Introduction, Reaction Rate & Collision Theory Mr. Shields Regents Chemistry U13 L04
Introduction, Reaction Rate &Collision Theory
Mr. Shields Regents Chemistry U13 L04
Introduction To KineticsIntroduction To Kinetics
In the Past we’ve discussed such concepts as
- Kinetic Energy- Kinetic Molecular Theory
So … what does the word “KINETIC” mean?
- Movement, being in a state of motion
Introduction To KineticsIntroduction To KineticsSo … Kinetic Energy is the Energy of Motion
- What is Avg. KE a measure of?
- Temperature
It can also be used to determine Molecular Velocity
- KE = ½ mv2
And Kinetic Molecular Theory as we’ve seen attemptsto Explain the behavior of ideal Gases in response toChanges in P, T or V.
Chemical KineticsChemical Kinetics
Chemical Kinetics attempts to describe how quickly aChemical Reaction will occur
For instance does reaction 1 and reaction 2 take placeAt the same or different rates?
1) N2 + 3H2 2NH3
2) PbNO3 + KI PbI + KNO3
Will these rxns be instantaneous or do they take placeover several minutes, several hours or even days?
Answering this Question involves understanding the kinetics of the reaction!
KI
Chemical KineticsChemical Kinetics
Consider for example some ground beef
- Cook up a few hamburgers and Leave them in a cold frig … and they may last a week
- leave them on the counter in a warm kitchen and they may go bad in a day
- Both hamburgers (in the frig and on the counter) are decaying over time (a chemical reaction) but these chemical rxns occur at DIFFERENT RATES
Chemical KineticsChemical Kinetics
Since RATE is the measure of Change per unit time …
We can express the Rate of a chemical reaction in thefollowing format …
Rate of reaction = Change Concentration Change in Time
(Concentration is usually expressed as Molarity (moles/L))
Example: 2NO2 2NO + O2
Time(sec) [NO2]mol/l Rate = 0.040mol/l – 0.031mol/l0 0.040 5 sec5 0.031 Rate = 1.8 x 10-3 mol liter-1/s
Collision TheoryCollision Theory
If molecules in a chemical reaction react at differentrates there must be specific conditions that play a role In determining the rate.
COLLISION THEORY explains how changing reactionconditions control reaction rates
Collision theory states there are three things that mustHappen before a chemical reaction will take place.
Collision TheoryCollision Theory1) Molecules must collide
The above is know as a “2 body collision” since onlyTwo reactant molecules need collide to produce Products.
N2 + O2 NO
Collision TheoryCollision TheoryCollision Theory states that MOST chemical reactionsInvolve 2 body collisions
So… If most reactions are only two body collisons thenHow do we explain reactions with more than 2 Reactants?
For example: 2NO(g) + F2 2NOF(g)
i.e. NO + NO + F2 NOF + NOF
According to this eqn, 2 nitrogen (II) oxide moleculesand a fluorine molecule must collide at the same time toyield 2 NOF. But this looks like a 3 body collision, right?
Nitrogen oxyfluoride
Collision TheoryCollision Theory
The answer is that a 3 body collision DOES NOT actuallyTake place. The reaction proceeds in 2 separate 2 bodycollisions:
Step 1: NO(g) + F2(g) NOF2 Intermediate Product
2 separate two body collisions
Step 2: NOF2 + NO 2NOF Product
Notice that the sum of the two separate steps equalsThe chemical equation as initially written
2NO + F2 2NOF
OrientationOrientation
In Collison theory we said there were three factorsNecessary for a chemical reaction to take place.
The first factor, as we’ve seen is that Molecules Must Collide.
The 2nd factor is:
During collision the Molecules Must be in the ProperOrientation
OK. So what does PROPER ORIENTATION mean?
OrientationOrientation
When two molecules react, bonds need to be brokenAnd bonds need to be created
For this to happen molecules need to be in the correct Orientation when collision occurs
For example: H2 + Cl2 2HCL
H H ClCl
Poor orientation forFormation of new bonds
H H
ClCl
Bond breaks
Bond breaks
Bonds beginTo form
Good orientation
CollisionOccurs
No Rxn Takes Place
Poor Orientation- No Reaction Occurs
Kinetic EnergyKinetic Energy
So to reiterate, collision theory says that for moleculesTo react they must:
1) Collide
2) Be in the right orientation at collision
And lastly besides these two requirements, moleculesMust have …
3) Sufficient kinetic energy when they collide for the reaction to occur – in other words they must collide with sufficient velocity
Even though the orientation may be right, without enoughKE there is insufficient energy to break bonds that needto be broken before new bonds can form
Kinetic EnergyKinetic Energy
Good Orientation
InsufficientKE
Kinetic EnergyKinetic Energy
At a given temperature some molecules have moreenergy than others (The Maxwell-Boltzman Distribution).
Even though the temperature increases (inc. KE) notall molecules will have sufficient energy to break bonds even if the orientation is correct.
At 100 deg thereIs a greater Fraction of Molecules withSufficient KE forRxn to occur
Point aboveWhich there is Sufficient KEFor Rxn tooccur
Inc. TV ( )