37 9.2 Solution Stoichiometry

In Binary Ionic Compounds and Their Properties we point out that when an ionic compound dissolves in water, the positive and negative

 originally present in the crystal lattice persist in solution. Their ability to move nearly independently through the solution permits them to carry positive or negative electrical charges from one place to another. Hence the solution conducts an electrical current.

Electrolytes

Substances whose solutions conduct electricity are called electrolytes. All soluble ionic compounds are

strong electrolytes

. They conduct very well because they provide a plentiful supply of

ions

 in solution. Some polar covalent compounds are also

strong electrolytes

. Common examples are HCl, HBr, HI and H₂SO₄, all of which react with H₂O to form large concentrations of

ions

. A solution of HCl, for example, conducts even better than one of NaCl having the same

concentration

.

: (a) 0.1 M NaCl (b) 0.05 M NaCl (c) 0.1 M HgCl₂. An electrolyte solution conducts electricity because of the movement of

in the solution (see above). The larger the concentration of ions, the better the solutions conduct.

The effect of the concentration of ions on the electrical current flowing through a solution is illustrated in Figure 11.2.1$\mathrm{11.2.}1$. Part a of the figure shows what happens when a battery is connected through an electrical meter to two inert metal strips (electrodes) dipping in ethanol. Each cubic decimeter of such a solution contains 0.10 mol NaCl (that is, 0.10 mol Na⁺ and 0.10 mol Cl^–). An electrical current is carried through the solution both by the Na⁺ ions moving toward the negative electrode and by the Cl^– ions which are attracted toward the positive electrode. The dial on the meter indicates the quantity of current.

Figure 1b shows that if we replace the 0.10-M NaCl solution with a 0.05-M NaCl solution, the meter reading falls to about one-half its former value. Halving the

Conductivity measurements reveal that most covalent compounds, if they dissolve in water at all, retain their original molecular structures. Neutral molecules cannot carry electrical charges through the solution, and so no current flows. A substance whose aqueous solution conducts no better than water itself is called a nonelectrolyte. Some examples are oxygen, O₂, ethanol, C₂H₅OH, and sugar, C₁₂H₂₂O₁₁.

Some covalent substances behave as weak electrolytes—their solutions allow only a small current flow, but it is greater than that of the pure solvent. An example is mercury(II) chloride (seen in the Figure above). For a 100-M HgCl₂ solution the meter reading shows only about 0.2 percent as much current as for 0.10 M NaCl. A crystal of HgCl₂ consists of discrete molecules, like those shown for HgBr₂ in Figure 11.2.2$\mathrm{11.2.}2$. When the solid dissolves, most of these molecules remain intact, but a few dissociate into ions according to the equation

(The double arrows indicate that the ionization proceeds only to a limited extent and an

Conductivity measurements can tell us more than whether a substance is a strong, a weak, or a nonelectrolyte. Consider, for instance, the data in Table 11.2.1$\mathrm{11.2.}1$ which shows the electrical current conducted through various aqueous

 HC₂H₃O₂ (acetic acid) and NH₃ (ammonia).

Closer examination of the data for

* All measurements refer to a cel1 in which the distance between the electrodes is 1.0 mm and the area of each electrode is 1.0 cm². A potential difference of 1.0 V is applied to produce the tabulated currents.

There is a simple reason for this behavior. Under similar conditions, most

A similar argument applies to

A second, slightly more subtle, conclusion can be drawn from the data in Table 11.2.1$\mathrm{11.2.}1$. When an electrolyte dissolves, each type of ion makes an independent contribution to the current the solution conducts. This can be seen by comparing NaCl with KCl, and NaI with KI. In each case the compound containing K⁺ conducts about 0.2 mA more than the one containing Na⁺. If we apply this observation to Na₂CO₃ and K₂CO₃, each of which produces twice as many Na⁺ or K⁺

Thus conductivity measurements confirm our statement that each ion exhibits its own characteristic properties in aqueous