An aqueous solution of which salt is acidic. Salt hydrolysis
Lecture: Salt hydrolysis. Environment of aqueous solutions: acidic, neutral, alkaline
Salt hydrolysisWe continue to study the patterns of chemical reactions. When studying the topic, you learned that during electrolytic dissociation in an aqueous solution, the particles involved in the reaction of substances dissolve in water. This is hydrolysis. Various inorganic and organic substances, in particular salts, are exposed to it. 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 a 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.
Salts with weak bases and strong acids. 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, we reduce:
2FeCl 3 + 3Na 2 CO 3 + 3H 2 O ↔ 6NaCl + 2Fe(OH) 3 ↓ + 3CO 2
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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.
Various substances undergo hydrolysis: 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 of salt cations or anions 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 of 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 the chemical equilibrium is shifted to the left (in the direction of 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 pure 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 form $NH_3 H_2O$ (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 alkaline earth metal cations, 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 - an acid and a base, 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 us write the ionic equations of 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
Salt hydrolysis 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 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.
In the composition of washing powders and other detergents, salts of inorganic acids (phosphates, carbonates) are specially introduced, 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:
$NH_4^(+)+H_2O⇄NH_3 H_2O$
The biological role of the hydrolysis of some salts that make up our body is invaluable. 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 hydrolysis
The topic “Hydrolysis of salts” is one of the most difficult for 9th grade students studying inorganic chemistry. And it seems that its difficulty lies not in the actual complexity of the material being studied, but in the way it is presented in textbooks. So, F.G. Feldman and G.E. Rudzitis from the corresponding paragraph have very little that can be understood. In the textbooks of L.S. Guzey and N.S. Akhmetov, this topic is generally excluded, although Akhmetov’s textbook is intended for students in grades 8–9 with in-depth study of chemistry.
Using the textbooks of these authors, the student is unlikely to be able to understand well the theory of solutions, the essence of the electrolytic dissociation of substances in an aqueous medium, correlate ion exchange reactions with hydrolysis reactions of salts formed by acids and bases of different strengths. In addition, at the end of each textbook there is a table of solubility, but nowhere is it explained why there are dashes in its individual cells, and in the texts of textbooks, students meet the formulas of these salts.
In a short lecture for teachers (especially for beginners, it is especially difficult for them to answer questions that arise in children), we will try to fill this gap and, in our own way, highlight the problem of compiling equations for hydrolysis reactions and determining the nature of the resulting medium.
Hydrolysis is the process of decomposition of substances by water (the word "hydrolysis" itself speaks of this: Greek - water and - decomposition). Different authors, giving a definition of this phenomenon, point out that this forms an acid or acid salt, base or basic salt(N.E. Kuzmenko); when salt ions react with water, a weak electrolyte is formed(A.E. Antoshin); as a result of the interaction of salt ions with water, the balance of electrolytic dissociation of water is shifted(A.A. Makarenya); the constituents of the solute combine with the constituents of the water(N.L. Glinka), etc.
Each author, giving a definition of hydrolysis, notes the most important, in his opinion, side of this complex, multifaceted process. And each of them is right in its own way. It seems that it is up to the teacher which definition to give preference to - what is closer to him in his way of thinking.
So, hydrolysis is the decomposition of substances by water. It is caused by the electrolytic dissociation of salt and water into ions and the interaction between them. Water dissociates slightly into H + and OH - ions (1 molecule out of 550,000), and during hydrolysis one or both of these ions can bind with ions formed during the dissociation of the salt into a low-dissociating, volatile or water-insoluble substance.
Salts formed by strong bases (NaOH, KOH, Ba (OH) 2) and strong acids (H 2 SO 4,
HCl, HNO 3), do not undergo hydrolysis, because the cations and anions that form them are not capable of binding H + and OH - ions in solutions (the reason is high dissociation).
