Hess's law
MCAT glossary · Chemical & Physical Foundations of Biological Systems
Definition
Enthalpy change for an overall reaction equals the sum of enthalpy changes for the steps; ΔH is a state function.
Hess's law in plain English
Chemical reactions release or absorb heat, and Hess's law says the total heat change doesn't care what path you take to get there — only where you start and where you end. So if a reaction is hard to measure directly, you can add up the heat from a series of easier steps that lead to the same result, and the numbers will match. Think of hiking: your net change in altitude from the trailhead to the summit is the same whether you go straight up or zigzag — only the start and end heights matter. That 'only endpoints matter' property is what people mean when they call enthalpy (Δ H) a 'state function.'
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