Topic: Kinetics

Video

2 things needed for a reaction to occur

• X amount of activation energy• Effective collisions

Particle Diagram of Collision

NO + O3 NO2 + O2

Activated Complex is NOT in equation!

Kinetics tells how fast a reaction will occur

• Reaction rate = speed of the reaction– Depends on Frequency of collisions (how often

they occur) and efficiency of collisions• Rate = Concentration of reactant or product

timeDisappearance of reactants

Appearance of products

Collision Theory• Molecules must collide in order to react• Effective collisions lead to formation of products– Effective Collisions =

• Energetic• Favorable orientation

• Ineffective collisions do not lead to products

Most collisions are NOT effective!

Why Do Collisions Have to be Energetic?

• KE of reactants is used to overcome reaction barrier

• KE is transformed into PE

Factors that determine reaction rates

1) Nature of reactants (ions vs molecules)

Nature of the reactants:Ions or Molecules?

• Ions in solution react quickly• Covalently bonded molecules react slowly– takes time to break all those bonds!

• 2 gas phase reactants react more quickly than 2 liquids or 2 solids

Factors that determine reaction rates

1) Nature of reactants (ions vs molecules)2) Temperature

Increase in Temperature

• Temp = measure of average KE of molecules in system

• faster molecules are moving, will collide more often

• faster molecules are moving, more energetic the collisions

Factors that determine reaction rates

1) Nature of reactants (ions vs molecules)2) Temperature3) Concentration

Concentration

• Increase in concentration:– more particles per unit volume – more collisions in given amount time

Factors that determine reaction rates

1) Nature of reactants (ions vs molecules)2) Temperature3) Concentration4) Pressure (gases only)

Pressure

• systems involving gases changing pressure analogous to changing concentration

• Pressure, # particles per unit volume– Increase reaction rate

• ↓ Pressure, ↓ # particles per unit volume– Decrease reaction rate

Factors that determine reaction rates

1) Nature of reactants (ions vs molecules)2) Temperature3) Concentration4) Pressure (gases only)5) Surface Area

Surface Area

• Higher surface area –more particles exposed for reaction

• Higher surface area means smaller particle size• (For heterogeneous reactions)

• Homogeneous Reaction: –all reactants in same phase

• Heterogeneous Reaction: –reactants in different phases

Factors that determine reaction rates

1) Nature of reactants (ions vs molecules)2) Temperature3) Concentration4) Pressure (gases only)5) Surface Area6) Presence of a catalyst

Catalyst

• Substance that increases rate of reaction without itself being consumed –does not participate in reaction

• Lowers the activation energy for the reaction

Catalyst/enzyme written above the arrow