Precipitate (Chemical Precipitation): Precipitation Reaction and Examples

The chemical precipitation it is a process that consists of the formation of an insoluble solid from the mixture of two homogeneous solutions. Unlike precipitation of rain and snow, 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 at the bottom of the glass material.

Precipitate (chemical precipitation)

The 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 able to establish this equilibrium, or what is the same, not all can saturate the solution at very low concentrations. For example, in order to precipitate NaCl it is necessary to evaporate water or add more salt.

A saturated solution means that it can no longer dissolve more solid, so it precipitates. It is for this reason that precipitation is also a clear signal that the solution is saturated.

Index

  • 1 Precipitation reaction
    • 1.1 Precipitate formation
  • 2 Solubility product
  • 3 Examples
  • 4 References

Precipitation reaction

Considering a solution with dissolved A ions and the other with B ions, when mixing 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)

The species A + displaces species D + to form the solid AB; in turn, species C - move to B - to form the soluble solid DC. That is, double displacements occur (metathesis reaction). Then, the precipitation reaction is a double ion displacement reaction.

For the example in the image above, the beaker contains golden crystals of lead (II) iodide (PbI). 2 ), product of the known reaction"golden shower":

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 - .

Precipitate formation

The walls of the beaker show condensed water due to the intense heat. For what purpose is the water heated? To slow down the process of formation of PbI crystals 2 and accentuate the effect of the golden shower.

When encountering two anions I - , the Pb cation 2+ 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 grow to form colloidal particles, responsible for the yellow clouding 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 results from the gelatinous type, with bright crystals of some crystals"wandering"through the solution. This is because the nucleation speed 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 oversaturated, which is achieved by increasing the temperature prior to precipitation.

Thus, as the solution cools, the cores have enough time to grow. In addition, as the concentration of the salts is not very high, the temperature controls the nucleation process. Consequently, both variables benefit the appearance of PbI crystals 2 .

Solubility product

The PbI 2 establishes a balance between this and the ions in solution:

PbI 2 (s) Pb 2+ (ac) + 2I - (ac)

The constant of this equilibrium is called the product constant of solubility, K $ . The term"product"refers to the multiplication of the concentrations of the ions that make up the solid:

K $ = [Pb 2+ ] [I - ] 2

Here the solid is composed of the ions expressed in the equation; however, it does not consider the solid in these calculations. Concentrations of Pb ions 2+ and the ions I - are equal to the solubility of PbI 2 . That is, by determining the solubility of one these can be calculated that of the other and the constant K $ .

What are the values ​​of K for? $ for the few compounds 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 $ , more insoluble is.

Therefore, when comparing this value against those of other compounds, it can be predicted which pair (for example, AB and DC) will precipitate first. In the case of the hypothetical compound DC, its K $ it may be so high that to precipitate it needs higher concentrations of D + or C - in solution.

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

However, in the case of KNO 3 there is no such balance, so it lacks K $ . In fact, it is a salt highly soluble in water.

Examples

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

AgNO 3 (ac) + NaCl (ac) => AgCl (s) + NaNO 3 (ac)

Precipitation (chemical precipitation)

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

BaCl 2 (ac) + K 2 SW 4 (ac) => BaSO 4 (s) + 2KCl (ac)

A white precipitate of barium sulfate is formed.

2CUS 4 (ac) + 2NaOH (ac) => Cu 2 (OH) 2 SW 4 (s) + Na 2 SW 4 (ac)

The bluish precipitate of copper (II) dibasic sulfate is formed.

2AgNO 3 (ac) + K 2 CrO 4 (ac) => Ag 2 CrO 4 (s) + 2KNO 3 (ac)

The orange precipitate of silver chromate is formed.

CaCl 2 (ac) + Na 2 CO 3 (ac) => CaCO 3 (s) + 2NaCl (ac)

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

Faith (NO 3 ) 3 (ac) + 3NaOH (ac) => Fe (OH) 3 (s) + 3NaNO 3 (ac)

Finally, the orange precipitate of iron (III) hydroxide is formed. In this way, 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.


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