SIR MIELANGEL REFAL SIXT FRM Annual Exam 2015 Subject: hemistry ADV 2 nd Time: 3 hours ANSWER ANY 6 QUESTINS. All questions carry equal marks. You are reminded of the importance of clear presentation in your answers and of the use of good English. Important Information: R.A.M. of = 12; = 1; = 16; Ag = 108; l = 35.5; r = 52. 1. This question is about acids, bases and salts. (a) A volume of 200 cm 3 of 0.0842 mol dm 3 l is mixed with 200 cm 3 of 0.0421 mol dm 3 Ba() 2. The initial temperature of the l and Ba() 2 solutions is 20.48. alculate the enthalpy of neutralisation of this reaction if the final temperature of the solution is 21.057. Assume the dilute solutions to have the same specific heat capacity as water, 4.18 J cm 3 K 1. (5 marks) (b) Explain why the enthalpy of neutralisation of propanoic acid by sodium hydroxide solution is less than the enthalpy of neutralisation that is calculated in (a). (c) The first step in the esterification of propanoic acid by propan-1-ol in the presence of a small quantity of sulfuric(vi) acid is the reaction: 3 2 + 2 S 4 3 2 2 + + S 4 Identify the two acid-base conjugate pairs. (d) Propanoic acid is described as a monobasic acid. State what is meant by the term in italics. (e) The pk a of propanoic acid, 3 2, is 4.87. (i) alculate the p of a 0.100 mol dm 3 solution of propanoic acid at 298K. (ii) 25.0 cm 3 of 0.100 mol dm 3 propanoic acid solution is titrated against a 0.100 mol dm 3 sodium hydroxide solution. alculate the p of the solution in the flask exactly at half neutralisation. (iii) n your foolscap, sketch a p-volume of alkali titration curve for the titration between propanoic acid and sodium hydroxide. Ensure that your sketch shows a correct estimate of the p of the solution in the flask at equivalence point. Page 1 of 7
(iv) State the name of a suitable indicator for the titration between propanoic acid and aqueous sodium hydroxide. Write down a one-sentence statement that explains your choice. (v) Describe the colour change that is expected to be seen in the flask up till the equivalence point. 2. This question is about the chemistry of oxygen and sulfur. (a) List the names of the two most common allotropes of elemental sulfur and elemental oxygen, and explain why the allotropes of oxygen can be distinguished by chemical means while the allotropes of sulfur cannot. (b) Write balanced equations, including the essential conditions, for two separate reactions by which (i) 2 S (g) and (ii) Na 2 S 2 3 (aq) may be prepared in the laboratory. (c) Describe, in a very brief way, how one may test for the presence of hydrogen sulfide gas and of sulfur dioxide gas. Your answer must include a description of how the test must be carried out and a description of what is observed to happen when each of the gases is present. (d) Explain why 2 is a liquid at room temperature while 2 S is a gas. (e) ne of the environmental problems that industrialised areas must keep under control is the emission of gaseous pollutants as N 2 and S 2. Explain, by including balanced equations in your answer, how such oxides may be harmful to the environment. (f) S 2 (g) and 2 (g) in the molar ratio of 2:1 were mixed at 600K and at constant pressure of 9 atmospheres in the presence of a catalyst. At equilibrium, one third of the S 2 (g) had been converted to S 3 (g) by the following reaction: 2S 3 (g) 2S 2 (g) + 2 (g) Δ = + 196 kj mol 1 (i) alculate K p at 600 K for the reaction. (ii) Describe the effect of an increase in pressure on the position of equilibrium. (iii) Describe the effect of a decrease in temperature on the position of equilibrium. (g) Elemental oxygen forms various compounds when it reacts with group 1 metals. Draw dot-cross diagrams to show the structure of the anion present in the oxides Na 2 and K 2 and explain why the compound Li 2 2 does not exist. (h) Explain why Ba 2 is more suitable than Na 2 2 to produce an aqueous solution of 2 2 by treating the peroxide with dilute sulfuric(vi) acid. (i) State the redox property that 2 2 (aq) shows in each of the following reactions: Reaction 1: 2Fe 2+ (aq) + 2 + (aq) + 2 2 (aq) 2Fe 3+ (aq) + 2 2 (l) Reaction 2: 2Mn 4 (aq) + 5 2 2 (aq) + 6 + (aq) 2Mn 2+ (aq) + 5 2 (g) + 8 2 (l) Page 2 of 7
