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Edexcel Jan 2010 Paper 4 Q23
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![PMT 23 Iodine and propanone react in the presence of an aqueous acid catalyst as follows CH3COCH3 + I2CH3COCH2I + HI To determine the rate equation for the reaction, propanone is reacted with iodine in the presence of aqueous hydrochloric acid at constant temperature. Samples are withdrawn at known times, quenched with sodium hydrogencarbonate solution, and the iodine remaining titrated with a standard solution of sodium thiosulfate. The rate equation for the reaction is rate = k[CH3COCH3]1 [H+]1 [I2]0 (a) The graph of [I2] against time is a straight line, showing that the order of reaction with respect to iodine is zero. (i) Explain why the propanone and the hydrogen ions must be in large excess in this experiment in order to give this straight line. (2)(ii) What further experiment could be done to show that the order of reaction with respect to propanone is one? State the effect of this change on the graph. (2)16 *N36289A01628*<br />
PMT (iii) Explain why the minimum number of steps in the mechanism for this reaction is (2) (1) two. acid.(b) Sodium hydrogencarbonate stops the reaction by neutralizing the acid catalyst. (i) Give the ionic equation for the reaction between sodium hydrogencarbonate and (ii) Sodium hydroxide cannot be used for neutralization because under very alkaline conditions a reaction occurs between propanone and iodine. Write the equation for this reaction. State symbols are not required. (3) (Total for Question 23 = 10 marks) *N36289A01728* 17 Turn over<br />](https://learnah.org/wp-content/uploads/2018/08/l_smalljpgJanuary_2010_QP_-_Unit_4_Edexcel_Chemistry_A-level_23-0.jpg)
![Mark Question Number 23 (a)(i) Acceptable Answers Reject These are stand alone marks First mark: (ensures that) [H+] and [propanone] (virtually) constant OR so that the [H+] and [propanone] do not affect the rate (1) Second mark: the [I2] / iodine concentration changes OR so that the overall order (of reaction) is not determined OR otherwise a curve (graph) is obtained (1) NOTE:- only the [I2] changes scores (2) OR only the I2 concentration changes scores (2) BUT only the iodine changes scores (1)<br />
Mark Acceptable Answers Reject First mark: double the concentration of propanone OR change/increase/decrease the concentration of propanone (1) Second mark (mark consequentially): slope/gradient of line doubles ALLOW rate doubles OR slope or gradient changes/increases/decreases by same factor ALLOW rate changes/increases/decreases by same factor (1) NOTE: may suggest a different procedure:- First mark: monitor/measure [propanone] over time (1) Second mark (mark consequentially): plot [propanone] v. time graph and state that t constant (1) Question Number 23 (a)(ii)<br />
Question Number 23 (a)(iii) Acceptable Answers Reject I2 not involved in rate-determining step/ I2 not involved in slow(est) step / H+ and propanone involved in rate-determining step/ H+ and propanone involved in slow(est)step I2 involved before rate- determining/slowest step (0) Mark (1) so there must be another step where I2 is involved/ so there must be a fast step where I2 is involved (1) BUT:- I2 not involved until after the rate-determining step/ I2 not involved until after the slow(est) step (2) ALLOW H+ involved in rate-determining step (1) and is regenerated as it is a catalyst (in another step) (1)<br />
Mark Mark Acceptable Answers Reject NaHCO3 + HCl NaCl + H2O + CO2 OR any equations with HA - + H+H2O + CO2 HCO3 OR - + H+H2CO3 HCO3 OR - + H3O+2H2O + CO2 HCO3 OR - + H3O+H2CO3 + H2O HCO3 ALLOW: NaHCO3 + H+ Na+ + H2O + CO2 OR Na+ + HCO3 IGNORE any correct or any incorrect state symbols+ H+ Na+ + H2O + CO2 Acceptable Answers Reject CH3COCH3 + 3I2 + 4NaOHCHI3 + CH3COONa + 3NaI +3H2O IGNORE any correct or any incorrect state symbols CHI3 on RHS of equation (1) remaining species correct (1) balanced equation (1) NOTE: balancing mark is CQ on all species correct Accept correct ionic equation (i.e. Na+ omitted) NOTE: If CH3I, can only access second mark above Question Number 23 (b)(i) Question Number 23 (b)(ii)<br />](https://learnah.org/wp-content/uploads/2018/08/January_2010_MS_-_Unit_4_Edexcel_Chemistry_A-level_23-0.jpg)
