Fundamentally, how are ΔHrxnand ΔHfdifferent? Which part of the experiment demonstrated the change inenthalpy per mole of a reaction?

The heat of reaction, ΔHrxn, is the change in enthalpy in a chemical reaction. It is the difference in enthalpies of the products and reactants. The reaction enthalpy dictates the amount of energy absorbed or released during a reaction.

The heat of formation, ΔHf, is the the energy required to form a compound from its elements in their standard states.

The heat of reaction can be found by using the heat of formation of the products and reactants and finding the difference. The heat of formation tells us the energy per mole required to form the product. The heat of reaction will tell us the change in enthalpy per mole of a reaction.