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Class Reg Number Candidate Name ....................................................................... Chemistry H2 9746 Tutor Tutee Revision Exercise 12: Integrated Questions

1 The conversion of sulphur dioxide to sulphur trioxide is a vital step in the industrial manufacture of sulphuric acid. The equation for this reaction is

2SO2 (g) + O2 (g) 2SO3 (g) ΔH = -197 kJ mol-1 To achieve the conversion, a mixture of sulphur dioxide and air is passed through a catalyst bed at 700 K. The emergent gases are cooled and passed through a second catalyst bed, again at 700 K, to increase the yield of sulphur trioxide. These operations are repeated twice more so that a minimum of 99.5 % conversion is achieved.

(a)(i) Write the full expression for the equilibrium constant, Kc, for the reaction, including its units.

(ii) Why are the gases cooled between successive passes through the catalyst beds? ................................................................................................................................................. .................................................................................................................................................

(iii) Suggest two conditions, other than temperature control, which will help to maximise the amount of sulphur dioxide converted to sulphur trioxide. ................................................................................................................................................. .................................................................................................................................................

(iv) For what reason must the escape to the environment of these oxides of sulphur be rigorously prevented? ................................................................................................................................................. .................................................................................................................................................

(b)(i) Predict whether there will be an increase or a decrease in the entropy of the system during the formation of sulphur trioxide in this reaction. Give a reason for your answer. ................................................................................................................................................. ................................................................................................................................................. .................................................................................................................................................

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(ii) Calculate the entropy change of the surroundings, assuming the reaction occurs at 700 K.

(iii) In the light of your answer above, what can you deduce about the numerical value of the entropy change of the system? Explain your answer. ................................................................................................................................................. .................................................................................................................................................

(c)(i) Calculate the pH of a 0.075 mol dm-3 solution of sulphuric acid, assuming it to be fully ionised.

(ii) A total of 25.0 cm3 of aqueous ammonia was added in small portions from a burette to 10.0 cm3 of 0.075 mol dm-3 sulphuric acid. The pH of the solution was followed as the ammonia was added. Sketch a graph showing how the pH changed assuming that 20.0 cm3 of aqueous ammonia was sufficient to neutralise the fully ionised sulphuric acid.

(iii) Calculate the concentration, in mol dm-3, of the aqueous ammonia.

(iv) Not all indicators are suitable for this titration. Explain why some indicators cannot be used. ................................................................................................................................................. .................................................................................................................................................

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2 This question is regarding elements found in the Periodic Table.

(a)(i) Give balanced equations describing two reactions of calcium metal in which Ca2+ ions are formed. Compare the reactions of zinc metal with the same reagents. ................................................................................................................................................. ................................................................................................................................................. ................................................................................................................................................. .................................................................................................................................................

(ii) State how the thermal stability of hydroxides and carbonates changes down Group II. Describe briefly a simple laboratory experiment which would confirm whether or not a carbonate is thermally stable or unstable. ................................................................................................................................................. ................................................................................................................................................. ................................................................................................................................................. ................................................................................................................................................. .................................................................................................................................................

(b) 14.78 g of a pure sample of carbonate of an element Z, which belongs to either Group I or Group II, was completely decomposed upon heating, producing exactly 4.48 dm3 of carbon dioxide at 0oC and 1 atom pressure. Calculate the number of moles of CO2 produced. Decduce the relative atomic mass of Z. Hence, using the Periodic Table, identify Z. [The molar volume of an ideal gas at 0oC and 1 atm pressure is 22.4 dm3]

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3(a) Using the bond energies given below, calculate the enthalpy change for the direct hydration of propene.

CH3CH=CH2 (g) + H2O (g) → CH3CHOHCH3 (g)

Bond Bond energy / kJ mol-1 C – C C – O C – H O – H C = C

348 360 412 463 612

(b) The reaction in (a) is reversible. Explain, giving your reasoning, the effect upon the equilibrium yield of propan-2-ol by

(i) Increasing the temperature at which equilibrium is attained ................................................................................................................................................. ................................................................................................................................................. .................................................................................................................................................

(ii) Increasing the total pressure at equilibrium ................................................................................................................................................. ................................................................................................................................................. .................................................................................................................................................

(c) For the equilibrium,

CH3CH=CH2 (g) + H2O (g) CH3CHOHCH3 (g) At a certain temperature, the equilibrium partial pressures of propene and steam in the system are 7.5 x 106 Pa respectively, and the total pressure is 2.0 x 107 Pa. Calculate the value of Kp at this temperature, clearly stating its units.

