Introduction, Reaction Rate & Collision Theory Mr. ShieldsRegents 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 ( )