Understanding Enthalpy Change in Closed Systems

[PPonte]

I was doing a True/False exercise and come across this one:

The enthalpy change measures the energy of the reaction when it occurs in a closed recipient with rigid walls.

With the knowledge I have, I think the energy of the reaction does not depend on the caracteristics of the recipient. What happens is that if it occurs in an open recipient the energy will disperse, but it continues to be the same energy.


The book says that it is false. And the correction is:

The enthalpy change measures the energy of the reaction when it occurs in a closed recipient with rigid walls and at constant pressure.

Could someone (Master Astronuc) clarify me? Thank you.

 

[Astronuc]

H = U + PV

H = enthalpy, U = internal energy

Internal energy = measures the energy of the reaction when it occurs in a closed recipient with rigid walls.

http://www.chem.arizona.edu/~salzmanr/480a/480ants/energy/energy.html - See equation 15 about midway down.

 

[PPonte]

Thank you, Astronuc! (bows to you) I already knew the equation of enthalpy but did not use it since I didn't learn it at school. How stupid I was.


But I need some clarifications.

The enthalpy change measures the energy of the reaction only when it occurs in a closed recipient with rigid walls, where the pressure/volume* is constant?

*To simplify I assume that the reactants and products are gases, so there is an inverse proportionality between volume and pressure.


This means that the energy of the reaction is always equal to the internal energy change, that in this case (a closed recipient with rigid walls) is equal to the enthalpy change because the pressure/volume is constant?

The questions are similar, but slightly different.


And I may conclude that the correction of the book is wrong, i.e., if a closed recipient garantees constant pressure/volume, it is a pleonasm to say that it is at constant pressure. Therefore, the initial statement is true.

 

[GCT]

Not sure what you're going for here, but as Astronuc implicated, stick with the equations and try not to deviate too much from formal mathematical implications. Enthalpy could mean many things, the utility of it is that, in addition to internal energy, it takes into account the pressure volume situation. There are times, when enthalpy equals zero for a particular context, however, I don't believe that there are any restrictions to when or when it isn't defined.

You'll need to read the complete derivations, thermodynamics, in a p. chemistry text. In gen. chem. they'll inform you on the basics, but nothing is really fundamental. The simple fact is that it's not particularly easy to keep track of energy conservation. You may think you have it all together, but if you ever get to it, the equations will tell you that it's fairly complicated.

 

[GCT]

This means that the energy of the reaction is always equal to the internal energy change, that in this case (a closed recipient with rigid walls) is equal to the enthalpy change because the pressure/volume is constant?

read upon the adiabatic case.

 

[Cyrus]

Enthalpy, is by definition, as Astronuc said:

h = u + Pv

where, h is the specific enthalpy, u is the specific internal energy, and v is the specific volume. (Specific means a per unit mass basis (KJ/Kg) )

I think the problem is that you do not understand the definition of enthalpy, which is the energy change due to internal energy and the Pv boundary work done by the system on the surroundings under constant pressure. *Note that enthalpy, h, and internal energy, U, are dependent on Temperature and not on pressure.

 

[Astronuc]

Adding to what my learned colleagues, GCT and cyrusabdollahi, have mentioned, the examples I have often seen in thermodynamics books are a rigid vessel (constant volume, increasing pressure) with heat input, and a piston with a constant weight (constant pressure, changing volume).

Heat added to the constant volume system changes the internal energy, and heat added to the piston, changes the enthalpy.

 

[vivek_mahali]

what will be the change in enthalpy when heat is supplied at constant volume?