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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


Acids
Acids

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
Bases

  • 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


Acids and bases
p(OH)

  • 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


Approximations
Approximations

  • 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