Reaction RatesReaction RatesAndAnd

Chemical equilibriumChemical equilibrium

Chemical KineticsChemical KineticsThe area of chemistry that concerns The area of chemistry that concerns reaction ratesreaction rates..

However, only a small fraction of collisions produces a reaction. Why?

Key Idea: Molecules must collide to react.

Collision ModelCollision Model

Orientation of Orientation of reactants must reactants must allow formation allow formation of new bonds.of new bonds.

Collisions must Collisions must have enough have enough energy to energy to produce the produce the reaction (must reaction (must equal or exceed equal or exceed the activation the activation energy).energy).

Heat of ReactionHeat of Reaction

The amount of heat released or absorbed during a chemical reaction.

Endothermic:

Exothermic:

Reactions in which energy is absorbed as the reaction proceeds.

Reactions in which energy is released as the reaction proceeds.

Reaction MechanismReaction Mechanism   The series of steps by which a

chemical reaction occurs.   A chemical equation does not tell us

how reactants become products

  It is a summary of the overall process.

Example:

6CO 6H O C H O O2 2light

6 12 6 2 6

has many steps in the reaction mechanism

Rate-Determining StepRate-Determining StepIn a multi-step reaction, the In a multi-step reaction, the slowest step is the rate-slowest step is the rate-determining step.determining step. It therefore It therefore determines the rate of reaction.determines the rate of reaction.

Factors Affecting RateFactors Affecting Rate

Temperature Increasing temperature always

increases the rate of a reaction. Surface Area

Increasing surface area increases the rate of a reaction Concentration

Increasing concentration USUALLY increases the rate of a reaction

Presence of Catalysts

CatalysisCatalysisCatalystCatalyst: A substance that speeds up : A substance that speeds up a reaction without being consumed a reaction without being consumed

EnzymeEnzyme: A large molecule (usually a : A large molecule (usually a protein) that catalyzes biological protein) that catalyzes biological reactions. reactions.

Homogeneous catalystHomogeneous catalyst: Present in : Present in the same phase as the reacting the same phase as the reacting molecules. molecules.

Heterogeneous catalystHeterogeneous catalyst: Present in : Present in a different phase than the reacting a different phase than the reacting moleculesmolecules..

Catalysts Increase the Number of Catalysts Increase the Number of Effective CollisionsEffective Collisions

Reaction RateReaction RateThe change in concentration of a The change in concentration of a reactant or product per unit of timereactant or product per unit of time

2 1

2 1

[ ] [ ]A at timet A at timetRate

t t

[ ]ARate

t

2NO2NO22(g) (g) 2NO(g) + O 2NO(g) + O22(g) (g) Reaction Rates:

2. Can measure appearance of products

1. Can measure disappearance of reactants

3. Are proportional stoichiometrically

Activation EnergyActivation EnergyThe minimum energy required to transform

reactants into the activated complex (The minimum energy required to produce an

effective collision)

Flame, spark, high temperature, radiation are all sources of activation energy

Endothermic ReactionsEndothermic Reactions

Exothermic ReactionsExothermic Reactions

Endothermic Reaction withEndothermic Reaction witha Catalysta Catalyst

Exothermic Reaction with a Exothermic Reaction with a CatalystCatalyst

Chemical Chemical EquilibriumEquilibriumReversible Reactions:

A chemical reaction in which the products

can react to re-form the reactantsChemical Equilibrium:

When the rate of the forward reaction

equals the rate of the reverse reaction

and the concentration of products and

reactants remains unchanged

2HgO(s) 2Hg(l) + O2(g)

Arrows going both directions ( ) indicates equilibrium in a chemical equation

LeChatelier’s PrincipleLeChatelier’s PrincipleWhen a system at equilibrium is placed When a system at equilibrium is placed under stress, the system will undergo a under stress, the system will undergo a change in such a way as to relieve that change in such a way as to relieve that stress.stress.

When you take something away from When you take something away from a system at equilibrium, the system a system at equilibrium, the system shiftsshifts in such a way as to in such a way as to replace replace what you’ve taken awaywhat you’ve taken away..

Le Chatelier Translated:Le Chatelier Translated:

When you add something to a system When you add something to a system at equilibrium, the system at equilibrium, the system shiftsshifts in in such a way as to such a way as to use up what use up what you’ve addedyou’ve added..

LeChatelier Example #1LeChatelier Example #1

A closed container of ice and water at equilibrium. The temperature is raised.

Ice + Energy Water

The equilibrium of the system shifts to the _______ to use up the added energy.

right

LeChatelier Example #2LeChatelier Example #2

A closed container of N2O4 and NO2 at equilibrium. NO2 is added to the container.

N2O4 (g) + Energy 2 NO2 (g)

The equilibrium of the system shifts to the _______ to use up the added NO2.

left

LeChatelier Example #3LeChatelier Example #3

A closed container of water and its vapor at equilibrium. Vapor is removed from the system.

water + Energy vapor

The equilibrium of the system shifts to the _______ to replace the vapor.

right

LeChatelier Example #4LeChatelier Example #4

A closed container of N2O4 and NO2 at equilibrium. The pressure is increased.

N2O4 (g) + Energy 2 NO2 (g)

The equilibrium of the system shifts to the _______ to lower the pressure, because there are fewer moles of gas on that side of the equation.

left