hess's law
Summary
Hess's law says the enthalpy change of an overall reaction equals the sum of the enthalpy changes of its steps, because enthalpy is a state function. Most testable: reversing a step flips the sign of ΔH, and scaling the coefficients scales ΔH by the same factor.
Detail
Meaning: Enthalpy is a state function, so a reaction's ΔH depends only on the initial and final states, not on the route. If an overall reaction equals the sum of several steps, its ΔH equals the sum of their ΔH values. Reversing a step flips the sign of ΔH. Multiplying the coefficients by n multiplies ΔH by n. Species that appear on opposite sides of the added steps cancel.
Equation: ΔH°rxn = ΔH°1 + ΔH°2 + …, where each term is the standard enthalpy change of one step, in J per mole of reaction as written (often given in kJ/mol).
On the MCAT: A passage gives two or three thermochemical equations, and you combine them to get a target reaction. Flip and scale the equations until the intermediates cancel, then add the ΔH values. Formation enthalpies use the same idea: ΔH°rxn = ΣnΔH°f(products) − ΣmΔH°f(reactants), where n and m are the coefficients in mol.
Common confusion: Students forget to change the sign when they reverse a step, or forget to scale ΔH when they double an equation. Hess's law also says nothing about rate. A different path, such as one with a catalyst, can change the activation energy, but the overall ΔH stays the same.
Sources
- Chemistry: The Central Science
- Chemistry (Zumdahl)
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.