When the salt is formed by a weak base or a weak acid, or both "parents" are weak, the salt in aqueous solution undergoes hydrolysis. In this case, the reaction of the medium depends on the relative strength of the acid and base. In other words, aqueous solutions of such salts can be neutral, acidic or alkaline, depending on the dissociation constants of the new substances formed.
So, during the dissociation of ammonium acetate CH 3 COONH 4, the reaction of the solution will be slightly alkaline, because dissociation constant NH 4 OH ( k dis \u003d 6.3 10 -5) is greater than the dissociation constant of CH 3 COOH
(k dis = 1.75 10 -5). In another salt of acetic acid - aluminum acetate (CH 3 COO) 3 Al - the reaction of the solution will be slightly acidic, because. k dis (CH 3 COOH) = 1.75 10 -5 more k dis (Al (OH) 3) \u003d 1.2 10 -6.
Hydrolysis reactions in some cases are reversible, while in others they go to completion. Quantitatively, hydrolysis is characterized by a dimensionless value r, called the degree of hydrolysis and showing what part of the total number of salt molecules in solution undergoes hydrolysis:
G = n/N 100%,
where n is the number of hydrolyzed molecules, N is the total number of molecules in a given solution. For example, if g \u003d 0.1%, then this means that out of 1000 molecules of salt, only one decomposed with water:
n = g N/100 = 0,1 1000/100 = 1.
The degree of hydrolysis depends on the temperature, the concentration of the solution, and the nature of the solute. So, if we consider the hydrolysis of a salt of CH 3 COONa, then the degree of its hydrolysis for solutions of various concentrations will be as follows: for a 1M solution - 0.003%, for 0.1M - 0.01%, for
0.01M - 0.03%, for 0.001M - 0.1% (data taken from G. Remy's book). These values are consistent with Le Chatelier's principle.
An increase in temperature increases the kinetic energy of molecules, their decomposition into cations and anions, and interaction with water ions (H + and OH -) - an electrolyte that is weak at room temperature.
Given the nature of the reactants, an acid can be added to the salt solution to bind OH - ions, and an alkali can be added to bind H + ions. You can also add other salts that hydrolyze at the opposite ion. In this case, the hydrolysis of both salts is mutually enhanced.
Hydrolysis can be weakened (if necessary) by lowering the temperature, increasing the concentration of the solution, introducing into it one of the hydrolysis products: acids, if H + ions accumulate during hydrolysis, or alkalis, if OH ions accumulate.
All neutralization reactions are exothermic, while hydrolysis reactions are endothermic. Therefore, the yield of the former decreases with increasing temperature, while the yield of the latter increases.
Ions H + and OH - cannot exist in solution in significant concentrations - they combine into water molecules, shifting the equilibrium to the right.
The decomposition of salt by water is explained by the binding of cations and / or anions of the dissociated salt into molecules of a weak electrolyte by water ions (H + and / or OH -), which are always present in solution. The formation of a weak electrolyte, precipitate, gas or the complete decomposition of a new substance is equivalent to the removal of salt ions from the solution, which, in accordance with the Le Chatelier principle (action is equal to reaction), shifts the equilibrium of salt dissociation to the right, and therefore leads to complete decomposition of the salt. Hence, dashes appear in the solubility table against a number of compounds.
If weak electrolyte molecules are formed due to salt cations, then they say that hydrolysis proceeds along the cation and the medium will be acidic, and if due to salt anions, then they say that the hydrolysis proceeds along the anion and the medium will be alkaline. In other words, whoever is stronger - acid or base - determines the environment.
Only soluble salts of weak acids and/or bases undergo hydrolysis. The fact is that if the salt is poorly soluble, then the concentrations of its ions in the solution are negligible and it makes no sense to talk about the hydrolysis of such a salt.
Drawing up equations for the reactions of hydrolysis of salts
Hydrolysis of salts of weak polybasic bases and/or acids occurs in steps. The number of hydrolysis steps is equal to the largest charge of one of the salt ions.
