"Moles of Reaction"

Email question:
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I know you answered this question in class on Friday but I'm still uncertain on how you get mols of rxn? Can you give me a hypothetical on the equation we did on Friday from exam 3a from Fall '08? In this it's 1 mol rxn per 2 mols of haxane.
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OK, this one always causes some trouble, largely because I try to tie together the units that we typically see on enthalpy and the balanced equations. It might be easier to think of it simply as "reaction" rather than "moles of reaction", so we could look at something like:
2 H₃PO₄(aq) + 3 Ca(OH)₂(aq) --> Ca₃(PO₄)₂(s) + 6 H₂O(l)
Calculating {delta}H_rxn for this process, we get...
2(1288.3^kJ/_mol) + 3(542.8^kJ/_mol) + 6(230.02^kJ/_mol) + 1(-4120.8^kJ/_mol) + 6(-285.8^kJ/_mol) = -250.48^kJ/_mol
{Note: These are numbers I pulled from a table similar to the one in your textbook. Change the sign on reactants because these are being consumed in the reaction, not formed.}
Since {delta}H is negative, this reaction is exothermic, but what exactly are those units? Remember when I ran through one of the first enthalpy problems in class I used very complete and expanded units, let's just look at the first term here. The units on the enthalpy of formation for phosphoric acid are "kilojoules per mole of phosphoric acid formed". If we want to properly add these terms together, they have to have the same unit, so we have to convert/relate "moles of phosphoric acid" into something that is consistent throughout this problem. That's where the units on the "2" become important, even through they are often left off. That "2" comes from the balanced chemical equation and is really "2 moles of phosphoric acid per balanced chemical equation" or "2 moles of phosphoric acid per reaction". OK, so we do that for every term in the problem and then add them together to get the final answer with units of "kilojoules per balanced chemical equation" or "kilojoules per reaction" and everything is great... except that these enthalpies of reaction are often reported with units of "kJ/mol". Mole of what? In the above reaction, we could say it's per mole of calcium phosphate because for each "reaction" there is 1 mole of calcium phosphate formed, but that seems to limit us to problems that only deal with calcium phosphate. Here comes the magic unit "mol of reaction". Using the terminology above, we can say that one "balanced chemical equation" is one "mol of reaction".
Many (most? maybe all?) textbooks get around this problem by being quite explicit in the way they present thermochemical reactions, in fact your textbook has a section in Chapter 6 called "Thermochemical Expressions" {Sec. 6.5, p 230) that gives a nice example. If the {delta}H for a chemical equation is shown right next to the balanced chemical equation to which it refers, it can be implied that the {delta}H is valid only for the exact balanced equation shown, so the "per mol" or "per mol rxn" is often omitted and {delta}H is just reported with units of "kJ".

OK, after that LONG explanation, let's try a shorter answer. You can think of "mol of rxn" simply as "rxn". Each time the reaction happens once (as balanced), the calculated heat is liberated or consumed. The reason I tend to use the "mol of rxn" label is because it naturally leads to the question "What reaction?" which means that every enthalpy you calculate MUST be related to a specific balanced chemical equation.

Other questions, let me know...