SRJC 2008 JC2 Prelim H2 Paper 2 Solutions

1 SERANGOON JUNIOR COLLEGE General Certificate of Education Advanced Level Higher 2 CHEMISTRY 9746/02 Preliminary Examination Paper 2 Solutions 1 Potassium manganate (VII) is an oxidising agent used in redox titrations. A 50.0 cm 3 sample of iron ( II) oxalate, FeC 2O4, was dissolved in water and the solution made up to 250 cm 3. A 25.0 cm 3 portion of this solution was acidified and titrated with 20.80 cm3 of 0.025 mol dm-3 potassium manganate (VII). (a) (i) State the change in oxidation number for manganese. The oxidation number of Mn decreases from +7 in MnO4 - to +2 in Mn2+. (ii) Write an ionic equation for the oxidation of oxalate ion to carbon dioxide. C 2O4 2- Æ 2CO2 + 2e (iii) Write an overall equation for the reaction between potassium manganate (VII) and iron (II) oxalate. 5Fe 2+ + 5C2O4 2- + 3MnO4 - + 24H+ Æ 5Fe3+ + 10CO2 + 3Mn2+ + 12H2O (iv) Calculate the concentration of iron (II) oxalate in the original sample. 5FeC 2O4 ≡ 3MnO4 - Amount of Fe 2C2O4 in 25.0 cm3 portion = 025 . 01000 80 . 20 3 5 × × = 8.667 x 10-4 mol Amount of Fe 2C2O4 in original sample = 410 667 . 80 . 25 250 −× × = 8.667 x 10 -3 mol Original [Fe 2C2O4] = 100050 10 667 . 83−× = 0.173 mol dm-3 (v) State the colour change at t he endpoint of the titration. Purple to yellow or purple to colourless

2 In the above redox titration, carbon dioxid e gas is produced. One of the uses of carbon dioxide is in the manufacture of carbonated drinks. Cola, a carbonated drink, was originally invented by chemist, John Pemberton, to stop headaches and calm nervousness. Cylinders of pressurised carbon dioxi de are used to produce cola. A commercial cola drink was manufactured using such cylinde rs, each with internal volume of 5 dm 3 and contains 2.58 kg of carbon dioxi de. The pressure inside each of the cylinder was found to be 1.0 x 104 kPa at room temperature. A glass of cola is fizzy because carbon dioxide has been dissolved in it under pressure. When 500 cm 3 of commercial cola drink is poured out from a bottle, the carbon dioxide is gradually released as bubbles of gas . On evolution of 1 dm 3 of carbon dioxide, the cola went flat, as the concentration of dissolved carbon dioxide decreases to its saturation level of 1.5 g dm -3 at room temperature. (b) (i) Using the ideal gas equation pV = nRT, calculate the pressure the carbon dioxide would exert inside the cylinder at room temperature. MV mRTp = 3 3 10 5 0 . 2 44 298 314 . 8 10 −× × × × ×58 . = = 2.91 x 107 Pa (ii) Suggest why the pressure you calculated in b(i) differs from the actual value. CO2 does not behave like an ideal gas at high pressure. (iii) Calculate the total mass of car bon dioxide dissolved in the 500 cm 3 bottle of cola under pressure. Mass of CO2 at saturation level = 5 . 11000 500 × = 0.75 g 0 . 440 . 24 1 ×Mass of CO2 evolved = = 1.833 g Total mass dissolved = 0.75 + 1.833 = 2.58 g (iv) Determine the number of 500 cm 3 bottles of cola that could be manufactured using one cylinder of pressurised carbon dioxide.. No. of bottled cola = 583 . 2 10 58 . 23× = 998 [ T o t a l : 1 0 ]