3. This question is about organic chemistry. onsider the following reaction scheme starting from hexa-2, 4-dienoic acid (sorbic acid): sorbic acid + E catalyst F + A l 4 (l) B dilute Na (aq) warm G J Na (s) + K Br K (a) Identify two functional groups in the sorbic acid molecule. 1-hexoxyhexane (b) Describe one chemical test (excluding the use of Br 2 ) for each functional group listed in question (a). (c) Identify the inorganic reagents A, E, F, G and J by listing their chemical formula. Write the additional essential conditions that are required by the inorganic reagent to react, next to the formula, if there is any. (5 marks) (d) Draw the displayed structural formulae of the organic compounds B,, and K. (5 marks) (e) omment on the water solubility of compound compared with that of the compound B. Give reasons for your answer. (f) Write down the systematic names of compounds and K. (g) Explain fully how 1-hexoxyhexane can be made directly from compound. State the essential experimental conditions and inorganic reagent/s for this reaction and write a balanced chemical equation. A diagram of the apparatus is not required. Page 3 of 7
(h) Describe one simple chemical test, together with the expected results, which can be used to distinguish between 3 ( 2 ) 4 and 3 2 2 2 3. Include balanced chemical equations for the reactions that occur. 4. This question is about chemical reaction kinetics. (a) ydrogen peroxide decomposes in aqueous solution according to the equation: 2 2 2 (l) 2 2 (l) + 2 (g) (i) Explain briefly NE experimental method which may be used to calculate the initial rate of this reaction. (ii) State TW practical changes which could be made to the reaction conditions used to increase the initial rate of this reaction. (b) The kinetics of the following chemical reaction was studied at 45 using the initial rate method: A (aq) + B (aq) (l) + D (g) The results of the study are as summarised in the table below: Experiment Initial [A] (mol dm 3 ) Initial [B] (mol dm 3 ) Initial rate at 45 (mol dm 3 s 1 ) 1 1.10 10 3 0.200 2.14 10 6 2 5.05 10 4 0.200 1.07 10 6 3 1.10 10 3 0.100 2.14 10 6 (i) Use the data in the table to determine the order of reaction with respect to both reactants, explaining your answer in each case. (ii) State the rate expression for the reaction. (iii) alculate the value of the rate constant for the reaction and state its units. (iv) alculate the rate of reaction when [A] = 2.00 10 3 mol dm 3 and [B] = 0.300 mol dm 3. (c) onsider the reaction between peroxydisulfate ions and iodide ions, catalysed by aqueous Fe 2+ : S 2 2 8 (aq) + 2I (aq) 2S 2 4 (aq) + I 2 (aq) Δ θ = 283.6 kj mol 1 (i) The catalyst used is described as a homogeneous catalyst. ther catalysts are described as being heterogeneous catalysts. Distinguish between the terms in italics and write down a balanced chemical equation of a reaction that is catalysed by a heterogeneous catalyst. Write the chemical formula of the heterogeneous catalyst that is used. (ii) Write balanced ionic equations that represent a mechanism for the above reaction when it is catalysed by Fe 2+ ions, and explain how the catalytic effect is brought about. Page 4 of 7
(iii) n your foolscap, sketch an energy profile diagram that shows the effect that the use of Fe 2+ (aq) has on the activation energy that peroxydisulfate ions and iodide ions must possess to react. (iv) n your foolscap, sketch on the same set of axes, two graphs to show the distribution of energies of the reactants ions at two different temperatures, T 1 and T 2, where T 2 = T 1 + 10K. Explain briefly how this graph shows that we may expect the rate of the redox reaction to be faster at the higher temperature. 5. This question is about electrochemistry. (a) A half cell made up of o 3+ (aq)/o 2+ (aq) is connected to a half cell made up of Br 2 (aq)/br (aq). The standard electrode potentials of the half cells at 25 are: o 3+ (aq) + e o 2+ (aq) ½ Br 2 (aq) + e Br (aq) E θ = +1.82 V E θ = +1.07 V (i) Draw a well-labelled diagram of the apparatus that may be used to measure the e.m.f. of the cell described above. (ii) Write down the cell diagram for the reaction using the usual cell notation. (iii) In which direction will electrons flow in the external circuit? Give one reason that justifies your answer. (iv) Describe what, if anything, will happen to the colour intensity of the solution of the Br 2 (aq)/br (aq) half cell. (v) Write a balanced equation for the overall reaction. (vi) alculate the e.m.f. of the cell. (vii) State, giving a reason in each case, which of the following: Br, Br 2, o 2+, o 3+ is: (I) the strongest oxidising agent. (II) the strongest reducing agent. (1 ½ marks) (1 ½ marks) (b) The standard electrode potentials of the reaction involved in the first stage of the rusting of iron are: Fe 2+ (aq) + 2e Fe (s) E θ = 0.44V 2 (g) + 2 2 (l) + 4e 4 (aq) E θ = + 0.40V (i) Write an overall equation for the first stage in the rusting of iron. (ii) Briefly explain why magnesium alloys are used to protect the hulls of steel ships and other underwater structures from corrosion. Page 5 of 7
