2007年第39屆IChO預備試題（英文）實驗部分

The 39th International Chemistry Olympiad Chemistry art science and fun PREPARATORY PROBLEMS Experimental July 15 24 2007 Moscow Russia The 39 th International Chemistry Olympiad Preparatory problems 2 TABLE OF CONTENTS RULES TO BE FOLLOWED IN LABORATORIES 3 LIST of R and S PHRASES 4 Problem 29 TITRIMETRIC DETERMINATION OF FE IN DIFFERENT OXIDATION STATES 6 Problem 30 ASYMMETRIC AUTOCATALYSIS THE NUMERICAL EXPERIMENT 10 Problem 31 OSCILLATING REACTIONS 13 Problem 32 DETERMINATION OF THE ACIDITY CONSTANT OF BROMOCRESOL BLUE 3 3 5 5 TETRABROMO M CRESOLSULFONEPHTHALEIN BCB 15 Problem 33 ACID ORANGE 7 18 Problem 34 DETERMINATION OF MOLECULAR WEIGHT OF A PROTEIN USING GEL FILTRATION 20 RULES TO BE FOLLOWED IN LABORATORIES As mentioned in the Preface we pay great attention to safety of experimental work Below you will find a list of rules to be followed during laboratory exam at IChO 2007 We also hope you will take this information into account while preparing for the Olympiad Students have to bring their own laboratory coats Prior to the exam students will be given Safety instructions in their mother tongue Each student must carefully read the text and then sign When students enter the lab they must familiarize themselves with the locations of emergency exits safety shower fire blanket and eye wash Laboratory coats eye protections and closed shoes must be worn while staying in the laboratory Coats and bags are forbidden in the laboratory Those have to be deposited in the cloakroom Eating drinking or smoking in the laboratory or tasting chemicals are strictly forbidden Pipetting by mouth is strictly forbidden Organizers do their best to avoid harmful chemicals at the exam All potentially dangerous materials if any will be labeled by international symbols Each student is responsible for recognizing these symbols and knowing their meaning Do not dispose of chemicals down the sink Follow all disposal instructions provided by Organizers Do not hesitate to ask your lab instructor if you have got any questions regarding safety issues Nobody can create rules that will cover all situations which may happen in reality We do rely on your common sense and responsibility Good luck during preparations and at the exam The 39 th International Chemistry Olympiad Preparatory problems 4 LIST of R and S PHRASES for the reagents used in Experimental problems R PHRASES R5 Heating may cause an explosion R8 Contact with combustible material may cause fire R9 Explosive when mixed with combustible material R10 Flammable R11 Highly flammable R20 Harmful by inhalation R22 Harmful if swallowed R23 Toxic by inhalation R25 Toxic if swallowed R34 Causes burns R35 Causes severe burns R36 Irritating to eyes R37 Irritating to respiratory system R40 Limited evidence of a carcinogenic effect R43 May cause sensitization by skin contact R50 Very toxic to aquatic organisms R61 May cause harm to the unborn child R20 21 22 Harmful by inhalation in contact with skin and if swallowed R23 24 25 Toxic by inhalation in contact with skin and if swallowed R36 38 Irritating to eyes and skin R36 37 38 Irritating to eyes respiratory system and skin R50 53 Very toxic to aquatic organisms may cause long term adverse effects in the aquatic environment S PHRASES S2 Keep out of the reach of children S7 Keep container tightly closed S16 Keep away from sources of ignition No smoking S17 Keep away from combustible material S22 Do not breathe dust The 39 th International Chemistry Olympiad Preparatory problems 5 S23 Do not breathe gas fumes vapor spray appropriate wording to be specified by the manufacturer S24 Avoid contact with skin S26 In case of contact with eyes rinse immediately with plenty of water and seek medical advice S28 After contact with skin wash immediately with plenty of to be specified by the manufacturer S30 Never add water to this product S35 This material and its container must be disposed of in a safe way S36 Wear suitable protective clothing S37 Wear suitable gloves S38 In case of insufficient ventilation wear suitable respiratory equipment S45 In case of accident or if you feel unwell seek medical advice immediately show the label where possible S60 This material and its container must be disposed of as hazardous waste S61 Avoid release to the environment Refer to special instructions safety data sheet S1 2 Keep locked up and out of the reach of children S36 37 Wear suitable protective clothing and gloves S36 37 39 Wear suitable protective