Acids and Bases
Acid-Base Theories
Arrhenius Acids and Bases (1884)
An acid is a substance that, when dissolved in water, increases the
concentration of hydrogen ions.
A base is a substance that, when dissolved in water, increases the
concentration of hydroxide ions.
H3O+(aq) + OH-(aq)
H2O(l) + H2O(l)
H3O+ and an anion.
A strong base is a substance that completely ionizes in aqueous solutions to give a
cation and a hydroxide ion.
NaOH(aq)
H2O
Na+(aq) + OH-(aq)
Bronsted Lowry Acid and Bases (1923) defines an acid and a base according to
its function in a proton transfer reaction.
Acid-base reactions can be seen as a proton transfer reaction.
An acid is a species donating a proton in a proton transfer reaction.
A base is a proton acceptor in a proton transfer reaction.
H+(aq) + Cl-(aq) + NH3(aq)
H2O(l) + NH4+(aq) + Cl-(aq)
Amphiprotic species is a species that can act as either an acid or a base depending
on the other reactant.
NH3(aq) + H2O(l)
CH3COOH(aq) + H2O(l)
NH4+(aq) + OH-(aq)
CH3COO-(aq) + H3O(aq)
Lewis Acids and Bases
An electron pair theory of covalent bonding for acid and bases.
Lewis acid is a species that can form a covalent bond by accepting an
electron pair from another species.
Lewis base is a species that can form a covalent bond by donating an
electron pair to another species.
NH3(aq) + H2O(l)
NH4+(aq) + OH-(aq)
Relative Strength of Acids and Bases
An acid base reaction normally goes in the direction of the weaker acid
The strongest acids have the weakest conjugate bases, and the strongest bases have
the weakest conjugate acids.
SO4-2(aq) + HCN(aq)
HSO4-(aq) + CN-(aq)
Molecular Structure and Acid Strength
The strength of an acid depends on how easily the hydrogen ion is lost or removed.
Two factors are important in determining this:
1. The polarity of the bond
2. The strength of the bond
The more polarized the bond is, the more easily the proton is removed.
High electron density is in red
and low electron density is in
blue.
The tighter the proton is held, the
greater the strength of the bond.
The larger the atom in the bond,
the weaker the bond and the
greater the acid strength.
The size of the atom is the dominant factor in determining the acid strength.
Following a column in the periodic table - as you go down the column of elements,
the size of atom X increases, the H - X bond strength decreases, and the strength of
the binary increases.
HF < HCl < HBr < HI
Going across a row of elements of the periodic table, the electronegativity
increases, the H - X bond polarity increases, and the acid strength increases.
Oxoacids have the structure H -O-Y . The acidic hydrogen is always attached to an
O atom, which in turn, is attached to an atom Y. Bond polarity appears to be the
dominant factor determining the relative strengths of the oxoacids
For a series of oxoacids of the same structure, differing only in the atom Y, the
acid strength increases with the electronegativity of Y.
Acid strength of oxoacids
Relative strength of polyprotic acids is dependent on the strength of the polyprotic
acid and its corresponding acid anions. The strength of the acids and its anion
decreases with increasing negative charge.
Self Ionization of water, or autoionization is a reaction in which two water
molecules react to give ions.
H2O(l) + H2O(l)
H3O+(aq) + OH-(aq)
Ion product constant for water, Kw, is the equilibrium value for the ion product
Kw = [H3O+] [OH-] = 1.0 x 10 -14 at 25ºC
Suppose you dissolve 0.10 mol HCl in 1.0 L of aqueous solution. You would like
to know the H3O+ concentration in this solution.
HCl(aq) + H2O(l)
H3O+(aq) + Cl-(aq)
Example: Calculate the concentrations hydronium ion and hydroxide ion at 25C
in: a. 0.15 M HNO3, b. 0.010 M Ca(OH)2
In an acidic solution, [H3O+] > 1.0 x 10-7 M
In a neutral solution, [H3O+] = 1.0 x 10-7 M
In a basic solution, [H3O+] < 1.0 x 10-7 M
Because these concentrations are very small, it is more convenient to give the
acidity in terms of pH
pH is the negative of the logarithm of the molar hydronium ion concentration
pH = - log [H3O+]
Example:
What is the pH of a sample of gastric acid whose hydronium concentration is 0.045 M?
A saturated solution of calcium hydroxide has a hydroxide ion concentration of 0.025 M. What is
the pH of the solution?