11.4. The reactions of chlorine
A subsection of Chemistry, 9701, through 11. Group 17
Listing 10 of 290 questions
Sodium chlorate(, NaCl O, is the active ingredient in commercial bleach. The concentration of chlorate(ions was determined by titration. • 10.0 cm3 of a bleach solution was diluted to 250 cm3 in a volumetric flask using distilled water. • Dilute sulfuric acid and an excess of potassium iodide solution were added to a 25.0 cm3 portion of this solution to liberate iodine. • The resulting solution required 20.80 cm3 of 0.100 mol dm–3 aqueous sodium thiosulfate solution to react with the iodine produced. The titration reactions are shown. Cl O– + 2I– + 2H+ → I2 + Cl – + H2O I2 + 2S2O3 2– → 2I– + S4O6 2– Calculate the concentration, in mol dm–3, of Cl O– ions in the bleach solution. concentration of Cl O– = mol dm–3 An indicator was used in the thiosulfate-iodine titration. Name a suitable indicator for this titration. State the expected colour change you would observe at the end-point in this titration. from to State when in the procedure you would add the indicator. The concentration of chlorate(ions can also be determined by adding an excess of hydrogenperoxide to the sample of bleach and measuring the volume of oxygen gas produced. H2O2 + NaCl O → NaCl + O2 + H2O water oxygen gas 100 cm3 measuring cylinder hydrogen peroxide 5.0 cm3 bleach When an excess of aqueous hydrogen peroxide was added to 5.0 cm3 of a different bleach solution, 82 cm3 of oxygen was produced at room temperature and pressure. Calculate the concentration of Cl O– ions in this bleach solution. concentration of Cl O– = mol dm–3 Trichlorocyanuric acid, C3Cl 3N3O3, acts as a chlorine buffer and disinfectant for swimming pools. It reacts with water to give chloric(acid, HCl O. C3Cl 3N3O3 + 3H2O C3H3N3O3 + 3HCl O Write the expression for Kc for this equilibrium. Kc = In outdoor swimming pools, the HCl O is decomposed by sunlight. The decomposition of HCl O is a redox reaction which forms a gas that relights a glowing splint. Describe and explain the effect of the decomposition of HCl O on the equilibrium in . State the effect on Kc. effect on Kc The decomposition of HCl O is a redox reaction. Suggest an equation for this reaction. The buffer solution in blood is a mixture of carbonic acid, H2CO3, and hydrogencarbonate ions, HCO3 –. Healthy blood has a pH of 7.40. H2CO3 + H2O HCO3 – + H3O+ Ka = 7.94 × 10–7 mol dm–3 Explain how this buffer system acts to control the blood pH. Include equations in your answer. A patient’s blood has a [HCO3 –] : ratio of 9.5 : 1. Calculate the pH of the patient’s blood. pH =
9701_w16_qp_41
THEORY
2016
Paper 4, Variant 1
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Sodium chlorate(, NaCl O, is the active ingredient in commercial bleach. The concentration of chlorate(ions was determined by titration. • 10.0 cm3 of a bleach solution was diluted to 250 cm3 in a volumetric flask using distilled water. • Dilute sulfuric acid and an excess of potassium iodide solution were added to a 25.0 cm3 portion of this solution to liberate iodine. • The resulting solution required 20.80 cm3 of 0.100 mol dm–3 aqueous sodium thiosulfate solution to react with the iodine produced. The titration reactions are shown. Cl O– + 2I– + 2H+ → I2 + Cl – + H2O I2 + 2S2O3 2– → 2I– + S4O6 2– Calculate the concentration, in mol dm–3, of Cl O– ions in the bleach solution. concentration of Cl O– = mol dm–3 An indicator was used in the thiosulfate-iodine titration. Name a suitable indicator for this titration. State the expected colour change you would observe at the end-point in this titration. from to State when in the procedure you would add the indicator. The concentration of chlorate(ions can also be determined by adding an excess of hydrogenperoxide to the sample of bleach and measuring the volume of oxygen gas produced. H2O2 + NaCl O → NaCl + O2 + H2O water oxygen gas 100 cm3 measuring cylinder hydrogen peroxide 5.0 cm3 bleach When an excess of aqueous hydrogen peroxide was added to 5.0 cm3 of a different bleach solution, 82 cm3 of oxygen was produced at room temperature and pressure. Calculate the concentration of Cl O– ions in this bleach solution. concentration of Cl O– = mol dm–3 Trichlorocyanuric acid, C3Cl 3N3O3, acts as a chlorine buffer and disinfectant for swimming pools. It reacts with water to give chloric(acid, HCl O. C3Cl 3N3O3 + 3H2O C3H3N3O3 + 3HCl O Write the expression for Kc for this equilibrium. Kc = In outdoor swimming pools, the HCl O is decomposed by sunlight. The decomposition of HCl O is a redox reaction which forms a gas that relights a glowing splint. Describe and explain the effect of the decomposition of HCl O on the equilibrium in . State the effect on Kc. effect on Kc The decomposition of HCl O is a redox reaction. Suggest an equation for this reaction. The buffer solution in blood is a mixture of carbonic acid, H2CO3, and hydrogencarbonate ions, HCO3 –. Healthy blood has a pH of 7.40. H2CO3 + H2O HCO3 – + H3O+ Ka = 7.94 × 10–7 mol dm–3 Explain how this buffer system acts to control the blood pH. Include equations in your answer. A patient’s blood has a [HCO3 –] : ratio of 9.5 : 1. Calculate the pH of the patient’s blood. pH =
9701_w16_qp_43
THEORY
2016
Paper 4, Variant 3
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The compound chlorine dioxide, Cl O2, can be prepared by the reaction shown. NaCl O2 + 1 2Cl 2 Cl O2 + NaCl Using oxidation numbers, explain why this reaction is a redox reaction. The central atom in the molecule of Cl O2 is chlorine. Draw the ‘dot-and-cross’ diagram for Cl O2. Show outer electrons only. The reaction between Cl O2 and F2 is shown. 2Cl O2 + F2 2Cl O2F The rate of the reaction was measured at various concentrations of the two reactants and the following results were obtained. experiment [Cl O2] / mol dm–3 / mol dm–3 initial rate / mol dm–3 s–1 0.010 0.060 2.20 × 10–3 0.025 0.060 to be calculated to be calculated 0.040 7.04 × 10–3 The rate equation is rate = k [Cl O2]. What is meant by the term order of reaction with respect to a particular reagent? Use the results of experiment 1 to calculate the rate constant, k, for this reaction. Include the units of k. rate constant, k = units Use the data in the table to calculate ● the initial rate in experiment 2, initial rate = mol dm–3 s–1 ● [Cl O2] in experiment 3. [Cl O2] = mol dm–3 What is meant by the term rate-determining step? The equation for the reaction between Cl O2 and F2 is shown. 2Cl O2 + F2 2Cl O2F rate = k [Cl O2] The mechanism for this reaction has two steps. Suggest equations for the two steps of this mechanism, stating which of the two steps is the rate-determining step. step 1 step 2 rate-determining step = By considering the rate equation, explain why the rate increases with increasing temperature.
9701_w17_qp_42
THEORY
2017
Paper 4, Variant 2
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