3 2(a) Brine is water saturated or nearly saturated with sodium chloride salt, NaC l, and was historically used to preserve vegetables, fish, and meat. Brine is electrolysed in the chloralkali pr ocess to make sodium hydroxide, chlorine and hydrogen, as well as the hypochlorite and chlorate salts on an industrial scale. In this case, the chloride ions are oxidised to chlorine, while water is reduced to hydrogen gas and hydroxide ions. (i) Write the reactions that occur at the anode and the cathode and hence the overall equation for the electrolysis of brine. Write the reactions that occur at the anode and the cathode and hence the overall equation for the electrolysis of brine. Anode : 2Cl – Æ ClAnode : 2Cl– Æ Cl2 + 2e– Cathode : 2H2O + 2e– Æ H2 + 2OH– Overall : 2NaCl (aq) + 2H2O (l) Æ Cl2 (g) + H2 (g) + 2NaOH(aq) or 2Cl- + 2H2O Æ Cl2 + H2 + 2OH- (ii) During a random sampling it was found that sodium chlorate (I) was detected. Write a chemical equation to illustrate a possible reaction that causes this observation. Cl 2 (g) + 2NaOH (aq) Æ NaClO (aq) + NaCl (aq) + H2O (l) (iii) The volume of chlorine gas collected 3 hours after the chloralkali process was 2700 cm 3. Assuming there is no contamination to the chlorine gas collected and the process efficiency was 80%, Calculate the current required to produce the specify quantity of chlorine gas at room temperature. Amount of Cl2 collected = 24 1000 2700 = 0.1125 mol 2Cl- Æ Cl2 + 2e Amount of Faraday required = 2 x 96500 C 0.1125 = 96500 2 ' × Q Q’ = 21712.5 C Actual charge required = 80 100 x 21712.5= 27140 C Q = I x t 21740 = I x 3 x 3600 Î I = 2.51 A

4 (b) Sedoneural is sodium bromide salt and was widely used as an anticonvulsant and a sedative. When concentrated sulphuric acid is warmed with sodi um bromide, 3 different gases were released. Write equations for the reaction: ite equations for the reaction: NaBr (s) + HNaBr (s) + H2SO4 (aq) Æ NaHSO4 (aq) + HBr (g) 2HBr (g) + H2SO4 (aq) Æ Br2 (g) + 2H2O (l) + SO2 (g) (c) Hydrogen halides are formed from the chemical reaction of hydrogen with one of the halogen elements (fluorine, chlorine, bromine, iodine), which are found in Group VII of the periodic table. Hydrogen halides can be abbr eviated as HX where H represents a hydrogen atom and X represents a halogen (fluorine, chlorine, bromine or iodine). Upon dissolving the hydrogen halides in water, acids are formed. (i) With reference from the Data Booklet , explain and rank the pK a of the following acids: HI, HBr and HCl. Down the group, • Bond length of H-X increases From Data Booklet, H-Cl 431 kJ mol -1 H-Br 366 kJ mol -1 H-I 299 kJ mol-1 • There is an decrease in H-X bond strength • Hence increase in ease of breaking the H-X bond ⇒ H+ (H3O+) can be formed more easily H—X + H 2O → H3O+ + X- • pKa: HI < HBr < HCl (ii) What would you expect to see when hy drobromic acid reacts with aqueous silver nitrate followed by concentrated ammonia? HBr reacts with AgNO 3: Cream ppt is formed On addition of concentrated NH3: Cream ppt dissolved (iii) Explain your observation made in c(ii) when concentrated ammonia was added. Include equations in your answer. When HBr reacts with AgNO3, cream AgBr is formed AgBr (s) Ag+ (aq) + Br- (aq) ------------------ (1) A g + (aq) + 2NH3 (aq) [Ag(NH3)2]+ (aq) ------------- (2) On addition of concentrated NH3, diamminesilver (I) complex is formed. [Ag+] in (1) decreases, by Le Chatelier’s Principle, equilibrium position in (1) will shift right so that solubility of AgBr increases or (and as such AgBr dissolved since ionic product of AgBr < Ksp of AgBr) [Total: 12]

5 3(a) Copper and iron are in the same period with chromium and are essential in all plants and animals. Most mollusks (suc h as mussels and scallops) and some arthropods (such as horseshoe crab and blue crab) use the copper-containing pigment hemocyanin rather than iron-containing haemoglobin for oxygen transport. (i) Comment on values of the first ionisation energy of copper and iron as