Chemistry / Unit 4 Chem 0712 worked example, 60 to 90 seconds
Manipulating Reactions to Reach a Target Equation
The idea
Manipulating known reactions for a Hess's-law calculation means reversing them, scaling them, or both, whichever combination of several known reactions sums exactly to the target equation, applying the matching sign change or multiplication factor to each reaction's own ΔH value in turn as that reaction is adjusted.
In practice
Building chlorine trifluoride's formation reaction from three known reactions requires halving one of them, reversing and halving a second, and reversing a third outright, with each structural change to an equation carried through to that same reaction's ΔH value. Once every intermediate species cancels out and only the target's own reactants and product remain, summing the three adjusted ΔH values gives the target reaction's full enthalpy change.
Where people slip
Reversing a reaction's equation without also flipping the arithmetic sign of its ΔH silently smuggles a wrong-signed enthalpy change into the final summed total.
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