Chemical Kinetics · Lesson 3 of 10
Elementary Steps, Molecularity and Mechanisms
“Distinguish measured reaction order from microscopic steps and use a catalytic cycle to connect mechanisms with observed rates.”
• Distinguish an elementary reaction from an overall stepwise conversion. • Assign molecularity only to a specified elementary event. • Identify an intermediate and a regenerated catalyst by adding reaction steps. • Explain how a proposed mechanism must account for the experimental law without assuming a universal slow-step shortcut.
The overall equation is a chemical balance sheet
A route from one city to another may involve several roads even if the map shows only the endpoints. Similarly, an overall equation states the net chemical conversion without listing every molecular event. An elementary reaction is one microscopic step in a kinetic model. A complex or stepwise reaction consists of several elementary events whose sum gives the overall conversion.
Stepwise processes can be consecutive, reversible or competing. Oxidation of a carbon compound may proceed through alcohol, aldehyde and acid functional groups before complete oxidation. Reversible steps can return material to earlier species, and competing reactions can form different products, as with different substituted products from a reaction of phenol. These examples describe possible kinds of pathways; they do not establish an elementary mechanism simply by naming the products.
The number of reactant molecular entities involved in one elementary reaction event. It is a positive integer and is not assigned to an arbitrary overall multistep equation.
| Specified elementary model | Reacting entities in one event | Molecularity |
|---|---|---|
| A → products | One A entity | Unimolecular |
| A + B → products | One A and one B | Bimolecular |
| 2A → products | Two A entities | Bimolecular |
| A + B + C → products | Three entities | Termolecular |
A unimolecular step need not involve a collision between two reactants at that instant; it can involve an already energised molecule changing structure or breaking apart. Simultaneous effective three-entity encounters are much less probable than pair encounters, so elementary termolecular steps are uncommon. An overall equation with several reactant molecules does not require them all to collide together.
Problem
Consider NH₄NO₂→N₂+2H₂O, 2HI→H₂+I₂ and 2NO+O₂→2NO₂. Can their overall equations alone establish elementary molecularities?
- 1.Each equation balances atoms, but balance is not evidence that the conversion happens in one elementary event.
- 2.If a specified model established the first as one reacting entity changing, its step molecularity would be one; a specified 2HI elementary event would have molecularity two.
- 3.For nitrogen monoxide oxidation, a third-order empirical law does not prove a simultaneous collision of two NO and one O₂. Stepwise mechanisms can generate the same overall order.
- 4.Use mechanistic evidence or an explicitly stipulated elementary model before assigning the microscopic label.
A peroxide catalytic cycle makes hidden steps visible
In a simplified iodide-catalysed decomposition of hydrogen peroxide in alkaline medium, the following two steps account for the overall conversion. The model labels the first step slow and the second fast under the conditions considered. The chemical species IO⁻ appears during the route even though it does not appear in the final balance.
| Step | Proposed chemical event | Role |
|---|---|---|
| 1, slow | H₂O₂ + I⁻ → H₂O + IO⁻ | Iodide reacts and hypoiodite is formed. |
| 2, fast | H₂O₂ + IO⁻ → H₂O + I⁻ + O₂ | Hypoiodite is consumed and iodide regenerated. |
| Sum | 2H₂O₂ → 2H₂O + O₂ | I⁻ and IO⁻ cancel from the overall equation. |
Check conservation separately for each step. Both have charge −1 on each side. In the first, two hydrogen atoms and two oxygen atoms enter and leave. In the second, three oxygen atoms enter and leave. Adding the steps cancels I⁻ because it is consumed and regenerated; iodide is a catalyst. IO⁻ is formed and then consumed; it is an intermediate. Both steps are bimolecular in this proposed model.
If the fast second step consumes intermediate almost as quickly as the first forms it, one overall oxygen-forming cycle occurs for each slow event. Then O₂ appearance rate is approximately r₁, while H₂O₂ disappearance is approximately 2r₁. A measured disappearance law v(H₂O₂)=k_obs[H₂O₂][I⁻] therefore has k_obs≈2k₁. Its partial orders are still one in each species. This is why identifying the measured rate matters even in a correct mechanism.
Problem
Suppose k₁=0.10 L mol⁻¹ s⁻¹, [H₂O₂]=0.20 M and [I⁻]=0.010 M in the stated slow/fast regime. Estimate oxygen appearance and peroxide disappearance.