For example:
However, hydrolysis in the second stage and especially in the third is very weak, since
r1 >> r2 >> r3. Therefore, when writing hydrolysis equations, one usually confines oneself to the first step. If the hydrolysis is practically completed at the first stage, then during the hydrolysis of salts of weak polybasic bases and strong acids, basic salts are formed, and during the hydrolysis of salts of strong bases and weak polybasic acids, acidic salts are formed.
The number of water molecules involved in the process of salt hydrolysis according to the reaction scheme is determined by the product of the cation valency and the number of its atoms in the salt formula
(author's rule).
For example:
Na 2 CO 3 2Na + 1 2 = 2 (H 2 O),
Al 2 (SO 4) 3 2Al 3+ 3 2 = 6 (H 2 O),
Co (CH 3 COO) 2 Co 2+ 2 1 \u003d 2 (H 2 O).
Therefore, when compiling the hydrolysis equation, we use the following algorithm(on the example of the hydrolysis of Al 2 (SO 4) 3):
1. Determine what substances salt is formed from:
2. We assume how hydrolysis could go:
Al 2 (SO 4) 3 + 6H–OH \u003d 2Al 3+ + 3 + 6H + + 6OH -.
3. Since Al (OH) 3 is a weak base and its Al 3+ cation binds OH ions - from water, the process actually goes like this:
Al 2 (SO 4) 3 + 6H + + 6OH - \u003d 2Al (OH) 2+ + 3 + 6H + + 2OH -.
4. We compare the amounts of H + and OH ions remaining in the solution and determine the reaction of the medium:
5. After hydrolysis, a new salt was formed: (Al (OH) 2) 2 SO 4, or Al 2 (OH) 4 SO 4, - aluminum dihydroxosulfate (or dialuminum tetrahydroxosulfate) - the main salt. Partially, AlOHSO 4 (aluminum hydroxosulfate) can also be formed, but in a much smaller amount, and it can be neglected.
Another example:
2. Na 2 SiO 3 + 2H 2 O \u003d 2Na + + + 2H + + 2OH -.
3. Since H 2 SiO 3 is a weak acid and its ion binds H + ions from water, the actual reaction goes like this:
2Na + + + 2H + + 2OH - \u003d 2Na + + H + H + + 2OH -.
4. H + + 2OH - \u003d H 2 O + OH - alkaline medium.
5. Na + + H \u003d NаНSiO 3 - sodium hydrosilicate - acid salt.
The acidity or alkalinity of the medium can be easily determined by the amount of H + or OH ions remaining in the solution, provided that new substances were formed and exist in the solution in equivalent ratios and no other reagents were added during the reaction. The medium can be acidic or slightly acidic (if there are few H + ions), alkaline (if there are many OH ions) or slightly alkaline, and also neutral if the values of the dissociation constants of a weak acid and a weak base are close and all H + and OH ions remaining in the solution are after hydrolysis, they recombined to form H 2 O.
We have already noted that the degree of salt hydrolysis is the greater, the weaker the acid or base that formed this salt. Therefore, it is necessary to help students bring the series of anions and cations corresponding to a decrease in the strength of acids and bases of their constituents (according to A.V. Metelsky).
| Anions: F - > > CH 3 COO - > H > HS - > > > > > . |
| Cations:
Cd 2+ > Mg 2+ > Mn 2+ > Fe 2+ > Co 2+ > Ni 2+ > > Cu 2+ > Pb 2+ > Zn 2+ > Al 2+ > Cr 2+ > Fe 2+. |
The more to the right in these rows the ion is located, the greater the hydrolysis of the salt formed by it, i.e. its base or acid is weaker than those to its left. Especially strong is the hydrolysis of salts formed simultaneously by a weak base and an acid. But even for them, the degree of hydrolysis usually does not exceed 1%. Nevertheless, in some cases, the hydrolysis of such salts proceeds especially strongly and the degree of hydrolysis reaches almost 100%. Such salts do not exist in aqueous solutions, but are stored only in dry form. In the solubility table, there is a dash against them. Examples of such salts are BaS, Al 2 S 3 , Cr 2 (SO 3) 3 and others (see textbook solubility table).
