REaction Rates and REaction Rates and EquilibriumEquilibrium

REaction Rates and REaction Rates and EquilibriumEquilibrium

How Fast and How FarHow Fast and How Far

ObjectivesObjectives

To understand collision model of chemical reactions

To understand activation energy

To understand how a catalyst speeds up a reaction

To explore reactions with reactants or products in different phases

To learn how equilibrium is established

To learn about characteristics of chemical equilibrium

To understand collision model of chemical reactions

To understand activation energy

To understand how a catalyst speeds up a reaction

To explore reactions with reactants or products in different phases

To learn how equilibrium is established

To learn about characteristics of chemical equilibrium

How Chemical reactions occurHow Chemical reactions occur

Molecules react by colliding with each other

Reactions go faster if concentration is increased

Reactions go faster with increased temperature

Molecules react by colliding with each other

Reactions go faster if concentration is increased

Reactions go faster with increased temperature

Conditions affecting reaction ratesConditions affecting reaction rates

Concentration - more molecules lead to more collisions

Why temperature?

Collisions must have enough energy to break bonds

Minimum energy to break bonds = activation energy

Concentration - more molecules lead to more collisions

Why temperature?

Collisions must have enough energy to break bonds

Minimum energy to break bonds = activation energy

Temperature and Reaction rateTemperature and Reaction rate

Higher

Temp

Higher

Temp

Higher

Speed

Higher

SpeedMore High-

energy Collisions

More High-

energy Collisions More bond

breakageMore bond breakage

Faster ReactionFaster Reaction

Reaction mechanismReaction mechanism

Step by step sequence of elementary reactions by which an overall chemical change occurs

For example: CO + NO2 --> CO2 + NO

The reaction mechanism for this might be:

2NO2 --> NO3 + NO (slow)

NO3 + CO --> NO2 +CO2 (fast)

Each step has its own rate

Rate of overall reaction is limited by rate of slowest reaction mechanism step

Step by step sequence of elementary reactions by which an overall chemical change occurs

For example: CO + NO2 --> CO2 + NO

The reaction mechanism for this might be:

2NO2 --> NO3 + NO (slow)

NO3 + CO --> NO2 +CO2 (fast)

Each step has its own rate

Rate of overall reaction is limited by rate of slowest reaction mechanism step

CatalystsCatalysts

Change reaction rate without changing temperature or concentration

Enzymes in your body are catalysts

Provides new pathway for reaction - lowers activation energy

Catalyst not used up in reaction

Change reaction rate without changing temperature or concentration

Enzymes in your body are catalysts

Provides new pathway for reaction - lowers activation energy

Catalyst not used up in reaction

How do catalysts work?How do catalysts work?

They increase the rate of effective collisions

How? Two main ways:

Help provide proper orientation of molecules so collisions “work”

Provide an alternate reaction mechanism that requires lower activation energy

Catalyst not used up or changed = can go on to catalyze more reactions

They increase the rate of effective collisions

How? Two main ways:

Help provide proper orientation of molecules so collisions “work”

Provide an alternate reaction mechanism that requires lower activation energy

Catalyst not used up or changed = can go on to catalyze more reactions

Catalyst exampleCatalyst example

Ozone Layer Destruction

Ozone absorbs high-energy radiation from the sun (e.g. UV light)

O + O3 --> 2O2

Chlorine catalyzes the reaction

One chlorine atom can catalyze the destruction of 1 million ozone molecules/second

Chlorine in the atmosphere from pollution by certain molecules called chlorofluorocarbons (e.g. CF2Cl2 - freon)

Ozone Layer Destruction

Ozone absorbs high-energy radiation from the sun (e.g. UV light)

O + O3 --> 2O2

Chlorine catalyzes the reaction

One chlorine atom can catalyze the destruction of 1 million ozone molecules/second

Chlorine in the atmosphere from pollution by certain molecules called chlorofluorocarbons (e.g. CF2Cl2 - freon)

EQUILIBRIUMEQUILIBRIUM

The exact balancing of two processes, one of which is the opposite of the other

For example - Physical equilibrium - Gas vaporizing over a liquid and gas condensing into liquid

H2O(g) <--> H2O(l)

This is a reversible process, indicated by double arrow

Equilibrium is reached when both processes are occurring at the same rate

The exact balancing of two processes, one of which is the opposite of the other

For example - Physical equilibrium - Gas vaporizing over a liquid and gas condensing into liquid

H2O(g) <--> H2O(l)

This is a reversible process, indicated by double arrow

Equilibrium is reached when both processes are occurring at the same rate

Chemical EquilibriumChemical Equilibrium

2NO2 (g) <--> N2O4 (g)

Dynamic Condition (process always happening)

Analogy - rate of cars leaving (reverse reaction) island on bridge = rate of cars arriving (forward reaction) on island on bridge - # of cars on island remains the same

Concentration of reactants and concentration of products remain the same indefinitely as long as the conditions are not changed

2NO2 (g) <--> N2O4 (g)

Dynamic Condition (process always happening)

Analogy - rate of cars leaving (reverse reaction) island on bridge = rate of cars arriving (forward reaction) on island on bridge - # of cars on island remains the same

Concentration of reactants and concentration of products remain the same indefinitely as long as the conditions are not changed

QuestionsQuestions

What must happen for two molecules to react together?

Name two factors that affect the rate of a reaction.

What is a catalyst? Give an example.

How do catalysts increase the rate of a reaction?

Define equilibrium. Give an example of physical equilibrium. Give an example of chemical equilibrium.

What must happen for two molecules to react together?

Name two factors that affect the rate of a reaction.

What is a catalyst? Give an example.

How do catalysts increase the rate of a reaction?

Define equilibrium. Give an example of physical equilibrium. Give an example of chemical equilibrium.

What if conditions Change?What if conditions Change?

Le Chatelier’s PrincipleLe Chatelier’s Principle