To understand how the hydrolysis of salts occurs in their aqueous solutions, we first give a definition of this process.
Definition and features of hydrolysis
This process involves the chemical action of water ions with salt ions, resulting in the formation of a weak base (or acid), and also changes the reaction of the medium. Any salt can be represented as a product of the chemical interaction of a base and an acid. Depending on their strength, there are several options for the process.
Types of hydrolysis
In chemistry, three types of reactions between salt and water cations are considered. Each process is carried out with a change in the pH of the environment, so it is assumed that different types of indicators are used to determine the pH value. For example, violet litmus is used for an acidic environment; phenolphthalein is suitable for an alkaline reaction. Let us analyze in more detail the features of each hydrolysis option. 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, consider ferric chloride (2). Iron(2) hydroxide is a weak base, while hydrochloric acid is a strong base. In the process of interaction with water (hydrolysis), the formation of a basic salt (iron hydroxychloride 2) occurs, and hydrochloric acid is also formed. An acidic environment appears in the solution; it can be determined using blue litmus (pH less than 7). In this case, the hydrolysis itself proceeds through the cation, since a weak base is used.
Let us give another example of the occurrence of hydrolysis for the described case. Consider the salt magnesium chloride. Magnesium hydroxide is a weak base, while hydrochloric acid is a strong base. In the process of interaction with water molecules, magnesium chloride is converted into a basic salt (hydroxychloride). Magnesium hydroxide, whose general formula is presented as M(OH) 2, is slightly soluble in water, but strong hydrochloric acid gives the solution an acidic environment.
Hydrolysis by anion
The next version of hydrolysis is typical for a salt, which is formed by a strong base (alkali) and a weak acid. As an example for this case, consider sodium carbonate.
This salt contains a strong sodium base as well as a weak carbonic acid. Interaction with water molecules occurs with the formation of an acidic salt - sodium bicarbonate, that is, hydrolysis occurs at the anion. In addition, the solution is formed which gives the solution an alkaline environment.
Let's give another example for this case. Potassium sulfite is a salt that is formed by a strong base - caustic potassium, as well as a weak one. In the process of interaction with water (hydrolysis), the formation of potassium hydrosulfite (acid salt) and potassium hydroxide (alkali) occurs. The solution will be alkaline, which can be confirmed using phenolphthalein.
Complete hydrolysis
The salt of a weak acid and a weak base undergoes complete hydrolysis. Let's try to find out what is special about it 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 using the example of aluminum sulfide. This salt is formed by aluminum hydroxide, which is a weak base, as well as weak hydrosulfide acid. When interacting with water, complete hydrolysis is observed, as a result of which gaseous hydrogen sulfide is formed, as well as aluminum hydroxide in the form of a precipitate. This interaction occurs through both the cation and the anion, so this version of hydrolysis is considered complete.
Also, as an example of the interaction of this type of salt with water, magnesium sulfide can be cited. This salt contains magnesium hydroxide, its formula is Mg(OH)2. It is a weak base and insoluble in water. In addition, inside magnesium sulfide there is hydrogen sulfide acid, which is weak. When interacting with water, complete hydrolysis occurs (by cation and anion), resulting in the formation of magnesium hydroxide in the form of a precipitate, and hydrogen sulfide is also released as a gas.
If we consider the hydrolysis of a salt that is formed by a strong acid and a strong base, it should be noted that it does not occur. The medium in solutions of salts such as potassium chloride remains neutral.
Conclusion
Strong and weak bases, acids that form salts, affect the result of hydrolysis and the reaction of the medium in the resulting solution. Similar processes are widespread in nature.
Hydrolysis is of particular importance in the chemical transformation of the earth's crust. It contains metal sulfides that are poorly soluble in water. As they hydrolyze, hydrogen sulfide is formed and released during volcanic activity to the surface of the earth.
When silicate rocks transform into hydroxides, they 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 also occurs in the World Ocean. and calcium, which are carried away by water, have a slightly alkaline environment. In such conditions, the process of photosynthesis in marine plants proceeds well, and marine organisms develop more intensively.
Oil contains impurities of water and calcium and magnesium salts. In the process of heating oil, they interact with water vapor. During hydrolysis, hydrogen chloride is formed, which, when interacting with metal, destroys the equipment.
After reading the article, you will be able to separate substances into salts, acids and bases. The article describes what the pH of a solution is and what general properties acids and bases have.
In simple terms, an acid is anything with H, and a base is anything with OH. BUT! Not always. To distinguish an acid from a base, you need to... remember them! Regret. To make life at least somehow easier, three of our friends, Arrhenius and Brønsted and Lowry, came up with two theories that are called after them.
