2021 RVHS Prelim Paper 2 (Solutions)

River Valley High School 9729/02/PRELIMS/21 2021 Preliminary Examination Suggested Answers [Turn over RIVER VALLEY HIGH SCHOOL JC 2 PRELIMINARY EXAMINATION CANDIDATE NAME Suggested Answers CLASS 2 0 J CENTRE NUMBER S INDEX NUMBER H2 CHEMISTRY 9729/02 Paper 2 Structured Questions 15 September 2021 2 hours Candidates answer on the Question Paper. Additional Materials: Data Booklet READ THESE INSTRUCTIONS FIRST Write your Centre number, index number, class and name on all the work that you hand in. 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. DO NOT WRITE IN ANY BARCODES. 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. The use of an approved scientific calculator is expected, where appropriate. A Data Booklet is provided. The number of marks is given in brackets [ ] at the end of each question or part question. For Examiner’s Use Question Number 1 2 3 4 5 6 7 Marks 12 9 12 9 8 18 7 significant figures units Total 75 This document consists of 23 printed pages and 1 blank page.

2 River Valley High School 9729/02/PRELIMS/02 2021 Preliminary Examination Suggested Answers 1 In recent years, there has been worldwide interest in the possible extraction of “shale gas” (a form of natural gas) as an important energy source. (a) One of the problems associated with using shale gas is its variable composition. Table 1 .1 shows the percentage composition of shale gas from four different sources J, K, L and M. Table 1.1 source CH4 C2Hx C3Hy CO2 N2 J 80.3 8.1 2.3 1.4 7.9 K 82.1 14.0 3.5 0.1 0.3 L 88.0 0.8 0.7 10.4 0.1 M 77.5 4.0 0.9 3.3 14.3 In the formulae above, x and y are variables. (i) Draw the structures of three possible hydrocarbons w ith 3 carbon atoms. (Any 3 of the compounds below)  CH3CH2CH3  CH3CH=CH2  CH3C≡CH  CH2=C=CH2  Δ (cyclopropane) [3] (ii) State the source of shale gas, J, K, L or M, that will likely provide the most energy when completely burned. Explain your answer. K, since it has the greatest % of hydrocarbons / carbon -containing compounds [1] (iii) Suggest a method by which carbon dioxide can be removed from shale gas. (Any one of these 3 answers)  reacted with lime / CaO / soda lime / Ca(OH)2 / KOH / NaOH /  liquefied under pressure  dissolved in water under pressure [1]

3 River Valley High School 9729/02/PRELIMS/02 2021 Preliminary Examination Suggested Answers [Turn over (b) Fracking is the process of drilling down into the earth before a high-pressure water mixture is directed at the rock to release the shale gas inside. Water, sand and chemicals are injected into the rock at high pressure which allows the gas to flow out to the head of the well. Magnesium hydroxide is one of the chemicals that can be used to control pH. (i) The equation for the formation of the gaseous hydroxide ion is shown. 1 2H2(g) + 1 2O2(g) + e–  OH–(g) H = Hf (OH–(g)) Use the data from Table 1.2 and the Data Booklet , construct a Born-Haber cycle to calculate Hf (OH–(g)). Table 1.2 H / kJ mol–1 standard enthalpy change of atomisation of Mg(s) +148 standard enthalpy change of formation of Mg(OH)2(s) –925 lattice energy of Mg(OH)2(s) –2993 (All values in kJ mol1) Let H = Hf (OH–(g)) be x kJ mol1 148 + 736 + 1450 + 2x + (2993) = 925 x = 133 Hf (OH–(g)) = 133 kJ mol1 [3]

