KINETICS AND KINETICS AND EQUILIBRIUMEQUILIBRIUM

HOW SUBSTACNCES REACT!HOW SUBSTACNCES REACT!

UNIT 6 UNIT 6

KINETICS AND EQUILIBRIUMKINETICS AND EQUILIBRIUM

CHEMICAL KINETICSCHEMICAL KINETICS

A. Definition: Branch of chemistry A. Definition: Branch of chemistry concerned with the rate of chemical concerned with the rate of chemical reactions and the mechanisms by reactions and the mechanisms by which chemical reactions occurwhich chemical reactions occur

B. Rate- B. Rate- measured in terms of measured in terms of moles of reactants consumed or moles of reactants consumed or moles of product formed in a unit moles of product formed in a unit of timeof time

C. Mechanism –describes the C. Mechanism –describes the sequence of events (reactions) by sequence of events (reactions) by which an overall reaction takes placewhich an overall reaction takes place

REACTION RATES AND COLLISION REACTION RATES AND COLLISION THEORYTHEORY

1. 1. Chemical reactions depend on Chemical reactions depend on collisions between reacting collisions between reacting species ( atoms, molecules, ions, species ( atoms, molecules, ions, or particles)or particles)

2. Reactions occur only when reactants 2. Reactions occur only when reactants collide with enough energy and in the collide with enough energy and in the correct orientation to form an correct orientation to form an activated complex – a high energy activated complex – a high energy intermediateintermediate

3. The rate of the reaction is affected by 3. The rate of the reaction is affected by the number of collisions occurring the number of collisions occurring and the fraction of these collisions and the fraction of these collisions that are effective. that are effective.

a. Effective collisions are a. Effective collisions are collisions in which the reacting collisions in which the reacting species collide at the correct species collide at the correct angle and at the right speed so angle and at the right speed so that they form an activated that they form an activated complexcomplex

b. Therefore, any factor that causes b. Therefore, any factor that causes more collisions to occur or results in more collisions to occur or results in more collisions being effective more collisions being effective (leading to an activated complex) (leading to an activated complex) will increase the rate of a will increase the rate of a reactionreaction

FACTORS AFFECTING REACTION RATEFACTORS AFFECTING REACTION RATE

1. Nature of reactants1. Nature of reactants

a. During a chemical reaction bonds a. During a chemical reaction bonds are broken and new bonds are madeare broken and new bonds are made

b. Covalent substance react b. Covalent substance react slower than ionic Ionic = metal + slower than ionic Ionic = metal + nonmetalnonmetal

Covalent = nonmetal + nonmetalCovalent = nonmetal + nonmetal

c. Ionic substances are generally c. Ionic substances are generally dissolved in water before being reacted dissolved in water before being reacted together. When an ionic substance is together. When an ionic substance is dissolved in water the crystal lattice is dissolved in water the crystal lattice is broken and ions are free to move about, broken and ions are free to move about, collide and react. Meaning no bonds collide and react. Meaning no bonds need to be broken for the reaction to need to be broken for the reaction to occur.occur.

2. Concentration of Reactants2. Concentration of Reactants

a. An increase in the concentration a. An increase in the concentration (amount) of one or more reactant results (amount) of one or more reactant results in an increase in the reaction rate in an increase in the reaction rate

because there are more molecules because there are more molecules reacting therefore there is an reacting therefore there is an increase in the frequency of increase in the frequency of collisions collisions

c. Concentration is generally measured in c. Concentration is generally measured in mole/ liters or Mmole/ liters or M

d. For reactants dissolved in a liquid d. For reactants dissolved in a liquid solvent solvent the concentration is increased the concentration is increased by removing some of the solvent by removing some of the solvent through evaporation or by adding through evaporation or by adding more of themore of the solutesolute. Conversely . Conversely decreases in concentration can be decreases in concentration can be achieved by adding more solventachieved by adding more solvent

2. Temperature2. Temperature

a. An increase in temperature will a. An increase in temperature will increase the rate of almost all increase the rate of almost all chemical reactions because chemical reactions because it it increases the KE(speed) of the increases the KE(speed) of the particles and if the particles particles and if the particles move faster the number of move faster the number of collisions will increase but so will collisions will increase but so will the number of effective collisionsthe number of effective collisions

3. Pressure3. Pressure

a. An increase in concentration for a a. An increase in concentration for a gas is achieved bygas is achieved by decreasing the decreasing the volume the gas occupies, this is volume the gas occupies, this is accomplished by increasing the accomplished by increasing the pressure on the gas.pressure on the gas.

