VJC H2 Chem 2013 Prelim P2 Soln

 VJC 2013 9647/02/PRELIM/13 [Turn over 1 Victoria Junior College 2013 H2 Chemistry Prelim Exam 9647/2 Suggested Answers 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. C6H14(l) + C2H5OH(l) + 25/2 O2(g)  8CO2(g) + 10H2O(l) [1] (b) Identify two possible sources of error and suggest an improvement to overcome each of them in the experiment. Errors: Heat loss to the surroundings by the calorimeter and water. Loss in amount of fuel mixture due to evaporation of the fuel mixture as ethanol and hexane are volatile liquids. Improvements: Cover and lag the calorimeter with non-flammable insulating material and conduct the experiment in a draught-free room, to minimize heat loss by the calorimeter and water to the surroundings. Cover the spirit burner to minimize rate of evaporation so as to minimize loss of fuel mixture. [3] spirit burner metal calorimeter 1503

 VJC 2013 9647/02/PRELIM/13 [Turn over 2 (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. 1) Weigh the spirit burner containing the ‘fuel mixture’ 2) Using a measuring cylinder, measure 100 cm 3 of water into the metal calorimeter. 3) Measure the initial temperature of the water using a thermometer. 4) Light the burner and allow it to heat the water in the calorimeter. 5) Monitor the temperature of water using the thermometer, and extinguish the flame when the temperature of the water reaches about 5 oC 6) Measure the final temperature of the water. 7) Cool and reweigh the spirit burner with the remaining ‘fuel mixture’.   Initial temperature of water/ oC T1 Final temperature of water/ oC T2 Initial mass of spirit burner with ‘fuel mixture’ / g M Final mass of spirit burner with remaining ‘fuel mixture’ /g N [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. Since density of water is 1 g cm-3, mass of the water in the calorimeter = 100 g Let the Heat capacity of the calorimeter be C J oC -1 Temperature rise of the water in the calorimeter = (T2 – T1 ) = T oC Heat gained by water and calorimeter = 100 x 4.2 x T + C x T = (420 + C)T J Mass of ‘fuel mixture’ burned = (M-N) g = P g Molar mass of ‘fuel mixture’ = 86 + 46 = 132 g mol-1 Hence, amount of ‘fuel mixture’ = P/132 mol Heat lost by ‘fuel mixture’ = 13200 x P/132 = 100P J Heat lost by ‘fuel mixture’ = H eat gained by water and calorimeter 100P = (420 + C)T C = 100P/ T – 420 [3] [Total: 12] 1504

 VJC 2013 9647/02/PRELIM/13 [Turn over 3 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. Amount of Cl 2  in 1 cylinder =  0 . 71 170000000  / 5730  = 4.18 x 102 mol (ii) Determine the concentration of chlorine gas, in mol dm3, at 1000 ppm. Concentration of Cl2 in g dm–3 in 1000 ppm  =  mg g dm mg 1000 1 11000 3   = 1.00 Concentration of Cl2 in mol dm–3 =  0 . 71 1 = 0.0141 mol dm–3    (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. Volume of factory room = 3 3 3 3 10 1 125 m dmm   = 25 000 dm3   Conc. of Cl2 in the factory   =  25000 10 18 . 42         = 0.0167 mol dm–3 Since 0.0167 mol dm–3 > 0.0141 mol dm–3, it is fatal. [4] (b) Hydrogen halides are dissociated at high temperatures according to the following equation: 2HX(g) H2(g) + X2(g) The approximate Kc values for the above equilibrium at various temperatures for the respective hydrogen halides are shown in the table: 1505

 VJC 2013 9647/02/PRELIM/13 [Turn over 4 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. From the table, it is observed that at each temperature, order of K c is HI > HBr > HCl. Since the larger the K c values, the higher the degree of dissociation of HX, the order of degree of dissociation is HI > HBr > HCl. From the Data Booklet, it is observed that order of bond energy is HC l > HBr > HI as shown: H – Cl H – Br H – I bond energy (kJ mol-1) 431 366 299 H–X bond energy decreases from HC l to H I due to increasing atomic size of X atom leading to decreasing H–X bond st rength as degree of effective overlap between H and X atoms decreases from Cl to I. The H–I bond thus requires the least amount of energy to break, causing HI to be least thermally stable followed by HBr and then HCl. Hence, order of thermal stability is HCl > HBr > HI. [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. It can be deduced that CH 3CO2H is a weaker base than H 2O and is less likely to accept H+ from HX. Thus dissociation of HX occurs to a lesser extent in CH3CO2H. The reason is because bond energy decreases from HC l to HI, as atomic radius of halogen increases, causing acid strength to increase from HC l to HI, as seen from increase in %ionisation. [2] By comparison with the reactions of sodium halides with concentrated H 2SO4, account for the behaviour of NaC l(s), NaBr(s) and Na I(s) when heated with the following concentrated acids: (d) (i) with phosphoric(V) acid, H3PO4, all give the hydrogen halide in good yield. H3PO4, unlike H 2SO4, is a non-oxidising acid, and a stronger acid than HX, thus undergoes acid-base reaction with X– to form HX. 1506

 VJC 2013 9647/02/PRELIM/13 [Turn over 5 (ii) with selenic(VI) acid, H2SeO4, all are