Standard 9: Chemical EquilibriumStandard 9: Chemical Equilibriumchapter 18chapter 18

Chemistry.Chemistry.Ms. SiddallMs. Siddall

VocabularyVocabulary::1.1. Equilibrium positionEquilibrium position2.2. Equilibrium constantEquilibrium constant3.3. Reversible reactionReversible reaction4.4. RateRate5.5. ConcentrationConcentration6.6. Le Chatelier’s PrincipleLe Chatelier’s Principle

Most reactions are ‘reversible’Most reactions are ‘reversible’ Forward reactionForward reaction: reactants make : reactants make

productsproducts

e.x. 3Oe.x. 3O2(g) 2(g) 2O 2O3(g)3(g)

Reverse reactionReverse reaction: products make : products make reactantsreactants

e.x. 2Oe.x. 2O3(g) 3(g) 3O 3O2(g)2(g)

In a reversible reaction the forward and In a reversible reaction the forward and reverse reactions occur at the same timereverse reactions occur at the same time

e.x. 3Oe.x. 3O2(g)2(g) 2 O 2 O3(g)3(g)

Reversible Reversible ReactionsReactions

Standard 9b: equilibrium conditions

Summary 1Summary 1

What is a reversible reaction?What is a reversible reaction?

Reversible reactions reach Reversible reactions reach equilibriumequilibrium: a balance between reactants : a balance between reactants and productsand products

Conditions of EquilibriumConditions of Equilibrium::1.1. rate of forward reaction = rate of reverse reactionrate of forward reaction = rate of reverse reaction2.2. Concentration of reactants and products is Concentration of reactants and products is

constantconstant ( (does not changedoes not change))NOTE:NOTE:RateRate = speed = speedConcentrationConcentration = number of particles or moles = number of particles or moles exampleexample: [HCl]: [HCl] = concentration of HCl = concentration of HCl

6M HCl = 6mole/L HCl 6M HCl = 6mole/L HCl = 6 moles of HCl per liter of solution = 6 moles of HCl per liter of solution

Summary 2Summary 2

What is equilibrium?What is equilibrium?

Reversible reaction: X Reversible reaction: X Y Y

[X]

[Y]

time

X Y

Y X

timera

te

conc

entr

atio

n

Reaction rates are equalConcentrations are constant

equilibrium

Summary 3Summary 3

Describe the part of each graph that Describe the part of each graph that illustrates equilibrium conditions.illustrates equilibrium conditions.

Le Chatelier’s PrincipleLe Chatelier’s Principle A system in equilibrium will react to relieve A system in equilibrium will react to relieve

stress (change) and re-establish equilibriumstress (change) and re-establish equilibrium

StressStress:: Adding reactants or productsAdding reactants or products Removing reactants or productsRemoving reactants or products Changing temperatureChanging temperature Changing pressure (for gases only)Changing pressure (for gases only)

9a: Le Chatelier’s Principle

Summary 4Summary 4

According to Le Chatelier’s Principle, what According to Le Chatelier’s Principle, what will happen to a system at equilibrium if will happen to a system at equilibrium if more reactants or products are added?more reactants or products are added?

Example:Example: N N2(g)2(g) + 3H + 3H2(g)2(g) NH NH3(g)3(g) + heat + heat

Stress:Stress: Add N Add N22

Stress relief:Stress relief: Forward reaction (Forward reaction (→)→) to get rid of N to get rid of N22

HH22 is used up ( is used up (↓)↓)

NHNH33 and Heat are produced ( and Heat are produced (↑)↑)

Summary 5:Summary 5: N N2(g)2(g) + 3H + 3H2(g)2(g) NH NH3(g)3(g) + heat + heat

Stress:Stress: remove N remove N22

Stress relief:Stress relief: Which way does equilibrium shift?Which way does equilibrium shift? What happens to [HWhat happens to [H22]?]?

What happens to [NHWhat happens to [NH33]?]? What happens to heat?What happens to heat?

Adding products or reactantsAdding products or reactants Equilibrium shifts to Equilibrium shifts to removeremove addition addition

Removing products or reactantsRemoving products or reactants Equilibrium shifts to Equilibrium shifts to replacereplace what has been what has been

removedremoved GassesGasses

Equilibrium shifts to produce:Equilibrium shifts to produce:• moremore gas at gas at lowlow pressure pressure• LessLess gas at gas at highhigh pressure pressure

Stress relief.

Summary 6Summary 6

Why would a gas equilibrium system Why would a gas equilibrium system produce more gas at low pressure and produce more gas at low pressure and less gas at high pressure? less gas at high pressure?

