How to write the equation of hydrolysis of salts? This topic often causes difficulties for graduates of secondary schools who choose chemistry at the exam. We analyze the main types of hydrolysis, consider the rules for the preparation of molecular and ionic equations.
Definition
Hydrolysis is a reaction between a substance and water, accompanied by a combination of the components of the starting substance with it. This definition indicates that this process occurs not only in inorganic substances, it is also characteristic for organic compounds.
For example, the hydrolysis reaction equation is written for carbohydrates, esters, proteins, fats.
Hydrolysis value
All chemical interactions that are observed in the process of hydrolysis are used in various industries. For example, this process is used to remove coarse and colloidal impurities from water. For these purposes, special precipitates of aluminum and iron hydroxides are used, which are obtained by hydrolysis of sulfates, chlorides of these metals.
What other significance is hydrolysis? The equation of this process indicates that this reaction forms the basis of the digestive processes of all living things. The bulk of the energy the body needs is focused as ATP. The energy release is possible due to the hydrolysis process in which ATP takes part.
Process features
The molecular equation for the hydrolysis of salts is written as a reversible reaction. Depending on which base and acid the inorganic salt is formed, various variants of the process are distinguished.
Enter into such an interaction of salt, which are formed:
- mild hydroxide and active acid (and vice versa);
- volatile acid and active base.
It is impossible to write the ionic hydrolysis equation for salts that are formed by active acid and base. The reason is that the essence of neutralization comes down to the formation of water from ions.
Process characteristic
How can hydrolysis be described? The equation of this process can be considered by the example of a salt, which is formed by a monovalent metal and monobasic acid.
If the acid is represented as HA, and the base is MON, then the salt they form is MA.
How can hydrolysis be recorded? The equation is written in molecular and ionic form.
For dilute solutions, the hydrolysis constant is used, which is defined as the ratio of the number of moles of salt involved in the hydrolysis to their total number. Its value depends on which acid and base form the salt.
Anion hydrolysis
How to write the molecular equation of hydrolysis? If the salt contains active hydroxide and volatile acid, the result of the interaction will be an alkali and an acid salt.
As a typical embodiment, one can imagine a process for sodium carbonate, as a result of which alkali and acid salt are formed.
Considering that the solution contains anions of the hydroxyl group, the solution is alkaline, hydrolysis proceeds along the anion.
Process example
How to record such hydrolysis? The process equation for iron sulfate (2) involves the formation of sulfuric acid and iron hydrosulfate (2).
In solution, an acidic environment created by sulfuric acid.
Complete hydrolysis
The molecular and ionic equations for the hydrolysis of salts, which are formed by an inactive acid and the same base, suggest the preparation of the corresponding hydroxides. For example, for aluminum sulfide formed by amphoteric hydroxide and volatile acid, the reaction products are aluminum hydroxide and hydrogen sulfide. The solution has a neutral environment.
Sequencing
There is a certain algorithm, following which high school students will be able to accurately determine the type of hydrolysis, identify the reaction of the medium, and also record the products of the ongoing reaction. First you need to determine the type of process and record the process of the ongoing dissociation of salt.
For example, for copper sulfate (2), decay into ions is associated with the formation of a copper cation and a sulfate anion.
This salt is formed by a weak base and active acid; therefore, a cation (weak ion) process occurs.
Next, the molecular and ionic equations of the process are written.
To determine the reaction of the medium, it is necessary to make an ionic view of the ongoing process.
The products of this reaction are: copper hydroxosulfate (2) and sulfuric acid, therefore, the solution is characterized by an acidic reaction of the medium.
Hydrolysis has a special place among various metabolic reactions. In the case of salts, this process can be represented as a reversible interaction of the ions of a substance with a hydration shell. Depending on the strength of this effect, the process can proceed with different intensities.
Between cations and hydrating water molecules donor-acceptor bonds arise. The oxygen atoms contained in water will act as a donor, since they have lone electron pairs. The acceptors will be cations that have free atomic orbitals. The polarization effect on water depends on the charge of the cation.
A weak hydrogen bond forms between the anions and HOH dipoles. Under the strong influence of anions, a complete detachment from the proton molecule is possible, which leads to the formation of an acid or anion of the type 3‾. Hydrolysis is a reversible and endothermic process.
