Cambridge A Level Chemistry 9701 — 2021 May/June Paper 2 · Variant 3

1 The elements in Group 17 are known as the halogens. (a) Between the molecules of Group 17 elements van der Waals’ forces exist. (i) State the trend in the relative strength of van der Waals’ forces down Group 17. ....................................................................................................................................... [1] (ii) State the physical state of each of the halogens under room conditions. chlorine ................................................................................................................................ bromine ............................................................................................................................... iodine ................................................................................................................................... [1] (b) A solution of aqueous bromide ions, Br –(aq), is added to separate samples of Cl 2(aq) and I2(aq). Describe what is observed in each reaction. Explain your answer in terms of the relative reactivity of these elements as oxidising agents. observation on addition to Cl 2(aq) ............................................................................................. .................................................................................................................................................... observation on addition to I2(aq) ................................................................................................ .................................................................................................................................................... explanation ................................................................................................................................. .................................................................................................................................................... .................................................................................................................................................... [3] (c) Bleach is made by reacting Cl 2 with cold NaOH(aq). Write an equation for the reaction of Cl 2 with cold NaOH. .............................................................................................................................................. [1] (d) When Cl O–(aq) is added to water, it behaves as a Brønsted-Lowry base. (i) Define the term Brønsted-Lowry base. ....................................................................................................................................... [1] (ii) Write an ionic equation for the reaction between Cl O– and H2O. ....................................................................................................................................... [1] (e) The concentration of NaCl O in bleach S is x g dm–3. NaCl O reacts with H2O2(aq) as shown. H2O2(aq) + NaCl O(aq) → H2O(l) + NaCl (aq) + O2(g) A 5.00 cm3 sample of S completely reacts with H2O2(aq). The volume of O2(g) produced is 24.0 cm3 under room conditions. Assume that only the NaCl O in S reacts with H2O2(aq). Calculate x. Show your working. x = .............................. g dm–3 [3] (f) Sodium chlorate(I), NaCl O, oxidises dilute hydrochloric acid to form three products. The products which contain chlorine have chlorine species with oxidation number –1 or 0. No other species changes its oxidation number during the reaction. Use this information to complete the ionic equation. ........Cl O– + ......HCl → ............ + ............ + ............ [2] [Total: 13]

Mark scheme: 1(a)(i) increases 1 1(a)(ii) chlorine gas bromine liquid iodine solid 1 1(b) M1 observation with Cl2(aq) (colourless / pale green to) orange / brown 1 M2 observation with I2(aq) no visible change 1 M3 explanation chlorine is a stronger oxidising agent (than bromine) AND iodine is a weaker oxidising agent 1 1(c) Cl2 + 2NaOH → NaCl + NaClO + H2O 1 1(d)(i) proton / H+ acceptor 1 1(d)(ii) ClO– + H2O → HClO + OH– 1 1(e) M1 Use volume O2 to express / find no mol O2 produced 24 / 24 000 mol O2 produced = 1 × 10–3 (mol) 1 M2 Use 1:1 ratio to deduce no mol NaClO in 1000 cm3 sample M1 × 1000 / 5 = 0.2 (mol dm–3) 1 M3 Use amount of NaClO and its molecular mass (74.5) to find the mass of NaClO added –24 / 24 000 × 1000 / 5 × 74.5 = 14.9 (g dm–3) 1 1(f) M1 products contain Cl– AND Cl2 1 M2 correct balanced equation ClO– + 2HCl → Cl– + H2O + Cl2 1

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2 Methanol, CH3OH, is soluble in water because it forms hydrogen bonds with water molecules. (a) Draw a fully labelled diagram to show how a hydrogen bond forms between a water molecule and a methanol molecule. [3] (b) Methanol has a melting point of –97.6 °C and a boiling point of 64.7 °C. A sample of pure liquid methanol is added to a flask and then sealed. The sealed flask is left for several days at constant temperature. The vapour pressure is then measured as 17 kPa. (i) Describe what is meant by the term vapour pressure of methanol. ............................................................................................................................................. ............................................................................................................................................. ....................................................................................................................................... [2] (ii) Explain why some of the liquid becomes a vapour. ............................................................................................................................................. ............................................................................................................................................. ....................................................................................................................................... [1] (iii) Suggest and explain why the vapour pressure of water at room temperature is lower than the vapour pressure of methanol at room temperature. Refer to the correct intermolecular forces in your answer. ............................................................................................................................................. ............................................................................................................................................. ....................................................................................................................................... [2] (c) Methanol is made by reacting carbon monoxide with hydrogen. CO(g) + 2H2(g) CH3OH(g) Carbon monoxide and hydrogen react at 1.0 × 107 Pa and 200 °C. Eventually the reaction mixture reaches dynamic equilibrium. The table shows the amounts of each species present in the mixture. CO(g) H2(g) CH3OH(g) initial amount / mol 1.0 2.0 0 equilibrium amount / mol 0.030 0.060 0.97 (i) Explain what is meant by dynamic equilibrium. ............................................................................................................................................. ............................................................................................................................................. ....................................................................................................................................... [2] (ii) Calculate the partial pressure of methanol vapour at equilibrium under these conditions. Show your working. ........................................................ Pa [2] (iii) Write an expression for the equilibrium constant, Kp, for this reaction. State the units in your answer. Kp = units = .............................. [2] [Total: 14]

