Signs of Enthalpy Changes in Reactions

Steps:

• Recall that enthalpy change (ΔH) is negative for exothermic reactions (heat released) and positive for endothermic (heat absorbed).
• Define each type: atomisation breaks bonds into atoms; combustion burns substances with oxygen; formation creates compounds from elements; neutralisation reacts acid and base to form salt and water.
• Evaluate signs: atomisation requires energy input (positive); combustion releases energy (negative); formation varies by compound stability; neutralisation always releases heat as bonds form in water (negative).
• Identify the always-true statement based on consistent reaction nature.

Why D is correct:

• Neutralisation involves H⁺ + OH⁻ → H₂O, forming strong O-H bonds and releasing energy, so ΔH is always negative by definition.

Why the others are wrong:

• A: Atomisation breaks bonds, requiring energy, so ΔH is always positive.
• B: Combustion is exothermic, releasing heat, so ΔH is always negative.
• C: Formation ΔH can be positive (endothermic, e.g., NO) or negative (exothermic, e.g., H₂O).

Final answer: D