Molecularity of a reaction
Molecularity is the number of species that collide at once in an elementary step. In a complex reaction, the slowest step decides the rate law.
Molecularity with examples
An elementary reaction takes place in one step. Its molecularity is the number of species that must collide at the same time for that step to happen. Unimolecular: NH₄NO₂ → N₂ + 2H₂O (one species). Bimolecular: 2HI → H₂ + I₂ (two molecules collide). Trimolecular: 2NO + O₂ → 2NO₂ (three molecules collide). Molecularity above three is rare, because the chance of four or more molecules hitting one another at the same moment is very small.
A complex reaction: decomposition of H₂O₂ with iodide
Overall: 2H₂O₂ → 2H₂O + O₂ (in alkaline medium, with I⁻ ions). The experimental rate law is rate = k[H₂O₂][I⁻], so the reaction is first order in each and second order overall.
- 1★H₂O₂ + I⁻ → H₂O + IO⁻Slow step: this is the rate-determining step. It is bimolecular.
- 2H₂O₂ + IO⁻ → H₂O + I⁻ + O₂Fast step, also bimolecular.
- 3Add the two steps: 2H₂O₂ → 2H₂O + O₂IO⁻ is formed and used up: it is an intermediate. I⁻ is used and given back: it acts as a catalyst.
★ key step
Order vs molecularity
| Point | Order | Molecularity |
|---|---|---|
| Meaning | Sum of the powers of concentration terms in the rate law | Number of species colliding at once in one elementary step |
| How found | By experiment | From the elementary step itself (theoretical) |
| Values | 0, 1, 2, 3 or a fraction | Only 1, 2 or 3; never zero or fractional |
| Applies to | Elementary and complex reactions | Elementary reactions only |
| Complex reaction | Given by the slowest step | Has no meaning for the overall reaction; each step has its own |
Points examiners look for
- The slowest step controls the overall rate, just as the slowest worker on a line decides how many items come out.
- For an elementary reaction, order and molecularity are the same.
- For a complex reaction, molecularity of the overall equation means nothing; talk about the molecularity of each step.
- An intermediate (such as IO⁻) appears in the steps but not in the overall equation.
Hydrogen peroxide poured on a cut bubbles because the enzyme catalase in blood breaks it into water and oxygen through a series of fast steps, much like iodide does in the textbook reaction.
2 marks: “Give two differences between the order and the molecularity of a reaction.” Order is found by experiment; molecularity comes from the elementary step. Order can be zero or fractional; molecularity is only 1, 2 or 3.
3 marks: “For 2H₂O₂ → 2H₂O + O₂ (alkaline medium, I⁻), rate = k[H₂O₂][I⁻]. (i) State the overall order. (ii) Write the two steps. (iii) Name the rate-determining step and the intermediate.” (i) 2. (ii) H₂O₂ + I⁻ → H₂O + IO⁻ (slow); H₂O₂ + IO⁻ → H₂O + I⁻ + O₂ (fast). (iii) The first, slow step; IO⁻ is the intermediate (I⁻ is a catalyst).
1 marks: “Why can the molecularity of a reaction not be zero?” At least one species must take part in an elementary step, so the smallest molecularity is 1.
Molecularity: Must be a Whole number, 1 to 3, for one step. Order: Observed by experiment.