MI H2 Chem 2012 Prelim P2 Soln

2 1 Planning The benzenediazonium ion, C6H5N2+, reacts with water as shown in the equation. C6H5N2+ (aq) + H2O (l) ® C6H5OH (aq) + N2 (g) + H+ (aq) At temperatures below 10 ºC, the reaction is very slow. However, nitrogen gas is evolved at a measureable rate at temperatures of 20 ºC and above. In this experiment, you are provided with an aqueous solution containing 0.100 mol dm-3 of the benzenediazonium ion at a temperature below 10 ºC. You are required to design an experiment to deduce the order of reaction with respect to C6H5N2+ of an aqueous solution at 20 ºC and an atmospheric pressure of 101 kPa. (a) Define the term order of reaction. [1] Order of reaction with respect to a reactant is the power of the concentration of that reactant in the experimentally determined rate equation. ; (b) Outline the steps you would take to determine the order of reaction with respect to C6H5N2+ (aq) at 20 ºC. In your plan, include the following details: - a suitable volume of C6H5N2+ (aq) used, - measurements to be taken, - plotting of a suitable graph, if any. [5] Assume 50 cm3 of nitrogen gas to be collected. (Volume chosen has to be large enough for sufficient volume of C6H5N2+ solution used) Using pV = nRT, Amount of nitrogen gas to be collected = 0.002074 mol = Amount of C 6H5N2+ required Thus, minimum volume of C6H5N2+ needed = 0.002074 / 0.1 = 20.7 cm3 (To use 25 cm3 of C6H5N2+ ) Use a water bath (no naked flame) to maintain a temperature of 20 ºC for the reaction mixture. Record the volume of nitrogen produced over regular time intervals of 5 minutes. A graph of volume of nitrogen produced against time is plotted. ; ; ; ; ;

[Turn over 3 (c) Draw a labelled diagram of the experimental set-up. [You may assume that you are given common apparatus found in a school laboratory.] [2] 1 mark for suitable apparatus (use of well-greased gas syringe to collect nitrogen gas), well-labeled. 1 mark for feasibility of set-up (d) Describe how you will use the results obtained to determine the order of reaction with respect to C6H5N2+ (aq). [2] From the graph of volume of nitrogen produced against time plotted, determine the time taken for nitrogen to be produced. (time taken for ½ of total volume, then ¾ of total volume, then all of nitrogen produced) If the time taken is constant, order of reaction with respect to C6H5N2+ (aq) is one. ; ; (e) Identify one potential hazard in this experiment and suggest the safety precaution you would take to overcome this. [2] The organic compounds are toxic and may cause irritation when in contact with the skin. Gloves should be worn during the experiment. OR Conduct the experiment in a fume cupboard to present inhaling of toxic fumes. ; ; [Total: 12 marks]

4 2 (a) A lower secondary Science st