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chemical kinetics

is the study of the rates of reactions, the effect of reaction conditions on these rates, and the mechanisms implied by such observations



of a reaction is the actual series of steps through which a chemical reaction occurs



does not appear in the overall reaction because it is neither a reactant nor a product


rate-determining step

the slowest step in a proposed mechanism because the overall reaction cannot proceed faster than that step



the disappearance of reactants over time or the appearance of products over time. Expressed in the units of moles per liter per second (mol/L x s) or molarity per second (molarity/sec)


reaction order

is defined as the sum of the exponents


zero-order reactions

has a constant rate, which is independent of the reactants' concentrations. Thus the rate law is: rate=k, where k has units of Msec-1


first order reactions

has a rate proportional to the concentration of one reactant. Have units of sec-1


second order reactions

has a rate proportional to the product of the concentration of two reactants, or to the square of the concentration of a single reactant; for example, rate = k[A]2, rate =k[B]2, or rate=k[A][B]. The units of second-order rate constants are M-1sec-1


higher order reactions

a higher order reaction has an order greater than 2


mixed order reactions

a mixed order reaction has a fractional order; e.g., rate=k[A]1/3


collision theory of chemical kinetics

states that the rate of a reaction is proportional to the number of collisions per second between the reacting molecules


effective collision

(one that leads to the formation of products) occurs only if the molecules collide with correct orientation and sufficient force to break the exisiting bonds and form new ones


activation energy

minimum energy of collision necessary for a reaction to take place


transition state

when molecules collide with sufficient energy in which the old bonds are weakened and the new bonds are beginning to form


potential energy diagram

illustrates the relationship between the activation energy, the heats of reaction, and the potential energy of the system


enthalpy change

is the difference between the potential energy of the products and the potential energy of the reactants


reactant concentrations

the greater the concentrations of the reactants (the more particles per unit volume), the greater will be the number of effective collisions per unit time, and therefore the reaction rate will increase for all but zero order reactions



for nearly all reactions, the reaction rate will increase as the temperature of the system increases. Since the temperature of a substance is a measure of the particles' average kinetic energy increasing the temperature increases the average kinetic energy of the molecules.



the rate of a reaction may also be affected by the medium in which it takes place. Certain reactions proceed more rapidly in aqueous solution, whereas other reactions may proceed more rapidly in benzene. The state of the medium (liquid, solid, or gas) can also have a significant effect the pH of a medium may also affect the rate of certain reactions



are substances that increase reaction rate without themselves being consumed; they do this by lowering the activation energy. Catalysts are important in biological sysstems and in industrail chemsitry; enzymes are biological catalysts. Catalysts may increase the frequency of collision between the reactants, change the relative orientation of the reactants making a higher percentage of collisions effective, donate electron density to the reactants, or reduce intramolecular bonding with reactant molecules.



only proceed in one direction, and that the reactions procee to completion


reaction quotient, Q

is a measure of the degree to which a reaction has gone to completion


equilibrium constant, Keq

pure solids and liquids do not appear in the equilibrium constant expression. Keq is characteristic of a given system at a given temperature. If the value of Keq is very large compared to 1, an equilibrium mixture of reactants and products will contain very ittle of the reactants compared to the products. if the value of Keq is very small compared to 1 (ie less than 0.1), an equilibrium mixture of reactants and products will contain very little of the products compared to the reactants. if the value of Keq is close to 1, an equilibrium mixture of products and reactants will contain approximately equal amounts of reactants and products


Le Chatelier's principle

is used to determine the direction in which a reaction at equilibrium will proceed when subjected to a stress, such as a change in concentration, pressure, temperature, or volume