In Hess’s Law, you reverse an equation by changing its sign of ΔH, and you multiply equations by scaling ΔH accordingly. This feels like balancing my grandfather’s old ledger books—the same careful adjustment of columns to make the final sum correct. But I must disagree with just memorizing the steps; in the lab, I’ve seen students rush and miss that the physical state of compounds matters immensely for those ΔH values. My own failed calorimetry experiment last term, where I neglected to account for water of hydration, taught me that the numbers must belong to the exact substances in your beaker, not just the symbols on paper.
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