Such salts, which have a high degree of hydrolysis, are completely and irreversibly hydrolyzed, since the products of their hydrolysis are removed from the solution in the form of a poorly soluble, insoluble, gaseous (volatile), low-dissociating substance or decomposed by water into other substances.
For example:
Salts that are completely decomposed by water cannot be obtained by ion exchange in aqueous solutions, because instead of ion exchange, the hydrolysis reaction proceeds more actively.
For example:
2AlCl 3 + 3Na 2 S Al 2 S 3 + 6NaCl (it could be so),
2АlCl 3 + 3Na 2 S + 6H 2 O 2Al(OH) 3 + 3H 2 S + 6NaCl (so it actually is).
Salts like Al 2 S 3 are obtained in anhydrous environments by sintering the components in equivalent quantities or by other methods:
Many halides, as a rule, actively react with water, forming a hydride of one element and a hydroxide of another.
For example:
СlF + H–OH HClO + HF,
PСl 3 + 3H–OH P(OH) 3 + 3HCl
(according to L. Pauling).
As a rule, in this kind of reactions, also called hydrolysis, the more electronegative element combines with H +, and the less electronegative - with OH -. It is easy to see that the above reactions proceed in accordance with this rule.
Acid salts of weak acids also undergo hydrolysis. However, in this case, along with hydrolysis, dissociation of the acid residue occurs. So, in a NaHCO 3 solution, hydrolysis of H occurs simultaneously, leading to the accumulation of OH - ions:
H + H–OH H 2 CO 3 + OH -,
and dissociation, albeit slight:
H + H + .
Thus, the reaction of an acid salt solution can be either alkaline (if the hydrolysis of the anion prevails over its dissociation) or acidic (in the opposite case). This is determined by the ratio of the salt hydrolysis constant ( TO hydr) and dissociation constants ( TO dis) of the corresponding acid. In the considered example TO hydr anion more TO dis acids, so the solution of this acidic salt has an alkaline reaction (which is used by those suffering from heartburn from high acidity of gastric juice, although they do it in vain). With the reverse ratio of the constants, for example in the case of hydrolysis of NaHSO 3 , the reaction of the solution will be acidic.
The hydrolysis of a basic salt, such as copper(II) hydroxochloride, proceeds as follows:
Cu(OH)Cl + H–OH Cu(OH) 2 + HCl,
or in ionic form:
CuOH + + Cl - + H + + OH - Cu (OH) 2 + Cl - + H + acidic medium.
Hydrolysis in a broad sense is the reaction of exchange decomposition between various substances and water (G.P. Khomchenko). This definition covers the hydrolysis of all compounds, both inorganic (salts, hydrides, halides, chalcogens, etc.) and organic (esters, fats, carbohydrates, proteins, etc.).
For example:
(C6H10O5) n + n H–OH n C6H12O6,
CaC 2 + 2H–OH Ca(OH) 2 + C 2 H 2,
Cl 2 + H–OH HCl + HClO,
PI 3 + 3H–OH H 3 PO 3 + 3HI.
As a result of the hydrolysis of minerals - aluminosilicates - the destruction of rocks occurs. Hydrolysis of some salts - Na 2 CO 3, Na 3 PO 4 - is used to purify water and reduce its hardness.
The rapidly growing hydrolysis industry produces a number of valuable products from waste (wood sawdust, cotton husks, sunflower husks, straw, corn stalks, sugar beet waste, etc.): ethyl alcohol, fodder yeast, glucose, dry ice, furfural, methanol, lignin and many other substances.
