Alkaline environment in salt solutions. Acidity of the environment. Concept of solution pH
Salts – these are ionic compounds; when they get into water they dissociate into ions. In an aqueous solution, these ions are HYDRATED - surrounded by water molecules.
It was found that aqueous solutions of many salts do not have a neutral environment, but are 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 weak acid, react with water, removing 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) A salt formed by a strong base and a strong acid - DOES NOT HYDROLYZE , in solution only dissociates into ions.The environment is neutral. EXAMPLE: Salts do not hydrolyze - NaCl, KNO3, RbBr, Cs2SO4, KClO3, etc. In solution, these salts only dissociate: Cs2SO4 à 2 Cs++SO42- | 2) A salt formed by a strong base and a weak acid - hydrolysis BY ANION . The anion of a weak acid abstracts hydrogen ions from water and binds them. An excess of ions is formed in the solution OH is an alkaline environment. EXAMPLE: Salts undergo hydrolysis at the anion - Na2S, KF, K3PO4, Na2CO3, Cs2SO3, KCN, KClO, and acid salts of these acids. K3 P.O. 4 – a salt formed by a weak acid and a strong base. The phosphate anion is hydrolyzed. P.O.4 3- + NON⇄ NPO42-+OH- K3 P.O.4 + H2O⇄ K2NPO4 + KON (this is the first stage of hydrolysis, the remaining 2 occur to a very small extent) |
3) Salt,formed by a weak base and a strong acid - hydrolysis BY CATION . A cation of a weak base abstracts the OH- ion from water and binds it. Excess ions remain in solution H+ - the environment is acidic. EXAMPLE: Salts undergo hydrolysis by cation - CuCl2, NH4Cl, Al(NO3)3, Cr2(SO4)3. Cu SO4 – a salt formed by 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 of BOTH CATION AND ANION. If any of the products are released as sediment or gas, then 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 trying to obtain, through 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) Salts of metals with oxidation state +3 and salts of volatile acids (carbonates, sulfides, sulfites)– during their mutual hydrolysis, a hydroxide precipitate and gas are formed:
2AlCl3 + 3K2S + 6H2O à 2Al(OH)3¯ + 3H2S + 6KCl
(Fe3+, Cr3+) (SO32-, CO32-) (SO2, CO2)
2) Salts of metals with oxidation state +2 (except calcium, strontium and barium) and soluble carbonates also hydrolyze 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, does not proceed to the end, but only until the moment of EQUILIBRIUM;
2) The hydrolysis process is the reverse of the NEUTRALIZATION reaction, therefore, hydrolysis is endothermic process (proceeds with heat absorption).
KF + H2O ⇄ HF + KOH – Q
What factors enhance hydrolysis?
1. Heating – with increasing temperature, the equilibrium shifts towards the ENDOTHERMIC reaction - hydrolysis increases;
2. Adding water– since water is the starting material in the hydrolysis reaction, diluting the solution enhances hydrolysis.
How to suppress (weaken) the hydrolysis process?
It is often necessary to prevent hydrolysis. To do this:
1. The solution is made as concentrated as possible (reduce the amount of water);
2. To shift balance to the left add one of the hydrolysis products –acid, if hydrolysis occurs at the cation or alkali, if hydrolysis occurs at the anion.
Example: how to suppress the hydrolysis of aluminum chloride?
Aluminum chlorideAlCl3 - is a salt formed by a weak base and a strong acid - hydrolyzes into a cation:
Al+3 + HOH ⇄ AlOH +2 + H+
The environment is acidic. Therefore, more acid must be added to suppress hydrolysis. In addition, the solution should be made as concentrated as possible.
