[offlinedoctor]

I think my understanding of enthalpy vs energy is rather poor as I get these types of questions consistently wrong.

C2H2 + 5/2O2 -> 2CO2 + H2O ΔH= -1299.5kJ.

Determine ΔU at 25°, constant pressure.

1) this h been nagging me, but shouldn't enthalpy be in energy per mol??

2) my logic was, if it was at constant pressure it would be conditions similar to open containers, so I thought, the energy lost would just be equal to enthalpy. The answer seemed to indicate that its actually -1298.3, how is it possible?

[Borek]

1) this h been nagging me, but shouldn't enthalpy be in energy per mol??

It is probably "per reaction as written" - so per mole of C₂H₂ burnt, or per mole of H₂O produced, or per two moles of CO₂ produced.

2) my logic was, if it was at constant pressure it would be conditions similar to open containers, so I thought, the energy lost would just be equal to enthalpy. The answer seemed to indicate that its actually -1298.3, how is it possible?

What is the dependence between ΔU and ΔH? What is the volume before and after the reaction?

[offlinedoctor]

Volume before and after the reaction, wouldn't it be unchanged, so ΔV would be zero hence, Energy would equal Enthalpy?

[Borek]

What is volume before the reaction, what is volume after the reaction?

You can assume STP.

[offlinedoctor]

The question makes no mention of volume changes. 

[Corribus]

Your assumption that ΔH is equal to ΔU is a great approximation in most cases involving condensed phases, because usually the volume change is small (and hence the work is small) compared to the amount of energy liberated/consumed by bond formation and breaking.  You can see the difference here between ΔH andΔU is small as well but not really negligible.  This is because with gasses the volume changes are typically a lot greater on a per mole basis.

As to what the volume change is, as Borek suggested you can determine the approximate volumes of reactants and starting materials by assuming everything is carried out at STP.  (Hint: what is the molar volume of an ideal gas at STP?)

[offlinedoctor]

I figured it out! Is this the right assumptions?

I) the change in mole is from 3.5 to 3, so some work will be done.

Ii)
using n=V/Vm, the total volume from the left hand side side is, 78.4L, total volume on right hand side is, 67.2L.
So volume change is 11.2L,
Multify by 101 to convert to joules, divide by 1000 for kJ,
Gives 1.13kJ,

So E=H-W
E=-1299.5+1.13 (add, as the work goes 'into' the system.

[Corribus]

Yes that looks right to me.

[MətHylFrÆtH]

I thought that

Δh=Δu + Pv

and for for ideal gases

Δh = Δu + RT

for ideal liquids and solids:

Δh ≈ Δu

[Corribus]

Yes.