Which of the following describes an exothermic reaction?

• A. A reaction that absorbs heat from its surroundings
• B. A reaction that releases heat to its surroundings
• C. A reaction with a positive enthalpy change (ΔH > 0)
• D. A reaction in which the temperature of the surroundings decreases

Answer: (B)

Verified step by step guidance

1. Understand the definition of an exothermic reaction: it is a chemical reaction that releases heat to its surroundings.
2. Recall that the enthalpy change (\( \Delta H \)) indicates whether a reaction absorbs or releases heat. For exothermic reactions, \( \Delta H < 0 \) because heat is released.
3. Analyze the options: a reaction that absorbs heat from its surroundings is endothermic, not exothermic.
4. A reaction with a positive enthalpy change (\( \Delta H > 0 \)) means heat is absorbed, so it is endothermic.
5. A reaction in which the temperature of the surroundings decreases means the system is absorbing heat, so it is not exothermic.