Problem book on general and inorganic chemistry
7. Aqueous solutions of protoliths. 7.1. Water. Neutral, acidic and alkaline environment. Strong protoliths
Look assignments >>>Theoretical part
The modern theory of acids and bases is proton theory Brønsted–Lowry, which explains the manifestation of acidic or basic function by substances by the fact that they react protolysis– exchange reactions of protons (hydrogen cations) H +:
NA+E A - +NOT +
acidbase base acid
According to this theory acid- This proton-containing substance HA, which is a donor of its proton; base is a substance E that accepts a proton donated by an acid. In general, the reactant is acid HA and the reactant is base E, and the product is base A - and the product - acid HE + compete with each other for the possession of a proton, which leads the reversible acid-base reaction to the state protolytic equilibrium. Therefore, the system contains four substances that make up two conjugate acid-base pairs: HA / A - and NOT + /E. Substances exhibiting 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 hydroxide ions OH - 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
A quantitative characteristic of water autoprotolysis is ionic product water:
K IN= [H 3 O + ][ OH – ] = 1 . 10 –14 (25 ° WITH)
Therefore, in clean water
[H 3 O + ] = [OH – ] =1. 10 –7 mol/l (25° WITH)
The content of oxonium cations and hydroxide ions is also expressed through pH value pHAnd hydroxyl index pOH:
pH = -lg ,pOH = -lg [ OH – ]
In clean water at 25 ° WITHpH = 7, pOH = 7, pH + pOH = 14.
In dilute (less than 0.1 mol/l) aqueous solutions of substances, the valuepHmay be equal, greater or lesspHclean 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 through the irreversible reaction of protolysis of substances such as hydrogen chloride, perchloric and sulfuric acids:
HCl+H2O= Cl – +H 3 O + ,pH< 7
HClO4+H 2 O=ClO 4 – +H 3 O + ,pH< 7
H2SO4+2H 2 O=SO 4 2– +2H 3 O + ,pH< 7
IonsCl – , ClO 4 – , SO 4 2– , conjugated with these acids, do not possess basic properties in water. Some hydroanions behave similarly in an aqueous solution, for example the hydrogen sulfate ion:
HSO 4 – + H 2 O=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 strong acid HA (i.e., at With AT less than 1 mol/l) the concentration of oxonium cations and pH are related to the analytical (by preparation) molar concentration With ON as follows:
[ H 3 O + ] = With ON,pH = - lg[ H 3 O + ] = - lgWith ON
Example 1 . Determine the pH value in a 0.006 M solution of sulfuric acid at25 ° WITH .
Solution
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pH = ? With B= 0.006 mol/l 2 With B |
H 2 SO 4 + 2H 2 O = SO 4 2– + 2H 3 O +, pH<7 pH = – lg = –lg (2With B) = –log(2´ 0,006) = 1, 9 2 |
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Answer : 0.006M solutionH 2 SO 4 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 dissolution and complete electrolytic dissociation of substances such as potassium and barium hydroxides, called alkalis:
KOH = K + + OH – ; Va(OH) 2 + 2OH – , pH >7
Substances KOH, B A(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 + , Va 2+ ,Na + etc., which do not have acidic properties in water. At a given analytical concentration of alkali MOH in a dilute solution ( With Bless than 0.1 mol/l) we have:
[OH – ] = With M OH; pH = 14 – pOH = 14 +lg[OH – ] = 14 +lgWith MOH
Example 2 . Determine the pH in a 0.012 M barium hydroxide solution at 25° WITH.
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pH = ? With B= 0.012 mol/l [OH – ] = 2 With B |
IN A(OH) 2 = Ba 2+ + 2OH – ,pH >7 pH = 14 – pOH = 14 + lg[OH – ] = 14 +lg(2With c) = 14+ lg(2 . 0,012)=12,38 |
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pH indicator and its impact on the quality of drinking water.
What is pH?
pH(“potentia hydrogeni” - the strength of hydrogen, or “pondus hydrogenii” - the weight of hydrogen) is a unit of measurement for the activity of hydrogen ions in any substance, quantitatively expressing its acidity.
This term appeared at the beginning of the twentieth century in Denmark. The pH indicator was introduced by the Danish chemist Soren Peter Lauritz Sorensen (1868-1939), although statements about a certain “power of water” are also found among his predecessors.
Hydrogen activity is defined as the negative decimal logarithm of the hydrogen ion concentration expressed in moles per liter:
pH = -log
For simplicity and convenience, the pH indicator was introduced in the calculations. pH is determined by the quantitative ratio of H+ and OH- ions in water, formed during the dissociation of water. It is customary to measure pH levels on a 14-digit scale.