clothing gloves and eye face protection S37 39 Wear suitable gloves and eye face protection The 39 th International Chemistry Olympiad Preparatory problems 6 Problem 29 TITRIMETRIC DETERMINATION OF FE IN DIFFERENT OXIDATION STATES Some methods of iron determination in the oxidation states 2 and 3 are discussed in Problem 12 You are invited to test one more approach to solving that problem in practice Reagents and solutions required KIO3 R9 R22 R36 37 38 S35 reagent grade solid Ascorbic acid solid KI R36 38 R42 43 R61 S26 S36 37 39 S45 5 aqueous solution HCl R34 R37 S26 S36 S45 conc and 2 M HNO3 R8 R35 S1 2 S23 S26 S36 S45 conc Sulfosalicylic acid 25 aqueous solution NH3 R10 R23 R34 R50 S1 2 S16 S36 37 39 S45 S61 10 aqueous solution EDTA R36 S26 standard solution about 0 05 M the exact value will be given 1 Preparation of a primary standard solution of KIO3 1 1 Calculate with the accuracy of 0 0001 g the weight of KIO3 necessary for the preparation of 200 0 mL of 0 01000 M KIO3 solution 1 2 Using analytical balance weigh out accurately a portion of KIO3 The weight of the portion may differ from the calculated one no more than by 0 05 g and it should be measured with a 0 0001 g accuracy 1 3 Transfer the portion into 200 0 mL volumetric flask dissolve it in water dilute to the mark and mix 1 4 Calculate the exact concentration of the solution prepared in mol L The 39 th International Chemistry Olympiad Preparatory problems 7 2 Preparation of the titrant solution ascorbic acid 2 1 Calculate with the accuracy of 0 01 g the weight of ascorbic acid necessary for preparation of 200 mL of 0 1 M solution 2 2 Using technical balance weigh out a portion of ascorbic acid Its weight may differ from the calculated one no more than by 0 05 g 2 3 Dissolve the portion in 200 mL of water mix well transfer the solution into a vessel and close it tightly with a stopper 3 Standardization of the ascorbic acid solution 3 1 Fill in a burette with the ascorbic acid solution 3 2 With a pipette transfer 10 00 mL of standard KIO3 solution into a 100 mL Erlenmeyer flask add 20 mL of 5 KI solution and 5 mL of 2 M HCl 3 3 Titrate the mixture with the ascorbic acid solution until the iodine color disappears Note When titrating iodine with solutions of reducing agents starch is usually added as an indicator Here it is not recommended to do so because the reaction rate decreases significantly in presence of starch 3 4 Repeat the titration until three titrant volumes differ no more than by 0 10 mL 3 5 Calculate the average titrant volume 3 6 Calculate the ascorbic acid concentration in the solution in mol L Questions 1 Write down the balanced equations of all the reactions taking place during standardization of ascorbic acid solution Ascorbic acid C6H8O6 is being oxidized to dehydroascorbic acid C6H6O6 The 39 th International Chemistry Olympiad Preparatory problems 8 2 KIO3 in presence of excess of KI can be used as a primary standard for HCl standardization as well The method is similar to that described above with the exception that no HCl is added to the titrated solution in this case Which compound s can be used as an indicator s for that titration starch sulfosalicylic acid methyl orange methyl orange Na2S2O3 in excess 4 Determination of Fe III by ascorbimetric titration 4 1 From your instructor obtain a sample solution containing Fe II and Fe III in 100 0 mL volumetric flask Dilute the solution to the mark with water and mix 4 2 Fill in the burette with the standardized ascorbic acid solution 4 3 With a pipette place 10 00 mL of the sample solution into a 100 mL Erlenmeyer flask add 40 mL of water and heat nearly to boiling 4 4 Into the hot solution add 4 5 drops of 25 sulfosalicylic acid solution as an indicator 4 5 Titrate the solution with the ascorbic acid solution until the violet color disappears During the titration and especially near the end point the solution must be hot You may need to heat it additionally if necessary Near the end point the ascorbic acid solution should be added slowly 4 6 Repeat the titrations until three titrant volumes differ no more than by 0 10 mL 4 7 Calculate the average titrant volume 4 8 Calculate the weight of Fe III in the sample solution given to you The 39 th International Chemistry Olympiad Preparatory problems 9 Note Ascorbic acid especially in aqueous solutions is instable and oxidizes with oxygen from the air Therefore the standardization of ascorbic acid solution and ascorbimetric determination of Fe III