2017 MGS Prelim P2

Page 2 of 24 Methodist Girls’ School Chemistry Paper 2 Sec 4 Preliminary Examination 2017 Section A (70 marks) Answer all the questions in this section in the spaces provided. A1 Carbon is an element with chemical symbol C and atomic number 6. Many substances are made from carbon atoms. (a) Graphene is a new 2-dimensional material made of carbon a toms. Graphene can be described as a ‘one-atom-thick’ layer of graphite. Using your understanding of bonding and structure, predict which of these statements would be true and which would be false. true false Graphene is slippery. Graphene is soluble in organic solvent. Graphene conducts in solid state. Graphene is a gas at room temperature. [2] (b) Carbon disulfide, CS 2, is a compound of carbon used in the m anufacture of fibres and polymers. Draw a ‘dot-and-cross’ diagram using the outer electrons to show the bonding in carbon disulfide. [2]

Page 3 of 24 Methodist Girls’ School Chemistry Paper 2 Sec 4 Preliminary Examination 2017 A2 Ethane reacts with chlorine in the presence of sunlight. The equation for this reaction is: Some bond energies are given in the table. Bond Bond energy in kJ per mole C−C 607 C−H 413 C−Cl 327 Cl−Cl 243 H−Cl 432 (a) Show that the enthalpy change, ΔH, for this reaction is −103 kJ. [2] (b) Draw a labelled energy profile diagram for the reaction of ethane with chlorine, including activation energy and enthalpy change. [2] Energy Progress of reaction

Page 4 of 24 Methodist Girls’ School Chemistry Paper 2 Sec 4 Preliminary Examination 2017 A3 0.03 g of magnesium ribbon reacted with excess dilute hydrochloric acid at room temperature. The volume of gas produced was recorded every 20 seconds. Mg + 2HCl → MgCl2 + H2 The results obtained in the experiment, using 0.03 g of magnesium ribbon and excess dilute hydrochloric acid, are shown as line C on the graph, shown. The experiment was repeated under different conditions and the results obtained plotted as lines A, B, D and E on the graph. (a) State which line, A, B, D or E, was obtained when 0.03 g of magnesium ribbon reacted with excess dilute hydrochloric acid at a temperature below room temperature. [1]

Page 5 of 24 Methodist Girls’ School Chemistry Paper 2 Sec 4 Preliminary Examination 2017 (b) State which line, A, B, D or E, was obtained when 0.015 g of magnesium ribbon reacted with excess dilute hydrochloric acid at room temperature. [1] (c) (i) State which line, A, B, D or E, was obtained when 0.03 g of magnesium ribbon were replaced by 0.03 g of magnesium powder. [1] (ii) Explain your answer based on the shape of the line chosen in comparison to line C. [1] (d) State an d explain, in terms of Collision T heory, the effect of increasing the concentration of the acid on the rate of the reaction between hydrochloric acid and magnesium. [2] (e) Calculate the mass of magnesium ribbon used to react with excess dilute hydrochloric acid at room temperature to produce 20 cm3 of hydrogen gas. [1]

Page 6 of 24 Methodist Girls’ School Chemistry Paper 2 Sec 4 Preliminary Examination 2017 A4 In 2008, many people in Zimbabwe died because of a cholera epidemic caused largely by drinking untreated water. It is possible to stop the spread of diseases like cholera by treating water with chlorine. Chlorine forms chloric(I) acid which kills the harmful bacteria found in the water. (a) A student decides to analyse some swimming pool water to determine the concentration of chlorine it contains. The student takes 250 cm 3 sample of water and treats it with an excess of potassium iodide solution. Equation 1 represents the reaction of chlorine with potassium iodide solution. Equation 1 2I- (aq) + Cl2 (aq) → I2 (aq) + 2Cl-(aq) (i) Equation 2 represents the reaction of sodium thiosulfate solution and iodine. The student titrates the 250 cm3 treated sample with sodium thiosulfate solution, Na2S2O3, to find out how much iodine has formed. Equation 2 2S2O32-(aq) + I2(aq) → S4O62- (aq) + 2I-(aq) The titration requires 12.30 cm 3 of 0. 10 mol/dm3 sodium thiosulfate solution. Calculate the number of moles of thiosulfate ion, S2O32-, used. [1] (ii) Using the answer in (i) and Equation 2, calculate the number of moles of iodine, I2, in the 250 cm3 sample of treated water. [1] (iii) Using the answer in (ii) and Equation 1, calculate the concentration of the chlorine, Cl2, in the original swimming pool water, in mol/dm3. [1]

Page 7 of 24 Methodist Girls’ School Chemistry Paper 2 Sec 4 Preliminary Examination 2017 (b) Chloric(I) acid, HClO, can also be produced by adding water to either solid calcium chlorate(I), which is a salt of chloric(I) acid or chlorine dioxide gas, ClO2. (i) Write the chemical formula of calcium chlorate(I). [1] (ii) State one reason why it may be preferable to use calcium chlorate(I) rather than chlorine dioxide for treating drinking water. [1] (iii) Chlorine dioxide reacts with water in a disproportionation reaction. Disproportionation happens when the oxidation state of the same element both increases and decreases in the reaction. 4ClO2 + 2H2O → HClO + 3HClO3 The oxidation state of chlorine in chlorine dioxide is +4. Explain why the reaction of chlorine dioxide with water is a disproportionation reaction. [2]

Page 8 of 24 Methodist Girls’ School Chemistry Paper 2 Sec 4 Preliminary Examination 2017 A5 Sodium hydroxide is a very useful industri al chemical. The diagram shows a set-up used to produce aqueous sodium hydroxide from concentrated sodium chloride solution. (a) Label on the diagram the anode and the cathode. [1] (b) Identify Y and explain your answer. [2] (c) Explain how aqueous sodium hydroxide is formed as a product. [1] (d) Describe a simple test for the presence of chlorine gas. [1] chlorine Y diluted sodium chloride solution concentrated sodium chloride solution water aqueous sodium hydroxide sodium ions membrane (a) (a) battery

Page 9 of 24 Methodist Girls’ School Chemistry Paper 2 Sec 4 Preliminary Examination 2017 (e) The membrane used in the set-up is porous. A porous membrane has pores of a specific size so that only particles with sizes smaller than the pores can pass through the membrane. The porous membrane is necessary for aqueous sodium hydroxide to be collected as the only product without any contamination from the chloride ions in the set-up. Explain why the porous membrane prevents contamination from chloride ion. [1] (f) If dilute sodium chloride solution is used in the set-up instead of concentrated sodium chloride solution, the results would be different. Describe and explain two differences in the results. [3]

Page 10 of 24 Methodist Girls’ School Chemistry Paper 2 Sec 4 Preliminary Examination 2017 A6 Most vehicles have diesel engines. In recent years, biodiesel is used as an alternative for diesel. Biodiesel is made from a vegetable oil while di esel is obtained from fractional distillation of crude oil. (a) Biodiesel and diesel have different effects on the environment when used as fuels. Suggest the effect of using these two fuels on the percentage of carbon dioxide in the environment. [3] (b) Cars are the most co