Kinetics , Thermodynamics Kinetics , Thermodynamics and Equilibriumand Equilibrium

Regents ChemistryRegents Chemistry

Kinetics and ThermodynamicsKinetics and Thermodynamics

Kinetics:Kinetics: deals with rates of reactions deals with rates of reactions

Thermodynamics:Thermodynamics: involves changes involves changes in energy that occur in reactionsin energy that occur in reactions

Kinetics: Collision TheoryKinetics: Collision Theory

Measured in:Measured in:#moles of reactant used per unit time#moles of reactant used per unit time

OrOr# moles of product formed per unit time# moles of product formed per unit time

Frequency of collisions:Frequency of collisions: more collisions = more collisions = faster ratefaster rate

Effective collisions:Effective collisions: must have proper must have proper orientation and enough energyorientation and enough energy

Factors Affecting RateFactors Affecting Rate

1. Type of substance: 1. Type of substance:

Ionic substances react faster: Ionic substances react faster: bonds require less bonds require less energy to breakenergy to break

Covalent react more slowly: Covalent react more slowly: bonds require more bonds require more energy to breakenergy to break

Factors Affecting RateFactors Affecting Rate

2. Temperature increase 2. Temperature increase

Kinetic energy increases and the Kinetic energy increases and the number of collisions increases. number of collisions increases.

So…reactants have more energy So…reactants have more energy when colliding. This increases rate.when colliding. This increases rate.

Factors Affecting RateFactors Affecting Rate

3. Concentration increase3. Concentration increase

Increases rate due to the fact that Increases rate due to the fact that more particles in a container, more particles in a container, which which creates more collisionscreates more collisions..

Factors Affecting RateFactors Affecting Rate

4. Surface Area Increase4. Surface Area Increase

Increases rate by increasing reactant Increases rate by increasing reactant interaction or collisionsinteraction or collisions

Factors Affecting RateFactors Affecting Rate

5. Pressure Increases5. Pressure Increases

Increases the rate of reactions Increases the rate of reactions involving involving gases onlygases only

As pressure Volume so:spaces between molecules frequency of effective

collisions

Factors Affecting RateFactors Affecting Rate

6. 6. Catalyst:Catalyst: substance that increases substance that increases rate of reaction, lowers the activation rate of reaction, lowers the activation energy of the reaction.energy of the reaction.

Catalysts remain unchanged during Catalysts remain unchanged during the reaction and can be reused.the reaction and can be reused.

Activation energy:Activation energy: amount of amount of energy required to “start” a reactionenergy required to “start” a reaction

Potential Energy Diagrams Potential Energy Diagrams

Graphs the Change in heat during Graphs the Change in heat during the course of a reaction.the course of a reaction.

Heat of reaction (ΔH) = Ep - ER

PE of Activated Complex

PE of reactants (ER) Activation Energy (Ea)

PE of products (EP)

Activation Energy (Ea)* reverse reaction

Exothermic: PE of products is less because energy was lost.

PE of reactants (ER)

PE of Activated Complex

Heat of reaction (ΔH) Activation Energy (Ea)

PE of products (EP)

Activation Energy (Ea)* reverse reaction

Endothermic: PE of products is more because energy was gained.

CatalystsCatalysts

ThermodynamicsThermodynamics

Heat content (Enthalpy):Heat content (Enthalpy): amount of amount of heat absorbed or released in a heat absorbed or released in a chemical reactionchemical reaction

Enthalpy (Enthalpy (ΔΔH = HH = Hproductsproducts – H – Hreactantsreactants))

ΔΔH = HH = Hproductsproducts – H – Hreactantsreactants

ΔΔH is positive when the reaction is H is positive when the reaction is endothermic. Heat of products are endothermic. Heat of products are greater than reactantsgreater than reactants

ΔΔH is negative when the reaction is H is negative when the reaction is exothermic. Heat of reactants were exothermic. Heat of reactants were greater than the products greater than the products

Table ITable I

Includes heats of reaction for Includes heats of reaction for combustion, synthesis (formation) combustion, synthesis (formation) and solution reactions.and solution reactions.

You must remember equation You must remember equation stoichiometry (balanced equations). stoichiometry (balanced equations).

Endothermic:Endothermic: heat is a reactant heat is a reactant Exothermic:Exothermic: heat is a product heat is a product

Table I- PracticeTable I- Practice

1.1. Which reaction gives off the most Which reaction gives off the most energy?energy?

2.2. Which reaction gives off the least Which reaction gives off the least energy?energy?

3.3. Which reaction requires the most Which reaction requires the most energy to occur?energy to occur?

Entropy (Entropy (ΔΔS)S)

Definition:Definition: randomness, disorder in a randomness, disorder in a sample of mattersample of matter

Gases have high entropyGases have high entropy Solids have low entropySolids have low entropy

Increasing Increasing ΔΔSS

Phase change from s Phase change from s l l g g Mixing gasesMixing gases Dissolving a substanceDissolving a substance

Spontaneous ReactionsSpontaneous Reactions

Nature favors low energy and high Nature favors low energy and high entropyentropy

Reactions are Reactions are spontaneousspontaneous when when heat (heat (ΔΔH) decreases and entropy H) decreases and entropy ((ΔΔS) increasesS) increases

ΔΔH = (-)H = (-) ΔΔS= (+)S= (+)

Chemical EquilibriumChemical Equilibrium

Regents Chemistry Regents Chemistry

Reversible ReactionsReversible Reactions

Most chemical reactions are able to Most chemical reactions are able to proceed in both directions under the proceed in both directions under the appropriate conditions.appropriate conditions.

