Final answer:
The initiating step in a chain reaction is the initial process that generates reactive intermediates, such as free radicals, necessary for the reaction to proceed, as seen in the formation of water from H₂ and O₂.
Step-by-step explanation:
The initiating step in a chain reaction is the process that begins the reaction by generating reactive intermediates, such as free radicals. In the context of the mixing of hydrogen (H₂) and oxygen (O₂) to form water (H₂O), an initiator such as a spark or light is required to produce the initial free radicals that trigger the reaction.
For instance, in the reaction 2 H₂ + O₂ → 2 H₂O, the initiating step may involve the dissociation of molecular hydrogen or oxygen to form radicals. This is followed by a series of propagation steps where these radicals react with other H₂ and O₂ molecules to produce water, continually generating new radicals that propagate the reaction.
The initiating step is crucial because it sets into motion the sequence of reactions that constitute the overall reaction mechanism, leading to the transformation of reactants into products, as in the synthesis of water from its elemental gases.