Mock Paper 3

CHEMISTRY 9729/03 Paper 3 Free Response Questions 2 hours READ THESE INSTRUCTIONS FIRST Write in dark blue or black pen. You may use an HB pencil for any diagrams or graphs. Do not use staples, paper clips, glue or correction fluid. Answer all questions in the spaces provided on the Question Paper. If additional space is required, you should use the pages at the end of this booklet. The question number must be clearly shown. Section A Answer all questions. Section B Answer one question. The use of an approved scientific calculator is expected, where appropriate. The number of marks is given in brackets [ ] at the end of each question, or part question. Question 1 2 3 4 5 Total Marks scored Maximum Marks 22 15 23 20 20 80 This document consists a total of 30 printed pages and 3 blank pages.

Paper by @ijustloveh2chem 2 BLANK

Paper by @ijustloveh2chem 3 Section A: Answer all questions. 1 Nitrogen, being the most abundant element in our atmosphere, is an element with much research interest and many applications. (a) Apart from N2, there are a few other poly-nitrogen species that are discovered to be stable enough at room temperature. One of these poly-nitrogen species is N5+. N5+ has 5 nitrogen atoms arranged in a straight-chain. Only the sigma bonds of N5+ is shown in the structure below. Na—Nb—Nc—Nd—Ne Some data is presented in Table 1.1. Bond length of Na—Nb and Nd—Ne 110 pm Bond length of Nb—Nc and Nc—Nd 130 pm Bond angle of Na—Nb—Nc 168° Bond angle of Nb—Nc—Nd 111° Table 1.1 (i) State the principles of the Valence Shell Electron Pair Repulsion theory. [2] (ii) State the geometry at the atoms Nb, Nc and Nd that most suitably explains the data in Table 1.1. [1] (iii) By considering your answer in (ii) and data in Table 1.1, draw a dot-and-cross diagram of N5+. [2] (iv) State the oxidation state of each of the nitrogen atoms Na to Ne from your dot-and-cross diagram in (iii). [1] …………………………………………………………………………………………………………… …………………………………………………………………………………………………………… …………………………………………………………………………………………………………… …………………………………………………………………………………………………………… …………………………………………………………………………………………………………… …………………………………………………………………………………………………………… ……………………………………………………………………………………………………………

Paper by @ijustloveh2chem 4 …………………………………………………………………………………………………………… …………………………………………………………………………………………………………… …………………………………………………………………………………………………………… …………………………………………………………………………………………………………… …………………………………………………………………………………………………………… …………………………………………………………………………………………………………… …………………………………………………………………………………………………………… …………………………………………………………………………………………………………… (b) The azide ion, N3-, is another poly-nitrogen species. The hypothetical salt [N5+] [N3-] has been postulated to be unstable at low temperatures. Some 2.267 mmol of [N5+] [SbF6-] was mixed with 2.267 mmol of CsN3 in SO2 at -196 °C. The mixture was warmed to -64 °C (the melting point of SO2), and a violent reaction started to take place which produced yellow flames together with bubbling. After the reaction, 0.8990 g of white solid was obtained at -64 °C. Equation 1.1 describes what this experiment was aiming to achieve. equation 1.1: [N5+] [SbF6-] + CsN3 → [N5+] [N3-] + CsSbF6 (i) Suggest an equation that accounts for the violent reaction. [1] (ii) With reference to your answer in (i), suggest why when the mixture was warmed, the reaction mixture became violent. [1] (iii) State the identity of the white solid, and comment on the amount of white solid obtained after the reaction. [2] (iv) At -64 °C, the value of Ksp of CsN3 in liquid SO2 is 9.86. Calculate the minimum volume of SO2 that needs to be added to dissolve all of the CsN3. [2] (v) Explain how the presence of dissolved CsN3 affects the solubility of CsSbF6 in liquid SO2. [1] …………………………………………………………………………………………………………… ……………………………………………………………………………………………………………

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Paper by @ijustloveh2chem 6 (c) Azide salts are highly unstable and decompose to give nitrogen gas. They have a variety of applications. For example, sodium azide is used in airbags. Electric detonators in the military contain lead(II) azide, Pb(N3)2. When an electric current is passed through lead(II) azide, it decomposes. (i) An electric detonator has a cylindrical aluminium covering that contains the detonating substance. In 1 electric detonator measuring 45.0 mm in length, and 7.00 mm in diameter, the detonator carries 0.600 g of lead(II) azide. A soldier passes a current through the detonator. Estimate the pressure inside the cylindrical aluminium covering after all the lead(II) azide has decomposed, and just before the covering is about to break. Assume that the covering is rigid. [3] Slight amounts of copper present in the electric detonator may lead to the formation of copper(II) azide, Cu(N3)2, over time. Copper(II) azide is much more sensitive than lead(II) azide, so this may lead to premature detonations and is very dangerous to the user. One way to detect copper(II) azide is through the iron(III) chloride test. A few drops of aqueous iron(III) chloride is added to the detonator. If a red colouration is seen, copper(II) azide is most likely present. Equation 1.2 represents the reaction taking place. equation 1.2: [Fe(H2O)5(OH)]2+(aq) + H+(aq) + N3-(aq) ⇌ [Fe(H2O)5(N3)]2+(aq) + H2O(l) numerical value of Kc = 0.5088 (ii) Explain why a solution of iron(III) chloride is acidic. [1] (iii) Explain why [Fe(H2O)5(N3)]2+ has a different colour compared to [Fe(H2O)5(OH)]2+. [1] (iv) 5.00 cm3 of a buffered solution containing [Fe(H2O)5(OH)]2+ with a concentration of 0.0200 mol dm-3 and pH of 2.72 was dropped onto a detonator suspected to have a contamination of copper(II) azide. Assume that this solution did not contain any azide ions initially. A red colouration was observed. 0.1 cm3 of this mixture, at equilibrium, was extracted for analysis. It was determined that the concentration of [Fe(H2O)5(N3)]2+ is 1.40 × 10-6 mol dm-3. Calculate the mass of copper(II) azide detected. [4] …………………………………………………………………………………………………………… …………………………………………………………………………………………………………… …………………………………………………………………………………………………………… ……………………………………………………………………………………………………………

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