SAJC H1 CHEM P2 student solutions

Name: Class: ST ANDREW’S JUNIOR COLLEGE JC2 Preliminary Examination ANSWERS 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: This document consists of 19 printed pages including this page. [Turn Over

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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] Alkane (ii) Write a balanced equation to show the cracking of kerosene into heptane, propene and ethene only. [1] C14H30 → C7H16 + C3H6 + 2 C2H4 (iii) Suggest a use of ethene during the cracking process. [1] Any one of the following:  form poly(ethene) used as plastic  form bromoethane to add to the petrol to remove lead  form ethanol which used as a solvent  form ethane-1,2-diol to act as anti freeze. (iv) With the aid of an equation, define standard enthalpy change of combustion of kerosene. [1] C14H30(l) + 43/2 O2 (g) → 14 CO2 (g) + 15 H2O (l) (b) Fill in the boxes with the appropriate structures and state the reagents and conditions of steps I and II.

[5] (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 cm3 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 the ethanoic acid. [1] CH3COOH + NaOH CH3COO−Na+ + H2O CaVa =CbVb Ca(20) =3.0(12.50)

Ca = 1.875 mol dm-3 (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] Heat release = mc∆T = 30x 4.2 x 12.5 = 1575J Amount of ethanoic acid = 1.875 x 20/1000 = 0.0375 mol Amount of NaOH = 3.0 x 10/1000 = 0.03 mol ∆Hn = - 1575/0.03 = -52,500 Jmol-1 = -52.5 kJmol-1 (iii) If HCl replaces ethanoic acid in the above experiment, w ould the temperature rise be higher or lower? Explain your answer. [2] The temperature rise would be higher. HCl is a strong acid , it could completely dissociate into H + ion, so no heat is required to dissociate the H + ion from the acid hence the te mperature rise would be higher. [Total: 14 marks] 2 Chlorine dioxide, ClO2, is used for b leaching 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] H2O2 O2 + 2H+ + 2 e (ii) Hence, write an equation for the overall reaction. [1] ClO3 −+ 2H+ + e ClO2 + H2O H2O2 O2 + 2H+ + 2 e Overall equation: 2 ClO3 − + H2O2 + 2H+ 2 ClO2 + 2 H2O + O2 (iii) Use oxidation numbers to show which species in your equation is oxidised and

which is reduced. [1] Cl in ClO3 ¯ has been reduced from +5 ON to +4 in ClO2 O in H2O2 has been oxidised from -1 to 0 in O2 (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] Disproportionation reaction (ii) Write the electronic configuration of K+. [1] K+ : 1s22s22p63s23p6 (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] ClO2 is bent, it has 2 bond pairs, 1lone pair and 1 electron around the central Cl atom, l one-pair-lone-pair repulsion is greater than lone pair - lone e lectron repulsion, hence, the bond angle would be larger than 104.5 0 (H2O), but less than 1070(NH3 -3bp + 1 lp). Accept value between 105-1060 (ii) Chlorine dioxide is highly soluble even in cold water. Explain your answer with reference to structure and bonding.

[2] ClO2 is a polar simple covalent molecule , the hydrogen bonding between ClO2 and water release sufficient energy to overcome the p ermanent dipole- permanent dipole interactions between the C lO2 molecules and the hydrogen bonding between water molecules; hence it is soluble in cold water. [Total: 10 marks] 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] Carbonic acid is a weak acid as [H+] is not equal to [carbonic acid]. OR

Carbonic acid did not fully dissociate to give H+ ions in aqueous solution. (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] HCO3 - + H+ H2CO3 (iii) Explain, with reference to the equilibrium reactions, why a reduction in respiratory activity to remove CO2 will result in acidosis. [2] When CO2 is not removed, the position of equilibrium in r eaction 1 will shift to the right to remove excess CO2, producing more carbonic acid. Therefore the position of equilibrium in reaction 2 will shift to the right to remove exce