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HESS'S LAW

enthalpy is a state function. it does not care about your path. this is the hack.

THE CORE CONCEPT

💡 What's the Point

Sometimes you cannot directly measure the ΔH for a reaction. Maybe it's too dangerous. Maybe the reaction doesn't go cleanly. Maybe it's not even feasible in a lab.

Hess's Law says: it doesn't matter. Enthalpy is a state function — it only depends on the initial and final states, not the path between them. So if you can get from reactants to products through any series of steps, the total ΔH is exactly the same as if you'd done it in one step.

You're building the target reaction out of other reactions like you're speedrunning a level using previously discovered glitches. The game only checks if you made it to the end screen. So does enthalpy.

enthalpy does not care about your process. it only cares about your outcome. very blunt of it honestly.
WHY THIS WORKS: Imagine every reaction secretly happens in two phantom steps: Step 1: Decompose all REACTANTS back into their elements. → This is the reverse of formation. Enthalpy = −ΔH°f each. Step 2: Form all PRODUCTS from those elements. → This is formation. Enthalpy = +ΔH°f each. By Hess's Law: total ΔH = step 1 + step 2 = Σ(+ΔH°f,products) + Σ(−ΔH°f,reactants) = products − reactants This is also why the formation enthalpy formula from page 5 works. It's all Hess's Law wearing different outfits.

THE FOUR-STEP PROTOCOL

  1. Write the target reaction. This is what you want ΔH for. Put it somewhere visible. Circle it. Give it a name. You'll be checking everything against it constantly. Every species in the target is either a reactant (left) or product (right) and needs to end up there.
  2. Look at each given reaction and check: are your target species on the correct side? If a species that should be a reactant is currently showing up as a product in your given equation — flip the entire equation. WHEN YOU FLIP, ΔH CHANGES SIGN. This is non-negotiable. Flip the reaction = flip the sign. Both things happen. You cannot flip one without the other. The universe is watching.
  3. Multiply any equation to match the coefficients needed in the target. Multiply the entire reaction AND ΔH by the same factor. Halve a reaction = halve ΔH. Triple a reaction = triple ΔH. It scales perfectly. It has always scaled perfectly.
  4. Add all the manipulated equations together. Cancel species that appear on both sides. What survives should be exactly the target reaction. ΔH_total = sum of all ΔH values. If something that shouldn't be in the target is still floating around uncancelled, you made an error in step 2 or 3. Go back. Stare at it. You missed a flip.
🎯 COMMON MISTAKE: Flipping the reaction but forgetting to flip ΔH.

These two things happen together. Always together. One does not happen without the other. There is no version of this where you flip the equation and keep the same ΔH sign. That version does not exist. It has never existed.

🔍 Cancellation Check

After adding everything up, anything that appears on BOTH sides (once as a reactant, once as a product) cancels out and disappears. If your cancellation leaves you with exactly the target reaction and nothing else, you're done. If there are leftover species, something went wrong.

The most common "leftover species" problem: you forgot to flip a reaction so an intermediate is on the wrong side and can't cancel. Go back, find the reaction that produced it wrong, flip it (and its ΔH), and redo the addition.

HESS'S LAW SUMMARY: 1. ΔH is a state function — start and end only, not the path 2. You can add reactions to get a target reaction 3. Flip a reaction → flip its ΔH sign (both. always both.) 4. Multiply a reaction → multiply its ΔH by the same factor 5. Add everything → species on both sides cancel 6. What remains = target reaction + total ΔH if you flip and forget the sign: wrong answer if you multiply and forget the ΔH: wrong answer if your stuff doesn't cancel: you made an error above
← CALORIMETRY ΔH°f →