chemical kinetics collision theory: how reactions takes place reaction rates: how fast reactions...
TRANSCRIPT
Chemical KineticsCollision Theory:
How reactions takes placeReaction Rates:
How fast reactions occurReaction Mechanisms
Resource: www.mwiseman.com
Why are kinetics important?
In order to control processes. speed up useful reactions that occur too slowly slow down reactions that are harmful
Example: Catalysts are used in our cars to
rapidly convert toxic substances into safer substances
Refrigerators are used to slow the process of spoiling in food
Collision Theory
How do reactions occur at the molecular level? Molecules collide with each other Form activated complex
http://www.chem.iastate.edu/group/Greenbowe/sections/projectfolder/animations/NO+O3singlerxn.html
collisions http://www.mhhe.com/physsci/chemistry/
essentialchemistry/flash/collis11.swf
correct and incorrect collisions
The area under the curve is a measure of the total number of particles present.
Svante Arrhenius Did some fancy math to figure out that
number of collisions alone don’t account for reaction rates
He found that reactants also require:Activation energy (Ea - energy to break bonds) Right orientation http://www.mhhe.com/physsci/chemistry/essentialch
emistry/flash/activa2.swftransition state
Not all collisions leads to a reaction For effective collisions proper orientation ofthe molecules must be possible
What affects reaction rate?
Temperature http://www.sciencepages.co.uk/keystage4/GCSEChemistry/rate5
concentration and temperature
Increased number of collisions More molecules have enough activation energy Remember Maxwell-Boltzmann distribution
Increased temperature, distribution flattens out More molecules
have Ea
What affects reaction rate?
Higher concentration Number of collisions increased
http://www.chem.iastate.edu/group/Greenbowe/sections/projectfolder/animations/O2+NO2%20kinetics8.html
concentration
Increased surface area Number of collisions increased
What affects reaction rate?
Catalysts Def’n: substance that speeds up a rxn w/o being used
up itself Number of collisions with Ea increase
Ea lowers Catalysts hold molecules in right orientation
• Homogeneous catalyst (same phase of matter) Demo: Catalysis by Co2+
• Heterogeneous catalyst (different phase)
http://www.chem.iastate.edu/group/Greenbowe/sections/projectfolder/animations/Catalyst2NOO2N28.html
catalyst
What is this?
How do we measure rxn rates?
Rates must be measured by experiment Indicators that a reaction is happening
Color change Gas formation Precipitate formation Heat and light
Many ways to measure the rate Volume / time Concentration / time Mass / time Pressure / time
How do we measure rxn rate?
A B How fast product appears
How fast reactant disappears
t
A
t
B
Forward vs Reverse Rxn
Some rxns are reversibleAfter a sufficient amount of product is
made, the products begin to collide and form the reactants
We will deal only w/ rxns for which reverse rxn is insignificant
2 N2O5(aq) 4 NO2(aq) + O2 (g)Why is reverse rxn not important here?
Rate Law
Math equation that tells how reaction rate depends on concentration of reactants and products
Rates = k[A]n
K = rate constant / proportionality constant n = order of reaction
Tells how reaction depends on concentration• Does rate double when concentration doubles?
• Does rate quadruple when concentration doubles?
