Lecture: Salt hydrolysis. Environment of aqueous solutions: acidic, neutral, alkaline
Salt hydrolysisWe continue to study the patterns of flow chemical reactions. While studying the topic, you learned that when electrolytic dissociation in an aqueous solution, the particles involved in the reaction of substances dissolve in water. This is hydrolysis. Various inorganic and organic matter in particular salt. Without understanding the process of hydrolysis of salts, you will not be able to explain the phenomena that occur in living organisms.
The essence of salt hydrolysis is reduced to the exchange process of interaction of ions (cations and anions) of the salt with water molecules. As a result, a weak electrolyte is formed - a low-dissociating compound. An excess of free H + or OH - ions appears in an aqueous solution. Remember, the dissociation of which electrolytes forms H + ions, and which OH -. As you guessed, in the first case we are dealing with an acid, which means that the aqueous medium with H + ions will be acidic. In the second case, alkaline. In the water itself, the medium is neutral, since it slightly dissociates into H + and OH - ions of the same concentration.
The nature of the environment can be determined using indicators. Phenolphthalein detects an alkaline environment and colors the solution crimson. Litmus turns red with acid and blue with alkali. Methyl orange - orange, in an alkaline environment it becomes yellow, in an acidic environment - pink. The type of hydrolysis depends on the type of salt.
Salt types
So, any salt is an interaction of an acid and a base, which, as you understand, are strong and weak. Strong are those whose degree of dissociation α is close to 100%. It should be remembered that sulfurous (H 2 SO 3) and phosphoric (H 3 PO 4) acid are often referred to as medium strength acids. When solving hydrolysis problems, these acids must be classified as weak.
Acids:
Strong: HCl; HBr; Hl; HNO3; HClO 4 ; H2SO4. Their acid residues do not interact with water.
Weak: HF; H2CO3; H 2 SiO 3 ; H2S; HNO2; H2SO3; H3PO4; organic acids. And their acidic residues interact with water, taking hydrogen cations H + from its molecules.
Reasons:
Strong: soluble metal hydroxides; Ca(OH) 2 ; Sr(OH) 2 . Their metal cations do not interact with water.
Weak: insoluble metal hydroxides; ammonium hydroxide (NH 4 OH). And metal cations here interact with water.
Based on this material, considersalt types :
Salts with a strong base and strong acid. For example: Ba (NO 3) 2, KCl, Li 2 SO 4. Features: do not interact with water, which means they do not undergo hydrolysis. Solutions of such salts have a neutral reaction medium.
Salts with a strong base and a weak acid. For example: NaF, K 2 CO 3 , Li 2 S. Features: acid residues of these salts interact with water, anion hydrolysis occurs. The medium of aqueous solutions is alkaline.
Salt co weak base and strong acid. For example: Zn (NO 3) 2, Fe 2 (SO 4) 3, CuSO 4. Features: only metal cations interact with water, cation hydrolysis occurs. Wednesday is sour.
Salts with a weak base and a weak acid. For example: CH 3 COONН 4, (NH 4) 2 CO 3 , HCOONН 4. Features: both cations and anions of acid residues interact with water, hydrolysis occurs by cation and anion.
An example of hydrolysis at the cation and the formation of an acidic environment:
Hydrolysis of ferric chloride FeCl 2
FeCl 2 + H 2 O ↔ Fe(OH)Cl + HCl(molecular equation)
Fe 2+ + 2Cl - + H + + OH - ↔ FeOH + + 2Cl - + H+ (full ionic equation)
Fe 2+ + H 2 O ↔ FeOH + + H + (abbreviated ionic equation)
An example of anion hydrolysis and the formation of an alkaline environment:
Hydrolysis of sodium acetate CH 3 COONa
CH 3 COONa + H 2 O ↔ CH 3 COOH + NaOH(molecular equation)
Na + + CH 3 COO - + H 2 O ↔ Na + + CH 3 COOH + OH- (full ionic equation)
CH 3 COO - + H 2 O ↔ CH 3 COOH + OH -(abbreviated ionic equation)
An example of co-hydrolysis:
- Hydrolysis of aluminum sulfide Al 2 S 3
Al 2 S 3 + 6H2O ↔ 2Al(OH) 3 ↓+ 3H 2 S
In this case, we see complete hydrolysis, which occurs if the salt is formed by a weak insoluble or volatile base and a weak insoluble or volatile acid. In the solubility table there are dashes on such salts. If during the ion exchange reaction a salt is formed that does not exist in an aqueous solution, then it is necessary to write the reaction of this salt with water.
