336/01 CHEMISTRY CH6a A.M. MONDAY, 26 June 2006 (1 hour 10 minutes)

Transcription

1 Candidate Name Centre Number Candidate Number WELSH JOINT EDUCATION COMMITTEE General Certificate of Education Advanced CYD-BWYLLGOR ADDYSG CYMRU Tystysgrif Addysg Gyffredinol Uwch 336/01 CHEMISTRY CH6a A.M. MONDAY, 26 June 2006 (1 hour 10 minutes) FOR EXAMINER S USE ONLY Section Question A B 1 2 Mark 3 C 4 ADDITIONAL MATERIALS TOTAL MARK In addition to this examination paper, you will need a: calculator; Data Sheet which contains a Periodic Table supplied by WJEC. Refer to it for any relative atomic masses you require. INSTRUCTIONS TO CANDIDATES JD* Write your name, centre number and candidate number in the spaces at the top of this page. Section A Answer the question in the spaces provided. Section B Answer the question in the spaces provided. Section C Answer both questions in the spaces provided. Candidates are advised to allocate their time appropriately between Section A (10 marks), Section B (15 marks) and Section C (25 marks). INFORMATION FOR CANDIDATES The number of marks is given in brackets at the end of each question or part-question. The maximum mark for this paper is 50. Your answers must be relevant and must make full use of the information given to be awarded full marks for a question. You are reminded that marking will take into account the Quality of Written Communication used in all written answers. Page 12 may be used for rough work. No certificate will be awarded to a candidate detected in any unfair practice during the examination.

2 2 Examiner SECTION A Answer the questions in the spaces provided. 1. (a) Group I metals are highly reactive and can form ionic compounds which are very soluble in water. (i) Write a balanced equation for the reaction of sodium with oxygen. [1] (ii) Explain why ionic compounds of the Group I elements are generally soluble in water. [2]

3 3 Examiner (b) By means of a Born-Haber cycle based on the data below, calculate the enthalpy of formation of potassium chloride. Process Enthalpy change of atomisation of potassium K(s) K(g) Enthalpy change of atomisation of chlorine 1 2 Cl 2 (g) Cl(g) 1st ionisation energy of potassium K(g) K + (g) + e Electron affinity (electron gain enthalpy) of chlorine Cl(g) + e Cl (g) Enthalpy change of lattice formation of potassium chloride K + (g) + Cl (g) KCl(s) H / kj mol 1 φ [4] Turn over.

4 4 Examiner (c) (i) The enthalpy change of lattice formation of rubidium chloride is 675 kj mol 1. The enthalpies of hydration of gaseous Rb + and Cl ions are 301 kj mol 1 and 364 kj mol 1, respectively. Calculate the enthalpy change of solution of rubidium chloride. [2] (ii) Give one reason why the enthalpy change of lattice formation of rubidium chloride is less exothermic than that of potassium chloride. [1] Section A Total [10]

5 5 Examiner SECTION B 2. Read the passage below and then answer the questions (a) to (g) in the spaces provided. Hydrogen Sulphide Hydrogen sulphide (H 2 S) is one of the chemicals that almost everyone has heard of, usually in connection with stink bombs or rotten eggs. In fact, hydrogen sulphide is very poisonous (about 1 part per thousand is fatal) but it rarely poisons anyone, because a concentration of the gas in air of about 1 part per million (1 ppm) is intolerable. 5 Parts per million means how many parts (mass or volume) in every million parts. A solution concentration of 1 ppm is equivalent to one milligram per cubic decimetre (1 mg dm 3 ). If a sample of hydrogen sulphide is needed in a laboratory, it is usually made by reacting iron(ii) sulphide with dilute hydrochloric acid Sulphur lies below oxygen and above selenium in Group VI of the Periodic Table and the hydrogen sulphide molecule has the same shape as a molecule of water. However, the physical states of hydrogen sulphide and water at room temperature and pressure are different. Water is a liquid (boiling temperature 100 C) but hydrogen sulphide is a gas (boiling temperature 61 C). Hydrogen selenide, H 2 Se, is also a gas (boiling temperature 42 C). Hydrogen sulphide can show many different chemical properties. (1) It takes part in redox reactions, e.g., it reacts with chlorine to form hydrogen chloride: H 2 S(g) + Cl 2 (g) 2HCl(g) + S(s) 20 (2) It is a weak acid and is not fully ionised when in aqueous solution. An equilibrium exists in aqueous solution and the equation representing this is H 2 S(aq) H + (aq) + HS (aq) (3) It will form precipitates of insoluble metal sulphides when it is bubbled into solutions of metal ions: 25 Cu 2+ (aq) + H 2 S(g) CuS(s) + 2H + (aq) The formation of black silver sulphide is responsible for the tarnishing of silver-plated spoons when they are used to eat eggs. Next time you get a whiff of bad eggs, think about all the interesting chemistry of hydrogen sulphide before you bolt for the door and some fresh air! End of passage Turn over.

