VJC H2 Chem 2013 Prelim P2 QP

 VJC 2013 9647/02/PRELIM/13 [Turn over   2 Answer all the questions in the space provided. 1 Planning A student was provided with a spirit burner containing a ‘fuel mixture’ which was prepared by mixing equimolar amounts of hexane and ethanol. The enthalpy change of combustion of this ‘fuel mixture’ is 13.2 kJ per mole of ‘fuel mixture’. He was told to use the enthalpy change of combustion of this ‘fuel mixture’ to find the heat capacity of a metal calorimeter using the apparatus shown below. Heat capacity is defined as the number of joules of heat needed to raise the temperature of the calorimeter by one Kelvin or one degree Celsius. Additional information: Specific capacity of water is 4.2 J cm3 K1 (a) Construct a balanced equation for the complete combustion of the ‘fuel mixture’ with state symbols. ………………………………………………………………………………………………… [1] (b) Identify two possible sources of error and suggest an improvement to overcome each of them in the experiment. Errors ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… Improvements ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… [3] spirit burner metal calorimeter 1490

 VJC 2013 9647/02/PRELIM/13 [Turn over   3 (c) Write a plan to determine the heat capacity of the metal calorimeter using the apparatus provided. In your plan you should give details of the procedure (number your steps) and provide a table to record the readings to be taken, including the units. [5] (d) Outline how you would determine the heat capacity of the metal calorimeter based on the plan that you have written and other information given in this question. [3] [Total: 12] 1491

 VJC 2013 9647/02/PRELIM/13 [Turn over   4 2 (a) World War I is sometimes known as 'the Chemists' War'. Knowledge of chemistry was applied towards developing high explosives and new methods of warfare such as the large scale use of poison gas. The first successful use of chlorine as a poison gas was at Ypres, Belgium, on 22 April 1915. 170 tonnes (1 tonne = 1000 kg) of chlorine contained in 5730 cylinders was released forming a grey-green cloud which drifted across French troops. Chlorine can damage the eyes, nose, throat and lungs and is fatal at concentrations of 1000 ppm and above (1 ppm = 1 mg dm −3). Early counter- measures to chlorine included instructing troops to cover their mouths with gauze pads soaked in sodium hydrogen carbonate solution. Eventually, more effective counter-measures to chlorine were developed and thus other poison gases were introduced. (i) Calculate the maximum amount of chlorine gas that could have been released from one of the cylinders that was used at Ypres on 22 April 1915. (ii) Determine the concentration of chlorine gas, in mol dm 3, at 1000 ppm. (iii) In an accident, the chlorine gas from one such cylinder was released into a factory room of volume 25.0 m 3. Determine if the concentration of chlorine gas was fatal. Assume that the gas was released at room temperature and pressure. [4] 1492

 VJC 2013 9647/02/PRELIM/13 [Turn over   5 (b) Hydrogen halides are dissociated at high temperatures according to the following equation: 2HX(g) H2(g) + X2(g) The approximate K c values for the above equilibrium at various temperatures for the respective hydrogen halides are shown in the table: Temperature / oC Kc values for dissociation of HX HCl HBr HI 800 1013 109 10 5 1000 1010 10 7 10 4 1200 109 10 5 10 3 1400 107 10 4 10 2 Using the above information and relevant data from the Data Booklet, describe and explain the relative thermal stability of the hydrogen halides. [3] (c) Aqueous solutions of HC l, HBr and H I (approximately 1 mol dm –3) are almost completely ionised, but solutions in concentrated ethanoic acid are ionised to approximately 5 %, 20 % and 50% respectively. Explain the phenomenon. ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… [2] 1493

 VJC 2013 9647/02/PRELIM/13 [Turn over   6 (d) By comparison with the reactions of sodium halides with concentrated H 2SO4, account for the behaviour of NaCl (s), NaBr(s) and Na I(s) when heated with the following concentrated acids: (i) with phosphoric(V) acid, H3PO4, all give the hydrogen halide in good yield. …………………………………………………………………………………………. …………………………………………………………………………………………. …………………………………………………………………………………………. (ii) with selenic(VI) acid, H 2SeO4, all are converted into the free halogens. (iii) …………………………………………………………………………………………. …………………………………………………………………………………………. …………………………………………………………………………………………. with telluric(VI) acid, H 6TeO6, there is no reaction. …………………………………………………………………………………………. …………………………………………………………………………………………. …………………………………………………………………………………………. [3] (e) When chlorine is bubbled through a concentrated aqueous solution of ammonium chloride, a yellow oily liquid, nitrogen trichloride, is formed together with a solution of hydrochloric acid. A pungent gas together with a solution of sodium chlorate( I) is obtained when nitrogen trichloride is added to aqueous sodium hydroxide. (i) Write balanced equations for the formation of nitrogen trichloride and its reaction with sodium hydroxide. …………………………………………………………………………………………. …………………………………………………………………………………………. (ii) Sodium chlorate(I) can be produced using another method. Write a balanced equation for its production. …………………………………………………………………………………………. (iii) Give one domestic use for sodium chlorate(I). ...…………………………..………………...………………………………………… [3] [Total; 15] 1494

 VJC 2013 9647/02/PRELIM/13 [Turn over   7 3 (a) The graph below shows the trend in boiling points of some of the chlorides of the elements in Period 3 of the Periodic Table. However, there are two boiling points that are wrongly represented. Account, in terms of structure and bonding of the chlorides, for the error in (i) region I …………………..………………………………….………………………………….. ……………………………………………………….………………………………… ……………………………………………………….………………………………… …………………………………………………….…………………………………… (ii) region II …………………………………………………………….…………………………… …………………………………………………………….…………………………… …………………………………………………………….…………………………… ………………………………………………………….……………………………… [4] (b) When chlorides of the elements in Period 3 are added to water, some simply dissolve while others are observed to hydrolyse in water. (i) Write equations to show the behaviour of A lCl 3 and PC l3 in water, clearly indicating whether they refer to dissolution or hydrolysis. ………………………………………………….……………………………………… …………..……………………………………...……………………………………… ……………………………………………….………………………………………… ……………………………………………….………………………………………… boiling point NaCl  MgCl2 AlCl3 SiCl4 PCl3 SCl2 region I region II  1495

 VJC 2013 9647/02/PRELIM/13 [Turn over   8 (ii) Phosphorus also forms the higher chloride, PC l5. When PCl 5 is added to water, liquid phosphorus oxychloride, POC l3 is formed as an intermediate compound. Reactions of covalent chlorides with water can be rationalised as step–wise replacement of –Cl with –OH. Deduce a three–step reaction sequence for the formation of POCl 3 from PCl5. Step 1 ……