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 student described an atom of Silicon as having ‘fourteen bees buzzing around a space the size of a football stadium, as though they are being trapped.’ (i) Explain using Chemistry concepts, what do the bees in the description represent and what stopped the bees from flying away from the stadium? The bees represent electrons and the electrostatic attraction of the positively charged protons in the nucleus for the electrons presents the electrons from being lost. ; (ii) The teacher commented that the student’s description of Silicon was incomplete. What is missing from the student’s description of Silicon? The positively charged nucleus. ; (iii) 28Si, 29Si, 30Si are isotopes of Silicon. Explain why isotopes of Silicon undergo identical chemical reactions? [3] Chemical reactions depend on the electronic configuration of the substance. Since isotopes have the same number of electrons but different number of neutrons, they still have identical chemical properties. ; (b) Amongst all the elements in Period 3 of the periodic table, Silicon has the highest melting point. Sketch a graph to illustrate the trend of the melting points of elements in Period 3. Explain your sketch. [4] Na to Al giant metallic structure strong electrostatic forces of attraction between metal cations and delocalised electronlarge amount of energy required to overcome those forces \ high m.p. delocalised electrons ↑, size of cations ↓, (charge on cation ↑) ; ;

[Turn over 5 strength of metallic bond ↑ \m.p. ↑ Si giant covalent structure large amount of energy required to break strong covalent bonds between Si atoms. \ high m.p. P to Ar Simple molecular structure weak temporary dipole – induced dipole forces of attraction between molecules relatively smaller amount of energy required to overcome those forces \ low m.p. size of electron cloud of molecules varies, S8>P4>Cl2>Ar size of electron cloud of molecules ↑, strength of temporary dipole – induced dipole forces of attraction ↑. \ m.p.: S8>P4>Cl2>Ar ; ; (c) Silicon reacts with oxygen to form silicon dioxide as a possible product. In dentistry, a composite material based on SiO2 has been developed to be used as dental fillings. (i) State one property of SiO2 that makes it suitable for use as dental fillings. SiO2 is hard and resistant to chemical attack (Unreactive towards acids in food or saliva) ; (ii) Draw a diagram to illustrate the type of bonding involved in SiO2. [2] Tetrahedral, similar to diamond. Each Si is bonded to 4 O atoms, and each O is bonded to 2 Si atoms ; [Total: 9 marks] 3 Ammonia gas decomposes into nitrogen and hydrogen when passed over a platinum gauze. The rate of decomposition is found to be independent of the partial pressure of ammonia at very high pressures, but the rate was directly proportional to the partial pressure of ammonia at low pressures. (a) Explain this observation as far as you can. [2] 2NH3 ↔ N2 + 3H2 When the pressure is high enough, the catalyst surface would be saturated with adsorbed ammonia molecules (By LCP, equilibrium shifts left at high pressure). Any decomposition products that are desorbed from the surface would immediately be replaced by incoming ;

6 adsorbed ammonia. Hence, rate of decomposition is constant. At low pressure, the catalyst surface is not saturated with adsorbed ammonia molecules. Hence, rate of decomposition depends on the partial pressure of ammonia. ; (b) Given that 45% of ammonia dissociated into nitrogen and hydrogen at moderately high temperatures, and a total pressure of 1 atm, calculate the value of Kp, stating its units. [3] 2NH3 ↔ N2 + 3H2 I / mol 2 0 0 C/ mol -0.9 +0.45 +1.35 E / mol 1.1 0.45 1.35 Total amount at eqm = 2.90 PNH3 = 1.1 / 2.9 = 0.3793 atm PN2 = 0.1552 atm PH2 = 0.4655 atm Kp = (0.1552) (0.4655)3 / 0.37932 = 0.109 atm-2 (3s.f.) (with units stated) ; ; ; (c) Ammonia, nitrogen and hydrogen are non-ideal gases. (i) State two assumptions of the kinetic theory as applied to an ideal gas. Gas particles have negligible volume as compared to the volume of the whole gas. Gas particles have negligible intermolecular forces of attraction. ; ; (ii) Which of these three gases deviates the most from an ideal gas? Explain your answer. Ammonia, due to formation of hydrogen bonding (strongest intermolecular force) between its molecules. Thus, the intermolecular forces of attraction are significant. ; (iii) For a given fixed mass of an ideal gas, sketch graphs of: I: P against V at constant T (where P represents pressure) II: V ag