A-LevelChemistryGroup 29701/42 · May/June 2021 · Q212 Marks
State and explain the trend observed in the thermal stability of the Group 2 nitrates. ........................................................................................................................................................... ........................................................................................................................................................... ...........................................................................................................................................................
- (a)State and explain the trend observed in the thermal stability of the Group 2 nitrates. ........................................................................................................................................................... ........................................................................................................................................................... ...........................................................................................................................................................[3]
- (b)(i)Lead(II) nitrate, Pb(NO3)2, decomposes on heating in a similar manner to the Group 2 nitrates. Write an equation for the decomposition of lead(II) nitrate. ...........................................................................................................................................................[1]
- (b)(ii)Suggest how the ease of decomposition of Pb(NO3)2 would compare to that of Ba(NO3)2. Explain your answer. You may find it useful to refer to the Data Booklet. ........................................................................................................................................................... .........................................................................................................................................................[1]
- (c)(i)Barium ethanedioate, BaC2O4, decomposes on heating to produce barium oxide and a mixture of two different gases. Construct an equation for the decomposition of barium ethanedioate. ...........................................................................................................................................................[1]
- (c)(ii)An impure sample of BaC2O4, of mass 0.500g, is added to 50.0cm³ of 0.0200moldm-³ acidified MnO4-(aq), an excess. A redox reaction takes place and all the BaC2O4 reacts. The resulting solution, containing unreacted acidified MnO4-, is titrated with 0.0500 mol dm-³ Fe2+(aq). The end-point is reached when 30.40 cm³ of 0.0500 mol dm-³ Fe2+(aq) has been added. C2O42- ⇌ 2CO2 + 2e- MnO4- + 8H+ + 5e- ⇌ Mn2+ + 4H2O Fe2+ ⇌ Fe3+ + e- Calculate the percentage by mass of BaC2O4 in the 0.500g impure sample. Show your working. [Mr: BaC2O4, 225.3] percentage by mass of BaC2O4 = ..............................................................[4]
- (d)Barium hydroxide, Ba(OH)2, is completely dissociated in aqueous solution. Calculate the pH of 0.120 moldm-³ Ba(OH)2(aq) at 298K. pH = ..............................................................[2]
Mark scheme
Marking points from the official mark scheme (May/June 2021). The mark or mark type is shown in brackets.
- [3] M1: increases down the group M2: radius / size of cation / M2+ increases OR charge density of cation / M2+ decreases M3: less polarisation / less distortion of anion / NO3¯ ion OR less weakening of NO bond
- [1] Pb(NO3)2 → PbO + 2NO2 + 1/2O2
- [1] lead nitrate / Pb(NO3)2 would decompose more / easier AND as Pb2+ is smaller / Pb2+ has larger charge density (so more polarising)
- [1] BaC2O4 → BaO + CO2 + CO OR BaC2O4 → BaO + 2CO + 1/2O2
- [4] M1: [a] initial moles MnO4¯ = 0.0200 × 0.050 = 1.00 × 10-3 [b] moles Fe2+ = 0.050 × 0.0304 = 1.52 × 10-3 M2: [a] moles MnO4¯ unreacted = 1.52 × 10–3 / 5 = 3.04 × 10–4 [b] moles MnO4¯ reacted = 1.00 × 10–3 – 3.04 × 10–4 = 6.96 × 10–4 M3: moles C2O42- reacted = 6.96 × 10–4 × 5/2 = 1.74 × 10-3 M4: mass of BaC2O4 = 225.3 × 1.74 × 10–3 = 0.392 g % Purity of BaC2O4 = 100 × 0.392/0.50 = 78.4
- [2] M1: [OH-] = 2 × 0.12 = 0.24 (mol dm-³) [H+] = 1 × 10–14/0.24 = 4.17 × 10–14 / pOH = –log(0.24) OR 0.62 M2: pH = -log[H+] = 13.4 OR pH = 14 – 0.6 = 13.4
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