2 H2(g) + O2(g) → 2 H2O(g) When H2(g) and O2(g) are mixed together in a rigid reaction vessel at 25° — Kinetics Chemistry Question

STEPS:

1. Analyze the physical observation at room temperature: When $\text{H}_2(g)$ and $\text{O}_2(g)$ are mixed at $25^\circ\text{C}$, no reaction is observed. This indicates that the rate of the reaction under these conditions is virtually zero, which is a kinetic property rather than a thermodynamic one.
2. Analyze the effect of the spark: Introducing a spark provides a localized source of high-energy heat. This input allows the gases to begin reacting "vigorously," releasing a massive amount of thermal energy. Once started, this exothermic reaction is self-sustaining because the heat released by the reacting molecules provides the energy needed for neighboring molecules to react.
3. Relate the initiation barrier to Collision Theory:
* According to collision theory, for a chemical reaction to occur, reactant molecules must collide with sufficient kinetic energy to overcome a minimum energy barrier—known as the activation energy ($E_a$)—which is required to break the strong $\text{H–H}$ and $\text{O=O}$ covalent bonds in the reactants.
* At $25^\circ\text{C}$, the average kinetic energy of the gas molecules is far too low to overcome this barrier. The fraction of collisions that possess enough energy to successfully react is negligible, leaving the mixture kinetically stable (or "trapped").
* The spark provides the necessary localized kinetic energy to help the initial group of reactant molecules surmount this activation energy barrier.
4. Identify the matching option: The necessity of an initial energy source (the spark) to kickstart a highly favorable, exothermic reaction is due to a large activation energy at $25^\circ\text{C}$. This directly matches Option D.

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WHY_OTHERS_WRONG:

• Option A is incorrect: The reaction is highly thermodynamically favorable (spontaneous) at $25^\circ\text{C}$. Thermodynamics determines *if* a reaction can occur based on the relative free energy of reactants and products, but it does not dictate the *rate* at which it occurs. A reaction can be highly thermodynamically favored yet kinetically hindered due to a high activation energy.
• Option B is incorrect: The reaction "releases heat," which means it is an exothermic process. Therefore, the enthalpy change ($\Delta H^\circ$) for this reaction has a large negative value, not a positive one.
• Option C is incorrect: Although there is a decrease in entropy ($\Delta S^\circ < 0$) because the reaction goes from 3 moles of gas to 2 moles of gas, this thermodynamic factor does not explain why a spark is needed. The requirement of a spark is a kinetic phenomenon governed by activation energy, whereas entropy is a thermodynamic state function.