Aqueous Equilibria Acids and Bases

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Problem Set
Equilibrium Constant Determination
Handouts
General Solubilities of Common Compounds in Water
Guidelines for Identifying Electrolytes and
Non-electrolytes
Guidelines for Writing Net Ionic Equations
Ionization Constants for Acids and Bases
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Aqueous EquilibriaElectrolytic Character

• Edward A. Mottel
• Department of Chemistry
• Rose-Hulman Institute of Technology

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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 the amount of AgCl which
dissolves ionizes extensively. (strong
electrolyte)
This indicates that AgCl isnt very
soluble. (sparingly soluble)
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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 the concentration of the
solute in the solvent
Categorize each of the following compounds in
terms of its solubility and tendency to form ions.
Dissociation is related to the ability of the
compound to form ions in solution
oil NaCl sugar acetic acid Ca(OH)2
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Electrolytic Character ¹ SolubilityExamples

• Oil -- not very soluble, doesnt 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 the concentration of the
solute in the solvent
Dissociation is related to the ability of the
compound to form ions in solution
NaCl
Ca(OH)2
acetic acid
CO2
oil
sugar
The solubility of a compound does not predict its
electrolytic character.
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Equation Notation

• Each compound in solution is considered in its
  predominant form with proper annotation

• 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

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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
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Buff Colored SheetCompetency Assignment
Complete Experiment F in the laboratory 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
Rank the following compounds in terms of acid
strength (strongest to weakest).

• acetic acid
• phosphoric acid
• nitrous acid

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

• H3PO4 7.5 x 103

HNO2 4.5 x 104
CH3COOH 1.8 x 105
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Numeric Values ofIonization Constants

• Describe this reaction in a sentence.

• 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 ofIonization Constants
Ka 7.2 x 104 M
K 1 / 7.2 x 104 M1
1.4 x 103 M1
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Numeric Values ofEquilibrium 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)
Ka,1 4.2 x 107 M
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Carbonic Acid
Carbonic acid has more than one acidic proton,
and each ionization has its own constant.
Ka,2 4.8 x 1011
Write the mass-action expression for the second
ionization of carbonic acid.
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Carbonic Acid
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Determining UnlistedEquilibrium Constants
What is the numeric value of the equilibrium
constant for this reaction?
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Determining UnlistedEquilibrium Constants
What is the numeric value of the equilibrium
constant for this reaction?
The equilibrium constant is equal to the
mass-action expression at equilibrium, so write
the mass-action expression for this reaction.
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Determining UnlistedEquilibrium Constants
How is this ratio related to the
other mass-action expressions?
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Determining UnlistedEquilibrium Constants
What is similar, what is different?
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Determining UnlistedEquilibrium Constants

2.0 x 1017 M2
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Chemical EquationsCan Be Treated Algebraically
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Numeric Values ofDissociation Constants

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

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Dissociation of Ammonium Ion
(target)
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Dissociation of Ammonium Ion
(target)
What other equation can be added to the first
equation to give the target equation?
What is the numeric value of the equilibrium
constant?
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Assignments
Equilibrium Constant Determination Problem Set
Complete Experiment F in the laboratory manual
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Group Activity
Rank the following species in terms of acid
strength (strongest to weakest).

• hydrogen peroxide
• water
• ammonium ion

hydrogen peroxide
water
ammonium ion
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