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Hydrogen ions catalyse the iodination of acetone. The overall stoichiometric equation for the reaction is: CH3C(=O)CH3 + 12 - CH3C(=O)CH2I + HI (1) If the reaction mechanism is represented by Egn (1), the reaction rate would be proportional to both the acetone and iodine concentrations. However, this proportionality is not found, and the following mechanism has been proposed: CH3C(=O)CH3 + H+ CH3C(OH)-CH2 + H+ (2) CH3C(OH)-CH2 + 12 CH3C(-0)CH21+ HI (3) Step (3) is rapid and does not affect the overall rate of iodination, while step (2) - the smbolisation of acetone - is slow and thus rate-determining. The effects of varying the concentrations of acetone, iodine and hydrogen ions are studied, and the results are compared with the proposed mechanism.

= dt Theory The rate law of the rate-determining step (2) is: d[121 Rate = k[+][CH3C(=O)CH3] where k is the reaction rate constant. In the presence of excessive amount of acetone and constant concentration of H', the iodination should proceed at a constant rate. Therefore, a plot of iodine concentration (by monitoring the fading out of characteristic brown color of 12 in a UV-Vis spectrophotometer) against time should give a straight line of negative slope according to the eqn (4). In fact, as indicated from (1), the [H") concentration increases over time, i.e., the reaction is autocatalytic. In this experiment, the conditions are prescribed so that the autocatalytic rate is negligible, and the rate constant can be deduced from the slope of an absorbance-time graph. Acetone is always present in a large excess in the reaction

Experimental Procedure Using the stock solutions provided, eight mixtures were labelled 1A, 1B, 2A, 2B,... 4A and 4B. The composition (in cm?) of each mixture is listed below. А B Mixture Acetone Water 1 M HCI 0.05 M 12 in KI 20 1 10 60 10 2 10 50 30 10 3 فوا 3 5 65 20 10 4 10 65 20 5 Mixtures 1 and 2 differ only in iodine concentration, mixtures 1 anji 3 differ in acetone concentration, and mixtures 1 and 4 differ in acid concentration. The stopwatch was started, and at the same time, mixtures 1A and 1B were mixed thoroughly by shaking for 2-3 minutes, The mixture was quickly transferred to a 1 cm absorbance cell and placed in the colourimeter within 1 minute. The absorbance was recorded at 605 nm in the first minute and every minute until the absorbance fell to zero. If the absorbance did not fall to zero, the w absorbance was recorded for 25 minutes. Repeat the above procedure for mixtures 2 - 4

Results Mixtures 1 2 3 Reaction Time/min Abs Abs Abs Abs 3 4 0.3 0.293 0.285 0.278 0.22 0.218 0.214 0.21 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 0.203 0.196 0.188 0.18 0.173 0.165 0.157 0.149 0.141 0.133 0.125 0.117 0.11 0.102 0.094 0.086 0.078 0.07 0.062 0.054 0.047 0.047 0.047 0.047 0.047 0.047 0.27 0.263 0.255 0.248 0.241 0.233 0.226 0.218 0.21 0.203 0.194 0.187 0.179 0.172 0.163 0.156 0.148 0.14 0.132 0.124 0.116 0.108 0.207 0.203 0.199 0.196 0.193 0.189 0.185 0.181 0.178 0.174 0.17 0.166 0.163 0.159 0.155 0.152 0.148 0.144 0.14 0.136 0.132 0.128 0.214 0.211 0.207 0.203 0.199 0.194 0.19 0.187 0.183 0.178 0.174 0.17 0.167 0.162 0.158 0.154 0.15 0.146 0.142 0.138 0.134 0.13 0.126 0.121 0.117 0.113

Mixtures 1 2 3 -0.00624 Slope (m) -0.00768 -0.00368 -0.00404 Mixtures 2 3 0.22172 0.32304 0.23104 0.22612 Absorbance at t=0 0.35 0.3 0.25 0.2 Absorbance 0.15 0.1 0.05 0 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 Time (minutes) -234

this question is really difficult for me, please help ! 1a) Indicate how far you think your experimental results verify the suggested mechanism. 1b) Based on your graph, determine the effects of iodine, acetone and H+ on the reaction rate. 10) Determine the absorbance at t=0 by extrapolating your plots to + = 0. From the relationship A = e cl(where A is absorbance, e is molar absorptivity in the unit of mol-1 dm' cm-', c is the concentration of iodine and I = 1 cm), determine the values of e. 1d) Calculate the reaction rate (in mol dm-3 min-1) for one of your mixtures by multiplying your measured slope (in absorbance units min 1) by the factor ( e 1)^-1. Assuming the rate law (4) applies, determine the rate constant for the reaction from the results of one of your mixtures.