the time required for a reactant concentration to drop to one half of its initial value
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 __.
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
__ 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 __.
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
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
one that has rate law equal to k
the rate remains constant, independent of the concentration of the reactant
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
the sequence of reaction steps that describes the pathway from reactants to products
A single step in a reaction mechanism is called an __, or __.
elementary reaction (step)
Contrast elementary and overall reaction
e: describes an individual molecular event
o: describes the reaction stoichiometry
a species that is formed in one step of a reaction mechanism and consumed in a subsequent step
number of molecules (or atoms) on the reactant side of the chemical equation
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
Rate law for a single-step, elementary reaction follows directly from its __
rate law for a multistep, overall reaction depends on the __.
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.
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
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.
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 out
3a) if dont agree, devise anoter mechanism
3b) if agreed, look for additional supporting evidence
The case for a particular mechanism is strengthened considerably if a reaction intermediate __
can be isolated or if an unstable intermediate can be detected
As a rule of thumb, reaction rates tend to __ when temperature is increased by __.
10 degrees Celcius
According to the __, a bimolecular reaction reaction occurs when two properly oriented reactant molecules come together in a sufficiently energetic collision
collision theory model
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 __.
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.
The fraction of collisiosn having proper orientation for the conversion of reactants to products is called the __.
steric factor (p)
Reaction rates are affected not only by reactant concentrations and temperature but also by the presence of __.
substance that increases the rate of a transformation without itself being consumed in the process
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
one that exists in the same phase as the reactants
one that exists in a different phase from that of the reactants
- ordinarily, it was a solid
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