Acid-Base
Reactions and
Proton
Accounting
Lecture 20
Water
• Water is virtually omnipresent at the surface of the Earth.
• Consequently, there is continual reaction between
water and materials at the surface (rocks, soil,
atmosphere, life).
• As a consequence of these reactions, water is never
pure (though often pure enough that we will find it
convenient to assume its concentration is 1M).
• We’ll now apply our tools of physical chemistry to the the
problem of aqueous solutions and their interaction with
the atmosphere and, particularly, the solid Earth.
Aquatic Reactions
• Acid-Base
H2CO3 ⇋ H+ + HCO3–
• Complexation
Hg2+ + H2O ⇋ Hg(OH)- + H+
• Dissolution/Precipitation
KAlSI3O8 + H+ + 7H2O ⇋ Al(OH)3 + K+ + 3H4SiO4
• Adsorption/Desorption
≡S + Mn2+ ⇋ ≡S–Mn
• We’ll consider each of these in turn.
Importance of Acid-Base
Reactions
• The hydrogen and hydroxide ions are often participants
in all the foregoing reactions.
• As a result, these reactions are pH-dependent.
o In order to characterize the state of an aqueous solution, that is, to determine
how much CaCO3 a solution will dissolve, the complexation state of metal ions,
or the redox state of Mn, the first step is usually to determine pH.
• On a larger scale, weathering of rock and precipitation
of sediments depend critically on pH.
• Thus pH is sometimes called the master variable in
aquatic systems.
• The concentration of OH– is also known from pH since
[OH–][H+] = 10-14
o at 25˚C. (Strictly speaking, it is the product of activities equal to 10-14. For
simplicity, we will often assume ideality.
Defining Acids and Bases
• Arrhenius defined an acid as a substance that upon solution in
water releases free protons. He defined a base is a substance
that releases hydroxide ions in solution.
• Chemists generally prefer the definition of Brønstead, who
defined acid and base as proton donors and proton
acceptors respectively.
• The strength of an acid or base is measured by its tendency to
donate or accept protons. The dissociation constant for an
acid or base is a quantitative measure of its strength. For
example, dissociation of HCl:
HCl ⇋ H+ + ClK Diss =
aH + aCl aHCl
= 10 3
• Thus is a strong acid because only about 3% remains
undissociated.
• In contrast, for
H2S ⇋ H+ + HS–
• Kdiss = 10-7; very few hydrogens generally dissociate (except in
very allkaline solution).
Amphoteric Behavior
• Metal hydroxides can either donate or accept protons,
depending upon pH. For example, we can represent this
in the case of aluminum as:
Al(OH)2+ + H+ ⇋ Al(OH)2+ +H2O
Al(OH)2+ + OH– ⇋ Al(OH)3
• Metals dissolved in water are always surrounded by
solvation shells. The positive charges of the hydrogens in
the surrounding water molecules are to some extent
repelled by the positive charge of the metal ion. For this
reason, water molecules in the solvation shell are more
likely to dissociate and give up a proton more readily
than other water molecules. Thus the concentration of
such species will affect pH.
Proton Accounting
• Knowing the pH of an aqueous system is the key to
understanding it and predicting its behavior. This
requires a system of accounting for the H+ and OH–
in the system. There are several approaches to
doing this.
o proton balance equation
o TOTH proton mole balance equation
Proton Balance Equation
• The concentration of all species whose genesis
caused the production of OH– are written on one
side, and the concentration of all species whose
genesis caused the production of H+ are written on
the other side.
• For water:
[H+] = [OH–]
• For
HNO3 = H+ + NO3[H+] = [OH–] + [NO3-]
Proton Mole Balance
Equation
• In the Morel & Hering system, H+ and H2O are always chosen
as components of the system but OH– is not. The species OH– is
the algebraic sum of H2O less H+
•
OH– = H2O – H+
• When an acid, such as HCl, is present we choose the
conjugate anion as the component, so that the acid HCl is
formed from components:
•
HCl = Cl- + H+
• For bases, such as NaOH, we choose the conjugate cation as
a component. The base, NaOH, is formed from components
as follows:
•
NaOH = Na+ + H2O - H+
• Because we are generally dealing with dilute solutions, we
assume XH2O = 1 or 55.4M (this is only 2-3% different in
seawater), H2O is an implicit component; presence assumed
by not written.
TOTH
• TOTH is the total amount of component H+, rather
than the total of the species H+.
•
o Every species containing H+ contributes positively to TOTH while every
species formed by subtracting H+ contributes negatively to TOTH.
For pure water: TOTH = [H+] - [OH–]
o Of course in pure water [H+] = [OH–] so TOTH = 0.
• Now we dissolve CaCO3 to our solution and chose
Ca2+ and CO32- as components.
•
o In near neutral pH, almost all the CO32- will react to form HCO3–:
CO3+ + H2O = HCO3- + OH–
o some Ca2+ (though generally not much) will form Ca(OH)+, so our mole
balance equation will be
TOTH = [H+] - [OH–] + [HCO3–] - [Ca(OH)+]
Since we have not added [H+], TOTH remains 0.
TOTH
• Now we dissolve CO2 in our solution:
• H2O + CO2 = H2CO3
o In near neutral pH, almost all the H2CO3 will react to form HCO3–:
H2CO3 ⇋ HCO3- + H+
o If we chose CO2 as our component,
HCO3– = CO2 + H2O - H+
TOTH = [H+] - [OH–] - [HCO3–]
• This time HCO3- contributes negatively.
•
•
Every species containing H+ contributes positively to TOTH while every
species formed by subtracting H+ contributes negatively to TOTH.
How we write the TOTH equation depends on how we defined
components.
• Since we have not added [H+], TOTH remains 0.