Name: Class: Form 7S Class Number: Total Mark:

Transcription

1 Paper I / 1 CCC HEEP WOH COLLEGE MOCK EXAMINATION F.7 AL CHEMISTRY PAPER I 8:30am 11:30am (3 hours) Question-Answer Book SETTER: MR. LAU C.K. Name: Class: Form 7S Class Number: Total Mark: 1. This paper must be answered in English. 2. There are THREE sections in this paper, Section A, Section B and Section C. 3. Section A carries 60 marks, Section B carries 20 marks, and Section C carries 20 marks. 4. All questions in Sections A and B are COMPULSORY. Answers are to be written in this Question-Answer Book. If you use supplementary answer sheet(s) for these two sections, write your Name and Class Number on each sheet and fasten them with string to this Question-Answer Book. 5. Answer ONE question in Section C. Answers are to be written in Supplementary Answer Sheets. 6. Some useful constants and a Periodic Table are printed below: Periodic Table / 100 Useful Constants Gas constant, R = 8.31 J K -1 mol -1 Speed of light in vacuum, c = 3.00 x 10 8 m s -1 Faraday constant, F = 9.65 x 10 4 C mol -1 Ionic product of water at 298K, Kw = 1.00 x mol 2 dm -6 Avogadro constant, L = 6.02 x mol -1 Specific heat capacity of water = 4.18 J g -1 K -1 Planck constant, h = 6.63 x J s Molar Volume of gas at r.t.p. = 24.0 dm 3 mol -1 Characteristic Infra-red Absorption Wavenumber Ranges (Stretching modes) Bond Compound type Wavenumber range / cm -1 C=C Alkenes 1610 to 1680 C=O Aldehydes, ketones, carboxylic acids, esters 1680 to 1750 C C Alkynes 2070 to 2250 C N Nitriles 2200 to 2280 O H Acids (hydrogen-bonded) 2500 to 3300 C H Alkanes, alkenes, arenes 2840 to 3095 O H Alcohols, phenols (hydrogen-bonded) 3230 to 3670 N H Amine 3350 to 3500

2 Paper I / 2 Section A: Answer ALL questions in this Section. Write your answers in the spaces provided. Question 1 [10 marks] (a) (i) Draw Lewis structures of N2O and N2O3 respectively. State oxidation numbers of nitrogen in N2O and N2O3. (b) (i) Complex with molecular formula of CoCl2(H2NCH2CH2NH2)2 which is optically active. Draw the structures of the complex and its mirror image. Write the equations which represent the displacement of ligands of all Cl - by CN -. (c) Account for the following: (i) The boiling point of NH3 is 36 C, but that of PH3 is 87 C. Unlike the other halogens, fluorine does not disproportionate in water.

3 Paper I / 3 Question 2 [9 marks] Nitrogen dioxide, NO2, is a constituent of photochemical smog caused by cars in cities. In gaseous state, NO2 undergoes dimerization to form N2O4 as follow: 2NO2(g) Brown N2O4(g) Pale yellow (a) Draw the structure and give the shape of NO2 molecule. (b) Draw the Lewis structure of N2O4 molecule. (c) If 0.2 mol of NO2(g) originally present in a closed container with 5 dm3 was heated to 350 K and the above equilibrium is established, 46% of the original NO2(g) was consumed. Calculate the Kc for the reaction at 350 K. (d) State, with explanation, the effect of increasing temperature on Kc for the reaction. (e) Explain, with reasons, whether the above dimerization is favoured by high or low pressure.

4 Paper I / 4 Question 3 [11 marks] (a) X and Y are chain isomers having the molecular formula C3H7CHO. The boiling points of X and Y are 75 C and 63 C respectively. (i) Deduce the structures of X and Y. Give the IUPAC names of X and Y. (iii) Explain why the boiling point of X is higher than that of Y. (iv) Suggest a separation method to separate a mixture of X and Y. Explain your answer. (b) Riboflavin is also known as vitamin B2. It dissolves in water to form a yellow solution. It can be found in certain food such as eggs, nuts, milk, meat and green vegetables. It is essential for maintaining healthy hair and skin, boosting immune system, slowing down aging, etc. A set of standard riboflavin solutions was used to construct a calibration curve. Their absorbances were recorded as follows: Concentration of riboflavin ( 10 5 M) Absorbance

5 Paper I / 5 (i) What is the relationship between the colour intensity of the riboflavin solution and the absorbance? By using the information in the above table, plot a calibration curve of riboflavin solutions. (iii) What would be the concentration of the riboflavin solution if it had an absorbance of 0.680?

6 Paper I / 6 Question 4 [10 marks] Read the passage below and answer the questions below. Melamine Melamine is a cyanamide which has a molecular formula of C3H6N6. Its structure is featured by the presence of N=C linkages. Under X-ray diffraction, a six-membered ring is found. Within the ring, there is only one type of nitrogen-carbon bond length which is shorter than the normal nitrogen-carbon single bond length. Melamine can be synthesized through decomposition and polymerization with urea ((NH2)2CO) as its starting material. Urea is decomposed into cyanic acid (HCNO) and an alkaline gas first. Then, the cyanic acid undergoes polymerization to give the final product and a gas with the release of heat. Moreover, melamine reacts with compound A to form amide with structure shown in Fig. 1 under room temperature. On the other hand, melamine has been used as non-protein nitrogen (NPN) for cattle feed in the past. However, it is added into wide variety of milk products to increase the apparent protein content of the products illegally. Melamine is not toxic in low dose. When it comes across cyanuric acid (Fig. 2), they form melamine-cyanuric acid complex by bonding which is similar to the bonding between polynucleotide chains in DNA molecules. If the complex concentrates in the body and forms crystals in the renal tubules, kidney damage and even complete kidney failure can be resulted. Fig. 1 Fig. 2 (a) (i) Write the chemical equations for the two steps involved in the production of melamine. State and explain whether the first step is an endothermic or exothermic reaction. (iii) Draw the structure of melamine.

