Thermodynamics Tells if a reaction will occur.. Kinetics Tells how fast a reaction will occur

Thermodynamics

Tells if a reaction will occur.

Kinetics

Tells how fast a reaction will occur.

Reaction RateReaction Rate

• Speed of the reaction.• Found experimentally.• Measure change in concentration of

a reactant or a product over time.• Rate = Conc

time

How do you measure rates?How do you measure rates?

• Measure the concentration of 1 or more reactants or products over time.

• Reactants disappearReactants disappear

• Products appearProducts appear

• The reaction rate is the change in concentration of reactants & products in a given amount of time.

Concentration of Reactants, Products

Disappearance of reactants

Appearance of products

How do reactions occur?How do reactions occur?

• Must haveMust have an effective collision between reacting particles for reaction to occur. “Collision TheoryCollision Theory”

– Collision must be energetic.

– Collision must occur at an effective angle.

Particle Diagram of CollisionParticle Diagram of Collision

Activated complex or transition state.ReactantsProducts

NO + O3 NO2 + O2

Activated Complex is NOT in equation!

Reaction Rates depend on …Reaction Rates depend on …

• The frequency of collisions – how often they occur

And

• The efficiency of the collisions – what percentage are effective

Collision TheoryCollision Theory

• Molecules must collide in order to react.

• Effective collisions lead to the formation of products.

• Ineffective collisions do not lead to products.

Effective CollisionsEffective Collisions

• Energetic

• Favorable Orientation

Effective vs. Ineffective Collision

Most collisions are NOT effective!

Why Do Collisions Have to be Energetic?

Activation Energy & ReactionActivation Energy & Reaction

Energy Diagram of a Reaction

Reaction Pathway

Ent

halp

y or

Pot

enti

al E

nerg

y

Reactants

Activated Complex

Products

Activation EnergyActivation Energy

• Energy needed to initiate the reaction.

• Energy needed to overcome the reaction barrier.

• The difference between the top of the hill & where you start.

• Difference between activated complex & reactants.

Activation EnergyActivation Energy

• Using a match to start a fire.

• The spark plug in a car engine.

Potential Energy Curve: Endothermic

Endothermic Endothermic Reaction: Reaction:

Products have Products have more P.E. than more P.E. than reactants.reactants.

Start low, end Start low, end high.high.

Potential Energy Curve: Exothermic

Exothermic Exothermic Reaction: Reaction:

Products have Products have less P.E. than less P.E. than reactants.reactants.

Start high, end Start high, end low.low.

Have to label 6 energies on curve.

1)Ea = Activation Energy

2)H = Hproducts – Hreactants

Pot

enti

al E

nerg

y

6 Energies to Label

1. P.E. of reactants

2. P.E. of products

3. P.E. of activated complex

4. Ea for the forward reaction

5. Ea for the reverse reaction

6. H

Label on both Label on both endo & exo endo & exo P.E. curves.P.E. curves.

They mix the arrows up!

• You can’t memorize them by location – they move them around.

• Have to memorize them by where they start and where they stop.

• The 3 arrows for “Potential Energy of …” start at the baseline.

• Ea’s start “where you are” & end at the top of the hill.

What kind of reaction is represented?

P.E. of P.E. of reactantsreactants

P.E. of P.E. of activated activated complexcomplex

P.E. of P.E. of productsproducts

EEaa for for

forward rxnforward rxn

EEaa for for

reverse reverse rxnrxn

H of H of reactionreaction

What kind of reaction is represented?

P.E. of P.E. of reactantsreactants

P.E. of P.E. of activated activated complexcomplex P.E. of P.E. of

productsproducts

EEaa

forwardforwardEEaa

reversereverse

H of H of reactionreaction

Why does the collision have to be energetic?

• The kinetic energy of the reactants is used to overcome the reaction barrier.

• The kinetic energy is transformed into potential energy.

Factors that determine reaction ratesFactors that determine reaction rates

• Nature of the reactants (ions vs. molecules)

• Temperature

• Concentration

• Pressure (for gases)

• Surface Area

• The presence of a catalyst

Nature of the reactants:Nature of the reactants:Ions or Molecules?Ions or Molecules?

• Ions in solution react quickly.

• Covalently bonded molecules react slowly. It takes time to break all those bonds!

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

TemperatureTemperature

• Rule of thumb:

• Increasing the temperature 10oC doubles the reaction rate.

TemperatureTemperature

• A measure of the average kinetic energy of the molecules in a system.

• The faster they are moving, the more the more oftenoften they will collide.

• The faster they are moving, the more the more energeticenergetic the collisions.

Maxwell-Boltzmann Distribution

Increase in TemperatureIncrease in Temperature

• Increases the frequency of collisions

• Increases the percentage of collisions that lead to reaction.

ConcentrationConcentration

• Increase in concentration means more particles per unit volume – so more collisions in a given amount of time.

PressurePressure

• For systems involving gases.

• Analogous to increasing concentration. Pressure, number of particles per unit

volume.

Surface AreaSurface Area

• Higher surface area – more particles exposed for reaction.

• Higher surface area means smaller particle size.

• (For heterogeneous reactions.)

Vocabulary InterludeVocabulary Interlude

• Homogeneous ReactionHomogeneous Reaction: all reactants are in the same phase.

• Heterogeneous ReactionHeterogeneous Reaction: reactants are in different phases.

CatalystCatalyst

• Substance that increases the rate of reaction without itself being consumed.

• Provides an alternate reaction pathway with a lower energy barrier.

Enzymes are catalysts!

Catalytic Converter in Engines

Hydrogenation & Surface Catalysis

Surface Science

Reaction MechanismReaction Mechanism

• A series of steps that leads from reactants to products.

• Describes how bonds break, atoms rearrange, and bonds form in a reaction.

Elementary StepElementary Step

• Each individual step in a reaction mechanism.

• The slowest elementary step is called the rate-determining step.rate-determining step.

P.E. Curve for Multi-Step Rxn