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VCE Stuff => VCE Science => VCE Mathematics/Science/Technology => VCE Subjects + Help => VCE Chemistry => Topic started by: Teen on August 07, 2012, 08:21:35 pm

Title: Multi-step reaction - sac help
Post by: Teen on August 07, 2012, 08:21:35 pm
can someone pls explain to me ...how find the change of H from the equations
For example:
2C(s)+O2 ------>2CO  change of H is =-221kjmol^-1
C(s)+O2------->CO2 change of H is =-787kjmol^-1

calculate the change of H for the reaction 2Co2(g)------>2CO+O2(g)
Title: Re: Multi-step reaction - sac help
Post by: Jenny_2108 on August 07, 2012, 08:29:05 pm
Quote from: Teen on August 07, 2012, 08:21:35 pm
can someone pls explain to me ...how find the change of H from the equations
For example:
2C(s)+O2 ------>2CO  change of H is =-221kjmol^-1
C(s)+O2------->CO2 change of H is =-787kjmol^-1

calculate the change of H for the reaction 2Co2(g)------>2CO+O2(g)

2C(s)+O2 ------>2CO (1) delta H=-221kjmol-1

Multiply with 2 for 2nd equation: 2C(s)+2O2------->2CO2 (2) -1574kjmol-1

(2)-(1): 2CO+O2 -------> 2CO2 delta H=-1353kjmol-1
 Inverse reaction: delta H=1353kjmol-1

Title: Re: Multi-step reaction - sac help
Post by: Teen on August 07, 2012, 08:32:32 pm
Quote from: Jenny_2108 on August 07, 2012, 08:29:05 pm
Quote from: Teen on August 07, 2012, 08:21:35 pm
can someone pls explain to me ...how find the change of H from the equations
For example:
2C(s)+O2 ------>2CO  change of H is =-221kjmol^-1
C(s)+O2------->CO2 change of H is =-787kjmol^-1

calculate the change of H for the reaction 2Co2(g)------>2CO+O2(g)

2C(s)+O2 ------>2CO (1) delta H=-221kjmol-1

Multiply with 2 for 2nd equation: 2C(s)+2O2------->2CO2 (2) -1574kjmol-1

(2)-(1): 2CO+O2 -------> 2CO2 delta H=-1358kjmol-1
 Inverse reaction: delta H=1353kjmol-1




why do we have to do 2-1....Can u pls explain to me .Thanks
Title: Re: Multi-step reaction - sac help
Post by: Jenny_2108 on August 07, 2012, 08:39:54 pm
Because the question asks about 2CO2(g)------>2CO+O2(g)
Thus, you need to get rid of C(s) and 1 O2 right?

Take (2)-(1) so 2C(s) and 1 O2 will be cancelled out, the remaining is 2CO2(g), 2CO and 1 O2(g)
Title: Re: Multi-step reaction - sac help
Post by: Teen on August 07, 2012, 08:47:40 pm
Quote from: Jenny_2108 link=topic=13628[sub
3.msg567935#msg567935 date=1344335994]
Because the question asks about 2CO2(g)------>2CO+O2(g)
Thus, you need to get rid of C(s) and 1 O2 right?

Take (2)-(1) so 2C(s) and 1 O2 will be cancelled out, the remaining is 2CO2(g), 2CO and 1 O2(g)


cool,however, when i retry to apply that knowledge in to a different question . i am getting the wrong answer...
the question was
2Fe+O2 ----->2FeO delta H=-272KJmol^-1
2Fe+(3/2)O2------>Fe2O3 delt= H is -kjmol^-1
delta H for
2FeO+(1/2)O2------>Fe2O3
Title: Re: Multi-step reaction - sac help
Post by: Jenny_2108 on August 07, 2012, 08:50:48 pm
Quote from: Teen on August 07, 2012, 08:47:40 pm
2Fe+(3/2)O2------>Fe2O3 delt= H is -kjmol^-1

give me the delta H value plz!
Title: Re: Multi-step reaction - sac help
Post by: Jenny_2108 on August 07, 2012, 08:54:46 pm
Quote from: Teen on August 07, 2012, 08:47:40 pm
the question was
2Fe+O2 ----->2FeO delta H=-272KJmol^-1
2Fe+(3/2)O2------>Fe2O3 delt= H is -kjmol^-1
delta H for
2FeO+(1/2)O2------>Fe2O3

Take equation (2)- equation (1), you will get 2FeO+(1/2)O2------>Fe2O3
=> Delta H= delta H of equation (2) + 272
You should get the right ans then
Title: Re: Multi-step reaction - sac help
Post by: Teen on August 07, 2012, 09:01:27 pm
yes .... I got it how do it....Thanks for ur assistance
You will get +1 respect
Title: Re: Multi-step reaction - sac help
Post by: Jenny_2108 on August 07, 2012, 09:11:48 pm
Quote from: Teen on August 07, 2012, 09:01:27 pm
yes .... I got it how do it....Thanks for ur assistance
You will get +1 respect

Your welcome. I help you partly because I enjoy the subject, no need +1 respect   :)