Types of effects on salts of water molecules
All anions and cations having insignificant charges and substantial sizes differ in insignificant polarizing effect on water molecules, therefore there is practically no reaction in an aqueous solution. As an example of such cations, mention may be made of hydroxyl compounds, which are alkalis.
Let us single out the metals of the first group of the main subgroup of the table of D. I. Mendeleev. Anions that meet the requirements are acid residues of strong acids. Salts that are formed by active acid and alkali do not undergo hydrolysis. For them, you can write the process of dissociation in the form:
H 2 O = H + + OH‾
Solutions of these inorganic salts have a neutral environment, therefore, hydrolysis of the destruction of salts is not observed.
For organic salts formed by the weak acid anion and alkali cation, anion hydrolysis is observed. As an example of such a salt, consider potassium acetate CH 3 COOK.
There is a binding of acetate ions of CH 3 COO - with hydrogen protons in the molecules of acetic acid, which is a weak electrolyte. A significant amount of hydroxide ions is accumulated in the solution, as a result of which it acquires an alkaline reaction of the medium. Potassium hydroxide is a strong electrolyte, so it cannot be bound, the pH is> 7.
The molecular equation of the process takes the form:
CH 3 COOC + H 2 O = KOH + CH 3 UN
In order to understand the essence of the interaction between substances, it is necessary to draw up a complete and abbreviated ionic equation.
The Na 2 S salt is characterized by a stepwise process of hydrolysis. Considering that the salt is formed by strong alkali (NaOH) and dibasic weak acid (H2S), the solution binds the sulfide anion with water protons and the accumulation of hydroxyl groups. In molecular and ionic form, this process will look as follows:
Na 2 S + H 2 O = NaHS + NaOH
First stage. S 2− + HOH = HS - + OH -
Second stage. HS - + HOH = H 2 S + OH -
Despite the possibility of a two-stage process of hydrolysis of this salt under normal conditions in the second stage, the process under consideration practically does not proceed. The reason for this phenomenon is the accumulation of hydroxyl ions, which impart a weak alkaline environment to the solution. This promotes a shift in chemical equilibrium according to the Le Chatelier principle and causes a neutralization reaction. In this regard, the hydrolysis of salts, which are formed by alkali and weak acid, can be suppressed by excess alkali.
Depending on the polarizing effect of the anions, it is possible to influence the rate of hydrolysis.
For salts containing strong acid anions and weak base cations, cation hydrolysis is observed. For example, a similar process can be considered on ammonium chloride. The process can be represented as follows:
NH 4 CL + H 2 O = NH 4 OH + HCL
NH 4 + + HOH = NH 4 OH + H +
Due to the fact that protons accumulate in the solution, an acidic environment is created in it. To shift the equilibrium to the left, acid is introduced into the solution.
The salt formed by the weak cation and anion is characterized by complete hydrolysis. For example, you can consider the option of hydrolysis of ammonium acetate CH 3 COONH 4 . In ionic form, the interaction has the form:
NH 4 + + CH 3 COO - + HOH = NH 4 OH + CH 3 COOH
Finally
Depending on what acid and base the salt is formed, the process of reaction with water has certain differences. For example, when salt is formed by weak electrolytes and when they interact with water, volatile products are formed. Complete hydrolysis is the reason that it is impossible to prepare some salt solutions. For example, for aluminum sulfide, you can write the process in the form:
Al 2 S 3 + 6H 2 O = 2Al (OH) 3 ↓ + 3H 2 S ↑
Such a salt can only be obtained in a “dry way” using heating of simple substances according to the scheme:
2Al + 3S = Al 2 S 3
To avoid decomposition of aluminum sulfide, it is necessary to store it in sealed containers.
In some cases, the hydrolysis process is quite complicated, therefore, the molecular equations of this process have a conventional form. In order to reliably establish the products of interaction, it is necessary to conduct special studies.
For example, this is characteristic of multinuclear complexes of iron, tin, and beryllium. Depending on which direction this reversible process needs to be shifted, it is possible to add ions of the same name and change its concentration and temperature.