Which of the following exothermic reactions is most likely to have the greatest activation energy?

• A. Formation of ammonia by Haber process: N_2 + 3 H_2 → 2 NH_3
• B. Neutralization of HCl with NaOH: HCl + NaOH → NaCl + H_2O
• C. Rusting of iron: 4 Fe + 3 O_2 → 2 Fe_2O_3
• D. Combustion of methane: CH_4 + 2 O_2 → CO_2 + 2 H_2O

Answer: (C)

Verified step by step guidance

1. Understand that activation energy (Ea) is the minimum energy required for a reaction to proceed, regardless of whether the reaction is exothermic or endothermic.
2. Recognize that reactions involving breaking strong bonds typically have higher activation energies because more energy is needed to reach the transition state.
3. Analyze each reaction: the Haber process involves breaking the very strong triple bond in N_2, neutralization is a fast acid-base reaction with low Ea, rusting involves solid-state reactions and formation of metal oxides which often have high Ea, and combustion of methane involves breaking C-H bonds but is generally fast once initiated.
4. Compare the bond strengths and reaction mechanisms: rusting of iron involves solid metal and oxygen reacting slowly with a complex mechanism, suggesting a higher activation energy compared to the others.
5. Conclude that among the given exothermic reactions, rusting of iron is most likely to have the greatest activation energy due to the nature of the bonds broken and the reaction environment.