AgNO3 + HCl = AgCl + HNO3.
3. Two different ions may form undissociated molecules. In the neutralization of sodium hydroxide by hydrochloric acid the ions H+ and OH- come to the equilibrium
H+ + OH- <--> H2O.
But since water is almost entirely undissociated, equilibrium can only be reached when there are very few hydroxyl or hydrogen ions present. Consequently the two ions keep uniting until one or the other of them is practically removed from the solution. When this occurs the neutralization expressed in the following equation is complete:
NaOH + HCl = H2O + NaCl.
Preparation of acids. The principle of reversible reactions finds practical application in the preparation of most of the common acids. An acid is usually prepared by treating the most common of its salts with some other acid of high boiling point. The mixture is then heated until the lower boiling acid desired distills out. Owing to its high boiling point (338°), sulphuric acid is usually employed for this purpose, most other acids boiling below that temperature.
EXERCISES
1. What would take place when solutions of silver nitrate and sodium chloride are brought together? What other chlorides would act in the same way?
2. Is the reaction expressed by the equation NH3 + H2O = NH4OH reversible? If so, state the conditions under which it will go in each direction.
3. Is the reaction expressed by the equation 2H + O = H2O reversible? If so, state the conditions under which it will go in each direction.