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Strong and Weak Acids and Bases: Assignment # 11
Definitions
 An Arrhenius acid is a compound that contains hydrogen and releases hydrogen ions (H +) in water. A
Brønsted acid is a proton donor (where a proton is a hydrogen ion, H +). A Lewis acid is an electron pair
acceptor.
 An Arrhenius base is a compound that produces hydroxide ions (OH –) in water. A Brønsted base is a
proton acceptor. A Lewis base is an electron pair donor.
Strong Acids
Strong acids are 100% ionized in aqueous solution to form the hydronium ion, H3O+ (also written as H+(aq))
and an anion. For example, HCl in water ionizes completely:
HCl + H2O  H3O+(aq) + Cl–(aq) [goes to completion]
(or, equivalently, HCl + water  H+(aq) + Cl–(aq) [goes to completion])
There are very few strong acids, but they are extremely important in chemistry since they are excellent sources of
H+(aq), a highly reactive ion!
Strong Acid Examples: HCl (hydrochloric acid), HBr (hydrobromic acid), HI (hydroiodic acid), HNO 3 (nitric acid),
H2SO4 (sulfuric acid), HClO4 (perchloric acid), and a small number of non-metallic oxides which react with water
to give a strong acid (eg., SO3(g) + H2O  H2SO4).
Weak Acids
Most acids are weak. Weak acids are typically less than 5% ionized in water; thus the predominant species is
the un-ionized form. Since relatively small amounts of H+(aq) are formed, weak acids are not very reactive.
Typical weak acid ionizations in water are
HC2H3O2 + H2O  H3O+(aq) + C2H3O2–(aq)
(or, equivalently, HC2H3O2 + water  H+(aq) + C2H3O2–(aq))
SO2(g) + H2O  H2SO3  H+(aq) + HSO3–(aq)
(or, equivalently, SO2(g) + 2 H2O  H3O+(aq) + HSO3–(aq))
In each case above, reaction proceeds only to a very limited extent; typically over 95% of the weak acid remains
un-ionized! Since the predominant form is un-ionized, chemists do not split up weak acids into ions when
writing an ionic equation.
Weak Acid Examples:
 Molecular compounds with an acidic hydrogen: HC2H3O2 = CH3COOH (acetic acid), HF (hydrofluoric acid),
HNO2 (nitrous acid), HCN (hydrocyanic acid), C6H5COOH (benzoic acid).
 Non-metallic oxides: SO2(g) (sulfur dioxide), CO2(g) (carbon dioxide), and NO(g) (nitrogen oxide). A few
non-metallic oxides such as SO3(g) (sulfur trioxide) and N2O5(g) (dinitrogen pentoxide) give strong acids when
dissolved in water. Acidic oxides of S and N are major contributors to acid rain.
 Most cations are acidic. Examples include H+, ammonium ion (NH4+), and amine-type cations such as
CH3NH3+. Numerous metallic cations are acidic; the more acidic metal cations are those such as Be +2, Al+3,
and Fe3+ which have a high charge to radius ratio.
 Anions of type HX- which are conjugate to strong or moderately strong acids H2X. An example is HSO4(conjugate base of H2SO4) which is a moderately strong acid. This is a tiny class!!
Strong Bases
Strong bases are 100% ionized in aqueous solution to form the hydroxide ion, OH–, and a cation. There are
very few strong bases, but they are extremely important in chemistry since they are excellent sources of OH –(aq), a
highly reactive ion! Typical ionization reactions are
NaOH(s) + water  Na+(aq) + OH–(aq) [goes to completion]
Na2O(s) + H2O  2 Na+(aq) + 2 OH–(aq) [goes to completion]
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Strong and Weak Acids and Bases: Assignment # 11
Strong Base Examples:
 Alkali metal hydroxides and the more soluble alkaline earth hydroxides: NaOH(s) (sodium hydroxide), KOH(s)
(potassium hydroxide), Ba(OH)2(s) (barium hydroxide).
 Alkali metal oxides and the more soluble alkaline earth oxides: Na2O(s) (sodium oxide), K2O(s) (potassium
oxide), BaO(s) (barium oxide).
 The less soluble hydroxides and oxides of the alkaline earth cations are weak bases. Since solubility increases
for these compounds as you go down Column II, the hydroxides and oxides of Ba 2+ and Sr2+ are generally
considered strong bases, while those for Ca2+ are on the borderline between strong and weak due to their
limited solubility in water.
Weak Bases
The vast majority of bases are weak. Much like weak acids, weak bases are typically less than 5% ionized.
Since their water solutions contain low concentrations of OH–(aq), they are not very reactive. Examples of weak
base ionization reactions include
NH3 + H2O  NH4+(aq) + OH–(aq)
CH3NH2 + H2O  CH3NH3+(aq) + OH–(aq)
Cu(OH)2(s) + water  Cu2+(aq) + 2 OH–(aq)
CuO(s) + H2O  Cu2+(aq) + 2 OH–(aq)
In each case above, reaction proceeds only to a very limited extent; typically over 95% of the weak base remains unionized! Weak bases are therefore not split up into ions when writing ionic equations.
Weak Base Examples
 Metal hydroxides and metal oxides other than those listed as strong bases above. Examples include
Mg(OH)2(s) (magnesium hydroxide), MgO(s) (magnesium oxide), and Cu(OH) 2(s) (copper(II) hydroxide).
 Ammonia (NH3) and amine-type bases such as CH3NH2. Typical amine-type bases have an ammonia-type
structure, but with one or more of the H atoms replaced by a hydrocarbon group.
 Most anions are basic. Common examples include HCO3–(aq) (hydrogen carbonate or bicarbonate ion) which
is present in sodium bicarbonate (NaHCO3(s)) and CO32–(aq) (carbonate ion), a constituent of calcium
carbonate (CaCO3(s)).
Neutral Compounds and Ions
So many compounds and ions are acidic or basic, you may wonder whether anything is neutral! Examples of
neutral substances include:
 Water, H2O.
 Hydrocarbons, alcohols, sugars, starch, and many other organic molecules.
 The cations present in the strong hydroxide and oxide bases: Na+, K+, Ba2+, etc.
 Most anions produced upon ionization of the strong acids: Cl–, Br–, I–, NO3–, SO42–.
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