37 9.2 Solution Stoichiometry
Water is by far the most important liquid solvent, partly because it is plentiful and partly because of its unique properties. In your body, in other living systems, and in the outside environment a tremendous number of reactions take place in aqueous
There are three important classes of reactions which occur in
- Precipitation reactions are useful for detecting the presence of various
ions and for determining the concentrations ofsolutions.
- Acid-
base reactions and redox reactions are similar in that something is being transferred from one species to another.
- Acid-base reactions involve proton transfers, while redox reactions involve electron transfers.
- Redox reactions are somewhat more complicated, though, because proton transfers and other bond-making and bond-breaking processes occur at the same time as electron transfer.
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
. They conduct very well because they provide a plentiful supply of
in solution. Some polar covalent compounds are also
. Common examples are HCl, HBr, HI and H2SO4, all of which react with H2O to form large concentrations of
. A solution of HCl, for example, conducts even better than one of NaCl having the same
.
: (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. 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