6. This question is about the chemistry of the transition elements. (a) Describe NE property that distinguishes transition elements from d-block elements. Write any relevant electron configurations, in terms of orbitals, to support your answer. (b) Write down the systematic name of the following chemical species: (i) r 2 (S 4 ) 3.12 2 (ii) [rl 2 ( 2 ) 4 ]l (c) The glycinate ion, 2 N 2, formed by deprotonation of glycine, can function as a bidentate ligand with transition metal ions, such as u 2+. Define the term in italics and account for the fact that glycinate shows this property. (d) Write down the chemical formula of chrome alum and state what feature enables it to be described as an alum. (e) An isomer of hydrated aluminium chloride has the empirical formula rl 3.6 2. (i) What is the oxidation state of chromium in this compound? (ii) ne of the isomers of this compound has water molecules only acting as ligands. Draw a displayed structural formula of the complex ion present in this isomer, and state the name of the shape of such complex ion. (iii) Account for the fact that an aqueous solution of this compound has a p of 3. (iv) A 1.0g sample of this isomer reacts with excess silver nitrate(v) solution to produce 1.62g of silver chloride. Explain, by working out any relevant calculations, how this data supports the information about the complex ion given in (e) (ii). (v) Draw the displayed structural formula of another isomer of rl 3.6 2 which has a very low solubility in water and dissolves more in l 4 to produce a solution with negligible electrical conductivity. (f) Explain why chromium(iii) hydroxide dissolves in alkaline hydrogen peroxide to give a yellow solution. Write a balanced ionic equation to support your answer. 7. This question is about Raoult s Law and the laboratory preparation of ethanal. (a) Ethanal and propanal form an ideal mixture. They have the following properties: Substance Molar mass (g mol 1 ) Boiling Point ( ) Saturation vapour pressure at 20 (kpa) Ethanal 44 21 98.7 Propanal 58 48 34.4 For a liquid mixture containing 20.0g of ethanal and 50.0 g of propanal which is in equilibrium with its vapour, calculate: Page 6 of 7
(i) the mole fraction of each component in the liquid phase; (ii) the partial pressure of each component in the vapour phase; (iii) the mole fraction of propanal in the vapour phase. (b) Sketch a well-labelled T-x diagram for a mixture of ethanal and propanal and explain, also by drawing tie-lines, how it shows that this mixture may be separated by fractional distillation. (c) A mixture of ethanol and water exhibits a positive deviation from Raoult s Law. (i) Explain what the term in italics mean by referring to any intermolecular forces found in the pure liquids and in the mixture of these two liquids. (ii) Sketch a well-labelled T-x diagram for this mixture. (d) Ethanal may be prepared by reacting ethanol with a suitable oxidising agent. (i) Write down a balanced redox equation, including state symbols, for the reaction in which ethanol is oxidised to ethanal. (ii) Draw and label the apparatus which should be used to prepare ethanal from ethanol in the laboratory. (iii) Identify a side reaction by which the yield of ethanal may be reduced, and describe an important precaution that must be taken to prevent the loss of ethanal by such reaction. (iv) Write down two balanced chemical equations of two reactions which are used to purify ethanal by chemical purification. 8. This question is about organic chemistry. Discuss each of the following statements, giving the relevant examples, mechanisms and chemical equations wherever appropriate. (a) zone may be used, under suitable conditions, to distinguish between pent-1-ene and pent-2-ene. (b) The Lucas test can be used to distinguish between propan-1-ol and propan-2-ol, but a different reaction has to be used to distinguish between propan-2-ol and pentan-3-ol. (c) The rate of reaction of ( 3 ) 3 I with an aqueous solution of sodium hydroxide does not increase when the concentration of Na(aq) is increased, at constant temperature. (d) Grignard reagents are a source of nucleophiles which can be used to prepare primary, secondary and tertiary alcohols. (e) When ethene is subjected to a source of free radicals at the right temperature and pressure, it forms a non-biodegradable substance having high molecular mass which does not react with Br 2 dissolved in tetrachloromethane. Page 7 of 7