Question about thermochemistry

[t0ny]

Hey guys I was going over EK Chemistry lesson 3 and I keep getting caught up on this part.

It says that enthalpy change at constant pressure can be either of these and I don't know when to apply which scenario:

1) deltaH= deltaU + PdeltaV
2) deltaH= q

I know the second case is if you have constant pressure, your volume will be constant and this deltaV will be zero and all energy change of system is due to heat transfer but I am not quite sure of when to use the first one.

Any help clarifying this would be great!

---

Members don't see this ad.

 

[sps27]

The first equation is a generic equation. So I think the first equation is applicable everywhere where there is Heat exchange at constant pressure accompanied by some volume change which would mean experiments conducted in lab (such as Br (l) ----> Br (g) ) which are essentially under constant atmospheric pressure but also have a volumetric change as well. The second equation is more of a specific case of first equation where it so happens that Heat transfer takes place not only at constant pressure but also a constant volume, so the delta V term vanishes away and you are left with delta U = delta H. Not sure if this helps you......