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4 Methanol can be synthesised from carbon monoxide and hydrogen according to the equation as shown below.

CO (g) + 2H2 (g) CH3OH (g) Use the thermodynamic data below to answer the questions that follow.

Substance ΔHfθ / kJ mol-1 Sθ / J K-1 mol-1

CO (g) -110 198 H2 (g) 0 131

CH3OH (g) -201 240

(a) Determine the standard enthalpy change and the standard entropy change for the synthesis of methanol from carbon monoxide and hydrogen.

(b) Explain what is meant by the term feasible reaction and determine the temperature at which the methanol synthesis reaction is no longer feasible. ................................................................................................................................................. ................................................................................................................................................. .................................................................................................................................................

(c) Industrially, methanol can be manufactured from a 2:1 hydrogen : carbon monoxide mixture at high pressure and a temperatue of 300oC in the presence of a finely divided solid catalyst. Under these conditions, the equilibrium yield of methanol is only of the order of 1%, yet these are the conditions chosen for a continuous commercial process in which an overall conversion of 95 % can be achieved.

(i) Explain why the relatively low temperature of 300oC is chosen. ................................................................................................................................................. .................................................................................................................................................

(ii) Why is a catalyst necessary? ................................................................................................................................................. .................................................................................................................................................

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(iii) Explain how a 95 % conversion can be achieved in a reaction which has an equilibrium yield of only 1 %. ................................................................................................................................................. .................................................................................................................................................

5 Myrcene is a naturally occurring oil present in bay leaves. The structure of myrcene is shown below.

(a) State the molecular formula of myrcene. .................................................................................................................................................

(b) Reaction of a 0.100 mol sample of myrcene with hydrogen produced a saturated alkane A.

(i) Explain what is meant by the term saturated alkane. ................................................................................................................................................. .................................................................................................................................................

(ii) Determine the molecular formula of the saturated alkane A;

(iii) Construct a balanced equation for this reaction. .................................................................................................................................................

(iv) Calculate the volume of hydrogen, measured at room temperature and pressure (r.t.p.), that reacted with the sample of myrcene. [1 mole of gas molecules occupy 24.0 dm3 at r.t.p.]

(b) Squalene is a naturally occurring oil present in shark liver oil. A 0.100 mol sample of squalene reacted with 14.4 dm3 of hydrogen, measured at r.t.p., to form a saturated hydrocarbon C30H62.

(i) Calculate how many double bonds there are in each molecule of squalene.

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(ii) Suggest the molecular formula of squalene. .................................................................................................................................................

6 Salicylic acid, shown below, has been used as a painkiller.

(a) Name the functional groups present in salicylic acid. .................................................................................................................................................

(b) Deduce the molecular formula of salicylic acid. .................................................................................................................................................

(c) Show a displayed formula of a likely organic product formed when salicylic acid reacts with

(i) ethanol and concentrated sulphuric acid under reflux;

(ii) bromine;

(iii) aqueous sodium hydroxide.

(d) Salbutamol, shown below, is used in inhalers to relieve asthma.

(i) Salbutamol is a chiral compound. Mark the chiral centre with an asterisk (*) on the structure

above.

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(ii) Salbutamol is reacted with K2Cr2O7/H2SO4 under reflux. Predict the likely organic product of this reaction and draw a dsiplayed formula of this product below.

7 A student prepared benzoic acid, C6H5COOH by hydrolysing methyl benzoate, C6H5COOCH3 using the following method.

• Dissolve 4.0 g of sodium hydroxide in water to make 50 cm3 of an alkaline solution. • Add the aqueous sodium hydroxide to 2.70 g of methyl benzoate in a 100 cm3 flask and set up the apparatus for reflux. • Reflux this mixture for 30 minutes. • Distil the mixture and collect the first 2 cm3 of distillate. • Pour the residue from the flask into a beaker and add dilute sulphuric acid until the solution is acidic. • Filter the crystals obtained and re-crystallise from hot water to obtain the benzoic acid.

The overall equation for this hydrolysis is:

C6H5COOCH3 + H2O → C6H5COOH + CH3OH The student obtained 1.50 g of benzoic acid, C6H5COOH.

(a) Name the functional group that reacts during this hydrolysis. .................................................................................................................................................

(b)(i) Calculate how many moles of methyl benzoate were used.

