Ketones And Aldehydes 1.pdf

Persulfate ions, S2O8 2–, oxidize iodide ions in aqueous solution to form iodine and sulfate ions, SO4 2–.
(a) The ionic equation for this reaction is: 2I– + S2O8 2– → I2 + 2SO4 2–.
(b) (i) The final colour of the reaction mixture is blue-black due to the formation of iodine-starch complex.
(ii) If the reaction was carried out without the addition of sodium thiosulfate, the mixture would turn blue-black immediately due to the formation of iodine, which would then react with starch to form a blue-black complex.
(iii) The concentration of iodide ions remains constant until the mixture changes colour because the iodide ions are being consumed in the reaction with persulfate ions to form iodine, but the rate of this reaction is slow compared to the subsequent reaction of iodine with starch.
(c) (i) A graph of 1/time on the vertical axis against concentration of iodide ions would show a straight line passing through the origin, indicating a first-order reaction with respect to iodide ions.
(ii) The order of reaction with respect to iodide ions is 1, as the graph of 1/time against concentration of iodide ions is a straight line passing through the origin.
(iii) The overall rate equation for the reaction is Rate = k[S2O8 2–][I–], and the units for the rate constant are dm3 mol–1 s–1.
(d) (i) The activation energy of the reaction can be calculated using the equation: ln(1/time) = –Ea/R(1/T) + constant.
(ii) The reliability of the activation energy determination could be improved by repeating the experiment at more temperatures and using a more precise method to measure the time.
2 A flow chart for making 2-hydroxy-2-methylpropanoic acid from propan-2-ol is shown.
(a) (i) The reagents and conditions for Step 1 are: propan-2-ol, oxidizing agent (e.g. potassium dichromate), acid (e.g. sulfuric acid), heat.
(ii) A chemical test to identify the carbonyl group is the Tollens' test, which involves adding Tollens' reagent (ammoniacal silver nitrate) to the compound, resulting in the formation of a silver mirror.
A further test to show the presence of the H3C–C– group is the iodoform test, which involves adding iodine and sodium hydroxide to the compound, resulting in the formation of a yellow precipitate of iodoform.