question is about the transition metal copper.

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

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.

[Cu(H₂O)₆]²⁺  absorbs red light (700 nm) and the solution appears blue.

i) Give the equation which relates the energy change to the Planck constant, h, and the frequency of the visible light.

ii) Calculate a value for the energy in J associated with this wavelength of light. The Planck constant h = 6.63 x 10^-34J s and c = 3 x 10⁸ m s^-1.

iii) State two different features of a transition metal complex which cause a change in the energy change value.

EDTA is a multidentate ligand with a wide range of applications in both medicine and Industry.

Figure 1

When added to a solution containing [Ni(H₂O)₆^]2+ ions a ligand substitution reaction will occur:

[Ni(H₂O)₆]²⁺ + EDTA^4- → [NiEDTA]^2- + 6H₂O

i) EDTA^4- is a chelating ligand.  Explain this term.

ii) Explain why this ligand substitution reaction occurs.

iii) State the coordination number of the two complexes [Ni(H₂O)₆]²⁺ and [NiEDTA]^2-.

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.

Transition metals and their compounds are important catalysts.

i) Explain the term heterogeneous catalyst.

ii) Name the heterogeneous catalyst used in the Haber Process.

iii) Heterogeneous catalysts work by chemisorption. Explain this process.

iv) Explain why heterogeneous catalysts can become ineffective as a result of impurities. 

The reaction of manganate (VII) ions with ethanedioate ions is initially very slow. However, the reaction is catalysed by manganese (II) ions which are formed in the reaction.

i) Give an equation for the reaction between manganate (VII) ions and ethanedioate ions using the half equations below.

MnO₄^- + 8H⁺ + 5e^- → 2Mn²⁺ + 4H₂O

C₂O₄^2- → 2CO₂ + 2e^-

ii) Explain why the reaction can be monitored using colorimetry.

iii) Sketch a graph of concentration of manganate (VII) ions against time. Explain the shape of the graph.

Complete Table 1 below giving the formula, oxidation number and colour in solution of some vanadium ions.

Table 1

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

SO₂ + V₂O₅ → SO₃ + V₂O₄

V₂O₄ + O₂ → 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.

iii) The complex [Fe(H₂O)₆]²⁺ will react with sodium carbonate solution. Give an observation and an equation for the reaction with carbonate ions.

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.

iii) [Co(H₂O)₆]²⁺ is pink and [Co(NH₃)₆]²⁺ is yellow.  Explain why the two complexes have different colours.  

Aqueous Iron (II), Fe²⁺ and iron (III), Fe³⁺ ions catalyse the reaction between peroxodisulfate ions, S₂O₈^2-and iodide ions, I^-. It forms sulfate (VI) ions SO₄^2- and iodine, I₂ and is an example of homogeneous catalysis.

i) Explain what is meant by the term homogeneous catalysis.

ii) Give an equation for the conversion of S₂O₈^2- ions into SO₄^2- ions using I^- ions and explain why it is slow.

iii) Explain how the iron (II), Fe²⁺ or iron (III), Fe³⁺ ions catalyse the 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

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

ii) Predict the sign of ΔS for the reaction. Justify your prediction.

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 in Figure 1 below.

Figure 1

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.

Reduction potentials can be used to predict the feasibility of reactions. Consider the two half equations below:

VO²⁺ + 2H⁺ + e- → V³⁺ + H₂O             E = +0.34V

Zn²⁺ + 2e^- → Zn                                   E = -0.76V

i) Give an ionic equation for the reaction between zinc and the oxovanadium (IV) VO²⁺ ions.

ii) Calculate the EMF of this reaction.

iii) Predict the colour change which would be observed.

EDTA can sequester metal ions in a mixture by forming a stable complex that prevents the metal ions from reacting with other molecules or ions in the mixture.  The reaction is used in medicine to sequester calcium ions to prevent them from participating in clot formation.  It can also be used to determine the concentration of calcium compounds in hard water.

Figure 1

i) Give an equation to represent the reaction between [Ca(H₂O)₆]²⁺ and EDTA.

ii) Explain why the entropy value for the reaction is positive.

iii) A solution of calcium chloride was diluted by placing 25.0 cm³ into a volumetric flask and volume made up to 250.0 cm³ using deionised water.  25.0 cm³ of this solution was titrated with 0.0500 mol dm^-3 EDTA solution and the average titre was found to be 12.7 cm³.

Calculate the concentration of the original calcium chloride solution in g dm^-3.

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.

[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.

This question is about transition metals and transition metal complexes.

When chromium (III) sulfate dissolves in water, a green solution containing the [Cr(H₂O)₆]³⁺ ion forms.

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

ii) Explain why the chromium (III) complex ion is coloured.

This question is about transition metals and transition metal complexes.

The ligand ethylenediaminetetraacetate, EDTA⁴⁻, is shown in Figure 1.

Figure 1

When a solution of EDTA⁴⁻ is added to a solution of [Cr(H₂O)₆]³⁺ ions, a new complex ion is formed.

[Cr(H₂O)₆]³⁺ + EDTA^4-   ⇌  [Cr(EDTA)]^- + 6H₂O

The equilibrium constant for this equilibrium is 2.51 × 10²³  mol⁻¹dm³.

By considering the equilibrium for this reaction and changes in entropy, comment on the value of the equilibrium constant. No calculations are required.

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.

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.

Table 1 below shows the first and second ionisation energies of nickel, copper and zinc.

