Unit 5 · Kinetics

Reaction rate ● (core concept) — The amount of reactant converted to product per unit time in a chemical reaction.

Factors affecting reaction rate ● (core concept) — Concentration, temperature, surface area, the presence of a catalyst, and other environmental conditions all influence how fast a reaction proceeds.

Rate law ● (core concept) — An expression relating reaction rate to reactant concentrations, each raised to a power: rate ∝ [reactant]ⁿ.

Reaction order ● (core concept) — The exponent on a reactant's concentration in the rate law (e.g., first order, second order); the overall order is the sum of the individual orders.

Rate constant (k) ● (core concept) — The proportionality constant in the rate law; it depends on temperature and its units reflect the overall reaction order.

Method of initial rates ● (core concept) — A technique for determining reaction orders: run several experiments with different starting concentrations and compare the initial rates.

First-order integrated rate law ● (core concept) — ln[A]ₜ − ln[A]₀ = −kt. A plot of ln[A] versus time is a straight line for a first-order reaction.

Second-order integrated rate law ● (core concept) — 1/[A]ₜ − 1/[A]₀ = kt. A plot of 1/[A] versus time is a straight line for a second-order reaction.

Zeroth-order integrated rate law ● (core concept) — [A]ₜ − [A]₀ = −kt. A plot of [A] versus time is a straight line for a zeroth-order reaction.

Half-life (t₁/₂) ● (core concept) — The time for half of a reactant to be consumed. For a first-order reaction it is constant: t₁/₂ = 0.693/k; for other orders it depends on concentration.

Radioactive decay ● (core concept) — The disintegration of unstable nuclei; it follows first-order kinetics.

Elementary reaction ● (core concept) — A single step in a reaction mechanism. Its rate law can be written directly from the stoichiometry of the colliding particles in that step.

Termolecular collisions ○ — Collisions of three particles at once are rare, so elementary steps involving three particles are uncommon.

Collision model ● (core concept) — Reactants must collide to react, and a collision succeeds only if the particles have enough energy and the correct orientation.

Maxwell-Boltzmann distribution ● (core concept) — The distribution of particle energies in a system; it is used to estimate the fraction of collisions with enough energy to react and how that fraction changes with temperature.

Reaction energy profile ● (core concept) — A diagram of potential energy versus reaction coordinate showing reactants, products, the transition state, and the activation energy.

Transition state ● (core concept) — The configuration of maximum potential energy along the reaction coordinate, partway between reactants and products.

Activation energy (Eₐ) ● (core concept) — The minimum energy needed to overcome the reaction barrier: the energy of the transition state minus the energy of the reactants.

Arrhenius equation (concept) ● (core concept) — A relationship linking the rate constant to temperature and activation energy. The CED requires only conceptual understanding of it, not calculations.

Reaction mechanism ● (core concept) — The series of elementary steps by which a reaction occurs.

Reaction intermediate ● (core concept) — A species that is produced in one elementary step and consumed in a later one; it exists only while the reaction is occurring.

Catalyst (mechanism role) ● (core concept) — A species that appears in the steps of a mechanism but is regenerated, so it is not consumed overall; it speeds the reaction by providing a lower-energy pathway.

Rate-limiting (slow) step ● (core concept) — The slowest elementary step in a mechanism; its molecularity determines the rate law for the overall reaction.

Molecularity ● (core concept) — The number of reactant particles in an elementary step: one (unimolecular), two (bimolecular), or three (termolecular).

Pre-equilibrium approximation ● (core concept) — When the first step of a mechanism is not rate-limiting, it is treated as being at equilibrium to derive the overall rate law.

Multistep reaction energy profile ● (core concept) — The overall energy profile of a reaction, built by combining the energetics of each elementary step; the highest barrier corresponds to the rate-limiting step.

Catalyst (function) ● (core concept) — A substance that increases reaction rate by providing an alternative pathway with a lower activation energy; it is not consumed in the overall reaction.

Enzymes ○ — Biological catalysts that bind reactants and position them to lower the activation energy.

Acid-base catalysis ○ — A catalytic mechanism in which proton transfer creates new reaction intermediates.

Surface catalysis ○ — A catalytic mechanism in which reactants bind to a solid surface that facilitates the reaction.