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Hess's law - HSN forum

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Hess's law


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#1 doug_90

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Posted 21 February 2006 - 08:09 PM

HELP, would really appreciate it if someone could do this question.

Calculate the enthlapy of ethanol, C2h5OH, using the enthlapies of combustion of carbon and hydrogen and the ebthlapy of formation of ethanol.
(NOTE: Dont't forget to count the oxygen present in ethanol when balancing the combustion equation!)

#2 The Wedge Effect

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Posted 24 February 2006 - 12:23 PM

QUOTE(doug_90 @ Feb 21 2006, 08:09 PM)
HELP, would really appreciate it if someone could do this question.

Calculate the enthlapy of ethanol, C2h5OH, using the enthlapies of combustion of carbon and hydrogen and the ebthlapy of formation of ethanol.
(NOTE: Dont't forget to count the oxygen present in ethanol when balancing the combustion equation!)

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It would help if you gave us those enthalpies needed.

#3 ad absurdum

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Posted 24 February 2006 - 01:17 PM

I think this is what you do:

2C(s) + 2O2(g) --> 2CO2
delta.gif H = -(394*2)kJ (because 394 is the value for one mole, 394*2 is the value for two moles of graphite combusting)

6H2 + 3O2 --> 6H2O
delta.gif H = (-286*6)kJ (as before, except multiplied by six as there are six moles of hydrgoen combusting)

2CO2 + 6H2O --> C2H5OH + (9/2)O2
delta.gif H = + 1378kJ (because you have flipped the equation around, you have to change the sign for the enthalpy change)

Combine these equations:

2C(s) + 2O2(g) + 6H2 + 3O2 + 2CO2 + 6H2O --> 2CO2 + 6H2O + C2H5OH + (9/2)O2

Cancel stuff out to get:

2C(s) + 6H2 + (1/2)O2 --> C2H5OH
delta.gif H = (-394*2) + (-286*6) + 1378 kJ/mol
delta.gif H = -1126 kJ/mol

I really hate this type of question.
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#4 doug_90

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Posted 24 February 2006 - 05:33 PM

Thanks ad that really helped. biggrin.gif I dont mind hess's law but that question was annoyin me cse the equtions wouldnt balance.

BTW wedge if i had the enthlapies i would hav posted them!! tongue.gif

#5 ad absurdum

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Posted 24 February 2006 - 06:23 PM

QUOTE(doug_90 @ Feb 24 2006, 05:33 PM)
Thanks ad that really helped. biggrin.gif  I dont mind hess's law but that question was annoyin me cse the equtions wouldnt balance.

BTW wedge if i had the enthlapies i would hav posted them!! tongue.gif

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You're welcome. If you're looking for enthalpy values, then the data booklet has a few and I think we only get asked questions which can be solved using these ones.
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#6 mystic

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Posted 28 February 2006 - 12:31 PM

is it enphalthy( bad spellings) that you can work out by using albergare?
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#7 dfx

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Posted 28 February 2006 - 03:22 PM

albergare?

#8 Joel

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Posted 28 February 2006 - 03:26 PM

QUOTE(dfx @ Feb 28 2006, 03:22 PM)
albergare?

Algebra, perhaps? huh.gif

#9 doug_90

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Posted 28 February 2006 - 05:15 PM

thats the one gizmo. dont worry guys as long as some of us can interpret gizmo's ramblings tongue.gif

#10 mystic

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Posted 01 March 2006 - 10:10 PM

hey i make sense! most of the time, if only some listened boy! lol. biggrin.gif biggrin.gif biggrin.gif smile.gif laugh.gif u know who you are! so is it?
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