Reaction between hydrogen and oxygen

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Reaction between hydrogen and oxygen

The chain-branching explosion between hydrogen and oxygen is the reaction which were studied in the most details. The chemical equation of the reaction looks rather simple:

$\displaystyle \rm 2H_{2}+O_{2}\longrightarrow2H_{2}O \hspace{1cm} \Delta H_{298}^{0}=-115 kcal/mole$

Actually, the reaction mechanism includes more than 50 elementary reactions. However, in order to understand the main features of the process we will limit ourself by the five following reactions:

$\displaystyle \rm H+O_{2}\longrightarrow OH +O$

(1)

$\displaystyle \rm O+H_{2}\longrightarrow OH + H$

(2)

$\displaystyle \rm OH + H_{2}\longrightarrow H_{2}O + H$

(3)

$\displaystyle \rm H \longrightarrow wall~ decay$

(4)

$\displaystyle \rm H + O_{2} + M \longrightarrow HO_{2} + M$

(5)

Reactions (1) and (2) are the reactions of branching, since in these reactions one free valence produces three valences. Thus, the number of free valences increases as a result of these reactions. Since the reaction (1) has higher activation energy than reaction (2), the rate of this reaction is slower and it usually can be consider as a rate determining step. Reaction (3) is the reaction of the chain propagation, since it does not change the number of valences. In reactions (4) and (5) the number of valences decreases, and these reactions are called chain termination or chain breaking reactions. Note, that the reaction (5) actually looks as a chain propagation reaction, but radical HO $_{2}$ is rather non-active radical and therefore it will much faster distracted on the wall than react with other molecules in the gas phase. Thus, for the simplicity the reaction (5) can be considered as the termination reaction and HO $_{2}$ as a final product of the total reaction. Let us consider the behavior of the reactions (4) and (5) in more details:

Subsections

Next: Reactions of the surface Up: Theory Previous: Chain branch reaction kinetics

Sergey Cheskis 2007-02-26