OCR A Level Chemistry

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A LevelChemistryOCRTopic Questions5. Physical Chemistry & Transition Elements (A Level Only)5.6 Transition Elements

5.6 Transition Elements

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1a2 marks

Transition metals can form complex ions where ligands are coordinately bonded to the central metal ion.

State the meaning of the terms ligand and bidentate with respect to transition metal complexes.

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1b2 marks

Transition metals are located in the d-block of the periodic table.

i)
State the electron configuration of Ti2+.

ii)
Explain, using the electron configuration, why zinc is found in the d-block of the periodic table but not classed as a transition metal.

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1c2 marks

Sodium ferrocyanide,  Na4[Fe(CN)6], is a commonly used anticaking agent.

i)
State the oxidation state of Fe in [Fe(CN)6]4−

ii)
State the name of the shape and bond angle between the ligands in [Fe(CN)6]4−

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1d2 marks

Transition metals can form complexes with different ligands.

Identify one species from the following list that does not act as a ligand and explain your answer.

CO         H2O         SCN-          H2 

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2a1 mark

A complex ion contains one Co2+ ion, four ammonia molecules and two chloride ions.

State the formula of this complex ion.

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2b2 marks

Transition metals have characteristic properties. One of these properties is that they form coloured ions.

State two other properties that transition metals or their ions exhibit.

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2c2 marks

Blood carries oxygen around the body due to the presence of haemoglobin. Haemoglobin consists of a haem molecule that has a central metal iron(II) ion which can bind to oxygen gas.

Figure 1

haemoglobin-png-ocr

 
 Explain why carbon monoxide binding to haemoglobin can be deadly.

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2d2 marks

The [Ni(NH3)4Cl2]+ ion exists as two isomers. One geometric isomer is shown in Figure 2 below.

Figure 2

isomer-ocr

Draw the structure of the other isomer and state the type of isomerism.

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3a2 marks

Cisplatin can be used to treat some types of cancer.

i)
Draw the structure of cisplatin.

ii)
Explain how cisplatin can work as an anti-cancer drug.

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3b2 marks

Draw the cis and trans isomers of NiCl2(H2O)2.

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3c2 marks

State whether the coordination number and/or shape of the complex ion changes in the following situations.

i)
Ligand exchange of similarly sized ligands.

ii)
Ligand exchange of differently sized ligands.

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3d2 marks

The H2O ligands in [Fe(H2O)6]3+ can be exchanged for other ligands. 

Predict the shape of the complex ions formed after the following substitutions.

i)
All the H2O ligands exchanged for OH- ligands.

ii)
The six H2O ligands exchanged for four Cl- ligands.

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4a1 mark

Write an equation for the formation of [MnCl4]2- from a hexa-aqua manganese (II) ion and chloride ligands.

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4b2 marks

The metal-aqua ion [Cu(H2O)6]2+ will undergo a ligand substitution reaction with hydroxide ions to form [Cu(OH)4(H2O)2]2-.

i)
Write an equation for this reaction.

ii)
State the shape of [Cu(OH)4(H2O)2]2-.

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4c1 mark

Write an equation for the reaction of copper(II) ions with sodium hydroxide solution.

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4d1 mark

What would you observe as sodium hydroxide solution is added to a solution of iron(III) ions.

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5a1 mark

Give an example of an oxidising agent which could be used in the conversion of Fe2+ to Fe3+.

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5b3 marks

Aqueous hydrogen peroxide is added to an aqueous chromium (III) solution. Aqueous chromate (VI) ions are produced.

i)
What is the change in oxidation state when Cr3+ ions are oxidised to CrO42- ions.

ii)
What colour change is observed during this reaction?

iii)
What would the product be if dilute sulfuric acid was added to the chromate (VI) solution?

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5c3 marks

Give the colour of the precipitate formed when NaOH (aq) is added to solutions of the following ions.

i)
Mn2+

ii)
Fe2+

iii)
Cr3+

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1a3 marks

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.

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1b3 marks

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.

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1c
Sme Calculator3 marks

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

Figure 2


chromium-piccolate

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.

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1d1 mark

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.

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2a5 marks

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, C2O42-, to form geometric isomers.

[Cr(H2O)6]2+ +2C2O42- ⇌ [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.

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2b
Sme Calculator3 marks

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+.

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2c2 marks

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

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3a4 marks

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.

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3b1 mark

Phosphine, PH3, has a similar structure to ammonia.

Explain how phosphine can act as a ligand.

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3c2 marks

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.

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3d6 marks

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

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4a
Sme Calculator3 marks

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: 

Cr  S N
15.5 % 38.15 % 29.2 %

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

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4b
Sme Calculator1 mark

Calculate the oxidation number of chromium in the complex.

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4c1 mark

Suggest a shape for the complex anion.

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4d3 marks

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

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5a2 marks

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
platins


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

[2]

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5b5 marks

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

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5c1 mark

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].

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1a6 marks

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.

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1b5 marks

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. 

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1c6 marks

[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.

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1d3 marks

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.

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2a
Sme Calculator7 marks

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.

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2b8 marks

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.

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2c3 marks

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.

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2d3 marks

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.

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3a3 marks

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.

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3b2 marks

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.

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3c
Sme Calculator5 marks

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.

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3d3 marks

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.

haemoglobin

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.

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4a6 marks

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

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4b2 marks

Hydrogen peroxide, H2O2, is used as an oxidising agent in the oxidation of Cr(OH)63- to form CrO42-.

CrO42- is formed from Cr3+ ions.

i)
Explain how Cr(OH)63- 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)

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4c3 marks

The half equations for the redox reaction between H2O2 and Cr(OH)63- are:

H2O2 + 2e- → 2OH-

2Cr(OH)63- + 4OH- → 2CrO42- +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, CrO42- is formed, suggest a method to convert it to dichromate, Cr2O72-.

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4d4 marks

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.

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5a3 marks

Reactions of aqueous copper ions are shown in Figure.

reactions-of-copper-ions


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.

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5b3 marks

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.

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5c6 marks

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.

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5d2 marks

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, C2O42-, 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.

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