MI H2 CHEM P2 Prelim

Class Adm No Candidate Name: This document consists of 19 printed pages and 1 blank page. [Turn over 2014 Preliminary Examination II Pre-university 3 H2 CHEMISTRY 9647/02 Paper 2 Structured Questions Friday, 19 Sep 2014 2 hours Additional Materials: Data Booklet READ THESE INSTRUCTIONS FIRST Write your name, class and index number in the spaces provided at the top of this page. Write in dark blue or black pen in the spaces provided. You may use a soft pencil for any diagrams, graphs or rough working. Do not use staples, paper clips, highlighters, glue or correction fluid. Answer all questions. A Data Booklet is provided. You are reminded of the need for good English and clear presentation in your answers. The number of marks is given in brackets [ ] at the end of each question or part question. For Examiner’s Use 1 /12 2 /18 3 /11 4 / 9 5 /22 Total /72

2 1 Planning (P) For Examiner’s Use In metal displacement reactions, a more reactive metal will displace a less reactive metal from a solution. Copper, silver and gold appear as elements in the earth due to their poor reactivity with the environment. These metals are easy to extract. You ar e required to plan an experiment to determine the enthalpy change of a metal displacement reaction between zinc and copper( II) sulfate solution. Zinc reacts with aqueous copper(II) sulfate as shown by the equation below. Zn(s) + CuSO4(aq) → ZnSO4(aq) + Cu(s) The reaction is exothermic and spontaneous, requiring only a few minutes to go to completion. You are provided with the following chemicals  Zinc powder,  0.8 mol dm-3 copper(II) sulfate, CuSO 4 as well as apparatus commonly found in the laboratory. (a) State the observations when the reaction goes to completion. ………………………………………………………………………………………………… ………………………………………………………………………………………………… [1] (b) Using the information above, write a plan to describe how you would determine the enthalpy change of the reaction between zinc and copper(II) sulfate. Your plan must include  a suitable mass of zinc to be used, and your justifications for choosing this mass,  the volume of copper(II) sulfate solution to be used,  detailed procedures of the experiment you would perform. Include details of the apparatus you would use, and  the precautions you would take to minimise heat loss.

3 [Turn over ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… For Examiner’s Use [4]

4 (c) A student carried out the reaction between zinc powder and copper sulfate solution using a data logger with a temperature probe. The zinc powder was added in at the third minute. He obtained a set of temperature readings at different time intervals. The student then plotted a graph of temperature against time, shown below. Using his graph, estimate a value for ∆T by extrapolating the curve to meet the vertical line representing the starting time of the experiment and hence determine the maximum temperature that could have been reached if heat loss to the surrounding were prevented. [2] For Examiner’s Use

5 [Turn over (d) Using your answers in (b) and (c), determine the enthalpy change of the metal displacement reaction. [4.2 J of heat energy raises the temperature of 1 cm3 of any solution by 1 oC] For Examiner’s Use [1] (e) Another student performed the same experiment but instead of using zinc powder, he used zinc metal strips. Identify one likely difference between this student’s experiment and the one done in (c). Explain your answer. ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… [2] (f) Reactive metals such as aluminium are more difficult to extract. They are always found as compounds in nature. Suggest one method by which these reactive metals can be extracted from its compounds. ………………………………………………………………………………………………… [1] (g) Predict a probable enthalpy change value of the metal displacement reaction if the metal is now replaced with magnesium instead of zinc. Explain your answer. ………………………………………………………………………………………………… ………………………………………………………………………………………………… [1] [Total:12]

6 2. This question concerns the conversion of a ketone to a tertiary alcohol using Grignard reagent. Grignard reagent, RMgX, can be formed by reacting magnesium with a halogenoalkane, as shown in the equation below. When Grignard reagent reacts with a ketone in the presence of dilute acid, the following reaction takes place. The table below shows some physical properties of the r eagents and the organic product of the reaction. Substance Formula Molar mass / g mol-1 Density / g cm-3 Solubility in water Boiling point / °C 1-Bromopropane CH3CH2CH2Br 122.9 1.35 Slightly soluble 71 Ethoxyethane C2H5OC2H5 74.0 0.713 Slightly soluble 35 Magnesium Mg 24.3 1.74 Insoluble 1110 Butanone CH3COC2H5 72.0 0.805 Soluble 80 Alcohol product 0.829 Slightly soluble 122 For Examiner’s Use

7 [Turn over (a) In Stage One of the conversion, 1.335 g of magnesium is added to a flask containing an equimolar quantity of 1 -bromopropane dissolved in 20 cm 3 of ethoxyethane. The mixture is then refluxed For Examiner’s Use (i) Calculate the amount of magnesium used and hence the volume of 1-bromopropane used in Stage One. Amount of Mg = ………………. Volume of 1-bromopropane = ……………..... (ii) Explain why ethoxyethane can be used to dissolve 1-bromopropane. ……………………………………………………………………………………… ……………………………………………………………………………………… ……………………………………………………………………………………… (iii) Suggest why prolonged heating at high temperature is required for reactions such as this. ……………………………………………………………………………………… ……………………………………………………………………………………… ………………………………………………………………………………………

8 (iv) Give the structural formula of the Grignard reagent formed. For Examiner’s Use ……………………………………………………………………………………… [8] (b) In Stage Two of the conversion, the mixture from Stage One is allowed to cool and 4.0 cm 3 of butanone is added dropwise. The mixture is then gently heated under reflux. (i) Assuming that the reaction between magnesium and 1 -bromopropane in Stage One had 100% yield, show with calculations that the Grignard reagent is in excess. (ii) State the type of reaction undergone in Stage Two. ……………………………………………………………………………………… [3]

9 [Turn over (c) In Stage Three of the conversion, the mixture from Stage Two is cooled using an ice bath, and then 25 cm3 of 4 mol dm−3 dilute hydrochloric acid is slowly added. For Examiner’s Use (i) Draw the structure of the tertiary alcohol produced from the reaction. (ii) Suggest why the mixture is cooled before adding the dilute hydrochloric acid.