For example:
2FeCl 3 + 3Na 2 CO 3 ↔ Fe 2 (CO 3) 3+ 6NaCl
Fe 2 (CO 3) 3+ 6H 2 O ↔ 2Fe(OH) 3 + 3H 2 O + 3CO 2
We add these two equations, then what is repeated in the left and right parts, reduce:
2FeCl 3 + 3Na 2 CO 3 + 3H 2 O ↔ 6NaCl + 2Fe(OH) 3 ↓ + 3CO 2
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During the lesson, we will study the topic “Hydrolysis. Medium of aqueous solutions. Hydrogen indicator". You will learn about hydrolysis - the exchange reaction of a substance with water, leading to decomposition chemical. In addition, a definition will be introduced for the hydrogen index - the so-called pH.
Topic: Solutions and their concentration, dispersed systems, electrolytic dissociation
Lesson: Hydrolysis. Medium of aqueous solutions. Hydrogen indicator
Hydrolysis - is the exchange reaction of a substance with water, leading to its decomposition. Let's try to understand the reason for this phenomenon.
Electrolytes are divided into strong electrolytes and weak ones. See Table. one.
Tab. one
Water belongs to weak electrolytes and therefore dissociates into ions only to a small extent. H 2 O ↔ H + + OH -
Ions of substances entering the solution are hydrated by water molecules. However, another process may also take place. For example, salt anions, which are formed during its dissociation, can interact with hydrogen cations, which, albeit to a small extent, are nevertheless formed during the dissociation of water. In this case, a shift in the equilibrium of water dissociation can occur. Let's denote the acid anion X - .
Let's assume the acid is strong. Then, by definition, it almost completely decays into ions. If a weak acid, then it dissociates incompletely. It will be formed when salt anions and hydrogen ions are added to water, resulting from the dissociation of water. Due to its formation, hydrogen ions will bind in the solution, and their concentration will decrease. H + + X - ↔ HX
But, according to Le Chatelier's rule, with a decrease in the concentration of hydrogen ions, the equilibrium shifts in the first reaction in the direction of their formation, i.e., to the right. The hydrogen ions will bind to the hydrogen ions of the water, but the hydroxide ions will not, and there will be more of them than there were in the water before the salt was added. Means, solution will be alkaline. The phenolphthalein indicator will turn crimson. See fig. one.
Rice. one
Similarly, we can consider the interaction of cations with water. Without repeating the whole chain of reasoning, we summarize that if the base is weak, then hydrogen ions will accumulate in the solution, and the environment will be acidic.
Salt cations and anions can be divided into two types. Rice. 2.
Rice. 2. Classification of cations and anions according to the strength of electrolytes
Since both cations and anions, according to this classification, are of two types, there are 4 different combinations in total in the formation of their salts. Let us consider how each of the classes of these salts relates to hydrolysis. Tab. 2.
|
What is the strength of the acid and base to form the salt? |
Salt examples |
Relation to hydrolysis |
Wednesday |
Litmus coloring |
|
Salt of a strong base and a strong acid |
NaCl, Ba(NO 3) 2 , K 2 SO 4 |
Hydrolysis is not subject. |
neutral |
violet |
|
Salt of a weak base and a strong acid |
ZnSO 4 , AlCl 3 , Fe(NO 3) 3 |
Hydrolysis at the cation. Zn 2+ + HOH ZnOH + + H + |
||
|
Salt of a strong base and a weak acid |
Na 2 CO 3, K 2 SiO 3, Li 2 SO 3 |
Anion hydrolysis CO 3 2 + HOH |
alkaline |
|
|
Salt of a weak base and a weak acid |
FeS, Al(NO 2) 3 , CuS |
Hydrolysis of both the anion and the cation. |
the medium of the solution depends on which of the compounds formed will be the weaker electrolyte. |
depends on the stronger electrolyte. |
HCO3+OHTab. 2.
Hydrolysis can be enhanced by diluting the solution or by heating the system.
Salts that undergo irreversible hydrolysis
Ion exchange reactions proceed to the end when a precipitate forms, a gas or a poorly dissociable substance is released.
2 Al (NO 3) 3 + 3 Na 2 S +6H 2 O→ 2 Al (OH) 3 ↓+ 3 H 2 S+6 NaNO 3(1)
If we take a salt of a weak base and a weak acid, and both the cation and the anion are multiply charged, then the hydrolysis of such salts will form both an insoluble hydroxide of the corresponding metal and a gaseous product. In this case, hydrolysis may become irreversible. For example, in reaction (1) no precipitate of aluminum sulfide is formed.
The following salts fall under this rule: Al 2 S 3, Cr 2 S 3, Al 2 (CO 3) 3, Cr 2 (CO 3) 3, Fe 2 (CO 3) 3, CuCO 3. These salts in the aquatic environment undergo irreversible hydrolysis. They cannot be obtained in aqueous solution.
AT organic chemistry hydrolysis is very great importance.
