Lech Atelier s

7/28/2019 Lech Atelier s

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o If you recall, we mentioned that there

are rxns that bounce back and forthfrom forming products to reformingreactants

A.K.A. reversible rxnso Reversible reactions occur simultan-

eously in both directions

An example of a reversible rxninvolves reactants A and B producingC.

A + B C

Reversible Reactions


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oAt chemical equilibrium there is no net

change in the actual amounts of thecomponents of the system.

And although the rates of the forward

& reverse rxns are equalat chemicalequilibrium, the concentrations of thecomponents on both sides of the chem-ical eqn are not necessarily the same.

In fact they can be dramaticallydifferent.

Chemical Equilibrium


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o Consider a set of escalators as beinglike the double arrows in a dynamic

equilibrium.o The # of people using the up escalator

must be the same as the # of peopleusing the down escalator for equilibriumto have been established

However, the # of peopleupstairs do not have to

equal the # of peopledownstairs

Just the transfer between

floors must be consistent


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o The equilibrium position of a rxn is

given by the concentrations of thesystems components at equilibrium

o The equilibrium position indicates

whether the components on the left orright side of a reversible rxn are at ahigher concentration.

If A reacts to give B and the mixture atequilibrium contains more of B say1% of A vs. 99% of Bthe formation ofB is said to be favored.

Which Direction is Favored?


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o On the other hand, if the mixture

contains 99% of A and 1% of B atequilibrium then the formation of A isfavored.

Reversedirection is

favored!

Forwarddirection isfavored!

Which Direction is Favored?


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o In principle, almost all rxns are

reversible to some extent under theright conditions

o In practice, one set of components is

often so favored at equilibrium that theother set cannot be detected. If one set of components has

established equilibrium by converting

mostly into products, the rxn has goneto completion

When no products can be detected,you can say there is no rxn

Reversibility vs. Reality


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o Reversible rxns occupy a middle

ground between the theoreticalextremes of irreversibility and no rxn.

o The addition of a catalyst will speed

up forward and reverse rxns equally By reducing the energy needed to

activate the rxn in both forward andreverse directions.

Does not effect the amount of reactantsand products present at equilibrium;simply decreases the time it takes toestablish equilibrium

Reversibility vs. Reality


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Equilibrium Expressiono Chemists can express the equilibrium

position in terms of a numericalconstant The equilibrium constant shows the

relationship between the amount ofproduct and reactant at equilibrium

o Consider this hypothetical rxnaA + bB cC + dD


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o We can write an expression to show

the ratio of product concentrations toreactant concentrations called amassaction expression

Equilibrium Expression

[C]c [D]d

[A]a [B]b

o The conc of each substance is raised

to a power equal to the # of mols ofthat substance in the balanced rxn eqn.

o The square brackets indicateconcentration in Molarity(mol/L)


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o The constant is dependent on the temp If the temp changes so does the

constant

Equilibrium Expression

NOTE: water and solid materials are not

included in mass action

o The resulting ratio of the equilibrium is

called theequilibrium constant or Keq, When the reactants and products

amnts are in molarity the constant iscalled a K

c

When the reactants andproducts amounts are inpressure units is called

a Kp

Keq= [C]c [D]d

[A]a [B]b


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1. Write the mass action expression for

each of the following reactions:A.2SO2(g) + O2(g) 2SO3(g)

B.Bi2S3(s) 2Bi+3(aq) + 3S-

2(aq)

For example:


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o Equilibrium constants provide valuable

chemical informationo They show whether the products

or the reactantsare favored in a rxn

(spontaneus or nonspontaneous) always written as a ratio ofproducts

over reactants

a value ofKeq > 1means that productsare favored

Keq < 1than reactants are favored

Equilibrium Constant


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o Molarity is a measure of how much

stuff is dissolved in water. The more stuff dissolved,

the more concentrated the solution

The higher the molarity


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1. What is the molarity of a solution thatwas made by dissolving 0.100 mols ofsugar in 100 ml of water?

2. How many moles of salt are contained in300.0 mL of a 0.40 M NaCl solution?

3. A chemist dissolves 98.4 g of FeSO4 inenough water to make 2.00 L of solution.What is the molarity of the solution?

Classwork:


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Sample Problem 1Dinitrogen tetroxide (N2O4), a

colorless gas, and nitrogen dioxide(NO2), a brown gas, exist in

equilibrium with each other according

to the following eqn:N2O4(g) 2NO2(g)A 1.0 liter of gas mixture at 10C atequilibrium contains .0045 mol N2O4

& .030 mol NO2. Write the mass actionexpression and calculate Kc for the rxn.


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o Known:

[N2O4] =.0045 mol/1.0 L

[NO2] =.030 mol/1.0 L

o Unknown: Mass action expression = ?

Kc = ?

