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Class Reg Number Candidate Name ....................................................................... Chemistry H2 9746 Tutor Tutee Revision Exercise 11: Miscellaneous

1 A porcelain boat was weighed. After a sample of the oxide of a metal M, of Ar = 119, was placed in the boat, the boat was reweighed. Then the boat was placed in a reduction tube, and heated while a stream of hydrogen was passed over it. The oxide was reduced to the metal M. The boat was allowed to cool with hydrogen, cooled again and reweighed. The following results were obtained.

Mass of boat = 6.10g Mass of boat + metal oxide = 10.63g

Mass of boat + metal (1) = 9.67g Mass of boat + metal (2) = 9.67g

(a) Explain why hydrogen was passed over the boat while it was cooling. ................................................................................................................................................. .................................................................................................................................................

(b) Explain why the boat + metal was reheated. ................................................................................................................................................. .................................................................................................................................................

(c) Find the empirical formula of the metal oxide.

(d) Write an equation for the reaction of the oxide with hydrogen. .................................................................................................................................................

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2 Ammonium iron (II) sulphate crystals have the following formula: (NH4)2SO4. FeSO4. nH2O. In an experiment to determine n, 8.492g of the salt were dissolved and made up to 250cm3 of solution with distilled water and dilute sulphuric acid. A 25.0cm3 portion of the solution was further acidified and titrated against potassium manganate (VII) solution of concentration 0.0150 mol dm-3. A volume of 22.5cm3 was required. Calculate the value of n.

3 A piece of iron wire of mass 2.225 g was put into a conical flask containing dilute sulphuric acid. The flask was fitted with a bug carrying a Bunsen valve, to allow the hydrogen generated to escape but prevent air from entering. The mixture was warmed to speed up reaction. When all the iron had reacted, the solution was cooled to room temperature and made up to 250 cm3 in a gradulated flask. With all these precautions, iron is converted to Fe2+ions only, and no Fe3+ ions are firmed. 25.0 cm3 of the solution were acidified and titrated against a 0.0185 mol dm-3 solution of potassium dichromate (VI). The volume required was 31.0 cm3. Calculate the percentage of iron in the iron wire.

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4 Chlorine displaces iodine from iodides. The iodine formed can be found by titration with a standard thiosulphate solution. Chlorate (I) solutions are often used as a source of chlorine as they liberate chlorine readily on reaction with acid:

ClO- (aq) + 2H+ (aq) + Cl- (aq) → Cl2 (aq) + H2O (l)

The amount of chlorine available in a domestic bleach which contains sodium chlorate (I) can be found by allowing the bleach to react with an iodide solution to form iodine, and then titrating with thiosulphate solution:

ClO- (aq) + 2H+ (aq) + 2l- (aq) → l2 (aq) + Cl- (aq) + H2O (l)

A domestic bleach in solution is diluted by pipetting 10.0 cm3 and making this volume up to 250 cm3. A 25.0 cm3 portion of the solution is added to an excess of potassium iodide and ethanoic acid and titrated against sodium thiosulphate solution of concentration 0.0950 mol dm-3, using starch as an indicator. The volume required is 21.3 cm3. Calculate the percentage of available chlorine in the bleach.

5 The problem is to find the percentage of iron that had rusted in a sample of rusty iron. A few grams of the rusty iron were completely dissolved in 1.00 ddm3 sulphuric acid. Iron reacted to form Fe2+ ions and rust reacted to form Fe3+ ions. A 25.0 cm3 sample of the solution was titrated with 0.0200 mol dm-3 aqueous potassium manganate (VII). It took 27.2 cm3 for complete reaction. Zinc was added to the remaining acidic solution to reduce Fe3+(aq) to Fe 2+ (aq). A 25.0 cm3 sample of this solution now took 29.0 cm3 of the same potassium manganate (VII) solution for titration.

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6(a) The ammonium ion content of fertilizers can be found by heating the fertilizer with sodium hydroxide and passing the ammonia produced into an excess of hydrochloric acid.

(i) Write an equation for the reaction between ammonium sulphate and sodium hydroxide. .................................................................................................................................................

(ii) 3.00 g of a fertiliser mixture containing ammonium sulphate was made up to 250 cm3 of solution. 25.0 cm3 portions of this were added to an excess of sodium hydroxide solution, and the ammonia produced passed into 50.0 cm3 of 0.100 mol dm-3 hydrochloric acid solution. The residual acid was then titrated with 0.100 mol dm-3 sodium hydroxide solution, 25.4 cm3 being required. Find the percentage by mass of ammonium sulphate in the fertiliser mixture.

(b) Some of the nitrogen content of a fertiliser could be present as nitrate ions, which are not detected by sodium hydroxide solution. Nitrate ions can be reduced to ammonia by the use of aluminium in sodium hydroxide solution.