Hydrolysis changes the concentration of hydrogen ions in solution, and many reactions use acids or bases. Therefore, if we know the concentration of hydrogen ions in a solution, it will be easier to monitor and control the process. To quantitatively characterize the content of ions in a solution, the pH of the solution is used. It is equal to the negative logarithm of the concentration of hydrogen ions.
pH = -lg [ H + ]
The concentration of hydrogen ions in water is 10 -7 degrees, respectively, pH = 7 in absolutely pure water at room temperature.
If you add an acid to a solution or add a salt of a weak base and a strong acid, then the concentration of hydrogen ions will become more than 10 -7 and pH< 7.
If alkali or salts of a strong base and a weak acid are added, the concentration of hydrogen ions will become less than 10 -7 and pH>7. See fig. 3. To know the quantitative indicator of acidity is necessary in many cases. For example, the pH of gastric juice is 1.7. An increase or decrease in this value leads to a violation of the digestive functions of a person. AT agriculture soil acidity is monitored. For example, soil with pH = 5-6 is the best for gardening. When deviating from these values, acidifying or alkalizing additives are introduced into the soil.
Rice. 3
Summing up the lesson
During the lesson, we studied the topic “Hydrolysis. Medium of aqueous solutions. Hydrogen indicator. You learned about hydrolysis - the exchange reaction of a substance with water, leading to the decomposition of a chemical substance. In addition, a definition was introduced for the hydrogen index - the so-called pH.
Bibliography
1. Rudzitis G.E. Chemistry. Basics general chemistry. Grade 11: textbook for educational institutions: basic level / G.E. Rudzitis, F.G. Feldman. - 14th ed. - M.: Education, 2012.
2. Popel P.P. Chemistry: Grade 8: a textbook for general education educational institutions/ P.P. Popel, L.S. Krivlya. - K .: Information Center "Academy", 2008. - 240 p.: ill.
3. Gabrielyan O.S. Chemistry. Grade 11. A basic level of. 2nd ed., ster. - M.: Bustard, 2007. - 220 p.
Homework
1. No. 6-8 (p. 68) Rudzitis G.E. Chemistry. Fundamentals of General Chemistry. Grade 11: textbook for educational institutions: basic level / G.E. Rudzitis, F.G. Feldman. - 14th ed. - M.: Education, 2012.
2. Why is the pH of rainwater always less than 7?
3. What causes the crimson color of a sodium carbonate solution?
In order to understand what hydrolysis of salts is, let us first recall how acids and alkalis dissociate.
What all acids have in common is that when they dissociate, hydrogen cations (H +) are necessarily formed, while when all alkalis dissociate, hydroxide ions (OH -) are always formed.
In this regard, if in a solution, for one reason or another, there are more H + ions, they say that the solution has an acid reaction of the environment, if OH − - an alkaline reaction of the environment.
If everything is clear with acids and alkalis, then what will be the reaction of the medium in salt solutions?
At first glance, it should always be neutral. And the truth is, where, for example, in a solution of sodium sulfide, an excess of hydrogen cations or hydroxide ions can come from. Sodium sulfide itself does not form ions of either type during dissociation:
Na 2 S \u003d 2Na + + S 2-
However, if you had, for example, aqueous solutions of sodium sulfide, sodium chloride, zinc nitrate and an electronic pH meter (a digital device for determining the acidity of a medium), you would find unusual phenomenon. The instrument would show you that the pH of the sodium sulfide solution is greater than 7, i.e. it has a clear excess of hydroxide ions. The environment of the sodium chloride solution would be neutral (pH = 7), and the solution of Zn(NO 3) 2 would be acidic.
The only thing that meets our expectations is the sodium chloride solution medium. It turned out to be neutral, as expected.
But where did the excess of hydroxide ions in the sodium sulfide solution and hydrogen cations in the zinc nitrate solution come from?
Let's try to figure it out. To do this, we need to learn the following theoretical points.
Any salt can be thought of as the reaction product of an acid and a base. Acids and bases are divided into strong and weak. Recall that those acids and bases, the degree of dissociation of which is close to 100%, are called strong.
note: sulfurous (H 2 SO 3) and phosphoric (H 3 PO 4) are often referred to as medium strength acids, but when considering hydrolysis tasks, they should be classified as weak.
Acidic residues of weak acids are capable of reversibly interacting with water molecules, tearing off hydrogen cations H + from them. For example, the sulfide ion, being the acid residue of a weak hydrosulfide acid, interacts with it in the following way:
S 2- + H 2 O ↔ HS - + OH -
HS - + H 2 O ↔ H 2 S + OH -
As can be seen, as a result of this interaction, an excess of hydroxide ions is formed, which is responsible for the alkaline reaction of the medium. That is, the acid residues of weak acids increase the alkalinity of the medium. In the case of salt solutions containing such acidic residues, it is said that for them anion hydrolysis.
