Kinetics and Equilibrium What determines the speed of a

reaction?What determines the extent of a reaction?

Thermodynamics, Kinetics and Equilibrium

• Thermodynamics is a topic that address whether a reaction will occur or not ?

• Kinetics is a concept that address, how fast will the reaction occur

• Equilibrium address, to what extent will the reaction proceed.

COLLISION THEORY• A mathematical description of the

number of collisions between molecules in a sample of matter per unit time, useful for predicting rates of reaction.

COLLISION THEORY• Reaction rate: the number of

atoms, ions or molecules that react in a given period of time to form products

I2

H2

HI

Initial State Final State

THE STATES OF THE COLLISION THEORY

COLLISION THEORY

• Collision Theory also states that in order for reactions to occur between substances, their particles (molecules, atoms, or ions) must collide.

I2 Molecule

H2 Molecule

Hydrogen Iodine Molecule

COLLISION THEORY

• Further,these collisions must result in interactions.

• These interactions, if effective will form two new molecules.

HIHI

COLLISION THEORY

• According to the Collision Theory, the two HI molecules must collide in order to react.

HI HI

A COLLISION THAT IS TO GENTLE

• This collision is not energetic enough to supply the required activation energy.

• Therefore the Collision is ineffective. HI HI

HI HI

HIHI

A COLLISION IN POOR ORIENTATION

• The colliding molecules are not oriented in a way that enables them to react with each other.

• If the Collision doesn’t have the right orientation then the collision is not effective.

HIHI

HI HI

HI HI

AN EFFECTIVE COLLISION

• This collision has the right orientation.

• This collision is powerful enough to cause a good effect.

• Everything is satisfied, and the collision turns out to be effective.

HIHI

HI HI

I I

H2

Temperature

• The higher the temperature the faster the molecules will move:– higher frequency of collisions

– more energy in each collision

Rate of reaction will increase!

How will an increase in temperature effect reaction rate?

Concentration

• The higher the concentration the more particles per unit area – higher probability of a collision

Rate of reaction will increase!

How will increasing the concentration effect reaction rate?

Nature of the reactants• Solids and liquids cannot undergo a

change in concentration since they occupy a given space determined by intermolecular bonding.

• Gases and solutions can alter their concentrations.

Rates of Reactions

• Ways to measure reaction rates:–Color change

–Pressure changes: indication of number of moles of gases produced

–Precipitate formation

–Temperature change

Activation Energy• Eact - minimum energy a reactant must

possess in order to convert to products.

• The activation energy (Eact) can determine how fast a reaction occurs. In general, the higher the activation energy, the slower the reaction rate. The lower the activation energy, the faster the reaction.

WS 17 - 1

Activation Energy

Consider the process of someone trying to roll a boulder over a hill. The higher the hill, the slower the task. The lower the hill the faster the process. The height of the hill (a) correspond to the energy of activation (Eact).

E act

Activated complex

–the atomic configuration at the top of the energy barrier

–short life time (10-13 s)–breaks apart to form reactants or

products both of which have lower potential energy

activated complex

activated complex

• This explains why some reactions do not take place at room temperature.

• CH4(g) + O2(g) → no reaction at room temp.

• The molecules can not overcome the activation energy.

•A reaction with a very low activation energy will occur spontaneously.

Types of Reactions

Spontaneous exothermic Spontaneous endothermic

Slow exothermic Slow endothermic

Reaction Mechanism

• Most reactions do not take place in a single step. A→B→C→D

• Each step is usually a simple one on one collision reaction.

• The set of steps is called the reaction mechanism.

• The slowest step in the reaction mechanism is the rate determining step.

Reaction MechanismE

ner

gy

Reaction Progress

I

II

III

Intermediates

Activated Complex

Reactants

Products

Reaction Mechanism

• II is the rate determining step.

• It has the highest activation energy and therefore the slowest rate.

Catalyst Affect

• A catalyst is a substance that increases the rate of a chemical reaction without being consumed in the reaction.

Catalyst Affect• Consider the task of moving coal over a barrier. A pathway with a lower barrier is analogous to a reaction affected by a catalyst. The task becomes easier for a pathway with a lower barrier .

Catalyst Affect

• 2H2(g) + O2(g) → 2H2O(l) Slow

• 2H2(g) + O2(g) → 2H2O(l) Fast

• The Pt is not used up and does not appear as a reactant or product

Pt

Catalyst Affect

• Enzymes are organic catalysts the allow chemical reactions in the human body to occur at a lower temperature than normal.

• Cellular respiration• Inhibitors effect catalysts by

rendering them useless.

Catalyst Affecting Reaction Rate•The catalyst lowers the E act for the forward and the reverse reaction because the reaction takes place through a different set of steps.

Catalytic Converter

•A catalytic converter works by taking exhaust gases from the engine, including CO and NO, passing them through the catalytic converter, where they are converted to harmless CO2 and N2 by catalyzed reactions.

