CIE Jun 2015 v2 Paper 3 Q2

Answers available below

In this experiment you will determine the percentage by mass of magnesium in a mixture of magnesium and iron. To do this you will measure the heat given out by the reaction of the mixture with excess hydrochloric acid. You should assume that only the magnesium in the mixture will react with the acid. FB 4 is the mixture of magnesium and iron. FB 5 is 2.00 mol dm3 hydrochloric acid, HCl. (a) Method Read through the instructions carefully and prepare a table below for your results before starting any practical work.Support the plastic cup in a 250 cm3 beaker. Use the measuring cylinder to transfer 40 cm3 of FB 5 into the plastic cup. Weigh the stoppered container with the FB 4 and record the mass. Measure and record the initial temperature of the solution in the plastic cup. Add all the sample of FB 4 to the acid in the plastic cup carefully to minimise acid spray. Stir the mixture and record the maximum temperature that is reached. Weigh the stoppered container and any residual metal mixture. Record the mass. Record the mass of metal added to the acid. II III IV [4] (b) Calculations Show your working and appropriate signi cantgures in thenal answer to each step of your calculations. (i) Use your value for the maximum temperature rise to calculate the heat energy produced in the reaction. (Assume that 4.2 J are required to increase the temperature of 1.0 cm3 of solution by 1.0 C.) heat energy produced =JUCLES 2015 9701/32/M/J/15<br /> (ii) The molar enthalpy change, H, for the reaction shown below is457 kJ mol1. Mg(s) + 2HCl (aq)MgCl 2(aq) + H2(g) Use this value and your answer to (i) to calculate the number of moles of magnesium in your reaction. (iii) Use your answer to (ii), to calculate the percentage by mass of magnesium in FB 4. (Ar: Mg, 24.3) moles of Mg =mol percentage by mass of Mg in FB 4 =% (iv) You have assumed that only the magnesium reacts with the acid. What other assumption have you made in using this method to determine the percentage by mass of magnesium in the mixture?[4] (c) (i) Calculate the maximum percentage error in the temperature rise. maximum percentage error =% (ii) Using the same FB 4, the same FB 5 and no additional apparatus, describe how you could modify the experiment to reduce this maximum percentage error in the temperature rise. Explain your answer.[3]UCLES 2015 9701/32/M/J/15 [Total: 11] [Turn over<br />

Show answer