Acid residues of strong acids, unlike weak ones, do not interact with water. That is, they do not affect the pH of the aqueous solution. For example, the chloride ion, being the acidic residue of a strong of hydrochloric acid, does not react with water:
That is, chloride ions do not affect the pH of the solution.
Of the metal cations, only those that correspond to weak bases are also able to interact with water. For example, the Zn 2+ cation, which corresponds to the weak base zinc hydroxide. In aqueous solutions of zinc salts, the following processes occur:
Zn 2+ + H 2 O ↔ Zn(OH) + + H +
Zn(OH) + + H 2 O ↔ Zn(OH) + + H +
As can be seen from the equations above, as a result of the interaction of zinc cations with water, hydrogen cations accumulate in the solution, which increase the acidity of the medium, that is, lower the pH. If the composition of the salt includes cations, which correspond to weak bases, in this case they say that the salt hydrolyzed at the cation.
Metal cations, which correspond to strong bases, do not interact with water. For example, the Na + cation corresponds to a strong base - sodium hydroxide. Therefore, sodium ions do not react with water and do not affect the pH of the solution in any way.
Thus, based on the foregoing, salts can be divided into 4 types, namely, formed:
1) strong base and strong acid,
Such salts contain neither acidic residues nor metal cations that interact with water, i.e. capable of affecting the pH of an aqueous solution. Solutions of such salts have a neutral reaction medium. Such salts are said to be do not undergo hydrolysis.
Examples: Ba(NO 3) 2 , KCl, Li 2 SO 4 etc.
2) strong base and weak acid
In solutions of such salts, only acid residues react with water. The environment of aqueous solutions of such salts is alkaline; in relation to salts of this type, they say that they hydrolyze at the anion
Examples: NaF, K 2 CO 3 , Li 2 S, etc.
3) weak base and strong acid
In such salts, cations react with water, and acidic residues do not react - salt hydrolysis at the cation, acidic environment.
Examples: Zn(NO 3) 2, Fe 2 (SO 4) 3, CuSO 4, etc.
4) weak base and weak acid.
Both cations and anions of acid residues react with water. The hydrolysis of salts of this kind is both cation and anion or. They also talk about such salts that they are exposed to irreversible hydrolysis.
What does it mean that they are irreversibly hydrolyzed?
Since in this case both metal cations (or NH 4 +) and anions of the acid residue react with water, both H + ions and OH − ions simultaneously appear in the solution, which form an extremely low dissociating substance - water (H 2 O).
This, in turn, leads to the fact that salts formed by acidic residues of weak bases and weak acids cannot be obtained by exchange reactions, but only by solid-phase synthesis, or cannot be obtained at all. For example, when mixing a solution of aluminum nitrate with a solution of sodium sulfide, instead of the expected reaction:
2Al(NO 3) 3 + 3Na 2 S \u003d Al 2 S 3 + 6NaNO 3 (- so the reaction does not proceed!)
The following reaction is observed:
2Al(NO 3) 3 + 3Na 2 S + 6H 2 O= 2Al(OH) 3 ↓+ 3H 2 S + 6NaNO 3
However, aluminum sulfide can be obtained without problems by fusing aluminum powder with sulfur:
2Al + 3S = Al 2 S 3
When aluminum sulfide is added to water, it, as well as when trying to obtain it in an aqueous solution, undergoes irreversible hydrolysis.
Al 2 S 3 + 6H 2 O \u003d 2Al (OH) 3 ↓ + 3H 2 S
Salt hydrolysis. Environment of aqueous solutions: acidic, neutral, alkaline
According to the theory of electrolytic dissociation, in an aqueous solution, solute particles interact with water molecules. Such an interaction can lead to a hydrolysis reaction (from the Greek. hydro- water, lysis disintegration, decay).
Hydrolysis is a reaction of the metabolic decomposition of a substance by water.
are subjected to hydrolysis various substances: inorganic - salts, carbides and hydrides of metals, non-metal halides; organic - haloalkanes, esters and fats, carbohydrates, proteins, polynucleotides.
Aqueous solutions of salts have different pH values and different types of media - acidic ($pH 7$), neutral ($pH = 7$). This is due to the fact that salts in aqueous solutions can undergo hydrolysis.
The essence of hydrolysis is reduced to the exchange chemical interaction cations or anions of salt with water molecules. As a result of this interaction, a low-dissociating compound (weak electrolyte) is formed. And in an aqueous salt solution, an excess of free $H^(+)$ or $OH^(-)$ ions appears, and the salt solution becomes acidic or alkaline, respectively.
Salt classification
Any salt can be thought of as the product of the interaction of a base with an acid. For example, the salt $KClO$ is formed by the strong base $KOH$ and the weak acid $HClO$.
Depending on the strength of the base and acid, four types of salts can be distinguished.
Consider the behavior of salts various types in solution.
