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

Types of Equilibrium:Physical Equilibrium (Phase equilibrium)Physical Equilibrium (Solution Equilibrium)

Chemical Equilibrium

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

Occurs when two phases exist at the same temperature.

Example: Ratevaporization = Ratecondensation

H2O (l) H2O (g)

Adding a solute to solvent (making a solution), lowers the vapor pressure which in turn

1.Lowers the Freezing Point of a solution

2.Raises the Boiling Point of a solution

The more solute added, the greater the change in freezing point and boiling point

Changing the container size Adding more of the solvent to the container

Rate of dissolving = rate of crystallization

Occurs in saturated solutions

SO far, we have only talked about chemical reactions that proceed in one direction, from Reactants to Products and then STOP

A + B C Equilibrium is not reached if one of the products is withdrawn as quickly as it is produced and no new

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

BUT most chemical reactions are able to proceed in both directions under the appropriate conditions. They are REVERSIBLE!

Example:Fe3O4 (s) + 4 H2 (g) ↔ 3 Fe(s) + 4 H2O(g)

WHEN DOES IT STOP?

Represented by a double arrow between

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

Ratefwd = Raterev

This is called chemical equilibrium.

Rate depends on concentration The forward rate of a reaction decreases over time

The reverse rate of a reaction increases over time

Eventually the 2 rates become equal

AABB

Once equilibrium is achieved, the amount of each reactant and product remains constant

CON CON REQUAL: Concentration of reactants and products are constant , not necessarily equal. Rates are Equal!

A double arrow identifies that a reaction is in equilibrium.

NN22OO4 (4 (gg)) 2 NO2 NO2 (2 (gg))

Ratio of Products over Reactants Only use aqueous or gaseous substances in the expression; never use solids or liquids

• The equilibrium expression The equilibrium expression for this reaction would befor this reaction would be

KK = = [C][C]cc[D][D]dd

[A][A]aa[B][B]bb

aaA + A + bbBB ccC + C + ddDD

K is the K is the equilibriequilibri

um um constantconstant

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The Equilibrium ExpressionThe Equilibrium Expression• Write the equilibrium expression Write the equilibrium expression

for the following reaction:for the following reaction:

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

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The Equilibrium ConstantThe Equilibrium ConstantKK is based on the molarities is based on the molarities (concentration) of reactants & (concentration) of reactants &

products at equilibrium.products at equilibrium.• We generally omit the units of We generally omit the units of the equilibrium constant.the equilibrium constant.

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

[N2] = .100M[N2] = .100M[H2] = .300 [H2] = .300

MM[NH3] = .200 [NH3] = .200

MM

Insert the concentrations Insert the concentrations into the expression to into the expression to

solve for Ksolve for K

If K >> 1, the reaction is product-favored; product predominates at equilibrium.

• If K << 1, the If K << 1, the reaction is reaction is

reactant-favoredreactant-favored; ; reactant reactant

predominates at predominates at equilibrium.equilibrium.

As you can see, the ratio of [NO2]2 to [N2O4] remains constant at this temperature no matter what the initial concentrations of NO2 and N2O4 are.

NN22OO4 (4 (gg)) 2 NO2 NO2 (2 (gg))

Whenever stress is applied to a reaction at equilibrium, the reaction will shift its point of equilibrium to offset the stress.

Stresses include:Temperature, pressure, changes in reactant or product concentrations

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Change in Reactant or Product Change in Reactant or Product ConcentrationsConcentrations

• Adding a reactant or product Adding a reactant or product shifts the equilibrium away shifts the equilibrium away

from the increase.from the increase.

• Removing a reactant or Removing a reactant or product shifts the product shifts the

equilibrium towards the equilibrium towards the decrease.decrease.

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Change in ConcentrationChange in Concentration

If HIf H22 is added while the system is at is added while the system is at equilibrium, the system must respond equilibrium, the system must respond

to counteract the added Hto counteract the added H22

• That is, the system must consume the HThat is, the system must consume the H22 and produce products until a new and produce products until a new

equilibrium is established.equilibrium is established.• Therefore, [HTherefore, [H22] and [N] and [N22] will decrease ] will decrease

and [NHand [NH33] increases.] increases.

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

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• Volume & Pressure are inversely Volume & Pressure are inversely related. related.

• Increasing pressure (decrease in Increasing pressure (decrease in volume) favors the direction that has volume) favors the direction that has

fewer moles of fewer moles of gasgas..• Decreasing pressure (increase in Decreasing pressure (increase in volume) favors the direction that has volume) favors the direction that has

greater moles of greater moles of gasgas..• In a reaction with the same number of In a reaction with the same number of

product and reactant moles of gas, product and reactant moles of gas, pressure has no effect.pressure has no effect.

