If we consider the two main branches of chemical science, inorganic and organic chemistry, it turns out that you will find they do not have many points of contact. But there is one truly global process that combines inorganic and organic substances. This is the process of hydrolysis.
Water \u0026 ndash; universal solvent
Hydrolysis \u0026 ndash; this interaction of substances with water, in which the constituent parts of chemical compounds form reaction products with hydrogen ions and hydroxo groups of H2 O. Given that 79% of the planet and up to 80% of the mass of all living organisms is water, it becomes clear that hydrolysis reactions cover all manifestations of natural processes, beginning with the destruction of rocks and ending with metabolism at all seven levels of organization of living matter, ranging from molecular and to the biosphere.
Mathematical language of hydrolysis
The more complex the chemical process, the more formulas and calculations it requires. For both types of exchange reactions of substances with water in both organic and inorganic chemistry, mathematical values are used \u0026 ndash; this degree and the hydrolysis constant, denoted as αg and Kin Their values are calculated and used in the technological processes of organic synthesis, for example, with saccharification of starch, hydrolysis of wood, saponification of fats.
What is the degree of hydrolysis
The hydrolysis constant
This value indicates the ability of the substance to hydrolyze. The higher it is, the faster the molecules of the solute interact with the ions of hydrogen and hydroxyl groups of water. It is denoted as Kg. the expression for the hydrolysis constant can be represented by the formula:
where: Kw \u0026 ndash; ion product of water [H +] * [OH -];
Kd \u0026 ndash; dissociation constant (splitting) of the solute.
For organic compounds, the degree and constant of hydrolysis are related to the formula:
TOg \u0026 ndash; hydrolysis constant.
\u0026 Ndash; concentration of ions of the dissolved substance.
αg \u0026 ndash; degree of hydrolysis.
Features of hydrolysis of organic compounds
Exchange reactions with water in proteins, carbohydrates and fats are multistage and quite difficult. Therefore, the hydrolysis constant, the formula of which is:
where: C \u0026 ndash; concentration of dissolved substance (mol / l).
αg \u0026 ndash; the degree of hydrolysis will also depend on the nature of the catalyst (enzyme), its activity and the temperature of the solution.
For example, in the technological process of exchange reactions of cellulose with water, specialists calculate all the parameters, the main of which are the hydrolysis constant and the hydrolysis rate constant. For the last value, we introduce such components as: α \u0026 ndash; the relative activity of the catalyst, N? its normality, that is, the concentration, b \u0026 ndash; the ability of cellulose to hydrolyze and λ \u0026 ndash; an indicator characterizing the dependence of the reaction rate on water on temperature: k = α * N * b * λ
In the reactions of hydrolysis of fats, chemists-technologists take into account its reversibility. To shift the equilibrium to the right towards the formation of the desired products, for example, glycerin, alkali is used in industrial synthesis. In this case, the hydrolysis of fats goes almost to the end: sodium or potassium hydroxides convert the multibasic carboxylic acids that form in the salt and thus prevent the reverse reaction of fat formation from passing. This method is used in the decomposition of esters in water in the saponification reaction. Increasing the concentration of hydroxide ions and diluting the reacting mixture, achieve an increase in the degree of hydrolysis of α, and hence the yield of reaction products of alcohols and organic acids.
Decomposition of water by inorganic substances
The hydrolysis of chemical compounds belonging to the class of salts is of practical importance. They are known to be metabolic products between acids and bases. So, their hydrolysis will depend precisely on what kind of hydroxides and acids formed salts. And the key here is the concept of the theory of electrolytic dissociation about the strength of electrolytes. The constant and the degree of salt hydrolysis will also vary, depending on the composition of the ions that form their molecules.
Why the pH of the salts is different
Experiments show that solutions of various salts can be acidic (pH \u003c7), neutral (pH 7), or alkaline (pH\u003e 7), although there are neither hydrogen nor hydroxyl ions in their molecules. The explanation for these contradictions must be sought in the process of their reaction with water:
Salt + water \u0026 lt; = \u0026 gt; acid + base.