1. Salts formed by a strong base and a weak acid.
For example, the potassium cyanide salt $KCN$ is formed by the strong base $KOH$ and the weak acid $HCN$:
$(KOH)↙(\text"strong monoacid base")←KCN→(HCN)↙(\text"weak monoacid acid")$
1) a slight reversible dissociation of water molecules (a very weak amphoteric electrolyte), which can be written in a simplified way using the equation
$H_2O(⇄)↖(←)H^(+)+OH^(-);$
$KCN=K^(+)+CN^(-)$
The $H^(+)$ and $CN^(-)$ ions formed during these processes interact with each other, binding into weak electrolyte molecules - hydrocyanic acid $HCN$, while the hydroxide - the $OH^(-)$ ion remains in solution, thus making it alkaline. Hydrolysis occurs at the $CN^(-)$ anion.
We write the full ionic equation of the ongoing process (hydrolysis):
$K^(+)+CN^(-)+H_2O(⇄)↖(←)HCN+K^(+)+OH^(-).$
This process is reversible and chemical equilibrium shifted to the left (toward the formation of the starting substances), because water is a much weaker electrolyte than hydrocyanic acid $HCN$.
$CN^(-)+H_2O⇄HCN+OH^(-).$
The equation shows that:
a) there are free hydroxide ions $OH^(-)$ in the solution, and their concentration is greater than in clean water, so the salt solution $KCN$ has alkaline environment($pH > 7$);
b) $CN^(-)$ ions participate in the reaction with water, in which case they say that there is anion hydrolysis. Other examples of anions that react with water are:
Consider the hydrolysis of sodium carbonate $Na_2CO_3$.
$(NaOH)↙(\text"strong monoacid base")←Na_2CO_3→(H_2CO_3)↙(\text"weak dibasic acid")$
The salt is hydrolyzed at the $CO_3^(2-)$ anion.
$2Na^(+)+CO_3^(2-)+H_2O(⇄)↖(←)HCO_3^(-)+2Na^(+)+OH^(-).$
$CO_2^(2-)+H_2O⇄HCO_3^(-)+OH^(-).$
Hydrolysis products - acid salt$NaHCO_3$ and sodium hydroxide $NaOH$.
The environment of an aqueous solution of sodium carbonate is alkaline ($pH > 7$), because the concentration of $OH^(-)$ ions increases in the solution. The acid salt $NaHCO_3$ can also undergo hydrolysis, which proceeds to a very small extent, and it can be neglected.
To summarize what you have learned about anion hydrolysis:
a) at the anion of the salt, as a rule, they hydrolyze reversibly;
b) the chemical equilibrium in such reactions is strongly shifted to the left;
c) the reaction of the medium in solutions of similar salts is alkaline ($рН > 7$);
d) during the hydrolysis of salts formed by weak polybasic acids, acidic salts are obtained.
2. Salts formed from a strong acid and a weak base.
Consider the hydrolysis of ammonium chloride $NH_4Cl$.
$(NH_3 H_2O)↙(\text"weak monoacid base")←NH_4Cl→(HCl)↙(\text"strong monobasic acid")$
Two processes take place in an aqueous solution of salt:
1) a slight reversible dissociation of water molecules (a very weak amphoteric electrolyte), which can be written in a simplified way using the equation:
$H_2O(⇄)↖(←)H^(+)+OH^(-)$
2) complete dissociation of salt (strong electrolyte):
$NH_4Cl=NH_4^(+)+Cl^(-)$
The resulting $OH^(-)$ and $NH_4^(+)$ ions interact with each other to obtain $NH_3 H_2O$ (weak electrolyte), while the $H^(+)$ ions remain in the solution, causing the most of its acidic environment.
Full ionic hydrolysis equation:
$NH_4^(+)+Cl^(-)+H_2O(⇄)↖(←)H^(+)+Cl^(-)NH_3 H_2O$
The process is reversible, the chemical equilibrium is shifted towards the formation of the starting substances, because water $Н_2О$ is a much weaker electrolyte than ammonia hydrate $NH_3·H_2O$.
Abbreviated ionic hydrolysis equation:
$NH_4^(+)+H_2O⇄H^(+)+NH_3 H_2O.$
The equation shows that:
a) there are free hydrogen ions $H^(+)$ in the solution, and their concentration is greater than in pure water, so the salt solution has acid environment($pH
b) ammonium cations $NH_4^(+)$ participate in the reaction with water; in that case they say it's coming cation hydrolysis.
Multicharged cations can also participate in the reaction with water: two-shot$M^(2+)$ (for example, $Ni^(2+), Cu^(2+), Zn^(2+)…$), except for cations alkaline earth metals, three-shot$M^(3+)$ (for example, $Fe^(3+), Al^(3+), Cr^(3+)…$).
Let us consider the hydrolysis of nickel nitrate $Ni(NO_3)_2$.
