Hess's law and energy cycles

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The total enthalpy change of a reaction is the **same** whatever route is taken, because ΔH depends only on the initial and final states.

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A property that depends only on the **current state** of the system, not on the path taken to reach it (enthalpy is one).

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Because enthalpy is a **state function**, so ΔH is **path-independent** — you can add up the steps of an alternative route.

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Its **sign is reversed** (the magnitude stays the same).

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ΔH is **doubled** — multiply ΔH by the same factor as the equation.

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$\Delta H^{\ominus} = \Sigma\,\Delta H_{f}^{\ominus}(\text{products}) - \Sigma\,\Delta H_{f}^{\ominus}(\text{reactants})$.

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**Zero** by definition (e.g. O_{2}(g), C(s) graphite).

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**With** an arrow → **add** its ΔH; **against** it (reverse) → **subtract** its ΔH (flip the sign).

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Forgetting to **multiply** a step by the number of moles in the target equation.

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To find a ΔH that **cannot be measured directly** (e.g. the reaction is too slow or has side reactions).

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The ΔHf equation **is** a Hess cycle — going down to the elements (reverse ΔHf of reactants) and up to the products (ΔHf of products).