AP Chemistry Unit 5 (Kinetics): Understanding Energy Profiles and Catalysis

Collision model

A model for reaction rates that views reactions as resulting from collisions between particles; only some collisions lead to products.

Effective collision

A collision that leads to reaction because particles collide with sufficient energy to overcome the activation energy and with proper orientation to rearrange bonds.

Activation energy (Ea)

The minimum energy required for reactants to reach the transition state; the energy barrier that must be overcome for reaction to occur.

Transition state

An unstable, high-energy arrangement of atoms at the peak of an energy profile; the point reached when activation energy is overcome.

Proper orientation (orientation factor)

The requirement that particles collide in the correct geometry so the necessary bonds can break and form; without it, even energetic collisions may not react.

Collision frequency

How often reactant particles collide per unit time; increases with higher concentration (or pressure for gases) and can affect reaction rate.

Concentration (rate effect)

Increasing concentration increases the number of particles per volume, raising collision frequency and generally increasing reaction rate.

Pressure (for gases)

Increasing gas pressure increases particle concentration, increasing collision frequency and typically increasing reaction rate.

Temperature (rate effect)

Increasing temperature raises average kinetic energy and, crucially, increases the fraction of particles with energy at or above Ea, increasing the rate.

Maxwell–Boltzmann energy distribution (qualitative)

A description of the spread of particle kinetic energies at a given temperature; higher temperature increases the area of the distribution above Ea.

Surface area (rate effect for solids)

For reactions involving a solid, increasing surface area (e.g., crushing to powder) exposes more reactive sites, increasing collisions and rate.

Stirring/mixing (rate effect)

Agitation that can increase rate when the reaction is limited by bringing reactants together (especially in heterogeneous mixtures), without changing the intrinsic probability per collision.

Reaction energy profile (reaction coordinate diagram)

A graph of potential energy versus reaction progress showing how energy changes from reactants to products along a reaction pathway.

Reaction coordinate (reaction progress)

The horizontal axis on an energy profile indicating progress along the pathway; it is not time.

Overall enthalpy change (ΔH)

The energy difference between products and reactants on an energy diagram; ΔH = E(products) − E(reactants).

Exothermic reaction

A reaction where products are lower in energy than reactants; ΔH < 0.

Endothermic reaction

A reaction where products are higher in energy than reactants; ΔH > 0.

Forward activation energy (Ea,forward)

The energy difference from reactants up to the transition state on an energy diagram.

Reverse activation energy (Ea,reverse)

The energy difference from products up to the transition state on an energy diagram; for exothermic reactions it is larger than Ea,forward.

Arrhenius equation

k = A e^(−Ea/RT); relates the rate constant k to activation energy Ea and temperature T.

Frequency factor (A)

The Arrhenius parameter related to collision frequency and the effectiveness of collisions (including orientation).

Multistep reaction energy profile

An energy diagram with multiple peaks and valleys representing multiple elementary steps, transition states (peaks), and intermediates (valleys).

Reaction intermediate

A species formed in one elementary step and consumed in a later step; appears as a local minimum (valley) between peaks on a multistep diagram.

Rate-determining step (RDS)

The slowest elementary step in a mechanism; typically corresponds to the largest activation energy barrier measured from the preceding valley to the next peak.

Catalyst

A substance that increases reaction rate without being consumed overall by providing an alternative pathway with lower Ea; it speeds reaching equilibrium but does not change ΔH or the equilibrium constant.