JJC H2 Chem 2012 Prelim P2 Soln

2 Jurong Junior College 9647/02/Prelim 2012 [Turn over 1. Planning (P) You are to plan an investigation into the thermal decomposition of caesium nitrate, CsNO3. You may make use of some or all of the following data when planning your investigation. Data: Group I element cation Ionic radius / nm lithium Li+ 0.060 sodium Na+ 0.095 potassium K+ 0.133 rubidium Rb+ 0.148 caesium Cs+ 0.176 Equation for the thermal decomposition of lithium nitrate and sodium nitrate are given below: 1. 4LiNO3(s) ® 2Li2O(s) + 4NO2(g) + O2(g) 2. 2NaNO3(s) ® 2NaNO2(s) + O2(g) Nitrogen dioxide gas Oxygen gas NO2 O2 brown in colour colourless soluble in water almost insoluble in water poisonous powerful oxidant 1 mol of any gas occupies a volume of approximately 24 dm 3 at room temperature and atmospheric pressure. Ar: Cs, 133; N, 14.0; O, 16.0 1. Planning (P) (a) Predict which of the equations below will represent the thermal decomposition of caesium nitrate. Place a tick against the equation of your choice. 4CsNO3(s) ® 2Cs2O(s) + 4NO2(g) + O2(g) 2CsNO3(s) ® 2CsNO2(s) + O2(g) ü [1m] Use the data provided to explain your prediction. As the size/ionic radius of Cs + is larger than that of Na +, Cs+ has a lower charge density and hence has a lower polarising power/less able to polarise the large NO3- anion than Na+. [1m] (b) You are to plan an experiment in which · caesium nitrate is heated

3 Jurong Junior College 9647/02/Prelim 2012 [Turn over · gas is collected · the volume of gas collected is measured · the experimental results are used in a calculation to confirm or reject your prediction (i) Draw a diagram of the apparatus you would use in this experiment. Your apparatus should use only standard items found in college laboratory. Show clearly how the solid will be heated, the gas collected and its volume measured. Label each piece of apparatus used, indicating its size or capacity and state the gas or gases collected on your diagram. [1m] Heat solid CsNO 3 in a hard glass tube or boiling tube with a stoppered delivery tube. Must indicate ‘Heat’ with or without arrow. [1m] Collect gas over water into an inverted burette or a measuring cylinder; or collect gas directly into a graduated frictionless gas syringe. [1m] Connect the two apparatus (no gas loss), label the capacity of apparatus for gas volume measurement, state the gas(es) collected in the set-up. 1. (b) (ii) Calculate the volume of gas you would expect to collect in your apparatus if 1 mol of caesium nitrate completely decomposed according to your predicted equation in (a). OR Following equation 2, 2CsNO 3 º O2 Expected amount of O2 = ½ x 1 = 0.5 mol Expected volume of O2 gas at r.t.p. = 0.5 x 24 dm3 = 12 dm3 Following equation 1 (for direct collection), 4CsNO3 º 4NO2 º O2 Expected amount of gases (NO2 and O2) = 5/4 x 1 = 1.25 mol Expected volume of O2 gas at r.t.p. = 1.25 x 24 dm3 = 30 dm3 Following equation 1 (for collection of gas over water), 4CsNO3 º O2 Expected amount of O2 gas = 1/4 x 1 = 0.25 mol Expected volume of O2 gas at r.t.p. = 0.25 x 24 dm3 = 6 dm3 [1m]