must be carried out during one workday Questions 1 Write down the balanced equations of all the reactions taking place during Fe III determination Ascorbic acid C6H8O6 is being oxidized to dehydroascorbic acid C6H6O6 2 In what media does ascorbic acid exhibit its reducing properties most markedly in acidic in neutral in alkaline reducing properties of ascorbic acid do not depend on the pH 5 Determination of total iron by complexometric titration 5 1 Fill in the burette with an EDTA standard solution 5 2 With a pipette transfer 10 00 mL of the sample solution into a 100 mL Erlenmeyer flask Add 5 mL of conc HCl and 2 mL of conc HNO3 to oxidize Fe II present in the sample to Fe III Cover the flask with a watch glass heat until boiling and continue heating for 3 5 min avoiding splashing 5 3 Cool down the solution and neutralize it carefully adding 10 NH3 dropwise until color changes from lemon yellow to yellowish brown and slight turbidity persists 5 4 Add 1 2 drops of 2 M HCl to dissolve the precipitate then 0 5 mL of 2 M HCl more dilute up to 50 mL with distilled water and heat nearly to boiling 5 5 Into the hot solution add 4 5 drops of 25 sulfosalicylic acid solution as an indicator The 39 th International Chemistry Olympiad Preparatory problems 10 5 6 Titrate the solution until color changes from violet to clear yellow During the titration and especially near the end point the solution must be hot You may need to heat it additionally if necessary Near the end point the EDTA solution should be added slowly 5 7 Repeat the titrations until three titrant volumes differ no more than by 0 10 mL 5 8 Calculate the average titrant volume 5 9 Calculate the total weight of iron in the sample solution given to you 5 10 Calculate the weight of Fe II as a difference between the results obtained in 5 9 and 4 8 Questions 1 Write down the balanced equations of all the reactions taking place during total Fe determination 2 One of the crucial items in the Fe III determination by complexometric titration is strict maintenance of solution acidity What are the reasons for that If the acidity is too low Fe OH 3 precipitates If the acidity is too high complex of Fe III with sulfosalicylic acid does not form If the acidity is too high complex of Fe III with EDTA acid does not form If the acidity is too low and or too high the titrant decomposes Problem 30 ASYMMETRIC AUTOCATALYSIS THE NUMERICAL EXPERIMENT Nature exhibits a curious asymmetry between the left and the right which is generally called chiral asymmetry Indeed living organisms contain mostly L amino acids and D carbohydrates One of the possible explanations of this phenomenon is based on the idea of autocatalysis Chiral asymmetric autocatalysis is a reaction in which every chiral product serves as the catalyst of its own formation In such reactions small initial excess of one of the enantiomers can increase exponentially in time The 39 th International Chemistry Olympiad Preparatory problems 11 Consider the kinetic scheme explaining this phenomenon Two Enantiomers XL and XD are reversibly formed from achiral reagents T and S 1 1 1 1 2 2 2 2 L D LL DD S T X 1 S T X 2 S T X 2X 3 S T X 2X k k k k k k k k 3 LD 4 X X P 5 k Enantiomers react with each other giving the product P The reactions take place in an open system where constant concentrations of reagents S and T are maintained The system of rate equations can be solved numerically using any of the mathematical packages for example Mathematica MathCad etc Alternatively you may use the program KINET posted on the official website www icho39 chem msu ru Let us assume the following values of rate constants in arbitrary units k1 0 5 k 1 0 1 k2 0 5 k 2 0 2 k3 0 5 Procedure For numerical solution of the systems of differential equations mathematical packages use different commands In Mathematica it is done by the function NDSolve The arguments are the list of equations initial conditions and a time interval For example the system of equations 2 a ta t p t p ta t p tp t with the initial conditions a 0 2 p 0 0 5 in a time interval from t 0 to t 10 is solved numerically by the command sol NDSolve a t a t p t p t a t p t 2 p t a 0 2 p 0 0 5 a p t 0 10 The 39 th International Chemistry Olympiad Preparatory problems 12 The obtained solution is presented on the graph by the command Plot Plot Evaluate a t p t sol t 0 10 PlotRange All Questions 1 Compare equations 1 and 2 or 3 and 4 in the Scheme above Why are the rate constants identical for enantiomers