ALS5A - Section 5 / Chemical Kinetics / Suggested solutions...

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Section 5 / Chemical Kinetics / Suggested solutions / SKH Bishop Baker Secondary School A-Level Chemistry Section 5 Chemical Kinetics 80 IB Q.8 (a) (i) A small and fixed amount of sodium thiosulphate (Na 2 S 2 O 3 ) solution and 1 cm 3 of starch solution are added to each sample. Iodine, which is produced by the studied reaction, reacts with sodium thiosulphate. When this fixed amount of sodium thiosulphate is used up by the iodine produced, excess iodine, in the presence of starch, will cause a blue colouration . This indicates the time for formation of a fixed amount of iodine. [3] (ii) In this experiment, [I 2 ]/ t = rate, [I 2 ] = constant. 1/ t = 1/t, [H 2 O 2 ] = constant, [H + ] = constant, [I - ] = (0.1)V KI 40.0 V KI = volume of KI [I 2 ] = Rate = k [H 2 O 2 ] a [H + ] b [I - ] c t 1 = k [H 2 O 2 ] a [H + ] b 0.1 c V KI c t [I 2 ] 40.0 = k’V KI c k’ = constant log(1/t) = logk’ + c logV KI log (1/t) - 1.886 - 1.658 - 1.495 - 1.357 - 1.009 log V KI 0.699 1.000 1.176 1.301 1.398 A graph of log(1/t) versus log V KI is plotted, from the graph, slope = ( - 1.44) - (2.00) 1
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Section 5 / Chemical Kinetics / Suggested solutions / 1.22 - 0.62 = 0.93 1 The reaction is 1st order with respect to I - (aq). [4] (iii) The concentration of H 2 O 2 (aq) and I - (aq) are kept constant, e.g. 5.0 cm 3 of 0.1M H 2 O 2 and 5.0 cm 3 of 0.1 M KI are used in each run. The volume of 1.0 M H 2 SO 4 (aq) is varied, e.g. 4, 8, 12, 20 cm 3 are used in the separate run, and finally the total volume of the mixture, e.g. 40 cm 3 in each run is kept constant by varying the volume of water. [2] (iv) The rate determining step must be bimolecular involving collision of H 2 O 2 with I - . [1] 81 IB Q.5 (a) (i) No mark if the shape of the curve is wrong. Deduct ½ mark for missing out each coordinate. [1] (ii) The activation energy for the forward reaction is very high. [1] 82 IA Q.1 (a) (i) Take note of the time [1] when the same turbidity due to the formation of colloidal sulphur. [1] either by observing visually the same reference mark placed underneath the reaction flask to disappear. or by using a colorimeter to measure the transmittance. [1] [Take note of the time when colloidal S begins to appear. (1) Quench the reaction (1); titrate the unreacted S 2 O 3 2 - with I 2 solution of known concentration. (1)] (ii) Stop watch [ ½ ], thermostatic bath [ ½ ], thermometer [ ½ ], pipettes or measuring cylinders [ ½ ], small conical flask. [ ½ ]; 0.1 M Na 2 S 2 O 3 ; 0.5 M HCl; distilled water [ ½ ] white paper with mark / colorimeter / burette, 0.1 M iodine solution, ice for quenching, starch solution. [1] 82 IB Q.9 (b) (i) mol dm - 3 s - 1 or conc. time - 1 [1] (ii) s - 1 or time - 1 . [1] (c) (i) 14 N + 1 n 14 C + 1 H [1] 7 0 6 1 14 C 14 N + 0 e [1] 6 7 - 1 2
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Section 5 / Chemical Kinetics / Suggested solutions / (ii) t ½ = 0.693/k (k is the decay constant) [ ½ ] k = 0.693/5570 yr - 1 N = N o e - kt , ln N o /N = kt [ ½ ] 2.303 log 10 32/8 = 0.693/5570
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ALS5A - Section 5 / Chemical Kinetics / Suggested solutions...

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