Edexcel Jun 2017 Paper 5 Q23

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SECTION C Answer ALL the questions. Write your answers in the spaces provided. 23 This question is about complex ions. Complex ions are ions in which a number of molecules or anions are bonded to a central metal cation. Some of the first research on complex ions was carried out about 100 years ago, and demonstrated that there were two isomers with the molecular formula Co(NH3)4Cl3. In aqueous solution, many transition metal ions are surrounded by six water molecules forming ions such as [Cu(H2O)6]2+. Complex ions, such as haem, are also found in biological systems. The haem groups are large organic molecules containing nitrogen atoms, which form complex ions with Fe2+. The shape of a complex ion depends on the number of bonds from the ligands to the central metal ion, and the different shapes can result in different types of stereoisomerism. *(a) Explain why the formation of complex ions is a characteristic of transition metals, but Group 1 metals, such as sodium, do not show this property in general. (2)(b) (i) State all the types of bonding involving chloride ions in isomers of Co(NH3)4Cl3 (1)22 *P48070A02228* D O N O T W R I T E I N T H I S A R E A D O N O T W R I T E I N T H I S A R E A D O N O T W R I T E I N T H I S A R E A<br /> (ii) The ion [Co(NH3)4Cl2]+ has stereoisomers. Name the type of stereoisomerism and explain how their structures differ. The different structures may be shown on a diagram. (2)(c) Write the equation for the ligand exchange reaction of [Cu(H2O)6]2+ with excess chloride ions from concentrated hydrochloric acid. State symbols are not required. State the shape of the [Cu(H2O)6]2+ ion, and suggest the shape of the copper complex ion produced. (3) Equation Shape of [Cu(H2O)6]2+Shape of complex ion produced(d) (i) Explain the difference between monodentate and hexadentate ligands. (2)A E R A S I H T N I E T I R W T O N O D A E R A S I H T N I E T I R W T O N O D A E R A S I H T N I E T I R W T O N O D *P48070A02328* 23 Turn over<br /> *(ii) EDTA4 forms very stable complex ions. In solution, its reaction with copper ions can be represented as shown. [Cu(H2O)6]2+ + EDTA4 (cid:85) [Cu(EDTA)]2 + 6H2O Explain why the equilibrium constant for this reaction is greater than that for the reaction of [Cu(H2O)6]2+ with a monodentate ligand. (2)D O N O T W R I T E I N T H I S A R E A(e) Crystals of a hydrated salt were prepared by reacting a solution containing iron(II) ions with a mixture of ethanedioic acid, (COOH)2 and potassium ethanedioate, K2(COO)2 or K2C2O4. HO OH O O ethanedioic acid ethanedioate ion The resulting mixture was reacted with hydrogen peroxide, which oxidized the iron(II) ions and, after a suitable procedure was carried out, coloured crystals were produced. The crystals contain a complex ion and the formula of the crystals can be written as KxFey(C2O4)z(H2O)n The crystals were analysed. (i) The percentage of water in 2.00 g of the crystals was measured by gentle heating until the mass was constant. The final mass was 1.78 g. Calculate the percentage by mass of water in the crystals. (1) *P48070A02428* 24 D O N O T W R I T E I N T H I S A R E A D O N O T W R I T E I N T H I S A R E A<br /> (ii) The percentage by mass of ethanedioate ions in the crystals was found by titrating an acidified solution containing 0.150 g of the crystals with a solution of 0.0100 mol dm3 potassium manganate(VII). The titre was 36.60 cm3. The ethanedioate ions were oxidized by the manganate(VII) ions. The reactions which occur are C2O4 2 (cid:111) 2CO2 + 2e MnO4+ 8H+ + 5e (cid:111) Mn2+ + 4H2O Use the titration results to calculate the percentage by mass of ethanedioate ions in the crystals. (4) (iii) To find the percentage by mass of iron in the crystals, they were first heated with concentrated sulfuric acid to decompose the ethanedioate ions, taking suitable safety precautions. The iron ions in solution were then reduced to iron(II) ions with a suitable metal and these ions were then titrated with 0.0100 mol dm3 potassium manganate(VII) solution. The iron(II) ions were oxidized to iron(III) ions in the titration. Why must the ethanedioate ions be decomposed before determining the concentration of iron(II) ions? (1)A E R A S I H T N I E T I R W T O N O D A E R A S I H T N I E T I R W T O N O D A E R A S I H T N I E T I R W T O N O D *P48070A02528* 25 Turn over<br /> (iv) The percentage by mass of iron was found to be 11.4%. Use this value, and the percentages of water and ethanedioate ions you have calculated, to deduce the percentage by mass of potassium in the crystals. Species Percentage by mass water ethanedioate ions iron potassium 11.4 (v) The percentage by mass shows the mass of each species in 100 g. Calculate the number of moles of each species in 100 g of the crystals and hence the ratio in moles. Species Number of moles in 100 g Mole ratio (1) (2) water ethanedioate ions iron potassium (vi) Suggest a formula, including the charge, for the complex ion present in the crystals. (1) 26 *P48070A02628* D O N O T W R I T E I N T H I S A R E A D O N O T W R I T E I N T H I S A R E A D O N O T W R I T E I N T H I S A R E A<br /> (vii) Draw a diagram showing how one ethanedioate ligand is bonded to the central metal ion. (1) (Total for Question 23 = 23 marks) TOTAL FOR SECTION C = 23 MARKS TOTAL FOR PAPER = 90 MARKS A E R A S I H T N I E T I R W T O N O D A E R A S I H T N I E T I R W T O N O D A E R A S I H T N I E T I R W T O N O D *P48070A02728* 27<br />

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