Like metals and nonmetals, acids and bases are the division of substances based on similar properties. The first theory of acids and bases belonged to the Swedish scientist Arrhenius. An Arrhenius acid is a class of substances that, when reacting with water, dissociate (decay), forming the hydrogen cation H +. Arrhenius bases in aqueous solution form OH - anions. The next theory was proposed in 1923 by scientists Bronsted and Lowry. The Brønsted-Lowry theory defines acids as substances capable of donating a proton in a reaction (a hydrogen cation is called a proton in reactions). Bases, accordingly, are substances that can accept a proton in a reaction. The currently relevant theory is the Lewis theory. Lewis theory defines acids as molecules or ions capable of accepting electron pairs, thereby forming Lewis adducts (an adduct is a compound formed by combining two reactants without forming by-products).
In inorganic chemistry, as a rule, by acid we mean a Bronsted-Lowry acid, that is, substances capable of donating a proton. If they mean the definition of a Lewis acid, then in the text such an acid is called a Lewis acid. These rules apply to acids and bases.
Dissociation
Dissociation is the process of decomposition of a substance into ions in solutions or melts. For example, the dissociation of hydrochloric acid is the decomposition of HCl into H + and Cl -.
Properties of acids and bases
Bases tend to feel soapy to the touch, while acids generally taste sour.
When a base reacts with many cations, a precipitate is formed. When an acid reacts with anions, a gas is usually released.
Commonly used acids:
H 2 O, H 3 O +, CH 3 CO 2 H, H 2 SO 4, HSO 4 −, HCl, CH 3 OH, NH 3
Commonly used bases:
OH − , H 2 O , CH 3 CO 2 − , HSO 4 − , SO 4 2 − , Cl −
Strong and weak acids and bases
Strong acids
Such acids that completely dissociate in water, producing hydrogen cations H + and anions. An example of a strong acid is hydrochloric acid HCl:
HCl (solution) + H 2 O (l) → H 3 O + (solution) + Cl - (solution)
Examples of strong acids: HCl, HBr, HF, HNO 3, H 2 SO 4, HClO 4
List of strong acids
- HCl - hydrochloric acid
- HBr - hydrogen bromide
- HI - hydrogen iodide
- HNO 3 - nitric acid
- HClO 4 - perchloric acid
- H 2 SO 4 - sulfuric acid
Weak acids
Only partially dissolved in water, for example, HF:
HF (solution) + H2O (l) → H3O + (solution) + F - (solution) - in such a reaction more than 90% of the acid does not dissociate:
= < 0,01M для вещества 0,1М
Strong and weak acids can be distinguished by measuring the conductivity of solutions: conductivity depends on the number of ions, the stronger the acid, the more dissociated it is, therefore, the stronger the acid, the higher the conductivity.
List of weak acids
- HF hydrogen fluoride
- H 3 PO 4 phosphoric
- H 2 SO 3 sulfurous
- H 2 S hydrogen sulfide
- H 2 CO 3 coal
- H 2 SiO 3 silicon
Strong grounds
Strong bases completely dissociate in water:
NaOH (solution) + H 2 O ↔ NH 4
Strong bases include metal hydroxides of the first (alkalines, alkali metals) and second (alkalinotherrenes, alkaline earth metals) groups.
List of strong bases
- NaOH sodium hydroxide (caustic soda)
- KOH potassium hydroxide (caustic potash)
- LiOH lithium hydroxide
- Ba(OH) 2 barium hydroxide
- Ca(OH) 2 calcium hydroxide (slaked lime)
Weak foundations
In a reversible reaction in the presence of water, it forms OH - ions:
NH 3 (solution) + H 2 O ↔ NH + 4 (solution) + OH - (solution)
Most weak bases are anions:
F - (solution) + H 2 O ↔ HF (solution) + OH - (solution)
List of weak bases
- Mg(OH) 2 magnesium hydroxide
- Fe(OH) 2 iron(II) hydroxide
- Zn(OH) 2 zinc hydroxide
- NH 4 OH ammonium hydroxide
- Fe(OH) 3 iron(III) hydroxide
Reactions of acids and bases
Strong acid and strong base
This reaction is called neutralization: when the amount of reagents is sufficient to completely dissociate the acid and base, the resulting solution will be neutral.