If water has a reduced content of free hydrogen ions (pH greater than 7) compared to hydroxide ions [OH-], then the water will have alkaline reaction, and with an increased content of H+ ions (pH less than 7) - acid reaction. In perfectly pure distilled water, these ions will balance each other.
acidic environment: >
neutral environment: =
alkaline environment: >
When the concentrations of both types of ions in a solution are the same, the solution is said to be neutral. In neutral water the pH value is 7.
When various chemicals are dissolved in water, this balance changes, resulting in a change in pH value. When an acid is added to water, the concentration of hydrogen ions increases, and the concentration of hydroxide ions correspondingly decreases; when an alkali is added, on the contrary, the content of hydroxide ions increases, and the concentration of hydrogen ions decreases.
The pH indicator reflects the degree of acidity or alkalinity of the environment, while “acidity” and “alkalinity” characterize the quantitative content of substances in water that can neutralize alkalis and acids, respectively. As an analogy, we can give an example with temperature, which characterizes the degree of heating of a substance, but not the amount of heat. By putting our hand in the water, we can tell whether the water is cool or warm, but we will not be able to determine how much heat is in it (i.e., relatively speaking, how long this water will cool down).
pH is considered one of the most important indicators of drinking water quality. It shows the acid-base balance and influences how chemical and biological processes will proceed. Depending on the pH value, the rate of chemical reactions, the degree of corrosiveness of water, the toxicity of pollutants, etc. can change. Our well-being, mood and health directly depend on the acid-base balance of our body’s environment.
Modern man lives in a polluted environment. Many people purchase and consume food made from semi-finished products. In addition, almost every person is exposed to stress on a daily basis. All this affects the acid-base balance of the body's environment, shifting it towards acids. Tea, coffee, beer, carbonated drinks reduce pH in the body.
It is believed that an acidic environment is one of the main causes of cell destruction and tissue damage, the development of diseases and aging processes, and the growth of pathogens. In an acidic environment, building material does not reach the cells and the membrane is destroyed.
Externally, the state of the acid-base balance of a person’s blood can be judged by the color of his conjunctiva in the corners of his eyes. With an optimal acid-base balance, the color of the conjunctiva is bright pink, but if a person’s blood alkalinity increases, the conjunctiva becomes dark pink, and with an increase in acidity, the color of the conjunctiva becomes pale pink. Moreover, the color of the conjunctiva changes within 80 seconds after consuming substances that affect the acid-base balance.
The body regulates the pH of internal fluids, maintaining values at a certain level. The body's acid-base balance is a certain ratio of acids and alkalis that contributes to its normal functioning. The acid-base balance depends on maintaining relatively constant proportions between intercellular and intracellular waters in the tissues of the body. If the acid-base balance of fluids in the body is not constantly maintained, normal functioning and preservation of life will be impossible. Therefore, it is important to control what you consume.
Acid-base balance is our indicator of health. The more “sour” we are, the sooner we age and get sick. For the normal functioning of all internal organs, the pH level in the body must be alkaline, in the range from 7 to 9.
The pH inside our body is not always the same - some parts are more alkaline and some are acidic. The body regulates and maintains pH homeostasis only in certain cases, such as blood pH. The pH levels of the kidneys and other organs whose acid-base balance is not regulated by the body are affected by the food and drinks we consume.
Blood pH
The blood pH level is maintained by the body in the range of 7.35-7.45. The normal pH of human blood is considered to be 7.4-7.45. Even a slight deviation in this indicator affects the ability of the blood to carry oxygen. If the blood pH rises to 7.5, it carries 75% more oxygen. When the blood pH drops to 7.3, it is already difficult for a person to get out of bed. At 7.29, he can fall into a coma; if the blood pH drops below 7.1, the person dies.
Blood pH levels must be maintained within a healthy range, so the body uses organs and tissues to maintain a constant pH level. Because of this, the pH level of the blood does not change due to drinking alkaline or acidic water, but the tissues and organs of the body used to regulate the pH of the blood do change their pH.