$(Ni(OH)_2)↙(\text"weak diacid base")←Ni(NO_3)_2→(HNO_3)↙(\text"strong monobasic acid")$
The salt is hydrolyzed at the $Ni^(2+)$ cation.
Full ionic hydrolysis equation:
$Ni^(2+)+2NO_3^(-)+H_2O(⇄)↖(←)NiOH^(+)+2NO_3^(-)+H^(+)$
Abbreviated ionic hydrolysis equation:
$Ni^(2+)+H_2O⇄NiOH^(+)+H^(+).$
Hydrolysis products - basic salt$NiOHNO_3$ and Nitric acid$HNO_3$.
The medium of an aqueous solution of nickel nitrate is acidic ($ pH
The hydrolysis of the $NiOHNO_3$ salt proceeds to a much lesser degree and can be neglected.
To summarize what you have learned about cation hydrolysis:
a) by the cation of the salt, as a rule, they are hydrolyzed reversibly;
b) the chemical equilibrium of reactions is strongly shifted to the left;
c) the reaction of the medium in solutions of such salts is acidic ($ pH
d) during the hydrolysis of salts formed by weak polyacid bases, basic salts are obtained.
3. Salts formed from a weak base and a weak acid.
It is obviously already clear to you that such salts undergo hydrolysis both at the cation and at the anion.
A weak base cation binds $OH^(-)$ ions from water molecules, forming weak base; anion of a weak acid binds $H^(+)$ ions from water molecules, forming weak acid. The reaction of solutions of these salts can be neutral, slightly acidic or slightly alkaline. It depends on the dissociation constants of two weak electrolytes - acids and bases, which are formed as a result of hydrolysis.
For example, consider the hydrolysis of two salts: ammonium acetate $NH_4(CH_3COO)$ and ammonium formate $NH_4(HCOO)$:
1) $(NH_3 H_2O)↙(\text"weak monoacid base")←NH_4(CH_3COO)→(CH_3COOH)↙(\text"strong monobasic acid");$
2) $(NH_3 H_2O)↙(\text"weak monoacid base")←NH_4(HCOO)→(HCOOH)↙(\text"weak monobasic acid").$
In aqueous solutions of these salts, weak base cations $NH_4^(+)$ interact with hydroxide ions $OH^(-)$ (recall that water dissociates $H_2O⇄H^(+)+OH^(-)$), and anions weak acids $CH_3COO^(-)$ and $HCOO^(-)$ interact with $Н^(+)$ cations to form molecules of weak acids — acetic $CH_3COOH$ and formic $HCOOH$.
Let's write down ionic equations hydrolysis:
1) $CH_3COO^(-)+NH_4^(+)+H_2O⇄CH_3COOH+NH_3 H_2O;$
2) $HCOO^(-)+NH_4^(+)+H_2O⇄NH_3 H_2O+HCOOH.$
In these cases, hydrolysis is also reversible, but the equilibrium is shifted towards the formation of hydrolysis products—two weak electrolytes.
In the first case, the solution medium is neutral ($рН = 7$), because $K_D(CH_3COOH)=K+D(NH_3 H_2O)=1.8 10^(-5)$. In the second case, the medium of the solution is weakly acidic ($pH
As you have already noticed, the hydrolysis of most salts is a reversible process. In a state of chemical equilibrium, only part of the salt is hydrolyzed. However, some salts are completely decomposed by water, i.e. their hydrolysis is an irreversible process.
In the table "Solubility of acids, bases and salts in water" you will find a note: "decompose in the aquatic environment" - this means that such salts undergo irreversible hydrolysis. For example, aluminum sulfide $Al_2S_3$ in water undergoes irreversible hydrolysis, since the $H^(+)$ ions that appear during hydrolysis at the cation are bound by the $OH^(-)$ ions formed during hydrolysis at the anion. This enhances hydrolysis and leads to the formation of insoluble aluminum hydroxide and hydrogen sulfide gas:
$Al_2S_3+6H_2O=2Al(OH)_3↓+3H_2S$
Therefore, aluminum sulfide $Al_2S_3$ cannot be obtained by an exchange reaction between aqueous solutions of two salts, for example aluminum chloride $AlCl_3$ and sodium sulfide $Na_2S$.
Other cases of irreversible hydrolysis are also possible, they are not difficult to predict, because for the irreversibility of the process it is necessary that at least one of the hydrolysis products leave the reaction sphere.
To summarize what you have learned about both cation and anion hydrolysis:
a) if salts are hydrolyzed both by cation and anion reversibly, then the chemical equilibrium in hydrolysis reactions is shifted to the right;
b) the reaction of the medium is either neutral, or slightly acidic, or slightly alkaline, which depends on the ratio of the dissociation constants of the formed base and acid;
c) salts can be hydrolyzed by both the cation and the anion irreversibly if at least one of the hydrolysis products leaves the reaction sphere.
