A-level Chemistry (7405/3)

A-level Chemistry (7405/3) Paper 3 Specimen 205 v0.5 Session 2 hours Materials For this paper you must have: the Data Booklet, provided as an insert a ruler a calculator. Instructions Answer all questions. Show all your working. Information The maximum mark for this paper is 90. Please write clearly, in block capitals, to allow character computer recognition. Centre number Candidate number Surname Forename(s) Candidate signature Barcode v0.5 7405/3

2 Section A Answer all questions in this section. Ethanol can be oxidised by acidified potassium dichromate(vi) to ethanoic acid in a two-step process. ethanol ethanal ethanoic acid 0. In order to ensure that the oxidation to ethanoic acid is complete, the reaction is carried out under reflux. Describe what happens when a reaction mixture is refluxed and why it is necessary, in this case, for complete oxidation to ethanoic acid. [3 marks] 0. 2 Write a half-equation for the overall oxidation of ethanol into ethanoic acid. [ mark] Barcode Typesetter code

3 0. 3 The boiling points of the organic compounds in a reaction mixture are shown in Table. Table Compound ethanol ethanal ethanoic acid Boiling point / C 78 2 8 Use these data to describe how you would obtain a sample of ethanal from a mixture of these three compounds. Include in your answer a description of the apparatus you would use and how you would minimise the loss of ethanal. Your description of the apparatus can be either a description in words or a labelled sketch. [5 marks] Question continues on the next page Barcode Typesetter code Turn over

4 0. 4 Use your knowledge of structure and bonding to explain why it is possible to separate ethanal in this way. [2 marks] 0. 5 A student obtained a sample of a liquid using the apparatus in Question.3. Describe how the student could use chemical tests to confirm that the liquid contained ethanal and did not contain ethanoic acid. [5 marks] Barcode Typesetter code

5 2 Ethanol and ethanoic acid react reversibly to form ethyl ethanoate and water according to the equation: CH 3 COOH + CH 3 CH 2 OH CH 3 COOCH 2 CH 3 + H 2 O A mixture of 8.00 0 2 mol of ethanoic acid and.20 0 mol of ethanol is allowed to reach equilibrium at 20 C. The equilibrium mixture is placed in a graduated flask and the volume made up to 250 cm 3 with distilled water. A 0.0 cm 3 sample of this equilibrium mixture is titrated with sodium hydroxide added from a burette. The ethanoic acid in this sample reacts with 3.20 cm 3 of 2.00 0 mol dm 3 sodium hydroxide solution. 0 2. Calculate the value for K c for the reaction of ethanoic acid and ethanol at 20 C. Give your answer to the appropriate number of significant figures. [6 marks] K c = Question 2 continues on the next page Barcode Typesetter code Turn over

6 A student obtained the titration results given in Table 2. Table 2 Rough 2 3 Final burette reading / cm 3 4.60 8.65 2.85 6.80 Initial burette reading / cm 3 0.0 4.65 8.65 2.85 Titre / cm 3 0 2. 2 Complete Table 2. [ mark] 0 2. 3 Calculate the mean titre and justify your choice of titres. Calculation [2 marks] Mean titre = cm 3 Justification 0 2. 4 The ph ranges of three indicators are shown in Table 3. Table 3 Indicator ph range Bromocresol green 3.8 5.4 Bromothymol blue 6.0 7.6 Thymol blue 8.0 9.6 Select from Table 3 a suitable indicator for the titration of ethanoic acid with sodium hydroxide. [ mark] Barcode Typesetter code

7 0 2. 5 The uncertainty in the mean titre for this experiment is ±0.5 cm 3. Calculate the percentage uncertainty in this mean titre. [ mark] Percentage uncertainty = % 0 2. 6 Suggest how, using the same mass of ethanoic acid, the experiment could be improved to reduce the percentage uncertainty. [2 marks] Turn over for the next question Barcode Typesetter code Turn over

