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What is the half-life of a reaction
the time required for a reactant concentration to drop to one half of its initial value
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The half life of a first order reaction is a __ because it depends only on the __ and not on the __. This point is worth noting because reactions that are not first order have half-lives that do depend on __.
- constant
- rate constant
- concentration x2
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Because the half-life of a first order reaction is a constant, each successive half-life is __.
equal period of time in whihc the reactant concentration decreases by a factor of two
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__ is a first order process, whose rate is proportional to the number of radioactive nuclei N iin a sample times a first order rate constant k, called the __.
- radioactive decay
- decay constant
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second order reaction
one whose rate depends eiter on the concentration of a single reactant raised to the second power or on the concentrations of two different reactants, each raised to the first power
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In contrast with a first-order reaction, the time required for the concentration of A to drop to one-half of its initial value in a second order reactiond epends on __.
both the rate constant and the initial concentration
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zeroth-order reaction
- one that has rate law equal to k
- the rate remains constant, independent of the concentration of the reactant
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An equally important issue in chemical kinetics is the __, the sequence of molecular events, or reaction steps, that describes the pathway from reactants to products
reaction mechanism
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reaction mechanism
the sequence of reaction steps that describes the pathway from reactants to products
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A single step in a reaction mechanism is called an __, or __.
elementary reaction (step)
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Contrast elementary and overall reaction
- e: describes an individual molecular event
- o: describes the reaction stoichiometry
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reaction intermediate
a species that is formed in one step of a reaction mechanism and consumed in a subsequent step
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molecularity
number of molecules (or atoms) on the reactant side of the chemical equation
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For a bimolecular elementary reaction of the type A+B--> Products, the reaction rate depends on the __. The frequency of AB collisions involving any particualr A molecule is __; and the total frequency of AB collisions involving all A molecules is __
- frequency of collisions between A and B molecules
- proportional to the molar concentration of B,
- proportional to the molar concentration of A times the molar concentration of B
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Rate law for a single-step, elementary reaction follows directly from its __
rate law for a multistep, overall reaction depends on the __.
- molecularity
- reaction mechanism
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When an overall reaction occurs in two or more elementary steps, one of the steps is often much slower than the others. This slowest step in a reaction mechanism is called the __ becuase it limits the rate at which reactants can be converted to product.
rate-determining step
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A plausible mechanism must meet two criteria
- the elementary steps must sum up to give the overall reaction
- the mechanismmust be consistent with the experimental rate law for the overall reaction
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Just as the rates of vaporization and condensation are equal for a __, so the rates of teh forward adn reverse reactions are equal for a chemical equilibrium.
liquid-vapor equilibrium
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Procedure Used in Studies of Reaction Mechanisms
- 1) determine rate law by experiment
- 2) series of elementary steps is proposed
- 3) rate law predicted by proposed mechanism is worked outÂ
- 3a) if dont agree, devise anoter mechanism
- 3b) if agreed, look for additional supporting evidence
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The case for a particular mechanism is strengthened considerably if a reaction intermediate __
can be isolated or if an unstable intermediate can be detected
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As a rule of thumb, reaction rates tend to __ when temperature is increased by __.
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According to the __, a bimolecular reaction reaction occurs when two properly oriented reactant molecules come together in a sufficiently energetic collision
collision theory model
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The height of the barrier that atoms must overome is called __, and the configuration of atoms at the maximum in the potential energy profile is called the __, or the __.
- activation energy
- transition state
- activated complex
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As the temperature increases, the distribution of collision energies broadens and shifts to __, resulting in a rapid increase in the fraction of collisions that lead to products.
higher energies
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The fraction of collisiosn having proper orientation for the conversion of reactants to products is called the __.
steric factor (p)
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Arrhenius equation
k=Ae-Ea/RT
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Reaction rates are affected not only by reactant concentrations and temperature but also by the presence of __.
catalysts
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catalyst
substance that increases the rate of a transformation without itself being consumed in the process
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The catalyzed pathway for a reaction might have a faster rate than te uncatalyzed patway either because of a __ or a __.
- larger frequency factor (A)
- smaller activation energy in the Arrhenius equation
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homogeneous catalyst
one that exists in the same phase as the reactants
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heterogeneous catalyst
- one that exists in a different phase from that of the reactants
- - ordinarily, it was a solid
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The mechanism of heterogeneous catalysis is often complex and not well understood. Important steps frequently involve __
1) attachment of reactants to the surface of the catalyst, a process called adsorption
2) conversion of reactants to products on the surface - 3 desorption of products from the surface
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