calculating enthalpy

[Corky Cacas]

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hey, i need help with this question guys?

Given:


Fe(s) + H2O(g) --------> FeO(s) + H2(g) Delta H= -24.7KJ
2FeO(s) + 1/2 O2(g) --------> Fe2O3(s) Delta H= -289.2
H2(g) + 1/2 O2(g) ----------> H2O (g) Delta H= -241.8

Calculate Enthalpy change:

2Fe(s) +3H2O(g) ------>Fe2O3(s) + 3H2(g)

a. -96.8KJ
b. +96.8KJ
c. -72.1KJ
d. 72.1KJ
e.-580.4KJ

I calculate it in the following:
2(-24.7) + (-289.2) + 3(241.8)= which is not one of the answers.
this is a question on Achiever test 3 gchem.


the correct answer is A. but i get something else. any help?

 

[osims]

hey so the correct answer is A. this is how i did it. you always have to look at the balanced final equation they give you because that is your "map." first of all you see that from the final equation that you do not have O2's anymore so you must cancel out the O2 by reversing the third reaction. when you reverse a reaction it becomes the opposite of what it was before (-241 becomes +241). then you see that you need 3H2O's and 2Fe's so in the first reaction you must multiply that by 2. But you also must multiply the delta H by 2 as well so its 2(-24). after that you should be able to add and cancel out everything to equal the final equation. therefore it would be:

+241.8
2(-24.7)
-289.2

Equals---> -96.8 (answer A)

 

[Corky Cacas]

hey so the correct answer is A. this is how i did it. you always have to look at the balanced final equation they give you because that is your "map." first of all you see that from the final equation that you do not have O2's anymore so you must cancel out the O2 by reversing the third reaction. when you reverse a reaction it becomes the opposite of what it was before (-241 becomes +241). then you see that you need 3H2O's and 2Fe's so in the first reaction you must multiply that by 2. But you also must multiply the delta H by 2 as well so its 2(-24). after that you should be able to add and cancel out everything to equal the final equation. therefore it would be:

+241.8
2(-24.7)
-289.2

Equals---> -96.8 (answer A)

hey but we have three water. so why you don't multiply +241.8 by 3?

 

[osims]

because you multiplied the first equation by 2 so you have 2H2O's there and the 3rd equation has 1H2O, so when you add those two together, you get 3H2O.

 

[Corky Cacas]

because you multiplied the first equation by 2 so you have 2H2O's there and the 3rd equation has 1H2O, so when you add those two together, you get 3H2O.

thank. i got it.

 

[113zami]

is there a faster way to do Hess's law problems??

i mean its easy but they always take ~3min from me because of the fact that I have to rewrite all these 4 equations again and balance then add and multiply