Biological significance of the process
In biological systems, hydrolysis plays a fundamental role. This very process underlies digestion. Complex macromolecules ingested with food undergo hydrolytic cleavage.
For instance, proteins are hydrolyzed into amino acids, of which there are at least 20 types. The resulting monomers are absorbed into the bloodstream through the walls of the small intestine, providing the body with essential building blocks. In inorganic chemistry, salt hydrolysis is most commonly examined, which obeys strict regularities.
Lewis reaction mechanism
From the standpoint of Lewis theory, any salt is a combination of a cation (acting as a Lewis acid) and an anion (a Lewis base).
- Cation behavior: The particle experiences an electron deficiency and possesses vacant orbitals. In an aqueous solution, such a cation forms a bond with an electron pair donor—a hydroxide ion ($OH^-$)—which is split off from a water molecule.
- Change in the medium: The binding of hydroxide ions leads to the accumulation of excess hydrogen ions ($H^+$) in the solution. This causes a shift in pH towards the acidic side (the medium becomes acidic, $pH < 7$).
Anions of strong acids (e.g., the chloride ion $Cl^-$) undergo no changes upon interacting with water, as they are residues of strong electrolytes and do not form molecules in solution. However, anions of weak acids (e.g., $CO_3^{2-}$) exhibit basic properties and bind a proton from water, increasing the concentration of hydroxide ions and creating an alkaline medium.
Main types of salt hydrolysis
The direction and outcome of the exchange reaction depend on which acids and bases formed the initial salt. Observations show that clear solutions are not always formed upon the addition of water.
- By cation. Characteristic of salts formed by a weak base and a strong acid (e.g., aluminum chloride $AlCl_3$ or copper sulfate $CuSO_4$). It proceeds reversibly, and litmus turns red due to the acidic medium.
- By anion. Occurs in solutions of salts formed by a strong base and a weak acid (sodium carbonate $Na_2CO_3$). The process is reversible, $OH^-$ ions accumulate, and litmus turns blue (alkaline medium).
- By both ions. Observed in salts of a weak base and a weak acid (copper acetate). Both the cation and the anion actively bind the constituent parts of water. The medium remains close to neutral, since no significant excess of $H^+$ or $OH^-$ ions arises, yet the degree of hydrolysis is maximal here.
- Irreversible (complete). Salts formed by trivalent cations and anions of weak volatile acids (e.g., aluminum sulfide $Al_2S_3$) decompose completely. In experiments, this is manifested by the formation of a turbid mixture and the release of a gas with the odor of hydrogen sulfide. The medium changes insignificantly (around $pH = 7$), since the products are weak electrolytes.
Stepwise progression and degree of hydrolysis
To quantify the process, the degree of hydrolysis ($h$) is used—the ratio of the number of reacted particles (or their concentration) to the initial amount of the substance.
If a salt is formed by a multiply charged cation or an anion of a polybasic acid, the reaction proceeds in a stepwise manner. In this case, an important rule applies: the first stage always predominates over subsequent ones ($h_1 \gg h_2 \gg h_3$).
Example with aluminum chloride: At the first stage, a basic salt and a strong acid are formed ($pH < 7$). The second stage proceeds to an extremely small extent. If the reaction had proceeded to the third stage to a significant extent, the precipitation of insoluble aluminum hydroxide would be observed, which does not happen under normal conditions.
The activity of the process is directly affected by the strength of the initial components: the stronger the acid or base, the lesser the extent to which its corresponding salt is hydrolyzed.
Briefly: answers to topical questions
What classes of macromolecules, apart from proteins, undergo hydrolytic cleavage during digestion?
During digestion, apart from proteins, carbohydrates, lipids, and nucleic acids (RNA) undergo hydrolytic cleavage.
- Carbohydrates — broken down by amylase into disaccharides, as well as by enzymatic complexes of the enterocyte brush border (via the hydrolysis of glycosidic bonds).
- Lipids and phospholipids — broken down by lipase and phospholipase into fatty acids, mono- and diglycerides.
- Nucleic acids — RNA molecules are cleaved by ribonucleases into nucleotides.
What formula is used to calculate the salt hydrolysis constant?
For a salt formed by a weak acid and a strong base, the following formula is used: Kг = Kw / Ka, where Kw is the ionic product of water, and Ka is the dissociation constant of the weak acid.
Why do the second and subsequent stages of hydrolysis of multiply charged ions proceed to a significantly lesser extent than the first?
The second and subsequent stages of hydrolysis proceed to a significantly lesser extent because compounds with more pronounced acidic or basic properties are formed at each subsequent stage. The rule applies: the stronger the acid or base, the lesser the degree to which the corresponding ion is hydrolyzed. For example, during the hydrolysis of carbonates, the first stage produces a hydrocarbonate ion with very weak acidic properties, while the second stage produces carbonic acid, which is a stronger acid, thereby suppressing further hydrolysis.
How does heating a solution affect the degree of salt hydrolysis?
Heating a solution increases the degree of salt hydrolysis, meaning that hydrolysis is enhanced. Sources indicate that the degree of hydrolysis increases upon heating; hydrolysis is characterized as an endothermic process.
By what methods can chemical equilibrium be shifted to suppress the hydrolysis process?
To reduce the degree of hydrolysis, one can:
- decrease the temperature;
- increase the salt concentration;
- add an alkali; the presence of OH⁻ shifts the equilibrium towards the initial salt according to Le Chatelier's principle.
To determine the pH of a salt solution, find the "strong" parent: if the salt is formed by a strong acid, the medium is acidic; if by a strong base, it is alkaline.
Class and exam questions
What is the degree of hydrolysis?
This is a value showing the fraction of a substance that has reacted with water. It is equal to the ratio of the concentration of the hydrolyzed portion to the initial salt concentration.
Why does the medium remain neutral during the complete hydrolysis of aluminum sulfide?
Despite the complete breakdown of the initial salt, the resulting products (aluminum hydroxide and hydrogen sulfide) are very weak electrolytes and do not produce an excess of protons or hydroxide ions.
How does acid strength affect the hydrolysis of its salts?
An inverse relationship is observed: the stronger the acid, the weaker the ability of its anions to enter into an exchange reaction with water (all other things being equal).
What else to study on this topic
- Writing ionic and molecular equations of stepwise hydrolysis
- Specifics of the hydrolysis of salts formed by a weak base and a weak acid
- Examples of irreversible decomposition of polyvalent metal salts
- Mathematical calculation of the degree of hydrolysis via the amount of substance
- Dependence of hydrolysis on the nature of cations in Lewis acid-base theory
Master the entire topic in the "Chemistry" course
This is a summary. The course covers everything tested in classes and exams.
- Complete study guide on "Salt hydrolysis" and 3,009 other pages of the subject
- Assignments and quizzes after every topic — just like in a colloquium
- AI tutor answers questions on the topic anytime
- 95 video lessons for the subject
Other topics in "Chapter 7. Classification of chemical reactions by electronic processes"
Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.