Which term describes a reaction in which heat is released to the surroundings?

• A. Photosynthesis
• B. Exothermic
• C. Calorimeter
• D. Bond energy

Answer: (B)

Heat flow in reactions is what determines whether a reaction is exothermic or endothermic. In an exothermic reaction, energy stored in the reactants is released as heat to the surroundings, so the surrounding environment gets warmer. This corresponds to the system's enthalpy decreasing (negative ΔH). A familiar example is combustion, where burning a fuel releases heat.

Other terms here don’t describe that heat transfer direction: photosynthesis uses light energy to drive the reaction, making it endothermic overall; a calorimeter is simply an instrument used to measure heat, not a description of the reaction type; bond energy is the amount of energy needed to break bonds, not a classification of heat flow during a reaction.

So the term for a reaction that releases heat to the surroundings is exothermic.