Hydrolysis occurs in the body of humans and animals during the digestion of food (fats, carbohydrates, proteins) in an aquatic environment under the action of enzymes - biological catalysts. It plays an important role in a number of chemical transformations of substances in nature (the Krebs cycle, the tricarboxylic acid cycle) and industry. Therefore, we think that much more attention should be paid to the study of hydrolysis in the school chemistry course.
Below is an example transfer card, offered to students to consolidate the material after studying the topic "Hydrolysis of salts" in the 9th grade.
Algorithm for writing the Fe 2 (SO 4) 3 hydrolysis equation1. Determine what salt is formed by:
2. We assume how hydrolysis could go: Fe 2 (SO 4) 3 + 6H 2 O \u003d 2Fe 3+ + 3 + 6H + + 6OH -. 3. Since Fe (OH) 3 is a weak base, the Fe 3+ cations will be bound by OH anions - from water and the hydrolysis will actually proceed as follows: 2Fe 3+ + 3 + 6H + + 6OH – = 2Fe(OH) 2+ + 3 + 6H + + 2OH – . 4. Determine the reaction of the environment: 6H + + 2OH - \u003d 2H 2 O + 4H + acidic environment. 5. We determine the new salt by the ions remaining in the solution: 2Fe (OH) 2+ + = 2 SO 4 - iron (III) dihydroxosulfate Hydrolysis proceeds through the cation. |
Additional Information
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The application of this algorithm contributes to the conscious writing of hydrolysis equations by students and, with sufficient training, does not cause any difficulties.
LITERATURE
Antoshin A.E., Tsapok P.I. Chemistry. Moscow: Chemistry, 1998;
Akhmetov N.S.. Inorganic chemistry. M.: Education, 1990;
Glinka N.L. General chemistry. L.: Chemistry, 1978;
Eremin V.V., Kuzmenko N.E. Chemistry. M.: Exam, 1998;
Eremin V.V., Kuzmenko N.E., Popov V.A.. Chemistry. Moscow: Bustard, 1997;
Kuzmenko N.E., Churanov S.S. General and inorganic chemistry. M.: Publishing House of Moscow State University, 1977;
Metelsky A.V. Chemistry. Minsk: Belarusian Encyclopedia, 1997;
Pauling L., Pauling P. Chemistry. M.: Mir, 1998;
Pimentel D.S. Chemistry. Moscow: Mir, 1967;
Feldman F.G., Rudzitis G.E. Chemistry-9. M.: Enlightenment, 1997;
Kholin Yu.V., Sleta L.A. Chemistry tutor. Kharkov: Folino, 1998;
Khomchenko G.P.. Chemistry. Moscow: Higher school, 1998.
We study the effect of a universal indicator on solutions of some salts 
As we can see, the environment of the first solution is neutral (pH=7), the second one is acidic (pH< 7), третьего щелочная (рН >7). How to explain such an interesting fact? 🙂
First, let's remember what pH is and what it depends on.
pH is a hydrogen indicator, a measure of the concentration of hydrogen ions in a solution (according to the first letters of the Latin words potentia hydrogeni - the strength of hydrogen).
pH is calculated as the negative decimal logarithm of the concentration of hydrogen ions, expressed in moles per liter:
In pure water at 25 °C, the concentrations of hydrogen ions and hydroxide ions are the same and amount to 10 -7 mol/l (pH=7).
When the concentrations of both types of ions in a solution are the same, the solution is neutral. When > the solution is acidic, and when > - alkaline.
Due to what, in some aqueous solutions of salts, is there a violation of the equality of the concentrations of hydrogen ions and hydroxide ions?
The fact is that there is a shift in the equilibrium of water dissociation due to the binding of one of its ions (or) with salt ions with the formation of a poorly dissociated, hardly soluble or volatile product. This is the essence of hydrolysis.
- this is the chemical interaction of salt ions with water ions, leading to the formation of a weak electrolyte - an acid (or acid salt), or a base (or basic salt).
The word "hydrolysis" means decomposition by water ("hydro" - water, "lysis" - decomposition).
