Multi-step reaction - sac help

[Teen]

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)

[Jenny_2108]

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

[Teen]

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

[Jenny_2108]

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)

[Teen]

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

[Jenny_2108]

2Fe+(3/2)O₂------>Fe₂O₃ delt= H is -kjmol^-1

give me the delta H value plz!

[Jenny_2108]

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

[Teen]

yes .... I got it how do it....Thanks for ur assistance
You will get +1 respect

[Jenny_2108]

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