This equilibrium corresponds to the hydrolysis constant:
where: AT \u0026 ndash; acid,
MON \u0026 ndash; base,
MA \u0026 ndash; salt.
Proceeding from the fact that the concentration of water in dilute solutions is constant, the hydrolysis constant will have the form:
Simplifying the first formula, we get the value:
This is the salt hydrolysis constant, \u0026 ndash; Cg. Its value is characterized by the ability of the substance to decompose with water, the more it is, the faster (at the same temperature and salt concentration) reaction takes place. Since the hydrolysis constant, the formula of which is Kr = [OH] × [MA], the concentration of hydroxide ions will be:
For most salts, hydrolysis is a reversible process. The value of α is affected by the temperature and concentration of the salt solution. The higher both these parameters, the greater the degree and the hydrolysis constant. This is explained by the fact that with heating, Cn + and C0n- increase sharply. Increasing the concentration of water, as seen from the hydrolysis equation, shifts the equilibrium to the right. However, in real chemical processes it has been proved that the h salts formed by weak acid and base do not depend on the dilution of the solution.
Degree of hydrolysis and TED
In the light of the theory of electrolytic dissociation, the equilibrium of the hydrolysis process depends on the already known quantity h or αg \u0026 ndash; degree of hydrolysis. If the water decomposes a weak acid salt, for example Na2CO3 or K2S, then
The reaction of solutions of such salts will be alkaline. It can be determined with the help of a colorless indicator of phenolphthalein, which becomes red with an excess of hydroxyl ions. Violet litmus in the alkaline solution becomes blue in color, and methylorange, \u0026 ndash; yellow. Sodium carbonate, as a strong electrolyte, dissociates completely into metal cations and anions of the acid residue when dissolved in water. It is the latter that interact with hydrogen ions and hydroxo groups.
Sodium cations can not bind OH- ions to sodium hydroxide molecules, since it is a strong electrolyte and in solution is never present as a molecule. At the same time, carbonate ions bind to H + to form a weak electrolyte \u0026 ndash; carbonic acid - until the equilibrium is established in the solution.
At hydrolysis of salts of weak bases AlCl3, FeSO4:
The reaction of solutions of such salts will be acidic, the pH is less than 7. In this case, a weak electrolyte Al (OH) 3 is formed. Part of the hydrogen ions, remaining free, causes acidification of the solution: the litmus indicator lowers this by changing the violet color to red. As a result, the ionic equilibrium of water dissociation shifts and an excess of hydrogen ions is formed.
If the salts of a weak base and a weak acid (NH4) CH3COO, NH4CN enter the exchange reaction with water, then
Solutions of such salts are hydrolyzed especially easily, and their pH will depend on the degree of dissociation of both the acid and the base. If the concentration of H + ions is greater, then the pH will be less than 7. With an excess of hydroxydiones \u0026 ndash; pH is greater than 7, and in the case of approximately the same number of \u0026 ndash; the solution will become neutral. Thus, the hydrolysis of salts occurs when their ions, which are formed due to electrolytic dissociation, are able to form weak (slightly dissociated electrolytes) with water,
Add the following:The salt hydrolysis constant, determined by this formula, shows that Ostwald's law, having the form:
is applicable not only for electrolytic dissociation, but also for the process of decomposition of substances by water. If the salt is formed by a strong base and a strong acid, then it does not hydrolyze in aqueous solutions, since a weak electrolyte does not form. That is, in the solution four types of ions are constantly present in the free state: they are metal and hydrogen cations and anions of hydroxyl groups and acid residues. For solutions of such salts, the reduced ionic equation can not be written, and the entire reaction reduces to the formation of water molecules.
Calculation of the ammonium chloride hydrolysis constant
To determine this value by a weak base and a strong acid, we use the relation:
where: Kw \u0026 ndash; the product of water;
KOSN \u0026 ndash; the dissociation constant of the weak base NH4OH formed during hydrolysis.
Then the hydrolysis constant of ammonium chloride will be:
This proves that during hydrolysis the solution of this salt has an acid reaction.