Edexcel Jun 2015 Paper 2 Q18 (with explained solutions) |

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Edexcel Jun 2015 Paper 2 Q18

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SECTION B Answer ALL the questions. Write your answers in the spaces provided. 18 Halogenoalkanes react slowly with water to form alcohols. The equation for the reaction is RX + H2O (cid:314) ROH + H+ + X (a) The rate of this reaction for different halogenoalkanes was investigated using the apparatus below. In one experiment, equal amounts of 1-chlorobutane, 1-bromobutane and 1-iodobutane were placed in separate test tubes with 5 cm3 of ethanol. These test tubes were placed in the water bath together with a test tube containing aqueous silver nitrate. After about 5 minutes, 1 cm3 of the silver nitrate solution was added to each test tube containing a halogenoalkane and the time taken for a precipitate to form in each test tube was noted. The temperature of the water bath was maintained at 50C. (i) Why is ethanol used as a solvent in this experiment? (1)(ii) Explain why the apparatus was left for 5 minutes before the silver nitrate was added. (1)10 *P44881A01024*<br />
(iii) Name the precipitate formed in the test tube containing 1-bromobutane and write an ionic equation for the formation of this precipitate. State symbols are not required. (2) PrecipitateIonic equation (iv) Predict the order (fastest first) in which the halogenoalkanes form precipitates. Explain your answer. (2) OrderExplanation(b) Alcohols are usually prepared from halogenoalkanes using aqueous alkali, rather than water, as the reaction is faster. (i) Name the mechanism and type of reaction occurring when 1-bromobutane reacts with aqueous alkali. (2)*P44881A01124* 11 Turn over<br />
(ii) Explain why the formation of alcohols is faster with aqueous alkali than with water. (1)(iii) Give the mechanism for the reaction of 1-bromobutane with aqueous alkali. Show the lone pair involved in the mechanism and any relevant dipoles and curly arrows. (3) 12 *P44881A01224*<br />
(iv) One student suggested that the final reaction mixture could be tested for the presence of an alcohol using phosphorus(V) chloride while another suggested using potassium dichromate(VI) with sulfuric acid. Describe the result of a positive test for alcohols using each of these reagents. Explain which test is better for the final reaction mixture. (3) Observation with PCl5Observation with acidified K2Cr2O7Explanation(c) 1-bromobutane is classified as a primary halogenoalkane and is one of the four structural isomers with a molecular formula C4H9Br. Give the skeletal formula of the three isomers, other than 1-bromobutane, classifying the halogenoalkane in each case. (3) Skeletal formula Classification (Total for Question 18 = 18 marks) *P44881A01324* 13 Turn over<br />

