TJC H2 CHEM P3 W MARK SCHEME Prelim

9647/03/TJC Prelim 2011 [Turn Over 1 CHEMISTRY (H2) 9647/03 Paper 3 Free Response Tuesday 13th September 2011 2 hours Candidates answer on separate paper. Additional materials: Answer paper Data Booklet READ THESE INSTRUCTIONS FIRST Write your name, Civics Group and Index number in the spaces provided on the cover page provided and on all the work you hand in. Write in dark blue or black pen on both sides of the paper. You may use a soft pencil for any diagrams, graphs or rough working. Do not use staples, paper clips, highlighters, glue or correction fluid. Answer any four questions. 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 bracke ts [ ] at the end of each question or part question. At the end of the examination, fasten all your work securely together. This question paper consists of 11 printed pages.

9647/03/TJC Prelim 2011 [Turn Over 2 Answer any four questions. 1. (a) Nitrogen is chemically unreactive and will only react with oxygen under high temperatures to produce various atmospheric pollutants, such as nitrogen monoxide and nitrogen dioxide. The table below shows the standard enthalpy change of formation of some nitrogen oxides. Oxide NO NO 2 N 2O H f / kJ mol1 +90.4 +33.9 +81.6 (i) Determine the density, in g cm3, of nitrogen dioxide gas at room temperature and pressure. (ii) Nitrogen dioxide and dinitrogen oxide can react to give nitrogen monoxide: NO2(g) + N2O(g)  3NO(g)  S r = +171.4 J K1 mol1 By using the data provided above, calculate the G for the above reaction at 800 K and hence predict if the reaction is spontaneous at 800 K. Assume that H f and S r do not change with temperature. (iii) Dinitrogen oxide, N 2O, is a colourless gas and is also known as laughing gas. Draw a ‘dot-and-cross’ diagram to illustrate the type of bonding present within the N 2O molecule, where nitrogen is the central atom. Name the shape of the molecule. [7] (b) A solution contains 1.0  102 mol dm3 of silver ions and 2.0  102 mol dm3 of lead(II) ions. When an aqueous solution containing chloride ions is added to the solution, both silver chloride (Ksp value = 1.8  1010) and lead(II) chloride (Ksp value = 1.7  105) precipitate from the solution. Determine which salt precipitates first and the concentration of chloride which is necessary to begin the precipitation of each salt. [3] (c) Explain the following observations with the aid of relevant equations. (i) The second ionisation energies of the Group II elements decrease down the group. (ii) Magnesium nitrate produces a brown gas on gentle heating whereas barium nitrate gives a brown gas on prolonged heating. (iii) When 1.00 g of an unknown Group II carbonate, MCO3 (where M is Mg, Ca or Ba), is heated, there is a loss in mass and a white residue, X, is formed. When X is added to water, a weakly alkaline solution is obtained. No precipitate is observed when dilute sulfuric acid is added to the resulting solution formed. Deduce the identity of the Group II element, M. [10] [Total: 20]

9647/03/TJC Prelim 2011 [Turn Over 3 2 (a) Sodium hydroxide is often used for acid-base titrations. A student prepared a standard solution of sodium hydroxide by dissolving 10.0 g of sodium hydroxide in 250 cm 3 of water. In the process, he found the solution warm to the touch. Given the information below, calculate the heat evolved when solid sodium hydroxide is dissolved in water. Compound ∆H f / kJ mol1 NaOH(s) 425 NaOH(aq) 470 Hence, find the increase in temperature of the standard solution. [3] (b) 3-nitrophenol and benzoic acid are weak Bronsted acids with p Ka values of 8.35 and 4.20 respectively. (i) Calculate the pH of 0.1 mol dm3 of 3-nitrophenol. (ii) Explain, in molecular terms, which is the stronger acid. Phenolpthalein and bromothymol blue are common indicators for acid-base titrations. O O O OH H phenolpthalein pKa = 9.3 O S O O OH O Br Br H Bromothymol blue pKa = 7.1 A 25.0 cm 3 solution containing both 3-nitrophenol and benzoic acid was titrated against 1 mol dm 3 sodium hydroxide using 1 to 2 drops of phenolphthalein and bromothymol blue indicator. (iii) Explain why both indicators are added in small amounts. Phenolphthalein pKa = 9.3 Bromothymol blue pKa = 7.1

9647/03/TJC Prelim 2011 [Turn Over 4 It was found that 14.40 cm 3 was needed to change the colour of the first indicator and a further 7.60 cm3 was needed to change the colour of the second indicator. A sketch (not to scale) of the pH curve of the titration is shown below. (iv) State the values for A and B on the graph. (v) Calculate the concentration of benzoic acid and 3-nitrophenol in the solution. [8] (c) Methyl orange is another acid-base indicator. It can be synthesised from N,N-dimethylphenylamine and diazonium salt. N CH3 CH3 N,N-dimethylphenylamine Suggest a 3-step synthesis of N,N-dimethylphenylamine from benzene. [3] (d) Amino acid glycine (H2N-CH2-COOH) is often used as a buffer to assay the enzyme pepsin. The isoelectric point of glycine is 6.07. (i) What is meant by a buffer? (ii) Draw the structure of glycine at pH 6.07. (iii) Write equations to show how glycine acts as a buffer. In the Miller-Urey experiment, amino acids ca n be formed from the irradiation of the gases ammonia, methane and water vapour with ultraviolet light. In the process, a by-product is formed. (iv) Construct a balanced equation showing how glycine might have been produced in this experiment. (v) Suggest a reason why ultraviolet light needs to be used in this experiment [6] [Total: 20] A C pH 0 B Volume of titre added / cm3

9647/03/TJC Prelim 2011 [Turn Over 5 3. This question is about the chlorides of aluminium, silicon and phosphorus. (a) State and explain the pH and the colour observed when some universal indicator is added to the solution formed from A lCl3 dissolving in water. Write equations where appropriate. [3] (b) In electrophilic substitution reactions, A lCl3 can function as a Lewis acid catalyst to generate the electrophile from heterolytic fission. (i) Deduce whether A l2Cl6 can also function as a catalyst in electrophilic substitution reactions. (ii) Describe the mechanism of the reaction when benzene is reacted with ethanoyl chloride in the presence of AlCl3. [4] (c) When liquid SiCl4 is added to water, a white solid of SiO2 is immediately formed. (i) Explain why SiCl4 can be hydrolysed by water to form SiO2. (ii) The standard entropy change of the above reaction is positive. Write a balanced equation for the above reaction, with state symbols, and hence explain why the standard entropy change is positive. (iii) Unlike the reaction with water, when liquid SiC l 4 is reacted with excess methanol, SiO2 is not formed. Write a balanced equation for this reaction and describe what you may observe. [5] (d) The following table shows three compounds and their respective types of hybridisation. Compound HCN C H H O Si Cl Cl Cl Cl Number of σ bonds around central atom 2 3 4 Type of hybridisation sp sp 2 sp3 From the data above, deduce the type of hybridisation in PCl5. [1] Cl

9647/03/TJC Prelim 2011 [Turn Over 6 (e) PCl5 is often use