2020 SOTA Yr6 Chem HL Prelim P2 MS

Yr6/4/CHEM/HP2/Sep2020 22pages © School of the Arts, Singapore CHEMISTRY Higher level Paper 2 Preliminary Examinations Tuesday 1 September 2020 2 hours 15 minutes INSTRUCTIONS TO CANDIDATES • Write your name, class and index number in the blanks below. • Do not open this examination paper until instructed to do so. • Answer all questions. • Answers must be written within the answer boxes provided. • A calculator is required for this paper. • A clean copy of the chemistry data booklet is required for this paper. • The maximum mark for this examination paper is [90 marks]. Name: Class: Index: 90

Yr6/4/CHEM/HP2/Sep2020 Turn over - 2 - 1. Ammonia and ammonium salts have a wide range of uses. (a) In an experiment, 20.0 cm3 of aqueous ammonia is titrated with dilute sulfuric acid. The titration curve for this experiment is shown below. The chemical equation for the reaction between ammonia and sulfuric acid is as follows. 2 NH3 (g) + H2SO4 (aq) ® 2 NH4+ (aq) + SO42- (aq) (i) Outline why ammonia is classified as a weak base. [1] It dissociates/ ionises partially << in water>>. (ii) Using the titration curve, estimate the pH at equivalence point of this titration. [1] pH at equivalence point: Accept range from 4.8 to 5.8. (iii) Using a relevant chemical equation, explain why pH is low at the equivalence point. [2] NH4+ + H2O à NH3 + H3O+; Ammonium is conjugate acid of weak base/ acts as weak acid. 01234567891011121314 0.005.0010.0015.0020.0025.0030.0035.0040.0045.0050.00 pH Volume of H2SO4/ cm3 Initial pH = 11.3

Yr6/4/CHEM/HP2/Sep2020 Turn over - 3 - (iv) Using information from section 22 of the data booklet, state a suitable indicator that can be used to detect the equivalence point of this titration. [1] Methyl orange/ bromophenol blue/ bromocresol green/ methyl red/ bromothymol blue (v) The concentration of the sulfuric acid used for this titration is 0.100 mol dm-3. Using information from the graph only, calculate the initial concentration of the aqueous ammonia. [1] [NH3] = !.#!! × !.!&&! × &!.!&!! = 0.220 mol dm-3 (vi) The initial pH of the ammonia solution is 11.3. Using your answer from part (a)(v) and the initial pH of the ammonia solution, calculate the base dissociation constant, Kb, for ammonia. [2] pOH = 2.7, [OH-] = 2.00 x 10-3 mol dm-3 ; Kb = (2.00 x10-3)2 / 0.220 = 1.82 x 10-5 mol dm-3 Do not penalise if unit is missing or incorrect. Allow ECF from (v). (b) Ammonia reacts with boron trifluoride, BF3, to form NH3BF3. The chemical equation for this reaction is as shown. NH3 + BF3 ® NH3BF3 (i) Using Lewis acid-base theory, explain the role of NH3 in this reaction. [1] Lewis base as it is electron pair donor. (ii) Draw the Lewis structure for boron trifluoride, BF3. [1] Do not award mark if LP on F is missing.

Yr6/4/CHEM/HP2/Sep2020 Turn over - 4 - (iii) Use the VSEPR theory to predict the electron domain geometry of boron and the F-B-F bond angle in BF3 and NH3BF3. [2] BF3 NH3BF3 Electron domain geometry Trigonal planar Tetrahedral F-B-F bond angle 120o 109o/ 109.5o (c) Ammonia is produced industrially via the Haber process. This process involves a reversible reaction as shown. N2 (g) + 3 H2 (g) 2 NH3 (g) The industrial yield of ammonia produced under different temperature (oC) and pressure (atm) is shown in the graph below. (i) Using information from the graph, deduce if the forward reaction is an exothermic or endothermic reaction. [2] Yield decrease with increase in temperature; Forward reaction is not favoured/ POE shift left at high temperature, thus forward reaction is exothermic / OWTTE Source: https://qph.fs.quoracdn.net/main-qimg-fd67756fed8d30db42031075f8c50ecc-c Source: QS Study, Chemistry

