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Question 1
When NH3(aq) is added to Cu2+(aq), dropwise at first and then in excess, two chemical reactions occur as shown.
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a) | For each reaction:
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b) | When complex ion B, [Cu(NH3)4(H2O)2]2+, reacts with 1,2-diaminoethane (en), the ammonia molecules but not the water molecules are replaced.
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Write an equation for this reaction. en = 1,2-diaminoethane.
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c) | Suggest why the enthalpy change for the reaction in part b) is approximately zero.
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d) | Explain why the reaction in part b) is spontaneous despite having an enthalpy change that is approximately zero.
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Question 2
Ethanedioic acid, (COOH)2, is the active ingredient in a spray to treat infections of the outer ear. A chemist carries out a redox titration using aqueous cerium(IV) sulfate, Ce(SO4)2(aq), to determine the percentage, by mass, of ethanedioic acid in the spray. In the titration, Ce4+(aq) ions oxidise ethanedioic acid in acidic conditions, as shown by the equation below 2Ce4+(aq) + (COOH)2(aq) → 2Ce3+(aq) + 2CO2(g) + 2H+(aq) Ce4+(aq) ions have a yellow colour. Ce3+(aq) ions are colourless. Dilute sulfuric acid is added to 5.00 g of the spray to form a colourless solution which is made up to 250.0 cm3 with distilled water. 25.00 cm3 of this solution is titrated with 1.00 x 10-2 moldm-3 Ce(SO4)2 from the burette. The titration is repeated until concordant titres are obtained. The mean titre is 22.25 cm3.
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a) | What colour change would the chemist observe at the end point?
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b) | Calculate the percentage, by mass, of ethanedioic acid, (COOH)2, in the spray. Give your answer to 3 significant figures.
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Question 3
A chemist uses the following procedure to analyse iron tablets containing iron in the form of iron(II) sulfate, FeSO4.
The overall equation for the reaction occurring in the titration is: MnO4-(aq) + 8H+(aq) + 5Fe2+(aq) ⇌ Mn2+(aq) + 4H2O(l) + 5Fe3+(aq)
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The table below shows some standard electrode potential data. |
a) | Using data in the table, explain why the chemist used sulfuric acid and not hydrochloric acid in their titration.
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b) | Suggest why this titration does not require the addition of an indicator.
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c) | State the colour change at the end-point of this titration.
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d) | The chemist decides to take burette readings from the top of the meniscus rather than from the bottom. Explain the effect of this, if any, on the titre values.
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e) | The chemist's titration readings are shown in the table below. Complete the table.
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f) | Use the titration results in part e) and information from the procedure to calculate the mass of FeSO4 in 1 iron tablet. Give your answer to 3 significant figures.
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Question 4
This question look at the ions and complexes of cobalt. An octahedral complex ion Y, C9H30N6Co3+, exists as two optical isomers. In complex ion Y, Co3+ is bonded to three molecules of a bidentate ligand L.
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a) | Write the molecular formula of the bidenitate ligand L.
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The diagram below shows the skeletal formula of L. |
b) | Explain how L is able to act as a bidentate ligand.
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c) | Draw a diagram that shows the shape of the complex ion Y and shows the type of bond between the ligand L and the cobalt.
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d) | The compounds [Co(NH3)5Br]SO4 and [Co(NH3)5SO4]Br are structural isomers.
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Define the term structural isomer.
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e) | What is the oxidation number of cobalt in [Co(NH3)5Br]2+? A +1 B +2 C +3 D +4
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Question 5
Ammonium hexachloroplatinate, (NH4)2PtCl6, is a complex of platinum used in platinum plating. Ammonium hexachloroplatinate contains the hexachloroplatinate ion.
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a) | Draw a 3D diagram to show the shape of a hexachloroplatinate ion. On your diagram, show:
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b) | Platin, Pt(NH3)2Cl2, is a neutral complex of platinum(II).
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Explain why Pt(NH3)2Cl2 has no overall charge.
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Pt(NH3)2Cl2, displays cis-trans isomerism.
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c) | Draw the structure of cis-platin. State its shape and give a value of the Cl-Pt-Cl bond angle.
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d) | The use of cis-platin can cause side effects so oxaliplatin has been developed. The structure of oxaliplatin is shown below.
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Draw the structures of the two bidentate ligands in oxaliplatin.
