VJC Prelim P1 P2 ANS

1 2009 H2 Chemistry 9746/2 Suggested Answers Answer all the questions in the spaces provided. 1 Hydrogen cyanide was first isolated from a blue dye (Prussian blue) which had been known since 1704 and is now known to be a coordination polymer, FeIII 4[FeII(CN)6]3. In the year 2000, 732,552 tonnes of HCN were produced in the US. The most important process for HCN production is the Andrusso v oxidation in which methane and ammonia react in the presence of oxygen at about 1200 °C: 2CH 4 + 2NH3 + 3O2 → 2HCN + 6H2O The energy needed for the reaction is provided by the partial oxidat ion of methane and ammonia. (1 tonne = 1 000 kg) HCN is also obtainable from fruits that have a pit, such as cherries, apricots, apples, and bitter almonds. Many of these pits contai n small amounts of cyanohydrins such as mandelonitrile and amygdalin, which slowly release hydrogen cyanide. An article mentioned that 100 g of crushed apple seeds can yield about 10 mg of HCN. (a) (i) Draw the structure of the anion of Prussian blue to sh ow its shape. Indicate on the structure the bond angle and give the name of the shape. Structure: Fe CN CN CN CN NC NC 4- 90 O Bond angle must be ON the structure. Correct structure drawn (pls draw dotted lines to show square plane). Shape: Octahedral (ii) Using specific examples fr om the information above, state two characteristic properties of transition elements. They are able to show variable oxidation states. (e.g. FeIII 4[FeII(CN)6]3 , FeIII is +3 O.S. but FeII is +2 O.S.) © VJC 2009 9746/02/PRELIM/09 [Turn over

2 They are able to form stable complex compounds or ions. (e.g. FeII(CN)6]3 4- complex ion) They are able to form coloured compounds and ions. (e.g. Prussian blue) [Only two properties required. For each property, specific example must be given.] (iii) Find the percentage by mass of cyanide in Prussian blue. Percentage by mass of cyanide in Prussian blue = ) x55.8 ( + ] . +3[(26.0x6) × x . 4 8 55 3 6 0 26 x 100% = 858.6 468.0 x 100% = 54.5 % [5] (b) (i) Assuming that the amount of HCN produced in the US annually is the same, calculate the number of moles of HCN produced in the US from 2000-2002. Total mass of HCN produced in 3 years = 732,552 x 3 = 2.20 x 106 tonnes Mr of HCN = 1.0 + 12.0 + 14.0 = 27.0 No. of moles of HCN = 0 . 27 x102.20x10 6 6 = 8.15 x 1010 mol (ii) What is volume in dm 3 of oxygen gas required via the Andrussov oxidation to form the amount of HCN mentioned in (i) at r.t.p.? 3 O2  2 HCN No. of moles of O2 = 3/2 x 8.15 x 1010 = 1.22 x 1011 mol Volume of O2 at r.t.p. = 24 x 1.22 x 1011 = 2.93 x 1012 dm3 [2] (c) An apple has an average of 4 seeds and eac h seed weighs an average of 0.20 g. Find out the mass of HCN that can be yielded from seeds from 10 apples. Mass of seeds from 10 a