01Adding equations
Enthalpy is a state function, so ΔH depends only on start and end, not the path. Rearrange given equations to sum to the target, adjusting ΔH as you go.
Flip → change sign. Multiply → multiply ΔH.
02Notebook box
Match the target's reactants and products one species at a time.
Worked example: C + O₂ → CO₂ (−394); CO + ½O₂ → CO₂ (−283). Target C + ½O₂ → CO = −394 + 283 = −111 kJ.