## Enthalpy versus heat

isochoric/isometric: $\Delta V = 0$
isothermal: $\Delta T = 0$
isobaric: $\Delta P = 0$

Sanjana Borle 2K
Posts: 111
Joined: Fri Aug 09, 2019 12:15 am

### Enthalpy versus heat

For enthalpy and heat, the only differences between them (q and H) is that enthalpy is a state property and the heat is not, correct? When are we supposed to assume that they equal each other?

chemboi
Posts: 101
Joined: Sat Jul 20, 2019 12:16 am

### Re: Enthalpy versus heat

I think they equal each other when there is no work being done on the system. Enthalpy is the change in heat of a system at constant pressure, and yes it is a state function vs. heat which depends on the path of the reaction.

205291012
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Joined: Mon Nov 11, 2019 12:17 am

### Re: Enthalpy versus heat

yes, that is correct.
delta H = q when no work is done. So assume they're the same when no work is being done by or on the system.

Erik Buetow 1F
Posts: 96
Joined: Fri Aug 30, 2019 12:15 am

### Re: Enthalpy versus heat

Yes, in essence that is the only difference. A general rule of thumb is that enthalpy is always heat, but heat does not always equal enthalpy. This has to do with the fact that enthalpy is heat at a constant pressure, so if it is not a constant pressure then heat will not equal enthalpy.

Cavalli_1H
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Joined: Thu Sep 19, 2019 12:15 am

### Re: Enthalpy versus heat

enthalpy is a form of heat but heat is not a form of enthalpy

Charysa Santos 4G
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Joined: Wed Sep 18, 2019 12:21 am

### Re: Enthalpy versus heat

Delta H is equal to q + w, so when there is no work being done on or by the system (or when w = 0), then delta H = q

SarahCoufal_1k
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Joined: Thu Jul 25, 2019 12:17 am

### Re: Enthalpy versus heat

In the review session today they said enthalpy is always heat, but heat is not always enthalpy. The definition of enthalpy is heat at constant pressure. So if it isn't at constant pressure, the heat can't be described as enthalpy

kausalya_1k
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Joined: Wed Nov 14, 2018 12:23 am

### Re: Enthalpy versus heat

so enthalpy is basically like heat but only during isobaric (deltaP=0) situations
basically, enthalpy is always heat, but heat is not always enthalpy.

William Francis 2E
Posts: 101
Joined: Wed Sep 18, 2019 12:20 am

### Re: Enthalpy versus heat

I think it should be noted that enthalpy can never really be determined. When pressure is constant, ΔH=q, so the change in enthalpy is equal to heat, but enthalpy itself is not equal to heat.

William Francis 2E
Posts: 101
Joined: Wed Sep 18, 2019 12:20 am

### Re: Enthalpy versus heat

Also, I believe that ΔU is equal to q+w, not ΔH, so when no work is being done on or by a system, it doesn't necessarily mean that ΔH=q. Rather, ΔH=q under constant pressure conditions. When no work is being done by a system, ΔH=q+PΔV as derived from ΔH=ΔU+PΔV with (q+w) substituted for ΔU and w=0.

Leila_4G
Posts: 114
Joined: Sat Sep 14, 2019 12:17 am

### Re: Enthalpy versus heat

And to further clarify, standard enthalpy for a reaction and heat needed for that reaction are the same, right? As long as the q is for the same number of moles at the delta H is for.
Because I noticed that standard enthalpy only has the units of kJ (or J). (Not kJ per mol)

Sartaj Bal 1J
Posts: 101
Joined: Thu Jul 25, 2019 12:17 am

### Re: Enthalpy versus heat

Enthalpy is the study of heat released or absorbed in chemical reactions and physical changes. It is considered a state property as its value is determined by its current state. Heat is the transfer of energy due to temperature differences. It depends on the path taken and is therefore not a state property.

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