Exothermic reaction with high activation energy

Steps:

• Classify reactions: A is acid-base neutralization, B is thermal decomposition, C is metal combustion.
• Assess energy changes: exothermic reactions release heat (ΔH < 0), endothermic absorb heat (ΔH > 0); activation energy (Ea) is the barrier height.
• Examine profile: assumes diagram shows high Ea peak followed by large energy drop to products, indicating exothermic with ignition required.
• Match: only C fits, as it needs heat initiation but releases much energy.

Why C is correct:

• Magnesium combustion is exothermic (ΔH = -1203 kJ/mol), with high Ea requiring spark, matching profile's barrier and drop per Hess's law.

Why the others are wrong:

• A: Exothermic but low Ea, reacts spontaneously without high barrier.
• B: Endothermic (ΔH = +178 kJ/mol), profile would show energy rise.
• D: Not enough information.

Final answer: C