CIE Jun 2017 v3 Paper 4 Q2

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(a) One atom of each of the four elements H, C, N and O can bond together in different ways. Two examples are molecules of cyanic acid, HOCN, and isocyanic acid, HNCO. The atoms are bonded in the order they are written. (i) Draw dot-and-cross diagrams of these two acids, showing outer shell electrons only. PMT HOCN, cyanic acid HNCO, isocyanic acid (ii) Suggest the values of the bond angles HNC and NCO in isocyanic acid. HNCNCO(iii) Suggest which acid, cyanic or isocyanic, will have the shorter CN bond length. Explain your answer. [3] [1][1] (b) (i) Isocyanic acid is a weak acid. HNCO H+ + NCO Ka = 1.2104 mol dm3 Calculate the pH of a 0.10 mol dm3 solution of isocyanic acid. pH =[2] (ii) Sodium cyanate, NaNCO, is used in the production of isocyanic acid. Sodium cyanate is prepared commercially by reacting urea, (NH2)2CO, with sodium carbonate. Other products in this reaction are carbon dioxide, ammonia and steam. Write an equation for the production of NaNCO by this method.[1]UCLES 2017 9701/43/M/J/17<br /> (c) Barium hydroxide, Ba(OH)2, is completely ionised in aqueous solutions. During the addition of 30.0 cm3 of 0.100 mol dm3 Ba(OH)2 to 20.0 cm3 of 0.100 mol dm3 isocyanic acid, the pH was measured. (i) Calculate the [OH] at the end of the addition. PMT (ii) Use your value in (i) to calculate [H+] and the pH of the solution at the end of the addition. [OH] =mol dm3 [2] final [H+] =mol dm3 final pH =[2] (iii) On the following axes, sketch how the pH changes during the addition of a total of 30.0 cm3 of 0.100 mol dm3 Ba(OH)2 to 20.0 cm3 of 0.100 mol dm3 isocyanic acid. 14 pH 10 15 20 25 30 volume of Ba(OH)2 added / cm3UCLES 2017 9701/43/M/J/17 [3] [Turn over<br /> PMT (d) The cyanate ion, NCO, can act as a monodentate ligand. (i) State what is meant by the terms monodentate,ligand.[2] Silver ions, Ag+, react with cyanate ions to form a linear complex. (ii) Suggest the formula of this complex, including its charge.[2] (e) When heated with HCl (aq), organic isocyanates, RNCO, are hydrolysed to the amine salt, RNH3Cl, and CO2. RNCO + H2O + HCl RNH3Cl + CO2 A 1.00 g sample of an organic isocyanate, RNCO, was treated in this way, and the CO2 produced was absorbed in an excess of aqueous Ba(OH)2 according to the equation shown. The solid BaCO3 precipitated weighed 1.66 g. Ba(OH)2(aq) + CO2(g) BaCO3(s) + H2O(l) (i) Calculate the number of moles of BaCO3 produced. (ii) Hence calculate the Mr of the organic isocyanate RNCO. moles of BaCO3 =[1] Mr of RNCO =[1]UCLES 2017 9701/43/M/J/17<br /> The R group in RNCO and RNH3Cl contains carbon and hydrogen only. (iii) Use your Mr value calculated in (ii) to suggest the molecular formula of the organic isocyanate RNCO. PMT molecular formula of RNCO[1] (iv) Suggest a possible structure of the amine RNH2, which forms the amine salt, RNH3Cl. [1] [Total: 23]UCLES 2017 9701/43/M/J/17 [Turn over<br />

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