Thermochemistry (Enthelpy) queery

[Yehia11]

I know that: Change in Internal Energy = Heat change - (pressure X change in volume)

and at constant volume PV is negligable.

But i heard that ENTHALPY is like internal energy but at constant PRESSURE, so PV becomes negligable and enthalpy = heat change - (PV) PLUS (PV) = heat change.

How come at constant pressure, we just use enthalpy (heat change)??

I hope you understood the question, help is very much appreciated! thankyou!

 

[freezard7734]

Enthalphy and internal energy are two similar yet different things. Enthalpy is defined as the sum of internal energy and work due to expansion, that si:
H = U + pV
We usually use enthalpy because it is easier to keep pressure constant than to keep volume constant. Its just a matter of convenience.