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#Post#: 463--------------------------------------------------
Thermodynamics
DIR By: PIRAH SALEEM
Date: July 31, 2012, 1:26 pm
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Hess`s law of heat summation is the application of 1st law of
thermodynamics......Is it true or false?? give your answer with
a short but effective explanation
#Post#: 464--------------------------------------------------
Re: Thermodynamics
DIR By: Michel
Date: August 1, 2012, 4:50 am
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Firstly, I'd like to ask..what's your opinion? do you think it's
true or false?
how do you think Hess law relates to the 1st law of
thermodynamics?
#Post#: 466--------------------------------------------------
Re: Thermodynamics
DIR By: Chris
Date: August 2, 2012, 4:30 am
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I'd say it's true, because hess law means that the enthalpy of a
reaction can be gotten from the combination of other reactions
eg
a) C + O2==> CO2, dH=X
can be derived from
b)2C + O2==> 2CO, and
c)2CO + O2 ==> 2CO2
the dH for reaction A can be gotten by accurately combining the
dH for reaction b and c.
the first law of thermodynamics is about energy conservation, so
no energy is created or destroyed in any of the above reactions.
if energy is created in reaction b for instance, we would not be
able to calculate the dH for reaction a by combining the values
of dH for reactions b and c, because we would have to add the
energy created in reaction b. since this is not so, it means
that the 1st law of thermodynamics is applied in hess law.
That's the way i understand this. hope it helps
#Post#: 469--------------------------------------------------
Re: Thermodynamics
DIR By: PIRAH SALEEM
Date: August 2, 2012, 2:09 pm
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I thought that there is no relation between Hess`s law and 1st
law of thermodynamics cox Hess`s law is about path that whether
a reaction is carried out in 1 step or several steps the energy
is same but 1st law of thermodynamics is about energy
conservation i.e conversion of 1 form of energy into another not
about path
#Post#: 474--------------------------------------------------
Re: Thermodynamics
DIR By: Michel
Date: August 3, 2012, 4:35 am
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--- Quote from: PIRAH SALEEM link ---
>
> Hess`s law is about path that whether a reaction is carried
out in 1 step or several steps the energy is same
>
--- End Quote ---
In a strict sense, the energy change is the same, not the energy
of the molecules involved. We cannot measure internal energy.
Also, suppose energy is never conserved in any chemical
reaction, there will be no Hess's law, so Hess's law can be seen
as a product of the law of energy conservation (energy is
conserved in the universe).
#Post#: 475--------------------------------------------------
Re: Thermodynamics
DIR By: Michel
Date: August 3, 2012, 4:36 am
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--- Quote from: PIRAH SALEEM link ---
>
> Hess`s law is about path that whether a reaction is carried
out in 1 step or several steps the energy is same
>
--- End Quote ---
In a strict sense, the energy change is the same, not the energy
of the molecules involved. We cannot measure internal energy.
Also, suppose energy is never conserved in any chemical
reaction, there will be no Hess's law, so Hess's law can be seen
as a product of the law of energy conservation (energy is
conserved in the universe).
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