EquilibriumEquilibrium

Chapter 15Chapter 15

EquilibriumEquilibrium

Have you ever tried Have you ever tried to maintain your to maintain your balance as you balance as you walked across a walked across a narrow ledge?narrow ledge?

In a chemical In a chemical reaction balance or reaction balance or equilibrium is also equilibrium is also maintained.maintained.

You can think of the You can think of the yields yields sign as sign as the ledgethe ledge

Equilibrium Equilibrium systems exist in systems exist in ocean water, ocean water, blood, urine, and blood, urine, and many other many other biological systemsbiological systems

Chemical reactions Chemical reactions for the most part for the most part are reversible are reversible

You can think of You can think of the yield sign the yield sign as the ledge in a as the ledge in a chem systemchem system

I. Chemical Equilibrium ConceptI. Chemical Equilibrium Concept

Chemical Equilibrium occurs when Chemical Equilibrium occurs when opposing reactions are proceeding at opposing reactions are proceeding at equal ratesequal rates

At equil rate forward = rate reverseAt equil rate forward = rate reverse AA

B [] indicates a molar conc.B [] indicates a molar conc. Fr AFr AB rate kf [A] kf [A] = kr [B]B rate kf [A] kf [A] = kr [B] Rr BRr BA rate kr [B] A rate kr [B] [B][B] = = kf kf cons= cons=

KcKc Rearranging formula [A] krRearranging formula [A] kr

A dynamic equilibriumA dynamic equilibrium Individual molecules are Individual molecules are

undergoing change but undergoing change but there is no net exchange in there is no net exchange in the concentration of the concentration of reactants and productsreactants and products

Does not mean that the Does not mean that the concentrations are not concentrations are not changing just that the ratio changing just that the ratio equals a definite valueequals a definite value

Look at Habber reaction Look at Habber reaction figure 15.6 text.figure 15.6 text.

Dihydrogen monoxide: Dihydrogen monoxide: is also known as hydric is also known as hydric

acid, and is the major acid, and is the major component of acid rain. component of acid rain.

contributes to the contributes to the "greenhouse effect." "greenhouse effect."

may cause severe burns. may cause severe burns. contributes to the erosion contributes to the erosion

of our natural landscape. of our natural landscape. accelerates corrosion and accelerates corrosion and

rusting of many metals. rusting of many metals. may cause electrical may cause electrical

failures and decreased failures and decreased effectiveness of effectiveness of automobile brakes. automobile brakes.

has been found in excised has been found in excised tumors of terminal cancer tumors of terminal cancer patients. patients.

Law of Mass ActionLaw of Mass Action

Equilibrium can be reached from either Equilibrium can be reached from either directiondirection

Concentrations of reactants and products are Concentrations of reactants and products are expressed asexpressed as

aA + bB aA + bB pP + qQ pP + qQ

equil constant Kc = equil constant Kc = [P][P]p p [Q[Q]]q productsq products

[A][A]a a [Q[Q]]b reactants b reactants

Equilibrium Constant mass action Equilibrium Constant mass action

Kc depends on the stoichiometry not Kc depends on the stoichiometry not on mechanicson mechanics

Doesn’t depend on the initial conc of Doesn’t depend on the initial conc of reactants and productsreactants and products

Doesn’t depend on other added sub Doesn’t depend on other added sub as long as they do not reactas long as they do not react

Varies with temptVaries with tempt Catalyst do not effect just speeds – Catalyst do not effect just speeds –

reaching eq.reaching eq.

Putting it TogetherPutting it Together

Question?Question? Main Main IdeasIdeas

DetailsDetails MonitorMonitor

Writing equilibrium expressionsWriting equilibrium expressions

2O2O33 (g)(g) 3O 3O22 (g)(g)

2NO 2NO (g)(g) + Cl + Cl22 (g)(g) 2NOCl 2NOCl (aq)(aq)

AgCl AgCl (s)(s) Ag Ag++ (aq)(aq) + Cl + Cl-- (aq)(aq)

Kc = Kc = [O[O22]]33 Kc = Kc = [NOCl][NOCl]22

[O[O33]]22 [NO] [NO]22 [Cl [Cl22]]

Kc = [AgKc = [Ag++] [Cl] [Cl--]]

Pure solids and liquids do not effect the Pure solids and liquids do not effect the equilibrium because their conc remain equilibrium because their conc remain unchangedunchanged

Eq expressed as pressureEq expressed as pressure

CC33HH8(g)8(g) + O + O2(g)2(g) CO CO2(g)2(g) + H + H22OO(l)(l)

KKp p = = COCO2p2p33

CC33HH8p8p O O2p2p55

P = partial pressure of the gasP = partial pressure of the gas KKpp = k = kcc(RT)(RT)delta ndelta n

What does kc tell you?What does kc tell you?

