QUESTION?

ANSWER!Only a small fraction of collisions results in a reaction. (~1 in every

1013 collisions!)

Does every collision result in a reaction?

Collision Theory

REACTION RATE & TEMPERATURE

QUESTION?Why doesn't every molecular collision result in a reaction?

2AB A2 + B2

No Reaction

Reaction occursA - A B - B

Illu

stra

tion

1:

Collision Theory

No Reaction

Reaction occursA - A

B - B

If molecules are moving too slowly, they collide with insufficient energy (unable to overcome the repulsion between electron clouds), and just bounce off each other instead of reacting.

If the reacting species are colliding with sufficient kinetic energy--activation energy--(to stretch, bend and break the present bonds), this leads to the formation of products.

Illu

stra

tion

2:

Conditions for a Reaction to Occur

Reacting species must Collide Collide in a favourable orientation (appropriate geometry

or juxtaposition) – steric factor Collide with a certain minimum kinetic energy – (more

than the activation energy barrier).

Collision Theory

Reaction ProfileEn

ergy

Reaction Pathway

CH3 + HCl [H3C---H---Cl]

CH4 + Cl

CH3 + HCl

[ H3C---H---Cl ]

CH4 + ClH

Ea1

Ea2

A) One Essential Step

or Energy Profile Diagram

Characteristics of Activated Complex

Very unstable i.e. It has a pretty short half-life. Its potential energy is greater than reactants or products. The activated complex and the reactants are in chemical

equilibrium. It decomposes to form products or reactants.

ENERGY PROFILE DIAGRAMB) Two or more essential/elementary steps

Step 1: A BStep 2: B COverall: A C

Ene

rgy

Reaction Coordinate

H

Ea

AB C

B: reactive intermediate

Example: Reaction Mechanism

Step 1: CH3CHO + I2 CH3I + HI + COStep 2: CH3I + HI CH4 + I2

Draw an energy profile diagram for the above reaction.Draw an energy profile diagram for the above reaction.

Overall:Overall: CH3CHO CH4 + CO H = H = +ve+ve

Reactive Intermediates: CH3I and HI

Catalyst: I2

ENERGY PROFILE DIAGRAMen

ergy

Reaction progress

H

Ea

CH3CHO

CH4 + COCH3I + HI

Objectives: Explain the effects of concentration

change/pressure, temperature change , surface area and the presence of catalyst on the reaction rate

Factors that Affect the Reaction Rate

Factors that Affect the Reaction Rate

Reactant concentration @ pressure System temperature Catalyst Reactants’ physical appearance (particle size,

medium) Other agents: light, pH, presence of impurities Chemical nature of reactants

a) Reactant concentration / Pressure of gases

PV = nRT P = (n/V) RT P = c.R.T pressure, P concentration, c

P or c , Distance between particles is shorter, Effective collisions increaseMore molecules are able to overcome Ea

The no. of collisions per unit time will increaseReaction rate increases

Explanation:

b) TemperatureM

ole

fract

ion

Kinetic energyEa

T1

T2

T2 > T1

Maxwell-Boltzmanndistribution

At higher T

More molecules posses kinetic energy equal to or higher than Ea

i.e.: More effective collisions

reaction rate increases

c) Catalyst

A catalyst speeds up a chemical reaction by providing an alternative pathway (mechanism) that has a lower activation energy.

Catalyst are not consumed in a chemical reaction.

(A substance, which slows down the rate of a

reaction, is called an inhibitor )

Function of Catalyst Catalysts provide alternative reaction pathways of

lower activation energies than those of the uncatalysed reactions

ener

gy

Reaction progress

X

Y

catalysed

H

Ea

uncatalysed

d) Surface Area

Collisions can only occur on the surface of a solid. If you cut a potato in half, then the boiling water can reach the “inside” surface that it couldn’t previously. So again the number of collisions per unit time will increase