XL and XD 2 The control parameter for this problem is the product of concentrations of reagents Solve the system of kinetic equations numerically and draw on one graph the kinetic curves for XL and XD using the initial conditions XL 0 0 XD 0 0 01 Consider two opposite cases S T is small S T is large By varying the parameter S T determine its break value at which the shape of kinetic curve s changes drastically 3 At fixed value S T 5 study the influence of initial chiral asymmetry on kinetic curves Consider two cases XD 0 0 001 XD 0 0 1 Let us determine which elementary reactions are essential for chiral asymmetry amplification 4 Consider the role of reversibility For this purpose given the same initial concentrations compare kinetic curves for two mechanisms with reversible k 1 0 k 2 0 and with irreversible formation of the enantiomers k 1 k 2 0 5 Consider the simplified scheme in which the first two reactions are absent Whether or not amplification of chiral asymmetry is possible in such system 6 Compare the open and closed systems You have already treated the open system In the closed system the reagents S and T are no more introduced to a reaction vessel during reaction therefore they should be included in the system of kinetic equations Whether or not amplification of chiral asymmetry is possible in a closed system Draw the conclusions What conditions are necessary for amplification of chiral asymmetry to be observed What elementary stages appear to be essential for it The 39 th International Chemistry Olympiad Preparatory problems 13 Problem 31 OSCILLATING REACTIONS Introduction In 1921 W Bray published an article describing the oscillating reaction of oxidation of hydrogen peroxide with potassium iodate However thorough investigation of oscillating reaction mechanisms has begun only in 1951 when B P Belousov discovered oscillations of concentrations of reduced and oxidized forms of cerium catalyzing oxidation of citric acid by bromate ion Later it was shown that oscillating reactions are possible in other redox systems A M Zhabotinsky investigated the oxidation of malonic acid by bromate ion in the presence of manganese ions This reaction mechanism is very sophisticated and includes dozens of intermediate compounds We will investigate an oscillating reaction taking place in the malonic acid iodate ion system in the presence of manganese salt and hydrogen peroxide Reagents and equipment 1 40 H2O2 R5 R8 R20 R22 R35 S1 2 S17 S26 S28 S36 37 39 S45 2 KIO3 R9 R22 R36 37 38 S35 3 conc H2SO4 R23 24 25 R35 R36 37 38 R49 S23 S30 S36 37 39 S45 3 C3H4O4 malonic acid R20 21 22 S26 S36 37 39 4 MnSO4 5H2O R20 21 22 R36 37 38 R40 S26 S36 5 starch 6 KI solution R36 38 R42 43 R61 S26 S36 37 39 S45 7 AgNO3 solution R34 R50 53 S1 2 S26 S45 S60 S61 8 analytical balance 9 weighing dishes 10 flat bottom flasks or beakers 250 500 ml 4 items 11 stop watch Procedure Prepare three solutions may be prepared in advance The 39 th International Chemistry Olympiad Preparatory problems 14 1 solution of 80 ml 40 H2O2 in 120 ml of water 2 solution of 8 7 g KIO3 and 0 9 ml conc H2SO4 in 190 ml of water 3 solution of 3 g C3H4O4 2 4 g MnSO4 5H2O and 0 06 g starch in 195 ml of water Mix the solutions in the same vessel and observe the oscillating process Evaluate the oscillation period and its change in time Split the mixture into two parts and place them into beakers To one of the parts add AgNO3 solution first several drops then 3 ml Observe changes of the oscillation period Note the color of the solution upon completion of the oscillation reaction To the other part add KI solution several drops Observe changes of the oscillation period Questions 1 Oxidation of malonic acid by potassium iodate is an autocatalytic process Write down the net equation of the reaction Which product is the catalyst of the oscillating process Explain the effect of silver nitrate 2 B P Belousov used bromate ion as an oxidizing agent Suggest what would happen if we substitute iodate ion by bromate ion in the reaction with malonic acid What role does hydrogen peroxide play in the oxidation of malonic acid with iodate ion 3 It is well known that one of the stages of the oscillating process is formation of iodomalonic acid with its subsequent decomposition How can we explain the fact that potassium iodide inhibits the reaction 4 B P Belousov used the Ce4 Ce3 redox couple to study oscillating reactions Is it possible to use the following transient metal redox couples as a catalys