Reaction Conditions Affecting Rates• Conditions to Increase Rate:

– Nature of ReactantsNature of Reactants - Different substances will have different reaction rates.

– Concentration of ReactantConcentration of Reactant- More reactants result in more collision giving rise to a faster production rate.

– Surface area- More surface area, the greater the chance for reactants to encounter to form product.

– TemperatureTemperature- Higher temperature results in more energetic collisions.

– CatalystCatalyst- Lowers the activation energy for the reaction.

Reversible Reactions

• The conversion of reactants into products and the conversion of products into reactants occur simultaneously.

• 2SO2(g) + O2(g) 2SO3(g)

• In a reversible reaction the rate of the reverse reaction is zero at the start.

Terms and Symbols

•ΔHR → activation

energy for the forward reaction

•ΔHL → activation

energy for the reverse reaction

Comparing Types of Reactions

• Exothermic

• ΔHL╪ = ΔH + ΔHR

╪

• Endothermic

• ΔHR╪ = ΔH + ΔHL

╪

EpEp

ΔH

ΔHL╪

ΔHR╪

ΔH

ΔHL╪

ΔHR╪

Many reactions do not convert 100% of reactants to products. There is often a point in a reaction when the products will back react to form reactants.

Concept of Equilibrium

The extent of the reaction, 20% or 80%, can be determined by measuring the concentration of each component in solution.

In general the extent of the reaction is a function of temperature and concentration which is monitored by some constant value called the equilibrium constant (Keq).equilibrium constant (Keq).

(Dynamic) Equilibrium•Chemical Equilibrium is a dynamic state in which the rates of the forward and the reverse reaction are equal.•The left is in balance to the right. In a chemical reaction the Left (reactant) is changing at the same rate as the right (products).

Reactant Product

• When equilibrium is establish A B• For any general chemical process

at equilibrium.aA + bB pP + qQ

Equilibrium: Mass Action Expression

KeqA

a• B b

P p

• Q q

The Haber Process

• Ammonia (NH3) is manufactured in the Haber process.

• Raw materials are:1. Nitrogen from the air2. Hydrogen from natural gas

• Reaction conditions:Catalyst: IronTemperature: 450°CPressure: 200 atmospheres

• The reaction is reversible, so as well as nitrogen reacting with hydrogen to make ammonia, the ammonia also breaks back down again into nitrogen and hydrogen.

• Word Equation:

nitrogen + hydrogen ammonia

• Symbol Equation:

N2 + H2 NH323

BALANCED!

2. Compressor

N2 H2

1. Gases are mixed and scrubbed

3. Converter

4. Cooler

N2 H2 NH3

NH3 to storage

unreacted N2 and H2 are recycled

iron catalyst

200 atmosphere

s 450°C

Haber ProcessN2 (g) + 3H2 (g) 2NH3 (g)

• As soon as NH3 is formed, some of it back reacts and form N2 and H2. This takes place until the amount consumed is equal to the amount produced, i.e., equilibrium is reached.

eq

23

32 2

Law of Mass Action:

NHK

N • H

• Which is favored: Reactant or Product?

–For Keq > 1, at equilibrium

• Product is favoredProduct is favored.

–For Keq = 1, at equilibrium

• Product and Reactant are equal Product and Reactant are equal

–For Keq < 1, at equilibrium

• Reactant is favoredReactant is favored..

Meaning of Keq

P.419 #11 & 12

•Substances are in different phases at equilibrium •i.e., solid and aqueous.

Which solid is more concentrated?

•Concentration of a solid (and pure liquid) is Concentration of a solid (and pure liquid) is always a constant; always a constant;

•The concentration does not change !The concentration does not change !

100 g (1 cup)

200 g (2 cup)

Heterogeneous Systems

Heterogeneous Systems• Consider the following reaction:

• CaCO3(s) CaO (s) + CO2 (g)

2eq

3

3

1 2eq

2

eq 2

CaO • COK

CaCO

but, CaO = constant (solid)

and CaCO constant (solid)

Constant • COK

Constant

K CO

2eq

3

3

1 2eq

2

eq 2

CaO • COK

CaCO

but, CaO = constant (solid)

and CaCO constant (solid)

Constant • COK

Constant

K CO

LeChatelier’s Principle

• Henri-Louis LeChatelier–If a system at equilibrium is disturbed by a change in temperature, pressure or the concentration of one of the components, the system will shift its equilibrium position so as to counter-act the effect of the disturbance.

LeChatelier’s Principle: Concentration Effect

• If a chemical system is at equilibrium and then a substance is added (either a reactant or product), the reaction will shift so as to re-establish equilibrium by subtracting part of the added substance. Conversely, removal of a substance will result in the reaction moving in the direction that forms more of the substance.

LeChatelier’s Principle: Concentration Effect

Consider the Haber reaction at equilibrium:

N2 + 3H2 2NH3

If some H2 is added to the reaction which was at equilibrium, the system self-adjust to remove the excess H2 by converting it to NH3 until equilibrium is re-establish; in the process some N2 is also consumed.

