## Relationships of Enthalpy with Change in Internal Energy

$\Delta U=q+w$

Daniel Minassian 2D
Posts: 33
Joined: Fri Sep 25, 2015 3:00 am

### Relationships of Enthalpy with Change in Internal Energy

The textbook states that if there is a net change in moles of gas in a reaction then the enthalpy of the reaction is not the same as the change in internal energy of the system because work is done. If the reaction is done under constant volume, then can the change in internal Energy be said to be equal to the enthalpy of the reaction?

Aoife Galvin 2D
Posts: 9
Joined: Fri Sep 25, 2015 3:00 am

### Re: Relationships of Enthalpy with Change in Internal Energy

Work = -(Pressure external)(change in volume), so if the reaction is done under a constant volume, the change in volume would be 0, making work = -(Pressure external)(0) = 0. And going back to the equation for internal energy,
change in U = w + q, if w = 0, then the change in internal energy = 0 + q. And as q represents the energy transferred to a system as heat, it could also be called the enthalpy of the reaction, so yes if a reaction is done under constant volume, then the change in internal Energy is equal to the enthalpy of the reaction.