(ii) What was the concentration, in mol dm-3, of the aqueous sodium hydroxide used.

(iii) Calculate the percentage yield of the C6H5COOH obtained by the student.

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(iv) Suggest why the percentage yield was substantially below 100%. ................................................................................................................................................. .................................................................................................................................................

(c)(i) Why was the residue from the flask acidified before recrystallising? ................................................................................................................................................. ................................................................................................................................................. .................................................................................................................................................

(ii) Why were the crystals recrystallised? ................................................................................................................................................. .................................................................................................................................................

8 This question refers to different aspects of acid/base chemistry:

(a) Hydrochloric acid HCl is classed as a strong acid but it can have both concentrated and dilute solutions. Explain why this is so. ................................................................................................................................................. ................................................................................................................................................. .................................................................................................................................................

(b) Sodium phosphate, Na3PO4, a water-softening agent, can be prepared in the laboratory by neutralising phosphoric acid. A student prepared this compound in the laboratory from 20.0 cm3 of 0.100 mol dm-3

phosphoric acid and 0.250 mol dm-3 sodium hydroxide:

H3PO4(aq) + 3NaOH(aq) → Na3PO4(aq) + 3H2O(l)

(i) Deduce the oxidation state of phosphorus in sodium phosphate, Na3PO4. .................................................................................................................................................

(ii) Calculate the volume of NaOH(aq) that the student would need to use to just neutralise the phosphoric acid using the quantities above.

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(c) Calculate the pH of the NaOH(aq) used in (b) (Kw = 1.00 x 10-14 mol2 dm-6.)

9 Nitrogen oxides such as nitrogen monoxide, NO, and nitrogen dioxide, NO2, are formed unintentionally by man and cause considerable harm to the environment.

(a) The oxidation of nitrogen monoxide in car exhausts may involve the following reaction:

NO(g) + CO(g) + O2(g) → NO2(g) + CO2(g) This reaction was investigated in a series of experiments. The results are shown below in the table below.

(i) For each reactant, deduce the order of reaction. Show your reasoning.

NO ........................................................................................................................................... ................................................................................................................................................. CO ........................................................................................................................................... ................................................................................................................................................. ................................................................................................................................................. O2 ............................................................................................................................................ ................................................................................................................................................. .................................................................................................................................................

(ii) Deduce the rate equation and calculate the rate constant for this reaction.

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(iii) Suggest, with a reason, what would happen to the value of the rate constant, k, as the car’s exhaust gets hotter. ................................................................................................................................................. ................................................................................................................................................. .................................................................................................................................................

(b) State two environmental consequences of nitrogen oxides. ................................................................................................................................................. ................................................................................................................................................. .................................................................................................................................................

(c) Not all nitrogen compounds are harmful: some, such as nitrogen fertilisers, are beneficial to man. A nitrogen fertiliser, D, was analysed in the laboratory and was shown to have the composition by mass Na, 27.1%; N, 16.5%; O, 56.4%. On heating, 3.40 g of D was broken down into sodium nitrite, NaNO2, and oxygen gas. Showing your working, suggest an identity for the fertiliser, D, and calculate the volume of oxygen that was formed. [Under the experimental conditions, 1 mole of gas molecules occupy 24 dm3.]

10 Cans of ‘self-heating’ coffee were available until recently. Inside the can, in separate compartments, were calcium oxide and water. When a button was pressed these reacted together to give enough heat to warm up the coffee.

(a) What term is used to describe a reaction that gives out heat? .................................................................................................................................................

(b) The reaction between calcium oxide, CaO, and excess water forms calcium hydroxide solution. Write a balanced equation for the reaction below. Include state symbols. .................................................................................................................................................

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(c) A group of students set out to determine the enthalpy change of this reaction by placing a known mass of calcium oxide into 250 cm3 of water in an insulated flask and measuring the temperature rise. The group of students recorded the measurements shown in the table below.

Mass of calcium oxide used 10 g Volume of water used 250 cm3

Temperature rise 50oC

Calculate the heat transferred to the water (in kJ) by the reaction of 1.0 mol of CaO(s).

Give your answer to two significant figures. specific heat capacity of water = 4.2 kJ K–1 kg–1; density of water = 1.0 g cm–3

(d) The reaction will produce solid calcium hydroxide if the exact molar ratio of water to calcium oxide is used, as represented by the balanced equation in (b) above.

(i) Suggest one reason why it is very difficult to measure this enthalpy change directly. ................................................................................................................................................. .................................................................................................................................................

