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Acids and Bases. GLY 4241 - Lecture Fall, 2014. Acids. Hydronium ion:. Acids are substances that dissociate to donate protons (Brønsted definition) General Formula: H 2n+1 O n +. Lone Pair Acceptor. Lewis definition of an acid is any substance that accepts a lone pair of electrons

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acids and bases

Acids and Bases

GLY 4241 - Lecture

Fall, 2014


Hydronium ion:

Acids are substances that dissociate to donate protons (Brønsteddefinition)

General Formula: H2n+1On+

lone pair acceptor
Lone Pair Acceptor
  • Lewis definition of an acid is any substance that accepts a lone pair of electrons
  • Important when studying catalysis
acidic rocks
“Acidic” Rocks
  • Term means any rock containing an excess of nonmetallic oxide (principally silica) over metallic oxides
    • Many nonmetallic oxides (CO2, SO2, SO3, NO2, etc.) will dissolve in water to yield acids (H2CO3, H2SO4, etc.) and this is the reason that nonmetallic oxides were originally called acidic
    • However, SiO2 is almost insoluble in water and therefore the term, acidic, applied to silica is very misleading.
attempt to correct
Attempt to Correct
  • An effort to replace the term acidic by the word felsic occurred
  • The older literature still contains references to acidic rocks, however
  • Many geologists continued to use the term acidic
  • So the word has now been redefined as weight percent silica content, not referring to the pH of any solution in contact with the rock
  • Bases yield hydroxide ion to solution (classical)
  • Brønsted modified this definition to be that of a proton acceptor
basic and ultrabasic rocks
Basic and Ultrabasic Rocks
  • The terms basic and ultrabasic rocks also exist in the older literature
  • These terms describe rocks with an excess or a large excess of metallic oxides (MgO, CaO, FeO, etc.) over nonmetallic oxides
  • Today these terms are replaced by the words mafic and ultramafic
  • The terms basic and ultrabasic may be used with the silica weight percent classification of igneous rocks, however
  • Alkaline was and is used for rocks with an excess of Na2O or K2O over SiO2
dissociation constant of water
Dissociation Constant of Water
  • pH is the negative log of the hydrogen ion concentration
  • For water:
strong acid
Strong Acid
  • Strong acids dissociate completely in water
  • Thus a 0.1M solution of nitric acid will have [H+] = 0.1 = 10-1
strong base
Strong Base
  • A strong base will dissociate completely to yield OH- ions
  • Thus a 0.01M solution of NaOH will have [OH-] = 0.01 =10-2
  • If [OH-] = 10-2,
ionization constant
Ionization Constant
  • Many substances do not dissociate completely in water including weak acids, weak bases, and salts
  • For these substances an ionization constant similar to that for water is needed
  • An example is hydrofluoric acid.
    • KHF = 10-3.2
multifunctional acid groups
Multifunctional Acid Groups
  • First ionization constant
multifunctional acid groups 2
Multifunctional Acid Groups 2
  • Second ionization constant
multifunctional acid groups 3
Multifunctional Acid Groups 3
  • Third ionization constant
  • Five species (H+, PO43-, HPO42-, H2PO4-, H3PO4) will be present at equilibrium
charge balance
Charge Balance
  • There are more unknowns than equations, so we need additional equations
  • These can be charge balance equations:
total phosphate concentration
Total Phosphate Concentration
  • Or, if the substance is totally dissolved and the total concentration of phosphate is known
  • If the substance did not dissolve totally then a solubility product equation could be set up
multifunctional base groups
Multifunctional Base Groups
  • First ionization constant
  • Similar expressions hold for the second and third constants
  • K2 is weaker than K1 by a factor of 10-4- 10-6
  • Thus the total concentration of H+ or OH- is, for practical purposes, given by K1 alone
use of approximation
Use of Approximation
  • For a 0.01M solution of H3PO4:
approximation failure
Approximation Failure
  • If K1 is large (≥ 10-2) this approximation begins to fail because [H+] is not small in comparison to [HnA]
  • On the other hand, if the concentration is too low then the [H+] calculated will be less than that for pure water, and this approach fails again
ionization of salt
Ionization of Salt
  • Multiple ionization constants in salts
strong base weak acid
Strong Base & Weak Acid
  • The salt of a strong acid and a weak base or a strong base and a weak acid often produce an acidic or a basic solution, respectively.
  • Li+ will not interact with either H+ or OH- ions
  • Li+ ions do not affect the pH of the solution
strong base weak acid 2
Strong Base & Weak Acid 2
  • CO32- can and does interact with H+
weak base and strong acid
Weak Base and Strong Acid
  • This reduces the [OH-] and makes the solution acidic
heavy metal salts
Heavy Metal Salts
  • Soluble salts of heavy metal ions will produce acidic solutions