2 ND ROUND, PROBLEMS

Transcription

1 2 ND ROUND, PROBLEMS Solve all or some of he four problems given on he nex pages and wrie your answers in MS Excel answer shees provided. If here are some explanaions required, wrie hem in English or Russian. Please, send your answers o: kimijas_olimpiades@inbox.lv ill a 24:00. Answers which will be sen afer his ime will no be graded. Wrie all he answers in one workbook bu in differen workshees for each problem. You should fill in he green spaces provided. File name mus consis from your name, surname (in English) and counry, for example John_Black_England.xls (or *.xlsx). If you do no name he file as described you will receive 3 poin penaly. For all correcly solved problems you can ge he maximum of 30 poins. The exac amoun of poins for each ask is given a he op of each problem. All sudens who will be aking par in a leas one round will paricipae in he final round which will be held on he web on 27 h of February. During he final round you will have o solve he muliple choice es. Auhors for problems in his round: Kaspars Veldre Vladislav Ivaniššev Bernardas Morkunas PhD suden, Universiy of Lavia, Deparmen of Chemisry PhD suden, Taru Universiy, Insiue of Chemisry 3 rd year suden, Cambridge Universiy Karina Kizjakina Agris Bērziņš Mihails Arhangeļskis Toms Rēķis PhD suden, Virginia Polyechnic Insiue and Sae Universiy, Deparmen of Chemisry MS suden, Universiy of Lavia, Deparmen of Chemisry 3 rd year suden, Cambridge Universiy 1 s year suden, Universiy of Lavia, Deparmen of Chemisry Good luck wih problem solving! If you have any quesions, address hem o us in English and send hem o: kimijas_olimpiades@inbox.lv Feel free o ask!

2 Problem 1 (Lavia * ) Unknown highly flammable gas (6 poins) Compound A is a colorless highly flammable gas conaining 21.7% of Hydrogen by mass (use aomic masses wih one decimal place). I undergoes pyrolysis a emperaures above 100 o C. The process is very complicaed, bu he firs seps can be described by he following equaions: A { C} fas A + { B} 2{ B} fas rae deermining sep { B} + { D} E { C} D + H A + { D} { B} + E F + H Compounds in brackes {} are unsable inermediaes ) Wha is compound A? Draw is srucure and describe he bonding in is molecule. 2) Deermine he formulae of compounds B F. 3) A room emperaure B dimerises ino A. Why does no B exis in monomer form? * There will be wo Lihuanian problems in nex round.

3 Problem m 2 (Lavia) Chemisry of Ice I hockey (14 p poins) The ice hockey h defin niely is mos popular sspor in Lavvia, only heen come baskeball, soccerr and ohers. The highess poin of faan aciviy is in each year ss May, beccause hen Lavian naaional ice hockey eaam paricipaes in World ch hampionship p, division A. Saring from f 2008 year Riga has hockey eam Dinam mo Riga whicch paricipaes in Coninen nal Hockey League (KH HL) champio onship so fans can enjoy ho ockey game almos all year. y Home ggames of Din namo Riga are played in Arena A Riga which lies only half m mile form Deparmen D of Chemisry, Universiy U on n Lavia. Le s ake e a look o hings whicch we need d if we wan n o play icce hockey. n modern riinks ice are produced b by The firs hing we neeed is ice. In pipes spraying hin layer of cold waer on concrree (picureed). Then hiis hin layer is i pained whie w (for beer conras) and all a necessarry markings an nd logos are pained on i. Then ano oher layer of o cold waeer is sprayed on i. Thickne ess of ice is approximae a ely 2 3 cm. Marin n Prusek (Czech goalkeeper) and d Sandis Ozoliņš (#8, onlyy Lavian who ha as S cup) in forms fo of Dinamo o won Sanley Riga, R KHL season 1. Lee s assume ha ice hickkness in arena a is 2.5 cm. Calculae how w m many liers of waer you ca an gain if you mel down one o ice hockeyy rink which corresponds c o o Inernaiona al hockey rulees and one icee hockey rinkk which corressponds o NH HL rules! Use an ny appropriae informaion n source for searching s for size of rinks a and oher info o. 2. W Waer enhalpy of fusion is J/g. C Calculae how w much hea is required o o mel down in nernaional icce hockey areena. If price ffor elecriciy in Lavia is Ls perr kw h, hen ho ow much i will w cos? 3. Ho ow does cha ange in presssure affecs meling of ice? Explain n your answeer using Le Ch haellier princcipe. Please wrie w your exp planaions in English. E 4. Co ompare hea of fusion forr waer wih heas of fusion for oher liquids such as benzene, peenane, aceo one and ehan nol, explain differences d according o bo onding. Pleasee wrie your exxplanaions in n English. 5. W When hea is added a o waeer i can be evvaporaed. Skkech he grap ph showing emperaure ch hange agains hea added.. 6. Th he normal boiling poin of waer is: (cho ose one he mos m correc answer) a a oC a any cond diions b. is he emperaure a a which he va apor pressure is 1 am c. is he emperaure a a which liquid d and vapor arre in equilibriu um d. is he emperaure a a which he va apor pressure equals he exxernal pressurre e. is he emperaure a which herre is a coninuous formaio on of gaseous bubbles in liquid. 7. Th he vapor presssure of given n liquid will inccrease if: (cho ose one he m mos correc answer) a a. he liqu uid is moved o a conained d in which is surface is very much larger b. he vollume of liquid is increased c. he vollume of vapor phase is increeased d. emperraure is increased e. a moree volaile liquid d is added o he given liquiid. 8. Ho ow does cha ange in presssure affecs boiling of waer? w Explain n your answ wer using Le Ch haelier s prin nciple. Please wrie your exxplanaions in n English.

