IGCSE/GCSE/O & A Level/IB/University Student Forum

Qualification => Subject Doubts => GCE AS & A2 Level => Sciences => Topic started by: Sue T on December 26, 2009, 10:21:49 am

Title: more cie chem doubts
Post by: Sue T on December 26, 2009, 10:21:49 am

(ppr 2)in nov 04 ques 3 (a)iii
at the anode th oxidation number goes from -1 to 0 (oxidation)
at the cathode the oxidation num is supposed 2 go from +1 to 0 (reduction)
th ms says -1 to 0 at th cathode- is t a mistake?

one more question  ;D
nov05 Q2 (d)
shouldnt it be like : -395+(2*-298)+(delta H formation) = -1110      ???
its exothermic since bonds r formed so the answer must be negative
but in th ms its positive (they did it in a diff way)

Title: Re: more cie chem doubts
Post by: Lana Wolf on December 26, 2009, 10:34:48 am

the papers arent opening... :S
some prob in freeexampapers..??

Title: Re: more cie chem doubts
Post by: nid404 on December 26, 2009, 01:38:08 pm

nov 04....that's a mistake..

nov 05
 
You need to calculate enthalpy change of formation of CS2

so C+2S`--->CS2

S+O2--->SO2 -298kj/mol   X2
C+O2--->CO2   -395kj/mol
CS2+3O2---> CO2+2SO2 -1110kj/mol

-395+ (-298X2)+dh--->1110 (become postive because it is on the other(product)side of the main reaction)
dh=1110-(991)
=+119kj/mol

Title: Re: more cie chem doubts
Post by: Sue T on December 26, 2009, 05:14:05 pm

OOooo! got it
thx alot again!  ;)

Title: Re: more cie chem doubts
Post by: sweetie on January 01, 2010, 02:31:10 pm

Hey,
am also confused about Q4 parts "c" and"d" of paper2 nov04

Title: Re: more cie chem doubts
Post by: sweetie on January 06, 2010, 03:30:55 pm

can any1 plz help me in Q.10 of nov07 paper1????????????
how can the conc of 2NO₂ be 2.4??????????????? :'( :'( :'(

Title: Re: more cie chem doubts
Post by: nid404 on January 06, 2010, 05:09:44 pm

conc of NO2 will be conc of O2 +N2(0.8+1.6)
Ratio of O2 to N2 is 1:2...so [N2] is 0.8X2=1.6

Title: Re: more cie chem doubts
Post by: sweetie on March 31, 2010, 03:32:21 pm

Thank You :-*