2023 SAJC H2 Chem Prelim P3 (Ans)

ST ANDREW’S JUNIOR COLLEGE JC2 PRELIMINARY EXAMINATIONS HIGHER 2 CANDIDATE NAME CLASS 2 2 S CHEMISTRY Paper 3 Free Response Candidates answer on the Question Paper. Additional Materials: Data Booklet 9729/03 14 September 2023 2 hours READ THESE INSTRUCTIONS FIRST Write your name and class on all the work that you hand in. Write in dark blue or black pen. You may use a 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. A Data Booklet is provided. 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. This document consists of XX printed pages (including this cover page). For Examiner’s Use Q1 21 Q2 20 Q3 19 Q4 or Q5 20 Total 80

Section A Answer all the questions in this section. 1 In the manufacture of tinted glass for chapels, a variety of carbonates including calcium carbonate were smelted together to form the needed colourations. (a) (i) Draw a ‘dot-and-cross’ diagram to show the bonding present within a carbonate ion, CO32−. [1] Accept x instead of • for the added e on the O (ii) State and explain, with reference to the VSEPR theory, the shape of the carbonate ion. [2] The carbonate ion has 3 bond pairs and 0 lone pairs around the central C. To minimise (electrostatic) repulsion (between the electron pairs , the shape of carbonate ion will be trigonal planar. (iii) When sodium carbonate was added to water, a weakly alkaline solution is produced. Write an equation for this reaction. [1] CO32-(aq) + H2O(l) ⇌ HCO3- (aq) + OH–(aq) (b) (i) Write a balanced equation, with state symbols, for the thermal decomposition of calcium carbonate under standard conditions. [1] CaCO3 (s) → CaO (s) + CO2(g) (ii) Construct a fully labelled energy cycle to show that the standard enthalpy change of formation of carbon dioxide is −399 kJ mol-1. Your cycle should include relevant data from the Data Booklet together with the following data. Standard enthalpy change of atomisation of carbon = +715 kJ mol−1 [2] C O O O x x xx xx xxxx xx xx xx x x 2-

[1] ΔHf = +715 + 496 – 805 x 2 = –399 kJ mol–1 (iii) The standard enthalpy changes of formation for these species are shown in the following table. Species CaCO3(s) CaO (s) CO2(g) ΔHfº / kJ mol–1 –1207 –635 –399 Using your answer in (b)(i) and (ii), calculate the standard enthalpy change of decomposition of calcium carbonate. [1] ΔHreaction = ΔHfo(CaO(s)) + ΔHfo(CO2(g)) – ΔHfo(CaCO3(s)) = –635 + (–399) – (–1207) = +173 kJ mol–1 (iv) By considering the entropy change, Δ S, and enthalpy change, Δ H,, in the decomposition of calcium carbonate, predict if the reaction is spontaneous at high or low temperature. [2] ΔS > 0 as the number of gaseous particles increases. Hence, at high temperature, TΔS > ΔH (or words to the effect), ΔG < 0 and reaction is spontaneous. (v) Using relevant data from the Data Booklet, suggest how the decomposition temperature of nickel (II) carbonate would compare with calcium carbonate. Explain your reasoning. [2] C(s) + O2(g) CO2(g) C(g) + O2(g) +715 C(g) + 2O(g) +496 ΔHf -805 x 2

Ionic radii of Ca2+ = 0.099 nm and ionic radii of Ni2+ = 0.069 nm Since the radii of Ni 2+ is smaller as compared to Ca 2+, Ni 2+ would have a stronger polarising power / higher charge density as compared to Ca2+. This would result in a stronger extent of polarisation / distortion of the electron cloud of the carbonate anion and a stronger weakening of the covalent bonds within carbonate anion. Hence less energy would be required to break such covalent bonds and a lower decomposition temperature is expected for NiCO 3 as compared to CaCO3. (c) The reaction of propanone with iodine can be catalysed by an acid. CH3COCH3 + I2 → CH3COCH2I + HI Sketch a Boltzmann distribution curve for the reaction and use it to explain the effect on the rate of the reaction when the catalyst is removed by adding CaCO3. [3] where Ea’ is the original activation energy and E a is the new higher activation energy. • Removal of the catalyst / H + ions results in the removal of the alternative reaction pathway with a lower activation energy, E a’. Or removal of H + catalyst means the reaction proceeds in a reaction pathway with higher activation energy, Ea. • The number of reactant particles having energy greater than or equal to the activation energy decreases. • Frequency of effective collisions decreases. • Rate decreases. Energy, E Ea Ea’ Fraction of particles having energy E Uncatalysed reaction Number of particles having energy E ≥ Ea Catalysed reaction Number of particles having energy E ≥ Eal 0

(d) When aqueous Cu2+ and Ni are mixed, an equilibrium is set up. Ni + Cu2+ ⇌ Ni2+ + Cu (i) Draw a fully labelled diagram of the experimental set-up used to measure the standard cell potential for this equilibrium. [2] (ii) Calculate the standard cell potential in (d)(i) and hence calculate the standard free energy change, ΔGo, per mole of Cu formed. [2] Cu2+ + 2e– ⇌ Cu +0.34 V Ni2+ + 2e– ⇌ Ni -0.25 V Eocell = +0.34 – (–0.25) = +0.59 V ΔGo = –(2)(96500)(+0.59) = − 113879 J mol−1 = –114 kJ mol–1 (iii) Use the Data Booklet to suggest how the position of equilibrium would change when concentrated NH3 is added to the Ni2+/Ni half cell. Explain your answer and write a balanced equation for this reaction. [2] Ni2+ + 2e– ⇌ Ni – 0.25 V [Ni(NH3)6]2+ + 2e– ⇌ Ni + 6NH3 – 0.51 V [Ni2+] drops / Eanode(Ni2+/Ni) becomes more negative / increases the reducing ability of Ni / decreases the oxidising ability of Ni2+ / [Ni2+] decreases. Hence, position of equilibrium would shift right Ni + 6NH3 + Cu2+ → [Ni(NH3)6]2+ + Cu [Total: 21 marks] salt bridge V [Cu2+] = = 1 mol dm–3 [Ni2+] = 1 mol dm–3 Cu Ni (+) (–) T = 298 K (1 bar)

2 (a) Methylamine, CH3NH2, can function as a Lewis base. (i) Explain what is meant by this statement and illustrate your answer with an equation. [2] N in methylamine can serve as electron pair donors. CH3NH2 + AlCl3 → (ii) Describe and explain the relative basicities of the following compounds. Compound formula A CH3NH2 B CH3CH2CH2CONHCH3 C [3] Order of increasing basic strength: B < C < A B is an amide and is the least basic (effectively neutral) because the lone pair of electrons on N is delocalised over the N-C-O bond, hence unavailable for donation to a proton. C is more basic B as the delocalisation effect in C is less extensive. A is the most basic as the electron donating -CH3 group makes the lone pair of electrons on N in A more available for donation to a proton. C is less basic than A because the lone pair of electrons on N can delocalise into the benzene ring. (b) Methylamine can also serve as a monodentate ligand. (i) State what is meant by monodentate ligand. [2] A monodentate ligand is an ion or molecule (or species) which can donate one lone pair of electrons to the vacant orbitals of a (central) metal atom/ion to form one dative/coordinate bond. NH2

(ii) In the presence of aqueous methylamine, [ Ni(H2O)6]2+ reacts to form a mixture of two isome