To understand how the hydrolysis of salts proceeds 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, as a result a weak base (or acid) is formed, and the reaction of the medium also changes. Any salt can be represented as a chemical reaction product of a base and an acid. Depending on what their strength is, there are several options for the course of 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 medium, so it is expected to use different types of indicators to determine the pH value. For example, purple litmus is used for an acidic reaction, phenolphthalein is suitable for an alkaline reaction. Let us analyze in more detail the features of each hydrolysis variant. Strong and weak bases can be determined from the solubility table, and the strength of acids can be 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 hydroxochloride 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 one more example of hydrolysis proceeding for the described case. Consider the magnesium chloride salt. Magnesium hydroxide is a weak base, while hydrochloric acid is a strong base. In the process of interaction with water molecules, magnesium chloride turns into a basic salt (hydroxochloride). Magnesium hydroxide, the general formula of which is M(OH) 2 , is sparingly soluble in water, but strong hydrochloric acid makes the solution acidic.
Anion hydrolysis
The next variant 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 and a weak carbonic acid. Interaction with water molecules proceeds with the formation of an acid salt - sodium bicarbonate, that is, hydrolysis occurs along 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 (during hydrolysis), potassium hydrosulfite (acid salt) and potassium hydroxide (alkali) are formed. The environment in the solution will be alkaline, it 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 its peculiarity, 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 aluminum sulfide as an example. This salt is formed by aluminum hydroxide, which is a weak base, as well as a weak hydrosulphuric 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. Such an interaction occurs both in the cation and in the anion; therefore, this hydrolysis option is considered complete.
Magnesium sulfide can also be cited as an example of the interaction of this type of salt with water. This salt contains magnesium hydroxide, its formula is Mg (OH) 2. It is a weak base, insoluble in water. In addition, there is hydrosulfide acid inside magnesium sulfide, which is weak. When interacting with water, complete hydrolysis occurs (according to the cation and anion), as a result of which magnesium hydroxide is formed in the form of a precipitate, and hydrogen sulfide is also released in the form of a gas.
If we consider the hydrolysis of a salt, which is formed by a strong acid and a strong base, it should be noted that it does not proceed. 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, 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, which are sparingly soluble in water. As their hydrolysis occurs, the formation of hydrogen sulfide, its release in the process of volcanic activity to the surface of the earth.
Silicate rocks, when converted 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 also occurs in the oceans. and calcium, which are carried out by water, have a slightly alkaline environment. Under such conditions, the process of photosynthesis in marine plants proceeds well, and marine organisms develop more intensively.
Oil contains impurities of water and salts of calcium and magnesium. In the process of heating oil, they interact with water vapor. During hydrolysis, hydrogen chloride is formed, the interaction of which with the metal causes the destruction of 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, what common 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. In order to somehow make life easier, our three friends, Arrhenius and Bronsted with Lowry, came up with two theories that are called by their name.
Like metals and non-metals, acids and bases are the separation of substances according to similar properties. The first theory of acids and bases belonged to the Swedish scientist Arrhenius. An Arrhenius acid is a class of substances that, in reaction with water, dissociate (decompose), forming a hydrogen cation H +. Arrhenius bases in aqueous solution form OH - anions. The following theory was proposed in 1923 by the scientists Brönsted 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, respectively, are substances capable of accepting a proton in a reaction. The current 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 they mean 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 are valid for acids and bases.
Dissociation
Dissociation is the process of disintegration of a substance into ions in solutions or melts. For example, the dissociation of hydrochloric acid is the breakdown of HCl into H + and Cl - .
Properties of acids and bases
Bases tend to be soapy to the touch, while acids tend to taste sour.