Mark scheme: 2(a) M1 H-bond between an O and an H in OH groups in water and methanol molecules 1 M2 minimum three partial charges (in sequence) over one water and one methanol molecule, i.e.: 1 M3 either δ–O—Hδ+ δ–O or Hδ+ δ–O—Hδ+ lone pair of electrons on O of H-bond, in line with H-bond 1 2(b)(i) M1 (methanol) gas (particles / molecules) in equilibrium with liquid 1 M2 (methanol) gas (particles) exert a pressure (on the walls of a container / on the surface of the liquid) 1 2(b)(ii) (liquid) particles (at the surface) have enough energy (to overcome attractive forces / evaporate / to become a vapour) 1 2(b)(iii) M1 (liquid) H2O molecules are held by stronger hydrogen bonding OR it takes more energy to break the hydrogen bonds between water molecules (in the liquid state) OR each water molecule forms two hydrogen bonds (whereas methanol can (only) form one per molecule 1 M2 fewer H2O liquid molecules (able to) escape / become gaseous ora 1 2(c)(i) M1 rates of forward and reverse / backward reactions are equal 1 M2 closed / sealed system / container OR no change in measurable properties / no change in macroscopic properties 1 2(c)(ii) M1 mol fraction = 0.97 / (0.030 + 0.060 + 0.97) = 0.97 / 1.06 = (0.9151) 1 M2 M1 × 1.0 × 107 = 9.2 × 106 (Pa)  1 2(c)(iii) expression for Kp M1 Kp = p(CH3OH) / p(CO) × p(H2)2 1 M2 units = Pa–2 1

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3 Separate samples of R, S, T and U are added to cold water. The identity of each sample is unknown. However, each sample is known to be pure and can only be one of Ba(OH)2, NaCl, P4O10 or SiCl 4. (a) (i) Use the observations in the table to identify each sample as one of Ba(OH)2, NaCl, P4O10 and SiCl 4. Write your answers in the table. state at room observations on addition identity of sample temperature of sample to water alkaline, colourless solution is produced, R solid some white solid remains white solid disappears, S solid solution is neutral misty fumes produced, T liquid white solid is made in vigorous reaction acidic, colourless solution produced in U solid vigorous reaction [4] (ii) Identify the formula of the white solid made when sample T reacts with water. ....................................................................................................................................... [1] (iii) Name the solution formed when sample U reacts with water. ....................................................................................................................................... [1] (b) Magnesium oxide and aluminium oxide have properties typical of ceramic materials. (i) Name one physical property typical of ceramic materials. ....................................................................................................................................... [1] (ii) Give the formula of another Period 3 oxide which behaves as a ceramic material. ....................................................................................................................................... [1] (c) Tungsten oxide, WxOy, is used to give colour to ceramic materials. A sample of WxOy contains 79.29% tungsten by mass. Calculate the empirical formula of WxOy. Show your working. empirical formula = ............................................... [3] [Total: 11]

Mark scheme: 3(a)(i) state at room temp observations on add’n of sample to water identity of sample R solid alkaline, colourless solution is made but some white solid remains M1 Ba(OH)2 OR barium hydroxide S solid white solid disappears, solution is neutral M2 NaCl OR sodium chloride T liquid misty fumes, white solid is made in vigorous reaction M3 SiCl4 OR silicon(IV) chloride U solid acidic, colourless solution is made in vigorous reaction M4 P4O10 OR phosphorus(V) oxide 4 3(a)(ii) SiO2 1 3(a)(iii) phosphoric acid / phosphoric(V)acid 1 3(b)(i) any one physical property typical of ceramic materials from: • strong / retain strength (over certain temperatures / conditions) • non-conductors of electricity / electrical insulators • high melting points 1 3(b)(ii) SiO2 1 Question Answer Marks 3(c) % of O M1 100 – 79.29 (= 20.71) 1 M2 express W and O as mol by / 183.8 and / 16 respectively 1 M3 divide each by smallest number to give empirical formula WO3 1

