// CHEMISTRY L4 — ENTHALPY & HESS'S LAW

Breaking bonds costs energy; making bonds releases it. The difference is the enthalpy change of the whole reaction. Pick a reaction and watch the energy ledger balance.

// FIG. 01 — THE BOND-ENERGY LEDGER

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ΔH = (bonds broken) − (bonds made) = {{ brokenSum }} − {{ madeSum }} = {{ dh }} kJ/mol {{ dhLabel }}

// ENTHALPY CHANGE, ΔH

Enthalpy (H) is the heat energy stored in a substance at constant pressure. We can't measure it directly, but we can measure the change, ΔH, during a reaction. A negative ΔH means energy left the chemicals and warmed the surroundings — exothermic. A positive ΔH means the chemicals absorbed heat and cooled the surroundings — endothermic. Standard enthalpy changes are quoted at 298 K and 100 kPa, written ΔH°.

// FIG. 02 — HESS'S LAW: EVERY ROUTE COSTS THE SAME

Hess's law says the total enthalpy change depends only on the start and end points — not the path taken. So if a reaction is hard to measure directly, we can add up the ΔH values of an easier route that reaches the same products. It's simply conservation of energy applied to chemistry: you can't get a different amount of heat just by taking the scenic route.

// REFERENCES & FURTHER READING

Wikipedia — Hess's law ↗ Wikipedia — Bond-dissociation energy ↗ BBC Bitesize — Science (chemistry) ↗

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