Relationship between molecularity and the reaction order Should the molecularity be equal or less than the reaction order?
Consider the reaction:
$$\ce{H2(g) + I2(g) → 2 HI(g)}$$
If we consider the mechanism of this multi-step reaction, the molecularity is 3 (termolecular reaction):
$$
\begin{align}
\ce{I2(g) &amp;&lt;=&gt; 2I(g)} &amp;\quad&amp;\text{(equilibrium reaction)} \\
\ce{2I(g) + H2(g) &amp;-&gt; 2HI(g)} &amp;\quad&amp;\text{(rate-determining step)}
\end{align}
$$
The rate equation for the reaction is
$$\mathrm{rate} = k[\ce{H2(g)}][\ce{I2(g)}].$$
The order of the reaction is 2 here. The molecularity is greater than the reaction order. Is it possible?
Or is the actual mechanism for the above reaction is different?
Molecularity applies to single elementary reaction steps and is seldom higher than 2.
The reaction order is the &quot;effective molecularity&quot; of the whole set of linked reactions, if it had been a single step reaction. It need not to be integer and has just indirect relation to molecularities of elementary steps.
The reaction order is either determined experimentally, either derived from the reaction schema via determined conditions for steady states of intermediate products.