Kidney pH
The pH parameter of the kidneys is influenced by water, food, and metabolic processes in the body. Acidic foods (such as meat products, dairy products, etc.) and drinks (sweetened drinks, alcoholic drinks, coffee, etc.) lead to low pH levels in the kidneys because the body eliminates excess acidity through urine. The lower the urine pH level, the harder the kidneys have to work. Therefore, the acid load placed on the kidneys from such foods and drinks is called potential acid-renal load.
Drinking alkaline water benefits the kidneys - the urine pH level increases and the acid load on the body decreases. Increasing the pH of urine increases the pH of the body as a whole and rids the kidneys of acidic toxins.
Stomach pH
An empty stomach contains no more than a teaspoon of stomach acid produced at the last meal. The stomach produces acid as needed when eating food. The stomach does not produce acid when a person drinks water.
It is very useful to drink water on an empty stomach. The pH increases to a level of 5-6. The increased pH will have a mild antacid effect and will lead to an increase in beneficial probiotics (good bacteria). Increasing the pH of the stomach increases the pH of the body, which leads to healthy digestion and relief from the symptoms of indigestion.
pH of subcutaneous fat
The body's fatty tissues have an acidic pH because excess acids are deposited in them. The body must store acid in fatty tissues when it cannot be excreted or neutralized by other means. Therefore, a shift in the body’s pH to the acidic side is one of the factors for excess weight.
The positive effect of alkaline water on body weight is that alkaline water helps remove excess acid from tissues because it helps the kidneys work more efficiently. This helps control weight because the amount of acid the body must “store” is greatly reduced. Alkaline water also improves the results of a healthy diet and exercise by helping the body deal with excess acidity produced by fat tissue during weight loss.
Bones
Bone has an alkaline pH because it is primarily composed of calcium. Their pH is constant, but if the blood needs pH adjustment, calcium is pulled from the bones.
The benefit of alkaline water to the bones is to protect them by reducing the amount of acid that the body has to fight against. Studies have shown that drinking alkaline water reduces bone resorption - osteoporosis.
Liver pH
The liver has a slightly alkaline pH, the level of which is affected by both food and drinks. Sugar and alcohol must be broken down in the liver, which leads to excess acid.
The benefits of alkaline water to the liver include the presence of antioxidants in such water; It has been found that alkaline water enhances the work of two antioxidants found in the liver, which contribute to more effective blood purification.
Body pH and alkaline water
Alkaline water allows the parts of the body that maintain the pH of the blood to function at greater efficiency. Increasing the pH levels in the parts of the body responsible for maintaining blood pH will help these organs stay healthy and functioning efficiently.
Between meals, you can help your body normalize its pH by drinking alkaline water. Even a small increase in pH can have a huge impact on your health.
According to research by Japanese scientists, the pH of drinking water, which is in the range of 7-8, increases the life expectancy of the population by 20-30%.
Depending on the pH level, water can be divided into several groups:
Strongly acidic waters< 3
acidic waters 3 - 5
slightly acidic waters 5 - 6.5
neutral waters 6.5 - 7.5
slightly alkaline waters 7.5 - 8.5
alkaline waters 8.5 – 9.5
highly alkaline waters > 9.5
Typically, the pH level of drinking tap water is within the range at which it does not directly affect the consumer quality of water. In river waters the pH is usually in the range of 6.5-8.5, in precipitation 4.6-6.1, in swamps 5.5-6.0, in sea waters 7.9-8.3.
WHO does not offer any medically recommended value for pH. It is known that at low pH water is highly corrosive, and at high levels (pH>11) water acquires a characteristic soapiness, an unpleasant odor, and can cause irritation to the eyes and skin. That is why the optimal pH level for drinking and domestic water is considered to be in the range from 6 to 9.
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Interesting to know: The German biochemist OTTO WARBURG, awarded the Nobel Prize in Physiology or Medicine in 1931, proved that lack of oxygen (acidic pH<7.0) в тканях приводит к изменению нормальных клеток в злокачественные.
The scientist discovered that cancer cells lose the ability to develop in an environment saturated with free oxygen with a pH of 7.5 or higher! This means that when body fluids become acidic, cancer development is stimulated.
