Acids and Bases GLY 4241 - Lecture Fall, 2016 The classical definition of an acid is a substance that dissociates to yield free protons:

Acids Hydronium ion: Later it was realized that existing in water was impossible for a “bare” proton (no electrons). Instead the "hydronium" ion is formed: (middle equation) In reality the hydronium ion is not only H3O+, but rather, some hydrated form with the general formula (lower equation) Nitric acid is a stronger acid than water. Therefore nitric acid acts as an acid in the presence of water by donating a proton to water and forming the hydronium ion. Brønsted defined an acid to be a "proton donor." Acids are substances that dissociate to donate protons (Brønsted definition) General Formula: H2n+1On+

Lone Pair Acceptor Lewis definition of an acid is any substance that accepts a lone pair of electrons Important when studying catalysis

“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. In geology the term "acidic rock" has long been used

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 yield hydroxide ion to solution (classical) Brønsted modified this definition to be that of a proton acceptor Here, the base or proton acceptor, KOH, has accepted a proton from the acid (H2O) and split, leaving K+ and H2O; the original water molecule has become an OH-. Lewis defined a base as a lone pair donor, again unimportant in geology.

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 of Water Water can be thought of as either an acid or a base. When pure it will dissociate as follows: (upper equation) Water's dissociation or ionization constant is: (middle equation) Dissociation constants are another example of a specific type of ionization constant. The value listed in equation 8-6 is temperature dependent and is strictly applicable only around room temperature. In a neutral solution: (Lower equation)

Dissociation Constant of Water pH is the negative log of the hydrogen ion concentration For water: The pH is the negative log of the hydrogen ion concentration. If [H+] = 10-7: In acidic solutions [H+] increases and [OH-] decreases, to maintain the value of 10-14 for the product. This means that the value of pH decreases in acidic solutions and increases in basic solutions.

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 A strong base will dissociate completely to yield OH- ions Thus a 0.01M solution of NaOH will have [OH-] = 0.01 =10-2 A strong base will dissociate completely to yield OH- ions. Thus a 0.01M solution of NaOH will have [OH-] = 0.01 =10-2. (Equations)

p(OH) If [OH-] = 10-2, An analogous function is p(OH) defined as: (upper equation) Lower equation If [H+] = 0.1 then pH = 1 and p(OH) = 13. Normally, pH is perfectly adequate for describing a system, but the p(OH) form is sometimes used in the literature.

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

Weak Acids and Bases Many acids and bases have more than one functional group (acid or base) and thus have more than one dissociation constant. Examples: Phosphoric acid is H3PO4.

Multifunctional Acid Groups First ionization constant

Multifunctional Acid Groups 2 The second ionization requires pulling a positive ion (H+) from a negative cation (H2PO4-) which is much more difficult: Second ionization constant

Multifunctional Acid Groups 3 The third ionization is more difficult yet: To specify the concentration of each of these five species, more equations are needed. Third ionization constant Five species (H+, PO43-, HPO42-, H2PO4-, H3PO4) will be present at equilibrium

Charge Balance There are more unknowns than equations, so we need additional equations These can be charge balance equations:

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 Solving sets of simultaneous equations is possible, such as the five for H3PO4, if the number of equations equals or exceeds the number of variables. However the procedure is tedious and approximations may be tried. Any approximations made must be checked against the results to be sure they are valid. First ionization constant Similar expressions hold for the second and third constants

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 For a 0.01M solution of H3PO4:

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 Multiple ionization constants in salts

Salts Which Produce Neutral Solutions The solution of a salt in water may give a neutral solution. This will happen when the salt is the product of a strong acid and a strong base (equations) The ions of such salts show no tendency to react with either H+ or OH-, so the resulting solution is neutral.

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 CO32- can and does interact with H+ The CO3-2 ions unite with some H+ ions. Water dissociation equilibrium must be maintained so the [OH-] goes up as [H+] goes down. Therefore, the pH increases and the solution is basic.

Weak Base and Strong Acid This reduces the [OH-] and makes the solution acidic Ammonia is a weak base: (equations) This type of process is known as hydrolysis. Certain salts will produce either acid or basic solutions. Soluble sulfides (S2-) or carbonates (CO32-) produce basic solutions. Soluble salts of heavy metal ions will produce acidic solutions.

Heavy Metal Salts Soluble salts of heavy metal ions will produce acidic solutions

Hydrolysis Constant (Kw must be raised to the coefficient of H+ or OH- if greater than 1).

Application of Hydrolysis Constant

Calculation of Hydrolysis Constant