4. Salts formed by a strong base and a strong acid do not undergo hydrolysis.
You obviously came to this conclusion yourself.
Consider the behavior of $KCl$ in potassium chloride solution.
$(KOH)↙(\text"strong monoacid base")←KCl→(HCl)↙(\text"strong monobasic acid").$
Salt in an aqueous solution dissociates into ions ($KCl=K^(+)+Cl^(-)$), but when interacting with water, a weak electrolyte cannot be formed. The solution medium is neutral ($рН=7$), because the concentrations of $H^(+)$ and $OH^(-)$ ions in the solution are equal, as in pure water.
Other examples of such salts may be alkali metal halides, nitrates, perchlorates, sulfates, chromates and dichromates, alkaline earth metal halides (other than fluorides), nitrates and perchlorates.
It should also be noted that the reversible hydrolysis reaction is completely subject to Le Chatelier's principle. That's why salt hydrolysis can be enhanced(and even make it irreversible) in the following ways:
a) add water (reduce concentration);
b) heat the solution, thus increasing the endothermic dissociation of water:
$H_2O⇄H^(+)+OH^(-)-57$ kJ,
which means that the amount of $H^(+)$ and $OH^(-)$, which are necessary for salt hydrolysis, increases;
c) bind one of the hydrolysis products into a sparingly soluble compound or remove one of the products into the gas phase; for example, the hydrolysis of ammonium cyanide $NH_4CN$ will be greatly enhanced by the decomposition of ammonia hydrate with the formation of ammonia $NH_3$ and water $H_2O$:
$NH_4^(+)+CN^(-)+H_2O⇄NH_3 H_2O+HCN.$
$NH_3()↖(⇄)H_2$
Salt hydrolysis
Legend:
Hydrolysis can be suppressed (significantly reduced the amount of salt undergoing hydrolysis) by proceeding as follows:
a) increase the concentration of the solute;
b) cool the solution (to weaken hydrolysis, salt solutions should be stored concentrated and at low temperatures);
c) introduce one of the hydrolysis products into the solution; for example, acidify the solution if its medium is acidic as a result of hydrolysis, or alkalinize if it is alkaline.
Significance of hydrolysis
The hydrolysis of salts has both practical and biological significance. Since ancient times, ash has been used as a detergent. The ash contains potassium carbonate $K_2CO_3$, which is hydrolyzed as an anion in water, the aqueous solution becomes soapy due to the $OH^(-)$ ions formed during hydrolysis.
At present, we use soap, washing powders and other detergents in everyday life. The main component of soap is sodium and potassium salts of higher fatty acids. carboxylic acids: stearates, palmitates, which are hydrolyzed.
The hydrolysis of sodium stearate $C_(17)H_(35)COONa$ is expressed by the following ionic equation:
$C_(17)H_(35)COO^(-)+H_2O⇄C_(17)H_(35)COOH+OH^(-)$,
those. the solution is slightly alkaline.
Salts are specially added to the composition of washing powders and other detergents. inorganic acids(phosphates, carbonates), which enhance the washing effect by increasing the pH of the medium.
Salts that create the necessary alkaline environment of the solution are contained in the photographic developer. These are sodium carbonate $Na_2CO_3$, potassium carbonate $K_2CO_3$, borax $Na_2B_4O_7$ and other salts hydrolyzed by the anion.
If the acidity of the soil is insufficient, the plants develop a disease - chlorosis. Its signs are yellowing or whitening of the leaves, lag in growth and development. If $pH_(soil) > 7.5$, then ammonium sulfate $(NH_4)_2SO_4$ fertilizer is added to it, which increases acidity due to hydrolysis by the cation passing in the soil:
$NH_4^(+)+H_2O⇄NH_3 H_2O$
invaluable biological role hydrolysis of some salts that make up our body. For example, the composition of the blood includes bicarbonate and sodium hydrogen phosphate salts. Their role is to maintain a certain reaction of the environment. This occurs due to a shift in the equilibrium of hydrolysis processes:
$HCO_3^(-)+H_2O⇄H_2CO_3+OH^(-)$
$HPO_4^(2-)+H_2O⇄H_2PO_4^(-)+OH^(-)$
If there is an excess of $H^(+)$ ions in the blood, they bind to the hydroxide ions $OH^(-)$, and the equilibrium shifts to the right. With an excess of $OH^(-)$ hydroxide ions, the equilibrium shifts to the left. Due to this, the acidity of the blood of a healthy person fluctuates slightly.
Another example: human saliva contains $HPO_4^(2-)$ ions. Thanks to them, a certain environment is maintained in the oral cavity ($рН=7-7.5$).
salt - These are ionic compounds, when they enter the water, they dissociate into ions. In an aqueous solution, these ions are HYDRATED - surrounded by water molecules.