2 kinds of rate laws
Both determined by experimentDifferential Rate Law
How rate depends on [ ]
Integrated Rate Law How rate depends on time
Differential Rate Law
2 methods Graphical analysis Method of initial rates
Graphical Analysis
1. Graph [ ] vs. time
2. Take slope at various pts
3. Evaluate rate for various concentrations
[N2O5]
(M)
Rate
(M/s)
1.0 2
0.5 1.0
0.25 0.5
Graphical Analysis
When concentration is halved… Rate is halved Order = 1 Rate = k[N2O5]1
[NO2]
(M)
Rate
(M/s)
1.0 2
2.0 8
4.0 32
Graphical Analysis
When concentration is doubled… Rate is quadrupled Order = 2 Rate = k[N2O5]2
Method of Initial Rates
Initial rate calculated right after rxn begins for various initial concentrations
NH4+(aq) + NO2
-(aq) N2(g) + 2H2O(l)
Rate = k [NH4+]n[NO2
-]m
[NH4+] [NO2
-] Rate (M/s)
0.1 0.1 2
0.1 0.2 4
0.2 0.2 6
[NH4] [NO2-] Rate
0.1 0.1 2
0.1 0.2 4
0.2 0.2 8
[NH4] [NO2-] Rate
0.1 0.1 2
0.1 0.2 4
0.2 0.2 6When [NO2] doubles, rate doubles,
First order with respect to (wrt) NO2
m = 1
When [NO2] doubles, rate doubles,
First order with respect to (wrt) NO2
n = 1
Rate = k[NH4+] [NO2-]
Try this one:
Rate = k [NO2-]2
[NH4+] [NO2
-] Rate (M/s)
0.1 0.1 2
0.1 0.2 8
0.2 0.2 8
Calculate k, using any of the trials, you should get the same value
Integrated Rate Law
Tells how rate changes with timeLaws are different depending on orderOverall reaction order is sum of exponents
Rate = k zero order Rate = k[A] first order Rate = k[A]2 second order Rate= k[A][B] second order
First order integrated rate law
Rearrange and use some calculus to get:
][][
Akt
A
0]ln[]ln[ AktA This is y = mx + b form
A plot of ln[A] vs time will give a straight line
If k and [A]0 (initial concentration) known, then you know the concentration at any time
Second order integrated rate law
Rearrange and use some calculus to get:
2][][
Akt
A
0][
1
][
1
Akt
A
This is y = mx + b form A plot of 1/[A] vs time will give a straight line
If k and [A]0 (initial concentration) known, then you can now the concentration at any time
Zero order integrated rate law
Rearrange and use some calculus to get:
kt
A
][
0][][ AktA This is y = mx + b form
A plot of [A] vs time will give a straight line
If k and [A]0 (initial concentration) known, then you can now the concentration at any time
Graphs give order of rxn
Use graphs to determine order If [A] vs time = zero order If ln [A] vs time = first order If 1/ [A] vs time = second order
Half-life
Def’n: time it takes for concentration to halve
Depends on order of rxnAt t1/2 [A]=[A]0/2
Half-Life
First order
Second order
Zero Order
kt
693.02/1
02/1 ][
1
Akt
k
At
2
][ 02/1
Reaction Mechanism
Reactions occur by a series of steps =
Reaction mechanism Example:
Overall reaction: NO2 + CO NO + CO2
occurs by following steps Step 1:
Step 2:
Intermediates
Two molecules of NO2 collide
Oxygen is transferred, making NO3, the intermediate Intermediates are temporarily formed during a
reaction They are neither a reactant nor a product & Get used up in reaction
Rules for Reaction Mechanisms
Sum of elementary steps = overall balanced rxn
Mechanism must agree with experimental rate law
Elementary Step
Steps in reaction from which a rate law for step can be directly written
2 molecules of NO2 need to collide, therefore…
Rate = k [NO2]2
Molecularity
Rate law written based on molecularity Number of things that have to collide
Unimolecular – rxn depends on 1 moleculeBimolecular – rxn depends on 2 molecules
Termolecular – rxn depends on 3 molecules • Very rare!
Give molecularity and rate law:
Unimolecular (first order) rate=k[A]
Bimolecular (second order) rate=k[A][B]
Rate Determining Step
The slowest step in mechanism determines overall rate
Rate cannot be faster than slowest step Demo: Filling bottle with funnel
Overall rate law can be written from molecularity of slowest step
How are mechanisms determined?
1. Rate law is determined using experiment (method of initial rates, etc.)
2. Chemist uses intuition to come up w/ various mechanisms
3. Narrows down choices using rules for mechanisms
No mechanism is ever absolutely proven