JJC H2 Chemistry 9746 Prelim P3 Question paper

JURONG JUNIOR COLLEGE PRELIMINARY EXAMINATION 2008 H2 CHEMISTRY PAPER 3 9746 / 3 Monday 18 th August 2008 2 hours INSTRUCTIONS TO CANDIDATES Do not open this question booklet until you are told to do so. Write your name and class in the spaces provided on the cover sheet and writing papers provided. Write in dark blue or black pen. You may use a pencil for any diagrams, graphs, or rough working. Do not use highlighters or correction fluid. Answer any four questions. Begin your answer to each new question on a fresh sheet of writing paper. At the end of the examination, arrange the answers in numerical order and attach the cover sheet on top. INFORMATION FOR CANDIDATES A Data Booklet is provided. Do not write anything on the Data Booklet. You are reminded of the need for good Englis h and clear presentation in your answers. You are advised to show all workings in calculations. The number of marks is giv en in brackets [ ] at the end of each question or part question. ___________________________________________________________________________________ _ This question booklet consists of 11 printed pages, including this page.

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Answer any four questions. 1. Sulfuric acid, H2SO4, is a strong mineral acid wh ich is an important chemical used in the manufacture of phosphate fe rtilisers. It is also used as an oxidising agent to remove rust in the iron and steel-making industry. (a) The manufacture of sulfuric acid in volves the reaction between sulfur dioxide and oxygen. 2SO2(g) + O2(g) = 2SO3(g) ΔH = −197 kJ mol−1 The reaction is carried out at a temperature of 450 °C in the presence of a catalyst. (i) Explain why a temperature of 450 °C is usually employed for the reaction. (ii) When a 2:1 ratio of SO 2 and O2 at a total initial pressure of 3 atm is passed over a catalyst at 450 °C, the percentage conversion of SO2 into SO3 was found to be 98 %. Calculate the equilibrium partial pressures of SO 2 and O 2 and hence find the value for Kp. (iii) State and explain the effect on Kp and the percentage conversion of SO 2 into SO 3 when the volume of the reaction vessel is reduced. [6] (b) SO2, the key reactant for the manufacture of sulfuric acid, is an oxide of a Period 3 element. The table below shows the melting points of SO 2 and oxides of other Period 3 elements. Compound Formula eltM ing point / °C sodiu Na2O 1132 m oxide alum l2O 2980inium oxide A 3 sulfu SO2 −70 r dioxide (i) Briefly relate the melting points of these oxides to their structure and bonding. (ii) Describe the reactions, if any, of each of these oxides with water, stating the approximate pH of any solution formed, and writing a balanced equation for any reaction that takes place. [6] Page 3 of 11

1. (c) Concentrated sulfuric acid is also an important reagent in many organic reactions where it can perform ro les such as an acid, dehydrating agent or a catalyst as illustrated in the following reaction scheme. conc. H2SO4, conc. HNO3 a few drops of conc. H2SO4, CH3COOH, D + E excess conc. H2SO4, heat heat I 1-phenyl-2-methylpropan-2-ol II B + C A CH3 OH CH3 III (i) Give the structural formulae of the compounds A to E. (ii) The first step in reaction II is the generation of an electrophile, NO2 +. Using NO2 + as an example, explain the term electrophile. (iii) With the aid of an equation, expl ain the role of concentrated H2SO4 during the generation of the electrophile, NO 2 +, for reaction II. (iv) When phenol undergoes the same reaction as 1-phenyl-2-methylpropan-2-ol in reaction II, concentrated H2SO4 is not required. Explain why this is so. [8] [Total: 20] Page 4 of 11

