Bases (hydroxides)– complex substances whose molecules contain one or more hydroxy OH groups. Most often, bases consist of a metal atom and an OH group. For example, NaOH is sodium hydroxide, Ca(OH) 2 is calcium hydroxide, etc.
There is a base - ammonium hydroxide, in which the hydroxy group is attached not to the metal, but to the NH 4 + ion (ammonium cation). Ammonium hydroxide is formed when ammonia is dissolved in water (the reaction of adding water to ammonia):
NH 3 + H 2 O = NH 4 OH (ammonium hydroxide).
The valency of the hydroxy group is 1. The number of hydroxyl groups in the base molecule depends on the valence of the metal and is equal to it. For example, NaOH, LiOH, Al (OH) 3, Ca(OH) 2, Fe(OH) 3, etc.
All reasons - solids that have different colors. Some bases are highly soluble in water (NaOH, KOH, etc.). However, most of them are not soluble in water.
Bases soluble in water are called alkalis. Alkali solutions are “soapy”, slippery to the touch and quite caustic. Alkalies include hydroxides of alkali and alkaline earth metals (KOH, LiOH, RbOH, NaOH, CsOH, Ca(OH) 2, Sr(OH) 2, Ba(OH) 2, etc.). The rest are insoluble.
Insoluble bases- these are amphoteric hydroxides, which act as bases when interacting with acids, and behave like acids with alkali.
Different bases have different abilities to remove hydroxy groups, so they are divided into strong and weak bases.
Strong bases in aqueous solutions easily give up their hydroxy groups, but weak bases do not.
Chemical properties of bases
The chemical properties of bases are characterized by their relationship to acids, acid anhydrides and salts.
1. Act on indicators. Indicators change color depending on interaction with different chemicals. In neutral solutions they have one color, in acid solutions they have another color. When interacting with bases, they change their color: the methyl orange indicator turns yellow, the litmus indicator turns blue, and phenolphthalein becomes fuchsia.
2. Interact with acid oxides with formation of salt and water:
2NaOH + SiO 2 → Na 2 SiO 3 + H 2 O.
3. React with acids, forming salt and water. The reaction of a base with an acid is called a neutralization reaction, since after its completion the medium becomes neutral:
2KOH + H 2 SO 4 → K 2 SO 4 + 2H 2 O.
4. Reacts with salts forming a new salt and base:
2NaOH + CuSO 4 → Cu(OH) 2 + Na 2 SO 4.
5. When heated, they can decompose into water and the main oxide:
Cu(OH) 2 = CuO + H 2 O.
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After reading the article, you will be able to separate substances into salts, acids and bases. The article describes what the pH of a solution is and what general properties acids and bases have.
In simple terms, an acid is anything with H, and a base is anything with OH. BUT! Not always. To distinguish an acid from a base, you need to... remember them! Regret. To make life at least somehow easier, three of our friends, Arrhenius and Brønsted and Lowry, came up with two theories that are called after them.
Like metals and nonmetals, acids and bases are the division of substances based on similar properties. The first theory of acids and bases belonged to the Swedish scientist Arrhenius. According to Arrhenius, an acid is a class of substances that, when reacting with water, dissociate (decay), forming the hydrogen cation H +. Arrhenius bases in aqueous solution form OH - anions. The next theory was proposed in 1923 by scientists Bronsted and Lowry. The Brønsted-Lowry theory defines acids as substances capable of donating a proton in a reaction (a hydrogen cation is called a proton in reactions). Bases, accordingly, are substances that can accept a proton in a reaction. The currently relevant theory is the Lewis theory. Lewis theory defines acids as molecules or ions capable of accepting electron pairs, thereby forming Lewis adducts (an adduct is a compound formed by combining two reactants without forming by-products).
In inorganic chemistry, as a rule, an acid means a Bronsted-Lowry acid, that is, substances capable of donating a proton. If they mean the definition of a Lewis acid, then in the text such an acid is called a Lewis acid. These rules apply to acids and bases.
Dissociation
Dissociation is the process of decomposition of a substance into ions in solutions or melts. For example, the dissociation of hydrochloric acid is the decomposition of HCl into H + and Cl -.
Properties of acids and bases
Bases tend to feel soapy to the touch, while acids generally taste sour.
When a base reacts with many cations, a precipitate is formed. When an acid reacts with anions, a gas is usually released.
Commonly used acids:
H 2 O, H 3 O +, CH 3 CO 2 H, H 2 SO 4, HSO 4 −, HCl, CH 3 OH, NH 3
Commonly used bases:
OH − , H 2 O , CH 3 CO 2 − , HSO 4 − , SO 4 2 − , Cl −
Strong and weak acids and bases
Strong acids
Such acids that completely dissociate in water, producing hydrogen cations H + and anions. An example of a strong acid is hydrochloric acid HCl:
HCl (solution) + H 2 O (l) → H 3 O + (solution) + Cl - (solution)
Examples of strong acids: HCl, HBr, HF, HNO 3, H 2 SO 4, HClO 4
List of strong acids
- HCl - hydrochloric acid
- HBr - hydrogen bromide
- HI - hydrogen iodide
- HNO 3 - nitric acid
- HClO 4 - perchloric acid
- H 2 SO 4 - sulfuric acid
Weak acids
Only partially dissolved in water, for example, HF:
HF (solution) + H2O (l) → H3O + (solution) + F - (solution) - in such a reaction more than 90% of the acid does not dissociate:
= < 0,01M для вещества 0,1М
Strong and weak acids can be distinguished by measuring the conductivity of solutions: conductivity depends on the number of ions, the stronger the acid, the more dissociated it is, therefore, the stronger the acid, the higher the conductivity.