Analyze: list what we know

oAt equil, there is no net change in theamount of N2O4 or NO2 at any giveninstant


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The only product of the rxn is NO2,

which has a coefficient of 2 in thebalanced eqn

The only reactant N2O4 has acoefficient of 1 in the balanced eqn

o The mass action expression is:

Calculate: solve for unknowns

[.030M]2

[.0045M]1Kc=

[NO2]

2

[N2O

4]1

Kc =

o Kc is equal to: Kc =0.20

o Kc< 1,therefore rxn doesnt favor

products


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1. Find the equilibrium constant if [SO2] =

1.0 M; [O2]=1.0 M; [SO3]=2.0 M; usingthe mass action expression written inthe examples

2. Find the equilibrium constant if [Bi+3] =

0.00058 M; [S-2] = 0.00087 M; usingthe mass action expression written inthe examples

Classwork:


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o We can also determine if a reaction has

reached equilibrium by calculating areaction quotient (Q). Its like taking a snapshot of a reaction

at a given time and interpreting how faralong the reaction is.

o Once the reaction quotient is solved,it is compared to the equilibrium

constant The following picture helps us decide

how to interpret the direction the

reaction will continue.

Reaction Quotient


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Sample Problem 2At a certain temperature Keq = 55 and

a reaction vessel contains a mixturewith the following concentrations :

[SO3] = 0.85 M, [NO] = 1.2 M ,

[SO2] = 1.5 M and [NO2] = 2.0 M.SO3(g) + NO(g) SO2(g) +

NO2(g)

Is the reaction at equilibrium and if notwhich direction will the rxn proceed?


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Solve just as if you were solvingfor the equilibrium constant.

Then analyze the resulting quotientwith the given Keq

][NO][SO

]][NO[SOQ

3

22

M)(.85M)(1.2

M)(1.5M)(2.0Q

o Q = 2.94 which is < Keq (55)

o If Q < Keq then the numeratorof our quotient must increase

therefore the rxn continues in order

to increase [products] until it reaches


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1. 0.035 moles of SO2, 0.500 moles ofSO

2Cl

2, and 0.080 moles of Cl

2are

combined in an evacuated 5.00 L flaskand heated to 100C. What is Q beforethe reaction begins? Which direction will

the reaction proceed in order to establishequilibrium?

SO2Cl2(g) SO2(g) + Cl2(g)

Kc = 0.078 at 100C

Classwork:

Manipulating the


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Manipulating theEquilibriumo There is a principle that can be studiedto govern changes in equilibriumLe ChateliersPrinciple.

o Le Chateliers Principle states:

If a stress is applied to a systemin dynamic equilibrium, thesystemchanges to relieve the stress.

Stresses are changes in temperature,

pressure, concentration of reactants,or concentration of products


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oAdjusting the concentrations of either

reactants or products can havedramatic impact on the equilibrium

If we add more of reactant A to asystem at equilibrium the system willstrive to reestablish equilibrium at anew equilibrium position.

The reaction will push to use up the

extra A and generate more C

Concentration & Equilibrium

A + 2B C H= - 250 kJ

[A], rxn will shift toward products


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oAdjusting the concentrations of either

reactants or products can havedramatic impact on the equilibrium

If we add more of product C to asystem at equilibrium the system willstrive to reestablish equilibrium at anew equilibrium position.

The reaction will push to use up the

extra C and generate more A and B

Concentration & Equilibrium

A + 2B C H= - 250 kJ

[C], rxn will shift towardreactants


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o The impact of temperature changes on

an equilibrium is dependent on if theprocess is endothermic or exothermic

Endothermic processes use energy asa reactant, while exothermic processesproduce energy

Keq is temperature dependent

Temp effects on Equilibrium

A + 2B C H= - 250 kJ

250 kJ is a product

If T, the equilibrium shifts left


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o The impact of temperature changes on

an equilibrium is dependent on if theprocess is endothermic or exothermic

Endothermic processes use energy asa reactant, while exothermic processesproduce energy

Temp effects on Equilibrium

A + 2B H= + 250 kJC

energy is a reactant

If T, the equilibrium shifts right


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o If A, B, and C are all gases, then the

equil they establish is pressuredependent

o When the pressure is increased,the system relieves the pressure byfavoring the direction thatproducesfewer gas molecules. Pressure is # of particles dependent,

the more particles the higher thepressure

Fewer gas molecules will exertlesspressure.

Pressure & Equilibrium


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o Conversely, a decrease in pressure will

favor the rxn that produces the mostmolecules

So we have a shift to theleft

Pressure & Equilibrium

A + 2B C H= - 250 kJ

P, this equilibrium shifts right

If P, this equilibrium shifts left


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Predict the effect of the following changes

on the reaction in which SO3 decomposesto form SO2 and O2.

2SO3(g) 2SO2 (g) + O2 (g) Ho = 197.78

kJ(a) Increasing the temperature of thereaction.

(b) Increasing the pressure on the

reaction.(c) Adding more O2 when the reaction is at

equilibrium.

(d) R i O2 f th t h

Classwork

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