OCR_2018 HCI H2 Chemistry Promo Answers

HWA CHONG INSTITUT~ON 2018 C1 H'2 ~ CHI EMISTRY PIROMOTIONAL EXAM~NAT~ ~ ONS .SUGGESTED SOILUTIO INS Paper 1 1 2 3 4 5 6 7 8 9 10 D B D ,c B ,c B .A A c 11 12 13 14 15 16 17 18 19 20 D ,c A B D B A D ,c D Comments 1 ID The Ia rge diP' between D and E shows ttl at ttl'e electron to be remo .ved from E is. in the next quantum shell . Hence D has the e~e ctron ic oonfig uration of a noble g.as ns2 n p6 and belongs to' Group 18. 2 IB To do this question quickly ~ you shou lid clhedk tlhe group number of the given e~ e1me nts a.nd recall tile gro up,'s val ,ence e~ ectJronie conflg uration. Ga - Group 13 (ns 2 np1) Se - Group 16 (ns2 np4 ) Cs - Group 1 1(ns 1) H!enae on~ Ga .and s ,e ·fulfil bo~h criteria stated . Cs does not fulfil the 1st crit,erion . 3 ID For this que stl on 1· you wi II ha1ve to wolik out the sllru ctures for all tlhe given species and ~h e1i r number of bond pairs (bp) vs number of lone pairs (I p) to deduce their shapes and bond angles . ] ]I! A N03- 3 bp NO- 2: bp, 1 lp (120°) .2 (<1120°) B GlE.- 4 bp·, 2 lp SIFG 6 bp (QOO) (90°) c XeF4 4 bp, 2 lp BrFs 5 bp, 1 lp (QOa) (<QOa) D Bro~- 3 bp', 1 lp ClDl- 3 bp, 1 lp (104°) (1 OSO.) For option D ~ although both have the same shape and number of b p and lp· ~ Br is less ~el ~ectronegative than Cl. H.enoo its bonding electrons are furttler from the central atom . Hence the bond angle is small ,er as mpu~sion between the bond pa.irs is sm.aller . 2018 HC I C 1 H2 Chemistry Pro motional Exam 4 Cl CJ H Option A: H HI has a dipo1 e moment and he nee is polar . There are permanent dipal ,e-permanent dipo~e interactions betwe .en the molecul ,es. in addition to C! IH H d is.pers.ion foroes.. lrhe r'e is on ~y dispersion forces between molecul ,es of H CJ as the mol,ecule has no, dipo~e moment and is non-polar. Hence tile tis isomer has higher bailing point (bp) than the tr:ans isomer. Option B: Hydrogen bonding (CHJCH120H) is stronger than permanent dipol ,e­ '0 II CH .... C v \ permanent dipole ( H ) interacUons. Hence C HJCH 20 H has higher boiling paint . Option tC: I Bo~h m ol,ecul ,es have hydrogen bonds pres ·ent. One r~ eason for why ethano ic acid has higher boiling point is because mo~erules of ethanoic acid form dim em and this also ' leads to' stronger dis pen~~ on forces lbetwee n its m·ol,ecul ,es .. 1(refer to Chapter 21 pa.ge 95 of Foundation Topic boo ld,et) OHI OH Option 1!) : has intr.amo ~ecu lar hyd rag en bonding between the two OH g mu ps,t hen ae it will have ~ess extens ive intermol eru lar hydrogen bonds and ~ower lbp. 5 B For C02~ since bolll C=O bonds have dipo lles o,f ~ equa. l m.agnitude and in opposit~e d irectio llS 1 they ca noel each oilier off and r~ esulted in overall 0 po~a rity . For COS~ the overa ll! polarity is lower than that ofCOSe . That means thatC=S bond is more pol.ar (larg er dipole moment) than C=Se as it is able to cance~ out mo:re of C=O dipo 'e moment This leads t~o a lower o,ve