Found that aqueous solutions of many salts are not neutral, but either slightly acidic or alkaline.
The explanation for this is the interaction of salt ions with water. This process is called HYDROLYSIS.
Cations and anions formed a weak base or a weak acid, interact with water, tearing off H or OH from it.
The reason for this: the formation of a STRONGER bond than in the water itself.
In relation to water, salts can be divided into 4 groups:
1) Salt formed by a strong base and a strong acid - NOT HYDROLYZED , in solution only dissociates into ions.Medium is neutral. EXAMPLE: Salts are not hydrolyzed - NaCl, KNO3, RbBr, Cs2SO4, KClO3, etc. In solution, these salts are only dissociate: Cs2SO4 à 2 Cs++SO42- | 2) Salt formed by a strong base and a weak acid - hydrolysis by anion . An anion of a weak acid detaches hydrogen ions from water, binds them. There is an excess of ions in solution. OH - alkaline environment. EXAMPLE: Salts undergo anion hydrolysis - Na2S, KF, K3PO4, Na2CO3, Cs2SO3, KCN, KClO, and acid salts of these acids. K3 PO 4 – a salt formed from a weak acid and a strong base. The phosphate anion is hydrolyzed. PO4 3- + NON⇄ HPO42-+OH- K3 PO4 + H2O⇄ K2HPO4 + KOH (this is the first stage of hydrolysis, the other 2 go to a very small extent) |
3) Salt,formed by a weak base and a strong acid - hydrolysis by cation . The cation of a weak base separates the OH- ion from the water and binds it. An excess of ions remains in the solution H+ - acidic environment. EXAMPLE: Salts undergo cation hydrolysis - CuCl2, NH4Cl, Al(NO3)3, Cr2(SO4)3. Cu SO4 A salt formed from a weak base and a strong acid. The copper cation is hydrolyzed: Cu+2 + NON⇄ CuOH+ + H+ 2 CuSO4 +2 H2 O ⇄ (CuOH)2 SO4 + H2 SO4 | 4) Salt formed by a weak base and a weak acid - hydrolysis BOTH CATION AND ANION. If any of the products are released as a precipitate or gas, then the hydrolysis irreversible , if both hydrolysis products remain in solution - hydrolysis reversible. EXAMPLE: Salts are hydrolyzed Al2S3,Cr2S3 (irreversible): Al2S3 + H2Oà Al(OH)3¯ + H2S NH4F, CH3COONH4 (reversible) NH4F+H2 O⇄ NH4OH + HF |
Mutual hydrolysis of two salts.
It occurs when an attempt is made to obtain, by means of an exchange reaction, salts that are completely hydrolyzed in an aqueous solution. In this case, mutual hydrolysis occurs - i.e., the metal cation binds OH groups, and the acid anion binds H +
1) Metal salts with an oxidation state of +3 and salts of volatile acids (carbonates, sulfides, sulfites)- during their mutual hydrolysis, a precipitate of hydroxide and gas is formed:
2AlCl3 + 3K2S + 6H2O à 2Al(OH)3¯ + 3H2S + 6KCl
(Fe3+, Cr3+) (SO32-, CO32-) (SO2, CO2)
2) Metal salts with an oxidation state of +2 (except calcium, strontium and barium) and soluble carbonates are also hydrolyzed together, but in this case a precipitate of BASIC metal CARBONATE is formed:
2 CuCl2 + 2Na2CO3 + H2O à (CuOH)2CO3 + CO2 + 4 NaCl
(all 2+ except Ca, Sr, Ba)
Characteristics of the hydrolysis process:
1) The hydrolysis process is reversible, proceeds not to the end, but only until the moment of EQUILIBRIUM;
2) The hydrolysis process is the reverse of the NEUTRALIZATION reaction, therefore, hydrolysis - endothermic process (occurs with the absorption of heat).
KF + H2O ⇄ HF + KOH - Q
What factors enhance hydrolysis?
1. The heating - with an increase in temperature, the equilibrium shifts towards an ENDOTHERMIC reaction - hydrolysis intensifies;
2. Adding water- since water is the starting material in the hydrolysis reaction, dilution of the solution enhances hydrolysis.
How to suppress (weaken) the hydrolysis process?
It is often necessary to prevent hydrolysis. For this:
1. Make a solution the most concentrated (reduce the amount of water);
2. To shift the balance to the left add one of the hydrolysis products –acid if there is hydrolysis at the cation or alkali, if there is an anion hydrolysis.
Example: how to suppress the hydrolysis of aluminum chloride?
aluminum chlorideAlCl3 - this is a salt formed by a weak base and a strong acid - hydrolyzes at the cation:
Al+3 + HOH ⇄ AlOH +2 + H+
Wednesday is sour. Therefore, more acid must be added to suppress hydrolysis. In addition, the solution should be made as concentrated as possible.
