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activated complex
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(also, transition state) unstable combination of reactant species formed during a chemical reaction
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activation energy (
E
a
)
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minimum energy necessary in order for a reaction to take place
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Arrhenius equation
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mathematical relationship between a reaction’s rate constant, activation energy, and temperature
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average rate
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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
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bimolecular reaction
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elementary reaction involving two reactant species
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catalyst
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substance that increases the rate of a reaction without itself being consumed by the reaction
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collision theory
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model that emphasizes the energy and orientation of molecular collisions to explain and predict reaction kinetics
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elementary reaction
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reaction that takes place in a single step, precisely as depicted in its chemical equation
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frequency factor (
A
)
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proportionality constant in the Arrhenius equation, related to the relative number of collisions having an orientation capable of leading to product formation
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half-life of a reaction (
t
l/2
)
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time required for half of a given amount of reactant to be consumed
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heterogeneous catalyst
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catalyst present in a different phase from the reactants, furnishing a surface at which a reaction can occur
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homogeneous catalyst
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catalyst present in the same phase as the reactants
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initial rate
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instantaneous rate of a chemical reaction at
t
= 0 s (immediately after the reaction has begun)
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instantaneous rate
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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
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integrated rate law
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equation that relates the concentration of a reactant to elapsed time of reaction
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intermediate
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species produced in one step of a reaction mechanism and consumed in a subsequent step
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method of initial rates
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common experimental approach to determining rate laws that involves measuring reaction rates at varying initial reactant concentrations
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molecularity
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number of reactant species involved in an elementary reaction
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overall reaction order
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sum of the reaction orders for each substance represented in the rate law
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rate constant (
k
)
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proportionality constant in a rate law
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rate expression
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mathematical representation defining reaction rate as change in amount, concentration, or pressure of reactant or product species per unit time
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rate law
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(also, rate equation) (also, differential rate laws) mathematical equation showing the dependence of reaction rate on the rate constant and the concentration of one or more reactants
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rate of reaction
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measure of the speed at which a chemical reaction takes place
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rate-determining step
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(also, rate-limiting step) slowest elementary reaction in a reaction mechanism; determines the rate of the overall reaction
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reaction diagram
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used in chemical kinetics to illustrate various properties of a reaction
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reaction mechanism
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stepwise sequence of elementary reactions by which a chemical change takes place
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reaction order
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value of an exponent in a rate law (for example, zero order for 0, first order for 1, second order for 2, and so on)
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termolecular reaction
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elementary reaction involving three reactant species
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unimolecular reaction
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elementary reaction involving a single reactant species
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