Term | Definition |
chemical kinetics | The area of chemistry concerned with the speed, or rates, at which chemical reactions occur. |
reaction rates | The decrease in concentration of a reactant or the increase in concentration of a product with time |
instantaneous rate | The reaction rate at a particular time as opposed to the average rate over an interval of time |
Beer's Law | The light absorbed by a substance (A) equals the product of its molar absorptivity constant (a), the path length through which the light passes (b), and the molar concentration of the substance (c): A=abc |
Rate Law | An equation that relates the reaction rate to the concentrations of reactions (and sometimes of products also) |
Rate constant | A constant of proportionality between the reaction rate and the concentrations of reactants that appear in the rate law. |
reaction orders | The power to which the concentration of a reactant is raised in a rate law |
overall reaction order | The sum of the reaction orders of all the reactants appearing in the rate expression when the rate can be expressed as
rate=k[A]^a [B]^b |
first-order reaction | A reaction in which the reaction rate is proportional to the concentration of a single reactant, raised to the first power. |
second-order reaction | A reaction in which the overall reaction order ( the sum of the concentration-term exponents) in the rate law is 2 |
half-life | The time required for the concentration of a reactant substance to decrease to half its initial value; the time required for half of a sample of a particular radioisotope to decay |
collision model | A model of reaction rates based on the idea that molecules must collide to react; it explains the factors influencing reaction rates in terms of the frequency of collisions, the number of collisions with energies exceeding the activation energy, and the p |
activation energy ( E sub a) | The minimum energy needed for reaction; the height of the energy barrier to formation of products. |
activated complex (transition site) | The particular arrangement of atoms found at the top of the potential-energy barrier as a reaction proceeds from reactants to products |
Arrhenius equation | An equation that relates the rate constant for a reaction to the frequency factor; A, the activation energy, E sub a, and the temperature, T: k=A sub e ^-E sub a/RT. In its logarithmic form its written lnk=-E sub a/RT+lnA. |
frequency factor | A term in the Arrhenius equation that is related to the frequency of collision and the probability that the collisions are favorably oriented for reaction. |
reaction mechanism | A detailed picture, or model, of how the reaction occurs, that is, the order in which bonds are broken and formed and the changes in relative positions of the atoms as the reaction proceeds |
elementary reactions | A process in a chemical reaction that occurs in a single event or step. An overall chemical reaction consists of one or more elementary reactions of steps. |
molecularity | The number of molecules that participate as reactants in an elementary reaction. |
unimolecular | An elementary reaction that involves a single molecule |
bimolecular | An elementary reaction that involves two molecules |
termolecular | An elementary reaction that involves three molecules. Termolecular reactions are rare. |
intermediate | A substance formed in one elementary step of multistep mechanism and consumed in another; it is neither a reactant nor an ultimate product of the overall reaction. |
rate determining step | The slowest elementary step in a reaction mechanism |
catalyst | A substance that changes the speed of a chemical reaction without itself undergoing a permanent chemical change in the process |
homogeneous catalyst | A catalyst that is in the same phase as the reactant substances |
heterogeneous catalyst | A catalyst that is in a different phase from that of the reactant substances |
adsorption | The binding of molecules to a surface |
enzymes | A protein molecule that acts to catalyze specific biochemical reactions |
substrates | A substance that undergoes a reaction at the active site in an enzyme |
active site | Specific site on a heterogeneous catalyst or an enzyme where catalysis occurs |
lock-and-key model | A model of enzyme action in which the substrate molecule is pictured as filtering rather specifically into the active site on the enzyme. It is assumed that in being bound to the active site, the substrate is somehow activated for reaction |