Edexcel Jun 2015 Paper 2 Q21 (with explained solutions) |

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Edexcel Jun 2015 Paper 2 Q21

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Answer ALL the questions. Write your answers in the spaces provided. SECTION C Swimming Pool Chemistry The circulation of water in swimming pools is much slower than that in most natural water courses, but the number of people using a given volume of water is often far greater. If steps are not taken to keep microorganisms and other contaminants under control, the water will become hazardous. Filters are used to remove solid material and chemicals are added to disinfect the water. The most common method of disinfection involves the use of chlorine compounds, but systems using bromine have advantages. These systems depend on their oxidizing properties. With chlorine systems, the key species is the chlorate(I) ion (OCl) which kills bacteria by damaging the structure of their cell walls and disrupting enzyme activity. A simple way of adding chlorate(I) ions to water is by using chlorine. The weak acid, chloric(I) acid (HOCl), is formed and this dissociates producing the chlorate(I) ion. (a) The equation for the reaction of chlorine with water is: Cl2(aq) + H2O(l) (cid:85) HOCl(aq) + HCl(aq) By referring to the relevant oxidation numbers, explain why this is a disproportionation reaction. (3) 2118 *P44881A01824*<br />
(b) (i) Complete the dot and cross diagram for chloric(I) acid (HOCl). Use a dot ((cid:118)) to represent the hydrogen electron, circles (o) to represent the oxygen electrons and crosses () to represent the chlorine electrons. Show the outer electrons only, but include non-bonding electrons. Cl *(ii) Predict the bond angle in chloric(I) acid. Explain your answer fully. Bond angle =(2) (5)*P44881A01924* 19 Turn over<br />
(c) If the concentration of chlorate(I) ions in a swimming pool is too low, bacteria will flourish. If it is too high, swimmers will be harmed. In the United Kingdom, it is recommended that the concentration of chlorine be in the range 12 mg dm3. The total amount of chlorine, Cl2, present can be measured by titration. In such a determination, excess sulfuric acid and potassium iodide were added to a 1.00 dm3 sample of swimming pool water and the resulting solution required 9.65 cm3 of 0.00550 mol dm3 sodium thiosulfate for complete reaction. The equations for the reactions are: Cl2 + 2I (cid:314) I2 + 2Cl I2 + 2S2O3 2 2 (cid:314) 2I + S4O6 (i) Show that the concentration of chlorine, Cl2, in this sample meets the UK recommended requirements. All steps in your calculation must be shown. (3) 20 *P44881A02024*<br />
(ii) Analysing a single sample of swimming pool water in this way is likely to give unreliable results because it is not possible to repeat the titration. Suggest another way in which this method is unreliable. (1)(d) One advantage of the bromine disinfection system is that, while chloric(I) acid decomposes in sunlight forming hydrogen chloride and oxygen, bromic(I) acid (HOBr) is stable. Another advantage is that bromine is less volatile than chlorine. (i) Write an equation for the decomposition of chloric(I) acid in sunlight. State symbols are not required. (ii) Explain why bromine is less volatile than chlorine. (1) (2)*P44881A02124* 21 Turn over<br />
(e) The pH of the swimming pool affects the performance of the disinfectant. The data below show how the concentrations of bromine species vary with pH. pH 6.0 7.0 8.0 8.5 % bromine as HOBr % bromine as OBr 100 98.0 83.0 57.0 0.0 2.0 17.0 43.0 The equation for the reaction of bromic(I) acid in water is HOBr(aq) (cid:85) H+(aq) + OBr(aq) (i) Assume the pH of the swimming pool is neutral. Explain how any changes in the pH of the swimming pool affect the concentration of the bromine species. (3)(ii) The higher the concentration of bromate(I) ions, the more effective the disinfectant. Suggest a disadvantage of too high a pH. (1)(Total for Question 21 = 21 marks) TOTAL FOR SECTION C = 21 MARKS TOTAL FOR PAPER = 80 MARKS 22 *P44881A02224*<br />