Example:Example:

FeFe33OO4 (s)4 (s) + 4 H + 4 H2 (g)2 (g) ↔↔ 3 Fe 3 Fe(s)(s) + 4 H + 4 H22OO(g)(g)

Reversible Reactions IIReversible Reactions II

In a closed system, as products are In a closed system, as products are produced they will react in the produced they will react in the reverse reaction until the reverse reaction until the ratesrates of of the forward and reverse reactions the forward and reverse reactions are are equalequal..

RateRatefwdfwd = Rate = Raterevrev

This is called This is called chemical equilibriumchemical equilibrium..

EquilibriumEquilibrium

Equilibrium is dynamic condition Equilibrium is dynamic condition where rates of opposing processes where rates of opposing processes are equal.are equal.

Types of Equilibrium:Types of Equilibrium: Phase equilibriumPhase equilibrium Solution EquilibriumSolution Equilibrium Chemical EquilibriumChemical Equilibrium

Phase EquilibriumPhase Equilibrium

Rate of one phase change is equal to Rate of one phase change is equal to the rate of the opposing phase the rate of the opposing phase change.change.

Occurs when two phases exist at the Occurs when two phases exist at the same temperature.same temperature.

Example: RateExample: Ratemeltingmelting = Rate = Ratefreezingfreezing

HH22OO (s) (s) HH22OO (l) (l)

Solution EquilibriumSolution Equilibrium

Rate of dissolving = rate of Rate of dissolving = rate of crystallizationcrystallization

Occurs in saturated solutionsOccurs in saturated solutions

Chemical EquilibriumChemical Equilibrium

RateRateforward reactionforward reaction = Rate = Ratereverse reactionreverse reaction

Concentration of reactants and Concentration of reactants and products are products are constantconstant NOT NOT necessarily equal.necessarily equal.

The Concept of EquilibriumThe Concept of Equilibrium As a system approaches equilibrium, both As a system approaches equilibrium, both

the forward and reverse reactions are the forward and reverse reactions are occurring.occurring.

At equilibrium, the forward and reverse At equilibrium, the forward and reverse reactions are proceeding reactions are proceeding at the same rateat the same rate..

Le Chatelier’s PrincipleLe Chatelier’s Principle

Whenever stress put on a reaction, Whenever stress put on a reaction, the reaction will shift its point of the reaction will shift its point of equilibriumequilibrium

Stresses include:Stresses include: Temperature, pressure, changes in Temperature, pressure, changes in

reactant or product concentrations reactant or product concentrations

Example: The Haber ProcessExample: The Haber Process

NN2 (g) 2 (g) + 3 H+ 3 H2 (g)2 (g) 2 NH 2 NH3 (g)3 (g) + heat + heat

[N[N22]] [H[H22]] [NH[NH33]] [NH[NH33]] pressurepressure pressurepressure temperaturetemperature temperaturetemperature

Example: The Haber ProcessExample: The Haber Process

NN2 (g) 2 (g) + 3 H+ 3 H2 (g)2 (g) 2 NH 2 NH3 (g)3 (g) + heat + heat

[N[N22]] shift towards products (right)shift towards products (right) [H[H22]] shift towards reactants (left)shift towards reactants (left) [NH[NH33]] shift towards reactants (left)shift towards reactants (left) [NH[NH33]] shift towards products (right)shift towards products (right) pressurepressure shift towards products (right)shift towards products (right) pressurepressure shift towards reactants (left)shift towards reactants (left) temperaturetemperature shift towards reactants (left)shift towards reactants (left) temperaturetemperature shift towards products (right)shift towards products (right)

Equilibrium shifts due to stresses:Equilibrium shifts due to stresses:

Concentration increase Concentration increase shift awayshift away from from increaseincrease

Concentration decrease Concentration decrease shift towardshift toward decrease decrease pressure pressure shifts in direction of fewer gas shifts in direction of fewer gas

moleculesmolecules.. pressure pressure shifts in direction of more gas shifts in direction of more gas

moleculesmolecules temperature favors endothermic reactiontemperature favors endothermic reaction

Shift away from heatShift away from heat temperature favors exothermic reactiontemperature favors exothermic reaction

Shift towards heatShift towards heat

Effect of Catalyst:Effect of Catalyst:

Addition of catalysts changes the Addition of catalysts changes the rate of both the forward and reverse rate of both the forward and reverse reactions. reactions.

There is no change in concentrations There is no change in concentrations but equilibrium is reached more but equilibrium is reached more rapidly.rapidly.

Reactions that go to completion:Reactions that go to completion:

Equilibrium is not reached if one of the Equilibrium is not reached if one of the products is withdrawn as quickly as it is products is withdrawn as quickly as it is produced and no new reactants are added.produced and no new reactants are added.

Reaction continues until reactants are Reaction continues until reactants are used up.used up.

Products are removed if:Products are removed if: Gases in liquid solutionGases in liquid solution Insoluble products (precipitate)Insoluble products (precipitate)

The Haber ProcessThe Haber Process Application of LeChatelier’s PrincipleApplication of LeChatelier’s Principle

NN2 (g) 2 (g) + 3 H+ 3 H2 (g)2 (g) 2 NH 2 NH3 (g)3 (g) + 92 kJ + 92 kJ

increase pressureincrease pressureShift Shift decrease Temp decrease Temp Shift Shift remove NHremove NH33 add N add N2 2 and H and H22

Shift Shift ****Maximum yields of NH****Maximum yields of NH33 occurs under occurs under

high pressures, low temperatures and by high pressures, low temperatures and by constantly removing NHconstantly removing NH33 and adding N and adding N22 & H & H22