Chemical Kinetics
What is it?
How fast rxns take place, facts that affect rates, and mechanisms
Chemical Kinetics
Factors affecting Kinetics
- Concentration
- Temp
- Surface
- Catalyst
Chemical Kinetics
what does Rate of Rxn equal?
-(1/a)(Rate of Reactant A) = -(1/b)(Rate of Reactant B) = (1/c)(Rate of Reactant C) = (1/d)(Rate of Reactant D)
Rate Law
What does Rate Law equal?
= k[A]^m[B]^n
* for aA + bB –> cC + dD
* m≠a and n≠b
* m = order of rxn wrt A
* n = order of rxn wrt B
Rate Law
Overall Order of Rxn
m+n
Rate Law
K
- Proportionality constant
- rate constant
- function of T
Reaction Rate
Methods to monitor change in reactant/product concentration
- Change in color using spectrophotometer
- Change in Pressure with manometer
- Change in Electrical Conductance
Reaction rates
Zero Order
- Rxn Rate: r=k
- Integrated Rxn Rate: [A]t = -kt + [A]0
- Half Life: ([A]0)/2k
Reaction Rate
First Order
- Rxn Rate: r=k[A]
- Integrated Rxn Rate: ln[A]t= -kt + ln[A]0
- Half Life: .693/k
Reaction Rate
Second Order
- Reaction Rate: r=k[A]^2
- Integrated Reaction Rate: 1/[A]t = kt + 1/[A]0
- Half Life: 1/([A]0* k)
Activation Energy
What is it?
Energy barrier that prevents less energetic molecules from reacting
Separatesineffective and effective collisions wrt energy
Activation Energy
Area under Maxwell-Boltzmann
- Represents fraction of collisions that are effective with correct orientation
- A= e ^(-Ea/RT)
- R = 8.314 J/mol/k
- T = K
Activation Energy
Rate constant equation with Activation Energy
- k = A * e ^(-Ea/RT)
- A = frequency factor = constant of soln to find rate constant
- R = 8.314 J/mol/k
- T = K
Activation Energy
To compare K values at different temperatures…
ln(k1/k2) = Ea/R(1/(T1) - 1/(T2))
Reaction Mechanism
What is it?
- Sequence of steps(each step = elementary step) and sum of all steps
- In the ES, rxn order for each reactant = stoic coeff.
Reaction Mechanism
Rate determining step
- Slowest ES = Speed of overall rxn
Reaction Mechanism
How to determine plausible mechanism
- ES must sum up to overall rxn
- Rate-determining step must have same rate law as overall exp.aly-det. rate law
Reaction Mechanism
Intermediate
- Produced in ES and consumed in later ES
Reaction Mechanism
Rate determining step senarios
- If the 1st step slow: 1st = rate of rxn and rest fast
- If 2 + steps slow: Step right before slow step = equilibrium and slow step = RDS
Activation Energy - Catalyst
What do they do?
- Increases rate of rxn(faster rxn) by decreasing Ea
- Present at beginning and end of rxn
Activation Energy - Catalyst
RDS relation to Ea
Highest Ea = RDS
