Elementary Steps and Mechanisms

Introduction

A net chemical equation reports reactants and products, but it can conceal a sequence of molecular events. A mechanism proposes those elementary steps. Any acceptable proposal must sum to the observed overall equation and must be consistent with kinetic evidence; matching atom counts alone does not prove it is correct.

Core explanation

Consider a proposed two-step process: A+B → I, followed by I+C → P. Adding the left sides gives A+B+I+C; adding the right sides gives I+P. Cancel intermediate I to obtain A+B+C → P. The intermediate is produced and then consumed, so it does not appear in the net equation. Each step must itself conserve atoms and charge in a real chemical example; abstract letters merely show the cancellation pattern.

An elementary step is one event, so its molecularity can guide a simple step-level rate expression. For an elementary A+B → I under ideal mass-action conditions, rate may be k[A][B]. An overall equation A+B+C → P does not imply rate=k[A][B][C], because C may react only after I forms. Rate depends on which steps and intermediates control the measured kinetics.

Mechanisms often include reversible steps. If A+B ⇌ I is fast and I+C → P is slower, a pre-equilibrium approximation may express [I] in terms of [A] and [B], producing an observed rate with all three concentrations. Conversely, if formation of I is slow, the rate may have no immediate C dependence. Both sets of steps sum to the same net equation, illustrating why stoichiometry cannot select the pathway.

A proposed mechanism must also agree with other evidence: reaction order, detected intermediates, isotope effects, product distributions and temperature dependence. An intermediate may be short-lived and difficult to detect; failure to observe one directly does not always disprove it. On the other hand, inventing an intermediate solely to fit one rate exponent is weak evidence.

Catalysts differ from intermediates by their position in the cycle. A catalyst is consumed in one step and regenerated later; an intermediate is formed and then consumed. Both cancel from the summed net equation, so one must inspect the direction in which each appears. Species can be labeled differently under different chosen mechanism boundaries, but the elementary bookkeeping should be explicit.

Mechanisms are models, not videos of individual molecules. An elementary step should be chemically plausible, respecting charge, mass and reasonable collision molecularity. A proposed one-step collision of five independent molecules is usually implausible even if the net equation has five reactants. Building mechanisms from smaller events is often more realistic.

Step-by-step reasoning

1. Write every proposed elementary step with atoms and charges balanced. 2. Add all reactant sides and all product sides. 3. Cancel species appearing on both sides. 4. Compare the remainder with the measured net equation. 5. Derive kinetic predictions and compare them with experiments.

Visual explanation

Draw a pathway A+B → I → P, with C entering the second arrow. Cross I out of the summed equation, while leaving A, B, C and P. Under the first arrow place a possible step rate expression and a warning that net stoichiometry alone does not select it.

Real-world analogy

A bakery's receipt lists flour, water and bread, but actual production includes dough as an intermediate. The receipt does not reveal which stage limits the bread-per-hour rate; that requires observing the process.

Real-world example

Many atmospheric reactions produce short-lived radical intermediates. Their net equations can be simple, but predicting pollution formation requires the sequence and rates of radical steps rather than atom balance alone.

Why?

Why must an intermediate cancel from the net equation? It is generated during the process and consumed later, so it is not an initial supply or final accumulated product in the idealized overall reaction.

Common misconception

“A mechanism is proved if its steps add to the correct equation.” Many different sequences can have the same net stoichiometry. Kinetic and structural evidence is also required.

Worked example

Consider elementary NO₂ + NO₂ → NO₃ + NO followed by NO₃ + CO → NO₂ + CO₂. Adding gives 2NO₂+NO₃+CO on the left and NO₃+NO+NO₂+CO₂ on the right. Cancel NO₃ and one NO₂, leaving NO₂+CO → NO+CO₂. NO₃ is an intermediate. The net equation alone would not reveal the initial two-NO₂ event or its possible kinetic role.

Quick check

1. What must a valid mechanism sum to? Answer: The observed balanced overall reaction after intermediates and catalysts cancel.

Exam focus

Show explicit cancellation and identify intermediates versus catalysts by formation and regeneration order. Check rate-law predictions rather than taking overall coefficients as kinetic exponents.

Advanced insight

Mechanisms can be kinetically indistinguishable under one experiment. Designing perturbations that change one intermediate or isotope selectively can separate candidate pathways that share the same apparent rate law.

Summary

Elementary steps propose a path behind a net equation. Their sum must conserve atoms and cancel intermediates, but kinetic and other evidence are needed to test the mechanism. Net stoichiometry alone does not give rate law.

Practice questions

1. In A+B→I and I+C→P, what is the net reaction? Answer: A+B+C→P after I cancels. 2. Does the net reaction A+B+C→P prove a three-particle collision? Answer: No. The proposed two-step mechanism uses smaller events. 3. What extra evidence besides correct summation helps test a mechanism? Answer: A measured rate law, detected intermediates, isotope effects or product distributions can help.