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Chemistry Forums for Students => Physical Chemistry Forum => Topic started by: f389#F on March 04, 2021, 07:42:05 PM

Title: Heat of combustion
Post by: f389#F on March 04, 2021, 07:42:05 PM

Dear forum

For the calculation of the heat of combustion at isochoric conditions, one can use Qv = ∆U, and ∆U=∆H-P∆V.
At isochoric conditions, ∆U=∆H and therefore Qv = ∆H.

However,

∆U=∆H-P∆V can be rewritten as ∆U=∆H-RT.∆n

My concerns are:

1) may we use such expression ∆U=∆H-RT.∆n in a combustion reaction? (why considering isothermic reactions in a combustion?)

2) can such expression be used in isochoric conditions?

3) we defined 2 expressions for ∆U at isochoric conditions: 1) ∆U=∆H-P∆V (being P∆V=0) and 2) ∆U=∆H-RT∆n. Therefore we will probably find two different ∆U values by using the first or the second equation. Which one should be used?

Thank you in advance.

Title: Re: Heat of combustion
Post by: mjc123 on March 05, 2021, 08:38:08 AM

The most general expression is Δ(PV) = Δ(nRT).
Δ(PV) = PΔV only if the pressure is constant.
Δ(nRT) = RTΔn only if the temperature is constant.
So PΔV = RTΔn only if both pressure and temperature are constant.