Activation energy barrier – why H2 and O2 do not explode on contact: Hydrogen and oxygen release huge energy when reacting to form water, yet a balloon containing mixed H2 and O2 can remain inert. What is the best explanation for the lack of spontaneous reaction?

Introduction / Context:
Thermodynamics and kinetics both govern chemical reactions. A reaction may be highly exergonic (favorable) yet proceed imperceptibly slowly if the kinetic barrier (activation energy) is high. The classic example is the combination of hydrogen and oxygen to form water.

Given Data / Assumptions:

• Hydrogen–oxygen reaction is energetically favorable (releases energy).
• No catalyst or ignition source is provided inside the balloon.
• We are asked why nothing happens despite a favorable thermodynamic drive.

Concept / Approach:
The reaction requires formation of high-energy transition-state complexes and radicals. Without sufficient activation energy—typically supplied by a spark, flame, or catalyst—molecules do not cross the barrier at an appreciable rate. Thus, the reaction is kinetically inhibited, not thermodynamically impossible. Enzyme-related answers are irrelevant because this is a non-enzymatic gas-phase reaction.

Step-by-Step Solution:
Separate thermodynamic favorability (Delta G < 0) from kinetic accessibility (Ea high). Recognize that H2 + O2 needs initiation energy to form reactive intermediates. Conclude that, absent spark/catalyst, the system stays in a metastable state. Choose the activation-energy explanation.

Verification / Alternative check:
When a spark is introduced, the mixture detonates, confirming that energy input to surpass the activation barrier triggers rapid reaction.

Why Other Options Are Wrong:
No enzyme or active site is involved (a); contamination (b) is unnecessary as an explanation; (d) is overinclusive; (e) is false—thermodynamics allows the reaction.

Common Pitfalls:
Assuming “favorable” means “fast”; kinetics often limits reaction rates in real systems.

Final Answer:
The activation energy to reach the transition state is too high without a spark/catalyst, so the reaction does not proceed spontaneously.