2019 DHS Prelim H2 Chem P3 QP

© DHS 2019 This paper consists of 16 printed pages. [Turn Over Name: Index Number: Class: DUNMAN HIGH SCHOOL Preliminary Examination 2019 Year 6 H2 CHEMISTRY 9729/03 Paper 3 Free Response 25 September 2019 2 hours Additional Materials: Data Booklet Answer Booklet INSTRUCTIONS TO CANDIDATES 1 Write your name, index number and class on this cover page. 2 Write your answers on the separate answer booklet provided. 3 Write in dark blue or black pen. 4 You may use an HB pencil for any diagrams or graphs. 5 Start each question on a fresh page. *[Marks will be deducted if you fail to do so.] 6 Do not use staples, paper clips, glue or correction fluid. Section A 7 Answer all questions. Section B 8 Answer one question. The number of marks is given in brackets [ ] at the end of each question or part question. You are advised to show all workings in calculations. You are reminded of the need for good English and clear presentation in your answers.

2 © DHS 2019 9729/03 Section A Answer all the questions from this section. 1 To facilitate better plant growth, there is widespread use of fertilisers and soil additives in the agricultural sector. (a) Agricultural lime is a soil additive used to increase soil pH so as to facilitate uptake of plant nutrients such as nitrogen and phosphorous. It is usually made up of a combination of calcium carbonate and magnesium carbonate. A manufacturer claims that a 2.00 g sample of agricultural lime contains 92 % calcium carbonate (Mr = 100.1) and 8 % magnesium carbonate (Mr = 84.3) by mass. To verify the manufacturer’s claim, the following steps were carried out: Step 1 The solid carbonat e mixture was dissolved in 250 cm3 of 0.25 mol dm–3 hydrochloric acid. Step 2 A 25 cm 3 aliquot of this resultant solution was titrated with 0.14 mol dm–3 potassium hydroxide. It was found that 12.50 cm 3 of potassium hydroxide was required for complete neutralisation. (i) Based on the manufacturer’s claim of 92 % calcium carbonate and 8 % magnesium carbonate by mass, calculate the theoretical amount, in moles, of each carbonate present in the sample of agricultural lime. [1] (ii) Use the information above to calculate the amount of HC l that reacted with the carbonate mixture in step 1. [2] (iii) The manufacturer’s claim is considered valid if the difference between the actual and theoretical total amount of carbonate present in the sample is less than 0.0010 mol. Using your answers in (a)(i) and (a)(ii), determine if the manufacturer’s claim is valid. [2] (iv) When solid magnesium chloride is dissolved in water, it produces a slightly acidic solution of pH 6.5. Using appropriate equation(s), explain the above observation. [2]

3 © DHS 2019 9729/03 [Turn Over (v) However, the resultant solution is neutral when solid calcium chloride is dissolved in water. By quoting relevant values from the Data Booklet, explain why a solution of calcium chloride is neutral while a solution of magnesium chloride is slightly acidic. [3] (b) Nitrogen fertilisers are also important in enhancing the growth of plants. Almost all nitrogen fertilisers are produced from ammonia, which in turn, is manufactured in the Haber Process. This process combines nitrogen and hydrogen gas to form ammonia according to the following equilibrium. N2(g) + 3H2(g) 2NH3(g) (i) State how the percentage yield of ammonia will change with increasing pressure. [1] (ii) N2 and H2 was introduced into an enclosed vessel at 300 C. The initial partial pressures of N 2 and H 2 were 2 atm and 3 atm respectively. After an equilibrium has been established, the total pressure in the vessel was 4.7 atm. Calculate the value of Kp at 300 C, stating its units. [3] (iii) 1 atm of N2 was added to the equilibrium mixture in (b)(ii) at 300 C and time was allowed for a new equilibrium to be established. State and explain how the Kp value at this point will compare to that in (b)(ii). [1] [Total: 15]

4 © DHS 2019 9729/03 2 Mandelic acid, C6H5CH(OH)COOH, is a white crystalline solid that is soluble in water. (a) Benzaldehyde is a useful starting material for the preparation of mandelic acid. Suggest a two –step synthesis of mandelic acid from benzaldehyde, stating clearly the reagents and conditions used and the structure of the intermediate. [3] (b) Suggest how you would distinguish between aqueous solutions of mandelic acid and nitric acid of the same concentration by means of a physical method. Explain your reasoning. [3] (c) (i) 20 cm3 of 1.0 mol dm3 mandelic acid solution was titrated with 2.0 mol dm3 aqueous sodium hydroxide. Calculate the concentration of the salt, sodium mandelate, formed at equivalence point. [1] (ii) Hence calculate the pH of the solution at equivalence point, given p Ka of mandelic acid is 3.41. [2] (iii) Use your answer in (c)(ii) to identify a suitable indicator from Table 2.1 for the titration. Explain your choice. Table 2.1 Indicator pH range Methyl red 4.2 – 6.3 Phenolphthalein 8.2 – 10.0 Titan yellow 12.0 – 13.0 [2] (d) (i) To another 20 cm 3 sample of 1.0 mol dm 3 mandelic acid solution, excess sodium carbonate was added and the gas liberated was collected in a gas syringe at 50 °C and atmospheric pressure. Calculate the volume of gas you would expect to obtain at the end of th e reaction. [2] (ii) The volume of gas collected was found to be smaller than the calculated value in (d)(i). Assuming that no gas escaped in the experiment, suggest two reasons for the discrepancy. [2] [Total: 15]

5 © DHS 2019 9729/03 [Turn Over 3 (a) High levels of acrylamide, which is harmful to human health , are produced when starchy foods are heated at high temperature. CH C O NH2CH2 acrylamide The formation of acrylamide begins with the reaction between an aldehyde and asparagine, H 2NCH(CH2CONH2)COOH. This process involves the formation of carbinolamine and a Schiff base. C O HR C COOHNH2 H CH2 C O NH2 asparagine + C COOHNH H CH2 C O NH2 CH OH R stage 1 C COOHN H CH2 C O NH2 CH R stage 2 Schiff base carbinolamine + H2O CH C O NH2CH2 acrylamide decarboxylation decomposition (i) State the type of reaction occurring in stages 1 and 2 respectively. [2]

6 © DHS 2019 9729/03 The mechanism for stage 1 is thought to involve the following three steps. step 1 Nucleophilic attack of NH2 of asparagine on carbonyl carbon atom to form the following intermediate where R 1 is CH2CONH2. C COOHN + H C H O R HH H R1 step 2 The intermediate is deprotonated by a water molecule, breaking the NH bond. step 3 The protonated water molecule from step 2 then transfers the proton to the negatively charged oxygen to form carbinolamine. (ii) Using the information above, draw the mechanism for stage 1 by showing relevant lone pairs of electrons, dipoles and curly arrows to represent the movement of electron pairs. [3] (iii) The experiment was repeated with the use of an aparaginase enzyme which hydrolyses asparagine, H2NCH(CH2CONH2)COOH to aspartic acid, H2NCH(CH2COOH)COOH. The level of acrylamide, CH2=CHCONH2 in food products is reduced by 99 %. Suggest a reason why this is so. [1]

7 © DHS 2019 9729/03 [Turn Over (b) Several aldehydes can be found in starchy foods. Fixed amount of each aldehyde was heated with asparagine. The mass of acrylamide formed from each aldehyde is shown in Table 3.1 below. Table 3.1 Aldehyde Structure Mass of acrylamide (g) glucose C C C O H OH H OH CC OH H OH C H H OH H H 1454 ribose C C