In the spaces provided, explain the meanings of the following terms. You may use an equation or diagram where appropriate.

Transcription

1 CEM J-2 June 2007 In the spaces prvided, explain the meanings f the fllwing terms. Yu may use an equatin r diagram where apprpriate. 5 (a) hydrgen bnding An unusually strng diple-diple interactin that frms when a hydrgen atm is bnded t ne f the very electrnegative atms F, r. (b) clligative prperties Prperties f a slutin that depend nly upn the number f mles f slute present, nt the nature f the slute. (c) hyptnic slutin A slutin with lwer smtic pressure than cell fluid. (d) iselectric pint The p at which there is n nett charge n a mlecule cntaining bth acidic and basic grups. (e) half life The time required fr the cncentratin f a reactant t fall t half its initial value. Cmmnly applied t radiactive materials where it is the time taken fr the activity f a sample t fall t half its initial value.

2 CEM J-4 June 2007 Give the xidatin number f carbn in each f the fllwing. CF 2 Cl 2 (g) +4 2 a 2 C 2 4 (s) +3 C 3 (aq) +4 C(s) 0 Cnsider a vltaic cell that uses the fllwing half-reactins: Mn 4 (aq) (aq) + 5e Mn 2+ (aq) (l) Sn 4+ (aq) + 2e Sn 2+ (aq) Write a balanced equatin fr the verall reactin. 3 The tw cell ptentials are V (Mn 4 - /Mn 2+ ) and V (Sn 4+ /Sn 2+ ). The least psitive (Sn 4+ /Sn 2+ ) is reversed s that it is the xidatin reactin. Cmbining the half cells in this way gives, after balancing: 2Mn 4 (aq) (aq) + 5Sn 2+ (aq) 2Mn 2+ (aq) (l) + 5Sn 4+ (aq) Which species is the xidising agent? Mn 4 - (aq) Which species is the reducing agent? Sn 2+ (aq) Calculate the standard cell ptential. (Refer t the table f standard reductin ptentials.) As nted abve, the Sn 4+ /Sn 2+ half cell is reversed s that its xidatin ptential is V. ence, the standard cell ptential is: E = E + E = ( ) + ( 0.15) = V cell red x Answer: V

3 CEM J-3 June 2007 Cmplete the table belw shwing the number f valence electrns, the Lewis structure and the VSEPR predicted shape f each f the fllwing species. Frmula umber f valence electrns Lewis structure Gemetry f species 9 e.g. 3 8 trignal pyramidal C 4 8 C tetrahedral C 2 16 C linear F F PF 5 40 P F F F trignal bipyramidal 3 24 trignal planar Which ne f the species abve displays resnance, and hw many resnance frms are pssible? 3 - shws resnance three frms are pssible:

4 CEM J-5 June 2007 Stearic acid, C , is a fatty acid cmmn in animal fats and vegetable ils and is a valuable energy surce fr mammals. The net reactin fr its metablism in humans is: C (s) (g) 18C 2 (g) (l) Calculate fr this reactin given the fllwing heats f frmatin. f (C (s)) = 948 kj ml 1, f ( 2 (l)) = 285 kj ml 1 f (C 2 (g)) = 393 kj ml 1 and 5 Using rxn f f = m (prducts) n (reactants), the heat f the reactin as written is: rxn f 2 f 2 f f 2 = [18 (C (g)) + 18 ( (g))] f 2 [ (C (s)) 26 ( (g))] = [(18 393) + (18 285)] [( 948) + (26 0)] = 11256kJ ml ( (g)) = 0 fr an element already in their standard states. 1 Answer: kj ml -1 If the cmbustin f stearic acid is carried ut in air, water is prduced as a vapur. Calculate the fr the cmbustin f stearic acid in air given that 2 (l) 2 (g) = +44 kj ml 1 Energy must be prvided t vaprize the water s the enthalpy f cmbustin is reduced: cmb = (18 44) = kj ml -1 Answer: kj ml -1 Will S be different fr the tw xidatin reactins? If s, hw will it differ and why? The entrpy f 2 (g) is greater than that f 2 (l) s S will be greater. In the xidatin reactin which prduces 2 (l), 26 ml f gas are cnverted int 18 ml f gas s that S will be negative. In the xidatin reactin which prduces 2 (g), 26 ml f gas are cnverted int ( ) = 36 ml f gas and S will be psitive. ASWER CTIUES TE EXT PAGE

