EquilibriumEquilibrium

Reaction DynamicsReaction Dynamics

If the products of a reaction are removed If the products of a reaction are removed from the system as they are made, then a from the system as they are made, then a chemical reaction will proceed until the chemical reaction will proceed until the limiting reactants are used up.limiting reactants are used up.

However, if the products are allowed to However, if the products are allowed to accumulate; they will start reacting accumulate; they will start reacting together to form the original reactants - together to form the original reactants - called the called the reverse reaction.reverse reaction.

Reaction DynamicsReaction Dynamics

The forward reaction slows down as the The forward reaction slows down as the amounts of reactants decreases because the amounts of reactants decreases because the reactant concentrations are decreasing reactant concentrations are decreasing

At the same time the reverse reaction speeds up At the same time the reverse reaction speeds up as the concentration of the products increases. as the concentration of the products increases.

Eventually the forward reaction is using Eventually the forward reaction is using reactants and making products as fast as the reactants and making products as fast as the reverse reaction is using products and making reverse reaction is using products and making reactants. This is called reactants. This is called chemical equilibriumchemical equilibrium.. raterateforwardforward = rate = ratereversereverse

Chemical EquilibriumChemical Equilibrium

Equilibrium only occurs in a closed system!!Equilibrium only occurs in a closed system!! When a system reaches equilibrium, the When a system reaches equilibrium, the

amounts of reactants and products in the system amounts of reactants and products in the system stays constant stays constant the forward and reverse reactions still continue, but the forward and reverse reactions still continue, but

because they go at the same rate the amounts of because they go at the same rate the amounts of materials don't change.materials don't change.

There is a mathematical relationship between There is a mathematical relationship between the amounts of reactants and products at the amounts of reactants and products at equilibrium equilibrium no matter how much reactants or products you start no matter how much reactants or products you start

with.with.

Equilibrium ConstantEquilibrium Constant xA + yB ↔ nC + mD

Law Of Chemical EquilibriumLaw Of Chemical Equilibrium In this expression, K is a number called the In this expression, K is a number called the equilibrium equilibrium

constantconstant. . Do not include solids or liquids, only solutions and gasesDo not include solids or liquids, only solutions and gases For a reaction, the value of K for a reaction depends on For a reaction, the value of K for a reaction depends on

the temperaturethe temperature K is independent of the amounts of reactants and K is independent of the amounts of reactants and

products you start withproducts you start with..

yx

mn

BA

DC

][][

][][

yx

mn

eq BA

DCK

][][

][][

Position of EquilibriumPosition of Equilibrium

The relative concentrations of reactants The relative concentrations of reactants and products when a reaction reaches and products when a reaction reaches equilibrium is called equilibrium is called the position of the position of equilibriumequilibrium

Different initial amounts of reactants (and Different initial amounts of reactants (and or products) will result in different or products) will result in different equilibrium concentrations but the same equilibrium concentrations but the same equilibrium constantequilibrium constant

Position of EquilibriumPosition of Equilibrium

If K is large then there will be a larger If K is large then there will be a larger concentration of products at equilibrium than of concentration of products at equilibrium than of reactants; we say the position of equilibrium reactants; we say the position of equilibrium favors the products.favors the products.

If K is small then there will be a larger If K is small then there will be a larger concentration of reactants at equilibrium than of concentration of reactants at equilibrium than of products; we say the position of equilibrium products; we say the position of equilibrium favors the reactants.favors the reactants.

The position of equilibrium is not affected by The position of equilibrium is not affected by adding a catalyst.adding a catalyst.

Example – Determine the value of the Example – Determine the value of the Equilibrium Constant for the Reaction Equilibrium Constant for the Reaction

2 SO 2 SO22 + O + O22 2 SO 2 SO33Determine the Equilibrium Determine the Equilibrium

ExpressionExpressionPlug the equilibrium Plug the equilibrium

concentrations into to concentrations into to Equilibrium ExpressionEquilibrium Expression

Solve the EquationSolve the Equation3.503.00SO3

1.251.50O2

1.502.00SO2

[Equilibrium][Initial]Chemical

4.36 1.25)((1.50)

(3.50)

]O[][SO

][SO K

2

2

22

2

23

Le ChLe Châtelier’s Principleâtelier’s Principle

Le Châtelier's Principle guides us in Le Châtelier's Principle guides us in predicting the effect various changes have predicting the effect various changes have on the position of equilibrium on the position of equilibrium

When a change is imposed on a system When a change is imposed on a system at equilibrium, the position of at equilibrium, the position of equilibrium will shift in the direction equilibrium will shift in the direction that will reduce the effect of that that will reduce the effect of that changechange

Concentration Changes and Le Châtelier’s Concentration Changes and Le Châtelier’s PrinciplePrinciple

The position of equilibrium can be affected without The position of equilibrium can be affected without changing the equilibrium constant. changing the equilibrium constant.

Adding a reactant will decrease the amounts of the other Adding a reactant will decrease the amounts of the other reactants and increase the amount of the products until a reactants and increase the amount of the products until a new position of equilibrium is found new position of equilibrium is found

Position shifts toward the products Position shifts toward the products has the same Khas the same K

Removing a reactant will increase the amounts of the Removing a reactant will increase the amounts of the other reactants and decrease the amounts of the other reactants and decrease the amounts of the products.products. Position shifts toward the reactantsPosition shifts toward the reactants

Removing a product can allow us to drive a reaction to Removing a product can allow us to drive a reaction to completion! completion!

