15.3 Precipitation Reactions
Learning Objectives
By the end of this section, you will be able to:
- Predict the solubility of common inorganic compounds by using solubility rules
- Define precipitation reactions
- Recognize and identify examples of precipitation reactions
- Apply the solubility rules of common inorganic compounds to predict the products formed when two aqueous solutions are mixed
Scientists have found it convenient (or even necessary) to classify chemical interactions by identifying common patterns of reactivity. This section of this chapter will focus on a specific type of double displacement reaction called a precipitation reaction.
Precipitation Reactions and Solubility Rules
Solubility of Inorganic Compounds
The idea of solubility was introduced in the solutions chapter. The extent to which a substance may be dissolved in water, or any solvent, is quantitatively expressed as its solubility, defined as the maximum concentration of a substance that can be achieved under specified conditions. Substances with relatively large solubilities are said to be soluble and are found as dissolved ions in aqueous solution. A substance will precipitate when solution conditions are such that its concentration exceeds its solubility. Substances with relatively low solubilities are said to be insoluble, and these are the substances that readily precipitate from solution to form a solid (s). For purposes of predicting the identities of solids formed by precipitation reactions, one may simply refer to the solubility guidelines for many ionic compounds in Table 15.3a to predict whether a precipitation reaction will occur when solutions of soluble ionic compounds are mixed together. First, it is important to become familiar with using the solubility table to determine if a given salt will dissolve (aq), or not (s) in aqueous solution. If a salt is said to be insoluble, or has low solubility, or is slightly soluble, it will form a precipitate – a solid – and the symbol (s) will be used to represent that observation. If a salt is soluble, the salt will dissociate (ionize) in aqueous solution, and the symbol (aq) will be used to show that chemistry. It is important to mention that Table 15.3a does not include every possible soluble, insoluble salt combination. Since there are other possibilities, Table 14.2a and Table 14.2b can also be referenced to determine solubility of inorganic compounds.
| Negative Ion (Anion) | Positive Ion (Cation) | Solubility | Phase, Phase Symbol |
|---|---|---|---|
| All | Li+, Na+, K+, Rb+, Cs+ NH4+ | Soluble | aqueous, (aq) |
| Chloride (Cl–), Bromide (Br–), Iodide (I–) | Ag+, Pb2+, Hg22+, Cu+ | Low solubility | solid, (s) |
| Chloride (Cl–), Bromide (Br–), Iodide (I–) | All others | Soluble | aqueous, (aq) |
| F– | compounds with group 2 metal cations, Li+, Al3+, Pb2+, Fe2+and Fe3+ | Low solubility | solid, (s) |
| F– | All others | Soluble | aqueous, (aq) |
| Hydroxide (OH–) | Li+, Na+, K+, Rb+, NH4+, Sr2+, Ba2+ | Sol