8 3 A peptide is hydrolysed to form a solution containing a mixture of amino acids. This mixture is then analysed by silica gel thin-layer chromatography (TLC) using a toxic solvent. The individual amino acids are identified from their R f values. Part of the practical procedure is given below.. Wearing plastic gloves to hold a TLC plate, draw a pencil line.5 cm from the bottom of the plate. 2. Use a capillary tube to apply a very small drop of the solution of amino acids to the mid-point of the pencil line. 3. Allow the spot to dry completely. 4. In the developing tank, add the developing solvent to a depth of not more than cm. 5. Place your TLC plate in the developing tank. 6. Allow the developing solvent to rise up the plate to the top. 7. Remove the plate and quickly mark the position of the solvent front with a pencil. 8. Allow the plate to dry in a fume cupboard. 0 3. Parts of the procedure are in bold text. For each of these parts, consider whether it is essential and justify your answer. [4 marks] Barcode Typesetter code

9 0 3. 2 Outline the steps needed to locate the positions of the amino acids on the TLC plate and to determine their R f values. [4 marks] 0 3. 3 Explain why different amino acids have different R f values. [2 marks] Barcode Typesetter code Turn over

0 4 Ethanedioic acid is a weak acid. Ethanedioic acid acts, initially, as a monoprotic acid. 0 4. Use the concept of electronegativity to justify why the acid strengths of ethanedioic acid and ethanoic acid are different. [6 marks] Barcode Typesetter code

0 4. 2 A buffer solution is made by adding 6.00 0 2 mol of sodium hydroxide to a solution containing.00 0 mol of ethanedioic acid (H 2 C 2 O 4 ). Assume that the sodium hydroxide reacts as shown in the following equation and that in this buffer solution, the ethanedioic acid behaves as a monoprotic acid. H 2 C 2 O 4 (aq) + OH (aq) HC 2 O 4 (aq) + H 2 O(l) The dissociation constant K a for ethanedioic acid is 5.89 0 2 mol dm 3. Calculate a value for the ph of the buffer solution. Give your answer to the appropriate number of significant figures. [5 marks] ph = Question 4 continues on the next page Barcode Typesetter code Turn over

2 0 4. 3 In a titration, the end point was reached when 25.0 cm 3 of an acidified solution containing ethanedioic acid reacted with 20.20 cm 3 of 2.00 0 2 mol dm 3 potassium manganate(vii) solution. Deduce an equation for the reaction that occurs and use it to calculate the original concentration of the ethanedioic acid solution. [4 marks] Equation Calculation Original concentration = mol dm 3 Barcode Typesetter code

3 5 A sample of ethanedioic acid was treated with an excess of an unknown alcohol in the presence of a strong acid catalyst. The products of the reaction were separated and analysed in a time of flight (TOF) mass spectrometer. Two peaks were observed at m/z = 04 and 8. 0 5. Identify the species responsible for the two peaks. [2 marks] 0 5. 2 Outline how the TOF mass spectrometer is able to separate these two species to give two peaks. [4 marks] Barcode Typesetter code Turn over

4 Section B Answer all questions in this section. Only one answer per question is allowed. For each answer completely fill in the circle alongside the appropriate answer. CORRECT METHOD WRONG METHODS If you want to change your answer you must cross out your original answer as shown. If you wish to return to an answer previously crossed out, ring the answer you now wish to select as shown. 0 6 Which change requires the largest amount of energy? A He + (g) He 2+ (g) + e B Li(g) Li + (g) + e C Mg + (g) Mg 2+ (g) + e D N(g) N + (g) + e [ mark] 0 7 A sample of 2.8 g of oxygen gas has a volume of 870 cm 3 at a pressure of 0 kpa. What is the temperature of the gas? The gas constant is R = 8.3 J K mol. [ mark] A B C D 67 K 334 K 668 K 334 000 K Barcode Typesetter code

5 0 8 An ester is hydrolysed as shown by the following equation. RCOOR / + H 2 O RCOOH + R / OH What is the percentage yield of RCOOH when 0.50 g of RCOOH (M r = 00) is obtained from.0 g of RCOOR / (M r = 50)? [ mark] A 33% B 50% C 67% D 75% 0 9 A saturated aqueous solution of magnesium hydroxide contains.7 0 3 g of Mg(OH) 2 in 00 cm 3 of solution. In this solution, the magnesium hydroxide is fully dissociated into ions. What is the concentration of Mg 2+ (aq) ions in this solution? [ mark] A 2.82 0 2 mol dm 3 B 2.0 0 3 mol dm 3 C 2.82 0 3 mol dm 3 D 2.0 0 4 mol dm 3 Turn over for the next question Barcode Typesetter code Turn over