Ex CO Ex CO (g)(g) + Cl + Cl22 (g)(g) COCl COCl2 (g)2 (g)

Kc = Kc = [COCl[COCl22]] = 4.57 X 10^9 = 4.57 X 10^9

[CO] [Cl[CO] [Cl22]]

Kc>>>1 larger numerator reaction Kc>>>1 larger numerator reaction goes almost totally to products – eq goes almost totally to products – eq lies right –favors productslies right –favors products

Kc<<<1 larger denominator Kc<<<1 larger denominator

Le Chatelier’s PrincipleLe Chatelier’s Principle

If a system at eq is If a system at eq is disturbed by a disturbed by a change in change in temperature, temperature, pressure, or the pressure, or the concentration of one concentration of one of the components, of the components, the system will shift the system will shift it’s eq pos so as to it’s eq pos so as to counter the effects counter the effects of the distrubanceof the distrubance

Henri-Louis Le Henri-Louis Le Chatelier (1858-Chatelier (1858-1936)1936)

Change in ConcentrationChange in Concentration

Le Châtelier's principle states that if the Le Châtelier's principle states that if the concentration of one of the components of concentration of one of the components of the reaction (either product or reactant) is the reaction (either product or reactant) is changed, changed, the system will respond in such a the system will respond in such a way as to counteract the effectway as to counteract the effect

If a substance (either reactant or product) If a substance (either reactant or product) is is removed from a system, the equilibrium removed from a system, the equilibrium will shift so as to produce more of that will shift so as to produce more of that component (and once again achieve component (and once again achieve equilibrium)equilibrium)

Change in concentrationChange in concentration

If a substance (either reactant or If a substance (either reactant or product) is product) is added to a system, the added to a system, the equilibrium will shift so as to equilibrium will shift so as to consume more of that component consume more of that component (and once again achieve equilibrium)(and once again achieve equilibrium)

Putting it Together Putting it Together NN2(2(gg)) + 3H + 3H2(2(gg)) <=> 2NH <=> 2NH3(3(gg))

Question?Question? Main IdeasMain Ideas DetailsDetails MonitorMonitor

How would How would inceasing inceasing HH2 2 change change eqeq

Predict the Predict the relative relative conc of conc of each each reactant reactant and and productproduct

The reaction is driven The reaction is driven "to the right" by the "to the right" by the effects of added Heffects of added H22

The eq conc’s will not The eq conc’s will not be identical to the be identical to the original state. original state. However, Kc will be However, Kc will be the same.the same. The new The new equilibrium state equilibrium state contains a slightly contains a slightly higher concentration higher concentration of NHof NH33((gg), and slightly ), and slightly lower concentration of lower concentration of NN22((gg) (as well as a ) (as well as a slightly higher slightly higher concentration of Hconcentration of H22((gg). ).

Change in Volume and PressureChange in Volume and Pressure A chemical system in equilibrium can A chemical system in equilibrium can

respond to the effects of pressure also. respond to the effects of pressure also. According to Le Châtelier's Rule, if the According to Le Châtelier's Rule, if the pressure is increased on a system, pressure is increased on a system, it will it will respond by trying to reduce the pressure. respond by trying to reduce the pressure. How does it do this?How does it do this?

We are primarily concerned with We are primarily concerned with homogeneous gaseous reactions homogeneous gaseous reactions

The stoichiometry of the reaction may lead The stoichiometry of the reaction may lead to a to a greater number of molecules on one greater number of molecules on one side of the equation. side of the equation.