His followers in the 60s of the last century proved that any pathogenic flora loses the ability to reproduce at pH = 7.5 and above, and our immune system easily copes with any aggressors!
To preserve and maintain health, we need proper alkaline water (pH=7.5 and above). This will make it possible to better maintain the acid-base balance of body fluids, since the main living environments have a slightly alkaline reaction.
Already in a neutral biological environment, the body can have an amazing ability to self-heal.
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In order to understand what hydrolysis of salts is, let us first remember 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, the solution is said to have an acidic reaction of the medium, if OH - - an alkaline reaction of the medium.
If everything is clear with acids and alkalis, then what reaction of the medium will be in salt solutions?
At first glance, it should always be neutral. And really, where, for example, in a sodium sulfide solution does the excess of hydrogen cations or hydroxide ions come from? Sodium sulfide itself upon dissociation does not form ions of one or another type:
Na 2 S = 2Na + + S 2-
However, if you were faced with, 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 an unusual phenomenon. The device would show you that the pH of the sodium sulfide solution is greater than 7, i.e. there is a clear excess of hydroxide ions. The medium of the sodium chloride solution would be neutral (pH = 7), and the Zn(NO 3) 2 solution would be acidic.
The only thing that meets our expectations is the sodium chloride solution environment. She turned out to be neutral, as expected.
But where did the excess of hydroxide ions in a solution of sodium sulfide and hydrogen cations in a solution of zinc nitrate come from?
Let's try to figure it out. To do this, we need to understand the following theoretical points.
Any salt can be thought of as the product of the interaction of an acid and a base. Acids and bases are divided into strong and weak. Let us recall that those acids and bases whose degree of dissociation is close to 100% are called strong.
note: sulfur (H 2 SO 3) and phosphoric (H 3 PO 4) are often classified 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, removing hydrogen cations H + from them. For example, the sulfide ion, being the acidic residue of a weak hydrogen sulfide acid, interacts with it as follows:
S 2- + H 2 O ↔ HS − + OH −
HS − + H 2 O ↔ H 2 S + OH −
As you can see, 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 acidic residues of weak acids increase the alkalinity of the environment. In the case of salt solutions containing such acidic residues, it is said that for them there is anion hydrolysis.
Acidic 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 strong 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 able to interact with water. For example, the Zn 2+ cation, which corresponds to the weak base zinc hydroxide. The following processes occur in aqueous solutions of zinc salts:
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, increasing the acidity of the environment, that is, lowering the pH. If the salt contains cations that correspond to weak bases, in this case it is said that the salt hydrolyzes 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 above, salts can be divided into 4 types, namely those formed:
1) a strong base and a strong acid,
Such salts do not contain either acidic residues or 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 environment. They say about such salts that they 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 acidic residues react with water. The medium 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, but acidic residues do not react - hydrolysis of salt by cation, the environment is acidic.
Examples: Zn(NO 3) 2, Fe 2 (SO 4) 3, CuSO 4, etc.
4) a weak base and a weak acid.
Both cations and anions of acidic residues react with water. Hydrolysis of salts of this kind occurs both cation and anion or else. They also say about such salts that they are subject 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 acidic residue react with water, both H + ions and OH − ions appear in the solution, which form an extremely poorly 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 = Al 2 S 3 + 6NaNO 3 (− the reaction does not proceed this way!)
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 easily obtained by fusing aluminum powder with sulfur:
2Al + 3S = Al 2 S 3
When aluminum sulfide is added to water, it, just like when trying to obtain it in an aqueous solution, undergoes irreversible hydrolysis.
Al 2 S 3 + 6H 2 O = 2Al(OH) 3 ↓ + 3H 2 S
We study the effect of a universal indicator on solutions of certain salts 
As we can see, the medium of the first solution is neutral (pH = 7), the second is acidic (pH< 7), третьего щелочная (рН >7). How can we 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 hydrogen ion concentration 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 equal, the solution is neutral. When > the solution is acidic, and when > it is alkaline.
What causes a violation of the equality of concentrations of hydrogen ions and hydroxide ions in some aqueous solutions of salts?
