 Jun 2nd 2018, 02:59 PM #1 Junior Member   Join Date: Jun 2018 Posts: 2 Heat of Reaction (Enthalpy) Well, suppose we have a reaction: $\displaystyle |\nu _1| A_1 +|\nu _2| A_2+\cdots \rightleftharpoons |\nu _3| A_3 +|\nu _4| A_4 +\cdots$ At standard conditions $\displaystyle (P_0,T_0)=(1$ bar $\displaystyle , 298.15$ K $\displaystyle )$ we calculate the enthalpy of reaction via tabulated values of enthalpy of formation: $\displaystyle \Delta H_r(P_0,T_0)=\sum\limits _i \nu _i H_{f,i}(P_0,T_0)$ Until now that's ok! If I want to evaluate the change in enthalpy of reaction at another condition, say $\displaystyle (P,T)$ most textbooks come with the following expression: $\displaystyle \Delta H_r(P,T)=\Delta H_r(P_0,T_0)+\int\limits _{T_0}^{T} \Delta C_P d\tau$ where $\displaystyle \Delta C_P\equiv \sum\limits _i\nu _i C_{P,i}$ For me that's all right. I understand the path (changing the reactants from $\displaystyle (P,T)$ to $\displaystyle (P_0,T_0)$, proceeding with the reaction and than taking the products from $\displaystyle (P_0,T_0)$ to $\displaystyle (P,T)$). What I can't understand is: why the pressure change is not considered? I mean, the expression is correct only for ideal gases, which $\displaystyle H=H(T)$. Shouldn't I put the residual terms in the expression? Like these: $\displaystyle \Delta H_r(P,T)=\Delta H_r(P_0,T_0) + \int\limits _{T_0}^{T} \Delta C_P d\tau + H^R_{reac.}(\overline{y}_{reac.},P_0,T_0) - H^R_{reac}(\overline{y}_{reac.},P,T)+H^R_{prod.}( \overline{y}_{prod.},P,T)-H^R_{prod.}(\overline{y}_{prod.},P_0,T_0)$ and then use some EOS to compute the residuals... I would be grateful if someone help me... Thanks!   Jun 4th 2018, 02:44 AM   #2
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 What I can't understand is: why the pressure change is not considered?
It seems that the enthalpy is defined as the amount of heat absorbed or released at constant pressure, so if you consider pressure changes, you're no longer calculating enthalpy.

For pressure changes in a chemical reaction, maybe this text will help? https://www.springer.com/cda/content...044-p174060664   Jun 4th 2018, 09:49 AM #3 Junior Member   Join Date: Apr 2017 Posts: 2 when [p,t] is not constant ?  Tags enthalpy, heat, reaction Thread Tools Show Printable Version Email this Page Display Modes Linear Mode Switch to Hybrid Mode Switch to Threaded Mode Similar Physics Forum Discussions Thread Thread Starter Forum Replies Last Post Rome Thermodynamics and Fluid Mechanics 1 Oct 9th 2017 12:23 PM simina11 Thermodynamics and Fluid Mechanics 2 Feb 22nd 2015 05:22 PM Wobzter Advanced Thermodynamics 2 Jul 4th 2012 08:02 AM rimmer Thermodynamics and Fluid Mechanics 1 Mar 9th 2010 03:54 AM Natla Energy and Work 0 Nov 6th 2008 04:19 PM 