6 6 Examiner (a) Explain why hydrogen sulphide rarely poisons people, despite it being so poisonous (lines 2-4). [1] (b) Calculate the minimum concentration, in mol dm 3, of a fatal dose of hydrogen sulphide (line 3). [2] (c) Write a balanced equation for the formation of hydrogen sulphide from iron(ii) sulphide and hydrochloric acid (lines 8-9). [1]... (d) (i) By considering the atoms in a hydrogen sulphide molecule, state the shape of the molecule (line 11). [1]... (ii) Explain why the molecule has this shape. [2] (e) Explain the differences in boiling temperature between the three molecules, i.e., water, hydrogen sulphide and hydrogen selenide (lines 13-15). [3]

7 7 Examiner (f) Explain, by the use of oxidation states of the reactants and products, why the reaction of hydrogen sulphide with chlorine (line 19) can be classified as a redox reaction. [2] (g) An excess of hydrogen sulphide was bubbled into 25 0 cm 3 of a solution of copper(ii) sulphate of concentration mol dm 3 (line 25). Calculate the mass of copper(ii) sulphide formed, to three significant figures. [3] Section B Total [15] Turn over.

8 8 Examiner SECTION C Answer both questions in the spaces provided. 3. (a) Ethanol is an important industrial chemical and is usually made by fermentation of sugars, e.g. C 6 H 12 O 6 2C 2 H 5 OH + 2CO 2 or by the direct hydration of ethene: C 2 H 4 (g) + H 2 O(g) C 2 H 5 OH(g) H = 46 kj mol 1 (i) State, giving a reason, which process is more environmentally friendly. [1] (ii) State, giving your reasons, the general conditions of temperature and pressure required to give a high equilibrium yield of ethanol in the direct hydration process. [2] (b) Ethanol can be used as an alternative fuel to petrol. Write a balanced equation for the complete combustion of ethanol. [1]... (c) Drinking and driving is a problem in society. All alcoholic drinks contain ethanol and in 1967 the Breathalyser was introduced. Drivers had to blow through a tube containing potassium dichromate(vi) crystals and sulphuric acid on a silica support. If alcohol was present in the breath, the crystals turned colour because the Cr 2 O 7 2 ions were reduced to Cr 3+ ions. (i) State the colour change in the crystals that showed a positive test. [1] (ii) Write the half equation for this reduction of acidified dichromate(vi) ions. [1]

9 9 Examiner (iii) The half equation for the oxidation of ethanol in the Breathalyser is CH 3 CH 2 OH + H 2 O CH 3 COOH + 4H + + 4e - Write a balanced overall equation for the process that takes place in a Breathalyser. [2] (d) Ethanol can be converted into propanoic acid in the laboratory in a three stage synthesis. [HBr](aq) Step 1 C 2 H 5 OH Compound A Step 2 Compound A Compound B H Step 3 Compound B 2 O/H + (aq) C 2 H 5 COOH (i) During step 1, aqueous hydrogen bromide is prepared in situ. Name the chemical to which sodium bromide must be added in this preparation. [1] (ii) State the names of compounds A and B. [2] Compound A... Compound B... (iii) State the reagent(s) and condition(s) required for step 2. [2] Total [13] Turn over.

10 10 Examiner 4. (a) A compound A, C 8 H 16 O 2, on hydrolysis, produced two straight-chained compounds B, C 3 H 6 O 2, and C, C 5 H 12 O. Compound B, on decarboxylation, produced compound D. Compound C did not undergo the triiodomethane (iodoform) reaction but on oxidation, produced a carboxylic acid E. (i) Give the structures of compounds A - E. [5] (ii) I. Give the structure of an isomer of compound C that gives a positive triiodomethane (iodoform) test and shows optical activity. [1] II. State the feature required for a molecule, such as compound C, to show optical activity. [1]......

11 11 Examiner (b) Describe the structure of, and bonding in, benzene and explain why the reaction of benzene with electrophiles is different from that of alkenes with electrophiles. [5] Total [12] Section C Total [25] Turn over.