(ii) This enthalpy change can, however, be measured indirectly using an enthalpy cycle as shown below.

Explain how the cycle can be used to calculate the enthalpy change ∆H. ................................................................................................................................................. .................................................................................................................................................

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(e) Magnesium oxide is a possible alternative substance to use in the self-heating cans. Use your knowledge of the Periodic Table to suggest why magnesium oxide might be considered a possible alternative to calcium oxide. ................................................................................................................................................. ................................................................................................................................................. ................................................................................................................................................. .................................................................................................................................................

11 Hydrofluorocarbons, HFCs, have replaced CFCs for many of their uses. They are broken down in the troposphere before they have time to reach the stratosphere.

(a)(i) Give the formula of a CFC. .................................................................................................................................................

(ii) CFCs were used as the refrigerant in domestic fridges. The presence of CFCs makes disposing of old fridges difficult. Give one property of CFCs that made them suitable as refrigerants. ................................................................................................................................................. .................................................................................................................................................

(b) CFCs cause depletion of the ozone layer. Describe how they do this. ................................................................................................................................................. ................................................................................................................................................. ................................................................................................................................................. ................................................................................................................................................. ................................................................................................................................................. .................................................................................................................................................

(c) Initially, studies of changes in the Earth’s atmosphere did not reveal the problem of ozone depletion. Explain why the information about ozone depletion was overlooked. ................................................................................................................................................. ................................................................................................................................................. ................................................................................................................................................. .................................................................................................................................................

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(d) Other atmospheric pollutants can contribute to a build-up in tropospheric ozone. For example, hydrocarbons can interfere with the normal reactions for the formation and breakdown of ozone. The reaction for the breakdown of ozone involves naturally occurring NO2 and NO.

NO2 → NO + O NO + O3 → NO2 + O2

O + O2 → O3

(i) Combine two of these equations to show how ozone is broken down. .................................................................................................................................................

(ii) Hydrocarbons lead to reactions in which NO is converted into NO2. Explain how this leads to a build-up of ozone. ................................................................................................................................................. ................................................................................................................................................. .................................................................................................................................................

(iii) Suggest one disadvantage of a build-up of tropospheric ozone. ................................................................................................................................................. .................................................................................................................................................

(e) One example of an HFC is CH2F2. The C–F bond is polar.

(i) Mark partial charges on the C and F atoms in the structure below.

(ii) Explain what determines where the partial charges are placed on this molecule. ................................................................................................................................................. ................................................................................................................................................. .................................................................................................................................................

(iii) Does the whole molecule have a dipole? Explain your answer. ................................................................................................................................................. .................................................................................................................................................

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(f) If molecules of CH2F2 reach the stratosphere, they do not break down to produce F radicals.

(i) Suggest why C–F bonds are not broken in the stratosphere. ................................................................................................................................................. .................................................................................................................................................

(ii) The bond enthalpy of the C–F bond is +467 kJ mol–1. Calculate the minimum energy (in joules) needed to break a single C–F bond. Avogadro constant, NA = 6.02 × 1023 mol–1

12 The substance GHB was originally designed for use in sleeping pills. However, other drug-abuse issues were identified with the substance and its sale was restricted in 2003. GHB stands for hydroxybutyric acid, an old name for the structure shown below.

(a) GHB is a weak acid. Weak acids can be represented as HA.

(i) Write an equation to show how a weak acid HA behaves when dissolved in water. .................................................................................................................................................

(ii) Use ions and molecules from this equation to explain the meaning of the term conjugate base. ................................................................................................................................................. ................................................................................................................................................. .................................................................................................................................................

(iii) Write an expression for the acidity constant Ka of an acid HA.

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(iv) A 0.10 mol dm–3 solution of GHB has a pH of 2.9. Calculate the value of Ka for GHB and give its units.

(v) State one simplifying assumption that you made when carrying out your calculation in (iii). ................................................................................................................................................. .................................................................................................................................................

(b) A mixture of GHB and its sodium salt acts as a buffer solution.

(i) Explain the meaning of the term buffer solution and explain why buffer solutions are found in our bodies. ................................................................................................................................................. ................................................................................................................................................. ................................................................................................................................................. ................................................................................................................................................. ................................................................................................................................................. ................................................................................................................................................. ................................................................................................................................................. .................................................................................................................................................

(ii) Calculate the pH of a buffer solution containing equal amounts of GHB and its sodium salt.

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