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4 (a) 1,3-dichloropropan-2-ol can be made by reacting M. M 1,3-dichloropropan-2-ol Cl Cl O HO Cl Cl (i) Give the systematic name of M. ....................................................................................................................................... [1] (ii) Name the functional group present in M that changes during this reaction. ....................................................................................................................................... [1] (iii) State a suitable reagent for this reaction. ....................................................................................................................................... [1] (b) Separate samples of 1,3-dichloropropan-2-ol and 3-chloropropane-1,2-diol are heated with excess acidified Cr2O72– until there is no further reaction. In each reaction, a different organic product, Q or R, is made. 1,3-dichloropropan-2-ol 3-chloropropane-1,2-diol Cl Cl Q R HO HO Cl OH Q and R are tested separately with 2,4-dinitrophenylhydrazine solution, 2,4-DNPH, and sodium carbonate solution, Na2CO3(aq). Complete the table to give any relevant observations. If no reaction occurs, write ‘no visible change’. reagent observation with Q observation with R 2,4-DNPH Na2CO3(aq) [4] (c) Citric acid can be made from M in a four-step reaction. M Cl Cl Cl O N O step 1 step 2 OH HO HO Cl Cl Cl step 3 citric acid O O N O HO OH O step 4 OH OH OH HO N Complete the table for each step of the reaction sequence to identify: ● the reagents and conditions required ● the type of reaction. step reagent and conditions type of reaction 1 2 dilute sulfuric acid 3 4 dilute sulfuric acid [5] [Total: 12]

Mark scheme: 4(a)(i) 1,3-dichloropropan(-2-)one 1 4(a)(ii) carbonyl / ketone 1 4(a)(iii) NaBH4 OR LiAlH4 1 4(b) reagent observation with Q observation with R 2,4-DNPH M1 red / orange / yellow AND precipitate / solid / crystals M2 red / orange / yellow AND precipitate / solid / crystals Na2CO3(aq) M3 no visible change M4 fizz / effervescence 4 4(c) step reagent and conditions type of rxn 1 M1 HCN and (trace) KCN / NaCN M3 Addition 2 dilute sulfuric acid Hydrolysis 3 M2 KCN/NaCN in ethanol / alcohol M4 Substitution 4 dilute sulfuric acid Hydrolysis M5 type of reaction for step 2 AND step 4 5

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5 Compound X contains the same functional groups as citric acid. citric acid O O HO OH O OH OH The table describes some of the similarities and differences between citric acid and compound X. citric acid X chiral centre no yes reaction with Na fizzing fizzing reaction with H+ / Cr2O72– remains orange orange to green (a) Complete the equation to show the reaction of excess sodium with citric acid. Show the skeletal structure of the product. O O HO OH O + ......Na OH OH [3] (b) (i) Use the information in the table to deduce the skeletal formula of X, C3H6O3. Draw the skeletal formula of X in the box. Label the chiral centre of compound X with an asterisk (*). [2] (ii) Explain why compound X reacts with acidified Cr2O72– but citric acid does not. ............................................................................................................................................. ............................................................................................................................................. ....................................................................................................................................... [2] (c) Compound X is one of a pair of stereoisomers. Stereoisomerism occurs when a molecule has at least one of two key features. State the two key features that give rise to stereoisomerism. 1 ................................................................................................................................................. .................................................................................................................................................... 2 ................................................................................................................................................. .................................................................................................................................................... [2] (d) A structural isomer of compound X does not fizz when added to sodium. Explain what is meant by structural isomer. .................................................................................................................................................... .............................................................................................................................................. [1] [Total: 10]

Mark scheme: 5(a) M1all 3 COOH form COONa / CO2Na / COO–Na+ 1 M2 the OH group forms CONa / CO–Na+ 1 M3 equation describes H2 as a product and equation balanced correctly +4Na + 2H2  OR C(CH2COOH)2(COOH)(OH) + 4Na  C(CH2COONa)2(COONa)(ONa) + 2H2  1 5(b)(i) M1 skeletal formula of 2-hydroxypropanoic acid / similar chiral isomer that meet the criteria in table 1 M2 chiral carbon marked with * 1 5(b)(ii) M1 citric acid has a tertiary alcohol group so is not oxidised 1 M2 X has a secondary alcohol group which is oxidised (to a ketone) 1 5(c) M1 chiral centre / non superimposable mirror image / has a C attached to 4 different groups 1 M2 C=C where each C is attached to (two) different groups 1 5(d) (molecules / isomers with) the same molecular formula / same number of atoms of each element AND different structural formulae / different structures 1

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