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If I've got the standard heats of formation and average heat capacities for the reactants and products, how do I figure out the reaction's ΔH when cooling from 350 K down to 298 K?
If I've got the standard heats of formation and average heat capacities for the reactants and products, how do I figure out the reaction's ΔH when cooling from 350 K down to 298 K?
Use the standard ΔHf values at 298 K, then subtract the integral of ΔCp from 350 K to 298 K. Basically, adjust the enthalpy change using the heat capacities over that temperature drop.
If you're cooling from 350 K to 298 K, and you already have standard heats of formation (ΔHf° at 298 K) and average heat capacities (Cp), here's a controversial take:
Stop relying solely on ΔHf° , you're missing real thermodynamic behavior if you don't account for heat capacity integration. Use: ΔH₃₅₀→₂₉₈ = ΔHf° + ∑(Cp × ΔT) for each species, then apply stoichiometry.
Use the standard ΔHf values at 298 K, then subtract the integral of ΔCp from 350 K to 298 K. Basically, adjust the enthalpy change using the heat capacities over that temperature drop.
Thanks, that helps clarify things. So just to be sure, youre saying i use the standard ΔHf at 298 K, then correct it by subtracting the ∫ΔCp dT from 350 K to 298 K, right? Would I need to account for any phase changes in that range, or just use the heat capacities directly if everything stays in the same phase??
If you're cooling from 350 K to 298 K, and you already have standard heats of formation (ΔHf° at 298 K) and average heat capacities (Cp), here's a controversial take:
Stop relying solely on ΔHf° , you're missing real thermodynamic behavior if you don't account for heat capacity integration. Use: ΔH₃₅₀→₂₉₈ = ΔHf° + ∑(Cp × ΔT) for each species, then apply stoichiometry.
If I've got the standard heats of formation and average heat capacities for the reactants and products, how do I figure out the reaction's ΔH when cooling from 350 K down to 298 K?
I know I'm jumping in a bit late but I wanted to offer a different approach. The first thing you should do is calculate the standard heat of reaction at the reference temperature, 298 K, using your heats of formation. Next, imagine a hypothetical path to get from your reactants at 350 K to your products at 298 K.
So the path has three parts, and the total enthalpy change is the sum of these three steps:
(1) Cool the reactants from 350 K to 298 K. The enthalpy change for this is the reactants' average heat capacity times the temperature difference (298 - 350).
(2) The reaction itself at the standard 298 K, which is the standard heat of reaction you just calculated.
(3) Cool the products from 350 K down to 298 K. The enthalpy change is the products' average heat capacity times the temperature difference (298 - 350).
The most common mistake is getting the signs wrong for the temperature changes, so double-check that. Good luck!
@ch3m3ist_
Thanks for jumping in, really appreciate the detailed breakdown! That step-by-step path makes the concept a lot clearer, especially the idea of breaking it into cooling, reaction and then more cooling. Just to clarify, shouldnt step (3) actually be heating the products from 298 K up to 350 K if we want everything at the same final temperature? Or is this assuming the reaction brings everything back to 298 K in the end ? Just want to make sure I'm not missing the intended direction of the process.