4 RT 9. Change of pressure over emperaure can be calculaed by equaion p = p 0 e, where p pressure; p 0 consan, R = J/(mol K), ΔH enhalpy of vaporizaion, T emperaure (in K). Use any mehod o deermine p 0 and ΔH as precise as you can if such hermodynamical daa are given: Triple poin of waer Normal boiling poin Criical poin Show your calculaions! Pressure, kpa Temperaure, o C Afer 1 s and 2 nd period of hockey game ice washed and cleaned for nex par of games. Washing process removes any foreign maerial ha migh oherwise become embedded in he ice surface. A he rear of he condiioner, a sprinkler pipe wes a cloh owel ha lays down clean waer o fill he residual grooves, and forms a new ice surface. Ho waer (60 C) is frequenly used where available because i mels and smoohes he rough op layer of ice o creae a fla, smooh surface. This waer in many rinks is filered and reaed before being heaed o remove any residual minerals and chemicals in he waer. These chemicals and minerals could oherwise make he ice brile or sof, give i pungen odors, or make i cloudy. 10. How do chemicals and minerals dissolved in waer affec emperaure of freezing? Use Raul s rules o explain his. Please wrie your explanaions in English. 11. Calculae emperaure of freezing for ap waer whichh conains sum of calcium sulfae and magnesium sulfae equal o 1.5 mmol per L. 12. Wrie molecular and ne ionic equaion which can show how o reduce number of Ca 2+ and Mg 2+ in waer! The second hing you need is skaes. I is made ou of carbon seel raher han sainless seel. Goalie skaes differ from regular hockey skaes. The blade is longer, wider, and flaer o provide he goalie wih more sabiliy. The blade is shorer verically so ha he goalender is lower o he ice. Iron can exis in more crysalline forms. The crysalline form of iron, known as α Fe, has a body cenered cubic (bcc) uni cell wih an edge lengh of 2.87 Å. Is densiy a 25 C is 7.86 g/cm 3. Anoher higher emperaure crysalline form, known as γ Fe, has a face cenered cubic (fcc) uni cell wih an edge lengh of 3.59 Å. 13. Calculae he aomic radius of iron in α Fe and use he above facs o esimae Avogadro s number. 14. Calculae he aomic radius of iron in γ Fe as well as he densiy of γ Fe. 15. Calculae radius of larges aom which can be insered in holes of α Fe and γ Fe. How much oversize in boh casess is a carbon aom, having a radius of nm? Nex hing we have o use for hockey game is sick. Tradiional sicks were usually made compleely of wood, mos modern sicks are reinforced wih graphie and maerial X (picured) and he paddle and blade are injeced wih foam o make hem ligher. X ray fluorescence specra for maerial X is given in nex page. Knowing ha each specral lines can be calculaed by equaion E = a * z + b, wheree E energy of specral line, kev; z posiion in specra from edge of paper, cm; a and b empirical consans. Using given specra and able of specral lines for heavy elemens answer quesions A specrum is obained by x ray fluorescence specromee r S8 Tiger Bruker, Universiy of Lavia, Deparmen of Chemisry. ΔH

