Which of the following reactions are always exothermic? 1. Neutralization Il. Decomposition III. Combustion

Analyze the question: We need to identify which reaction types always release heat (ΔH negative).

Concept: An exothermic reaction releases heat to the surroundings. Let's check each type:

• I. Neutralization: An acid reacting with a base to form salt and water always releases heat. For a strong acid and strong base, H⁺ + OH⁻ → H₂O has ΔH ≈ -57.3 kJ/mol. Always exothermic.
• II. Decomposition: Breaking compounds into simpler substances usually requires energy (for example, CaCO₃ → CaO + CO₂ is endothermic, and so is the electrolysis of water). It is not always exothermic. Some decompositions (such as that of hydrogen peroxide) are exothermic, but that is not true in general.
• III. Combustion: Burning a substance in oxygen always releases heat and light. Always exothermic.

Solution: The reactions that are always exothermic are I (neutralization) and III (combustion).

Correct answer: B (I and III only)

Tip: Remember the pairing "Neutralization and Combustion = always release heat," while decomposition usually needs heat (think of "thermal decomposition," where you must heat the substance). Watch for the word "always" in exam questions, because it rules out types with exceptions.