What do you call a reaction that releases heat to the surroundings?

• A. An endothermic reaction
• B. An exothermic reaction
• C. A reversible reaction
• D. A catalytic reaction

Answer: (B)

Exothermic reactions release heat to the surroundings. The energy difference between reactants and products goes out as heat, so you often feel the surroundings warming up—think of burning fuel or the heat you feel when acids react with metals. In thermodynamic terms, the enthalpy change is negative for these processes. By contrast, endothermic reactions absorb heat from the surroundings, making things feel cooler. A reversible reaction can proceed in both directions, but that doesn’t specify heat release or absorption. A catalytic reaction simply uses a catalyst to speed up the reaction without changing whether heat is released or absorbed. So, the reaction that releases heat is exothermic.