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.

1. (a)(i)Draw 'dot-and-cross' diagrams of these two acids, showing outer shell electrons only. HOCN, cyanic acid HNCO, isocyanic acid[3]
2. (a)(ii)Suggest the values of the bond angles HNC and NCO in isocyanic acid. HNC ______________ NCO ______________[1]
3. (a)(iii)Suggest which acid, cyanic or isocyanic, will have the shorter C–N bond length. Explain your answer.[1]
4. (b)(i)Isocyanic acid is a weak acid. HNCO ⇌ H⁺ + NCO⁻ Kₐ = 1.2 × 10⁻⁴ mol dm⁻³ Calculate the pH of a 0.10 mol dm⁻³ solution of isocyanic acid. pH = ______________[2]
5. (b)(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]
6. (c)(i)Barium hydroxide, Ba(OH)₂, is completely ionised in aqueous solutions. During the addition of 30.0 cm³ of 0.100 mol dm⁻³ Ba(OH)₂ to 20.0 cm³ of 0.100 mol dm⁻³ isocyanic acid, the pH was measured. Calculate the [OH⁻] at the end of the addition. [OH⁻] = ______________ mol dm⁻³[2]
7. (c)(ii)Use your value in (i) to calculate [H⁺] and the pH of the solution at the end of the addition. final [H⁺] = ______________ mol dm⁻³ final pH = ______________[2]
8. (c)(iii)On the following axes, sketch how the pH changes during the addition of a total of 30.0 cm³ of 0.100 mol dm⁻³ Ba(OH)₂ to 20.0 cm³ of 0.100 mol dm⁻³ isocyanic acid. [Figure X.X][3]
9. (d)(i)The cyanate ion, NCO⁻, can act as a monodentate ligand. State what is meant by the terms monodentate, ligand.[2]
10. (d)(ii)Silver ions, Ag⁺, react with cyanate ions to form a linear complex. Suggest the formula of this complex, including its charge.[2]
11. (e)(i)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 1.66g. Ba(OH)₂(aq) + CO₂(g) → BaCO₃(s) + H₂O(l) Calculate the number of moles of BaCO₃ produced. moles of BaCO₃ = ______________[1]
12. (e)(ii)Hence calculate the Mᵣ of the organic isocyanate RNCO. Mᵣ of RNCO = ______________[1]
13. (e)(iii)The R group in RNCO and RNH₃Cl contains carbon and hydrogen only. Use your Mᵣ value calculated in (ii) to suggest the molecular formula of the organic isocyanate RNCO. molecular formula of RNCO ______________[1]
14. (e)(iv)Suggest a possible structure of the amine RNH₂, which forms the amine salt, RNH₃Cl.[1]

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