Electrolytic Properties of Aqueous Solutions
Difference between Solution, solute, solvent
** A solution
is a homogeneous mixture of two or more substances.
** The solute
is the substance present in a smaller amount, and the solvent is the substance
present in a larger amount.
** A solution
may be gaseous (such as air), solid (such as an alloy), or liquid (seawater,
for example).
Difference between electrolytes and nonelectrolytes.
** In this subject
we will discuss only aqueous solutions, in which the solute initially is a
liquid or a solid and the solvent is water.
** All solutes
that dissolve in water fit into one of two categories: electrolytes and nonelectrolytes.
** An
electrolyte is a substance that, when dissolved in water, results in a solution
that can conduct electricity.
** A
nonelectrolyte does not conduct electricity when dissolved in water .
An Experiment distinguishes between electrolytes and nonelectrolytes
** Figure (1)
shows an easy and straightforward method of distinguishing between electrolytes
and nonelectrolytes.
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Figure (1) |
** A pair of
inert electrodes (copper or platinum) is immersed in a beaker of water.
** To light the
bulb, electric current must flow from one electrode to the other, thus
completing the circuit.
** Pure water
is a very poor conductor of electricity. However, if we add a small amount of sodium
chloride (NaCl), the bulb will glow as soon as the salt dissolves in the water.
** Solid NaCl,
an ionic compound, breaks up into Na+ and Cl- ions when
it dissolves in water. The Na+ ions are attracted to the negative electrode,
and the Cl- ions to the positive electrode. This movement sets up an
electric current that is equivalent to the flow of electrons along a metal
wire.
** Because the
NaCl solution conducts electricity, we say that NaCl is an electrolyte. Pure
water contains very few ions, so it cannot conduct electricity.
** Comparing the
lightbulb’s brightness for the same molar amounts of dissolved substances helps
us distinguish between strong and weak electrolytes. A characteristic of strong
electrolytes is that the solute is assumed to be 100 percent dissociated into
ions in solution. (By dissociation we mean the breaking up of the compound into
cations and anions.) Thus, we can represent sodium chloride dissolving in water
as:
** This equation says that all sodium chloride
that enters the solution ends up as Na+ and Cl- ions;
there are no undissociated NaCl units in solution.
Classification of Solutes in Aqueous Solution
** Solutes are divided according to Electrolytic Properties to strong electrolytes, weak
electrolytes, and nonelectrolytes.
** Figure (1) shows
An arrangement for distinguishing between strong electrolytes, weak electrolytes
and nonelectrolytes.
** A solution’s
ability to conduct electricity depends on the number of ions it contains:
(a) A
nonelectrolyte solution does not contain ions, and the lightbulb is not lit.
(b) A weak
electrolyte solution contains a small number of ions, and the lightbulb is
dimly lit.
(c) A strong electrolyte
solution contains a large number of ions, and the lightbulb is brightly lit.
The molar amounts
of the dissolved solutes are equal in all three cases.
** The
following table shows lists examples of strong electrolytes, weak electrolytes,
and nonelectrolytes.
** Ionic
compounds, such as sodium chloride, potassium iodide (KI), and calcium nitrate [Ca(NO3)2], are strong electrolytes. It is interesting to note that human
body fluids contain many strong and weak electrolytes.
Water is a very effective solvent for ionic compounds
** Water is a
very effective solvent for ionic compounds. Although water is an electrically neutral
molecule, it has a positive region (the H atoms) and a negative region (the O
atom), or positive and negative “poles”; for this reason it is a polar solvent.
** When an
ionic compound such as sodium chloride dissolves in water, the three-dimensional
network of ions in the solid is destroyed. The Na+ and Cl-
ions are separated from each other and undergo hydration, the process in which
an ion is surrounded by water molecules arranged in a specific manner. Each Na+
ion is surrounded by a number of water molecules orienting their negative poles
toward the cation. Similarly, each Cl- ion is surrounded by water molecules with
their positive poles oriented toward the anion (Figure 2). Hydration helps to
stabilize ions in solution and prevents cations from combining with anions.
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Figure (2) |
Acids and bases are electrolytes
** Some acids,
including hydrochloric acid (HCl) and nitric acid (HNO3 ), are
strong electrolytes. These acids are assumed to ionize completely in water; for
example, when hydrogen chloride gas dissolves in water, it forms hydrated H+
and Cl- ions:
** In other
words, all the dissolved HCl molecules separate into hydrated H+ and
Cl- ions. Thus, when we write HCl (aq), it is understood that it is
a solution of only H+ (aq) and Cl- (aq) ions and that
there are no hydrated HCl molecules present.
** On the other
hand, certain acids, such as acetic acid (CH3COOH), which gives vinegar
its tart flavor, do not ionize completely and are weak electrolytes. We represent
the ionization of acetic acid as:
where CH3COO-
is called the acetate ion.
** We use the
term ionization to describe the separation of acids and bases into ions. By
writing the formula of acetic acid as CH3COOH, we indicate that the
ionizable proton is in the COOH group.
** The
ionization of acetic acid is written with a double arrow to show that it is a reversible
reaction; that is, the reaction can occur in both directions. Initially, a number
of CH3COOH molecules break up into CH3COO- and
H+ ions. As time goes on, some of the CH3COO-
and H+ ions recombine into CH3COOH molecules. Eventually,
a state is reached in which the acid molecules ionize as fast as the ions
recombine. Such a chemical state, in which no net change can be observed
(although activity is continuous on the molecular level), is called chemical
equilibrium.
** Acetic acid,
then, is a weak electrolyte because its ionization in water is incomplete. By
contrast, in a hydrochloric acid solution the H+ and Cl-
ions have no tendency to recombine and form molecular HCl. We use a single
arrow to represent complete ionizations.
Reference: Chemistry / Raymond Chang ,Williams College /(10th edition).
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