Precipitate Precipitation Reaction and Examples

The precipitate or chemical precipitation It is a process that consists of 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 equilibrium, 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 can no longer dissolve any more solid, so it precipitates. It is for this reason that precipitation is also a clear sign that the solution is saturated..

Article index

• 1 Precipitation reaction
  • 1.1 Formation of the precipitate
• 2 Solubility product
• 3 Examples
• 4 References

Precipitation reaction

Considering a solution with dissolved A ions and the other with B ions, when mixed, the chemical equation of the reaction predicts:

TO⁺(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 species C^-, and B^- does the same with species D⁺. 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, species C^- shifts to 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_two), product of the reaction known as "golden shower":

Pb (NO₃)_two(ac) + 2KI (aq) => PbI_two(s) + 2KNO₃(aq)

According to the previous equation, A = Pb^two+, C^-= NO₃^-, 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 PbI crystals_two and accentuate the effect of the golden shower.

When encountering two anions I^-, the Pb cation^two+ it 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 originate clots, and these clots with others, to finally originate the precipitate..

However, when this occurs, the precipitate is of the gelatinous type, 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 crystals._two.

Solubility product

The PbI_two establishes a balance between this and the ions in solution:

PbI_two(s) <=> Pb^two+(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= [Pb^two+] [I^-]^two

Here the solid is composed of the ions expressed in the equation; however, this does not consider the solid in these calculations.

The concentrations of the Pb ions^two+ and I ions^- are equal to the solubility of PbI_two. That is, by determining the solubility of one of these, that of the other and the constant K can be calculated_ps.

What are the values ​​of K_ps for compounds poorly soluble in water? It is a measure of the degree of insolubility of the compound at a certain temperature (25ºC). Thus, the smaller a K_ps, more insoluble 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 it may be so high that it requires higher concentrations of D to precipitate⁺ or from C^- in solution.

This is the key to what is known as fractional precipitation. Also, knowing the K_ps for an insoluble salt, the minimum quantity can be calculated to precipitate it in a liter of water.

However, in the case of the KNO₃ there is no such equilibrium, so it lacks K_ps. In fact, it is a highly water soluble salt..

Examples

Precipitation reactions are one of the processes that enrich the world of chemical reactions. Some additional examples (besides the golden shower) are:

AgNO₃(aq) + NaCl (aq) => AgCl (s) + NaNO₃(ac)

The upper image illustrates the formation of the white silver chloride precipitate. In general, most silver compounds have white colors.

BaCl_two(ac) + K_twoSW₄(ac) => BaSO₄(s) + 2KCl (aq)

A white precipitate of barium sulfate forms.

2 COURSE₄(aq) + 2NaOH (aq) => Cu_two(OH)_twoSW₄(s) + Na_twoSW₄(ac)

The bluish precipitate of dibasic copper (II) sulfate forms.

2AgNO₃(ac) + K_twoCrO₄(ac) => Ag_twoCrO₄(s) + 2KNO₃(ac)

The orange precipitate of silver chromate forms.

CaCl_two(ac) + Na_twoCO₃(ac) => CaCO₃(s) + 2NaCl (aq)

The white precipitate of calcium carbonate, also known as limestone, forms.

Faith (NO₃)₃(aq) + 3NaOH (aq) => Fe (OH)₃(s) + 3NaNO₃(ac)

Finally, the orange precipitate of iron (III) hydroxide forms. Thus, precipitation reactions produce any compound.

References

1. Day, R., & Underwood, A. Quantitative Analytical Chemistry (fifth ed.). PEARSON Prentice Hall, p 97-103.
2. Der Kreole. (March 6, 2011). Gold rain. [Figure]. Retrieved on April 18, 2018, from: commons.wikimedia.org
3. Anne Marie Helmenstine, Ph.D. (April 9, 2017). Precipitation Reaction Definition. Retrieved on April 18, 2018, from: thoughtco.com
4. le Châtelier's Principle: Precipitation Reactions. Retrieved on April 18, 2018, from: digipac.ca
5. Prof. Botch. Chemical Reactions I: Net ionic equations. Retrieved on April 18, 2018, from: lecturedemos.chem.umass.edu
6. Luisbrudna. (October 8, 2012). Silver chloride (AgCl). [Figure]. Retrieved on April 18, 2018, from: commons.wikimedia.org
7. Whitten, Davis, Peck & Stanley. Chemistry. (8th ed.). CENGAGE Learning, p 150, 153, 776-786.