Ch 4

• Arrhenius concept: Acids produce hydrogen ions in aqueous solution,

  and bases produce hydroxide ions

  • Limited application since acids and bases also occur in gaseous state.
• Bronsted–Lowry model: Acids are proton (H+) donors, and bases are

  proton acceptors. It’s a broader definition when compared to the

  Arrhenius one.
• HCl(g) + H2O(aq) - H3O+ + Cl

  • The proton is transferred from the HCl molecule to the water molecule

  • Leads to the formation of hydronium ion, H3O+ and chloride ion.
• Conjugate base: Remains of an acid molecule after it loses a proton
• Conjugate acid: It is the protonated base; formed when a proton is

  transferred to a base
• Conjugate acid–base pair: Two substances related to each other by accepting

  and donating a single proton
• Ka is the acid dissociation constant

  • Water is omitted from the acid dissociation reaction since it is a liquid

  with a constant concentration.

  • Does not affect the equilibrium position
• Strong acid: Equilibrium lies far to the right

  • Yields a weak conjugate base
•  Weak acid: Equilibrium lies far to the left

  • Weaker the acid, stronger its conjugate base
• Monoprotic acids: Contain one acidic proton

  • Examples - Hydrogen sulfate ion (HSO4

  –) and phenol (HOC6H5)
• Polyprotic acids: Composed of two or more acidic protons; example

  H3PO4 (aq)
•  Diprotic acids: Composed of two acidic protons

  • Example - Sulfuric acid [H2SO4 (aq)]

  • In Sulfuric acid the first proton Completely dissociate in water.
• Organic acids: Contain a carbon atom backbone

  • Contain the carboxyl group
  • Generally weak in nature

  • Examples - Acetic acid (CH3COOH) and benzoic acid (C6H5COOH)
•  Hydrohalic acids (HX):

  • Acidic proton is attached to an atom other than oxygen

  • X - Halogen atom
•  Oxyacids:

  • Acidic proton is attached to an oxygen atom

  • Examples - Nitric acid [HNO3(aq)] and phosphoric acid (H3PO4)
• • Water is an amphoteric substance

  • Amphoteric: Substance that behaves either as an acid or as a base.

  • Depicted in the autoionization of water
• Autoionization: Transfer of a proton from one water molecule to

  another to produce a hydroxide ion and a hydronium ion.
• One water molecule acts as an acid by furnishing a proton, and the

  other acts as a base by accepting the proton.
•  KW is temperature dependent
• • [H+] = [OH–]

  Neutral solution
• • [OH–] > [H+]

  Basic solution
• • [H+] > [OH–]

  Acidic solution
•  pOH = –log [OH–]
• Log scale based on 10, where

  pH = – log [H+]

  • pH decreases as [H+] increases
• pK = –log K
• pH is a log scale based on 10
• pH Meter:

  Electronic device with a probe that can be inserted into a solution of

  unknown pH
• Percent dissociation = amount dissociated (mol/L) / initial concentration(mol/L) = answer x 100
• For a solution of any weak acid HA:

  • Percent dissociation increases as [HA]0 decreases
• Dilution shifts equilibrium to the side with the greater

  number of particles (% dissociation increases)
• Triprotic acid: Have the ability to furnish three protons

  • Example - Phosphoric acid (H3PO4)

  • For a typical weak polyprotic acid, Ka1

  > Ka2

  > Ka3
• Polyprotic acids:• Acids that can furnish more than one proton

  • Dissociate one proton at a time

  • Conjugate base of the first dissociation equilibrium becomes the acid in the

  second step
• Salts: • Another name for ionic compounds

  • Disintegrate into ions when dissolved in water

  • Have independent movement in dilute solutions

  • Behave as acids or bases under certain conditions
• When dissolved in water, salts that consist of cations of strong bases

  and anions of strong acids have no effect on [H+]

  • Gives a neutral solution (pH = 7)

  • Examples - KCl, NaBr, KNO3, and LiNO3
•  A basic solution is formed if the anion of the salt is the conjugate base

  of a weak acid

  • Kb value for the anion can be obtained from the relationship

  Kb = Kw/Ka
• Ka > Kb = pH < 7 (acidic)

  Kb > Ka = pH > 7 (basic)

  Ka = Kb = pH = 7 (neutral)
• • Factors that determine whether a molecule containing an X—H bond

  behaves as a Brønsted–Lowry acid

  • Bond strength

  • Bond polarity

  • Classes of acids - Hydrogen halides, oxyacids, and hydrated metal ions
• Oxyacids :

  • Contain the grouping H—O—X

  • For a given series, the acid strength increases with an increase in the

  number of oxygen atoms attached to the central atom

  • Acid strength is successively greater as the number of oxygen atoms

  increases

  • Oxygen atoms become more effective at drawing electrons away from the X

  atom and the O—H bond

  • This actions weakens and polarizes the O—H bond
• Hydrated Metal Ions:

  • Exhibit behavior similar to oxyacids

  • Greater the charge on the metal ion, more acidic the hydrated ion

  becomes
• Acidic Strength in Oxides:

  • A compound containing the O—X group will produce:

  • An acidic solution in water if the O—X bond is strong and covalent

  • A basic solution in water if the O—X bond is ionic

  • High electronegativity of X will lead to covalent and strong O—X bond

  • Low electronegativity will lead to ionic and weak O—X bond