SAJC H1 CHEM P2 student

1 Name: Class: ST ANDREW’S JUNIOR COLLEGE JC2 Preliminary Examination Chemistry Higher 1 8872/2 Paper 2 17th September 2015 1400 - 1600 2 hours Additional Materials: Writing paper, Data Booklet, OAS READ THESE INSTRUCTIONS FIRST Write your name and civics group on all the work you hand in. Write in dark blue or black pen. You may use a soft pencil for any diagrams, graphs or rough working. Do not use staples, paper clips, highlighters, glue or correction fluid. Section A: Answer all the questions in this section in the spaces provided. Section B: Answer two questions from this section on separate answer paper. 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. For Examiners use only: Section A Question 1 Question 3 Question 2 Question 4 Total for Section A Section B Total for Section B Total This document consists of 17 pages including a blank page. [Turn Over 14 10 9 7 40 20 20 40 80

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3 Section A Answer all the questions in this section in the spaces provided. 1 Jet airliners use kerosene as fuel. The formula of kerosene can be taken as C14H30. It can be converted into more useful small molecules by the process of cracking. (a) (i) To which homologous series does kerosene belong? [1] (ii) Write a balanced equation to show the cracking of kerosene into heptane, propene and ethene only. [1] (iii) Suggest a use of ethene during the cracking process. [1] (iv) With the aid of an equation, define standard enthalpy change of combustion of kerosene. [1] [Turn over]

4 1 (b) Fill in the boxes with the appropriate structures and state the reagents and conditions of steps I and II. Step I: Step II: [5]

5 1 (c) The propene obtained from cracking of kerosene is bubbled into acidified potassium manganate(VII) solution to form ethanoic acid. 20 cm 3 of the ethanoic acid produced is titrated with 3.0 mol dm -3 aqueous NaOH using a suitable indicator. The average volume of NaOH used is 12.50 cm3. Another 20 cm 3 of the same ethanoic acid is placed into a plastic cup of negligible heat capacity. A 10.0 cm 3 sample of 3.0 mol dm -3 aqueous NaOH, at the same initial temperature, is added into the plastic cup. The temperature of the mixture rises by 12.5 K. (i) Determine the concentration of ethanoic acid. [1] (ii) If the specific heat capacity per unit volume of the mixture is 4.2 J K -1 cm-3, what is the enthalpy change of neutralization of ethanoic acid? [2] (iii) If HC l replaces ethanoic acid in the above experiment, w ould the temperature rise be higher or lower? Explain your answer. [2] [Total: 14 marks] [Turn over]

6 2 Chlorine dioxide, ClO2, is used for bleaching paper pulp and for water treatment. Over 95% of the chlorine dioxide produced in the world today is made from acidified sodium chlorate, NaClO3, with a suitable reagent such as hydrogen peroxide, H2O2. The half-equation for the reduction of ClO3  ions to ClO2 is shown below. ClO3  + 2H+ + e ClO2 + H2O (a) (i) Using the Data Booklet, write the half equation for H2O2 in this reaction. [1] (ii) Hence, write an equation for the overall reaction. [1] (iii) Use oxidation numbers to show which species in your equation is oxidised and which is reduced. [1] (b) Metal chlorates can be produced by adding chlorine to hot metal hydroxides as shown below. 3 Cl2 + 6 KOH 5 KCl + KClO3 + 3 H2O (i) State the type of the reaction above. [1] (ii) Write the electronic configuration of K+. [1]

7 2 (b) (iii) Draw a dot and cross diagram to illustrate the bonding in KClO3, showing the outermost shell electrons only. [1] (c) (i) Using VSEPR Theory, predict and explain the shape and bond angle of ClO2. [2] (ii) Chlorine dioxide is highly soluble even in cold water. Explain your answer with reference to structure and bonding. [2] [Total: 10 marks] [Turn over]

8 3 The body's chemical buffer system consists of three individual buffers: the carbonate/carbonic acid buffer, the phosphate buffer and the buffering of plasma proteins. The first buffer system is usually considered the most important since it is coupled to t he respiratory system. In this buffer system, carbon dioxide combines with water to form carbonic acid (H2CO3), which in turn rapidly dissociates to form hydrogen and bicarbonate (HCO3 ) ions according to the reactions below: 2 H2O + CO2 H2CO3+ H2O reaction (1) H2CO3+ H2O H3O++ HCO3  reaction (2) As a buffer, it maintains a relatively constant blood plasma pH and counteracts any change that would alter it. Thus blood pH is regulated to stay within the narrow range of 7.35 to 7.45, making it slightly alkaline. If the pH of the body gets too low (below 7.4), a very serious condition known as acidosis results. Since carbonic acid is not stable in aqueous solutions , some of it decomposes to form carbon dioxide and water. The respiratory system is responsible for removing the carbon dioxide. It is the production of carbon dioxide from this reaction that couples the carbonic acid/bicarbonate buffer to the respiratory system. (a) (i) The pH of 0.10 mol dm-3 carbonic acid was found to be 3. 54. Suggest if carbonic acid is a weak or strong acid. Explain your answer. [1] (ii) With the aid o f an equation, explain how the carbonate/ carbonic acid buffer system helps to maintain a relatively constant pH when H + ions are added to the blood plasma. [1]

9 3 (a) (iii) Explain, with reference to the equilibrium reactions, why a reduction in respiratory activity to remove CO2 will result in acidosis. [2] (b) The acid strength of carbonic acid is determined by titration with aqueous NaOH. 25 cm 3 of 0.10 mol dm -3 carbonic acid was pipetted into a conical flask and titrated with 0.10 mol dm -3 NaOH in the burette. The titration curve of pH against volume of aqueous NaOH added is shown below. (You may wish to consider carbonic acid as a monobasic acid.) (i) Write the acid dissociation constant, K a, expression for the following equilibrium. H2CO3 (aq) + H2O(l) H3O+(aq) + HCO3 − (aq) [1] [Turn over] Q Vol of NaOH (aq) / cm3 pH 3.54 25 7

10 3 (b) (ii) Hence, calculate the acid dissociation constant, Ka, for the equilibrium in b(i). [2] (ii) At point Q, the amount of NaOH added just neutralises the acid present to from a salt. State the nature of the salt formed and hence illustrate its behaviour with the aid of an equation. [2] [Total: 9 marks] 4 Compound A is converted to compound B as shown in the synthetic route below. (a) Draw a labelled diagram to illustrate the sigma and pi bonds formed between the orbitals of C1 and C2 in compound A. [2]