Chemistry / Unit 4 Chem 0703 worked example, 60 to 90 seconds
Writing and Solving Formation-Reaction Equations
The idea
Writing a formation-reaction equation means balancing elements in their standard states so the equation forms exactly one mole of the target compound, which then lets a known overall enthalpy change be scaled down to solve for an unknown formation value.
In practice
Ozone forms from oxygen through the reaction 3O2(g) → 2O3(g), which carries a known overall ΔH measured for the equation as written. Because the standard enthalpy of formation is always defined for exactly one mole of product, and this equation produces two moles of O3, the known ΔH must first be divided by 2 before it can be reported as ozone's own standard enthalpy of formation.
Where people slip
Reporting the full reaction's measured ΔH as the formation enthalpy without dividing by the moles of product actually formed miscounts a two-mole value as if it were a one-mole one.
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