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Chemistry Forums for Students => Physical Chemistry Forum => Topic started by: simonsci on July 02, 2015, 04:37:35 AM

Title: Symbol of standard enthalpy change
Post by: simonsci on July 02, 2015, 04:37:35 AM

Hello, I found in Atkins textbook (9th edition) pp.65, the author writes that:

A note on good practice:
The attachment of the name of the
transition to the symbol Δ, as in
ΔvapH, is the modern convention.
However, the older convention,
ΔHvap, is still widely used. The new
convention is more logical because
the subscript identifies the type of
change, not the physical observable
related to the change.

What does the last sentence mean? Why the current notation is preferred?

Title: Re: Symbol of standard enthalpy change
Post by: mjc123 on July 02, 2015, 06:07:21 AM

I suppose it means that ΔH_vap might be interpreted as the change in a quantity H_vap, which doesn't exist. The real quantities are H_gas and H_liq, the enthalpies of the substance in the gaseous and liquid states, and the difference between them is the enthalpy change of vaporization, Δ_vapH. The subscript of H denotes a substance or state, which can have a value of enthalpy. A process or change, like vaporization, can only be associated with a change in enthalpy, the difference between the enthalpies of the final and initial states, so the subscript is attached to the Δ.

Title: Re: Symbol of standard enthalpy change
Post by: simonsci on July 02, 2015, 06:40:04 AM

I see, thank you very much  :) :)

Title: Re: Symbol of standard enthalpy change
Post by: Corribus on July 02, 2015, 10:11:44 AM

Lol, I've never seen this "modern convention" before.
I'll go to my grave using ΔH_vap, thank you very much. :)

Title: Re: Symbol of standard enthalpy change
Post by: Dan on July 03, 2015, 11:00:48 AM

Quote from: Corribus on July 02, 2015, 10:11:44 AM

Lol, I've never seen this "modern convention" before.

Atkins was my thermodynamics lecturer 10 years ago - I can tell you he pushes this modern convention very strongly.