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Why does a spark make hydrogen and oxygen react much faster to form water?

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A graph showing a potential energy curve for a chemical reaction, with the y-axis labeled potential energy and the x-axis labeled progress of reaction. The curve rises from the initial state (reactants) to a peak labeled transition state (activated complex), indicating activation energy, then drops to the final state (products). — chemical kinetics In the Arrhenius equation, the activation energy (E) represents the minimum amount of energy required to transform reactants into products in a chemical reaction. On a potential energy curve, the value of the activation energy is equivalent to the difference in potential energy between particles in an intermediate configuration (known as the activated complex, or transition state) and particles of reactants in their initial state. The activation energy thus can be visualized as a barrier that must be overcome by reactants before products can be formed.

A spark causes hydrogen and oxygen to react rapidly because it provides the necessary energy to overcome the activation energy barrier required for the reaction to proceed.12

At room temperature, the reaction rate between hydrogen and oxygen is negligible because the molecules lack sufficient energy to break their chemical bonds. A spark, flame, or catalyst provides the localized heat or energy needed to initiate the process. This initiation step generates reactive intermediates, such as atoms or radicals, which then participate in a chain reaction.3245

In the case of hydrogen and oxygen, this process involves branched-chain reactions where the number of reactive intermediates increases rapidly, leading to a self-sustaining and often explosive transformation into water. By supplying the initial energy to form these reactive species, the spark effectively bypasses the kinetic limitations that otherwise keep the mixture stable at lower temperatures.5432