8. ACIDS AND BASES
The term acid is derived from the latin word acidus (sour), which is related to acer
(sharp) and acetum (vinegar). Vinegar is simply an aqueous solution of acetric acid. The
term alkali (base) is derived from the Arabic al-qali, which describes the ashes of certain
plants associated with salt marshes and desert regions. Until relatively recent times, the
principal source of bases or alkalis has been wood ashes. Also known for a very long
time is that acids and bases can neutralize each other’s distinctive properties and
produce a type of substance known as a salt.
The properties that are most commonly associated with acids are: their sour taste, the
prickling sensation they produce on the skin, their ability to react with most metals to
form hydrogen gas (H2), and their ability to dissolve limestone and other carbonate
minerals form carbon dioxide (CO2). Their solutions conduct electricity because acids
form ions when dissolved in water.
All bases also share several common properties: They have a bitter taste and their
solutions feel slippery like soapy water (basic properties commonly found in soap and
other household cleaners). Solutions of bases also conduct electricity because they, too,
form ions in water.
Both acids and bases have the ability to affect the colour of certain naturally occurring
plant constituents. For example, litmus, obtained from lichens, is red in acidic solutions
and blue in basic solutions.
ARRHENIUS DEFINITION OF ACIDS AND BASES
Arrhenius proposed that:
+
Acids are substances that dissociate to produce hydrogen ions (H ).
-
Bases are substances that dissociate to produce hydroxide ions (OH ).
Chemical and electrical activity of the acids and bases are directly related to the extent
to which these H+ and OH– are present in the solution.
Some acids/ bases are strong electrolytes while others are weak electrolytes. A strong
electrolyte is completely dissociated into ions in a water solution; a weak electrolyte is
only partially ionized.
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, Examples:
(1) When HCl gas dissolves in water, it is almost completely ionized (hardly any
reverse reaction).
HCl → H+ + Cl– , Kc very large
(2) When sulfuric acid dissolves in water, it is almost completely ionized (hardly any
reverse reaction). H2SO4 → H+ + HSO4 – , Kc very large
(3) When acetic acid dissolves in water, most of it remains as undissociated molecule.
Concentration of H+ is very small.
CH3COOH H+ + CH3COO –
[H ][CH 3 COO ]
Kc = = 1.8x10 -5 . Kc is very low and it is a weak acid.
[CH 3 COOH]
(4) NaOH, KOH and barium hydroxide Ba(OH)2 are strong bases, almost 100%
ionized in solution.
NaOH → Na+ + OH–
KOH → K+ + OH–
Ba(OH)2 → Ba2+ + 2 OH–
(5) Ammonia is a weak electrolyte and a weak base. Most of ammonia remains as NH3
molecule dissolved in water.
NH3 + H2O NH4+ + OH–
[NH 4 ][OH ]
Kc = = 1.8x10 –5
[NH 3 ]
Neutralization
This refers to the reaction of an acid and a base yielding water and a salt. It involves the
combination of hydrogen and hydroxide ions to form water.
For reactions between strong acid and strong base: For example,
HCl + NaOH → H2O + NaCl
Since HCl and NaOH are almost 100% ionized, the above can be written in hydrated
ionic form,
Page 2 of 8
The term acid is derived from the latin word acidus (sour), which is related to acer
(sharp) and acetum (vinegar). Vinegar is simply an aqueous solution of acetric acid. The
term alkali (base) is derived from the Arabic al-qali, which describes the ashes of certain
plants associated with salt marshes and desert regions. Until relatively recent times, the
principal source of bases or alkalis has been wood ashes. Also known for a very long
time is that acids and bases can neutralize each other’s distinctive properties and
produce a type of substance known as a salt.
The properties that are most commonly associated with acids are: their sour taste, the
prickling sensation they produce on the skin, their ability to react with most metals to
form hydrogen gas (H2), and their ability to dissolve limestone and other carbonate
minerals form carbon dioxide (CO2). Their solutions conduct electricity because acids
form ions when dissolved in water.
All bases also share several common properties: They have a bitter taste and their
solutions feel slippery like soapy water (basic properties commonly found in soap and
other household cleaners). Solutions of bases also conduct electricity because they, too,
form ions in water.
Both acids and bases have the ability to affect the colour of certain naturally occurring
plant constituents. For example, litmus, obtained from lichens, is red in acidic solutions
and blue in basic solutions.
ARRHENIUS DEFINITION OF ACIDS AND BASES
Arrhenius proposed that:
+
Acids are substances that dissociate to produce hydrogen ions (H ).
-
Bases are substances that dissociate to produce hydroxide ions (OH ).
Chemical and electrical activity of the acids and bases are directly related to the extent
to which these H+ and OH– are present in the solution.
Some acids/ bases are strong electrolytes while others are weak electrolytes. A strong
electrolyte is completely dissociated into ions in a water solution; a weak electrolyte is
only partially ionized.
Page 1 of 8
, Examples:
(1) When HCl gas dissolves in water, it is almost completely ionized (hardly any
reverse reaction).
HCl → H+ + Cl– , Kc very large
(2) When sulfuric acid dissolves in water, it is almost completely ionized (hardly any
reverse reaction). H2SO4 → H+ + HSO4 – , Kc very large
(3) When acetic acid dissolves in water, most of it remains as undissociated molecule.
Concentration of H+ is very small.
CH3COOH H+ + CH3COO –
[H ][CH 3 COO ]
Kc = = 1.8x10 -5 . Kc is very low and it is a weak acid.
[CH 3 COOH]
(4) NaOH, KOH and barium hydroxide Ba(OH)2 are strong bases, almost 100%
ionized in solution.
NaOH → Na+ + OH–
KOH → K+ + OH–
Ba(OH)2 → Ba2+ + 2 OH–
(5) Ammonia is a weak electrolyte and a weak base. Most of ammonia remains as NH3
molecule dissolved in water.
NH3 + H2O NH4+ + OH–
[NH 4 ][OH ]
Kc = = 1.8x10 –5
[NH 3 ]
Neutralization
This refers to the reaction of an acid and a base yielding water and a salt. It involves the
combination of hydrogen and hydroxide ions to form water.
For reactions between strong acid and strong base: For example,
HCl + NaOH → H2O + NaCl
Since HCl and NaOH are almost 100% ionized, the above can be written in hydrated
ionic form,
Page 2 of 8