4. Surface Area4. Surface Area

a. Increase the surface area of a. Increase the surface area of reactants increases the rate of reactants increases the rate of the reactionthe reaction

b. Important for heterogeneous b. Important for heterogeneous reactions( one where the reactions( one where the reactants are in different phases)reactants are in different phases)

c. To increase the surface area of a c. To increase the surface area of a solid grind it up.solid grind it up.

d. Example.d. Example.

A given amount of Zinc will react A given amount of Zinc will react faster with dilute HCl if the zinc is faster with dilute HCl if the zinc is ground up due to increased surface ground up due to increased surface area.area.

5. Reaction Mechanism5. Reaction Mechanism

a. Most reactions do not occur in a a. Most reactions do not occur in a single step, but in a series of steps single step, but in a series of steps known as the reaction mechanismknown as the reaction mechanism

b. More steps required the b. More steps required the slower the reactionslower the reaction

c. Reaction Mechanism is not c. Reaction Mechanism is not indicated by a chemical equationindicated by a chemical equation

6. Presence of a Catalyst6. Presence of a Catalyst

a. Increase the rate of a chemical a. Increase the rate of a chemical reaction by providing an easier path reaction by providing an easier path for the reactionfor the reaction

b. They take part in the reaction by b. They take part in the reaction by lowering the activation energy lowering the activation energy but they are not used up.but they are not used up.

POTENTIAL ENERGY DIAGRAMSPOTENTIAL ENERGY DIAGRAMS

1. A diagram that shows the change in 1. A diagram that shows the change in potential (stored energy) that occurs potential (stored energy) that occurs during the course of a chemical during the course of a chemical reaction.reaction.

2. During the course of a chemical 2. During the course of a chemical reaction if reactants collide with both reaction if reactants collide with both the proper orientation and sufficient the proper orientation and sufficient energy an activated complex is energy an activated complex is formed.formed.

a. activated complex-a. activated complex-temporary temporary intermediate product that may intermediate product that may either break apart and reform the either break apart and reform the reactants or rearrange the atoms reactants or rearrange the atoms and form a new product.and form a new product.

3.3. Activation EnergyActivation Energy

a. Definition –a. Definition –Minimum energy Minimum energy necessary to initiate (start) a necessary to initiate (start) a chemical reaction.chemical reaction.

b. All reactions require some “start up” b. All reactions require some “start up” energy.energy.

4. Heat of reaction (ENTHALPY), 4. Heat of reaction (ENTHALPY), H,H,

a. Heat energy is released or absorbed a. Heat energy is released or absorbed in the formation of products. That is in the formation of products. That is reactions are either endothermic or reactions are either endothermic or exothermicexothermic

b. b. ΔΔ H represents the difference H represents the difference in Potential Energy (PE) between in Potential Energy (PE) between the products and reactantsthe products and reactants. .

c. c. H = Hproducts – HreactantsH = Hproducts – Hreactants

5. Exothermic Reactions5. Exothermic Reactions

a. Overall Energy is releaseda. Overall Energy is released

b. Products have lower potential b. Products have lower potential energy than reactantsenergy than reactants

c. c. H is negativeH is negative

6. Endothermic Reactions6. Endothermic Reactions

a. Overall energy is absorbeda. Overall energy is absorbed

b. Products have a higher PE than b. Products have a higher PE than reactantsreactants

c. c. H is positiveH is positive

7. Sign used when energy is included in 7. Sign used when energy is included in a chemical reaction should not be a chemical reaction should not be confused with the sign for confused with the sign for H. H.

a. The sign of a. The sign of H tells us whether a H tells us whether a reaction is endothermic or exothermic.reaction is endothermic or exothermic.

b. If b. If H is positiveH is positive the energy term the energy term is found on the reactant side the is found on the reactant side the reaction is endothermicreaction is endothermic..

c. conversely if c. conversely if H is negative the H is negative the energy term is found on the product energy term is found on the product side the reaction is exothermic. side the reaction is exothermic.

d.d. The units of The units of H and the energy term H and the energy term is kJ. is kJ.

