Weak base and strong acid during hydrolysis of salts

In order to understand how the hydrolysis of salts proceeds in their aqueous solutions, we begin by defining the process.

Determination and features of hydrolysis

This process involves the chemical action of water ions with salt ions, resulting in a weak base (or acid), as well as the reaction of the medium. Any salt can be represented as a product of the chemical reaction of a base and an acid. Depending on what their strength is, several variants of the process flow are singled out.

Types of hydrolysis

In chemistry, three types of reaction between salt and water cations are considered. Each process is carried out with a change in the pH of the medium, therefore, it is proposed to use different types of indicators to determine the hydrogen index. For example, violet litmus is used for acidic medium, phenolphthalein is suitable for an alkaline reaction. Let us analyze in more detail the features of each variant of hydrolysis. Strong and weak bases can be determined from the solubility table, and the strength of acids is determined from the table.

Hydrolysis by cation

As an example of such a salt, we consider ferric chloride (2). Ferric hydroxide (2) is a weak base, and hydrochloric acid is strong. In the process of interaction with water (hydrolysis), the formation of the basic salt (iron hydroxochloride 2) occurs, as well as hydrochloric acid is formed. An acidic medium appears in the solution , it can be determined with the help of blue litmus (pH less than 7). In this case, hydrolysis itself proceeds through the cation, since a weak base is used.

Let us give one more example of the course of hydrolysis for the described case. Consider the salt of magnesium chloride. Magnesium hydroxide is a weak base, and hydrochloric acid is a strong base. In the process of interaction with water molecules, magnesium chloride is converted to the basic salt (hydroxochloride). Magnesium hydroxide, the formula of which is generally represented in the form M (OH) ₂ , is slightly soluble in water, but strong hydrochloric acid gives the solution an acidic medium.

Hydrolysis by anion

The next variant of hydrolysis is characteristic for the salt, which is formed by a strong base (alkali) and a weak acid. As an example for this case, consider sodium carbonate.

In this salt there is a strong base of sodium, as well as a weak carbonic acid. Interaction with water molecules proceeds with the formation of an acid salt - sodium hydrogencarbonate, that is, hydrolysis proceeds by anion. In addition, sodium hydroxide is formed in the solution , which gives the solution an alkaline medium.

Let us give one more example for this case. Potassium sulphite is a salt that is formed by a strong base - caustic potassium, and also weak sulfuric acid. During the interaction with water (during hydrolysis), the formation of potassium hydrosulfite (acid salt) and potassium hydroxide (alkali) occur. The medium in the solution will be alkaline, it can be confirmed with phenolphthalein.

Complete hydrolysis

The salt of a weak acid and a weak base undergoes complete hydrolysis. Let's try to find out what its peculiarity is, and what products will be formed as a result of this chemical reaction.

Let us analyze the hydrolysis of a weak base and a weak acid by the example of aluminum sulphide. This salt is formed by aluminum hydroxide, which is a weak base, and also weak hydrogen sulphide. When interacting with water, complete hydrolysis is observed, as a result of which gaseous hydrogen sulfide is formed, and also aluminum hydroxide as a precipitate. This interaction takes place both over the cation and on the anion, so this variant of hydrolysis is considered complete.

Also, as an example of the interaction with this type of salt with water, magnesium sulphide can be cited. In the composition of this salt there is magnesium hydroxide, its formula is Mg (OH) 2. It is a weak base, insoluble in water. In addition, there is hydrogen sulphide inside the magnesium sulfide, which is weak. When interacting with water, complete hydrolysis takes place (by cation and anion), as a result of which magnesium hydroxide is formed in the form of a precipitate, and hydrogen sulphide is also released as a gas.

If we consider the hydrolysis of a salt, which is formed by a strong acid and a strong base, then it should be noted that it does not leak. The medium in solutions of such salts as sodium chloride , potassium nitrate , remains neutral.

Conclusion

Strong and weak bases, the acids with which the salts are formed, affect the result of hydrolysis, the reaction of the medium in the resulting solution. Such processes are widespread in nature.

Hydrolysis is of particular importance in the chemical transformation of the earth's crust. It contains metal sulphides that are slightly soluble in water. As they hydrolyze, hydrogen sulfide forms and releases into the surface of the earth during volcanic activity.

Silicate rocks during the transition to hydroxides, cause gradual destruction of rocks. For example, a mineral such as malachite is a product of the hydrolysis of copper carbonates.

An intensive process of hydrolysis occurs also in the World Ocean. The magnesium and calcium bicarbonates, which are carried out by water, have a slightly alkaline medium. In such conditions, the process of photosynthesis in marine plants perfectly proceeds, and marine organisms develop more intensively.

In oil there are impurities of water and salts of calcium and magnesium. During the heating of oil, their interaction with water vapor occurs. In the course of hydrolysis hydrogen chloride is formed, in the interaction of which with the metal, the equipment is destroyed.