4 Jurong Junior College 9647/02/Prelim 2012 [Turn over (c) Use your answer to (b )(ii) and the size of the apparatus selected in ( b)(i) to calculate the maximum mass of CsNO3 that can be used in your experiment. OR OR Following equation 2: Mass of 1 mol of CsNO 3 = 195 g Using 100 cm3 measuring cylinder or graduated gas syringe, Maximum volume of gas collected = 100 cm3 Maximum mass of CsNO3 = 100/12000 x 195 = 1.625 g » 1.63 g (1-2 d.p.) Or, Using 50.00 cm3 burette or graduated gas syringe, Maximum volume of gas collected = 50 cm3 Maximum mass of CsNO3 = 50/12000 x 195 = 0.8125 g »0.81 g (1-2 d.p.) Following equation 1 (for direct collection of gas): Using 100 cm3 measuring cylinder or graduated gas syringe, Maximum volume of gas collected = 100 cm3 Maximum mass of CsNO3 = 100/30000 x 195 = 0.65 g (1-2 d.p.) Or, Using 50.00 cm3 burette or graduated gas syringe, Maximum volume of gas collected = 50 cm3 Maximum mass of CsNO3 = 50/30000 x 195 = 0.325 g » 0.33 g (1-2 d.p.) Following equation 1 (for collection of gas over water): Using 100 cm3 measuring cylinder or graduated gas syringe, Maximum volume of gas collected = 100 cm3 Maximum mass of CsNO3 = 100/6000 x 195 = 0.65 g (1-2 d.p.) Or, Using 50.00 cm3 burette or graduated gas syringe, Maximum volume of gas collected = 50 cm3 Maximum mass of CsNO3 = 50/6000 x 195 = 1.625 g » 1.63 g (1-2 d.p.) [1m] 1. (d) Outline, in a series of numbered steps, the method to be used in the experiment. Make certain that the steps you describe are in the correct order. You need not explain how the apparatus is assembled. Indicate clearly how you will know when decomposed is complete. 1. Weigh accurately *1.50 g (should be slightly less than the maximum mass) of solid CsNO3 into a dry boiling tube and record the mass. 2. Stopper the boiling tube with a delivery tube. Measure and record the initial volume reading on the gas syringe (or in the inverted measuring cylinder or burette). (ü) 3. Heat the solid sample gently at first and then strongly until a constant volume of gas is collected (or there is no movement of piston of the gas syringe or no more gas bubbles is observed in water). 4. When the whole set-up has been cooled to room temperature, measure [1m] 2(ü) [1m] [1m]

5 Jurong Junior College 9647/02/Prelim 2012 [Turn over and record the final volume reading on the gas syri nge (or in the inverted measuring cylinder or burette). (ü) (e) What should be done when decomposition is complete to ensure that the volume of the gas measured in the apparatus is the “correct” volume. The gas or whole set-up needs to be cooled to room temperature before measuring the final volume (or repeat the whole experiment with the same mass of solid and take the average readings) [1m] (f) Identify a risk present in the method you have described and suggest how you would minimise this risk. O2 is a powerful oxidant and support burning. Remove any oxidisable material. (no mark if student commented that O2 is flammable…..) Or, Only for those who chose equation 1: NO2 is poisonous gas. Conduct the experiment i nside the fume cupboard.) Or, Only for those who collected the gas over water: Potential suck back may occur and crack the hot tube. Remove the delivery tube from water when heating stops. [1m] [Total: 12]

6 Jurong Junior College 9647/02/Prelim 2012 [Turn over 2. (a) (i) Write an equation for the reaction between chlorine and cold aqueous sodium hydroxide and state the type of reaction that occurs. Type of reaction: Disproportionation ( ) ( ) ( ) ( ) ( ) ( ) ( ) ( ) ( ) ( ) ¾ ¾® ¾ ¾® - -- 22 22 Cl g + 2OH aq Cl aq + ClO aq + H O l OR Cl g + 2NaOH aq NaCl aq + NaClO aq + H O l [1m] [1m] (ii) The resultant solution obtained in (a)(i) is heated and a further reaction occurs. Write an equation for the overall reaction between chlorine and sodium hydroxide. ( ) ( ) ( ) ( ) ( ) ( ) ( ) ( ) ( ) ( ) ¾ ¾® ¾ ¾® - -- 2 32 2 32 Overall eqn : 3Cl g + 6OH aq 5Cl aq + ClO aq + 3H O l OR 3Cl g + 6NaOH aq 5NaCl a q + NaClO aq + 3H O l [1m] (b) Chlorine forms a variety of oxides and oxoanions . A series of standard reduction potentials involving the chlorine oxoanions, in alkaline medium, are shown: Half-equation E,/V ClO− + H2O + 2e− = Cl− + 2OH− −0.89 ClO2− + H2O + 2e− = ClO− + 2OH− −0.67 ClO3− + H2O + 2e− = ClO2− + 2OH− −0.33 ClO4− + H2O + 2e− = ClO3− + 2OH− −0.35 Using the above data