Chapter 15: Chemical Kinetics Flashcards
Rate of reaction is impacted by what factors?
- Collision model (concentration of reactant particles) - Temperature of reactant mixture (generally occur faster with increasing temperature) - Structure and relative orientation of colliding particles
Rate of Reaction (definition)
- Measure of how fast the reaction occurs - Large fraction of molecules is produced from a faster rate
Speed Equation
change in distance / change in time = ∆x / ∆t
Instantaneous Rate of Reaction: aA + bB –> cC + dD
Rate = -1/a (∆[A]/∆t) = -1/b (∆[B]/∆t) = +1/c (∆[C]/∆t) = +1/d (∆[D]/∆t)
Rate Law
Express relationship between rate of reaction and concentration of reactant:
Rate = k[A]n
k = constant of proportionality (rate constant) n = reaction order
Reaction Order:
n = 0
- Zero order
- Rate is independent of concentration
- Concetration decreases linearly over time
- Occurs when amount of reactant available for reaction unaffected by changes in overall quantity of reactant
Reaction Order:
n = 1
- First order
- Directly proportional to concentration
- Rate slows down as reaction proceeds
Reaction Order:
n = 2
- Rate of reaction proportional to square concentration of reactant
- Rate is more sensitive to concentration
How to determine order of reaction?
- Order of reaction is determined by experiment
- Common method is to use method of initial rates
k = rate 2 / rate 1 = k[A]n2 / k[A]n1
Integrated Rate Law
- Relationship between concentrations of reactants and time
First Order Integrated Rate Law
ln[A]t = -kt + ln[A]0
slope = -k
Second Order Integrated Rate Law
-∆[A] / ∆t = k[A]2 or
1/[A]t = kt + 1/[A]0
NONLINEAR
Half-Life
t1/2 = 0.693/k
Arrhenius Equation
k = Ae-Ea/RT
- Ea = activation energy
- Frequency factor (A): number of times the reactants approach activation barrier/time
- Frequency factor (Ea): Energy barrier to be surmounted by reactants
- In a graph, E of products will be contained on the right side after bump in activation energy
Activated Complex
- Transition state in which it has a higher energy than either reactant or product
Exponential Factor
- Number between 0 and 1 representing fraction of molecules that have enough E to make it over the activation barrier on given approach
= e-Ea/RT
Arrhenius Plot
- Plot of natural log of rate constant (lnk) versus the inverse of temperature in Kelvins (1/T)
- Creates straight line with slope -Ea/R and y-intercept lnA
Two-Point Arrhenius
lnk2/k1 = Ea/R (1/T1 -1/T2)
Collision Model
- Chemical reaction occurs after a sufficiently energetic collision between two molecules
Orientation Factor
- Measure of how specific the orientation of the collision must be
Reaction Mechanism
- Series of individual chemical steps by which an overall chemical reaction occurs
- Comprised of elementary steps (each step in the reaction mechanism)
Molecularity
- Number of reactatnt particles involved in step; used to characterize elementary steps
- Unimolecular, bimolecular, termolecular
Rate Determining Step
- Limits the overall reaction and determines the rate law for the overall reaction
- Obtain rate law from “slow step”
Catalyst
- Substance that increases rate of chemical reaction but is not consumed by the reaction
- Provides alternative mechanism for reaction (in which rate-determining step has lower Ea)
Homogenous Catalyst
- Catalyst exists in same state as reactants
Heterogenous Catalyst
- Catalyst exists in different phase than reactants
Hydrogenation
- Involves hydrogenation of double bonds of alkenes
Catalyst Procedure
1) Absorption: Reactants absorbed onto metal surface
2) Diffusion: Reactants diffuse on surface until they approach each other
3) Reaction: Reactants react to form products
4) Desorption: Products desorb from surface to react
Enzymes
- Biological catalysts that increase rates of biochemical reaction
- Usually large protein molecule with 3D structures
- Active site: Binds reactant molecule
- Substrate: Fits into active site (only catalyze one substrate)
