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

Qualification => Subject Doubts => GCE AS & A2 Level => Sciences => Topic started by: ruby92 on March 17, 2010, 01:23:44 pm

Title: chemistry:enthalpy changes and moles
Post by: ruby92 on March 17, 2010, 01:23:44 pm
1)A typical jumbo jet burns fuel at a rate of 200kg per minute assuming jet fuel is c11h24 what mass of water does the jumbo jet produce per minute?

2)(a) calculate the bond enthalpy of the N-H bond in NH3 using the data given below.
N2+3H2 (reversible reaction sign) 2NH3   DetaH=-92kj/mol

N(triple bond)N has a bong enthalpy of 946
and h-h bond has a bond enthalpy of 436

b)The enthalpy of formation of N2H4 is 94kj/mol
using the N-H bond enthalpy derieved in (a), calcualte the bond enthalpy of N-N bond
Title: Re: chemistry:enthalpy changes and moles
Post by: nid404 on March 17, 2010, 02:09:48 pm
1) Complete combustion of C11H24

2C11H24 +17 O2 ----> 11CO2 + 12 H2O

1 mole of C11H24= (11X12) + (24X1)=156g
no of moles in 200000g=1282moles

2moles of fuel is to 12 moles of water
1282 moles= 1282X12/2
                =7692moles
1mole of water=18g
7692moles=138462g= 138.5kg (approx)

Title: Re: chemistry:enthalpy changes and moles
Post by: nid404 on March 17, 2010, 02:18:08 pm
2)  N2=+946 kjmol-1
     3 H-H= 3X436=+1308kj

Enthalpy of the reactants=2254

Enthalpy change= -92kj/mol

Bond breaking-bond making=-92
2254- bond making=-92
bond making=2346kj for 6N-H bonds(since it's 2NH3)
so for 1 N-H bond 2346/6=391kj/mol
Title: Re: chemistry:enthalpy changes and moles
Post by: nid404 on March 17, 2010, 02:28:08 pm
 N2+ 2H2---->N2H4
946               + 2X436                                X

1818-X=+92
X=-92+1818=1726

1726=4N-H + N-N
       =4X391  + Z

Z=1726-1564=162kj/mol
Title: Re: chemistry:enthalpy changes and moles
Post by: ruby92 on March 18, 2010, 09:46:36 am
in the 1st question the balancing is wrong.
there are 22 C on the left side whereas only 11 on the right?
Title: Re: chemistry:enthalpy changes and moles
Post by: nid404 on March 18, 2010, 01:54:00 pm
srry ill do it agn if u like...unless u've gotten it
Title: Re: chemistry:enthalpy changes and moles
Post by: ruby92 on March 18, 2010, 03:31:29 pm
ive been trying for like d past hour...,, :-[....
i hate moles...
cld u please do it again...
thnk u
Title: Re: chemistry:enthalpy changes and moles
Post by: nid404 on March 18, 2010, 06:00:29 pm
ok fine here u go :)

 Complete combustion of C11H24

C11H24 +17O2 ----> 11CO2 + 12 H2O

1 mole of C11H24= (11X12) + (24X1)=156g
no of moles in 200000g=1282moles

1moles of fuel is to 12 moles of water
no of moles of water= 15385moles
mass of 1 mole of water=18g
mass of 15385moles=276923g or approx 277kg