Chemical Kinetics & Catalysis — 60 Terms Defined

Half-Life of a Reaction Chemical Kinetics & Catalysis s The time required for the concentration of a reactant to fall to half its initial value. t½ = 0.693 / k (first order)

Heterogeneous Catalysis Chemical Kinetics & Catalysis Dimensionless Catalysis in which the catalyst is in a different phase from the reactants, usually a solid with gaseous or liquid reagents.

Homogeneous Catalysis Chemical Kinetics & Catalysis Dimensionless Catalysis in which the catalyst is in the same phase as the reactants.

Inhibitor Chemical Kinetics & Catalysis Dimensionless A substance that decreases the rate of a chemical reaction.

Initial Rate Method Chemical Kinetics & Catalysis mol/(L·s) A technique for determining reaction orders by measuring the starting rate in a series of experiments with different initial concentrations. rate₂/rate₁ = ([A]₂/[A]₁)^m

Instantaneous Rate Chemical Kinetics & Catalysis mol/(L·s) The rate of a reaction at one particular moment, given by the gradient of the concentration–time curve at that point. rate = d[A]/dt

Integrated Rate Law Chemical Kinetics & Catalysis mol/L The equation obtained by integrating a differential rate law, giving concentration directly as a function of time. varies with order

Kinetic and Thermodynamic Control Chemical Kinetics & Catalysis Dimensionless The distinction between a reaction giving the product that forms fastest and one giving the product that is most stable.

Langmuir Adsorption Isotherm Chemical Kinetics & Catalysis Dimensionless A model describing monolayer adsorption on a surface with a fixed number of equivalent, independent sites. θ = Kp / (1 + Kp)

Michaelis–Menten Kinetics Chemical Kinetics & Catalysis mol/(L·s) The standard model of enzyme kinetics, in which rate rises with substrate concentration until the enzyme becomes saturated. v = V_max[S] / (K_m + [S])

Molecularity Chemical Kinetics & Catalysis Dimensionless The number of reacting species that collide simultaneously in a single elementary step.

Order of Reaction Chemical Kinetics & Catalysis Dimensionless The power to which the concentration of a given reactant is raised in the experimentally determined rate law. rate ∝ [A]^m

Overall Order Chemical Kinetics & Catalysis Dimensionless The sum of the individual orders with respect to every reactant appearing in the rate law. n = m₁ + m₂ + …

Photochemical Quantum Yield Chemical Kinetics & Catalysis Dimensionless The number of molecules reacting for each photon of light absorbed by the system. Φ = molecules reacted / photons absorbed

Physisorption Chemical Kinetics & Catalysis kJ/mol Weak adsorption in which molecules are held to a surface by van der Waals forces alone. ΔH ≈ −20 to −40 kJ/mol

Pre-exponential Factor Chemical Kinetics & Catalysis Various The constant A in the Arrhenius equation, representing the frequency of appropriately oriented collisions. A = pZ

Promoter Chemical Kinetics & Catalysis Dimensionless A substance that has little catalytic activity itself but greatly increases the effectiveness of a catalyst.

Pseudo-First-Order Reaction Chemical Kinetics & Catalysis s⁻¹ A higher-order reaction that behaves as first order because one reactant is present in such large excess that its concentration is effectively constant. rate = k′[A], k′ = k[B]₀

Rate Constant Chemical Kinetics & Catalysis Various The proportionality constant in a rate law, characteristic of a given reaction at a given temperature. k = A e^(−Ea/RT)

Rate Law Chemical Kinetics & Catalysis mol/(L·s) An experimentally determined equation expressing reaction rate as a function of reactant concentrations. rate = k[A]^m[B]^n

Rate-Determining Step Chemical Kinetics & Catalysis Dimensionless The slowest step in a reaction mechanism, which limits the overall rate.

Reaction Intermediate Chemical Kinetics & Catalysis mol/L A species produced in one step of a mechanism and consumed in a later one, so it does not appear in the overall equation.

Reaction Mechanism Chemical Kinetics & Catalysis Dimensionless The detailed sequence of elementary steps by which reactants are converted into products.

Reaction Rate Chemical Kinetics & Catalysis mol/(L·s) The change in concentration of a reactant or product per unit time. rate = −Δ[A]/(aΔt) = Δ[P]/(pΔt)