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