5 CEM J-5 June 2007 Calculate the mass f carbn dixide prduced by the cmplete xidatin f 1.00 g f stearic acid. The mlar mass f stearic acid, C , is: (C) () () = g therefre cntains: number f mles f stearic acid = mass 1.00 = = ml mlar mass As 18 ml f C 2 are prduced fr every mle f stearic acid: number f mles f C 2 = = ml The mlar mass f C 2 is (12.01 (C) ()) = The mass f C 2 is thus: mass f C 2 = number f mles mlar mass = = 2.78 g Answer: 2.78 g

6 CEM J-6 June 2007 Lactic acid, C 3 CC, is prduced in the bdy during nrmal exercise. It is a mnprtic acid with a pk a f (a) What is the p f a 0.10 M water slutin f lactic acid? 6 The reactin table is: lactic acid(aq) 2 (l) lactate(aq) 3 + (aq) start 0.10 large 0 0 change -x -x +x +x equilibrium 0.10-x large x x As pk a = -lg 10 (K a ) = 3.86, K a = and: K a = + 2 [lactate(aq)][3 (aq)] (x)(x) x = = = 10 [lacticacid(aq)] (0.10 x) (0.10 x) 3.86 As K a is very small, x is tiny and 0.10 x ~ x. ence, K a ~ 2 x (0.10) 3.86 = 10 r x 2 = (0.10) ( ) s x = [ 3 + (aq)] = M As p = -lg 10 ([ 3 + (aq)]: p = -lg 10 ( ) = 2.43 Answer: 2.43 (b) Calculate the p f the slutin frmed when 0.02 ml f Ca() 2 (s) is added t 1.0 L f 0.10 M lactic acid. 1.0 L f 0.10M lactic acid cntains 0.10 ml f acid. Ca() 2 is a strng base. It will dissciate cmpletely t give 2 - (aq) fr every 1 mle f Ca() 2. (2 0.02) = 0.04 ml f - (aq) will be prduced. This will neutralize 0.04 ml f the acid leaving ( ) = 0.06 ml f acid. Assuming that the vlume des nt change frm the additin f the slid, [lactic acid] = M. The neutralizatin prduces lactate anin with [lactate] = M. The slutin nw cntains acid and its cnjugate base. It is a buffer and the p can be calculated using the endersn-asselbalch equatin can be used: [base] p = pk a + lg10 [acid] = lg = 3.86 Answer: 3.86 ASWERS CTIUES TE EXT PAGE

7 CEM J-6 June 2007 (c) Using equatins, cmment n hw the final slutin in (b) will respnd t additins f small amunts (e.g. less than 0.01 ml) f acid r base in cmparisn t additins f the same amunts f acid r base t 1 L f water. The slutin in (b) will act as a buffer. As it cntains bth an acid (lactic acid) and a base (lactate), it can react with bth added base and acid t maintain a near cnstant p. Added 3 + will be cnsumed by the reactin f the lactate anin: lactate(aq) (aq) lactic acid(aq) + 2 (l) Added - will be cnsumed by the reactin f the lactic acid: lactic acid(aq) + (aq) lactate(aq) + 2 (l) As lng as the amunts f lactic acid and lactate are large in cmparisn t the added acid r base, the p is apprximately cnstant and is described by the endersn-asselbalch equatin. If acid r base is added t water, the [ 3 + (aq)] r [ - (aq)] will change accrding t the amunt added and the p will change rapidly.

