A reaction mechanism is the step-by-step molecular story of how reactants become products. The overall chemical equation shows only the starting and ending substances, but the mechanism shows which bonds break, which bonds form, and in what order. This matters because reactions with the same overall equation can happen by different pathways and have different speeds.
Mechanisms help chemists explain observations and design better reactions in the lab and industry.
Each elementary step in a mechanism represents a single molecular event with its own transition state and activation energy. Unstable species called intermediates may form in one step and be consumed in a later step, so they do not appear in the overall equation. The slowest elementary step is often the rate-determining step because it limits how fast the whole reaction can proceed.
For many reactions, the mechanism also explains the rate law, which connects reaction speed to the concentrations of specific reactants.
Understanding Chemistry: Reaction Mechanisms
Chemists usually cannot watch individual molecules react directly, so a mechanism is built from evidence. They measure reaction speed, identify products, and test how changes in conditions affect the result. Electron movement is often tracked with curved arrows in organic chemistry.
An arrow begins at an electron pair or a bond and ends where those electrons make a new bond or become a lone pair. The arrows do not show atoms physically flying across the page.
They show a bookkeeping model for electrons. This helps explain why a bond can break unevenly, leaving one atom electron poor and another electron rich.
Molecules need more than a collision to react. They must collide with enough energy and in a useful orientation. A crowded site can block an incoming molecule even when the collision is energetic.
Solvents matter because they surround charged particles and can make some pathways easier than others. A transition state is the brief, high energy arrangement reached while bonds are partly breaking or forming. It is not a substance that can be collected in a flask.
An intermediate is different. It has a short lifetime but exists between steps, sometimes long enough to detect with special instruments. On an energy diagram, each peak represents a transition state and each valley between peaks represents an intermediate.
Rate data are a major test for any proposed mechanism. The measured rate law must agree with the pathway, not just with the balanced equation. This is especially important when a step involves an intermediate.
Since an intermediate is usually not placed in the final rate law, chemists use information from a fast earlier step to replace its concentration with concentrations of reactants that can be measured. A catalyst provides another route with lower energy barriers. It participates in early steps and is regenerated later.
It can speed both the forward and reverse reactions, but it does not change the equilibrium position at a fixed temperature. Enzymes are biological catalysts that use shaped active sites to hold reactants in favorable positions.
Mechanism ideas appear in many real settings. Drug makers need to know which route creates the desired molecule rather than an unwanted byproduct. Combustion, rusting, food spoilage, and reactions in the atmosphere often involve chains of reactive intermediates.
In class, pay close attention to what cancels when steps are added. Check that atoms and charge balance in every individual step, not only in the final equation. Keep intermediates separate from catalysts, since intermediates are made then used up, while catalysts are used then restored.
Treat a proposed mechanism as an explanation that must survive experiments. One observation rarely proves it completely, but several matching tests can make it convincing.
Key Facts
- A reaction mechanism is a sequence of elementary steps that add up to the overall balanced equation.
- An elementary step describes one molecular event, so its rate law follows directly from its molecularity.
- For an elementary step A + B -> products, rate = k[A][B].
- For an elementary step 2A -> products, rate = k[A]^2.
- An intermediate is formed in one step and consumed in a later step, so it cancels when steps are added.
- The rate-determining step is the slowest step and usually has the largest activation energy, Ea.
Vocabulary
- Reaction mechanism
- A reaction mechanism is the detailed sequence of elementary steps that explains how reactants are converted into products.
- Elementary step
- An elementary step is a single molecular event in a mechanism, such as one collision or one bond rearrangement.
- Intermediate
- An intermediate is a short-lived species that is produced during a reaction mechanism and then consumed before the final products form.
- Transition state
- A transition state is a high-energy arrangement of atoms at the top of an energy barrier during an elementary step.
- Rate-determining step
- The rate-determining step is the slowest elementary step in a mechanism and has a major effect on the overall reaction rate.
Common Mistakes to Avoid
- Treating the overall balanced equation as the mechanism is wrong because the equation only shows reactants and products, not the actual sequence of molecular events.
- Including intermediates in the final overall equation is wrong because intermediates are made in one step and consumed in another, so they cancel out.
- Writing a rate law directly from the overall equation is wrong for most multi-step reactions because the rate law must come from experiment or from the rate-determining elementary step.
- Confusing intermediates with transition states is wrong because intermediates can exist briefly as species, while transition states are unstable energy maxima that cannot be isolated.
Practice Questions
- 1 A proposed mechanism is Step 1: NO2 + NO2 -> NO3 + NO, slow. Step 2: NO3 + CO -> NO2 + CO2, fast. Write the overall reaction and the predicted rate law.
- 2 For the elementary step 2A + B -> products, write the rate law. If [A] is doubled and [B] is tripled, by what factor does the rate change?
- 3 A reaction has two possible mechanisms. Mechanism 1 has one large activation energy barrier, while Mechanism 2 has three smaller barriers with a stable intermediate between the first and second steps. Explain how the energy diagrams would differ and how you would identify the rate-determining step.