MI H2 CHEM P2 Answer Prelim

1 Planning (P) 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 are 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. Blue CuSO4 solution decolourises and pink copper solid is deposited at the bottom of the solution. [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. [4] Assuming 25.0 cm 3 of CuSO4 (aq) used, Amount of CuSO 4 = (25/1000) x 0.8 = 0.0200 mol = amount of zinc required. Minimum mass of Zinc = 0.02 x 65.4 = 1.31 g

Procedure: 1. Weigh an empty weighing bottle and record its mass. 2. Place between 1.40 g and 1.50 g of zinc powder into the weighing bottle ( excess zinc is used to ensure complete displacement). Record all required weighings in an appropriate table. 3. Place a styrofoam cup in the 250 cm 3 beaker and measure , using a measuring cylinder, 25.0 cm3 of CuSO4 into the styrofoam cup. 4. Stir gently with the thermometer and record the initial temperature of the solution. 5. Add zinc from the weighing bottle to the plastic cup. Immediately stir with the thermometer and record the highest temperature reached. 6. Reweigh the weighing bottle with residual zinc powder , if any, and record the mass. Accept any other suitable method. To minimise heat loss, a lid should be added to the styrofoam cup to prevent heat from escaping to the surroundings. This would lead to a more reliable ∆T result. 1m for calculation of mass of zinc 1m for steps 1,2,6 1m for steps 3,4,5 1m for method to minimise heat loss (c) A student carried out the reaction between zinc powder and copper sulfate solution using a data logger with a temperature probe. The zinc powde r 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] 1m for correct extrapolation shown on graph plot 1m for calculation of ∆T = 69.0 – 29.5 = 39.5ºC (accept any value between 36 to 43, based on students’ graph)

(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] [1] Q = mc∆T = (25.0)(4.2)(39.5) = 4147.5 J Limiting reagent is CuSO4 as Zinc is added in excess. ∆H = - Q / nLR = 3727.5 / 0.02 = - 207 kJ mol-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] The rate of reaction will be slower. ; This is due to the larger size of the zinc metal strips, giving rise to a smaller surface area of contact with the reactants. ; (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] Electrolysis ; 0 10 20 30 40 50 60 70 80 0 1 2 3 4 5 6 7 8 9 10 11 temperature/°C time/min

(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. Student can state any value that is more exothermic than their answer obtained in (d). Magnesium is higher up in the reactivity series, thus is it more reactive than Zinc (oxidises more readily than zinc), leading to a more vigorous reaction, enthalpy change is more exothermic. ; [1] [Total:12] 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 (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. (i) Calculate the amount of magnesium used and hence the volume of 1-bromopropane used in Stage One. Amount of Mg = 1.335 / 24.3 = 0.0549 mol ; Amount of Mg = 0.0549 mol Mass of 1-bromopropane = 0.0549 × 122.9 = 6.747 g ; Volume = 6.747 / 1.35 = 5.00 cm3 ; Volume of 1-bromopropane = 5.00 cm3 (ii) Explain why ethoxyethane can be used to dissolve 1-bromopropane. Energy evolved from the formation of permanent dipole -permanent dipole between ethoxyethane and 1 -bromopropane is sufficient to overcome the pd -pd between ethoxyethane and between 1 - bromopropane. identification of solute -solvent interaction and solute -solute and solvent solvent interactions; energy requirement; (iii) Suggest why prolonged heating at high temperature is required for reactions such as this. It has high activation energy / reaction is slow ; It involves breaking of a strong covalent bond or C-Br bond ;

(iv) Give the structural formula of the Grignard reagent formed. CH3CH2CH2MgBr ; [8] (b) In Stage Two of the conversion, t he 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. Amount of butanone used = 4.0 × 0.805 / 72.0 = 0.04472 mol ; Amount of butanone required = Amount of Grignard reagent = Amount of Mg = 0.0549 mol Since amount of butanone used is less than the amount required, it is a limiting reagent, hence Grignard reagent is in excess. ; (ii) State the type of reaction undergone in Stage Two. Nucleophilic addition ; [3] (c) In Stage Three of the conversion, t he mixture from Stage T wo is cooled using an ice bath, and then 25 cm 3 of 4 mol