When a base reacts with many cations, a precipitate is formed. When an acid reacts with anions, 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
Dissolve in water only partially, 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: the 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 hydrofluoric
- 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 bases
Strong bases completely dissociate in water:
NaOH (solution) + H 2 O ↔ NH 4
Strong bases include hydroxides of metals of the first (alkalins, alkali metals) and the second (alcaline terrenes, 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 bases
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
Such a reaction is called neutralization: if 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 view of the reaction:
Weak base (solution) + H 2 O ↔ Weak acid (solution) + OH - (solution)
Strong base and weak acid
The base completely dissociates, the acid partially dissociates, the resulting solution has weak base properties:
HX (solution) + OH - (solution) ↔ H 2 O + X - (solution)
Strong acid and weak base
The acid completely dissociates, the base does not completely dissociate:
Water dissociation
Dissociation is the breakdown of a substance into its constituent 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 takes place: = 10 -14 , which is called the dissociation constant of water. For pure water = = 10 -7 , whence -lg = 7.0.
This value (-lg) is called pH - the potential of hydrogen. 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 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 range of pH values 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 O 2- . In a weak aqueous solution, 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: neither H + nor OH - ions are released 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 for metals of the first and second groups, have acidic properties.
buffer solution
Solutions that maintain their pH when a small amount of a strong acid or strong base is added generally consist of:
- A mixture of a weak acid, the 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 corresponding salt, while taking into account:
- pH range in which the buffer solution will be effective
- The capacity of a solution is the amount of strong acid or strong base that can be added without affecting the pH of the solution.
- No undesired reactions should occur that could change the composition of the solution
Test:
All acids, their properties and bases are divided into strong and weak. But don't you dare confuse concepts like "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 strongly 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, as it receives a proton from hydrochloric acid. Acids that dissociate completely in aqueous solutions are called strong acids.
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 iodine;
4)
HNO3, nitric acid;
5)
HClO4 - perchloric acid;
6)
H2SO4 is sulfuric acid.
All known acids (with the exception of sulfuric acid) are listed 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.
Electrolytes are strong bases; they completely dissociate in aqueous solutions to form a hydroxide ion.
Like with acids, calculating the concentration of hydroxide ion is very easy once 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. Foundations and properties
As for weak acids, they do not completely dissociate, that is, partially. Distinguishing strong and weak acids is very simple: if in the reference table next to the name of the acid its constant is shown, 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 a dissociation constant K.
ELECTROLYTES Substances whose solutions or melts conduct electricity.
NON-ELECTROLYTES Substances whose solutions or melts do not conduct electricity.
Dissociation- decomposition of compounds into ions.
Degree of dissociation is 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 the equations of dissociation of strong electrolytes put an equal sign.
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 the solubility table);
Bases of alkali (LiOH, NaOH, KOH) and alkaline earth (Ca (OH) 2, Sr (OH) 2, Ba (OH) 2) metals ( see strong electrolyte bases in the solubility table).
WEAK ELECTROLYTES in aqueous solutions only partially (reversibly) dissociate into ions.
When writing the dissociation equations for weak electrolytes, the sign of reversibility is put.
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) ( see basescweak electrolytes in the solubility table).
The degree of electrolytic dissociation is influenced by a number of factors:
the 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, have solvents with a high dielectric constant, the molecules of which are polar (for example, water);
temperature: since dissociation is an endothermic process, an increase in 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⇄Н++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 PO3−4 phosphate ions is the lowest.
1. Solubility and the degree of dissociation of a substance are not related to each other. For example, a weak electrolyte is acetic acid, which is highly (unrestrictedly) soluble in water.
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 by addition of 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 the Le Chatelier principle, an increase in the concentration of formate ions shifts the equilibrium of the formic acid dissociation process to the left, i.e. the degree of dissociation decreases.
Ostwald dilution law- ratio 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 the equivalent electrical conductivity at concentration and at infinite dilution, respectively. The ratio is a consequence of the law of mass action and equality
where is the degree of dissociation.
The Ostwald dilution law was developed by W. Ostwald in 1888 and confirmed by him experimentally. The experimental establishment of the correctness of the Ostwald dilution law was of great importance for substantiating the theory of electrolytic dissociation.