6 0 The rate equation for the hydrogenation of ethene C 2 H 4 (g) + H 2 (g) C 2 H 6 (g) is Rate = k[c 2 H 4 ][H 2 ] At a fixed temperature, the reaction mixture is compressed to triple the original pressure. What is the factor by which the rate of reaction changes? [ mark] A 6 B 9 C 2 D 27 When one mole of ammonia is heated to a given temperature, 50% of the compound dissociates and the following equilibrium is established. NH 3 (g) 2 N 2(g) + 3 2 H 2(g) What is the total number of moles of gas present in this equilibrium mixture? [ mark] A.5 B 2.0 C 2.5 D 3.0 2 Which change would alter the value of the equilibrium constant (K p ) for this reaction? 2SO 2 (g) + O 2 (g) 2SO 3 (g) [ mark] A B C D Increasing the total pressure of the system. Increasing the concentration of sulfur trioxide. Increasing the concentration of sulfur dioxide. Increasing the temperature. Barcode Typesetter code

7 3 What is the ph of a 0.020 mol dm 3 solution of a diprotic acid which is completely dissociated? [ mark] A.00 B.40 C.70 D 4.00 4 The acid dissociation constant, K a, of a weak acid HA has the value 2.56 0 4 mol dm 3 What is the ph of a 4.25 0 3 mol dm -3 solution of HA? [ mark] A 5.96 B 3.59 C 2.98 D 2.37 5 Magnesium reacts with hydrochloric acid according to the following equation. Mg + 2HCl MgCl 2 + H 2 A student calculated the minimum volume of 2.56 mol dm 3 hydrochloric acid required to react with an excess of magnesium to form 5.46 g of magnesium chloride (M r = 95.3). Which of the following uses the correct standard form and the appropriate number of significant figures to give the correct result of the calculation? [ mark] A 4.476 0 2 dm 3 B 4.48 0 2 dm 3 C 4.50 0 2 dm 3 D 44.8 0 3 dm 3 Barcode Typesetter code Turn over

8 6 In which reaction is hydrogen acting as an oxidising agent? A Cl 2 + H 2 2HCl B (CH 3 ) 2 CO + H 2 (CH 3 ) 2 CHOH [ mark] C N 2 + 3H 2 2NH 3 D 2Na + H 2 2NaH 7 In which reaction is the metal oxidised? [ mark] A 2Cu 2+ + 4I 2CuI + I 2 B [Fe(H 2 O) 6 ] 3+ + Cl [Fe(H 2 O) 5 (Cl)] 2+ + H 2 O C [CoCl 4 ] 2 + 6H 2 O [Co(H 2 O) 6 ] 2+ + 4Cl D Mg + S MgS 8 The following cell has an EMF of +0.46 V. Cu Cu 2+ Ag + Ag Which statement is correct about the operation of the cell? [ mark] A B C D Metallic copper is oxidised by Ag + ions. The silver electrode has a negative polarity. The silver electrode gradually dissolves to form Ag + ions. Electrons flow from the silver electrode to the copper electrode via an external circuit. Barcode Typesetter code

9 9 In an experiment to identify a Group 2 metal (X), 0.02 g of X reacts with an excess of aqueous hydrochloric acid according to the following equation. X + 2HCl XCl 2 + H 2 The volume of hydrogen gas given off is 65 cm 3 at 99 kpa pressure and 303 K. The gas constant is R = 8.3 J K mol. Which is X? [ mark] A B C D Barium Calcium Magnesium Strontium 2 0 What forms when a solution of sodium carbonate is added to a solution of gallium(iii) nitrate? [ mark] A B C D A white precipitate of gallium(iii) carbonate. A white precipitate of gallium(iii) hydroxide. A white precipitate of gallium(iii) carbonate and bubbles of carbon dioxide. A white precipitate of gallium(iii) hydroxide and bubbles of carbon dioxide. 2 Which compound gives a colourless solution when an excess of dilute aqueous ammonia is added? [ mark] A MgCl 2 B AgCl C CuCl 2 D AlCl 3 Barcode Typesetter code Turn over