2. In 1727, German scientist Johann Heinrich Schulze noticed that the sun had a darkening effect on a flask of solution. He placed another flask of the same solution in a hot and dark oven and notic ed that there is no change. He then concluded that light, and not heat, was re sponsible for this reaction. This light-sensitive compound is silver nitrate. Silver nitrate crystals c an be prepared by reacting silver metal with nitric acid, followed by evaporating the solution. 4Ag(s) + 6HNO3(aq) → 4AgNO3(aq) + NO(g) + NO2(g) + 3H2O(l) (a) (i) Write an equation, with state symbols, to represent the standard enthalpy change of formation of AgNO3(aq). (ii) Using the following data, calculat e the standard enthalpy change for the reaction of silver metal with nitric acid. Species ΔHf , /kJ mol−1 Species ΔHf , /kJ mol−1 Ag+(aq) +106 NO(g) +90 NO3 −(aq) −207 NO2(g) +33 H+(aq) 0 H2O(l) −286 (iii) The enthalpy change of solution of silver nitrate is +22.8 kJ mol−1. Explain fully why silver nitrate dissolves spontaneously in water even though the process is endothermic. [5] (b) In an experiment, a cell containing aqueous silver nitrate is connected in series with another cell containing aqueous XCl3. When a current of 10 A is applied for t minutes, 1.44 g of silver and 0.120 g of metal X were deposited. Calculate the time t and the Ar of X. [3] (c) Aqueous silver nitrate, followed by an excess of aqueous ammonia, can be used to identify halide ions in aqueous solution. (i) State and explain the observations when aqueous silver nitrate is added to aqueous sodium chloride, followed by an excess of aqueous ammonia. (ii) A precipitate is formed when sodium iodide is added to the resulting mixture in part (c)(i). Identify the precipitate and explain the observation. [4] Page 5 of 11

2. (d) Tollens’ reagent is prepared by adding aqueous ammonia to aqueous silver nitrate. It is a test reagent used to identify aldehydes. For example, when benzaldehyde is warm ed with Tollens’ reagent, a silver mirror is formed. Benzaldehyde can undergo a series of reactions as follows: benzaldehyde 4 mol dm-3 CHCH2COCH3 OH NaOHCHO CH=CHCOCH3 CH3COCH3 I II compound Y elimination + C CC O C H 3 OH Br HH HOBr III compound Z IV reducing agent CH2CH2COCH3 V (i) In reaction I , benzaldehyde reacts wit h propanone in a strongly alkaline medium to form compound Y. The first step is the formation of negatively charged carbanions. CH3COCH3 + OH− → −CH2COCH3 + H2O State the type of reaction for reaction I to form compound Y. (ii) Compound Y exists as a pair of isomers. State the type of isomerism involved and draw the tw o isomers which illustrate this isomerism in Y. (iii) By considering the number of bonding and non-bonding electron pairs around oxygen atom, draw the shape of HOBr. Hence, describe the mechanism for reaction III. (iv) Suggest the structural formula for compound Z and state the reagent and condition required for reaction V. [8] [Total: 20] Page 6 of 11

3. (a) Blood plasma is a biological fluid that plays an impor tant role in maintaining pH in the body. In the blood plasma, the equilibrium between carbonic acid, H2CO3(aq), and hydrogencarbonate ion, HCO3 −(aq), buffers pH changes. H2CO3(aq) = H+(aq) + HCO3 −(aq) The Ka value for this equilibrium is 7.90 × 10−7 mol dm−3. At body temperature, t he pH of the arterial blood plasma is 7.40. If the pH falls below this normal value, a condition termed acidosis is produced. If the pH rises above this normal value, the condition is termed alkalosis. The concentration of H2CO3(aq) is controlled by respiration through the lungs. H2CO3(aq) is in equilibrium with dissolved CO2. H2CO3(aq) = CO2(aq) + H2O(l) In the lungs, excess dissolved CO2(aq) is exhaled as CO2(g). CO2(aq) = CO2(g) During heavy exercise, lactic acid is released into the blood and is buffered by the blood plasma. This ev entually leads to an increase in CO2(aq) concentration and stimulates increased breathing. (i) Explain what is meant by a buffer solution. (ii) Write an e