Many processes can be broken down into a series of steps, each with its own energy change.

EK 6.9.B.1 gives the justification, and it is worth reading closely because it is a first-law argument: because total energy is conserved, and each individual reaction in a sequence transfers thermal energy to or from the surroundings, the net thermal energy transferred in the sequence equals the sum of the transfers in each step. Those transfers result from potential-energy changes among the species, so at constant pressure the enthalpy change of the overall process equals the sum of the enthalpy changes of the individual steps.

EK 6.9.B.2 lists the two manipulation rules:

• Reversing a reaction keeps the magnitude of ΔH but reverses its sign.
• Multiplying a reaction by a factor c multiplies ΔH by the same factor c.

The strategy for solving a Hess's law problem:

1. Write the target equation and identify where each species must end up.
2. Find the given equation containing each species. Reverse it if the species is on the wrong side.
3. Scale it so the coefficient matches the target.
4. Apply the same operations to each ΔH.
5. Add all the manipulated equations; everything not in the target must cancel.
6. Sum the manipulated ΔH values.

Practical anchor: start with a species that appears in only one of the given equations. That fixes the required operation on that equation with no ambiguity, and the rest follows.

The same logic reappears for equilibrium constants (7.6) — except that combining equilibria multiplies K rather than adding it.