Equilibrium Constant Determination

General Solubilities of Common Compounds in Water

Guidelines for IdentifyingElectrolytes and Non-electrolytes

Guidelines for Writing Net Ionic Equations

Ionization Constants for Acids and Bases

Problem Set

Handouts

Aqueous EquilibriaElectrolytic Character

Edward A. Mottel

Department of Chemistry

Rose-Hulman Institute of Technology

04/19/23

Electrolytes

• Reading Assignment: • Chang: Chapter 14.5; Lab manual Exp. F

• The degree that a compound dissociates into ions is a measure of its strength, not its concentration.• Strong electrolytes - dissociate virtually

100% into ions, at all concentrations• Weak electrolytes - dissociate less than

50% into ions• Non-electrolytes - do not form ions when

dissolved in a solvent.

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Strong Electrolytes

• Strong Acids

• HCl, HBr, HI, HNO3, H2SO4, HClO4, etc.

• Strong Bases• Group IA hydroxides: NaOH, KOH

• Ca(OH)2, Sr(OH)2, Ba(OH)2

• Most ionic compounds

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Strong Electrolytes

• Soluble Salts• Binary compounds of metal cations with

nonmetal anions• Compounds of metal cations with

polyatomic anions• Ammonium cation with nonmetal or

polyatomic anions.

Which of these are soluble, strong electrolytes?

KCl, Zn(NO3)2, CuSO4, NH4I

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Strong Electrolytes

• Slightly soluble salts• Compounds which have a solubility

product constant (Ksp) are strong electrolytes, but are not very soluble.

• AgCl, BaSO4, Mg(OH)2, etc.

This indicates that AgClisn’t very soluble.(sparingly soluble)

AgCl (s) AgCl (aq) Ag+(aq) + Cl–(aq)

This indicates that theamount of AgCl which dissolves

ionizes extensively.(strong electrolyte)

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Weak Electrolytes

• Weak acids, weak bases, and other soluble compounds which do not ionize extensively.

• A weak electrolyte will usually have a dissociation constant associated with it such as Ka or Kb.

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Weak Electrolytes

• Weak Acids

• HCOOH, CH3COOH, HF, HCN, HNO2, H3PO4, H2S, HS–, H2CO3, HCO3

–, etc.

• Weak Bases

• NH3, CH3NH2, N2H4, etc.

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Non-electrolytes

Compounds which dissolve but do not form a significant number of ions.

Most covalent compounds: compounds of two or more nonmetals excluding those which are strong or weak electrolytes.• water, sugar, methanol, ethylene glycol,

etc.

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Solubility is related to theconcentration of the solute

in the solvent

Dissociationis related

to the ability ofthe compoundto form ionsin solution

Categorize each of thefollowing compounds

in terms of its solubilityand tendency to form ions.

oilNaClsugar

acetic acidCa(OH)2

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Electrolytic Character SolubilityExamples

• Oil -- not very soluble, doesn’t ionize• NaCl -- very soluble and very conductive• Sugar -- very soluble but non-conducting• Acetic acid -- very soluble but poorly

conducting• Calcium hydroxide -- slightly soluble but

the amount that dissolves, completely ionizes in water

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Solubility is related to theconcentration of the solute

in the solvent

Dissociationis related

to the ability ofthe compoundto form ionsin solution

solubleslightlysoluble

strong

weak

non-electrolyte

oil

NaCl Ca(OH)2

sugar

acetic acid CO2

The solubility of a compound does not predictits electrolytic character.

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Equation Notation

Each compound in solution is considered in its “predominant” form with proper annotation:

HCl(aq) H+(aq) + Cl– (aq)

KOH (aq) K+(aq) + OH– (aq)

• Strong Electrolytes as completely dissociated.

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Equation Notation

Each compound in solution is considered in its “predominant” form with proper annotation:• Weak Electrolytes and Non-electrolytes as

molecular compounds

HF(aq) H+(aq) + F– (aq)

CH3OH (aq) H+(aq) + CH3O–

(aq)

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Group Activity

Identify the predominant form and write a dissociation reaction for the following compounds• nitric acid• hydrofluoric acid• potassium chlorate• aqueous ammonia• sodium hydroxide

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Equilibrium Arrows

HNO3(aq) H+(aq) + NO3–

(aq)

HF(aq) H+(aq) + F– (aq)

KClO3(aq) K+(aq) + ClO3–

(aq)

NH3(aq) + H2O (l) NH4+(aq) + OH–

(aq)

NaOH (aq) Na+(aq) + OH– (aq)

Buff Colored SheetCompetency Assignment

Complete Experiment F

in thelaboratory manual

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Ionization Constants

Acid dissociation constants, Ka

• The larger the constant, the stronger the acid.