Example:
H 3 O + + OH - ↔ 2H 2 O
Weak base and weak acid
General type of reaction:
Weak base (solution) + H 2 O ↔ Weak acid (solution) + OH - (solution)
Strong base and weak acid
The base dissociates completely, the acid dissociates partially, the resulting solution has weak properties of a base:
HX (solution) + OH - (solution) ↔ H 2 O + X - (solution)
Strong acid and weak base
The acid dissociates completely, the base does not completely dissociate:
Dissociation of water
Dissociation is the breakdown of a substance into its component molecules. The properties of an acid or base depend on the equilibrium that is present in water:
H 2 O + H 2 O ↔ H 3 O + (solution) + OH - (solution)
K c = / 2
The equilibrium constant of water at t=25°: K c = 1.83⋅10 -6, the following equality also holds: = 10 -14, which is called the dissociation constant of water. For pure water = = 10 -7, hence -lg = 7.0.
This value (-lg) is called pH - hydrogen potential. If pH< 7, то вещество имеет кислотные свойства, если pH >7, then the substance has basic properties.
Methods for determining pH
Instrumental method
A special device, a pH meter, is a device that transforms the concentration of protons in a solution into an electrical signal.
Indicators
A substance that changes color in a certain pH range depending on the acidity of the solution; using several indicators you can achieve a fairly accurate result.
Salt
A salt is an ionic compound formed by a cation other than H+ and an anion other than O2-. In a weak aqueous solution, the salts completely dissociate.
To determine the acid-base properties of a salt solution, it is necessary to determine which ions are present in the solution and consider their properties: neutral ions formed from strong acids and bases do not affect pH: they do not release either H + or OH - ions in water. For example, Cl -, NO - 3, SO 2- 4, Li +, Na +, K +.
Anions formed from weak acids exhibit alkaline properties (F -, CH 3 COO -, CO 2- 3); cations with alkaline properties do not exist.
All cations except metals of the first and second groups have acidic properties.
Buffer solution
Solutions that maintain pH when a small amount of a strong acid or a strong base are added are mainly composed of:
- A mixture of a weak acid, its corresponding salt and a weak base
- Weak base, corresponding salt and strong acid
To prepare a buffer solution of a certain acidity, it is necessary to mix a weak acid or base with the appropriate salt, taking into account:
- pH range in which the buffer solution will be effective
- Solution capacity - the amount of strong acid or strong base that can be added without affecting the pH of the solution
- There should be no unwanted reactions that could change the composition of the solution
Test:
All acids, their properties and bases are divided into strong and weak. But don’t dare confuse concepts such as “strong acid” or “strong base” with their concentration. For example, you cannot make a concentrated solution of a weak acid or a dilute solution of a strong base. For example, hydrochloric acid, when dissolved in water, gives each of the two water molecules one of its protons.
When a chemical reaction occurs in the hydronium ion, the hydrogen ion binds very tightly to the water molecule. The reaction itself will continue until its reagents are completely exhausted. Our water in this case plays the role of a base, since it receives a proton from hydrochloric acid. Acids that dissociate completely in aqueous solutions are called strong.
When we know the very initial concentration of a strong acid, then in this case it is not difficult to calculate the concentration of hydronium ions and chloride ions in the solution. For example, if you take and dissolve 0.2 mol of gaseous hydrochloric acid in 1 liter of water, the concentration of ions after dissociation will be exactly the same.
Examples of strong acids:
1)
HCl - hydrochloric acid;
2)
HBr—hydrogen bromide;
3)
HI—hydrogen iodide;
4)
HNO3 - nitric acid;
5)
HClO4 - perchloric acid;
6)
H2SO4 is sulfuric acid.
All known acids (with the exception of sulfuric acid) are presented in the list above and are monoprotic, since their atoms donate one proton each; Sulfuric acid molecules can easily donate two of their protons, which is why sulfuric acid is diprotic.
Strong bases include electrolytes; they completely dissociate in aqueous solutions to form a hydroxide ion.
Similar to acids, calculating the concentration of the hydroxide ion is very simple if you know the initial concentration of the solution. For example, a NaOH solution with a concentration of 2 mol/L dissociates into the same concentration of ions.
Weak acids. Bases and properties
As for weak acids, they do not dissociate completely, that is, partially. It is very simple to distinguish between strong and weak acids: if the reference table next to the name of the acid shows its constant, then this acid is weak; if the constant is not given, then this acid is strong.
Weak bases also react well with water to form an equilibrium system. Weak acids are also characterized by their dissociation constant K.
ELECTROLYTES– substances whose solutions or melts conduct electric current.
NON-ELECTROLYTES– substances whose solutions or melts do not conduct electric current.
Dissociation– decomposition of compounds into ions.
Degree of dissociation– the ratio of the number of molecules dissociated into ions to the total number of molecules in the solution.
