When ΔH is negative, a reaction is said to be

1. Analyze the question

The question asks us to identify the type of chemical reaction that occurs when the change in enthalpy ($\Delta H$) is negative.

2. Explain the concept

In thermochemistry, enthalpy ($\Delta H$) represents the heat content of a system.

• Exothermic reactions: These are reactions that release heat energy into the surroundings. Because the system loses energy, the final enthalpy is lower than the initial enthalpy, resulting in a negative $\Delta H$ value ($\Delta H < 0$).
• Endothermic reactions: These are reactions that absorb heat energy from the surroundings. Because the system gains energy, the final enthalpy is higher than the initial enthalpy, resulting in a positive $\Delta H$ value ($\Delta H > 0$).

3. Show the solution

• If $\Delta H$ is negative, the reaction releases heat.
• By definition, a reaction that releases heat is an exothermic reaction.
• Option A (Endothermic) corresponds to a positive $\Delta H$.
• Option C (Reversible) and D (Ionic) describe the nature of the reaction mechanism or bond type, not the heat change.

4. Identify the answer

Based on the principles of thermodynamics, a negative $\Delta H$ indicates an exothermic process.

Correct Option: B

5. Exam Tip

Think of the prefix "Exo-" like "Exit". Just as you exit a room to leave it, heat "exits" the reaction system in an exothermic process, leaving the system with less energy (negative). Conversely, "Endo-" sounds like "Into"; heat goes "into" the system, increasing its energy (positive).