The precipitate or chemical precipitation is a process that consists in the formation of an insoluble solid from the mixture of two homogeneous solutions. Unlike the precipitation of rains and snows, in this type of precipitation “it rains solid” from the surface of the liquid.
In two homogeneous solutions ions are dissolved in water. When these interact with other ions (at the time of mixing), their electrostatic interactions allow the growth of a crystal or a gelatinous solid. Due to the effect of gravity, this solid ends up depositing on the bottom of the glass material.
Precipitation is governed by an ionic balance, which depends on many variables: from the concentration and nature of the intervening species to the water temperature and the allowed contact time of the solid with the water.
Additionally, not all ions are capable of establishing this equilibrium, or what is the same, not all can saturate the solution at very low concentrations. For example, to precipitate NaCl, it is necessary to evaporate the water or add more salt.
A saturated solution means that it cannot dissolve any more solid, so it precipitates. It is for this reason that precipitation is also a clear sign that the solution is saturated.
Precipitation reaction
Considering a solution with dissolved A ions and the other with B ions, when mixed, the chemical equation of the reaction predicts:
A + (ac) + B - (ac) <=> AB (s)
However, it is "almost" impossible for A and B to be alone initially, necessarily needing to be accompanied by other ions with opposite charges.
In this case, A + forms a soluble compound with the C - species, and B - does the same with the D + species. Thus, the chemical equation now adds the new species:
AC (ac) + DB (ac) <=> AB (s) + DC (ac)
Species A + displaces Species D + to form solid AB; in turn, the species C - displaces B - to form the soluble solid DC.
That is, double displacements occur (metathesis reaction). So the precipitation reaction is a double ion displacement reaction.
For the example in the image above, the beaker contains gold crystals of lead (II) iodide (PbI 2), a product of the so-called "gold shower" reaction:
Pb (NO 3) 2 (ac) + 2KI (aq) => PbI 2 (s) + 2KNO 3 (aq)
According to the previous equation, A = Pb 2+, C - = NO 3 -, D = K + and B = I -.
Formation of the precipitate
The walls of the beaker show condensed water from the intense heat. For what purpose is the water heated? To slow down the formation process of the PbI 2 crystals and accentuate the effect of the golden shower.
When meeting two I - anions, the Pb 2+ cation forms a tiny nucleus of three ions, which is not enough to build a crystal. Likewise, in other regions of the solution other ions also gather to form nuclei; This process is known as nucleation.
These nuclei attract other ions, and thus it grows to form colloidal particles, responsible for the yellow cloudiness of the solution.
In the same way, these particles interact with others to cause clots, and these clots with others, to finally originate the precipitate.
However, when this occurs, the precipitate is gelatinous, with bright hints of some crystals "wandering" through the solution. This is because the nucleation rate is greater than the growth of the nuclei.
On the other hand, the maximum growth of a nucleus is reflected in a brilliant crystal. To guarantee this crystal, the solution must be slightly supersaturated, which is achieved by increasing the temperature prior to precipitation.
Thus, as the solution cools, the nuclei have sufficient time to grow. Furthermore, since the concentration of the salts is not very high, the temperature controls the nucleation process. Consequently, both variables benefit the appearance of PbI 2 crystals.
Solubility product
PbI 2 establishes an equilibrium between it and the ions in solution:
PbI 2 (s) <=> Pb 2+ (ac) + 2I - (ac)
The constant of this equilibrium is called the solubility product constant, K ps. The term "product" refers to the multiplication of the concentrations of the ions that make up the solid:
K ps = 2
Here the solid is made up of the ions expressed in the equation; however, it does not consider the solid in these calculations.
The concentrations of Pb 2+ ions and I - ions are equal to the solubility of PbI 2. That is, by determining the solubility of one of these, that of the other and the constant K ps can be calculated.
What are the K ps values for low water soluble compounds for? It is a measure of the degree of insolubility of the compound at a certain temperature (25ºC). Thus, the smaller a K ps, the more insoluble it is.
Therefore, by comparing this value against those of other compounds, it can be predicted which pair (eg, AB and DC) will precipitate first. In the case of the hypothetical compound DC, its K ps may be so high that it requires higher concentrations of D + or C - in solution to precipitate.
This is the key to what is known as fractional precipitation. Likewise, knowing the K ps for an insoluble salt, the minimum quantity to precipitate it in one liter of water can be calculated.
However, in the case of KNO 3 there is no such equilibrium, so it lacks K ps. In fact, it is a highly soluble salt in water.
Examples
Precipitation reactions are one of the processes that enrich the world of chemical reactions. Some additional examples (besides the golden shower) are:
AgNO 3 (aq) + NaCl (aq) => AgCl (s) + NaNO 3 (aq)
The top image illustrates the formation of the white silver chloride precipitate. In general, most silver compounds have white colors.
BaCl 2 (aq) + K 2 SO 4 (aq) => BaSO 4 (s) + 2KCl (aq)
A white precipitate of barium sulfate forms.
2CuSO 4 (aq) + 2NaOH (aq) => Cu 2 (OH) 2 SO 4 (s) + Na 2 SO 4 (aq)
The bluish precipitate of dibasic copper (II) sulfate forms.
2AgNO 3 (aq) + K 2 CrO 4 (aq) => Ag 2 CrO 4 (s) + 2KNO 3 (aq)
The orange precipitate of silver chromate forms.
CaCl 2 (aq) + Na 2 CO 3 (aq) => CaCO 3 (s) + 2NaCl (aq)
The white precipitate of calcium carbonate, also known as limestone, forms.
Fe (NO 3) 3 (aq) + 3NaOH (aq) => Fe (OH) 3 (s) + 3NaNO 3 (aq)
Finally, the orange precipitate of iron (III) hydroxide forms. In this way, precipitation reactions produce any compound.
References
- Day, R., & Underwood, A. Quantitative Analytical Chemistry (5th ed.). PEARSON Prentice Hall, p 97-103.
- Der Kreole. (March 6, 2011). Gold rain.. Retrieved on April 18, 2018, from: commons.wikimedia.org
- Anne Marie Helmenstine, Ph.D. (April 9, 2017). Precipitation Reaction Definition. Retrieved on April 18, 2018, from: thoughtco.com
- le Châtelier's Principle: Precipitation Reactions. Retrieved on April 18, 2018, from: digipac.ca
- Prof. Botch. Chemical Reactions I: Net ionic equations. Retrieved on April 18, 2018, from: lecturedemos.chem.umass.edu
- Luisbrudna. (October 8, 2012). Silver chloride (AgCl).. Retrieved on April 18, 2018, from: commons.wikimedia.org
- Whitten, Davis, Peck & Stanley. Chemistry. (8th ed.). CENGAGE Learning, p 150, 153, 776-786.