ACJC_Chemistry_Answerto P2 Prelims

2008 H2 Chemistry Preliminary Examinations Paper 2 Mark Scheme 1 (a) On heating, Group I metal nitrates such as sodium nitrate(V) decompose giving the metal nitrate(III) and oxygen, while Group II metal nitrates, for example magnesium nitrate(V), decompose giving different products. (i) Write balanced equations for the decomposition of sodium nitrate(V) and magnesium nitrate(V) respectively. NaNO 3 NaNO2 + ½O2 Mg(NO3)2 MgO + 2NO2 + ½O2 15.35 g of a mixture of sodium nitrate(V) and magnesium nitrate(V) was heated in a fume cupboard until no more gases were evolved. The water soluble part of the residue was dissolved in water to prepare 1.00 dm 3 of solution. 10.00 cm3 of this solution was reacted with 20.00 cm3 (in excess) of 0.0200 mol dm-3 potassium manganate(VII) solution, acidified with dilute sulphuric acid. (ii) The nitrate(III) half equation is NO 2 - + H2O NO3 - + 2H+ + 2e-. Write a balanced equation for the reaction between nitrate(III) ions and manganate(VII) ions. 2MnO 4 - + 5NO2 - + 6H+ 2Mn2+ + 5NO3 - + 3H2O The excess potassium manganate(VII) required 12.00 cm3 of 0.0500 mol dm-3 ethanedioic acid solution for complete reaction. [ 2MnO4 - + 5C2O4 2- + 16H+ 2Mn2+ + 10CO2 + 8H2O] (iii) Calculate the amount in moles of the nitrate(III) ions in the 10.00 cm3 solution. no of mols of C2O4 2- ions in 12.00 cm3 = 0.012 x 0.05 = 6.00 x10-4 no of mols of excess MnO4 - = (2/5) x 0.0006 = 2.40 x 10-4 no of mols of MnO4 - in 20.00 cm3 = 0.02 x 0.02 = 4.00 x 10-4 no of mols of MnO4 - reacted with NO2 - in 10.00 cm3 = 4 x 10-4-2.40 x 10-4 = 1.60 x 10-4 l no of mols of NO2 - in 10.00 cm3 = (5/2) x 1.60 x 10-4 = 4.00x 10-4 1

(iv) Hence, calculate the mass of each nitrate in the mixture. NaNO3≡ NaNO2 mass of NaNO3 in 1 dm3 = (1000/10) x 4.00 10-4 x (23.0+14.0+16.0x3) = 0.0400 x 85 = 3.40g mass of Mg(NO3)2 = 15.35-3.40 =12.0g ( 3s.f.) (or 11.95 g) (b) Magnesium nitrate(V) and strontium nitrate(V) decompose similarly on heating. Magnesium nitrate decomposes at a lower temperature than strontium nitrate. Explain why these two nitrates decompose at different temperatures. • Mg 2+ has a smaller ionic radius than Sr2+ • higher charge density and greater polarising power • hence Mg2+ distorts electron cloud around NO3 - to a greater extent, thus decomposing at a lower temperature.