Reactants must collide with proper orientation and sufficient energy.

kinetics

Reactants must collide with proper orientation and sufficient energy

Collision Theory

Explains what happens once colliding particles react◦ Transition state is the in-between state when

reactants are being converted to products◦ Kinetic energy is converted to potential energy

(think about bouncing a basketball)

Transition State Theory

Potential Energy

Kinetic Energy

Potential Energy Diagrams

Negative

Exothermic reactions- give off heat (products more stable, less potential energy)

Endothermic reactions require heat (products less stable than reactants, higher PE)

Exo- Vs Endothermic

Exothermic Reaction Endothermic Reaction

CO reacts with NO2 to form CO2 and NO. The activation energy of the forward reaction is 134 kJ and the ΔE is -226 kJ.

Try ItDraw a PE diagram. Include: axes labels, the transition state, activated complex, Ea (forward and reverse), and ΔE.

Chemical reactions typically occur as a series of steps. This series of steps that make up the overall reaction is called the reaction mechanism.

Elementary reactions are a single step in the overall reaction mechanism.◦ Singular molecular event, such as a simple

collision of atoms, molecules or ions.◦ Cannot be broken down into further simpler steps.

Reaction Mechanisms

For example, the reaction 2NO(g) + O2(g) 2NO2(g)

involves a two-step reaction mechanism:◦ Step 1: NO(g) + O2(g) NO3(g)

◦ Step 2: NO3(g) + NO(g) 2NO2(g)

Each step is an elementary reaction, both steps together give the overall reaction mechanism.

Notice NO3(g)

◦ Not a product or reactant of overall reaction◦ Produced then consumed = reaction intermediate

Elementary Reactions

Describes the number of reactant particles in an elementary step◦ Unimolecular = one reactant

(CH3)3CBr(aq) (CH3)3C+ + Br-

◦ Bimolecular = 2 reactants come together. Ex: Step 1: NO(g) + O2(g) NO3(g)

Step 2: NO3(g) + NO(g) 2NO2(g)

◦ Termolecular = 3 reactants Extremely rare!!! Why?

Molecularity

One step in a reaction mechanism is always much slower than the others◦ Since it is so much slower, it determines the rate

of the overall reaction. Hence, this slow step is called the rate

determining step◦ Consider the process of making toast:

Rate Determining Step

slow

fast

RDS and PE Diagrams Activation energy for rds is always higher

◦ 2 STEP REACTION MEANS 2 TRANSITION STATES AND 1 INTERMEDIATE

Increases the rate of a reaction◦ Homogeneous catalyst: Same phase as reactants◦ Heterogeneous catalyst: Different phase as

reactants

Catalysts

Pd

Not consumed!◦ There in beginning and end of reaction◦ Works by lowering the activation energy

Therefore, greater number of collisions have sufficient energy to react

Provide alternative reaction mechanism

Catalysts

Reaction Rate Concentration Surface Area Catalyst Reactivity Temperature Collision Theory Orientation Rate Expression Exothermic Endothermic Homogeneous Heterogeneous Kinetic Energy Unimolecular

Kinetics Taboo Rate Constant Elementary Reaction Mechanism Catalyst Rate Law Reaction Order Molecularity Activation Energy Reverse Reaction Potential Energy Transition State Activated Complex Rate Determining Step Biomolecular Termolecular