For example, in the Haber reaction, NFor example, in the Haber reaction, N2(g)2(g) + + 3H3H2(g)2(g) <=> 2NH <=> 2NH3(g)3(g) there are twice as many there are twice as many moles of reactants as productsmoles of reactants as products

If the Haber reaction were in If the Haber reaction were in equilibrium, and the pressure was equilibrium, and the pressure was increased, the reaction would respond increased, the reaction would respond to to oppose the increase in pressure. It oppose the increase in pressure. It could accomplish this by shifting the could accomplish this by shifting the equilibrium to the right (producing equilibrium to the right (producing NH3(g)) NH3(g))

This would reduce the overall number of This would reduce the overall number of moles in the reaction, and therefore, moles in the reaction, and therefore, lower the pressurelower the pressure

Systems shift to the side with the Systems shift to the side with the fewest number of moles if both are the fewest number of moles if both are the same then no change in eq con’s occurssame then no change in eq con’s occurs

Changes in TemperatureChanges in Temperature

The intrinsic value of K does not change The intrinsic value of K does not change when we increase concentrations or when we increase concentrations or pressures of components in a reaction. pressures of components in a reaction. However, However, almost every equilibrium constant almost every equilibrium constant (K) changes in response to changes in (K) changes in response to changes in temperature.temperature.

We will consider reaction conditions under We will consider reaction conditions under which which no work is doneno work is done, and therefore all , and therefore all energy changes associated with reactions energy changes associated with reactions will be manifested by temperature changes)will be manifested by temperature changes)

Temperature ChangesTemperature Changes

Exothermic reactions are associated Exothermic reactions are associated with with heat release heat release when the reaction when the reaction proceeds in theproceeds in the forward direction forward direction

Endothermic reactions are associated Endothermic reactions are associated with heat release when the reaction with heat release when the reaction proceeds in the reverse direction (i.e. proceeds in the reverse direction (i.e. heat is heat is absorbed absorbed inin the forward the forward direction)direction)

These two types of reactions and their These two types of reactions and their associated heat changes can be written as: associated heat changes can be written as:

Exothermic: Reactants yield Products + HeatExothermic: Reactants yield Products + Heat Endothermic: Reactants + Heat yield Endothermic: Reactants + Heat yield

ProductsProducts If temperature is increased, the equilibrium If temperature is increased, the equilibrium

will shift so as to will shift so as to minimizeminimize the effect of the the effect of the added heat added heat

The reaction will shift in the The reaction will shift in the appropriate direction such that the appropriate direction such that the added heat is added heat is absorbedabsorbed

When heat is added to When heat is added to exothermic exothermic reactions at equilibrium, products will reactions at equilibrium, products will be consumed to producebe consumed to produce reactants reactants ((shift to the LEFT)shift to the LEFT) May also be May also be written delta t is negative.written delta t is negative.

When heat is added to When heat is added to endothermic endothermic reactions at equilibrium, reactants reactions at equilibrium, reactants will be consumed to produce will be consumed to produce products products (shift to the RIGHT) (shift to the RIGHT) May May also be written delta t is positive.also be written delta t is positive.

Based on this behavior, what is the Based on this behavior, what is the effect of effect of TT upon upon KK? ?

Assume K = 1.0 for an Assume K = 1.0 for an exothermic exothermic reaction at equilibrium. reaction at equilibrium.

Added heat causes the reaction to Added heat causes the reaction to shift to the left. Reactants <= shift to the left. Reactants <= Products + HeatProducts + Heat Thus, 1.0 must represent a reaction Thus, 1.0 must represent a reaction

quotient, Q, that is too large in quotient, Q, that is too large in comparison to the new value of K. comparison to the new value of K.

Thus, the effect of increasing temperature Thus, the effect of increasing temperature on an on an exothermicexothermic reaction is to reaction is to lowerlower the the value of K. value of K.

Conversely, the effect of increasing Conversely, the effect of increasing temperature on an temperature on an endothermicendothermic reaction reaction is to is to increaseincrease the value of K the value of K

Putting it Together Putting it Together Calc Delta H of formation for Calc Delta H of formation for

CC33HH8(g)8(g) + O + O2(g)2(g) CO CO2(g)2(g) + H + H22OO(l)(l)

Question?Question?