Table 1

The values for the first ionisation energies of copper and nickel are similar, but the values of the second ionisation energies are significantly different.

i) Explain how these data give evidence for the electronic configuration of a copper atom.

ii) Suggest why you might expect the third ionisation energies of the three elements to increase from nickel to zinc.

The equilibrium reaction for the transport of oxygen by haemoglobin (Hb) in blood can be represented as shown.

Hb (aq) + O₂ (aq) ⇌ HbO₂ (aq)

i) Explain how ligand substitution reactions allow haemoglobin to transport oxygen in blood.

ii) Write an expression for the equilibrium constant, K_c, for the equilibrium involved in the transport of oxygen by haemoglobin. Use the simplified species in the equation.

iii) In the presence of carbon monoxide, less oxygen is transported in the blood. Explain why, in terms of bond strength and K_c.

Iron and zinc are in the d-block of the Periodic Table.

Iodide ions, I^–, react with peroxodisulfate (VI) ions, S₂O₈^2- as shown in the reaction below.

2I^- (aq) + S₂O₈^2- (aq) →  I₂ (aq) + 2SO₄^2- (aq)

This reaction is catalysed by iron (II) ions, Fe²⁺(aq).

Write two ionic equations to show how iron (II) ions act as a catalyst in this reaction.

State symbols are not required.

Prince’s metal is a type of brass alloy that is used as imitation gold due to its yellow colour.

The copper content of a 5.00 g sample of Prince’s metal was analysed.

As part of the reaction process, copper ions are reacted with excess potassium iodide.

2Cu²⁺ + 4I^- → 2CuI + I₂

The iodine formed is analysed by titration with sodium thiosulfate.

I₂ + 2S₂O₃^2- → 2I^- + S₄O₆^2-

The sample containing iodine was diluted to 250 cm³ and then 10.0 cm³ aliquots were titrated with 0.100 mol dm^-3 sodium thiosulfate solution. The mean titre used was 22.65 cm³.

i) Calculate the percentage of copper, by mass, in the sample of Prince’s metal.

ii) The burette used had a margin of error of ±0.15 cm³. Calculate the percentage error in the burette for the titration.

A complex of cobalt has the following composition by mass:

Co, 21.98%; N, 31.35%; H, 6.81%; Cl, 39.86%

i) Calculate the empirical formula of this complex.

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

EDTA^4- is very soluble in water and is a multidentate ligand which can react with iron (II) ions to form complex ions. The concentration of iron (II) ions in a sample of river water was determined by colorimetry to be 7.84 × 10⁻⁵ mol dm⁻³.

This result was checked by titrating a sample of the water with a solution containing EDTA⁴⁻(aq) ions.

The EDTA⁴⁻(aq) had a concentration of 1.50 × 10⁻³ mol dm⁻³.

i) Write an equation for the reaction between [Fe(H₂O)₆]²⁺ and EDTA⁴⁻ ions.

ii) Calculate the volume of the EDTA⁴⁻ solution needed to react with a 30.0 cm³ sample of the water.

iii) Justify whether this titration will give an accurate value for the concentration of iron (II) ions. If necessary, suggest a practical step that would improve the accuracy.

1,2-diaminoethane is a bidentate ligand which can form a complex with [Co(NH₃)₄(H₂O)₂]²⁺. In this reaction only the ammonia molecules are replaced.

i) Write an equation for this reaction.

ii) Suggest why the enthalpy change for the reaction is approximately zero.

iii) Explain why the reaction proceeds despite having an enthalpy change that is approximately zero.

State and draw the number of stereoisomers that Co(H₂NCH₂CH₂NH₂)₂Cl₂ can form?

Iron (II) gluconate, C₁₂H₂₂FeO₁₄, is the active ingredient in some brands of iron supplements.

A student carries out an experiment to determine the mass of iron (II) gluconate in one tablet of an iron supplement, using the method below.

1. The student crushes two tablets and dissolves the powdered tablets in dilute sulfuric acid

2. The student makes up the solution from Stage 1 to 250.0 cm³ in a volumetric flask.

3. The student then titrated 25.0 cm³ portions of the solution obtained in Stage 2 with 0.00200 mol dm⁻³ potassium manganate (VII).

The student obtains a mean titre of 13.50 cm³.

In this titration, 1 mol of manganate (VII) ions reacts with 5 mol of iron (II) ions.

Explain why 0.00200 mol dm^-3 potassium manganate (VII) solution was used in this titration.

Use the student’s results from part (b) to determine the mass, in mg, of iron (II) gluconate in one tablet.

Give your answer to 3 significant figures.

Some iron supplements contain iron (II) sulfate or iron (II) fumarate.

Table 1 shows the information taken from the labels of two iron supplements, A and B.

Table 1

State which iron supplement, A or B, would provide the greater mass of iron per tablet. Show your workings.

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

In this test, aqueous iron (II) sulfate is added to a solution containing NO₃⁻ ions, followed by 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) Write equations for these two reactions.

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

When an acidified solution of potassium manganate (VII) is reacted with a sample of sodium ethanedioate, the reaction is quite slow initially for the purple colour to decolourise.

Write the overall equation for the reaction between manganate (VII) ions and ethanedioate ions in acidic solution.

Explain how the use of a colorimeter can be used to measure the increasing concentration of manganate ions formed in part (b).

Iron is a transition element that forms a number of ions with the iron in different oxidation states.

State the shorthand electron configuration of the iron cation that can form the complex ion [Fe(CN)₆]⁴⁻.