5 Table of specral line energies

6

7 X ray fluorescence specra of maerial X:

8 16. Which is he mos abundan chemical elemen in maerial X? 17. Deermine he qualiaive composiion of maerial X. Wha is maerial X (in English)? 18. A maerial X has no exac meling poin. Wha does i mean abou solid sae srucure of X?? Explain. Please wrie your explanaions in English. 19. The mos abundan elemen in maerial X is widely spread in naure. Wrie formula for compound Y conaining his elemen which is he mos abundan form of menioned elemen in naure. Draw srucure for his compound in solid sae and deermine coordinaion numbers for all elemens in his compound. 20. Wrie equaions for reacions which show how o obain pure elemens saring from compound Y. The fourh hing in hockey play we need is he puck. Hockey puck is made from vulcanized rubber and is parameers are: 25 mm hick, 76 mm in diameer and i weighs 160 g. Normally rubber is made synheicallyy from isoprene by freee radical polymerizaion wih 86 % yield. In synhesis benzoyl peroxidee is added o mixure. In synhesis polyisoprene forms wih molecular weigh g/mol (in your calculaions assume ha i is g/mol). I was deermined, ha hockey puck conains 10 % of sulfur. 21. Wrie down srucures of isoprene and benzoyl peroxide. 22. Wha role in synhesis benzoyl peroxide has? Wrie down reacion mechanism for is conversaion. 23. Wrie down mechanism of synhesis of polyisoprene in hese condiions by showing precisely all seps: a) chain iniiaion, b) chain growh and c) chain erminaion. 24. By assuming all given informaion, calculae how much isoprene is necessary for his reacion. 25. By assuming all given informaion, calculae how many macromolecules form one hockey puck. How many sulfur aoms has per one macromolecule? How many sulfur sulfur bonds form one macromolecule? Calculae how many monomers here are per one sulfur aom? 26. By assuming all given informaion, calculae he volume of one macromolecule. Volume for wha acually is calculaed in his way? 27. Show he sulfur binding in his hockey puck for hree bonded macromolecules. The fifh and oday he las hing we need is masks (inroduced only in 1959 by Canadian goalkeeper Jacques Plane, Monreal Canadiens, NHL). Nowadays here are no players o play wihou mask (i is prohibied). 1 s ype of masks was only in from of face, 2 nd ype is helme cagee ype mask nowadays used only by some goalkeepers (among hem Marin Prusek from Dinamo Riga, picured a he beginning) and 3 rd ype of mask wih a cage aached in he middle is used by mos of goalies (picured Edgars Masaļskis represening Dinamo Riga, Lavian Naional eam). The las ype of masks is hough o be safer. I is made from fiberglass and maerial Q. This maerial is synhesized in soluion from he monomers para phenylenediamine and erephhalo yl chloride in a condensaion reacion yielding compound W as a byproduc. 28. Wrie equaion for synhesiss of maerial Q. Wha is he name of i? Wha is compound W? 29. Wha makes maerial Q so srong? Explain i from poin of chemisry by drawing corresponding srucure. 30. Concenraed sulfuric acid is used for synhesis of compound Q. Explain why sulfuric acid is necessary for his synhesis (ake in accoun a leas wo reasons)? Pleasee wrie your explanaion in English.

9 Problem 3 (Esonia) Everyone loves specra Colour of a firework or a flame in a flame es is an evidence of phoon emission a frequency range THz. Le s find ou how emission happens in alkaline elemens aoms. When he elecrons in he aom are excied (by heaing for example), hey jump o higher energy levels. As he elecrons fall back down, energy is re emied, he wavelengh of which refers o he discree lines of he emission specrum. The phoon energy due o an elecron ransiion beween an exied aomic level k (of energy E k ) and a lower level i is ΔE = E k E i = hν = hc/λ, where ν is he frequency, and λ he wavelengh. Since a phoon carries energy and angular momenum (1 uni), any elecronic ransiion from one allowed energy level o anoher mus conserve energy and angular momenum for he aomand hey phoon sysem. These resricions are known as selecion rules are, ΔS = 0, ΔL = ±1, and ΔJ = 0, ±1, wih J = 0 o J = 0 forbidden. Allowed ransiion beween elecronic saes in alkaline elemens aoms obeys hese selecion rules. S P D 3d 3p 3s 2p 2s Energy level diagram 1. Schemaic presenaion of energy levels energy level diagram, is shown. Mark allowed ransiions by arrows which lead o 2s and 2p configuraions. All he allowed ransiions may be easily calculaed, if elecronic configuraion energies are known. Persisen lines in he visible specrum of alkaline elemens mainly due prior o ransiions from np 1 or (n + 1)p 1 o he ns 1 configuraion. The energy of corresponding levels of an upper elecron may be obained by: E = 13.6 Z 2 2 eff n, where Z eff. Value of Li ionizaion energy ( ev) may be used o esimae Z eff (2s). Z eff of elecron on 2p and 3p levels in Li aom is 0.25 and 0.75 bigger han Z eff (2s) respecively. Z eff (ns) in oher alkaline elemens aoms increased by in he ransiion o he nex period. Similarly Z eff (np) and Z eff ((n + 1)p) increased by and Calculae phoon wavelengh due o an elecron ransiion beween np or (n + 1)p and ns levels of alkaline elemens from Li o Cs. Calculaed values may be now used o predic he colours of he alkali compounds in firework or flame. Assume, ha calculaed values are overesimaed; hey are higher, han experimenal daa by 100 nm in average. 3. In visible specra of Li is anoher persisen line (610 nm). Which ransiion does i belong o? 4. Compounds of which alkali elemens give colour o yellow, red and viole firework? 1 These simplified daa were adjused based on phoon wavelengh values from he experimenal specra.