Is the Is the value value

Exo,or Exo,or endotherendothermm

Main Main IdeasIdeas

DetailsDetails MonitorMonitor

How How would inc. would inc. temp temp effect eqeffect eq

How How would dec would dec temp eff temp eff eq keq k

Calculations with eq KCalculations with eq K

Example calculating unknown Example calculating unknown concentrations using the eq constantconcentrations using the eq constant

COCO(g)(g) + 3H + 3H2(g)2(g) <-> CH <-> CH4(g)4(g) + H + H2200(g)(g)

At eq 0.3 mol of CO, 0.1 mol HAt eq 0.3 mol of CO, 0.1 mol H22 and and 0.02 mol of H0.02 mol of H220 are in 1.0 liter of a 0 are in 1.0 liter of a vessel at 1200 k kC is 3.92 what is vessel at 1200 k kC is 3.92 what is the conc of CHthe conc of CH44??

Kc = Kc = [CH[CH44] [H] [H22O]O]

[CO] [H[CO] [H22]]33

3.93 = 3.93 = [CH[CH44] (.020) ] (.020)

(0.30) (0.10)(0.30) (0.10)33

[CH4] = [CH4] = (0.30)(0.10)(0.30)(0.10)33 3.93 3.93

(0.020)(0.020)

0.059 mol/l0.059 mol/l

Learning CheckLearning CheckPClPCl5(g)5(g)<-> PCl<-> PCl3(g)3(g) + Cl + Cl2(g)2(g)

A l.0 liter vessel has a unknown A l.0 liter vessel has a unknown amount of PClamount of PCl55 at eq Kc at 250 at eq Kc at 250 00C is C is 0.0415. Calc the unknow conc. if 0.0415. Calc the unknow conc. if 0.02moles of PCl0.02moles of PCl33 and Cl and Cl22 are in the are in the container. (0.0096)container. (0.0096)

Solving linear eq equasionsSolving linear eq equasions CO(g) + H20(g) <-> C02(g) + H2(g)CO(g) + H20(g) <-> C02(g) + H2(g)

Given 1.0 mol of COGiven 1.0 mol of CO22 and H and H220 in a 50.0 l 0 in a 50.0 l vessel. How many moles are in an eq vessel. How many moles are in an eq mix at 1000 mix at 1000 ooC Ec = 0.58 at 1000C Ec = 0.58 at 1000ooCC

CO(g) + HCO(g) + H220(g) <-> C00(g) <-> C022(g) + H(g) + H22(g(g

I 0.02 0.02 0 0I 0.02 0.02 0 0

C -x -x +x +xC -x -x +x +x

E 0.02-x 0.02-x x x E 0.02-x 0.02-x x x

0.58 = 0.58 = [CO[CO22][H][H22] ] = = XX22

[CO] [H[CO] [H22O] (0.02-X)O] (0.02-X)22

+,-+,- 0.76 = 0.76 = X X22

(0.02-X)(0.02-X)

(the neg one gives a neg answer x can’t (the neg one gives a neg answer x can’t be neg)be neg)

_ _ +0.76(0.02-X)=X+0.76(0.02-X)=X

0.0152-0.76X=X0.0152-0.76X=X

0.0152= 1.76X0.0152= 1.76X

X= 0.0086X= 0.0086

HH2(g)2(g) + I + I2(g)2(g) <-> 2HI <-> 2HI(g)(g)

What is the eq comp of a reaction What is the eq comp of a reaction mixture starting with 0.500 mol each mixture starting with 0.500 mol each of Hof H22 and I and I22 in a 1 l vessel? Kc = 49.7 in a 1 l vessel? Kc = 49.7 at 458 at 458 ooC (HC (H22 &I &I22 = 0.11 mol/l HI = = 0.11 mol/l HI = 0.78 mol/l0.78 mol/l

Equil with quadratic expressionsEquil with quadratic expressions

Calc the conc. of the previous problem Calc the conc. of the previous problem with 1.00 molar H2 and 2.00 molar I2 as with 1.00 molar H2 and 2.00 molar I2 as the starting concentrations.the starting concentrations.

HH2(g)2(g) + I + I2(g)2(g) <-> 2HI <-> 2HI(g)(g)

I 1.00 2.00 0I 1.00 2.00 0

C -x -x 2x 49.9 = C -x -x 2x 49.9 = (2x)(2x)22

E 1.00-x 2.00-x 2x (1.00-x)(2.00-x)E 1.00-x 2.00-x 2x (1.00-x)(2.00-x)

(1.00-x)(2.00-x)= (1.00-x)(2.00-x)= (2x)2 (2x)2

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