8. Table I 8. Table I

a. Heats of reactions for certain a. Heats of reactions for certain chemical reactions and processeschemical reactions and processes

b. b. H is measured in kJ. H is measured in kJ.

c. the reaction is exactly equal to c. the reaction is exactly equal to H if the H if the equation is identical to the one listed. equation is identical to the one listed.

Examples: CHExamples: CH44 + 2O + 2O22 CO CO22 + 2H + 2H22O O

H = H = -890.4 kJ-890.4 kJ

Rewrite the equation with the heat term addedRewrite the equation with the heat term added

CHCH44 + 2O + 2O22 CO CO22 + 2H + 2H22O + 890.5kJO + 890.5kJ

NN22 (g) + O (g) + O22 (g) (g) 2NO (g) 2NO (g) H = H = +182.6kJ+182.6kJ

Rewrite the equation with the heat term addedRewrite the equation with the heat term added

NN22 (g) + O (g) + O22 (g) + 182.6kJ (g) + 182.6kJ 2NO (g) 2NO (g)

d. If it is the same except that the d. If it is the same except that the coefficients are multiples of the ones coefficients are multiples of the ones listed then listed then H and the energy written H and the energy written in the reaction will also be a multiple.in the reaction will also be a multiple.

ExamplesExamples

2H2H22 (g) + 2I (g) + 2I22(g) (g) 4HI (g) 4HI (g) H = H = 2(+53kJ)2(+53kJ)

Rewrite the equation with the heat term Rewrite the equation with the heat term addedadded

2H2H22 (g) + 2I (g) + 2I22(g) + 106kJ (g) + 106kJ 4HI (g) 4HI (g)

e. If the reactants and products are e. If the reactants and products are switched then change the sign of switched then change the sign of HH

2NO (g) 2NO (g) N N22 (g) + O (g) + O2 2 (g) (g)

H = H = -(+182.6kJ)-(+182.6kJ)

Rewrite with the heat term in the Rewrite with the heat term in the equationequation

2NO (g) 2NO (g) N N22 (g) + O (g) + O2 2 (g) + 182.6kJ(g) + 182.6kJ

EQUILIBRIUMEQUILIBRIUM

A. Definition: Equilibrium is said to A. Definition: Equilibrium is said to exist when any reaction takes place exist when any reaction takes place under fixed conditions so that the rate under fixed conditions so that the rate of the forward reaction is equal to the of the forward reaction is equal to the rate of the reverse reaction.rate of the reverse reaction.

B. At equilibrium the rate at which B. At equilibrium the rate at which reactants are converted to reactants are converted to products is equal to the rate at products is equal to the rate at which products are reconverted to which products are reconverted to reactantsreactants

C. C. THE QUANTITIES OF PRODUCTS AND THE QUANTITIES OF PRODUCTS AND REACTANTS AT EQUILIBRIUM ARE REACTANTS AT EQUILIBRIUM ARE NOT NECESSARILY EQUAL IT IS THE NOT NECESSARILY EQUAL IT IS THE RATES OF THE FORWARD AND RATES OF THE FORWARD AND REVERSE REACTION THAT ARE REVERSE REACTION THAT ARE EQUAL.EQUAL.

D. A closed system is necessary so D. A closed system is necessary so neither reactant nor product escapes.neither reactant nor product escapes.

EEEE = = forwardforward reaction reaction

EFEF = = ReverseReverse reaction reaction

PHYSICAL EQUILIBRIUMPHYSICAL EQUILIBRIUM

A. Phase equilibriaA. Phase equilibria

1. In general for a closed system there 1. In general for a closed system there will exist an equilibrium between phases will exist an equilibrium between phases where the rate of escape (from the phase) where the rate of escape (from the phase) = the rate of return (to that phase)= the rate of return (to that phase)

2. Equilibrium existing between a solid 2. Equilibrium existing between a solid and liquid phase at a substances melting and liquid phase at a substances melting point. At the melting point the rate at point. At the melting point the rate at which a solid melts to become a liquid is which a solid melts to become a liquid is equal to the rate at which the liquid freezes equal to the rate at which the liquid freezes to reform the solid. to reform the solid. Rate of melting = Rate of melting = Rate of freezingRate of freezing