- 1.The slow event rate is r₁=0.10×0.20×0.010=2.0×10⁻⁴ M s⁻¹.
- 2.Assuming intermediate does not accumulate substantially, oxygen appearance is approximately r₁=2.0×10⁻⁴ M s⁻¹.
- 3.The overall cycle consumes two peroxide units for each O₂ formed, so peroxide disappearance is approximately 4.0×10⁻⁴ M s⁻¹.
- 4.The catalyst is regenerated, but its concentration can still affect how rapidly the cycle runs.
A rate-controlling step is not a universal shortcut
A slow step can control the overall timescale when other steps adjust rapidly. However, its rate expression may contain an intermediate whose concentration is itself set by earlier equilibria or other steps. Therefore the order of the overall reaction need not equal the molecularity of the slowest step. A proposed mechanism must be consistent with both the balanced overall reaction and the observed rate law; matching a law supports a model without proving it uniquely.
Problem
In a hypothetical model, 2A ⇌ A₂ is a rapid pre-equilibrium and A₂+B→P is a slow bimolecular step. Explain the possible observed law.
- 1.The slow event law is r=k₂[A₂][B], which contains an intermediate concentration.
- 2.If the rapid pre-equilibrium gives [A₂]=K[A]² over the conditions considered, substitute it into the slow law.
- 3.Then r=k₂K[A]²[B]. Its observed overall order is three, while the slow step has molecularity two.
- 4.This is a conditional illustration, not a mechanism assigned to a real reaction without evidence. It shows why an intermediate must be eliminated using an appropriate assumption.
A balanced reaction such as KClO₃+6FeSO₄+3H₂SO₄→KCl+3Fe₂(SO₄)₃+3H₂O illustrates why counting ten reactant formula units is not a valid method of assigning reaction order or molecularity. Only kinetic evidence can establish its observed law, and individual elementary steps require a mechanistic model.
An intermediate has a finite lifetime between steps and can correspond to a valley on an energy profile. A transition state is the activated arrangement at a barrier between such states. A short-lived species is not automatically a transition state, and a barrier-top drawing should not be labelled a stable intermediate.
Quick Quiz
Quiz
Molecularity applies directly to:
Explanation: Molecularity counts reacting entities in one elementary event; an arbitrary multistep overall reaction has no single molecularity.
Which molecularity is possible for an elementary event?
Explanation: It counts a positive integer number of reacting entities. Fractional and negative values can describe empirical orders, not molecularities.
In the peroxide cycle, IO⁻ is:
Explanation: IO⁻ is produced in step one and removed in step two, so it cancels from the overall balance as an intermediate.
Why does iodide still affect rate although it cancels overall?
Explanation: The catalyst can occur in a rate-controlling elementary event even though its net consumption over the complete cycle is zero.
Must an overall reaction’s order equal the molecularity of its slowest step?
Explanation: Substituting an intermediate concentration obtained from earlier equilibria can produce a different overall concentration dependence.
Practice Problems
- Compare reaction order and molecularity using their definitions, allowed values and evidence.
- Add the two peroxide steps and identify the catalyst and intermediate.
- For the slow law r₁=k₁[H₂O₂][I⁻], predict the effect of doubling iodide and distinguish O₂ appearance from peroxide disappearance.
- Why does the overall equation 2NO+O₂→2NO₂ not prove a termolecular mechanism even when the observed law is third order?
- In the hypothetical 2A⇌A₂, A₂+B→P model, what assumptions are needed to write r=k₂K[A]²[B]?
Order describes measured macroscopic concentration dependence and may be zero, fractional or negative. Molecularity counts reactant entities in one elementary event and is a positive integer. A rate experiment determines order; mechanistic evidence or a specified elementary model is needed for molecularity. The two coincide for a simple elementary mass-action step, not for every overall conversion.
Key Takeaways
• An overall equation gives net stoichiometry and can conceal several elementary steps. • Molecularity counts reacting entities in one specified elementary event. • Order is experimental concentration dependence and has a different meaning. • A catalyst participates and is regenerated; an intermediate is formed and later consumed. • Adding proposed steps must reproduce atom and charge balance for the net reaction. • A slow step can control the timescale, but intermediate concentrations can alter the observed law. • A transition state at a barrier is distinct from an intermediate between barriers.