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Section C Question Number 21(a) Question Number 21(b)(i) Reject Mark Acceptable Answers Chlorine / Cl2 / same species / element / atom is oxidized and reduced (in the same reaction) (1) Chlorine oxidized from 0 to +1 in HOCl /OCl /chlorate(I) (1) Chlorine reduced from 0 to 1 in HCl / Cl / chloride (1) If oxidized and /or reduced omitted or the wrong way round, max 1 (out of final 2 marks) Mark Acceptable Answers Reject X X X O Cl X X X X Two bonding pairs (1) Five non-bonding electron pairs (1) IGNORE Lines representing bonds ALLOW Bonding pairs on the same horizontal line Different symbols for electrons max 1<br />
Reject Mark Bonds for electron pairs Penalise omission of pairs once only First mark Bond angle = 104.5 ALLOW 102106 (1) Second mark 2 bond pairs and 2 lone pairs (of electrons in valence shell of the oxygen atom) (1) Third & fourth marks (stand alone) (valence) electron pairs at minimum repulsion ALLOW maximum separation / distance apart (1) lone pair repulsion > bond pair repulsion (1) Fifth mark So tetrahedral bond angle reduced ALLOW 109 / 109.5 /109 28 (angle) reduced (1) Acceptable Answers Question Number 21(b)*(ii) No TE on incorrect structure in b(i)<br />
Acceptable Answers Reject Mark Incorrect units 2 = 9.65 x 0.005501000 * Amount of S2O3 (= 5.3075 x 105 mol) (1) Amount of Cl2 ( in 1 dm3) = 0.5 x * = 0.5 x 9.65 x 0.00550 1000 ** (= 2.65375 x 105 mol) (1) Mass of Cl2 (in 1 dm3) = 71 x 1000 x ** = 1.8842 (mg dm3) (1) (so within limits) ALLOW 1.8842x10-3 g dm-3 and so within limits An answer lower than 1 or higher than 2 mg dm3 only scores a TE mark if there is a comment relating to the limits Correct answer with no working scores 1 Ignore SF except 1 SF Note If 0.5 omitted in MP2 and 35.5 used in MP3 then final answer is numerically correct; this scores only MP1 Mark Acceptable Answers Reject Concentration of chlorine might be different in different parts of the pool / at different times OR Sample size small in relation to pool volume IGNORE Just sample size is small References to experimental uncertainty Reject Mark Acceptable Answers 2HOCl2HCl + O2 OR HOClHCl + O2 OR Other multiples ALLOW HClO or H+ + ClO for HOCl Question Number 21c(i) Question Number 21c(ii) Question Number 21d(i)<br />
Acceptable Answers Reject Mark dipole-dipole forces London forces / dispersion forces / induced dipole- induced dipole attractions (ALLOW van der Waals / vdw forces) (1) Stronger because bromine (molecule) has more electrons / electron shells ALLOW greater surface area more for stronger (1) Mark Acceptable Answers Reject Just repeating information from the table All three marks are stand alone Lowering pH increases [H+] OR Increasing pH reduces [H+] ALLOW More/less H+ IGNORE More acidic/alkaline (1) Lowering pH / increased [H+] shifts equilibrium to the left (so [HOBr] increases) (1) Increasing pH / reduced [H+] shifts equilibrium to the right (so [OBr] increases) (1) Explanations must refer to equilibrium, but this may be implied Acceptable Answers Reject Mark Alkaline solutions are irritant (to the eyes) ALLOW caustic / corrosive saponifies / burns skin / chemical burns stings eyes IGNORE Harmful toxic Total for Question 21 = 21 marks Total for Section C = 21 marks Question Number 21d(ii) Question Number 21e(i) Question Number 21e(ii)<br />

Question 18 19 20 21 22 23

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