3. Exists between liquid and gas 3. Exists between liquid and gas

Rate of evaporation = Rate of Rate of evaporation = Rate of condensationcondensation

B. B. Solution equilibriumSolution equilibrium

1. Solids and liquids1. Solids and liquids

a. Process that goes on in a a. Process that goes on in a saturated solution if any additional saturated solution if any additional material is added. We see the added material is added. We see the added chemical fall to the bottom. What chemical fall to the bottom. What actually occurs is the added particle actually occurs is the added particle dissolves and anther particles falls out dissolves and anther particles falls out of solution. of solution.

b. Solution equilibrium exists b. Solution equilibrium exists when the rate of dissolving equals when the rate of dissolving equals the rate of crystallization.the rate of crystallization.

c. A saturated solution is defined as : c. A saturated solution is defined as : A A solution in which an equilibrium solution in which an equilibrium exists between dissolved and exists between dissolved and undissolved solute. It cannot hold undissolved solute. It cannot hold any more solute at a given any more solute at a given temperature.temperature.

2. Gases in liquids.2. Gases in liquids.

a. In a closed system, equilibrium may a. In a closed system, equilibrium may exist between a gas dissolved in a liquid exist between a gas dissolved in a liquid and the undissolved gas above the liquidand the undissolved gas above the liquid

b. Equilibrium between dissolved and b. Equilibrium between dissolved and undissolved gas is affected by undissolved gas is affected by temperature and pressuretemperature and pressure

c. Increase in temperature c. Increase in temperature decreases solubility, converse is decreases solubility, converse is also truealso true

d. Increases in pressure increases d. Increases in pressure increases solubility, converse is also truesolubility, converse is also true

C. Chemical EquilibriumC. Chemical Equilibrium

1. State in which the concentration 1. State in which the concentration of the reactants and products of a of the reactants and products of a reaction remain constant.reaction remain constant.

2. When equilibrium is reached all 2. When equilibrium is reached all observable properties of the system, observable properties of the system, such as color, pressure and such as color, pressure and temperature remain constant.temperature remain constant.

3. When equilibrium is reached the 3. When equilibrium is reached the rate of the forward reaction is equal rate of the forward reaction is equal to the to the rate rate of the reverse reactionof the reverse reaction

a. During a certain unit of time, a. During a certain unit of time, the the rate at which the product is rate at which the product is made is equal to the rate at made is equal to the rate at which the Product is broken which the Product is broken downdown

b. This results in the mass of each b. This results in the mass of each substance remaining constant at substance remaining constant at equilibriumequilibrium

c. This results in a system that is c. This results in a system that is dynamic, constantly changing, but dynamic, constantly changing, but appears to be static, unchangingappears to be static, unchanging

d. The mass of each substance may d. The mass of each substance may only be changed if the system is only be changed if the system is disturbed, disturbed, meaning something is meaning something is done so that the rate of the done so that the rate of the forward and reverse reactions are forward and reverse reactions are no longer equal.no longer equal.

LE CHATLIER’S PRINCIPLELE CHATLIER’S PRINCIPLE

A. STATEMENT: A. STATEMENT:

If a stress (disturbance) such as a If a stress (disturbance) such as a change in concentration, pressure or change in concentration, pressure or temperature, is applied to a system in temperature, is applied to a system in equilibrium, the equilibrium is shifted in equilibrium, the equilibrium is shifted in such a way that tends to relieve the such a way that tends to relieve the effects of the stress.effects of the stress.

B. How to predict which direction the B. How to predict which direction the equilibrium will shift.equilibrium will shift.

1. Choose the arrow (forward or reverse) that 1. Choose the arrow (forward or reverse) that points away from the substance in the reaction points away from the substance in the reaction that was increased.that was increased.