12 12 Examiner Rough Work

13 CHEMISTRY CH6a Section A 1. (a) (i) 4Na + O 2 2Na 2 O [1] (accept 2Na + O 2 Na 2 O 2 ) (ii) Enthalpy of solution is the sum of the endothermic lattice breaking enthalpy and exothermic enthalpy of hydration of ions. [1] Generally, ionic salts are soluble if the enthalpy of solution is exothermic overall (therefore they are soluble) / hydration enthalpy > lattice enthalpy/ for Group I compounds, there is a small difference between the lattice and hydration enthalpies. [1] (accept δ + of water attracts anion / δ - attracts cation / ions attracted to polar water molecules for 1 mark) (b) Correct cycle [2] ΔH f = [1] ΔH f = 421 kj mol -1 [1] (c) (i) ΔH sol = ΔH lat break + ΔH hyd (or correct cycle) [1] ΔH sol = ( 665) ΔH sol = 10 kj mol -1 [1] (ii) Rb + ion is larger than K + / attractive forces in RbCl are weaker than those in KCl. [1] Total [10] 30

14 Section B 2. (a) Since it is intolerable at 1 ppm you would have to leave the area before the concentration approached a dangerous level. [1] (b) 1 ppt 1 g dm -3 [1] 1 concentration = (accept or 0.03) = mol dm -3 [1] (c) FeS + 2HCl H 2 S + FeCl 2 [1] (d) (i) V-shaped / bent / angular [1] (ignore non-linear) (ii) H 2 S contains two bonding and two lone pairs of electrons. [1] Lone pairs repel more than bonding pairs. [1] Explanation in terms of VSEPR [1] (e) H 2 O contains hydrogen bonds between molecules, H 2 S and H 2 Se contain van der Waals forces between molecules. [1] Hydrogen bonds are stronger, therefore H 2 O has highest boiling temperature. [1] Se has more electrons than S (or larger atom than S), therefore van der Waals forces between H 2 Se molecules are stronger than between H 2 S molecules. [1] (f) Oxidation number of S changes from 2 to 0, therefore oxidised [1] oxidation number of Cl changes from 0 to 1, therefore reduced. [1] (if oxidation numbers given, allow 1 mark; if no oxidation numbers given, 0 marks) (g) Moles Cu 2+ = = [1] Moles CuS = [1] Mass CuS = x 95.6 = g [1] Total [15] 31

15 Section C 3. (a) (i) Hydration of ethene since CO 2 is not given off / fermentation since sugars occur naturally. [1] (ii) Low temperature. As temperature is decreased equilibrium moves in exothermic direction. [1] High pressure. As pressure is increased equilibrium moves towards side with smaller number of gas moles. [1] (b) C 2 H 5 OH + 3O 2 2CO 2 + 3H 2 O [1] (c) (i) Orange to green. [1] (ii) Cr 2 O H + + 6e - 2Cr H 2 O [1] (iii) 2Cr 2 O H + + 3CH 3 CH 2 OH + 3H 2 O 4Cr H 2 O + 3CH 3 COOH (1 mark if equation not balanced properly) [2] (d) (i) Concentrated (½) sulphuric acid (½) [1] (ii) A = Bromoethane [1] B = Propanenitrile [1] (iii) Reflux/heat with ethanolic [1] potassium cyanide [1] Total [13] 32

16 4. (a) (i) A is C 2 H 5 COOC 5 H 12 [1] B is C 2 H 5 COOH [1] C is C 4 H 9 CH 2 OH [1] D is C 2 H 6 [1] E is C 4 H 9 COOH [1] (ii) I CH 3 CH(OH)C 3 H 7 [1] II Compound must contain an asymmetric carbon atom /carbon atom is bonded to four different groups / chiral centre [1] Q. 4(b) is to be assessed for the quality of Written Communication. [L]. Candidates must satisfy the following criteria. Selection of a form and style of writing which is appropriate to this question. Organisation of the relevant information clearly and coherently. Use of legible text with accurate spelling, grammar and punctuation. (b) Cyclic/ring/hexagonal structure, each carbon bonded (covalently) to 2 other carbons and 1 hydrogen. [1] Equal C-C bond lengths, planar structure [1] Explanation of sideways overlap of p-orbitals. [1] Formation of delocalised π electron system. [1] Alkenes have localised double bond and attract electrophiles readily / react by electrophilic addition. [1] In benzene addition would lead to loss of delocalisation energy / stable delocalised system would be disturbed. [1] (appropriate labelled diags can give first three marks) Total [12] Section C Total [25] 33

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