59
CHAPTER TWO
Chemical Basis of Life
Substances that combine with hydrogen ions are called
bases.
The compound sodium hydroxide (NaOH) in water
releases hydroxide ions (OH
), which can combine with hydro-
gen ions to form water. Thus, sodium hydroxide is a base:
NaOH
Na
+
+ OH
(
Note:
Some ions, such as OH
, consist of two or more atoms.
However, such a group usually behaves like a single atom
and remains unchanged during a chemical reaction.)
Bases can react with acids to neutralize them, forming
water and electrolytes called
salts.
For example, hydro-
chloric acid and sodium hydroxide react to form water and
sodium chloride:
HCl + NaOH
H
2
O + NaCl
Table 2.4
summarizes the three types of electrolytes.
Acid and Base Concentrations
Concentrations of acids and bases affect the chemical reac-
tions that constitute many life processes, such as those con-
trolling breathing rate. Thus, the concentrations of these
substances in body fl
uids are of special importance.
Hydrogen ion concentration can be measured in grams of
ions per liter of solution. However, because hydrogen ion con-
centration can cover such a wide range (gastric juice has 0.01
grams H
+
/liter; household ammonia has 0.00000000001 grams
H
+
/liter), a shorthand system called the
pH scale
is used. This
system tracks the number of decimal places in a hydrogen ion
concentration without writing them out. For example, a solu-
tion with a hydrogen ion concentration of 0.1 grams per liter
has a pH of 1.0; a concentration of 0.01 g H
+
/L has pH 2.0;
0.001 g H
+
/L is pH 3.0; and so forth. Each whole number on
the pH scale, which extends from 0 to 14, represents a tenfold
difference in hydrogen ion concentration. As the hydrogen ion
concentration increases, the pH number decreases. For exam-
ple, a solution of pH 6 has ten times the hydrogen ion concen-
tration as a solution with pH 7. Small changes in pH can refl ect
large changes in hydrogen ion concentration.
In pure water, which ionizes only slightly, the hydrogen
ion concentration is 0.0000001 g/L, and the pH is 7.0. Water
ionizes to release equal numbers of acidic hydrogen ions and
basic hydroxide ions, so it is
neutral.
H
2
O
H
+
+ OH
or reactants. A
reversible reaction
is symbolized using a
double arrow:
A + B
AB
Whether a reversible reaction proceeds in one direc-
tion or another depends on the relative proportions of reac-
tant (or reactants) and product (or products) as well as the
amount of energy available.
Catalysts
(kat
ah-listz) are mol-
ecules that infl uence the rates (not the direction) of chemical
reactions but are not consumed in the process.
Acids, Bases, and Salts
When ionically bound substances are placed in water, the
ions are attracted to the positive and negative ends of the
water molecules and tend to leave each other, or dissociate.
In this way, the polarity of water dissociates the salts in the
internal environment. Sodium chloride (NaCl), for example,
ionizes into sodium ions (Na
+
) and chloride ions (Cl
) in
water
(f g. 2.9)
. This reaction is represented as
NaCl
Na
+
+ Cl
The resulting solution has electrically charged par-
ticles (ions), so it conducts an electric current. Substances
that release ions in water are, therefore, called
electrolytes
(e-lek
tro-lıˉtz). Electrolytes that dissociate to release hydro-
gen ions (H
+
) in water are called
acids
(as
idz). For exam-
ple, in water, the compound hydrochloric acid (HCl) releases
hydrogen ions (H
+
) and chloride ions (Cl
):
HCl
H
+
+ Cl
Salt crystal
Ions in
solution
Na
+
Na
+
Cl
Cl
TABLE
2.4
|
Types of Electrolytes
Characteristic
Examples
Acid
Substance that releases
hydrogen ions (H
+
)
Carbonic acid, hydrochloric acid,
acetic acid, phosphoric acid
Base
Substance that releases
ions that can combine with
hydrogen ions
Sodium hydroxide, potassium
hydroxide, magnesium
hydroxide, sodium bicarbonate
Salt
Substance formed by the
reaction between an acid and
a base
Sodium chloride, aluminum
chloride, magnesium sulfate
FIGURE 2.9
The polar nature of water molecules dissociates
sodium chloride (NaCl) in water, releasing sodium ions (Na
+
) and
chloride ions (Cl
).
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