(i) Complete the half equation below for the reduction of nitrate ions:

NO3- (aq) + 6H2O (l) + ...... e- → NH3 (g) + ......

(ii) The reaction occurs in strongly alkaline solution; suggest a formula for the aluminium-

containing species present at the end of the reaction. ..............................................................................................................................................

(c) In cold aqueous sodium hydroxide ammonium salts produce the following equilibrium:

NH4+ (aq) + OH- (aq) NH3 (aq) + H2O (l)

(i) Identify the two acid-base conjugate pairs in the equilibrium.

.................................................................................................................................................

(ii) Explain the effect of raising the temperature on this eqilibirum. ................................................................................................................................................. .................................................................................................................................................

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(d) Pure liquid ammonia ionises as follows:

NH3 (l) + NH3(l) NH4+ (am) + NH2

- (am) Where (am) represents solutions in liquid ammonia. The Bronsted-Lowry theory of acid-base behaviour applies to solutions in liquid ammonia.

(ii) The salt sodium amide Na+NH2- reacts with ammonium chloride in liquid ammonia in an

acid-base reaction. Write an equation to represent the reaction. .................................................................................................................................................

7 Lemonade contains carbonated water, citric acid, sodium citrate and other ingredients. It has been suggested that the following equilibrium exists in carbonate water.

H2CO3 (aq) + H2O (l) HCO3- (aq) + H3O+ (aq)

(a) Use this equation to explain why carbonic acid, H2CO3, is classified as an acid.

................................................................................................................................................. .................................................................................................................................................

(b)(i) Write an expression for the acid dissociation constant of carbonic acid.

(ii) Calculate the pH of 0.010 mol dm-3 carbonic acid, given that Ka for carbonic acid is 2.0 x 10-4 mol dm-3.

(c) The pKa of citric acid is 3.1. Suggest, with a reason, which is the stronger acid: citric acid or carbonic acid. ................................................................................................................................................. .................................................................................................................................................

(d) What is the name commonly given to a solution of a mixture of a weak acid and the salt of the weak acid? .................................................................................................................................................

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(e) Describe the behaviour of a solution containing citric acid and sodium citrate if small quantities of (i) acid, (ii) alkali are added separately. Use the formula HA to represent citric acid. ................................................................................................................................................. ................................................................................................................................................. .................................................................................................................................................

8 The table below includes some values of standard enthalpies of formation (ΔHfθ).

Substance H2O (l) LiOH (s) Li (s) ΔH /kJ mol-1 -286 -487 0

The standard enthalpy of solution of lithium hydroxide is given below.

LiOH (s) → Li+ (aq) + OH- (aq) ΔHθ = -21 kJ mol-1

(a) State why the standard enthalpy of formation of lithium is quoted as zero.

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(b) Write an equation for the chemical reaction which represents the formation of lithium hydroxide from its elements, in which the enthalpy change if equal to its standard enthalpy of formation. .................................................................................................................................................

(c) Write an equation, including state symbols, for the reaction of lithium with water in which lithium ions are formed. .................................................................................................................................................

(d) Use the data given above to calculate a value for the enthalpy change for the reaction of lithium with water.

(e) State the observed trend in the reactivity of the Group I elements with water, from lithium to caesium. ................................................................................................................................................. ................................................................................................................................................. .................................................................................................................................................

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(f) When caesium reacts with water, the heat energy released (ΔHfθ) is less than that for

lithium reacting with water. State how this fact relates, if at all, to the observed difference in reactivity of lithium and caesium with water. Give a reason for your answer. ................................................................................................................................................. ................................................................................................................................................. ................................................................................................................................................. .................................................................................................................................................

9(a) Some mean bond enthalpy values are shown below.

Bond Mean bond enthalpy/ kJ mol-1 H – H 436 C – H 413 C – C 348 C = C 612

(i) The hydrogenation of 1- ethylcyclohexene occurs according to the equation

H2C

CH2

CH2

CH2

C

CH

CH2CH3 (g) + H2 (g) H2C

CH2

CH2

CH2

CHCH2CH3 (g)

CH2

Use mean bond enthalpy values to determine the standard enthalpy of hydrogenation of this compound to form ethylcyclohexene.

(ii) Deduce the enthalpy change when the hypothetical compound

HC

CH

CH

CH

C

CH

CH2CH3

is fully hydrogenated to form ethylcyclohexene.

(1-ethylcyclohexa-1,3,5- triene)

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(iii) The standard enthalpy change for the hydrogenation of ethylbenzene, C6H5CH2CH3, is -210 kJ mol-1. Which of the two compounds, ethylbenzene or 1-ethylcyclohexa-1,3,5-triene, is the more stable? Explain your answer. ................................................................................................................................................. ................................................................................................................................................. .................................................................................................................................................

(b) At 298 K, the enthalpy of solution of calcium chloride is -123 kJ mol-1 and the enthalpy of lattice formation of this salt is -2255 kJ mol-1. The enthalpy of hydration of the calcium ion is -1650 kJ mol-1.

