I pulled the following chemical equation from the CollegeBoard AP® Chemistry Course and Exam Description, Fall 2020:
2SO2(g) + O2(g) → 2SO3(g)
∆H° = -198 kJ/mol-rxn
This equation means that:
when two moles of gaseous sulfur dioxide react with one mole of gaseous oxygen in standard conditions of 298 K and atmospheric pressure, the reaction yields two moles of gaseous sulfur trioxide and releases 198 kJ of excess energy.
While the chemical equation itself is standard, the notation for the reaction’s change in enthalpy is nuanced and can be a sore point sowing confusion. Let’s break the enthalpy term down piece by piece:
• H denotes a state of enthalpy.
• ∆H denotes a change in enthalpy and implies that:
Because ∆H is negative in this reaction, the products have a lower enthalpy than the reactants; this reaction releases energy; and the released energy would be observed as excess heat emerging from the source. All to say that the reaction is exothermic.
The prime or degree mark in ∆H° denotes standard reaction conditions, such that the reaction is conducted at 298 K and 1 atmosphere of pressure.
∆H° = -198 kJ denotes that the reaction, with the written stoichiometry and conducted at standard reaction conditions, releases exactly 198 kJ of energy.
∆H° = -198 kJ/mol-rxn contains an additional mark “/mol-rxn” that was introduced by the CollegeBoard. This mark has been a source of significant confusion because it is not present in traditional chemistry texts, is thus foreign to most chemists, and was introduced into AP® exam preparation materials without explicit definition or justification.
The “/mol-rxn” mark is especially confusing because in most reactions, the stoichiometry of each reactant and product differs. There is simply no such thing as a reaction mole unless we refer to a mole of reactions, i.e., 6.022E23 reactions occurring in concert, which we are certainly not.
Thus, the only way to interpret the “/mol-rxn” mark is as “the reaction stoichiometry as written”. Thus, “/mol-rxn” simultaneously refers to the heat released “per mole of O2 reactant”, “per 2 moles of SO2 reactant”, and “per 2 moles of SO3 product.” And so, there is no real difference between “∆H = -198 kJ” and “∆H = -198 kJ/mol-rxn“; the two notations mean the same thing, except that the second notation is ripe for controversy and confusion.
My best advice is to teach students to read the “/mol-rxn” mark as “with the reaction stoichiometry as written.” Please ensure students understand that senior chemists will be unfamiliar with this notation, rightly find it unnecessary and arbitrary, and will resist using it in modern chemistry practices. But students who will take the AP® exam must be explicitly taught how to interpret the “/mol-rxn” mark, else they could easily mistake its meaning and come to the wrong answer on the AP® exam, even if they fundamentally understand all relevant chemical concepts involved.
Author: J Matt Jackson, PhD