8 CEM J-7 June 2007 rlistat (shwn belw) is a drug fr besity management which acts by inhibiting the absrptin f dietary fats. 6 amide ester (R) * (R) * * (S) *(S) ester Is it likely t be sluble in water? Why? The mlecule cntains lng-chain hydrcarbn areas which are hydrphilic. Althugh xygen atms are present, there are n - grups t allw - bnding. The mlecule is thus unlikely t be sluble in water. Indicate n the abve structure all steregenic centres. Select ne f these centres and clearly assign its sterechemical cnfiguratin. ame the functinal grups present in rlistat. Ester 2 and amide (see structure).

9 CEM J-8 June 2007 Glycine, 2 C 2 C, is the simplest f all naturally ccurring amin acids. The pk a f the acid grup is 2.35 and the pk a assciated with the amin grup is Draw a structure that indicates the charges n the mlecule at the physilgical p f C 2 C 2 Use yur structure t illustrate the cncept f resnance. 3 C 2 C 3 C 2 C What are the hybridisatin states and gemetries f the tw carbn atms and the nitrgen atm in glycine? C a C b has 4 bnds and n lne pairs: sp 3 with a tetrahedral gemetry. C a has 4 bnds and n lne pairs: sp 3 with a tetrahedral gemetry. C b has 3 bnds and n lne pairs: sp 2 wit a trignal planar gemetry. Prpinic acid, C 3 C 2 C, has a melting pint f 20.7 C while glycine has a melting pint f 292 C. Suggest a reasn why these tw mlecules have such different melting pints. The majr intermlecular frce in glycine is inic bnding between the psitively and negatively charged ends f the mlecule. The majr intermlecular frce in prpinic acid is hydrgen bnding between the carbxylic acid grups. Inic bnding is much strnger than -bnding s glycine has a much higher melting pint.

10 CEM J-9 June 2007 Alanine (R = C 3 ) and lysine (R = C 2 C 2 C 2 C 2 2 ) are tw cmmn amin acids. Using ala and lys t represent the tw amin acids, represent all cnstitutinal ismers f the tripeptide frmed frm ne ala and tw lys units. 6 Three tripeptides are pssible: ala-lys-lys, lys-ala-lys and lys-lys-ala Cmment, giving yur reasn, n whether the tripeptide(s) will be acidic, neutral r basic in character. Lysine has a basic, amine sidechain (R = C 2 C 2 C 2 C 2 2 ) whilst alanine has a neutral sidechain (R = C 3 ). The tripeptide(s) will thus be basic. Draw the cnstitutinal frmulas, indicating the crrect inic state, f the prducts frmed frm acid hydrlysis f ne f yur tripeptides. Each tripeptide will be hydrlysed t give alanine and lysine. In acid, these will be present with the basic amine grups (including the sidechain f lysine) and the carbxylic acid grups in the prtnated frms: 3 C 2 C 3 C 2 C C 2 C 2 C 2 C 2 3

11 CEM J-10 June 2007 AD + and AD are cenzymes used by animals in xidatin and reductin reactins. They are related by the fllwing half-reactins e 2 R R Which f these cenzymes is used in the bilgical xidatin f ethanl, C 3 C 2? An xidizing agent is itself reduced. As shwn in the equatin, it is AD + which is reducible. 2 R AD + What is the prduct f the bilgical xidatin f ethanl, C 3 C 2? C 3 C Which f AD + and AD is armatic? Give reasns fr yur answer. AD + is armatic. It is cyclic, planar, cnjugated, and has 4n+2 π electrns. AD is nt fully cnjugated and is therefre nt armatic. Benzic acid has a lw slubility in water at p 7, but is very sluble in aqueus slutins f greater p. Explain this bservatin, using chemical equatins where apprpriate. 2 Benzic acid has a lw slubility in water because f the relatively large hydrphbic armatic ring. It is a weak acid s exists primarily as the undissciated acid at p 7. At high p, it can react with ins t frm the benzate in. This species is water sluble because it is charged and hence is easily slvated by the plar water mlecules. C C 2