NJC H2 CHEM P3 Prelim

NATIONAL JUNIOR COLLEGE PRELIMINARY EXAMINATIONS Higher 2 CANDIDATE NAME SUBJECT CLASS REGISTRATION NUMBER CHEMISTRY Paper 3 Free response 9647/03 Tues 13 Sep 2011 2 hours READ THESE INSTRUCTIONS FIRST Answer any four questions. Start your answer to each question on a fresh piece of paper. 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. At the end of the examination, fasten all your work securely behind the cover page. This paper consists of 13 printed pages and 1 cover page.

NJC/H2 Chem/03/2011 [Turn over 2 Answer any four questions. 1 Barium carbonate, BaCO 3, also known as witherite, is often used as a rat poison. (a) Draw the dot-and-cross diagram of barium carbonate. [1] (b) Given the following data, construct an energy level diagram to calculate the standard enthalpy change of solution of barium carbonate. H θ lattice energy (BaCO3) / kJmol1 2554 Hθ hydration (Ba2+) / kJmol1 1305 Hθ hydration (CO3 2– ) / kJmol1 1245 [3] (c) (i) Given that the solubility of barium carbonate in water is 0.024 g dm 3, calculate the value of the solubility product of barium carbonate, stating its units. (ii) G θ solution can be calculated in J mol1 using the formula Gθ solution =  2.303 RT lg Ksp. Calculate a value for Gθ solution of BaCO3 in kJ mol–1. [3]

NJC/H2 Chem/03/2011 [Turn over 3 1 (d) (i) Write an equation, with state symbols, for the thermal decomposition of barium carbonate. (ii) When the same amount of MgCO 3 and BaCO 3 was heated as shown in the above setup, the following data was obtained. Time taken for white precipitate to be observed in X MgCO3 3 minutes BaCO3 9 minutes Explain the above observations. [5] 1 (e) When water is added to the white residue from (di), an aqueous solution of barium hydroxide is formed. Barium hydroxide could be used in organic synthesis as a strong base under suitable conditions. One example is the aldol reaction where barium hydroxide can act as a base to generate a nucleophile from ethanal. CH 3CHO + OH CH2CHO + H2O (nucleophile) (i) Explain why a nucleophile can be generated from an aldehyde molecule but not from an alkane molecule as shown below: CH3CH3 + OH CH2CH3 + H2O (ii) In the aldol reaction between 2 molecules of ethanal, the following product is obtained. OH CH3CHO + CH3CHO  CH3CH(OH)CH2CHO aldol By considering the nucleophile generated from ethanal, state the type of reaction and describe the mechanism for the above aldol reaction. [5] heat Test tube containing X MCO3 (s)

NJC/H2 Chem/03/2011 [Turn over 4 1 (f) Sodium hydride, NaH, is another inorganic compound which can take part in organic synthesis. Reaction I H O Cl O H ether heat + NaH + NaCl + H2 Reaction II + NaCl H O Cl ether heat + NaH H O H (i) State the type of reaction that has occurred in reaction I. (ii) Suggest and explain why reaction I is more feasible than reaction II in terms of free energy and entropy changes. [3] [Total: 20]

NJC/H2 Chem/03/2011 [Turn over 5 2 The use of medicine in the treatment of illnesses was revolutionised in the 19 th and 20th century due to rapid advancements in chemistry, in terms of research work, laboratory techniques and equipments. Aspirin and paracetamol are examples of drugs that have gained world-wide recognition for their medicinal properties. Aspirin and paracetamol are known as analgesics which is a group of drugs used to relieve pain. (a) The structures of paracetamol and aspirin are given below: N H C O CH3 HO O C O CH3 COOH Paracetamol Aspirin (i) Devise a three-step synthesis, starting with phenol, for the manufacture of paracetamol. (ii) Suggest a method by which aspirin can be distinguished from paracetamol. (iii) Predict, with a reason, whether aspirin or paracetamol has a lower pH. (iv) Give the displayed formula of an isomer of aspirin which is not soluble in both hydrochloric acid and sodium hydroxide. [10] (b) The hydrolysis reaction of aspirin is catalysed by an acid. Four experiments were carried out to find the relationship between the rate of reaction and the initial concentrations of aspirin and the acid catalyst. The results are recorded in the table below. Experiment [aspirin] /mol dm 3 [H+] /mol dm3 Initial rate / mol dm3 h1 I 0.04 0.08 1.00 II 0.05 0.08 1.25 III 0.04 0.06 0.75 IV 0.05 0.06 0.94 (i) Write an equation to represent the hydrolysis of aspirin.

NJC/H2 Chem/03/2011 [Turn over 6 2 (b) (ii) Using the data in the table, deduce the order of reaction with respect to aspirin and the acid catalyst, showing how you arrive at your answers. Hence determine the rate equation and calculate the rate constant for this reaction, giving its units. (ii) Given that the half-life of aspirin in experiment I was 3 hours, predict the half-life of aspirin in experiment II and in experiment III, giving your reasoning. (iii) Sketch a graph showing how the concentration of one of the products of the hydrolysis reaction varies with time, by making use of the data given for experiment I. [10] [Total: 20]

NJC/H2 Chem/03/2011 [Turn over 7 3 Ibuprofen is an over-the-counter painkiller for mild to moderate pain which also reduces inflammation and fever. O HO Ibuprofen In this question, ibuprofen is represented by RCOOH. (a) Ibuprofen has a pKa value of 4.4. (i) Define the term pKa. (ii) 800 cm3 of sodium hydroxide was added to 200 cm 3 of 0.5 mol dm 3 of ibuprofen to make a 1.00 dm3 buffer solution of pH 5.00. Prove that the ratio of the concentrations of the salt of ibuprofen to the concentration of unionised ibuprofen is 3.98. Hence, calculate the concentration of the sodium hydroxide solution added to make the buffer solution. [4] (b) The values for ∆H θ for the reactions of NaOH with HCl and ibuprofen are given below: HC l(aq) + NaOH(aq) → NaCl(aq) + H2O(l) ∆H1 θ = –57.3 kJmol–1 RCOOH (aq) + NaOH(aq) → RCOONa(aq) + H2O(l) ∆H2 θ = –50.0 kJmol–1 Comment on the values of ∆H1 θ and ∆H2 θ given and deduce the value of the enthalpy change of acid dissociation of ibuprofen by drawing a suitable energy cycle. [4]

NJC/H2 Chem/03/2011 [Turn over 8 3 (c) Ibuprofen can be synthesized using the following steps. The starting material is 2-methylpropylbenzene. AlCl3; Step 1 O C CH3 Step 2 CH3CH2ONa;CH3COCl