Haber Process: NHaber Process: N2(g)2(g) + 3H + 3H2(g)2(g) NH NH3(g)3(g) + heat + heat

ChangeChange Equilib.Equilib. [N[N22]] [H[H22]] [NH[NH33]]

↑ ↑ [N[N22]]

↓ ↓ [N[N22]]

↑ ↑ [H[H22]]

↓ ↓ [H[H22]]

↑ ↑ [NH[NH33]]

↓ ↓ [NH[NH33]]

Haber Process: NHaber Process: N2(g)2(g) + 3H + 3H2(g)2(g) 2NH 2NH3(g)3(g) + heat + heat

ChangeChange Equilib.Equilib. [N[N22]] [H[H22]] [NH[NH33]]

↑ ↑ heatheat

↓ ↓ heatheat

↑↑pressurepressure

↓↓ppressureressure

Summary 7Summary 7 According to Le Chatelier’s Principle:According to Le Chatelier’s Principle: Increasing reactant concentration will Increasing reactant concentration will

cause:cause: other reactants to __________?other reactants to __________? products to __________?products to __________?

Decreasing reactant concentration will Decreasing reactant concentration will cause:cause: other reactants to __________?other reactants to __________? products to __________?products to __________?

A(g) + B(g) AB(g) + heat

Summary 8Summary 8 Complete the table of Complete the table of

equilibrium changes equilibrium changes

A(g) + B(g) AB(g) + heat

ChangeChange Equilib.Equilib. [A][A] [B][B] [AB][AB]

↑ ↑ heatheat

↓ ↓ [A][A]

↑↑pressurepressure

↓↓[AB][AB]

Equilibrium Constant: KEquilibrium Constant: Keqeq

At equilibrium concentrations are constantAt equilibrium concentrations are constant KKeqeq represents concentrations of reactants represents concentrations of reactants

and products at equilibriumand products at equilibrium Example: aA + bB Example: aA + bB cC + dD cC + dD KKeqeq = [C] = [C]cc[D][D]dd

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

HONORS Standard 9c: equilibrium constant

Summary 9Summary 9

write Keq expression for the Haber-Bosch write Keq expression for the Haber-Bosch Process: NProcess: N2(g)2(g) + 3H + 3H2(g)2(g) 2NH 2NH3(g)3(g)

Concentrations calculated in mol/L (M)Concentrations calculated in mol/L (M) Only solutions(aq) & gases(g) are consideredOnly solutions(aq) & gases(g) are considered

No solids (s)No solids (s) No liquids (l)No liquids (l)

Example: 2HExample: 2H22OO(l)(l) 2H 2H2(g)2(g) + O + O2(g)2(g)

KKeqeq = [H = [H22]]22[O[O22]]

Summary 10Summary 10

Fe(OH)Fe(OH)2(aq)2(aq) + 2HSO + 2HSO3(aq)3(aq) Fe(SO Fe(SO33))2(aq)2(aq) + 2H + 2H22OO(l)(l)

Find KFind Keqeq

What Keq tells usWhat Keq tells us

If Keq If Keq ≤≤ 1 There are more reactants than 1 There are more reactants than products at equilibriumproducts at equilibrium

If Keq If Keq ≤≤ 1/100 There are 1/100 There are mostlymostly reactants at reactants at equilibriumequilibrium

If Keq If Keq ≥≥ 1 There are more products than 1 There are more products than reactants at equilibriumreactants at equilibrium

If Keq If Keq ≥≥ 100 There are 100 There are mostlymostly products at products at equilibriumequilibrium

COCO(g)(g) + 2H + 2H2(g)2(g) CH CH33OHOH(g) (g) KKeqeq=290 at 430°C=290 at 430°C

Write the expression for KWrite the expression for Keqeq

Reaction is… Reaction is… (mostly products or reactants?)(mostly products or reactants?)

Summary 11Summary 11

SolubilitySolubility KKspsp is the equilibrium constant for solubility is the equilibrium constant for solubility

Example: AgClExample: AgCl(s)(s) Ag Ag++(aq)(aq) + Cl + Cl--(aq)(aq)

KKspsp AgCl = 1.77 x 10 AgCl = 1.77 x 10-10-10

Does not really dissolve, mostly solidDoes not really dissolve, mostly solid Example: AgNOExample: AgNO3(s)3(s) Ag Ag++

(aq)(aq) + NO + NO33--(aq)(aq)

KKspsp AgNO AgNO33 ~ 1 x 10~ 1 x 101010 Very solubleVery soluble

Summary 13Summary 13

Write the balanced equation for the Write the balanced equation for the dissolving of sodium sulfate. dissolving of sodium sulfate.

Write a Ksp expression for the reaction.Write a Ksp expression for the reaction.