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## Main Question or Discussion Point

What is the difference between [tex]\Delta[/tex]H and [tex]\Delta[/tex]E in a constant volume process ?????

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What is the difference between [tex]\Delta[/tex]H and [tex]\Delta[/tex]E in a constant volume process ?????

- #2

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[tex]

H=E+pV

[/tex]

At constant volume you have

[tex]

\Delta_V H=\Delta E+V\Delta p

[/tex]

Here you see the difference in the term.

What exactly did you want to know?

- #3

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i.e.

i.e. [tex]\Delta[/tex]h = [tex]\Delta[/tex]U + [tex]\Delta[/tex]W

and [tex]\Delta[/tex]W = [tex]\Delta[/tex](pv) and

[tex]\Delta[/tex](pv) = P[tex]\Delta[/tex]V + V[tex]\Delta[/tex]P and

for work done V[tex]\Delta[/tex]p is taken as zero so how come can we say here V[tex]\Delta[/tex]P is not taken as zero here?

- #4

Mapes

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Hi jeedoubts, welcome to PF. Can you give a reference for these two equations? I doubt very much that they're correct. For example, work is defined as [itex]P\Delta V[/itex], not [itex]\Delta(PV)[/itex].i.e. [tex]\Delta[/tex]h = [tex]\Delta[/tex]U + [tex]\Delta[/tex]W

and [tex]\Delta[/tex]W = [tex]\Delta[/tex](pv)

I agree with Gerenuk's answer.

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what does the quantity v[tex]\Delta[/tex]p refers to then????

- #6

Mapes

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The difference between enthalpy change and energy change in a constant-volume process.

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physically what does it represent??

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Mapes

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- #9

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[tex]\Delta[/tex]W = [tex]\Delta[/tex](pv) and

[tex]\Delta[/tex](pv) = P[tex]\Delta[/tex]V + V[tex]\Delta[/tex]P

[tex]

\mathrm{d}W=p\mathrm{d}V

[/tex]

(or with the other sign if you consider the work done on the system) and if you wish

[tex]

\mathrm{d}(pV)=p\mathrm{d}V+V\mathrm{d}p

[/tex]

It follows that only for constant volume or constant pressure processes the work can be described by

[tex]

W=p\Delta V\qquad\text{(const. p or const. V)}

[/tex]

Is it very important to know what is the general equation and what the special case. These special cases only apply if the conditions are met.

Mapes is correct.

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