Physics Help Forum Heat of Reaction (Enthalpy)

 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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Join Date: Oct 2017
Location: Glasgow
Posts: 364
 Originally Posted by tom89 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

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