VJC Prelim P2

2 Section A Answer all the questions in this section in the spaces provided. 1 (a) Methylacetylene is an alkyne with the chemical formula CH3C≡CH. It is a component of MAPP gas along with its isomer propadiene, which is commonly used in gas welding. Methylacetylene exists in equilibrium with it s isomer, the mixture of methylacetylene and propadiene being called MAPD: C H3 CC H(g) H⇋ H C2 CC 2(g) 1 2 3 methylacetylene propadiene (i) State the type of hybridisation, shape a nd number of sigma and pi bonds about I C 1 and C2 of methylacetylene, and II C 3 of propadiene in the table below. Carbon Hybridisation Shape No. of sigma bonds No. of pi bonds C1 C2 C3 (ii) When methylacetylene reacts with Br 2 in CCl 4, a compound having the molecular formula of C 3H4Br2 is formed. Name the type of isomerism shown by the compound and draw all its possible isomers. Type of isomerism: ……………………………………………………. Isomers: [5] © VJC 2009 8872/02/PRELIM/09 [Turn Over

3 (b) An experiment was carried out as follows to determine the standard ent halpy change of combustion of methylacetylene contained in the gas cylinder of a gas stove. A large beaker of water was plac ed on the stove and heated. The temperature rise was recorded. The cylinder was weighed before and after the experiment to determine the ma ss of methylacetylene used. This process was known to be only 70% e fficient. The following results were obtained. Mass of methylacet ylene before heating = 4.66 g Mass of methylacetylene after heating = 2.16 g Mass of water heated = 750 g Temperature rise = 25.8 oC Assume the heat capacity of water = 4.2 J K 1 cm3 Based on the given data, calculate (i) the heat absorbed by water; (ii) the heat given out by methylacetylene during the experiment; (iii) the enthalpy change of combustion of methylacetylene. © VJC 2009 8872/02/PRELIM/09 [Turn Over

4 (iv) Hence, use the answer in (b)(iii) to calculate the enthalpy change of reaction for the conversion of methylacetylene to propadiene, given that the enthalpy change of combustion of propadiene is 1830 kJ mol-1. [5] [Total:10] 2 In nature, chlorine is abundantly found as the chloride ion, a component of the salt that is deposited in the earth or dissolved in the oceans. The pH of the solutions of chlorides for the elements sodium to phosphorus is given below. compound Sodium chloride Magnesium chloride Alumin ium chloride Silicon tetrachloride Phosphorus pentachloride pH of solution 7 6.5 3 2 2 (a) State the type of structure and bonding present in sodium chloride an d silicon tetrachloride. NaCl ………………………………………………………….…………………………. SiCl 4 …………………………………………………….………………………………. [2] © VJC 2009 8872/02/PRELIM/09 [Turn Over

5 (b ) When solid aluminium chloride is heated above 180 oC, a vapour is formed which has Mr = 267. When this vapour is heated above 800 oC, the vapour has Mr = 133.5. Draw the displayed formula of the vapour at 180oC to show its bonding. [1] (c) Write equations to account for the pH of aqueous solutions of (i) aluminium chloride ................................................................................................................. (ii) phosphorus pentachloride .................................................................................................................. [2] Chlorine forms various oxides and oxoanions as shown in the following. Chlorine oxides Cl 2O C lO2 C l2O6 C l2O7 Chlorine oxoacids HClO HC lO2 HClO3 HClO4 (d) C l2O6 exists as singly charged ions in the solid state. The oxidation states of chlorine are +5 and +7 in the cation and anion respectively. Suggest the formulae of the ions and draw the dot-and-cross diagram for the cation. Formulae of the ions: ……………………… and ……………………… Dot-and-cross diagram: [2] © VJC 2009 8872/02/PRELIM/09 [Turn Over

6 (e) When chlorine gas reacts with water, hypochlorous acid, HC lO is for med according to the equation below. Cl2(aq) + H2O (l) HClO (aq) + H+ (aq) + Cl-(aq) At 25 oC, 0.10 mole of the chlo rine gas is bubbled into 2 dm 3 of distilled water. 55% chlorine remains in the equilibrium solution. (i) Write an expression for Kc, stating its units. (ii) Calculate the value of K c for this reaction. [3] [Total: 10] © VJC 2009 8872/02/PRELIM/09 [Turn Over

7 3 Hydrogen cyanide was first isolated from a blue dye (Prussian blue) which had been known from 1704 an d is now known to be a coordination polymer, Fe4[Fe*(CN)6]3. In the year 2000, 732,552 tonnes of HCN we re produced in the US. The most important process is the Andrussov ox idation invented by Leonid Andrussow at IG Farben in which met hane and ammonia react in the presence of oxygen and platinum at about 1200 °C: 2CH4 + 2NH3 + 3O2  2HCN + 6H2O The energy needed for the reaction is prov ided by the partial oxidation 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, from which almond oil and flav oring are made. Many of these pits contain sma ll amounts of cyanohydrins such as mandelonitrile and amygda lin, which slowly release hydrogen cyanide. An article mentioned that 100 g of crushed ap ple seeds can yield about 10 mg of HCN. (a) What is the oxidation number of (i) Fe and (ii) Fe* in Prussian blue? (i) …………………..……… (ii) ………………..……..…. (iii) Find the percentage by mass of cyanide in Prussian blue. (iv) Draw the dot-and-cross diagram for HCN and predict the shape of the molecule. Dot-and-cross diagram: Shape: ……………………………….……. [3] © VJC 2009 8872/02/PRELIM/09 [Turn Over

8 (b)(i) Assuming that the am ount of HCN produced in US annually is the same, calculate the number of mo les of HCN produced in US from 2000-2002. (ii) What is the volume (in dm 3) of oxygen gas required via the Andrussov oxidation to form the amount of HCN mentioned in (b)(i) at r.t.p.? (iii) What is the role of platinum in the Andrussov oxidation? ………………………………………………………………………………….. ………………………………………………………………………………….. [3] (c) An apple has an average of 4 seeds and each seed weighs an average of 0.20 g. Find out the mass (in mg) of HCN that can be yielded from seeds of 10 apples. [1] © VJC 2009 8872/02/PRELIM/09 [Turn Over

9 (d) (i) HCN gas is a highly poisonous gas that can cause permanent damage to the central nervous system. The to xic level of hydrogen cyanide gas in the air is about 0.001 mg dm-3. Express the toxic level of hydrogen cyanide gas in the air in terms of mol dm-3. (ii) How many molecules of HCN are pres