Hess's law: If ΔH1 = -100 kJ for A → B and ΔH2 = +50 kJ for B → C, what is ΔH for A → C?

• A. -150 kJ
• B. 0 kJ
• C. +50 kJ
• D. -50 kJ

Answer: (D)

Enthalpy changes add when you move from one state to another, because enthalpy is a state function. So the total change from A to C through B is the sum of the two steps: the first step releases 100 kJ, the second step absorbs 50 kJ. Net effect is a release of 50 kJ, since -100 kJ + +50 kJ = -50 kJ. This means going from A to C releases heat. The other possibilities would require either adding the signs incorrectly or canceling the first step, which doesn’t reflect the actual sum of the two steps.