Yr6/4/CHEM/HP2/Sep2020 Turn over - 5 - (ii) Essentially the Haber process involves “the mixing of nitrogen and hydrogen gases in a 1:3 ratio, and the gaseous mixture is then flushed through beds of finely divided iron filings( powder) at 400 oC and at 250 atm”. Explain why iron filings (powder) is used instead of iron blocks. [1] To maximise/ increase surface area of iron (iii) Write down the expression of the Kc for the Haber process. [1] Kc = [()*],[),]*[(,] (iv) At 472 oC, the value of the Kc for the Haber process is 0.104. Deduce how the position of equilibrium will shift when 0.100 mol N2, 0.300 mol H2 and 0.200 mol NH3 is mixed in a 1 dm3 reacting chamber at 472 oC. [2] Reaction quotient, Q = (!.&!!),[!./!!]*[!.#!!] = 14.8; Since Q > Kc( =0.104), POE will shift to the left/ backward reaction will be favoured. Do not award M2 if Q is not mentioned in the answer. (v) On the axes below, sketch how Kc will change with pressure of reacting conditions, keeping the temperature constant at 472 oC. [1] Kc pressure <<104>>

Yr6/4/CHEM/HP2/Sep2020 Turn over - 6 - (d) When some compounds dissolve in water, the process can be exothermic or endothermic. This enthalpy change is commonly used in heat packs or cold packs. In an experiment, 4.50 g of ammonium nitrate, NH4NO3, was added to a beaker containing 50.0 cm3 of deionized water. The temperature of the solution during the experiment was recorded and presented in the graph below. (Ammonium nitrate was added at t = 75 s.) (i) Using information from the graph only, estimate the lowest temperature reached in this investigation. [1] 18.6 oC / 0.0137(75) + 17.6) = 18.6 oC Accept range 18.5 - 18.7 oC. (ii) Using relevant information from section 2 of the data booklet, calculate the amount of heat (in kJ) involved in this process. [1] Q = 50.0 x 4.18 x (25.0 – 18.6) = 1337 J = 1.34 kJ Accept range 1.32 – 1.36 kJ (iii) Calculate the amount of ammonium nitrate (in mol) used for this experiment. [1] 0.1!2!.!3= 0.0562 mol y = 0.0137x + 17.6 18.019.020.021.022.023.024.025.026.0 050100150200250300350 Temperature/ oC time/ s

Yr6/4/CHEM/HP2/Sep2020 Turn over - 7 - (iv) Using your answers to part (d)(ii) and (iii), calculate the standard enthalpy change of the solution, ∆ H ¡sol , of ammonium nitrate (in kJ mol-1). [1] ∆𝐻;<=⊖ =#./0!.!13& = + 23.8 kJ mol-1 Allow ECF from (ii) (v) Predict the signs of standard Gibbs free energy change, ∆ G ¡ , and standard entropy change, ∆ S ¡ , for the process of dissolving of ammonium nitrate in deionized water. [2] Sign Standard Gibbs free energy change, ∆ G ¡ “-”/ < 0 Standard entropy change, ∆ S ¡ “+” / > 0 (vi) Explain your answer for the signs of standard Gibbs free energy change, ∆ G ¡ , and standard entropy change, ∆ S ¡. [2] ∆G is negative as process is spontaneous; As ∆H is positive, ∆S has to be positive such that value of T∆S is larger than ∆H / there is increase in disorderliness as solid salt dissolved into aqueous state.

Yr6/4/CHEM/HP2/Sep2020 Turn over - 8 - (vii) The standard enthalpy change of hydration, ∆ H ¡hyd , of ammonium and nitrate ions are shown in the table below. Ions ∆ H ¡hyd / kJ mol-1 NH4+ - 307 NO3- - 314 Source: Dasent, p. 152; D.W. Smith, J. Chem. Educ., 54, 540 (1977). Using your answer to part (iv) and the information provided, construct an energy cycle to determine the lattice enthalpy, ∆ H ¡lattice , of ammonium nitrate. [3] NH4NO3(s) NH4+(aq) +