Chemistry

# Key Terms

ChemistryKey Terms
activated complex
(also, transition state) unstable combination of reactant species representing the highest energy state of a reaction system
activation energy (Ea)
energy necessary in order for a reaction to take place
Arrhenius equation
mathematical relationship between the rate constant and the activation energy of a reaction
average rate
rate of a chemical reaction computed as the ratio of a measured change in amount or concentration of substance to the time interval over which the change occurred
bimolecular reaction
elementary reaction involving the collision and combination of two reactant species
catalyst
substance that increases the rate of a reaction without itself being consumed by the reaction
collision theory
model that emphasizes the energy and orientation of molecular collisions to explain and predict reaction kinetics
elementary reaction
reaction that takes place precisely as depicted in its chemical equation
frequency factor (A)
proportionality constant in the Arrhenius equation, related to the relative number of collisions having an orientation capable of leading to product formation
half-life of a reaction (tl/2)
time required for half of a given amount of reactant to be consumed
heterogeneous catalyst
catalyst present in a different phase from the reactants, furnishing a surface at which a reaction can occur
homogeneous catalyst
catalyst present in the same phase as the reactants
initial rate
instantaneous rate of a chemical reaction at t = 0 s (immediately after the reaction has begun)
instantaneous rate
rate of a chemical reaction at any instant in time, determined by the slope of the line tangential to a graph of concentration as a function of time
integrated rate law
equation that relates the concentration of a reactant to elapsed time of reaction
intermediate
molecule or ion produced in one step of a reaction mechanism and consumed in another
method of initial rates
use of a more explicit algebraic method to determine the orders in a rate law
molecularity
number of reactant species (atoms, molecules or ions) involved in an elementary reaction
overall reaction order
sum of the reaction orders for each substance represented in the rate law
rate constant (k)
proportionality constant in the relationship between reaction rate and concentrations of reactants
rate expression
mathematical representation relating reaction rate to changes in amount, concentration, or pressure of reactant or product species per unit time
rate law
(also, rate equation) mathematical equation showing the dependence of reaction rate on the rate constant and the concentration of one or more reactants
rate of reaction
measure of the speed at which a chemical reaction takes place
rate-determining step
(also, rate-limiting step) slowest elementary reaction in a reaction mechanism; determines the rate of the overall reaction
reaction mechanism
stepwise sequence of elementary reactions by which a chemical change takes place
reaction order
value of an exponent in a rate law, expressed as an ordinal number (for example, zero order for 0, first order for 1, second order for 2, and so on)
termolecular reaction
elementary reaction involving the simultaneous collision and combination of three reactant species
unimolecular reaction
elementary reaction involving the rearrangement of a single reactant species to produce one or more molecules of product