LeChatelier’s Principle: Pressure Effect

•Increasing the pressure of a gaseous mixture causes the system to shift in the direction that reduces the number of moles of gas.

N2O4 (g) 2NO 2 (g)

•The equilibrium shifts to the side that reduces the total number of moles of gas involved in the reaction. In this example, 2 moles of product (NO2) will change to 1 mol of reactant (N2O4). The total moles in the reaction mixture is reduced to compensate for the increase in pressure.

LeChatelier’s Principle: Temperature

• An increase in the temperature of a system at equilibrium will shift the reaction so that it will absorb the heat.

• The equilibrium constant changes with temperature. It will either increase or decrease depending on the exothermicity or endothermicity of the reaction.

LeChatelier’s Principle

Effect of temperature on the N2O4/NO2 equilibrium. The tubes in the photograph both contain a mixture of NO2 and N2O4 . As predicted by LeChatelier’s principle, the equilibrium favors colorless N2O4 at lower temperatures because the reaction N2O4 2NO2 is endothermic. This is clearly seen in the tube at the right, where the gas in the ice bath at 0 °C is only slightly brown because there is only a small partial pressure of the brown gas NO2 . At 50 °C (the tube at the left), the equilibrium is shifted toward NO2 shown by the dark brown color.

LeChatelier’s Principle: Temperature• The equilibrium constant changes with temperature. It will either increase

or decrease depending on the exothermicity or endothermicity of the reaction.

Endothermic: reactant + E products Temp increase→ Shift rxn to Right , (Keq ↑)

temp ↓ Shift rxn to Left, (Keq ↓)

Exothermic: reactant products + E

Temp increase → Shift rxn to Left, (Keq ↓)

temp ↓ Shift rxn to Right, (Keq ↑)

KKeqeq(old)(old) KKeqeq(new)(new)

KKeqeq(new)(new) KKeqeq(old)(old)

Le Châtelier’s PrincipleLe Châtelier’s Principle

• As the pressure increases, the amount of ammonia present at equilibrium increases.

• As the temperature decreases, the amount of ammonia at equilibrium increases.

• Can this be predicted?• Le Châtelier’s Principle: if a system at

equilibrium is disturbed, the system will move in such a way as to counteract the disturbance.

N2(g) + 3H2(g) 2NH3(g)

•Catalyst added to a system at equilibrium lowers the activation energy of a reaction and therefore accelerates the rate of the reaction in both direction. A catalyst does not change the value of the equilibrium constant.

LeChatelier’s Principle : Catalyst

LeChatelier Principle•Methods of disturbing Equilibrium and LeChatelier response.• Stress on Reactant• Stress on Product• Relief on Reactant• Relief on Product

Reactant Product

LeChatelier PrincipleA stress on the a system at equilibrium causes changes to the system to reduce the stress until a new equilibrium is established.

i) A system at equilibrium.

ii) A stress is added to the reactant.

iii) The system self-adjust itself to reduce the stress.

iv) Until a new equilibrium is re-established

Reactant Change: Stress added• Consider the affect on Equilibrium as a result of

increasing the concentration of the reactant. What shift in direction occurs on the reaction ?

i) A system at equilibrium.

ii) A stress is added to the reactant.

iii) The system self-adjust to reduce the stress..

iv) Until a new equilibrium is re-established

The reaction shifts to the The reaction shifts to the right.right.

Product Changes: Stress added•Consider the affect on Equilibrium as a result of increasing the concentration of the product. What shift in direction occurs on the reaction ?

i) A system at equilibrium.

ii) A stress is added to the product..

iii) The system self-adjust itself to reduce the stress..

iv) Until a new equilibrium is re-established

The reaction shifts to The reaction shifts to the left.the left.

Reactant Change: Stress Relief•Consider the affect on Equilibrium as a result of decreasing the concentration of the reactant. What shift in direction occurs on the reaction ?

i) A system at equilibrium.

ii) A de-stress is place to the reactant..

iii) The system self-adjust itself to replace the missing reactants.

iv) Until a new equilibrium is re-established

The reaction shifts to The reaction shifts to the left.the left.

Product Change: Stress Relief•Consider the affect on Equilibrium as a result of decreasing the concentration of the Product. What shift in direction occurs on the reaction ?

i) A system at equilibrium.

ii) A de-stress is place to the product.

iii) The system self-adjust itself to replace the displaced products.

iv) Until a new equilibrium is re-established

The reaction shifts to The reaction shifts to the rightthe right

Temperature Effect: Endothermic•Consider the affect on Equilibrium as a result increasing the temperature of an endothermic reaction. What shift in direction occurs on the reaction ?

• Recall that an endothermic reaction treats heat (energy) as a reactant.

• Increase in temperature, increase in stress of reactant.

i) A system at equilibrium.

ii) An increase in temperature is a stress to the reactant.

iii) The system self-adjust itself to reduce the stress..

iv) Until a new equilibrium is re-established

The reaction shifts to the The reaction shifts to the right.right.

EE EE EE