Elementary reaction - Sohannou

When a chemical reaction is considered to be divided into as many simple reactions as possible, each of these reactions is called an elementary reaction. It is extremely rare for a chemical reaction to consist of only one elementary reaction, and usually a series of elementary reactions proceed one after the other until the overall reaction is complete. For example, the hydrogenation of ethene over a nickel catalyst:

_{C2H4 + H2 → C2H6}

It consists of the following four elementary reactions:

_{C2H4 → C2H4 (} a)

_H2 → 2H(a)

C ₂ H ₄ (a) + H(a) → C ₂ H ₅ (a)

C ₂ H ₅ (a) + H(a) → C ₂ H ₆

Here, (a) indicates the state adsorbed on the catalyst. The stable or unstable substances that result from each elementary reaction are called reaction intermediates, excluding the final product. In the above elementary reaction examples, C ₂ H ₄ (a), H(a), and C ₂ H ₅ (a) are reaction intermediates.

Source: Morikita Publishing "Chemical Dictionary (2nd Edition)" Information about the Chemical Dictionary 2nd Edition

化学反応で原系から生成系に至る過程をできるだけ簡単ないくつかの反応に分けて考えるとき，その個々の反応を素反応という．化学反応がただ一つの素反応からなる場合はきわめてまれで，普通は，一連の素反応が逐次進行して全反応が完結する．たとえば，ニッケル触媒上でエテンが水素添加する反応，

　　　C₂H₄ ＋ H₂ → C₂H₆

は次の四つの素反応から成り立っている．

　　　C₂H₄ → C₂H₄(a)

　　　H₂ → 2H(a)

　　　C₂H₄(a) ＋ H(a) → C₂H₅(a)

　　　C₂H₅(a) ＋ H(a) → C₂H₆

ここで，(a)は触媒上に吸着した状態を示す．各素反応の結果として生じる安定または不安定な物質は，最終生成物を除いて反応中間体とよばれる．上記の各素反応の例では，C₂H₄(a)，H(a)，C₂H₅(a)が反応中間体である．

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