Activated Complex
In the mechanism of a reaction, a species which lies at an energy peak, and that must either fall apart to form products or reform reactants.

Activation energy

the kinetic energy (Ea) which reactant molecules must have before they can take up the transition state and undergo the reaction to form product molecules. Since the transition state is the highest energy state within the reaction, the activation energy is the difference in energy between the enthalpy of the reactants and the enthalpy of the transistion state.

Arrhenius equation

k = A exp(-Ea/RT), where A (SI unit: as for the corresponding rate constant) is termed the "pre-exponential factor".


The increase in rate of a reaction due to the presence of a substance that undergoes no net change during the reaction.


a substance which is not consumed in a reaction, but can increase the rate of the reaction. It does so by providing an alternative reaction mechanism which involves a lower activation energy. Catalysts do not alter equilibrium positions of reactions but they can greatly reduce the time taken to achieve equilibrium.

Chain reaction

A reaction in which a product reacts so as to continue the reaction.

Chemical reaction

A process that results in the interconversion of chemical species. Chemical reactions may be elementary reactions or stepwise reactions. Detectable chemical reactions normally involve sets of molecular entities, but it is often conceptually convenient to use the term also for changes involving single molecular entities (i.e. "microscopic chemical events").



Collision frequency

The rate at which chemical species collide; used in theories of chemical kinetics. Also, the frequency with which gaseous molecules collide.

Energy of activation

Ea (SI unit: kJ mol-1) An operationally defined quantity expressing the dependence of a rate constant on temperature as derived from the "Arrhenius equation", k = A exp(-Ea/RT), where A (SI unit: as for the corresponding rate constant) is termed the "pre-exponential factor".

Enthalpy of formation

the energy required to make a substance from its constituent elements with the elements and substance at standard temperature and pressure. It is important that all processes involved are carried out at constant pressure or that at least the final pressure of all products matches the initial pressure (which is the usual condition for reactions taking place which are open to the atmosphere. Enthalpy of formation can be thought of as energy needed to make something from its elements without too much error.


A biochemical catalyst made up primarily of protein.


A state in which no net change is occurring, but which is characterized by forward and reverse reactions occurring at the same rate.

Equilibrium constant

The value (Keq) of the equilibrium constant expression for a system at equilibrium; a value greater than one indicates the position of equilibrium lies toward products, and a value less than one indicates the position of equilibrium lies toward reactants.

Equilibrium constant expression

The ratio of the concentrations of products in a reaction raised to the powers of their coefficients divided by the same for reactants; at equilibrium this ratio has the value of the equilibrium constant.

First-order reaction

A reaction for which the rate is directly proportional to the concentration of one reactant.

Free energy

A thermodynamic function corresponding to the tendency for spontaneous change in a system; represented by the symbol G. A negative free energy change for a reaction indicates this tendency towards spontaneous change.


In chemical kinetics, the time it takes for one half of the limiting reactant to be consumed. In nuclear chemistry, the time for half of a sample to undergo radioactive decay.

Integrated rate equation

An equation that describes the concentrations of reactants (and products) as a function of time.


The order of an experimental rate equation describes how the rate depends on the power of the concentration terms.(See first-order reactions and second-order reactions)


the chemicals or particles produced in the reaction.

Rate Constant
The proportionality factor k [SI units(dm3 mol-1)n-1)] is called the (nth order) "rate coefficient". Rate coefficients referring to (or believed to refer to) elementary reactions are called "rate constants". If the overall rate of reaction is v = k[A]a[B]b but [B] stays constant, then the order of the reaction (with respect to time), as observed from the concentration change of A with time, will be "a", and the rate of disappearance of A can be expressed in the form vA = kobs[A]a. The proportionality factor kobs deduced from such an experiment is called the "observed rate coefficient" and it is related to the (a + b)th order rate coefficient k by the equation
kobs = k[B]b
For the common case when a = 1, kobs is often referred to as a "pseudo-first order rate coefficient". For a simple (elementary) reactions a partial order of reaction is the same as the stoichiometric number of the reactant concerned and must therefore be a positive integer.

Rate-determining step

The mechanistic step in a reaction that by its relatively slow rate limits the overall rate of a reaction.

Rate law

The rate law (also called differential rate equation or rate expression) is an equation that relates the rate of disappearance of reactants , or appearance of products, to the reactant.



the chemicals or particles consumed by a chemical reaction.

Reaction mechanism

The set of elementary steps by which a reaction occurs; the individual steps correspond to molecular collisions.


Second-order reaction

A reaction for which the rate is directly proportional to the square of the concentration of one species or to the product of the concentrations of two species.


The molecule or ion that an enzyme uses as a reactant for the reaction that it catalyzes.

Transition state

the intermediate configuration of particles between reactants and products which has the highest enthalpy of formation. It represents the energy barrier to the reaction proceeding.


Zero-order reaction

A reaction for which the rate of reaction is independent of the concentration of reactants.