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Chem II Ch. 14

Chemical Kinetics

Chemical Kinetics The area of chemistry concerned with the speeds, or rates, at which chemical reactions occur.
Reaction Rates A measure of the decrease in concentration of a reactant or the increase of 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.
Rate Law An equation that relates the reaction rate to the concentration of reactants (and sometimes of products also).
Rate Constant A constant of proportionality between the reaction rate and the concentration 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.
Zero-Order Reaction One in which the overall reaction order is 0.
Half-Life The time required for the concentration of a reactant substance to decrease its initial value; the time required for half or 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.
Activation Energy (Ea) The minimum energy needed for reaction; the height of the energy barrier to formation of products.
Activated Complex (transition state) The particular arrangement of atoms found at the top of the potential-energy barrier as a reaction proceeds from the reactants to products.
Arrhenius Equation An equation that relates the rate constant for a reaction to the frequency factor, A, the activation energy, Ea, and the temperature, T: k= Ae^(-Ea/RT). In its logarithmic form it is: Ln k= -Ea/RT + Ln A
Frequency Factor (A) 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 Reaction 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 or steps.
Molecularity The number of molecules that participate as reactants in an elementary reaction.
Unimolecular Reaction An elementary reaction that involves a single molecule.
Bimolecular Reaction An elementary reaction that involves two molecules.
Termolecular Reaction An elementary reaction that involves three molecules. These types of reactions are rare.
Intermediate A substance formed in one elementary step of a 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 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 the same phase as the reactant substances.
Heterogeneous Catalyst A catalyst that is in a different phase from that of the reactant substances.
Absorption The binding of molecules to a surface.
Enzyme A protein molecule that acts to catalyze specific biochemical reactions.
Substrate A substance that undergoes a reaction at the active site in an enzyme.
Lock-and-Key Model A model of enzyme action in which the substrate molecule is pictured as fitting 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.
Active Site Specific site on a heterogeneous catalyst or an enzyme where catalysis occurs.
Created by: mmcclure



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