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Question 6
Steel pipelines carrying natural gas can corrode to form ‘green rust’, Fe(OH)2. After some time, green rust can change into ‘black rust’ via the equation shown below. 3Fe(OH)2 ➔ Fe2+Fe3+2O4 + H2 + 2H2O
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a) | Use oxidation states to show that this is a redox reaction.
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b) | If seawater is present, the green rust is often oxidised to [FeCl4]- ions. Draw diagrams to show two possible shapes for [FeCl4]-. Name the shapes.
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c) | State the type of bonding between the chloride ligands and the iron ion in [FeCl4]-.
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d) | Cu+ also forms complexes with chloride ligands. One example is the colourless complex [CuCl2]-. Explain why [CuCl2]- ions are colourless.
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Question 7
The complex [Co(NH3)4Cl2]+ shows isomerism. |
a) | Suggest the type of isomerism shown in [Co(NH3)4Cl2]+ and draw 3D structures of the two isomers.
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b) | The diagram below shows the structure of ethane-1,2-diamine, en. en is an example of a bidentate ligand. |
Explain what is meant by the term bidentate ligand.
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c) | [Cr(en)2Cl2]+ shows geometrical and optical isomerism. Complete the 3D diagrams to show the 3 stereoisomers of [Cr(en)2Cl2]+.
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d) | Using the model answer diagram, give the letters of the two isomers of [Cr(en )2Cl2]+ which show optical isomerism.
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e) | Using the model answer diagram, give the letter of the isomer which has no overall dipole moment.
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Question 8
a) | Explain what is meant by the term transition element.
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b) | Draw electrons in the boxes below to show the electronic configurations of Cu, Cu+ and Cu2+.
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c) | Complete the table by predicting the appropriate metal ions, co-ordination numbers, formulae and charges for the complexes A-D.
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Question 9
Transition metals have characteristic properties due to their electron arrangements.
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a) | Give two characteristic properties of transition metals.
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b) | Sketch the shapes of the following atomic orbitals in the table below.
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c) | Give the electronic configurations of a chromium atom, a Cr2+ ion and a Cr3+ ion.
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d) | Explain why chromium has variable oxidation states.
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Question 10
a) | Explain why complexes formed from transition metal ions are coloured.
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b) | This diagram represents the energy change that occurs when a d electron in a transition metal ion is excited by visible light.
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The energy change shown in the diagram represents the energy of green light and results in a solution that appears violet. Violet light has a higher frequency than green light.
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Suggest how the energy change, ΔE, will differ for a transition metal ion that forms a green solution. Explain your answer.
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c) | State 3 different features of transition metal complexes that cause a change in the value of ΔE.
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Question 11
The equation below represents the dissociation of [Fe(H2O)6]3+ ions in aqueous solution. [Fe(H2O)6]3+(aq) ⇌ [Fe(H2O)5OH]2+(aq) + H+(aq) The Ka for this dissociation is 6.00 × 10-3 mol dm-3
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a) | Calculate the pH of a 1.50 x 10-2 mol dm-3 solution of [Fe(H2O)6]3+. Give your answer to 2 decimal places.
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b) | The diagram below shows some reactions of iron ions in aqueous solution. |
Give the formulae of A and B and state their colours.
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c) | The green precipitate A, Fe(OH)2, turned brown on standing in air. Explain why.
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Question 12
When concentatd hydrochloric acid is added to aqueous Cu2+ ions, the following equilibirum is established. [Cu(H2O)6]2+ + 4Cl- ⇌ [CuCl4]2- + 6H2O
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a) | State the colours of the [Cu(H2O)6]2+ and [CuCl4]2- complex ions.
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b) | Draw the shapes of the [Cu(H2O)6]2+ and [CuCl4]2- complex ions. Give the name of the shape for each ion.
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c) | State the coordination numbers of the copper ion in [Cu(H2O)6]2+ and [CuCl4]2-. Explain the change in coordination number in this equilibrium.
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d) | In aqueous solution, the [Cu(H2O)6]2+ ion reacts with an excess of ethane-1,2-diamine (en) to form the complex ion Z.
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Write an equation for this reaction. en = 1,2-diaminoethane
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e) | Explain, in terms of the chelate effect, why the complex ion Z is formed in preference to the [Cu(H2O)6]2+ complex ion.
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