STRONG ELECTROLYTES when dissolved in water, they almost completely dissociate into ions.
When writing equations for the dissociation of strong electrolytes, an equal sign is used.
Strong electrolytes include:
· Soluble salts ( see solubility table);
· Many inorganic acids: HNO 3, H 2 SO 4, HClO 3, HClO 4, HMnO 4, HCl, HBr, HI ( Look acids-strong electrolytes in solubility table);
· Bases of alkali (LiOH, NaOH, KOH) and alkaline earth (Ca(OH) 2, Sr(OH) 2, Ba(OH) 2) metals ( see bases-strong electrolytes in the solubility table).
WEAK ELECTROLYTES in aqueous solutions only partially (reversibly) dissociate into ions.
When writing dissociation equations for weak electrolytes, the sign of reversibility is indicated.
Weak electrolytes include:
· Almost all organic acids and water (H 2 O);
· Some inorganic acids: H 2 S, H 3 PO 4, HClO 4, H 2 CO 3, HNO 2, H 2 SiO 3 ( Look acids-weak electrolytes in the solubility table);
· Insoluble metal hydroxides (Mg(OH) 2 , Fe(OH) 2 , Zn(OH) 2) ( look at the grounds-cweak electrolytes in the solubility table).
The degree of electrolytic dissociation is influenced by a number of factors:
nature of the solvent and electrolyte: strong electrolytes are substances with ionic and covalent strongly polar bonds; good ionizing ability, i.e. the ability to cause dissociation of substances is possessed by solvents with a high dielectric constant, the molecules of which are polar (for example, water);
temperature: since dissociation is an endothermic process, increasing the temperature increases the value of α;
concentration: when the solution is diluted, the degree of dissociation increases, and with increasing concentration it decreases;
stage of the dissociation process: each subsequent stage is less effective than the previous one, approximately 1000–10,000 times; for example, for phosphoric acid α 1 > α 2 > α 3:
H3PO4⇄H++H2PO−4 (first stage, α 1),
H2PO−4⇄H++HPO2−4 (second stage, α 2),
НPO2−4⇄Н++PO3−4 (third stage, α 3).
For this reason, in a solution of this acid the concentration of hydrogen ions is the highest, and the concentration of phosphate ions PO3−4 is the lowest.
1. Solubility and the degree of dissociation of a substance are not related to each other. For example, acetic acid, which is highly (unlimitedly) soluble in water, is a weak electrolyte.
2. A solution of a weak electrolyte contains less than others those ions that are formed at the last stage of electrolytic dissociation
The degree of electrolytic dissociation is also affected adding other electrolytes: e.g. degree of dissociation of formic acid
HCOOH ⇄ HCOO − + H +
decreases if a little sodium formate is added to the solution. This salt dissociates to form formate ions HCOO − :
HCOONa → HCOO−+Na+
As a result, the concentration of HCOO– ions in the solution increases, and according to Le Chatelier’s principle, an increase in the concentration of formate ions shifts the equilibrium of the dissociation process of formic acid to the left, i.e. the degree of dissociation decreases.
Ostwald's dilution law- a relationship expressing the dependence of the equivalent electrical conductivity of a dilute solution of a binary weak electrolyte on the concentration of the solution:
Here is the dissociation constant of the electrolyte, is the concentration, and are the values of equivalent electrical conductivity at concentration and at infinite dilution, respectively. The relationship is a consequence of the law of mass action and equality
where is the degree of dissociation.
Ostwald's dilution law was derived by W. Ostwald in 1888 and he also confirmed it experimentally. The experimental establishment of the correctness of Ostwald's dilution law was of great importance for substantiating the theory of electrolytic dissociation.