Electrolytic dissociation of water. Hydrogen indicator pH Water is a weak amphoteric electrolyte: H2O H+ + OH- or, more precisely: 2H2O \u003d H3O + + OH- The dissociation constant of water at 25 ° C is: can be considered constant and equal to 55.55 mol / l (water density 1000 g / l, mass 1 l 1000 g, amount of water substance 1000g: 18g / mol \u003d 55.55 mol, C \u003d 55.55 mol: 1 l \u003d 55 .55 mol/l). Then This value is constant at a given temperature (25 ° C), it is called the ion product of water KW: The dissociation of water is an endothermic process, therefore, with an increase in temperature, in accordance with the Le Chatelier principle, dissociation increases, the ion 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 acid is added to pure water, the concentration of hydrogen ions will increase and become more than 10-7 mol / l, the medium will become acidic, while the concentration of hydroxyl ions will instantly change so that the ion product of water retains its value of 10-14. The same thing will happen when alkali is added to pure water. The concentrations of hydrogen and hydroxyl ions are related to each other through the ion 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, it is not the concentrations of hydrogen or hydroxyl ions that are used, but the hydrogen pH or hydroxyl pOH indicators. The hydrogen index pH is equal to the negative decimal logarithm of the concentration of hydrogen ions: pH = - lg The hydroxyl index pOH is equal to the negative decimal logarithm of the concentration of hydroxyl ions: pOH = - lg It is easy to show by prolonging the ionic product of water that pH + pOH = 14 the medium is neutral, if less than 7 - acidic, and the lower the pH, the higher the concentration of hydrogen ions. pH greater than 7 - alkaline environment, the higher the pH, the higher the concentration of hydroxyl ions.
Salt hydrolysis" - To form an idea of chemistry as a productive force of society. Acetic acid CH3COOH is the oldest of the organic acids. In acids - carboxyl groups, But all the acids here are weak.
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, as it receives a proton from hydrochloric acid. Acids that dissociate completely in aqueous solutions are called strong acids.
For oxides hydrated with an indefinite number of water molecules, for example, Tl2O3 n H2O, it is unacceptable to write formulas like Tl(OH)3. Calling such compounds hydroxides is also not recommended.
For bases, one can quantify their strength, that is, the ability to split off a proton from an acid. All bases are solids with different colors. Attention! Alkalis are very caustic substances. If it comes into contact with the skin, alkali solutions cause severe long-healing burns, if they get into the eyes, they can cause blindness. When roasting cobalt minerals containing arsenic, volatile toxic arsenic oxide is released.
These properties of the water molecule are already known to you. II) and a solution of acetic acid. HNO2) - only one proton.
All bases are solids that have different colors. 1. They act on indicators. Indicators change their color depending on the 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 turns fuchsia.
Cool the containers, for example by placing them in a vessel filled with ice. Three solutions will remain clear, and the fourth will quickly become cloudy, a white precipitate will begin to fall out. This is where the barium salt is located. Set this container aside. You can quickly determine barium carbonate in another way. This is fairly easy to make, all you need are porcelain evaporating cups and a spirit lamp. If it is a lithium salt, the color will be bright red. By the way, if barium salt were tested in the same way, the color of the flame should have been green.
An electrolyte is a substance that in the solid state is a dielectric, that is, does not conduct electric current, however, in a dissolved or molten form 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 the temperature. Therefore, this division itself is to a certain extent conditional. 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, the acidity of the medium does not change. How does the acidity of the environment change? The reaction equations can not yet be written down. It remains for us to sequentially discuss 4 groups of salts and for each of them give a specific "scenario" of hydrolysis. In the next part, we will start with salts formed from 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 are the general properties of acids and bases. If they mean the definition of a Lewis acid, then in the text such an acid is called a Lewis acid.
The lower this value, the stronger the acid. Strong or weak - this is needed in the reference book of Ph.D. watch, but you need to know the classics. Strong acids are acids that can displace the anion of another acid from the salt.