AP Chem Unit 5

Reaction Rate

The change in concentration of a reactant or product per unit time; influenced by concentration, temperature, surface area, and catalysts.

Rate Law

An equation relating reaction rate to the concentrations of reactants: rate = k[A]ⁿ[B]ᵐ…; exponents are determined experimentally.

Rate Constant (k)

The proportionality constant in the rate law; temperature-dependent; units depend on overall reaction order.

Reaction Order

The exponent of a reactant's concentration in the rate law; must be determined experimentally (not from stoichiometry).

Overall Reaction Order

The sum of all individual orders in the rate law.

Method of Initial Rates

An experimental technique comparing initial rates at different initial concentrations to determine reaction orders.

Integrated Rate Law

Mathematical equations relating concentration to time for a given reaction order; used to determine concentration at any time or the order from graphical data.

Zero-Order Reaction

Rate is independent of reactant concentration; [A]t − [A]₀ = −kt; plot of [A] vs. t is linear.

First-Order Reaction

Rate is directly proportional to concentration; ln[A]t − ln[A]₀ = −kt; plot of ln[A] vs. t is linear.

Second-Order Reaction

Rate is proportional to concentration squared; 1/[A]t − 1/[A]₀ = kt; plot of 1/[A] vs. t is linear.

Half-Life (t₁/₂)

The time required for the concentration of a reactant to decrease by half; for first-order: t₁/₂ = 0.693/k (constant).

Radioactive Decay

A first-order process in which an unstable nucleus loses energy by emitting radiation.

Collision Model

Reaction occurs when molecules collide with sufficient energy and proper orientation; not all collisions are effective.

Activation Energy (E_a)

The minimum energy required for a collision to result in a reaction (to reach the transition state).

Transition State (Activated Complex)

The high-energy, unstable arrangement of atoms at the peak of the energy profile; neither reactant nor product.

Reaction Coordinate

The axis in an energy profile diagram representing the progress of the reaction from reactants to products.

Energy Profile (Reaction Energy Diagram)

A graph of potential energy vs. reaction coordinate; shows reactants, transition state, products, activation energy, and overall energy change.

Arrhenius Equation (conceptual)

The rate constant k increases with temperature because more collisions have energy ≥ activation energy; explains temperature dependence of reaction rates.

Reaction Mechanism

A series of elementary steps that describes the pathway from reactants to products.

Elementary Reaction

A single step in a mechanism; its rate law is written directly from stoichiometry.

Molecularity

The number of molecules involved in an elementary step (unimolecular = 1, bimolecular = 2, termolecular = 3, rare).

Intermediate

A species produced in one elementary step and consumed in a subsequent step; present during the reaction but not in the overall equation.

Rate-Determining Step

The slowest elementary step in a mechanism; determines the overall rate law.

Pre-Equilibrium Approximation

A method used when a fast reversible first step precedes the rate-determining step; the intermediate concentration is expressed in terms of reactants using the equilibrium constant of the first step.

Catalyst

A substance that increases reaction rate by providing an alternative pathway with lower activation energy; not consumed overall.

Homogeneous Catalyst

A catalyst in the same phase as the reactants.

Heterogeneous Catalyst

A catalyst in a different phase from the reactants (e.g., solid catalyst with gas-phase reactants); reactants bind to the surface.

Enzyme

A biological protein catalyst that binds reactants (substrates) at an active site, orienting them for reaction at lower activation energy.

Multistep Energy Profile

An energy diagram for a reaction with multiple elementary steps; shows multiple peaks (transition states) and valleys (intermediates); the rate-determining step has the highest activation energy.