(i) Write equations using calcium chloride or its ions to illustrate the terms enthalpy of solution, enthalpy of lattice formation and enthalpy of hydration. ................................................................................................................................................. ................................................................................................................................................. .................................................................................................................................................

(ii) Use the data above to determine the enthalpy of hydration of the chloride ion.

10 Benzene is the starting material for making a number of important organic compounds.

(a) It can be used to make cyclohexane which is needed for the manufacture of nylon.

(i) Draw the displayed formula of cyclohexane.

(ii) Name the reagent and catalyst used to make cyclohexane from benzene. .................................................................................................................................................

(iii) Severe conditions of 200(degree) and 30 atmospheres pressure are used. Explain why high pressure is needed when operating at the high temperature required for this reaction. ................................................................................................................................................. ................................................................................................................................................. .................................................................................................................................................

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(b) Benzene can also be used to make dodecylbenzenesulphunic acid.

C12H25C12H25

SO3H

step 1 step 2

fuming H2SO4

(i) Give the reagent and catalyst for Step 1.

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(ii) Both Step 1 and Step 2 involve attack by electrophiles on the benzene ring. Give the formula of the electrophile in Step 2 and explain what is meant by the term electrophile? …………………………………………………………………………………………………………. ………………………………………………………………………………………………………….

(i) Use the graph to predict the percentage conversion at 350°C. .................................................................................................................................................

(iii) State a use of substances like dodecylbenzenesulphonic acid. .................................................................................................................................................

(c) Another use for benzene is to make diazo compounds of using a sequence of reactions.

NO2 NH2 N2+Cl-

Cc. H2SO4

phenylamine benzene diazonium

chloride

D

(i) Name substances C and D.

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(ii) State the type of reaction that occurs when D is converted into phenylamine. ................................................................................................................................................. .................................................................................................................................................

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11 Furan is an organic compound containing only carbon, hydrogen and oxygen. It is used to make solvents and some forms of nylon. Furan consists of 70.6% of carbon and 5.8% of hydrogen, by mass

(a) Showing each stage of your working, calculate the empirical formula of furan.

(b) By experiment, the relative molecular mass of furan was found to be approximately 70. Use this, and your answer to a, to show that its molecular formula is C4H4O.

(c)(i) Write the equation, including state symbols, for the standard enthalpy change for the following reaction.

C4H4O (l) → 4C (g) + 4H (g) + O (g)

C (s) → C (g) ΔHθ = +717 KJ mol-1 ½ H2 (g) → H (g) ΔHθ = +218 kJ mol-1 ½ O2 (g) → O (g) ΔHθ = +248 kJ mol-1

ΔHθ (furan) = -63 kJ mol-1

(d) The structure of furan is shown below.

C C HH

C C HH

O

Use this structure, and bond energies the Data Booklet, to calculate a value for the enthalpy change for the following process.

C4H4O (g) → 4C (g) + 4H (g) + O (g)

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(e) Use your answers to (c)(ii) and (d) to calculate a value for the enthalpy change for the following process.

C4H4O (l) → C4H4O (g)

(v) Give the colour of the bromine after step 4 of the process. .................................................................................................................................................

(d) The hydrogen bromide for this process is made industrially by passing a mixture of hydrogen and bromine vapour over a heated catalyst. Using ideas of collision theory, explain how the use of a catalyst makes the reaction occur more quickly. ................................................................................................................................................. ................................................................................................................................................. ................................................................................................................................................. ................................................................................................................................................. .................................................................................................................................................

(e) International agreements, to reduce the amount of halogenated compounds being produced, concentrated initially on CFCs. The release of bromomethane at the Earth’s surface was not thought to be as serious as the release of CFCs. Explain how CFCs cause the breakdown of the ozone layer and why bromomethane has a smaller effect. ................................................................................................................................................. ................................................................................................................................................. ................................................................................................................................................. ................................................................................................................................................. .................................................................................................................................................

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12 This question is about benzene and its reactions.

(a) Describe the bonding in benzene and explain why benzene reacts less readily than alkeneswith electrophiles. ................................................................................................................................................. ................................................................................................................................................. ................................................................................................................................................. ................................................................................................................................................. ................................................................................................................................................. ................................................................................................................................................. ................................................................................................................................................. ................................................................................................................................................. .................................................................................................................................................

(b) Describe, including a mechanism, the nitration of benzene by electrophilic substitution. ................................................................................................................................................. ................................................................................................................................................. ................................................................................................................................................. ................................................................................................................................................. ................................................................................................................................................. ................................................................................................................................................. ................................................................................................................................................. ................................................................................................................................................. ................................................................................................................................................. ................................................................................................................................................. ................................................................................................................................................. ................................................................................................................................................. .................................................................................................................................................

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