Effects of Volume and Effects of Volume and Pressure on Gaseous Pressure on Gaseous

EquilibriumEquilibrium

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Change in Volume & Change in Volume & PressurePressure

If the volume of the container increases If the volume of the container increases while the system is at while the system is at

equilibrium(pressure of the gas equilibrium(pressure of the gas decreases), the system must respond to decreases), the system must respond to

counteract the decreased pressurecounteract the decreased pressure

• That is, the system will shift to the That is, the system will shift to the right ;more moles of product will form right ;more moles of product will form

until a new equilibrium is until a new equilibrium is established.- this raises the pressure established.- this raises the pressure

back to where it originally wasback to where it originally was• Therefore, [NOTherefore, [NO22] will increase and ] will increase and

[N[N22OO44] decreases.] decreases.

N2O4(g) 2NO2(g)colorlesscolorless brownbrown

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Effects of Volume and Effects of Volume and PressurePressure

• An increase in pressure (by decreasing An increase in pressure (by decreasing the volume) favors the formation of the volume) favors the formation of

colorless Ncolorless N22OO44..• The instant the pressure increases, The instant the pressure increases, the system is not at equilibrium and the system is not at equilibrium and the concentration of both gases has the concentration of both gases has

increased.increased.• The system moves to reduce the number The system moves to reduce the number

moles of gas: shifts leftmoles of gas: shifts left• A new equilibrium is established!A new equilibrium is established!

N2O4(g) 2NO2(g)colorlesscolorless brownbrown

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• The equilibrium constant is The equilibrium constant is temperature dependent.temperature dependent.

• For an endothermic reaction, For an endothermic reaction, HH > 0 or > 0 or HH is (+); heat can be is (+); heat can be

consideredconsidered as a reactant.as a reactant.

• For an exothermic reaction, For an exothermic reaction, HH < 0 or < 0 or HH is (-) heat can be considered as is (-) heat can be considered as

a product.a product.• Adding heat (i.e. heating the vessel) Adding heat (i.e. heating the vessel)

favors away from the increase:favors away from the increase:– if Endothermic, adding heat favors if Endothermic, adding heat favors

the forward reaction,the forward reaction,– if exothermic, adding heat favors if exothermic, adding heat favors

the reverse reaction.the reverse reaction.

Effect of Temperature ChangesEffect of Temperature Changes

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Effect of Temperature Effect of Temperature ChangesChanges

• Removing heat (i.e. cooling the Removing heat (i.e. cooling the vessel), favors towards the decrease:vessel), favors towards the decrease:– if endothermic, cooling favors if endothermic, cooling favors

the reverse reaction,the reverse reaction,– if exothermic, cooling favors if exothermic, cooling favors

the forward reaction.the forward reaction.

Cr(H2O)6(aq) + 4Cl-(aq) CoCl42-(aq) + 6H2O(l)

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Effect of Temperature Effect of Temperature ChangesChanges

HH =+50KJ ; If the temperature is =+50KJ ; If the temperature is lowered, the reaction will shift lowered, the reaction will shift to the left and the solution will to the left and the solution will

change to pink making more change to pink making more Co(HCo(H22O)O)66

2+2+Cr(H2O)6(aq) + 4Cl-(aq) CoCl4

2-(aq) + 6H2O(l)pale pinkpale pink blueblue

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

a) increase [N2]

b) decrease [H2]

c) increase [NH3]

d) decrease [NH3]

e) increase pressuref) increase volumeg) increase temperatureh) decrease temperature

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

)a [N2] shift towards products (right)

)b [H2] shift towards reactants (left)

)c [NH3] shift towards reactants (left)

)d [NH3] shift towards products (right)

)e pressure shift towards products (right))f volume shift towards reactants (left))g temp. shift towards reactants (left))h temp. shift towards products (right)

Concentration increase shift away from increase

Concentration decrease shift toward decrease

pressure shifts in direction of fewer gas molecules.

pressure shifts in direction of more gas molecules

temperature favors endothermic reaction◦Shift away from heat

temperature favors exothermic reaction◦Shift towards heat

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

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

Does not change the value of K

Application of LeChatelier’s Principle N2 (g) + 3 H2 (g) 2 NH3 (g) + 92 kJ

increase pressure Shift

decrease Temp Shift remove NH3 add N2 and H2 Shift

****Maximum yields of NH3 occurs under high pressures, low temperatures and by constantly removing NH3 and adding N2 & H2