2 ways to find ΔĤ of a reaction at STP:
As noted above, there are two ways to calculate the specific enthalpy change of a reaction at STP, ΔĤ°rxn (the "°" that follows the ΔH is a constant reminder that this is the ΔH at STP, 273.15 Kelvin and 1 atm).
Using the Heats of Formation is the general route, and we will only use Hess's law if we are given ΔĤ°rxn's of other reactions.
To introduce these two methods, let's use the following reaction:
Here, we would be given other heats of reaction that we can manipulate algebraically to up with the heat of reaction in this example. You have seen this before, in General Chemistry, so let's just show you the mechanics in an example:
So, we would have to be given other heat's of reaction, for example, we could be given the following:
Looking at the original equation, we want CO and H2 on the left, and CO2 and H2 on the right. So we must flip reaction B, to give,
Now, we would usually be concerned with getting the stoichiometry right, but here, if we simply add these equations, we get our reaction, thus, our ΔĤ°rxn, thus, adding these equations, we get:
Thus, by making use of Hess's law, we are able to determine the ΔĤ° rxn to be -41 kJ/mol.
Heats of Formation:
Thus, we have now used heats of formation to obtain ΔĤ° rxn. But what about ΔH°rxn? Well, this is even easier.
Two steps to find ΔH°rxn:
First, we find ΔĤ°
rxn, from above. Hey, we just did that. Yep, using either Hess's law or Heats of Formation.
Second, with ΔĤrxn now known, we apply the following new relation.
where x is as was defined in chapter 4, the extent of reaction,
where i can be any component in the reaction, and n is the number of moles that reacted and v is its stoichiometric coefficient.
For the previous example, if we had 5 moles of CO react (and thus, 5 moles of each of the other species involved in the reaction), we could calculate the ΔH°rxn, first obtaining x,
Here we will have streams coming into and out of the reactor at different temperatures with some components possibly undergoing phase changes, in addition to the reaction. We need to consider all of this when determining the heat exchange, or the ΔHrxn (again, because Q = ΔH, for ΔEp, ΔKE, and Ws all = 0). Before we set up a problem,
steps for solving energy reaction balances:
Now, for me, these reaction calculations are quite involving, so I would like to work and example from the book for you myself. Click here for an example.
5 moles of heptane is reacted to give toluene and hydrogen gas. Heptane at 500°C is fed to a reactor, which operates isothermally at this temperature. Determine the heat transferred to or from the reactor in kJ.
Given the following two reactions of ammonia with air:
Determine the rate of heat transfer to or from the reactor in kW.
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