Depending on which salt ion interacts with water, there are three types of hydrolysis:
- žhydrolysis by cation (only cation reacts with water);
- žanion hydrolysis (only anion reacts with water);
- ž joint hydrolysis - hydrolysis by cation and anion (both cation and anion react with water).
Any salt can be considered as a product formed by the interaction of a base and an acid:
Salt hydrolysis - the interaction of its ions with water, leading to the appearance of an acidic or alkaline environment, but not accompanied by the formation of a precipitate or gas.
The hydrolysis process proceeds only with the participation soluble salt and consists of two stages:
1)dissociation salt in solution irreversible reaction (degree of dissociation, or 100%);
2) actually , i.e. interaction of salt ions with water reversible reaction (degree of hydrolysis ˂ 1, or 100%)
The equations of the 1st and 2nd stages - the first of them is irreversible, the second is reversible - cannot be added!
Note that salts formed by cations alkalis and anions strong acids do not undergo hydrolysis, they only dissociate when dissolved in water. In solutions of salts KCl, NaNO 3 , NaSO 4 and BaI, the medium neutral.
Anion hydrolysis
In case of interaction anions dissolved salt with water the process is called salt hydrolysis at the anion.
1) KNO 2 = K + + NO 2 - (dissociation)
2) NO 2 - + H 2 O ↔ HNO 2 + OH - (hydrolysis)
The dissociation of the KNO 2 salt proceeds completely, the hydrolysis of the NO 2 anion - to a very small extent (for a 0.1 M solution - by 0.0014%), but this turns out to be enough for the solution to become alkaline(among the hydrolysis products there is an OH ion -), in it p H = 8.14.
Anions undergo hydrolysis only weak acids (in this example, the nitrite ion NO 2 corresponding to the weak nitrous acid HNO 2). The anion of a weak acid attracts the hydrogen cation present in water to itself and forms a molecule of this acid, while the hydroxide ion remains free:
NO 2 - + H 2 O (H +, OH -) ↔ HNO 2 + OH -
Examples:
a) NaClO \u003d Na + + ClO -
ClO - + H 2 O ↔ HClO + OH -
b) LiCN = Li + + CN -
CN - + H 2 O ↔ HCN + OH -
c) Na 2 CO 3 \u003d 2Na + + CO 3 2-
CO 3 2- + H 2 O ↔ HCO 3 - + OH -
d) K 3 PO 4 \u003d 3K + + PO 4 3-
PO 4 3- + H 2 O ↔ HPO 4 2- + OH -
e) BaS = Ba 2+ + S 2-
S 2- + H 2 O ↔ HS - + OH -
Please note that in examples (c-e) you cannot increase the number of water molecules and instead of hydroanions (HCO 3, HPO 4, HS) write the formulas of the corresponding acids (H 2 CO 3, H 3 PO 4, H 2 S). Hydrolysis is a reversible reaction, and it cannot proceed “to the end” (before the formation of an acid).
If such an unstable acid as H 2 CO 3 were formed in a solution of its NaCO 3 salt, then CO 2 would be released from the gas solution (H 2 CO 3 \u003d CO 2 + H 2 O). However, when soda is dissolved in water, a transparent solution is formed without gas evolution, which is evidence of the incompleteness of the hydrolysis of the anion with the appearance in the solution of only carbonic acid hydranions HCO 3 -.
The degree of salt hydrolysis by the anion depends on the degree of dissociation of the hydrolysis product, the acid. The weaker the acid, the higher the degree of hydrolysis. For example, CO 3 2-, PO 4 3- and S 2- ions undergo hydrolysis to a greater extent than the NO 2 ion, since the dissociation of H 2 CO 3 and H 2 S in the 2nd stage, and H 3 PO 4 in The 3rd stage proceeds much less than the dissociation of the HNO 2 acid. Therefore, solutions, for example, Na 2 CO 3, K 3 PO 4 and BaS will highly alkaline(which is easy to verify by the soapiness of soda to the touch) .