Base dissociation constants, Kb

• The larger the constant, the stronger the base.

Function of temperature

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Acid Strength

acetic acidphosphoric acidnitrous acid

Rank the following compounds in terms of acid strength (strongest to weakest).

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Acid Strength

H3PO4 7.5 x 10–3

Rank the following compounds in terms of acid strength (strongest to weakest).

HNO2 4.5 x 10–4

CH3COOH 1.8 x

10–5

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Numeric Values of

Ionization Constants

Describe this reaction in a sentence.

F–(aq) + H+(aq) HF(aq)

How is the equation listed in the Ionization Constants of Aqueous Acids table?

What is the numerical value of the reaction? Is the arrow notation correct?

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Numeric Values of

Ionization Constants

F–(aq) + H+(aq) HF(aq)

HF(aq) H+(aq) + F–(aq) Ka= 7.2 x 10–4 M

K = 1 / 7.2 x 10–4 M–1

= 1.4 x 103 M–1

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Numeric Values of

Equilibrium Constants

For carbonic acid• write the acid dissociation reaction.• determine the mass-action expression.• find the numeric value of the equilibrium

constant.• confirm the equilibrium arrow assignment.

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Carbonic Acid

H2CO3(aq) H+(aq) + HCO3–(aq)

[H+] [HCO3–]

[H2CO3]= Ka,1 = 4.2 x 10–7 M

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Carbonic Acid

H2CO3(aq) H+(aq) + HCO3–(aq) Ka,1= 4.2 x 10–7

HCO3–

(aq) H+(aq) + CO32–(aq) Ka,2= 4.8 x 10–11

Write the mass-action expression forthe second ionization of carbonic acid.

Carbonic acid has more than one acidic proton, and each ionization has its own

constant.

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Determining Unlisted

Equilibrium Constants

H2CO3(aq) 2 H+(aq) + CO32–(aq)

What is the numeric value of theequilibrium constant for this reaction?

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Determining Unlisted

Equilibrium Constants

[H+]2 [CO32–]

[H2CO3][H+] [HCO3

–]

[H2CO3]= 4.2 x 10–7

[H+] [CO32–]

[HCO3–]

= 4.8 x 10–11

What is similar, what is different?

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Determining Unlisted

Equilibrium Constants

[H+]2 [CO32–]

[H2CO3]=

[H+] [HCO3–]

[H2CO3]

[H+] [CO32–]

[HCO3–]

= 4.2 x 10–7 4.8 x 10–11

= 2.0 x 10–17 M2

H2CO3(aq) 2 H+(aq) + CO32–(aq)

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Chemical EquationsCan Be Treated Algebraically

H2CO3(aq) H+ (aq) + HCO3

– (aq)

HCO3–

(aq) H+ (aq) + CO3

2– (aq)

H2CO3(aq) 2 H+ (aq) + CO3

2– (aq)

target equation

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Numeric Values of

Dissociation Constants

Describe the reaction in a sentence. How is the listed Kb equation related to this

reaction?

NH4+(aq) NH3(aq) + H+(aq)

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Dissociation of Ammonium Ion

NH4+(aq) NH3(aq) + H+(aq) (target)

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Dissociation of Ammonium Ion

NH4+(aq) NH3(aq) + H+(aq) (target)

NH4+(aq) + OH–(aq) NH3 (aq) + H2O(l)

What other equation can be added to the first equationto give the target equation?

H2O(l) OH–(aq) + H+(aq)

What is the numeric value of the equilibrium constant?

Assignments

Equilibrium Constant Determination Problem Set

Complete Experiment F in the laboratory manual

04/19/23

Group Activity

hydrogen peroxide

water

ammonium ion

Rank the following species in terms of acid strength (strongest to weakest).

ammonium ion

hydrogen peroxide

water

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