2. Choose an arrow (forward or reverse) that 2. Choose an arrow (forward or reverse) that points at the substance that was decreasedpoints at the substance that was decreased

C. How to predict the change in C. How to predict the change in concentration (amount) of the concentration (amount) of the substances in the reactionsubstances in the reaction 1. All substances at the tip of the chosen 1. All substances at the tip of the chosen

arrow increasearrow increase

2. All substances at the tail of the chosen 2. All substances at the tail of the chosen arrow decreasearrow decrease

D. TerminologyD. Terminology

1. If the 1. If the FORWARD FORWARD reaction is reaction is favored, the products are favored, or the favored, the products are favored, or the equilibrium shifts to the right, it means equilibrium shifts to the right, it means that the forward reaction goes that the forward reaction goes fasterfaster in in the reverse reaction once the stress is the reverse reaction once the stress is appliedapplied

2. If the 2. If the reverse reverse reaction is favored, the reaction is favored, the reactants are favored, or the equilibrium reactants are favored, or the equilibrium shifts to the left, it means that the shifts to the left, it means that the reverse reaction goes reverse reaction goes fasterfaster than the than the forward once the stress is appliedforward once the stress is applied

E. Stresses that can be applied to a E. Stresses that can be applied to a chemical systemchemical system

1. Effect of changing the concentration1. Effect of changing the concentration

a. a. Increasing the concentration Increasing the concentration either a reactant or product in a either a reactant or product in a reaction atreaction at

equilibrium will cause the reaction to equilibrium will cause the reaction to go(shift) in such a direction as to go(shift) in such a direction as to consume the increase. This consume the increase. This direction is said to be favored.direction is said to be favored.

b. Which ever reaction, forward or b. Which ever reaction, forward or reverse that consumes the increase reverse that consumes the increase will be favored, meaning the rate of will be favored, meaning the rate of that reaction, (forward or reverse) that reaction, (forward or reverse) will be increased until the stress is will be increased until the stress is relieved. Eventually a new relieved. Eventually a new equilibrium is reached.equilibrium is reached.

c. Decreasing the concentration of c. Decreasing the concentration of either a reactant or product) in a either a reactant or product) in a reaction at equilibrium will cause the reaction at equilibrium will cause the reaction toreaction to go(shift) in such a go(shift) in such a direction as to produce more of direction as to produce more of the substance decreased. the substance decreased. Whichever reaction, forward or Whichever reaction, forward or reverse, that produces more of reverse, that produces more of the substance will be favored,the substance will be favored,

d. Example: [ ] = reads concentrationd. Example: [ ] = reads concentration

NN22 (g) + 3H (g) + 3H22 (g) ↔2NH (g) ↔2NH33 (g) This process is (g) This process is called the Haber Process it is used in called the Haber Process it is used in the commercial production of ammoniathe commercial production of ammonia

Stress: [ HStress: [ H22] is increased ] is increased

► Result: forward reaction is Result: forward reaction is favored favored because the forward reaction because the forward reaction

consumes Hconsumes H22 , ,

therefore [Ntherefore [N22] will ] will decreasedecrease and [ NH and [ NH33] ] will will increase.increase. The equilibrium will shift The equilibrium will shift to the rightto the right

Stress: [NStress: [N22] is increased ] is increased

►Result: forward reaction is Result: forward reaction is favoredfavored because the forward reaction consumes because the forward reaction consumes NN22 ,therefore [H ,therefore [H22]]will decreasewill decrease and [ NH and [ NH33] ]

increaseincrease. The equilibrium will shift . The equilibrium will shift to to the rightthe right

Stress: [ NHStress: [ NH33] is increased ] is increased

Result: reverse reaction is Result: reverse reaction is favoredfavored because the reverse reaction consumes because the reverse reaction consumes NHNH33 ,therefore the [N ,therefore the [N22] and [H] and [H22] will ] will increaseincrease. The equilibrium will shift . The equilibrium will shift to to the left.the left.

Stress: [ HStress: [ H22] is decreased] is decreased

Result: reverse reaction is Result: reverse reaction is favoredfavored because the reverse reaction produces because the reverse reaction produces [H[H22]]

therefore the [Ntherefore the [N22] will increase and the ] will increase and the [NH[NH33] will ] will decreasedecrease. The equilibrium . The equilibrium will shift will shift to the left.to the left.

►

Stress: [NStress: [N22] is decreased ] is decreased

►Result: reverse reaction is Result: reverse reaction is favoredfavored because Nbecause N22 is produced by the reverse is produced by the reverse reaction, therefore the [Hreaction, therefore the [H22] is ] is increasedincreased and the [ NH and the [ NH33] will ] will decrease.decrease. The equilibrium will shift The equilibrium will shift to the left.to the left.