Changing Pressure and Le Châtelier’s Changing Pressure and Le Châtelier’s PrinciplePrinciple

Changing the pressure of one gas is like changing Changing the pressure of one gas is like changing its concentrationits concentration Has the same effect as changing the concentration on the Has the same effect as changing the concentration on the

position of equilibriumposition of equilibrium

Increasing the pressure on the system causes the Increasing the pressure on the system causes the position of equilibrium to shift toward the side of the position of equilibrium to shift toward the side of the reaction with the fewer gas moleculesreaction with the fewer gas molecules

Decreasing the volume of the system increases its Decreasing the volume of the system increases its pressurepressure Reduces the pressure by reducing the number of gas Reduces the pressure by reducing the number of gas

moleculesmolecules Opposite effect happens if the system pressure is decreasedOpposite effect happens if the system pressure is decreased

Changing Temperature and Le Châtelier’s Changing Temperature and Le Châtelier’s PrinciplePrinciple

The equilibrium constant will change if the temperature The equilibrium constant will change if the temperature changeschanges

Exothermic reactions release heat, Endothermic Exothermic reactions release heat, Endothermic reactions absorb heat.reactions absorb heat.

For exothermic reactions, heating the system will For exothermic reactions, heating the system will decrease Kdecrease K Think of heat as a product of the reactionThink of heat as a product of the reaction Therefore shift the position of equilibrium toward the reactant Therefore shift the position of equilibrium toward the reactant

side side For endothermic reactions, heating the system will For endothermic reactions, heating the system will

increase Kincrease K Think of heat as a reactantThink of heat as a reactant The position of equilibrium will shift toward the productsThe position of equilibrium will shift toward the products

Cooling an exothermic or endothermic reaction will have Cooling an exothermic or endothermic reaction will have the opposite effects on K and equilibrium positionthe opposite effects on K and equilibrium position

Example – If the value of the Equilibrium Constant for Example – If the value of the Equilibrium Constant for the Reaction 2 SOthe Reaction 2 SO22 + O + O22 2 SO 2 SO33 is 4.36, is 4.36,

Determine the Equilibrium Concentration of SODetermine the Equilibrium Concentration of SO33

Determine the Equilibrium Determine the Equilibrium ExpressionExpression

Plug the equilibrium Plug the equilibrium concentrations and concentrations and Equilibrium Constant into Equilibrium Constant into the Equilibrium the Equilibrium ExpressionExpression

Solve the EquationSolve the Equation

?3.00SO3

1.251.50O2

1.502.00SO2

[Equilibrium][Initial]Chemical

3.51 12.3 ][SO

12.3 (1.25)x (1.50)x 4.36 ][SO

4.36 1.25)((1.50)

][SO

]O[][SO

][SO K

3

223

2

23

22

2

23

Solubility & Solubility ProductSolubility & Solubility Product

Even “insoluble” salts dissolve somewhat in Even “insoluble” salts dissolve somewhat in waterwater insoluble = less than 0.1 g per 100 g Hinsoluble = less than 0.1 g per 100 g H22OO The solubility of insoluble salts is described in The solubility of insoluble salts is described in terms of equilibrium between undissolved solid terms of equilibrium between undissolved solid and aqueous ions producedand aqueous ions produced

AAnnXXmm(s) (s) n A n A++(aq) + m Y(aq) + m Y--(aq)(aq) Equilibrium constant called Equilibrium constant called solubility productsolubility product

KKspsp = [A = [A++]]nn[Y[Y--]]mm

If undissolved solid in equilibrium with the If undissolved solid in equilibrium with the solution, the solution is solution, the solution is saturatedsaturated

Larger K = More SolubleLarger K = More Soluble for salts that produce same the number of ionsfor salts that produce same the number of ions

ExampleExample

Calculate the solubility of AgI in water at 25°C if the value of Ksp = 1.5 x 10-16

Determine the balanced equation for the Determine the balanced equation for the dissociation of the saltdissociation of the salt

AgI(s) AgI(s) Ag Ag++(aq) + I(aq) + I--(aq)(aq)Determine the expression for the solubility Determine the expression for the solubility

productproduct Same as the Equilibrium Constant ExpressionSame as the Equilibrium Constant Expression

KKspsp = [Ag = [Ag++][I][I--]]

ExampleExample

Calculate the solubility of AgI in water at 25°C if the value of Ksp = 1.5 x 10-16

Define the concentrations of dissolved Define the concentrations of dissolved ions in terms of ions in terms of xx

AgI(s) AgI(s) Ag Ag++(aq) + I(aq) + I--(aq)(aq)

Stoichiometry tells us that we get 1 mole of AgStoichiometry tells us that we get 1 mole of Ag++ and 1 mol Iand 1 mol I-- for each mole of AgI dissolved for each mole of AgI dissolved

Let Let xx = [Ag = [Ag++], then [I], then [I--] = ] = xx

ExampleExample

Calculate the solubility of AgI in water at 25°C if the value of Ksp = 1.5 x 10-16

Plug the ion concentrations into the expression for the Plug the ion concentrations into the expression for the solubility product and solve for Ksolubility product and solve for Kspsp

[Ag[Ag++] = [I] = [I--] = ] = xx

8-16-

16-2

2-16-sp

10x 2.110x 5.1

10x 5.1

)(x )( ][I][Ag 10x 5.1 K

x

x

xxx

[Ag+] = 1.2 x 10-8 mol/L = [AgI]The solubility of AgI = 1.2 x 10-8 M