Electrolytic dissociation of water. Hydrogen pH Water is a weak amphoteric electrolyte: H2O H+ + OH- or, more precisely: 2H2O = H3O+ + OH- The dissociation constant of water at 25°C is equal to: This value of the constant corresponds to the dissociation of one out of one hundred million water molecules, therefore the concentration of water can be considered constant and equal to 55.55 mol/l (density of water 1000 g/l, mass of 1 l 1000 g, amount of water substance 1000 g: 18 g/mol = 55.55 mol, C = 55.55 mol: 1 l = 55 .55 mol/l). Then This value is constant at a given temperature (25°C), it is called the ionic product of water KW: Dissociation of water is an endothermic process, therefore, with increasing temperature in accordance with Le Chatelier’s principle, dissociation intensifies, the ionic product increases and reaches a value of 10-13 at 100°C. In pure water at 25°C, the concentrations of hydrogen and hydroxyl ions are equal to each other: = = 10-7 mol/l Solutions in which the concentrations of hydrogen and hydroxyl ions are equal to each other are called neutral. If an acid is added to pure water, the concentration of hydrogen ions will increase and become greater than 10-7 mol/l, the medium will become acidic, and the concentration of hydroxyl ions will instantly change so that the ionic product of water retains its value of 10-14. The same thing will happen when adding alkali to clean water. The concentrations of hydrogen and hydroxyl ions are related to each other through the ionic product, therefore, knowing the concentration of one of the ions, it is easy to calculate the concentration of the other. For example, if = 10-3 mol/l, then = KW/ = 10-14/10-3 = 10-11 mol/l, or if = 10-2 mol/l, then = KW/ = 10-14 /10-2 = 10-12 mol/l. Thus, the concentration of hydrogen or hydroxyl ions can serve as a quantitative characteristic of the acidity or alkalinity of the medium. In practice, they do not use the concentrations of hydrogen or hydroxyl ions, but the hydrogen pH or hydroxyl pH indicators. The hydrogen pH indicator is equal to the negative decimal logarithm of the concentration of hydrogen ions: pH = - lg The hydroxyl indicator pH is equal to the negative decimal logarithm of the concentration of hydroxyl ions: pH = - lg It is easy to show by taking the logarithm of the ionic product of water that pH + pH = 14 If the pH of the medium is 7 - the medium is neutral, if less than 7 it is acidic, and the lower the pH, the higher the concentration of hydrogen ions. pH greater than 7 means the environment is alkaline; the higher the pH, the higher the concentration of hydroxyl ions.
Hydrolysis of salt" - To form an idea of chemistry as a productive force of society. Acetic acid CH3COOH is the oldest of organic acids. In acids there are carboxyl groups, but all the acids here are not strong.
All acids, their properties and bases are divided into strong and weak. For example, you cannot make a concentrated solution of a weak acid or a dilute solution of a strong base. Our water in this case plays the role of a base, since it receives a proton from hydrochloric acid. Acids that dissociate completely in aqueous solutions are called strong.
For oxides hydrated by an indefinite number of water molecules, for example Tl2O3 n H2O, it is unacceptable to write formulas like Tl(OH)3. It is also not recommended to call such compounds hydroxides.
For bases, you can quantify their strength, that is, the ability to abstract a proton from an acid. All bases are solids that have different colors. Attention! Alkalis are very caustic substances. If they come into contact with the skin, alkali solutions cause severe, long-healing burns; if they come into contact with the eyes, they can cause blindness. When cobalt minerals containing arsenic are fired, volatile, toxic arsenic oxide is released.
You already know such properties of the water molecule. II) and acetic acid solution. HNO2) - only one proton.
All bases are solid substances that have different colors. 1. Act on indicators. Indicators change color depending on interaction with different chemicals. When interacting with bases, they change their color: the methyl orange indicator turns yellow, the litmus indicator turns blue, and phenolphthalein becomes fuchsia.
Cool the containers, for example by placing them in a bowl of ice. Three solutions will remain clear, but the fourth will quickly become cloudy and a white precipitate will begin to form. This is where the barium salt is found. Set this container aside. You can quickly determine barium carbonate in another way. It's quite easy to do, all you need are porcelain steaming cups and a spirit lamp. If it is a lithium salt, the color will be bright red. By the way, if barium salt had been tested in the same way, the color of the flame should have been green.
An electrolyte is a substance that in its solid state is a dielectric, that is, it does not conduct electric current, but when dissolved or molten it becomes a conductor. Remember that the degree of dissociation and, accordingly, the strength of the electrolyte depend on many factors: the nature of the electrolyte itself, the solvent, and temperature. Therefore, this division itself is to a certain extent arbitrary. After all, the same substance can, under different conditions, be both a strong electrolyte and a weak one.
Hydrolysis does not occur, no new compounds are formed, and the acidity of the medium does not change. How does the acidity of the environment change? You don’t have to write down the reaction equations for now. All we have to do is discuss 4 groups of salts sequentially and give a specific “scenario” of hydrolysis for each of them. In the next part, we'll start with salts formed by a weak base and a strong acid.
After reading the article, you will be able to separate substances into salts, acids and bases. H solution, what common properties do acids and bases have. If they mean the definition of a Lewis acid, then in the text such an acid is called a Lewis acid.
The lower this indicator, the stronger the acid. Strong or weak - this is needed in the Ph.D. reference book. watch, but you need to know the classics. Strong acids are acids that can displace the anion of another acid from a salt.