20 2 2 What is the final species produced when an excess of aqueous ammonia is added to aqueous aluminium chloride? [ mark] A [Al(NH 3 ) 6 ] 3+ B [Al(OH) 3 (H 2 O) 3 ] C [Al(OH) 4 (H 2 O) 2 ] D [Al(OH)(H 2 O) 5 ] 2+ 2 3 The following equation represents the oxidation of vanadium(iv) ions by manganate(vii) ions in acid solution. 5V 4+ + MnO 4 + 8H + 5V 5+ + Mn 2+ + 4H 2 O What volume of 0.020 mol dm 3 KMnO 4 solution is required to oxidise completely a solution containing 0.00 mol of vanadium(iv) ions? [ mark] A 0 cm 3 B 25 cm 3 C 50 cm 3 D 00 cm 3 2 4 How many isomers have the molecular formula C 5 H 2? A 2 B 3 C 4 D 5 [ mark] Barcode Typesetter code

2 2 5 Which molecule is not produced when ethane reacts with bromine in the presence of ultraviolet light? [ mark] A C 2 H 4 Br 2 B HBr C H 2 D C 4 H 0 2 6 How many structural isomers have the molecular formula C 4 H 9 Br? A 2 B 3 C 4 D 5 [ mark] 2 7 What is the major product of the reaction between but--ene and DBr? (D is deuterium and represents 2 H) [ mark] A CH 2 DCH 2 CH 2 CH 2 Br B CH 2 DCH 2 CHBrCH 3 C D CH 3 CH 2 CHBrCH 2 D CH 3 CH 2 CHDCH 2 Br 2 8 Why are fluoroalkanes unreactive? [ mark] A B C D Fluorine is highly electronegative. The F ion is very stable. They are polar molecules. The C F bond is very strong. Barcode Typesetter code Turn over

22 2 9 Which alcohol could not be produced by the reduction of an aldehyde or a ketone? [ mark] A B C D 2-methylbutan--ol 2-methylbutan-2-ol 3-methylbutan--ol 3-methylbutan-2-ol 3 0 Which compound forms optically active compounds on reduction? [ mark] A CH 3 CH 2 C(CH 3 )=CHCH 3 B CH 3 CH 2 C(CH 3 )=CH 2 C CH 3 COCH 3 D CH 3 CH 2 COCH 3 3 How many secondary amines have the molecular formula C 4 H N? A 2 B 3 C 4 D 5 [ mark] 3 2 Which compound has the highest boiling point? [ mark] A C 2 H 4 B C 2 H 6 C CH 3 NH 2 D CH 3 F Barcode Typesetter code

23 3 3 Which compound can polymerise by reaction with itself? [ mark] A NH 2 CH 2 CH 2 NH 2 B CH 3 CH 2 CONH 2 C D HOOCCH 2 COOH NH 2 CH 2 COCl 3 4 A drug is designed to simulate one of the following molecules that adsorbs onto the active site of an enzyme. Which molecule requires the design of an optically active drug? [ mark] A H CH COOH OH B CH 3 C COOH O C CH 3 CH COOH OH D CH 2 CH 2 COOH OH Turn over for the next question Barcode Typesetter code Turn over

24 3 5 Which amine has only three peaks in its proton NMR spectrum? [ mark] A B C D Methylamine Trimethylamine Diethylamine Propylamine END OF QUESTIONS Copyright 204 AQA and its licensors. All rights reserved. Barcode Typesetter code

AQA Qualifications A-level Chemistry Paper 3 (7405/3) Mark scheme 7405 Specimen paper Version 0.5

MARK SCHEME A-level Chemistry Specimen paper 3 Section A Question Marking guidance Mark AO Comments 0. A mixture of liquids is heated to boiling point for a prolonged time Vapour is formed which escapes from the liquid mixture, is changed back into liquid and returned to the liquid mixture Any ethanal and ethanol that initially evaporates can then be oxidised AOb AOb AO2g 0.2 CH 3 CH 2 OH + H 2 O CH 3 COOH + 4H + + 4e AO2d 0.3 Mixture heated in a suitable flask / container With still head containing a thermometer AO3 2a AO3 2a A labelled sketch illustrating these points scores the marks Water cooled condenser connected to the still head and suitable cooled collecting vessel AO3 2a Collect sample at the boiling point of ethanal Cooled collection vessel necessary to reduce evaporation of ethanal AO3 2a AO3 2a 0.4 Hydrogen bonding in ethanol and ethanoic acid or no hydrogen bonding in ethanal AOa Intermolecular forces / dipole-dipole are weaker than hydrogen bonding AOa 2 of 0

