Which direction of the reaction is exothermic?

• A. Forward
• B. Reverse
• C. Both
• D. Neither

Answer: (B)

Heat flow in reactions is described by enthalpy change. If a direction releases heat, that direction is exothermic and has a negative ΔH. In a reversible reaction, the forward and reverse directions have equal magnitudes of enthalpy change but opposite signs. So if the reverse direction is exothermic (releases heat), its ΔH is negative, meaning the forward direction must be endothermic (absorbs heat, with ΔH positive). That’s why the reverse path is the exothermic one here. The idea that both directions could release heat or neither would conflict with the fact that reversing a reaction flips the sign of its enthalpy change.