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Acceptable Answers Section B Question Number 18 (a)(i) Ethanol dissolves silver nitrate / silver ions and halogenoalkanes OR Ethanol (molecule) is polar and non-polar (solvent) OR Ethanol dissolves ionic and covalent compounds ALLOW Ethanol dissolves ionic and non- polar compounds Ethanol dissolves both types (of compound) So that the reactants can mix miscible for dissolves IGNORE Any references to rate Question Number 18(a)(ii) To allow the temperature (of all the Acceptable Answers liquids) to equilibrate / to reach 50C OR So that all the substances are at the same temperature ALLOW So that the temperature is constant Question Number 18(a)(iii) Silver bromide Acceptable Answers Mark Reject Ethanol is non- polar Just ethanol dissolves halogenoalkanes Just water does not dissolve halogenoalkanes Just they dissolve in ethanol Reject Mark Mark Reject Non-ionic equations IGNORE Formula even if incorrect (1) Ag+ + BrAgBr (1) TE on incorrect silver halide ALLOW Ionic equations with uncancelled ions Ag+Br as product IGNORE state symbols even if incorrect<br />
Acceptable Answers Question Number 18(a)(iv) Order: iodo, bromo, chloro ALLOW AgI, AgBr, AgCl OR I, Br, Cl OR Iodine, bromine, chlorine (1) CI is the weakest bond OR I is best leaving group ALLOW (if MP1 awarded) Rate depends on the strength of the CX bond (1) IGNORE Explanations of the bond strengths, even if incorrect. References to bond length and atomic radius/size ALLOW Reverse argument for MP2 Acceptable Answers Question Number 18(b)(i) nucleophilic (1) substitution (1) Stand alone marks SN2 alone scores one mark Mark Reject I2, Br2, Cl2 Rate depends on the reactivity of X / X Mark Reject SN1<br />
Mark Reject Use of NaOH/OH for OH Just NaOH/alkali forms OH more readily Question Number 18(b)(ii) Some comparison is required. Acceptable Answers Hydroxide ion /OH is a stronger nucleophile (than water) ALLOW OH is a better electron pair donor (than water) Concentration of hydroxide ion / OH is higher OR Hydroxide ion / OH is charged More hydroxide ion / OH in NaOH (than water) IGNORE OH is more basic / alkaline Alkali is a stronger nucleophile OH is more reactive ALLOW Reverse argument<br />
Question Number 18 (b)(iii) Acceptable Answers Reject Mark OH with no / partial charge C+Br Penalise omission of charge on hydroxide ion once only (in MP2) First mark C3H7 C3H7 Both curly arrows First curly arrow from any part of the hydroxide ion (or the charge) to the carbon atom Second curly arrow from the CBr bond to the bromine atom or just beyond (1) Second mark Lone pair on oxygen of OH {HO:} (1) Third mark Partial charge on CBr bond { C+Br} (1) ALLOW Correct SN1 mechanism for full marks Curly arrow from hydroxide group from any part of the group including the charge. IGNORE transition state (even if incorrect) products (even if incorrect)<br />
Acceptable Answers Question Number 18(b)(iv) PCl5: misty /steamy /white fumes/gas IGNORE Tests on product (e.g. turns blue litmus red) (1) K2Cr2O7: orange solution turns green ALLOW Orange to blue (1) K2Cr2O7 preferred because PCl5 reacts with water (as well as alcohols) ALLOW K2Cr2O7 preferred because PCl5 reacts with alkali / OH /OH (1) IGNORE References to primary, secondary and tertiary alcohols Mark Reject smoke Just fumes/ effervescence PCl5 reacts with carboxylic acids Question Number 18(c) Acceptable Answers Reject Mark Just the classificat ions Skeletal formula Classification Primary/1o Secondary/2o Tertiary/3o Look at the structural formulae first: three structures correct scores 2 marks. two structures correct scores 1 If all three structures correct (any format), then all three classifications correct scores 1 Penalise displayed, partially displayed or structural formulae once only IGNORE Bond angles and names Total for Question 18 = 18 marks<br />
Question Number 19(a)(ii) ALLOW Acceptable Answers Reject Mark Mark Reject Just acid OR heating the carbonate Acceptable Answers Question Number 19(a)(i) Add hydrochloric acid / HCl(aq) / nitric acid / HNO3(aq) ALLOW Just acid only if a suitable acid is given in equation one Sulfuric acid / H2SO4((aq)) or HCl (1) IGNORE conc Gas / carbon dioxide / CO2 evolved turns lime water milky / cloudy / produces a white precipitate (1) MP2 is a stand alone mark but there must be some indication that a gas is being tested 2(s) + 2H+(aq)H2O(l) + CO2(g) H2CO3(aq) for H2O(l) + CO2(g) BaCO3(s) + 2HCl(aq)BaCl2(aq) + H2O(l) + CO2(g) OR BaCO3(s) + 2HNO3(aq)Ba(NO3)2(aq) + H2O(l) + CO2(g) OR CO3 ALLOW BaCO3(s) + H2SO4(aq)BaSO4(s/aq) + H2O(l) + CO2(g) OR BaCO3(s)BaO(s) + CO2(g) (1) Ca(OH)2(aq) + CO2(g)CaCO3(s) + H2O(l) (1) All state symbols in both equations correct (1) ALLOW State symbols mark if first equation not balanced but ALL species are correct. No TE on other equations<br />

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