An excess of OH ions in a solution is easy to detect with an indicator or measure with special instruments (pH meters).
If in a concentrated solution of a salt that is strongly hydrolyzed by the anion,
for example, Na 2 CO 3, add aluminum, then the latter (due to amphoterism) will react with alkali and hydrogen evolution will be observed. This is additional evidence of hydrolysis, because we did not add NaOH alkali to the soda solution!
Pay special attention to salts of acids of medium strength - orthophosphoric and sulfurous. In the first stage, these acids dissociate quite well, so their acid salts do not undergo hydrolysis, and the environment of the solution of such salts is acidic (due to the presence of a hydrogen cation in the composition of the salt). And the average salts are hydrolyzed by the anion - the medium is alkaline. So, hydrosulfites, hydrophosphates and dihydrophosphates are not hydrolyzed by the anion, the medium is acidic. Sulfites and phosphates are hydrolyzed by the anion, the environment is alkaline.
Hydrolysis by cation
In the case of the interaction of a cation of a dissolved salt with water, the process is called
salt hydrolysis at the cation
1) Ni(NO 3) 2 = Ni 2+ + 2NO 3 - (dissociation)
2) Ni 2+ + H 2 O ↔ NiOH + + H + (hydrolysis)
The dissociation of the Ni (NO 3) 2 salt proceeds completely, the hydrolysis of the Ni 2+ cation - to a very small extent (for a 0.1 M solution - by 0.001%), but this is enough for the medium to become acidic (among the hydrolysis products there is an H + ion ).
Only cations of poorly soluble basic and amphoteric hydroxides and the ammonium cation undergo hydrolysis. NH4+. The metal cation splits off the hydroxide ion from the water molecule and releases the hydrogen cation H + .
The ammonium cation, as a result of hydrolysis, forms a weak base - ammonia hydrate and a hydrogen cation:
NH 4 + + H 2 O ↔ NH 3 H 2 O + H +
Please note that you cannot increase the number of water molecules and instead of hydroxocations (for example, NiOH +) write hydroxide formulas (for example, Ni (OH) 2). If hydroxides were formed, then precipitates would fall out of salt solutions, which is not observed (these salts form transparent solutions).
An excess of hydrogen cations is easy to detect with an indicator or measure with special instruments. Magnesium or zinc is introduced into a concentrated solution of a salt that is highly hydrolyzed by the cation, then the latter react with the acid with the release of hydrogen.
If the salt is insoluble, then there is no hydrolysis, because the ions do not interact with water.
Task book on general and inorganic chemistry
7. Aqueous solutions of protoliths. 7.1. Water. Neutral, acidic and alkaline environment. Strong protoliths
See tasks >>>Theoretical part
The modern theory of acids and bases is proton theory Bronsted - Lowry, which explains the manifestation of an acidic or basic function by substances by the fact that they enter into reactions protolysis– reactions of exchange of protons (hydrogen cations) H + :
HA+E A - +NOT+
acidbase base acid
According to this theory acid- this proton-containing substance HA, which is a donor of its own proton; A base is a substance E that accepts a proton donated by an acid. In the general case, the reactant is the acid HA and the reactant is the base E, as well as the product is the base A - and the product - acid HE + compete with each other for the possession of a proton, which leads to a reversible acid-base reaction to the state protolytic equilibrium. Therefore, there are four substances in the system that make up two conjugated pairs of "acid - base": HA / A - and NOT + /E. Substances that exhibit acidic or basic properties are called protoliths .