7 Paper I / 7 (iv) Draw the structure of the melamine-cyanuric acid complex. (v) Give the IUPAC name of compound A. (b) Cyanuric acid is found to have an alternate structure to that shown above. Both structures have the same molecular formula. IR absorption spectrum of that structure of cyanuric acid is shown below. Deduce the alternate structure. (Hint: Absorption peak of C=N is more or less the same as that of nitrile)

8 Paper I / 8 Question 5 [8 marks] (a) Consider the following data: / kj mol -1 Enthalpy change of hydration of Mg Enthalpy change of hydration of Ba Enthalpy change of hydration of OH 550 Lattice enthalpy of Mg(OH) Lattice enthalpy of Ba(OH) (i) Explain why the sign of the enthalpy change of hydration is always negative. Calculate the enthalpy of solution of Mg(OH)2(s) and Ba(OH)2(s). (iii) Use the data to explain the trend in the solubility of Group II hydroxides.

9 Paper I / 9 (b) Given the electronegativity values of the elements: H 2.1, N 3.0, C 2.5, F 4.0. (i) Define electronegativity. Based on the data of the elements given, state a bond formed between two elements which have the highest polarity. Explain briefly. Question 6 [12 marks] (a) Outline the experimental procedure for determining the equilibrium constant, Kc, of the reaction shown below at room temperature using 0.02 moles of FeSO4(NH4)2SO4 6H2O crystal. Fe 2+ + Ag + Fe 3+ + Ag

10 Paper I / 10 (b) (i) State ONE chemical properties of fluorine which are different from that of the other halogens. Explain why fluorine has only one oxidation state in its compounds while chlorine has oxidation states ranging from -1 to +7 in its compounds. (c) (i) Write down the observation and ionic equation for the reaction when excess potassium iodide solution is added to copper(ii) sulphate solution. What happens when sodium thiosulphate solution is added to the resulting solution above? Give the equation of reaction. (iii) Suggest a chemical method to determine the purity of copper(ii) chloride solution. End of Section A

11 Paper I / 11 Section B: Answer ALL questions in this Section. Write your answers in the spaces provided. Question 7 [10 marks] (a) Consider the following compounds and answer the questions. A: CH3CH2OCH2CH3 B: CH3CH2CONH2 C: (CH3CH2)2NH D: CH3CH2I E: CH3CH2COCH2CH3 (i) Which of the above compounds has a fishy smell? Which of the above compound is an ethylating agent? (iii) Which of the above compounds reacts with hydroxylamine? (iv) Which of the above compounds is a colourless crystallite solid? (b) Products A and B are formed from the reaction shown below. CH 3 CH 2 C CH 2 NH 2 Br NaOH(aq) reflux C 10 H 15 ON A + C 10 H 15 ON B (i) Give the structural formulae of A and B. Name the reaction that gives A and B respectively. (iii) How the structure of reactant favour the formation of A. (iv) Draw the energy profile of the formation of A.

12 Paper I / 12 Question 8 [10 marks] (a) An experiment is carried out to determine the percentage by mass of aspirin in a tablet. Two 500 mg tablets is dissolved in 100 cm 3 distilled water. 100 cm 3 of 0.1M NaOH is added into the solution formed cm 3 of 0.2M HCl is used in titration of the resultant solution. Structure of aspirin is COOH OCOCH3 (i) What will be the approximate ph of the solution at equivalent point? Briefly explain. Suggest a suitable indicator used in the above titration. (iii) Write a chemical equation of the reaction between aspirin and NaOH(aq). (iv) Calculate the percentage by mass of aspirin in one tablet. (b) At room temperature, a saturated aqueous solution of hydrogen sulphide contains about 0.1 mole per litre of hydrogen sulphide. If a saturated solution of hydrogen sulphide contains 0.1M Zn 2+, will ZnS precipitate from the solution at ph 3.0? Explain. [H2S(aq) 2H + (aq) + S 2- (aq); K = 1.2 x mol 2 dm -6 ; Ksp of ZnS = 8 x mol 2 dm -6 ] END OF SECTION B

13 Section C: Answer ONE question only and write your answers in the ANSWER SHEETS provided. Marks will be allocated approximately as follows: Chemical knowledge 50% Organization 30% Presentation (including use of language) 20% Equations, suitable diagrams and examples are expected where appropriate. Mr. Lau is looking for the ability to analyse, to evaluate to express ideas clearly. Paper I / 13 Question 9 [20 marks] Write an essay on characteristics of d-block elements. Use vanadium, manganese and iron to illustrate the above characteristics if appropriate. Compare the complex formation properties of d-block and s-block elements. Question 10 [20 marks] Write an essay on the chemistry of halogeno compounds. Hence, compare reaction mechanism of halogeno compounds and carbonyl compounds. END OF PAPER