Stress [NHStress [NH33] is decreased ] is decreased

►Result: forward reaction isResult: forward reaction is favored favored because the forward reaction produces because the forward reaction produces NHNH33 therefore the [H therefore the [H22] and [N] and [N22] will ] will both both decrease. decrease. The equilibrium will The equilibrium will shift to the shift to the right.right.

g. Removal of a product as it is formed g. Removal of a product as it is formed tends to cause the forward reaction to tends to cause the forward reaction to go more toward completion. go more toward completion. Continuous removal of the product may Continuous removal of the product may destroy the equilibrium system by destroy the equilibrium system by removing all of the substance removing all of the substance necessary for the reverse reactionnecessary for the reverse reaction

h. Products are removed from a reaction h. Products are removed from a reaction byby

the formation of a gas, an insoluble the formation of a gas, an insoluble precipitate (table F), or in an ionic precipitate (table F), or in an ionic reaction by producing water.reaction by producing water.

2. Effect of pressure2. Effect of pressure

a. A change in pressure effects a. A change in pressure effects chemical equilibria in which gases chemical equilibria in which gases are involved. There is no effect on a are involved. There is no effect on a system that does not contain gasessystem that does not contain gases

b. An increase in pressure will favor a b. An increase in pressure will favor a shift in equilibrium toward the side of shift in equilibrium toward the side of the reaction that contains the the reaction that contains the lesser lesser number of molesnumber of moles ( the side of the ( the side of the reaction with a smaller sum of the reaction with a smaller sum of the coefficients)coefficients)

c. A decrease in pressure will favor a c. A decrease in pressure will favor a shift in equilibrium toward the side of shift in equilibrium toward the side of the reaction with the greater number the reaction with the greater number of moles ( the side of the reaction with of moles ( the side of the reaction with a larger sum of the coefficients)a larger sum of the coefficients)

d. d. If there is not change in the If there is not change in the number of moles than a change in number of moles than a change in pressure will not shift the pressure will not shift the equilibriumequilibrium

e. Examplee. Example

NN22 (g) + 3H (g) + 3H22 (g) ↔2NH (g) ↔2NH33 (g) (g)

4 moles4 moles 2moles2moles

Stress: Increase in pressureStress: Increase in pressure► Result: The Result: The forwardforward reaction is reaction is

favored because the product side has favored because the product side has fewer total fewer total

moles therefore [Nmoles therefore [N22] and [H] and [H22] will ] will decreasedecrease and the [NH and the [NH33] will ] will increaseincrease

Stress: Decrease in pressureStress: Decrease in pressure►ResultResult: : The The reverse reverse reaction is favored reaction is favored

because it has a greater number because it has a greater number

of total moles therefore [Nof total moles therefore [N22] and [H] and [H22] ] will will increase increase and the [NHand the [NH33] will ] will decreasedecrease

HH22 (g) + Cl (g) + Cl22 (g)↔ 2HCl (g) (g)↔ 2HCl (g)

2 moles2 moles 2 moles2 moles

Stress: increasing the pressureStress: increasing the pressure

Result: Result: No change the equilibrium will No change the equilibrium will not shiftnot shift

3. Effect of temperature3. Effect of temperature

a. When the temperature of a system a. When the temperature of a system in equilibrium is raised, the in equilibrium is raised, the equilibrium is displaced (shifted) in equilibrium is displaced (shifted) in such a way that heat is absorbed. such a way that heat is absorbed. Meaning the endothermic Meaning the endothermic reaction is favored.reaction is favored.

b. When the temperature of a system b. When the temperature of a system in equilibrium is lowered, the in equilibrium is lowered, the equilibrium is shifted in such a way as equilibrium is shifted in such a way as to release heat. to release heat. Meaning Meaning exothermic reaction is favoredexothermic reaction is favored..

c. Note: The rates of all chemical c. Note: The rates of all chemical reactions, both endothermic and reactions, both endothermic and exothermic are increased when exothermic are increased when temperature is increased (by temperature is increased (by increasing the number of effective increasing the number of effective collisions). However, the opposing collisions). However, the opposing reactions are increased unequally, reactions are increased unequally, resulting in a shift in the equilibrium.resulting in a shift in the equilibrium.

