MJC H2 Chem 2013 Prelim P2 QP

Name _______________________ Class: ___________ Reg Number: ______ MERIDIAN JUNIOR COLLEGE Preliminary Examination Higher 2 ____________________________________________________________________________ Chemistry 9647 Paper 2 Structured Questions 19 September 2013 2 hours Additional Materials: Data Booklet ____________________________________________________________________________ INSTRUCTIONS TO CANDIDATES   Write your name, class and register number in the spaces provided at the top of this page. Write your calculator brand and model/number in the box provided above. Answer all questions in the spaces provided on the question paper. All working must be shown clearly. INFORMATION FOR CANDIDATES This document consists of 26 printed pages and 2 blank pages   ©chemistry@meridian jc Examiner’s Use Paper 1 MCQ / 40 Paper 2 Q1 / 12 Q2 / 9 Q3 / 15 Q4 / 12 Q5 / 10 Q6 / 14 / 72 Paper 3 / 80 Total / 192 % & Grade The number of marks is given in brackets [ ] at the end of each question or part question. You are reminded of the need for good English and clear presentation in your answers. Calculator Model / No. 890

1 ©chemistry@meridian jc   [Turn over  1 Planning (P) You are to design an experiment to determine the enthalpy change of the reaction of magnesium with sulfuric acid as follows: Mg (s) + H2SO4 (aq)  MgSO4 (aq) + H2 (g) H1 You are provided with the following information and a list of apparatus and reagents. You may also use other apparatus normally found in a school laboratory. Reagents and apparatus:  1.0 mol dm 3 H2SO4  Mg turnings  A thermometer with 0.2 C division  A 200 cm 3 styrofoam cup Assume that 4.18 J is required to raise the temperature of 1 cm 3 of any solution by 1 oC. (a) (i) The enthalpy change of reaction bet ween magnesium and sulfuric acid is determined to be approximately 140 kJ mol 1. A student attempted to plan the experiment and s uggested the use of 100 cm 3 of sulfuric acid with 0.1 g of Mg turnings but was told by his teacher that the quantity of Mg used was inappropriate.   Explain with the aid of suitable calc ulations why the quantity of Mg suggested was inappropriate. [2] 891

2 ©chemistry@meridian jc   [Turn over  (ii) Hence, write a detailed plan to veri fy the enthalpy change of reaction between magnesium and sulfuric acid, H1. Your plan should include the following:  Justification of the mass of Mg turnings to be used  Tabulation of raw data  Brief description of how the data can be used to determine the enthalpy change of reaction [6] 892

3 ©chemistry@meridian jc   [Turn over  (b) The standard enthalpy change of formation of magnesium oxide, Hf cannot be determined directly by experiment and is determined via an indirect approach using Hess’ Law. As such, in order to determine Hf, two other experiments were conducted and the enthalpy change of each of th e two reactions is found to be as follows: MgO (s) + H2SO4 (aq)  MgSO4 (aq) + H2O (l) H2 = +176 kJ mol1 H 2 (g) + 1 2 O2 (g)  H2O (l) H3 = 286 kJ mol1 (i) Define the standard enthalpy change of formation of magnesium oxide. (ii) Construct an energy cycle to deter mine the standard enthalpy change of formation of magnesium oxide, using the approximate value of H1 (140 kJ mol1) in (a), and the above values of H2 and H3. [4] [Total : 12] 893

4 ©chemistry@meridian jc   [Turn over  2 Grignard reagents are very useful in organic synthesis reactions. This question concerns the preparation and us e of Grignard reagents. A Grignard reagent has the formula RMgX where X is a halogen, and R is an alkyl or aryl group. RX + Mg RMg X Grignard reagents readily react with carbonyl compounds to form their corresponding alcohols as outlined in the general equations below. (a) State the type of reaction for steps I and II respectively. [1] Step I: ………………………………………… Step II: ………………………………………… (b) Grignard reagents are formed readily by iodoalkanes. Using the above general reaction scheme, the Grignard reagent, CH3CH2MgI is first generated from iodoethane. This is subsequently reacted with pentan–2–one, converting it into a tertiary alcohol. Draw the displayed formula of the final organic product formed in step II. [1]   Grignard reagent step I step II 894

5 ©chemistry@meridian jc   [Turn over  The experimental procedure below det ails the conversi on of pentan–2–one to the alcohol product, which involves the initial preparation of a Grignard reagent. The Grignard reagent is highly reactive and has to be carefully prepared in an anhydrous environment. Procedure 1 1.50 g of magnesium turnings is added to a pear-shaped flask with an equimolar quantity of iodoethane and 20 cm3 of diethylether. 2 A crystal of iodine is added to the mixture from step 1 to activate the magnesium, and this mixture is refluxed. 3 The mixture is allowed to cool and 6.0 cm3 of pentan–2–one is added dropwise. The mixture is then gently heated under reflux. 4 Next, the mixture is cooled using an ice bath, and then 25 cm 3 of 4 mol dm −3 hydrochloric acid is slowly added. 5 The mixture from step 4 is added to a separating f unnel. The aqueous layer is separated and shaken successively with two 10 cm 3 portions of diethylether, retaining the diethylether extracts and combining them with the original diethylether layer. 6 The combined diethylether layer is washed successively with 20 cm3 of (i) water, (ii) saturated sodium hydrogencarbonate solution, (iii) 1 mol dm−3 aqueous sodium thiosulfate, (iv) saturated sodium chloride solution. 7 The diethylether layer is allowed to stand over anhydrous magnesium sulfate. 895

6 ©chemistry@meridian jc   [Turn over  substance formula molar mass / g mol−1 density / g cm−3 solubility in water boiling point / °C iodoethane C2H5I 156 1.93 slightly soluble 72 diethylether C2H5OC2H5 74 0.713 slightly soluble 35 magnesium turnings Mg 24.3 1.74 insoluble 1110 pentan-2-one C5H10O 86 0.814 slightly soluble 102 product alcohol 0.823 slightly soluble 143 Table 2.1: Physical Properties of the reagents and the organic product (c) Suggest the role of the diethylether in step 1 of the procedure. [1] (d) (i) In step 2, the mixture is refluxed. S uggest why elevated temperatures and long periods of time are required for reactions such as this. (ii) Show, with the use of calc ulation whether t he Grignard reagent or pentan–2–one is in excess. Assume that the reaction between magnesium and iodoethane in step 2 had 100 % yield. [2] 896

7 ©chemistry@meridian jc   [Turn over  (e) In the diagram of the s eparating funnel below, draw the two layers that will be observed after step 4. Indicate the relative position of the two layers. [1] (f) In step 6 , the water and the three aqueous solutions used to wash the diethylether layer are intended to remo ve a particular impurity, either by dissolving or by chemical reaction followed by dissolving. Suggest the impurity removed in each step. (i) water ………………………………..   (ii) saturated sodium hydr ogencarbonate ……………………………….. (iii) sodium thiosulfate ……………………………….. (iv) saturated sodium chlori de ……………………………….. [2] (g) After step 7, further separation and purification has to be carried out in order to collect the alcohol product. Briefly describe how you would obtain