NJC Prelim_P4_Solutions

Suggested solution for 2017 SH 2 Prelim Practical 1 (a) (ii) Table 1: Weighings of FA1 Mass of weighing bottle and FA1 / g 3.696 Mass of weighing bottle and residual FA1 / g 3.497 Mass of FA1 transferred / g 0.199 Table 2: Titration of FA3 with FA4 using screened methyl orange as indicator 1 2 Final burette reading / cm3 30.00 36.90 Initial burette reading / cm3 9.00 16.00 Volume of FA4 used / cm3 21.00 20.90 (iii) average volume = 21.00 + 20.90 2 = 20.95 cm3 (b) (i) amount of HNO3 in 25.0 cm3 = amount of NaOH = 20.95 / 1000 x (9.10/40.0) = 4.77 x 10‒3 amount of HNO3 in 250.0 cm3 = 4.77 x 10‒2 (ii) amount of HNO3 in 25.0 cm3 = 25/1000 x 2 = 0.05 mol (iii) amount of HNO3 = 0.05 ‒ 4.77 x 10‒2 = 2.34 x 10‒3 (iv) amount of MgCO3.xH2O = 2.34 x 10‒3 / 2 = 1.17 x 10‒3 Mr = 0.208 / 1.15 x 10‒3 = 170.2 x = [180.7 – (24.3 + 12.0 + 16.0 x 3)] / 18 = 4.77 ≈ 5 (c) The acid will not be diluted and close to 2.00 mol dm−3, the titre value required to neutralise 25 cm3 of FA 3 will be about 250 cm3 / more than 10 times greater. Since this exceeds the capacity of burette, it is unsuitable.

2 (a) Table 1: Weighings of FA1 Mass of weighing bottle and FA1 / g 4.495 Mass of weighing bottle and residual FA1 / g 3.478 Mass of FA1 transferred / g 1.017 Table 2: Measurement of temperature Ti / oC 30.3 Ti / oC 33.3 T / oC 3.0 (i) Δqreaction3 = −mcΔT = − 40 x 4.18 x 3.0 = −502 J (iii) Hreaction1 Mg(s) + C(s) + (3+x)/2 O2(g) + xH2(g)  MgCO3.xH2O(s) + 2HNO3(aq) ‒490 −86.0 + 2HNO3(aq) −393.5 + 6 x −285.5 Mg(NO3)2(aq) + C(s) + (3+x)/2 O2(g) + (x+1)H2(g) Mg(NO3)2(aq) + CO2(g) + (x+1)H2O(l) By Hess’s law, Hreaction1 = ‒490 −393.5 + 6 x −285.5 – (−86.0) = −2510 kJ mol−1 OR If they use x = 3 By Hess’s law, Hreaction1 = ‒490 −393.5 + 4 x −285.5 – (−68.3) = −1957.2 kJ mol−1 = −1960 kJ mol−1 (c) (i) percentage error = (+ 0.1 x 2) / 3.0 x 100 = + 6.67 %

(ii) Hreaction3 = −502 / (1.017 / 174.3) = −86.0 kJmol−1 OR If they use x = 3 Hreaction3 = −502 / (1.017 / 138.3) = −68.3 kJmol−1 (ii) Error: The experiment did not take into account heat exchange with surrounding / calorimeter / heat loss to surrounding / calorimeter. Improvement 1: Use cooling curve method where temperature changes after mixing are measured at 30s intervals for about 3 min, then max temperature is extrapolated to time of mixing . This accounts for heat exchange with surrounding, hence a more accurate ∆T is obtained. Improvement 2: Calibration of the calorimeter by making use of reaction with accurately known enthalpy change. The calibrated heat capacity, C of