d. Exampled. Example

NN22 (g) + 3H (g) + 3H22 (g) ↔2NH (g) ↔2NH3 3 (g) + 22 kcal(g) + 22 kcal

Stress: Increase in temperatureStress: Increase in temperature► Result: TheResult: The reverse reaction is reaction is

favored because it is the favored because it is the endothermicendothermic

reaction therefore the [Nreaction therefore the [N22] and [H] and [H22] ] will will increaseincrease and the [NH and the [NH33] will ] will decreasedecrease

Stress: Decrease in temperatureStress: Decrease in temperature►Result: The Result: The forwardforward reaction is reaction is

favored because it is favored because it is the exothermic the exothermic reaction therefore the [Nreaction therefore the [N22] and [H] and [H22] ] will will decreasedecrease and the [NH and the [NH33] will ] will increase.increase.

4. Effect of a Catalyst4. Effect of a Catalyst

a. Increases the rate of both the a. Increases the rate of both the forward and reverse reaction forward and reverse reaction equally and causes no shift in the equally and causes no shift in the equilibriumequilibrium

b. May cause equilibrium to be b. May cause equilibrium to be reached more quickly but does not reached more quickly but does not affect the point of equilibriumaffect the point of equilibrium

SPONTANEOUS REACTIONS?SPONTANEOUS REACTIONS?

A. Definition: A. Definition: Spontaneous changes are Spontaneous changes are changes that are observed to occur changes that are observed to occur under given conditionsunder given conditions

B. Example: If the temperature is above 0C B. Example: If the temperature is above 0C ice melts but if the reverse, freezing is not ice melts but if the reverse, freezing is not observed at these temperatures.observed at these temperatures.

1. Chemical reactions are also observed to 1. Chemical reactions are also observed to go in one direction (forward or reverse) go in one direction (forward or reverse) under one set of conditions and in the under one set of conditions and in the opposite direction under other conditions, opposite direction under other conditions, or to remain in equilibrium under still or to remain in equilibrium under still another set of conditions. another set of conditions.

C. Factors determining the direction of C. Factors determining the direction of spontaneous changespontaneous change

1. There are two fundamental 1. There are two fundamental tendencies in nature which together tendencies in nature which together determine the direction (forward or determine the direction (forward or reverse) of a spontaneous change.reverse) of a spontaneous change.

a. Minimum Energya. Minimum Energy

b. Maximum Disorderb. Maximum Disorder

2. Energy Changes (Enthalpy Changes)2. Energy Changes (Enthalpy Changes)

a. At constant temperature and a. At constant temperature and pressure a system tends to under go a pressure a system tends to under go a reaction that in its final state it has lower reaction that in its final state it has lower energy than in its initial stateenergy than in its initial state

b. Therefore the tendency in nature b. Therefore the tendency in nature favors the favors the exothermic reaction, -exothermic reaction, -ΔΔHH

3. Tendency toward randomness (Entropy)3. Tendency toward randomness (Entropy)

a. Randomness is the disorder or lack a. Randomness is the disorder or lack of order in a systemof order in a system

b. Entropy is a measure of theb. Entropy is a measure of the randomness or disorder in a randomness or disorder in a systemsystem

c. Symbol c. Symbol SS

d. The more random the higher the d. The more random the higher the entropyentropy

e. e. Nature favors more disorder or higher entropy; +ΔS

f. Examples of entropy changesf. Examples of entropy changes

Phase changes illustrate entropy changes Phase changes illustrate entropy changes

Changes from SChanges from SLL G show an G show an increase in entropy because the increase in entropy because the particles are moving toward a state of particles are moving toward a state of greater disorder. The reverse, cooling, greater disorder. The reverse, cooling, shows a decrease in entropyshows a decrease in entropy

►Systems were the number of particles Systems were the number of particles increases show increases in entropy, increases show increases in entropy, more particles more disordermore particles more disorder

2NO2NO22(g) (g) 2NO(g) + O 2NO(g) + O22(g)(g)

The dissolving of a solid in a solvent The dissolving of a solid in a solvent (solutions) show increases in entropy (solutions) show increases in entropy because dissolving breaks up the because dissolving breaks up the crystal lattice of the solid giving the crystal lattice of the solid giving the particles increased freedomparticles increased freedom

g.g. Increase in entropy means that the Increase in entropy means that the final state of a system isfinal state of a system is more more disordered than the initial state.disordered than the initial state.