7.1. Water. Neutral, acidic and alkaline environment. Strong protoliths
The most common liquid solvent on Earth is water. In addition to H 2 O molecules, pure water contains OH hydroxide ions - and oxonium cations H 3 O + due to the ongoing reaction autoprotolysis water:
H 2 O + H 2 O OH − + H 3 O
acid base base acid
The quantitative characteristic of water autoprotolysis is ionic product water:
K IN\u003d [H 3 O + ][ OH – ] = 1 . 10 –14 (25 ° FROM)
Therefore, in pure water
[H 3 O +] \u003d [OH -] \u003d 1. 10 –7 mol/l (25° FROM)
The content of oxonium cations and hydroxide ions is also expressed through pH value pHAnd hydroxyl index pOH:
pH = -lg ,pOH = -lg [ oh- ]
In pure water at 25 ° FROMpH = 7, pOH = 7, pH + pOH = 14.
In dilute (less than 0.1 mol/l) aqueous solutions of substances, the valuepHcan be equal, greater or lesspHpure water. AtpH= 7 the medium of an aqueous solution is called neutral, whenpH < 7 – кислотной, при pH> 7 - alkaline. Significant increase in ion concentrationH 3 O + in water (creation acidic environment) is achieved with an irreversible protolysis reaction of substances such as hydrogen chloride, perchloric and sulfuric acids:
HCl+H2O= Cl - + H 3 O +,pH< 7
HClO 4 +H 2 O \u003d ClO 4 - + H 3 O +, pH< 7
H2SO4+2H 2 O \u003d SO 4 2– + 2H 3 O +,pH< 7
ionsCl – , ClO 4 – , SO 4 2– conjugated with these acids do not have basic properties in water. Some hydroanions behave similarly in an aqueous solution, for example, a hydrosulfate ion:
HSO 4 – + H 2 O \u003d SO 4 2– + H 3 O +,pH< 7
Due to the irreversibility of protolysis reactions, the ion itselfH 3 O + , substancesHCl, HClO 4 AndH 2 SO 4 , similar to them protolytic propertiesHClO 3 , HBr, HBrO 3 , HI, HIO 3 , HNO 3 , HNCS, H 2 SeO 4 , HMnO 4 , ionsHSO 4 – , HSeO 4 – and some others in aqueous solution are considered strong acids. In a dilute solution of a strong acid HA (i.e. at from less than 1 mol/l) oxonium cation concentration and pH are related to the analytical (by preparation) molar concentration from ON as follows:
[ H 3 O + ] = from ON THE ,pH = - lg[ H 3 O + ] = - lgfrom ON THE
Example 1 . Determine the pH value in a 0.006 M solution of sulfuric acid at25 ° FROM .
Solution
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pH = ? from B= 0.006 mol/l 2 from B |
H 2 SO 4 + 2H 2 O \u003d SO 4 2– + 2H 3 O +, pH<7 pH = - lg = -lg(2from B) = –lg (2´ 0,006) = 1, 9 2 |
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Answer : 0.006M solutionH 2 SO 4 It has pH 1 9 2 |
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A significant increase in the concentration of OH - ions in water (creation of an alkaline environment) is achieved by the dissolution and complete electrolytic dissociation of substances such as potassium and barium hydroxides, called alkalis:
KOH = K + + OH - ; Wa(OH) 2 + 2OH – , pH >7
Substances KOH, V but(OH) 2,NaOHand similar basic hydroxides in the solid state are ionic crystals; during their electrolytic dissociation in an aqueous solution, OH ions are formed (this strong base) , as well as ionsK + , Ba 2+ ,Na + etc., which do not possess acidic properties in water. At a given analytical concentration of alkali MOH in a dilute solution ( from Bless than 0.1 mol/l) we have:
[OH -] = from M Oh; pH = 14 – pOH = 14 +lg[OH -] \u003d 14 +lgfrom MOH
Example 2 . Determine the pH in a 0.012M barium hydroxide solution at 25° FROM.
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pH = ? from B= 0.012 mol/l [OH -] = 2 from B |
IN but(OH) 2 \u003d Ba 2+ + 2OH -,pH >7 pH = 14 – pOH = 14 + lg[OH -] \u003d 14 +lg(2from c) = 14+ lg(2 . 0,012)=12,38 |
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