## ΔH vs ΔU?

Anna O 2C
Posts: 98
Joined: Fri Sep 28, 2018 12:19 am

### ΔH vs ΔU?

Can someone explain why heat transfer is ΔU at constant volume and interpreted as ΔH at constant pressure?

Becky Belisle 1A
Posts: 81
Joined: Fri Sep 28, 2018 12:18 am

### Re: ΔH vs ΔU?

The formula for internal energy is: ΔU= q + w, so when volume is constant, w=0. Thus, when no work is done, ΔU = q. ΔH = q at constant pressure because if the pressure changed that would affect the enthalpy, but not the heat transfer.

haleyervin7
Posts: 61
Joined: Fri Sep 28, 2018 12:15 am

### Re: ΔH vs ΔU?

Can deltaU ever equal deltaH?

Cole Elsner 2J
Posts: 88
Joined: Fri Sep 28, 2018 12:25 am

### Re: ΔH vs ΔU?

Yes at constant pressure. If there is no net change in moles, there will be no work done. So there is the potential for deltaU=deltaH.

Soumya Ravichandran 4H
Posts: 62
Joined: Fri Sep 28, 2018 12:19 am

### Re: ΔH vs ΔU?

Yes, this can occur when there is no volume change (free expansion), but generally speaking, delta H will only equal delta U when there is no work done and pressure is held constant

Anjali_Kumar1F
Posts: 62
Joined: Fri Sep 28, 2018 12:25 am

### Re: ΔH vs ΔU?

If enthalpy is given do we automatically assume the reaction is at constant pressure.

Destiny Diaz 4D
Posts: 51
Joined: Fri Sep 28, 2018 12:28 am

### Re: ΔH vs ΔU?

When enthalpy is given I would not say it is safe to assume that pressure is constant, unless other wise stated.

Pritish Patil 1K
Posts: 60
Joined: Fri Sep 28, 2018 12:24 am

### Re: ΔH vs ΔU?

When volume is constant, work = 0 J. This means that deltaU = q.

CHEM 14B Lover
Posts: 33
Joined: Fri Sep 28, 2018 12:19 am

### Re: ΔH vs ΔU?

Can't there be expansion with constant pressure? Wouldn't that still be work done?