MARK SCHEME A-level Chemistry Specimen paper 3 0.5 Reagent to confirm the presence of ethanal: Add Tollens reagent / ammoniacal silver nitrate / aqueous silver nitrate followed by drop of aqueous sodium hydroxide, then enough aqueous ammonia to dissolve the precipitate formed AOb OR Add Fehling s solution Warm AOb M2 and M3 can only be awarded if M is given correctly Result with Tollen s reagent: Silver mirror / black precipitate AOb OR Result with Fehling s solution: Red precipitate / orange-red precipitate Reagent to confirm the absence of ethanoic acid Add sodium hydrogencarbonate or sodium carbonate AOb Result; no effervescence observed; hence no acid present AOb M5 can only be awarded if M4 is given correctly OR Reagent; add ethanol and concentrated sulfuric acid and warm Result; no sweet smell / no oily drops on the surface of the liquid, hence no acid present 3 of 0

MARK SCHEME A-level Chemistry Specimen paper 3 Question Marking guidance Mark AO Comments 02. Extended response Stage : Moles of acid at equilibrium Moles of sodium hydroxide in each titration = (3.20 2.00 0 ) / 000 = 6.40 0 4 AO2h Sample = 0 cm 3 so moles of acid in 250 cm 3 of equilibrium mixture = 25 6.40 0 4 =.60 0 2 AO2h M2 can only be scored if = answer to M 25 Stage 2: Moles of ester and water formed Moles of acid reacted = 8.00 0 2.60 0 2 = 6.40 0 2 = moles ester and water formed AO2h M3 is 8.00 0 2 M2 Stage 3: Moles of ethanol at equilibrium Moles of ethanol remaining =.20 0 6.40 0 2 = 5.60 0 2 AO2h M4 is.20 0 M3 Stage 4: Calculation of equilibrium constant K c = [CH 3 COOCH 2 CH 3 ] [H 2 O] / [CH 3 COOH] [CH 3 CH 2 OH] AOb = (6.40 0 2 ) 2 / (.60 0 2 )(5.60 0 2 ) = 4.574 = 4.57 AO2h M6 is M3 2 / M2 M4 Answer must be given to 3 significant figures 4 of 0

MARK SCHEME A-level Chemistry Specimen paper 3 02.2 Rough 2 3 Final burette reading / cm 3 4.60 8.65 2.85 6.80 Initial burette reading / cm 3 0.0 4.65 8.65 2.85 Titre / cm 3 4.50 4.00 4.20 3.95 AOb 02.3 Mean = 4.00 + 3.95 / 2 = 3.98 (cm 3 ) AO3 a Allow 3.975 (cm 3 ) Titres and 3 are concordant AO3 a Allow titre 2 is not concordant 02.4 Thymol blue AOb 02.5 Percentage uncertainty: 0.5/3.98 00 = 3.77% AO2h Allow consequential marking on mean titre from 2.3 02.6 Use a lower concentration of NaOH So that a larger titre is required (reduces percentage uncertainty in titre) AO3 2b AO3 2b 5 of 0

MARK SCHEME A-level Chemistry Specimen paper 3 Question Marking guidance Mark AO Comments 03. Wear plastic gloves: Essential to prevent contamination from the hands to the plate AO3 a Add developing solvent to a depth of not more than cm 3 : Essential if the solvent is too deep it will dissolve the mixture from the plate AO3 a Allow the solvent to rise up the plate to the top: Not essential the Rf value can be calculated if the solvent front does not reach the top of the plate AO3 a Allow the plate to dry in a fume cupboard: Essential the solvent is toxic AO3 a Allow hazardous 03.2 Spray with developing agent or use UV AOb Measure distances from initial pencil line to the spots (x) AO2h Measure distance from initial pencil line to solvent front line (y) AO2h R f value = x / y AOb 03.3 Amino acids have different polarities AOb Therefore, have different retention on the stationary phase or different solubility in the developing solvent AOb 6 of 0

