Endothermic reactions absorb heat from surroundings

Steps:

• Recall endothermic reactions have positive ΔH, absorbing energy.
• Note decompositions often require heat input, unlike formations.
• Evaluate options: A and B are neutralizations/formations (exothermic); C is oxidation (exothermic); D is decomposition.
• Confirm D as endothermic via known calcination process.

Why D is correct:

• CaCO₃ decomposition (calcination) requires heat to break bonds, yielding positive ΔH per Hess's law.

Why the others are wrong:

• A: Neutralization of acid-base releases heat (exothermic, ΔH < 0).
• B: HCl synthesis from elements is highly exothermic (ΔH = -184 kJ/mol).
• C: Metal oxide formation liberates energy (exothermic combustion).

Final answer: D