Kinetics Flashcards
Factors that Affect Reaction Rates
Physical state of reactants (increase surface area=increase # collisions)
Concentration of reactants (increase reactant=increase # collisions)
Temperature (increase temperature=increase KE=increase in # collisions and energy of collisions
Presence of a catalyst (decrease activation energy=require less energy for collisions)
Rate of appearence
change (B)/ Change (time) or -rate of disappearance of A
Rate of disappearance
change (A)/ Change (time)
Average Rate
Change in concentration over a specific time interval
Instantaneous Rate
Rate at a particular moment during the reaction
Initial Rate
Instantaneous rate at t=0
Rate Expression
Compares the rate of reactants and products relative to each other and relative to the overall rate of the reaction
Rate Law
Shows that rate depends on the concentration of reactants
Rate=
k A^m B^n
k= rate constant
m, n: reaction orders
Rate constant, k, depends on…
Temperature and a catalyst
Overall reaction orders =
m + n
Units of rate constant, k, depends on…
reaction order
Units of Rate Constant Chart
Units of Rate Constant Chart
Rate constant, k, does not depend on
Concentration
Plots of the Different Orders
Zero order-concentration vs. time
First order-ln concentration vs. time
Second order-1/concentration vs. time
Zero Order
Rate is independent of concentration
slope=-k
Rate Law: rate=k
Integrated Rate Law: [At]=-kt+[A0]
Half Life: t1/2=[A0]/2k
First Order
Rate depends on concentration of a single reactant
slope=-k
Rate Law: rate=k[A]
Integrated Rate Law: ln[At]=-kt+ln[A0}
Half Life: t1/2=0.693/k
Second Order
Rate depends on the concentration of [A^2] or [A] and [B]
slope=k
Rate Law: rate=k[A^2]
Integrated Rate Law: 1/[At]=kt + 1/[A0]
Half Life: t1/2= 1/k[A0]
Half Life
Time required for reactant to reach 1/2 initial value
In zero and second order, the half-life depends on the initial concentration
First order is constant
Elementary Reaction
A process in a chemical reaction that occurs in a single event or step
Can use stoichiometry to find rate law
Can involve simultaneous collision of three or more particles but rare
Elementary steps of a multistep mechanism adds up to overall chemical equation
To find rate law for an overall reaction
you must use experimental data
Reaction Mechanism
The process by which a reaction occurs
The Collision Model
Molecules must collide in order to react
Increase # collisions = increase rate
The Orientation Factor
Collisions must occur with molecules in the proper orientation for a reaction to occur
Activation Energy
Minimum energy to start reaction
Collision Theory
Collide, have proper orientation, have suffient energy
Change in energy does not relate to…
reaction rate
Endothermic vs. Exothermic
Endothermic=energy is absorbed so products have greater energy than reactants
Exothermic=energy is given off so products have less energy than reactants
The Arrhenius Equation
k=Ae^-Ea/RT
A: frequency factor, related to collisions and orientation
R: 8.314 j/molK
Increase in temp or decrease in Ea will lead to…
increase rate constant
Molecularity
of molecules that are reactants
Unimolecular= 1 molecule involved
Intermediate
A substance that is formed in on elementary step but used in a later one so it doesn’t show in the overall reaction
Catalyst
A substance that speeds up a chemical reaction; it is present at the start of reaction and re-foremed in a later step but does not show up in overall reaction
How do you know that you are dealing with an elementary step?
It is a single step in a multi step mechanism or it tells you
Which steps determines the rate of the overall mechanism?
Slowest step limits the overrallr eaction
Mechanisms with a Slow Initial Step
Use coefficients from the slow step to get the rate law
Mechanisms with Fast Initial Step
Write rate law for the last step
Write forward and backward rate law for the first step
Substitute for the compound backing up the reaction
Peaks
Transition State/Elementary Steps
Valley
Intermediates
How do catalysts work?
They provide an alternate pathway with lower potential energy
Leads to new reaction mechanism
Leads to greater rate constant
-Formation of a more stable activated complex, increase collision frequency, improves the orientation of the reactantants
Homogeneous Catalyst
In the same phase/state of matter as the reacting molecules
Heterogeneous Catalyst
In different phase/state of matter than the reacting molecules
Enzymes
Biological catalysts; usually proteins
