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Terms in this set (21)
The way we normally express a chemical reaction
Ignores everything that happens in the reaction. It only gives your start and end points
A series of simpler reactions that add up to the overall reaction
Mechanisms are influenced by
Elementary steps, molecularity, and reaction rate.
Elementary reactions (steps)
Simple reactions that describe a single molecular event in a proposed reaction mechanism
Elementary steps are characterized by
The number of reactant particles involved in a proposed reaction mechanism
An elementary reaction that involves just one reactant species. This is a decomposition or rearrangement of that molecule
A → products
An elementary reaction involving two reactant species.
A + A → products
A + B → products
Some termolecular reactions exist
But are rare because it is difficult for three things to collide at once.
The rate law of an overall reaction is never determined from the balanced equation
But through experimentation.
The rate law for an elementary step, however, can be determined from
The reaction stoichiometry.
The reaction order equals
A reaction can only move as fast as
Its slowest step
Rate-limiting (rate-determining) step
The slowest step in a reaction mechanism and therefore is the step that limits the overall rate of the reaction
A substance that is formed and used up during the overall reaction and therefore does not appear in the overall reaction
Intermediates are usually
Unstable relative to the reactants and products
Intermediates are molecules with
Normal bonds and are sometimes stable enough to be isolated
We can never fully prove that a particular mechanism is the way the chemical change actually occurs
-We can only look for evidence that supports the idea.
-Regardless of the elementary steps that are proposed for the mechanism, they must obey three criteria.
1. The elementary steps must add up to the overall equation.
2. The mechanism must be consistent with the rate law.
3. The elementary steps must be physically reasonable (unimolecular or bimolecular).
Mechanisms with a slow initial step.
-The overall rate law includes only species up to and including those in the rate-determining step.
-Each step in the mechanism has its own *transition state.*
Mechanisms with a fast initial step.
-Some mechanisms have a fast equilibrium step first that must be consumed by the slow step.
-In writing the rate law for the rate-determining step, keep in mind that an overall rate law must include only those substances in the overall equation.
-Write rate laws for both directions of the fast equilibrium step and for the slow step.
-Show that the slow step's rate law is equivalent to the overall rate law by expressing the intermediate concentration in terms reactant concentration; set the forward rate law of the fast, reversible step equal to the reverse rate law
-Solve for the concentration of the intermediate.
-Substitute the concentration of the intermediate into the rate law for the slow step to obtain the overall rate law.
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