Lesson 4 of 11 · 7 min
Molecularity and mechanism
NCERT §3.2.4
Iodide appears in the club's rate law but not in the balanced equation. Something must be happening between H₂O₂ and I⁻ that the equation 2H₂O₂ → 2H₂O + O₂ hides.
The lesson in notes
In short
Molecularity is the number of reacting species (atoms, ions or molecules) that must collide at once in an elementary step. It is always a whole number: 1, 2 or 3.
Examples: NH₄NO₂ → N₂ + 2H₂O is unimolecular, 2HI → H₂ + I₂ is bimolecular, and 2NO + O₂ → 2NO₂ is termolecular.
Molecularity above three is not seen, because a simultaneous collision of four or more particles is too improbable.
A balanced equation with many molecules, such as KClO₃ + 6FeSO₄ + 3H₂SO₄ → KCl + 3Fe₂(SO₄)₃ + 3H₂O, cannot be one step; it is found to be second order and runs through several steps.
In a sequence of steps, the slowest one limits the overall rate. It is called the rate-determining step, like the slowest runner holding back a relay team.
Decomposition of H₂O₂ in alkaline medium with I⁻ has rate = k[H₂O₂][I⁻]. The proposed mechanism: slow step H₂O₂ + I⁻ → H₂O + IO⁻, then fast step H₂O₂ + IO⁻ → H₂O + I⁻ + O₂. IO⁻ is an intermediate that is made and then used up.
Order applies to both elementary and complex reactions; molecularity applies only to elementary steps. For a complex reaction the order is set by the slowest step, and the molecularity of that step can equal its order.
Order can be zero or fractional; molecularity cannot be zero or fractional.