MJC H2 CHEM P2 answers Prelim

Name: _______________________ Class: 13S_________ Reg Number: ______ MERIDIAN JUNIOR COLLEGE Preliminary Examination Higher 2 _______________________________________________________________________________ Chemistry 9647/02 Paper 2 Structured Questions 19 September 2014 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 20 printed pages. Examiner’s Use Paper 1 MCQ / 40 Paper 2 Q1 / 12 Q2 / 12 Q3 / 11 Q4 / 13 Q5 / 14 Q6 / 10 Paper 3 / 80 Total / 192 Percentage 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.

2 [Turn over 1 Planning (P) The enthalpy change of neutralisation is the enthalpy change when one mole of water is formed in the reaction between an acid and a base. H+(aq) + OH−(aq)  H2O(l) You are required to determine the enthalpy change of neutralisation between a strong base and a carboxylic acid through a series of experiments. You may assume you are provided with the following:  FA 1, aqueous sodium hydroxide, NaOH  FA 2, 1.50 mol dm–3 propanoic acid, CH3CH2COOH  polystyrene (styrofoam) cups  apparatus normally found in a school laboratory. In separate experiments, different volumes of FA 1 and FA 2 are mixed while keeping the total volume of the reaction mixture constant. In each exper iment, the temperature rise, ∆T , is to be determined. When a graph of ∆T against volume of FA 1 used is plotted, it is observed that the t emperature rise will increase, and then decrease when different volumes of FA 1 are used. Data from the graph can then be used to determine:  the concentration of sodium hydroxide, NaOH, in FA 1.  the enthalpy change of neutralisation between NaOH and CH3CH2COOH. It is given that 4.18 J is required to raise the temperature of 1 cm 3 of any solution by 1 oC. (a) The total volume of the reaction mixture between FA 1 and FA 2 is constant. Explain why the temperature rise increases as more FA 1 is added to the mixture. As more FA 1 is added to the mixture, with amount of FA 2 being in excess (or FA 1 limiting), the number of moles of water produced in creases and amount of heat produced in creases in the presence of a same constant volume, hence the temperature rise increases. [1] (b) It is predicted that the maximum temperature change for the neutralisation would occur when the volume of FA 1 mixed is between 35 cm3 and 40 cm3. Write a detailed plan on how you could determine the temperature changes for the series of reactions betw een FA 1 and FA 2 . The volume of FA 1 used should be between 30 cm 3 and 45 cm 3 while the total volume of any mixture should be kept constant at 70 cm3. Your plan should contain the following:  all essential experimental details  appropriate volumes of solutions to be used  a tabulation of the experimental data to be collected (on the next page) Comment [M1]: Shifted the sentence down to make the flow of the question better.

3 [Turn over Procedure: 1. Use a 50.00 cm 3 burette to transfer 30.00 cm 3 of FA 1 into a polystyrene / styrofoam cup labelled FA 1. 2 Use another 50.00 cm 3 burette to transfer 40.00 cm 3 of FA 2 into another polystyrene/styrofoam cup labelled FA 2. Support this cup in a 250 cm 3 beaker. 3 Use a thermometer to stir and measure the initial temperature of the FA 1 and FA 2 solution respectively. 4 Calculate the average temperature of the two solutions. 5 Add the contents of the FA 2 cup to the FA 1 cup. Use the thermometer to stir the mixture and measure the highest temperature of the mixture. 6 Calculate the temperature change of the reaction. 7 Wash and dry both the FA 1 and FA 2 polystyrene / styrofoam cup. 8 Repeat steps 1 to 7 using different volumes of FA 1 and FA 2 as stated in the table below. Tabulation: Volume of FA 1 / cm3 Volume of FA 2 / cm3 Initial temperature of FA 1 / oC Initial temperature of FA 2 / oC Average temperature of mixture before mixing / oC Highest temperature of mixture after mixing / oC ∆T, / oC 30.00 40.00 33.00 37.00 35.00 35.00 40.00 30.00 42.00 28.00 45.00 25.00 [6]

4 [Turn over (c) (i) The following plots were plotted on a grid after an experiment. Draw suitable graphs through the plotted points. (ii) By using the graph in (i), calculate:  the concentration of sodium hydroxide in FA 1.  the enthalpy change of neutralisation between NaOH and CH3CH2COOH. Volume of FA 1 (NaOH) used when temperature change is maximum = 36.50 cm3 No of moles of CH3CH2COOH used = 70 36.5 1.51000   = 0.05025 NaOH ≡ CH3CH2COOH No of moles of NaOH (FA 1) used = 0.05025 Concentration of sodium hydroxide in FA 1 = -3 0.05025 36.5 10 = 1.38 mol dm–3 From graph, maximum ∆T = 9.5 oC Total heat absorbed when ∆T is highest = 70  4.18  9.5 = 2780 J 30 50 45 40 35 Volume of FA 1 / cm3 10.0 9.0 8.0 7.0 6.0 5.0 4.0 ∆ T / oC

5 [Turn over No of moles of water formed = 0.05025 Enthalpy change of neutralisation between NaOH and CH3CH2COOH = − 2780 0.05025 = − 55320 J mol–1 = − 55.3 kJ mol–1 [4] (d) Describe one major source of error for the experim ent and suggest an improvement which could significantly increase the accuracy of the experiment. Error: Heat loss to the surrounding Suggested Improvement: Use a lid to cover the styrofoam cup to minimise heat loss. or Provide lagging for the styrofoam cup to minimise heat loss. [1] [Total: 12]

6 [Turn over 2 Chlorine is a yellow -green gas that was first synthesised around 1630. Nearly all the chlorine in the Earth's crust occurs as chloride in various ionic compounds. It forms a variety of oxides such as chlorate( V), C lO3 – , chlorate(III), ClO2 – , and chlorine dioxide, ClO2. The table below shows the reduction potentials for some oxides of chlorine. Electrode Reaction Eo / V ClO3 – (aq) + H2O(l) + 2e – ClO2 – (aq) + 2OH – (aq) +0.33 ClO3 – (aq) + 2H+(aq) + e – ClO2(g) + H2O(l) +1.15 ClO2(g) + 4H+(aq) + 5e – Cl–(aq) + 2H2O(l) +1.50 (a) Draw a labelled diagram to show how the redox potential Eo(ClO3 – / ClO2) could be measured in the laboratory. [2] (b) Use the Data Booklet and the above information to predict what might happen when solutions of the two reagents are mixed together. In each case, calculate the Eo cell and write an overall equation for any reaction that occurs. (i) mixing ClO3 – (aq) with excess H2O2(aq) First reaction: ClO3 – + 2H+ + e – ClO2 + H2O Eo = +1.15 V O2 + 2H+ + 2e– H2O2 Eo = +0.68 V 2ClO3 – + 2H+ + H2O2  2ClO2 + 2H2O + O2 Eo cell = 1.15 – 0.68 = +0.47 V > 0 (feasible) [ClO3 – (aq)] = 1 mol dm–3 [H+(aq)] = 1 mol dm–3 Pt(s) Pt (s)  ClO2(g) Pressure = 1 atm Temperature = 298 K H2(g)  Pressure = 1 atm Temperature = 298 K [H+(aq)] = 1 mol dm–3 Comment [M2]: Follow A-level phrasing N2004 P3 Q2

7 [Turn over Second consecutive reaction: ClO2 + 4H+ + 5e – Cl– + 2H2O Eo = +1.50 V O2 + 2H+ + 2e– H2O2 Eo = +0.68 V