4 River Valley High School 9729/02/PRELIMS/02 2021 Preliminary Examination Suggested Answers (c) Hydrazine, N2H4, can be also used as an energy source and is stored as a liquid. It reacts exothermically with oxygen to give only gaseous products. hydrazine (i) Hydrazine reacts with oxygen according to the following equation. N2H4(l) + O2(g)  N2(g) + 2H2O(g) Given that the standard enthalpy change of vapourisation of hydrazine is +36.9 kJ mol –1, calculate the enthal py change of the above reaction with relevant data from the Data Booklet. Bonds broken Bonds formed 1 NN N≡N 4 NH 4 OH 1 O=O Ha = 160 + 496 + 4(390)  944  4(460) = 568 kJ mol–1 Hr = +36.9 + (568) = 531.1 = 531 kJ mol–1 [2] (ii) Explain, using Gibbs free energy change, why the reaction of hydrazine with oxygen is spontaneous at all temperatures. Gr = Hr – TSr Sr is positive since there is an increase in number of gaseous particles from 1 to 3. Since both the ∆Hr and T∆Sr terms are negative, ∆Gr is always negative. Hence, the reaction is spontaneous at all temperatures. [2] [Total: 12] H = + 36.9 Hr Ha

5 River Valley High School 9729/02/PRELIMS/02 2021 Preliminary Examination Suggested Answers [Turn over 2 (a) (i) Describe the reactions, if any, of the chlorides MgC l2 and PCl5 with water. Write equations for all reactions that occur, and suggest the pH of the resulting solutions. Relate the reactivity of these chlorides to their structures and bonding. MgCl2 has a giant ionic lattice structure and strong electrostatic attraction between Mg 2+ and C l ions. MgCl2 dissolves in water readily to form an acidic solution of pH 6.5. Both hydration of ions and partial hydrolysis of Mg 2+(aq) occurs. Hydrolysis occurs due to the polarisation of water molecules by the Mg2+ ion MgCl2(s) + 6H2O(l)  [Mg(H2O)6]2+(aq) + 2Cl(aq) [Mg(H2O)6]2+(aq) + H2O(l)  [Mg(H2O)5OH]+(aq) + H3O+(aq) PCl5 has a simple covalent structure . PC l5 dissolves and undergoes hydrolysis in water to form an acidic solution of pH 2. P atom in PC l5 has energetically accessible vacant 3d orbitals to form dative bonds with water molecules. PCl5(s) + 4H2O(l)  H3PO4(aq) + 5HCl(aq) [3] (ii) Carbon is in the same group as silicon. Suggest and explain if CC l4 will react with water. CCl4 does not react with water as carbon does not have energetically accessible empty 3d orbitals to accept the lone pair of electrons from water molecules. [1] (b) Phosphorus exist in isotopes, mainly 31P, 32P and 33P. 31P is the most stable isotope. (i) Define isotopes. Isotopes are atoms of the same element with the same number of protons but different number of neutrons. [1] (ii) 31P bei ng the most stable isotope can react with chlorine to form phosphorus chloride. Phosphorus in phosphorus chloride can exhibit variable oxidation states. Based on your knowledge in periodicity, state the two common oxidation states of phosphorus in phosphorus chloride +3, +5 [1]

6 River Valley High School 9729/02/PRELIMS/02 2021 Preliminary Examination Suggested Answers (iii) Phosphorus can exist as cations or anions. Given the angle of deflection for 31P+ is 6, calculate the angle of deflection for 33P3 and add labelled line to the figure below to represent the paths of beam of 33P3 ions, at high temperature, in an electric field. charge mass for 31P+ = +1 31 = 0.03225 charge mass for 33P3 = -3 33 = 0.09091 Since chargeangle of deflection mass  , Angle of deflection= 0.09091 6 16.90.03225  [2] (iv) 32P is unstable and d ecays rapidly to another element . During the decay, a neutron is converted to a proton with the release of an electron, forming a charged ion. Write an equation to show the reaction. 32P  32S+ + e− [1] [Total : 9]  + Beam of 33P3 ions 16.9

7 River Valley High School 9729/02/PRELIMS/02 2021 Preliminary Examination Suggested Answers [Turn over 3 The use of the Data Booklet is relevant in this question. Values of the standard reduction potentials for certain silver species are given in Table