MARK SCHEME A-level Chemistry Specimen paper 3 Question Marking guidance Mark AO Comments 04. This question is marked using levels of response. Refer to the Mark Scheme Instructions for Examiners for guidance on how to mark this question. Level 3 5 6 marks Level 2 3 4 marks All stages are covered and the explanation of each stage is generally correct and virtually complete. Answer is communicated coherently and shows a logical progression from stage and stage 2 to stage 3. Steps in stage 3 must be complete, ordered and include a comparison. All stages are covered but the explanation of each stage may be incomplete or may contain inaccuracies OR two stages are covered and the explanations are generally correct and virtually complete. 6 AO3 a Indicative Chemistry content Stage : difference in structure of the two acids The acids are of the form RCOOH but in ethanoic acid R = CH 3 whilst in ethanedioic acid R = COOH Stage 2: the inductive effect The unionised COOH group contains two very electronegative oxygen atoms therefore has a negative inductive (electron withdrawing) effect The CH 3 group has a positive inductive (electron pushing) effect Level 2 marks Level 0 0 marks Answer is mainly coherent and shows aprogression from Stage and stage 2 to stage 3. Two stages are covered but the explanation of each stage may be incomplete or may contain inaccuracies, OR only one stage is covered but the explanation is generally correct and virtually complete. Answer includes some isolated statements, but these are not presented in a logical order or show confused reasoning. Insufficient correct Chemistry to warrant a mark. Stage 3: how the polarity of OH affects acid strength The O H bond in the ethanedioic acid is more polarised / H becomes more δ+ More dissociation into H + ions Ethanedioic acid is stronger than ethanoic acid 7 of 0

MARK SCHEME A-level Chemistry Specimen paper 3 04.2 Extended response Moles of NaOH = Moles of HOOCCOO formed = 6.00 0 2 AO2h Moles of HOOCCOOH remaining =.00 0 6.00 0 2 = 4.00 0 2 AO2h K a = [H + ][A ]/[HA] [H + ] = K a [HA]/[A ] AO2h [H + ] = 5.89 0 2 (4.00 0 2 /V)/(6.00 0 2 /V) = 3.927 0 2 AO2h ph = log 0 (3.927 0 2 ) =.406 =.4 AOb Answer must be given to this precision 04.3 5H 2 C 2 O 4 + 6H + + 2MnO 4 2Mn 2+ + 0CO 2 + 8H 2 O AO2d OR 5C 2 O 4 2 + 6H + + 2MnO 4 2Mn 2+ + 0CO 2 + 8H 2 O Moles of KMnO 4 = 20.2 2.00 0 2 /000 = 4.04 0 4 AO2h Moles of H 2 C 2 O 4 = 5/2 4.04 0 4 =.0 0 3 AO2h Concentration = moles/volume (in dm 3 ) =.0 0 3 000/25 = 4.04 0 2 (mol dm 3 ) AO2h If : ratio or incorrect ratio used, M2 and M4 can be scored 8 of 0

MARK SCHEME A-level Chemistry Specimen paper 3 Question Marking guidance Mark AO Comments 05. [CH 3 OCOCOOH] + AO3 a Allow names [CH 3 OCOCOOCH 3 ] + AO3 a Do not allow molecular formula 05.2 Positive ions are accelerated by an electric field AOa To a constant kinetic energy AOa The positive ions with m/z of 04 have the same kinetic energy as those with m/z of 8 and move faster AO2e Therefore, ions with m/z of 04 arrive at the detector first AO2e 9 of 0

MARK SCHEME A-level Chemistry Specimen paper 3 Section B In this section, each correct answer is awarded mark. Question Key AO Question Key AO 6 A AO3 b 2 B AOa 7 B AO2f 22 B AOa 8 D AO2d 23 D AO2h 9 D AO2d 24 B AOa 0 B AO2b 25 C AOa A AO2b 26 C AOa 2 D AOb 27 C AO3 a 3 B AO2d 28 D AOa 4 C AO2h 29 B AO3 a 5 B AOb 30 D AO3 a 6 D AO2c 3 B AOa 7 D AO2c 32 C AO3 b 8 A AO3 b 33 D AO2a 9 B AO3 a 34 C AO3 a 20 D AO3 a 35 C AOa