The fact is that there is a shift in the dissociation equilibrium of water due to the binding of one of its ions ( or ) with salt ions with the formation of a slightly dissociated, sparingly 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, three types of hydrolysis are distinguished:
- hydrolysis by cation (only the cation reacts with water);
- hydrolysis by anion (only the anion reacts with water);
- joint hydrolysis - hydrolysis at the cation and at the anion (both the cation and the anion react with water).
Any salt can be considered as a product formed by the interaction of a base and an acid:
Hydrolysis of a salt is the interaction of its ions with water, leading to the appearance of an acidic or alkaline environment, but not accompanied by the formation of precipitate or gas.
The hydrolysis process occurs only with the participation soluble salts and consists of two stages:
1)dissociation salts 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%)
Equations of the 1st and 2nd stages - the first of them is irreversible, the second is reversible - you cannot add them!
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.
Hydrolysis by anion
In case of interaction anions dissolved salt with water the process is called hydrolysis of salt at 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 occurs completely, the hydrolysis of the NO 2 anion occurs to a very small extent (for a 0.1 M solution - by 0.0014%), but this is enough for the solution to become alkaline(among the products of hydrolysis there is an OH - ion), it contains 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 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 = Na + + ClO -
ClO - + H 2 O ↔ HClO + OH -
b) LiCN = Li + + CN -
CN - + H 2 O ↔ HCN + OH -
c) Na 2 CO 3 = 2Na + + CO 3 2-
CO 3 2- + H 2 O ↔ HCO 3 — + OH —
d) K 3 PO 4 = 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” (until the formation of acid).
If such an unstable acid as H 2 CO 3 were formed in a solution of its salt NaCO 3, then the release of CO 2 gas from the solution would be observed (H 2 CO 3 = 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 hydrolysis of the salt by 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 are hydrolyzed to a greater extent than the NO 2 ion, since the dissociation of H 2 CO 3 and H 2 S is in the 2nd stage, and H 3 PO 4 in The 3rd stage proceeds significantly less than the dissociation of the acid HNO 2. Therefore, solutions, for example, Na 2 CO 3, K 3 PO 4 and BaS will be highly alkaline(which is easy to see by how soapy the soda is to the touch) .
An excess of OH ions in a solution can be easily detected with an indicator or measured with special devices (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 amphotericity) will react with alkali and the evolution of hydrogen will be observed. This is additional evidence that hydrolysis is occurring, because we did not add NaOH alkali to the soda solution!
Pay special attention to salts of medium-strength acids - orthophosphoric and sulfurous. In the first step, these acids dissociate quite well, so their acidic salts do not undergo hydrolysis, and the solution environment of such salts is acidic (due to the presence of a hydrogen cation in the salt). And medium salts hydrolyze at the anion - the medium is alkaline. So, hydrosulfites, hydrogen phosphates and dihydrogen phosphates do not hydrolyze at the anion, the medium is acidic. Sulfites and phosphates are hydrolyzed by anion, the medium is alkaline.
Hydrolysis by cation
When a dissolved salt cation interacts with water, the process is called
hydrolysis of salt at 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 occurs completely, the hydrolysis of the Ni 2+ cation occurs to a very small extent (for a 0.1 M solution - by 0.001%), but this is enough for the medium to become acidic (the H + ion is present among the hydrolysis products ).
Only cations of poorly soluble basic and amphoteric hydroxides and ammonium cation undergo hydrolysis NH4+. The metal cation splits off the hydroxide ion from the water molecule and releases the hydrogen cation H +.
As a result of hydrolysis, the ammonium cation 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 write hydroxide formulas (for example, Ni(OH) 2) instead of hydroxocations (for example, NiOH +). If hydroxides were formed, then precipitation would form from the salt solutions, which is not observed (these salts form transparent solutions).
Excess hydrogen cations can be easily detected with an indicator or measured with special devices. Magnesium or zinc is added to a concentrated solution of a salt that is strongly hydrolyzed by the cation, and the latter react with the acid to release hydrogen.
If the salt is insoluble, then there is no hydrolysis, because the ions do not interact with water.
