Chemistry / Unit 4 Chem 0688 worked example, 60 to 90 seconds
Determining Molar Enthalpy Change from a Limiting Reactant
The idea
Determining a reaction's molar enthalpy change from measured data means identifying which reactant limits the reaction, then scaling the measured heat up to the amount the balanced equation, as written, would actually produce.
In practice
A 0.240-g strip of magnesium is dropped into 25.0 mL of 0.500 M hydrochloric acid, and the calorimeter records 2.89 kJ of heat. Converting both to moles gives 0.00987 mol of magnesium and 0.0125 mol of acid; since the balanced equation consumes two acid units per magnesium atom, the magnesium present would need 0.0197 mol, so the acid runs out first and limits the reaction. That 0.0125 mol of acid represents 0.00625 mol of the reaction as written, and dividing 2.89 kJ by that figure scales the measurement to a reaction enthalpy near −462 kJ.
Where people slip
Scaling the measured heat to whichever reactant happens to be present in excess, rather than to the limiting reactant, produces a molar enthalpy the data does not actually support.
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