List of weak acids
- HF hydrogen fluoride
- H 3 PO 4 phosphoric
- H 2 SO 3 sulfurous
- H 2 S hydrogen sulfide
- H 2 CO 3 coal
- H 2 SiO 3 silicon
Strong grounds
Strong bases completely dissociate in water:
NaOH (solution) + H 2 O ↔ NH 4
Strong bases include metal hydroxides of the first (alkalines, alkali metals) and second (alkalinotherrenes, alkaline earth metals) groups.
List of strong bases
- NaOH sodium hydroxide (caustic soda)
- KOH potassium hydroxide (caustic potash)
- LiOH lithium hydroxide
- Ba(OH) 2 barium hydroxide
- Ca(OH) 2 calcium hydroxide (slaked lime)
Weak foundations
In a reversible reaction in the presence of water, it forms OH - ions:
NH 3 (solution) + H 2 O ↔ NH + 4 (solution) + OH - (solution)
Most weak bases are anions:
F - (solution) + H 2 O ↔ HF (solution) + OH - (solution)
List of weak bases
- Mg(OH) 2 magnesium hydroxide
- Fe(OH) 2 iron(II) hydroxide
- Zn(OH) 2 zinc hydroxide
- NH 4 OH ammonium hydroxide
- Fe(OH) 3 iron(III) hydroxide
Reactions of acids and bases
Strong acid and strong base
This reaction is called neutralization: when the amount of reagents is sufficient to completely dissociate the acid and base, the resulting solution will be neutral.
Example:
H 3 O + + OH - ↔ 2H 2 O
Weak base and weak acid
General type of reaction:
Weak base (solution) + H 2 O ↔ Weak acid (solution) + OH - (solution)
Strong base and weak acid
The base dissociates completely, the acid dissociates partially, the resulting solution has weak properties of a base:
HX (solution) + OH - (solution) ↔ H 2 O + X - (solution)
Strong acid and weak base
The acid completely dissociates, the base does not completely dissociate:
Dissociation of water
Dissociation is the breakdown of a substance into its constituent molecules. The properties of an acid or base depend on the equilibrium that is present in water:
H 2 O + H 2 O ↔ H 3 O + (solution) + OH - (solution)
K c = / 2
The equilibrium constant of water at t=25°: K c = 1.83⋅10 -6, the following equality also holds: = 10 -14, which is called the dissociation constant of water. For pure water = = 10 -7 , whence -lg = 7.0.
This value (-lg) is called pH - the potential of hydrogen. If pH< 7, то вещество имеет кислотные свойства, если pH >7, then the substance has basic properties.
Methods for determining pH
Instrumental method
A special device, a pH meter, is a device that transforms the concentration of protons in a solution into an electrical signal.
Indicators
A substance that changes color in a certain pH range depending on the acidity of the solution; using several indicators you can achieve a fairly accurate result.
Salt
A salt is an ionic compound formed by a cation other than H+ and an anion other than O2-. In a weak aqueous solution, salts completely dissociate.
To determine the acid-base properties of a salt solution, it is necessary to determine which ions are present in the solution and consider their properties: neutral ions formed from strong acids and bases do not affect pH: they do not release either H + or OH - ions in water. For example, Cl - , NO - 3 , SO 2- 4 , Li + , Na + , K + .
Anions formed from weak acids exhibit alkaline properties (F -, CH 3 COO -, CO 2- 3); cations with alkaline properties do not exist.
All cations, except for metals of the first and second groups, have acidic properties.
Buffer solution
Solutions that maintain their pH level when a small amount of a strong acid or a strong base is added are mainly composed of:
- A mixture of a weak acid, the corresponding salt and a weak base
- Weak base, corresponding salt and strong acid
To prepare a buffer solution of a certain acidity, it is necessary to mix a weak acid or base with the appropriate salt, taking into account:
- pH range in which the buffer solution will be effective
- Solution capacity - the amount of strong acid or strong base that can be added without affecting the pH of the solution
- There should be no unwanted reactions that could change the composition of the solution
Test:
12.4. Strength of acids and bases
The direction of displacement of the acid-base equilibrium is determined by the following rule:
Acid-base equilibria are biased toward the weaker acid and weaker base.
An acid is stronger the more easily it gives up a proton, and a base is stronger the more easily it accepts a proton and holds it more firmly. A molecule (or ion) of a weak acid is not inclined to donate a proton, and a molecule (or ion) of a weak base is not inclined to accept it, this explains the shift in equilibrium in their direction. The strength of acids as well as the strength of bases can only be compared in the same solvent
Since acids can react with different bases, the corresponding equilibria will be shifted in one direction or another to varying degrees. Therefore, to compare the strengths of different acids, we determine how easily these acids donate protons to solvent molecules. The strength of the grounds is determined similarly.
You already know that a water (solvent) molecule can both accept and donate a proton, that is, it exhibits both the properties of an acid and the properties of a base. Therefore, both acids and bases can be compared with each other in strength in aqueous solutions. In the same solvent, the strength of the acid largely depends on the energy of the breaking of the A-H bond, and the strength of the base depends on the energy of the formed B-H bond.
To quantitatively characterize the strength of an acid in aqueous solutions, you can use the acid-base equilibrium constant of the reversible reaction of a given acid with water:
HA + H 2 O A + H 3 O. 
To characterize the strength of an acid in dilute solutions in which the water concentration is almost constant, use acidity constant:
,
Where K to(HA) = Kc·.
In a completely similar way, to quantitatively characterize the strength of a base, you can use the acid-base equilibrium constant of the reversible reaction of a given base with water:
A + H 2 O HA + OH, 
and in dilute solutions - basicity constant
, Where K o (HA) = K c ·.
In practice, to assess the strength of a base, the acidity constant of the acid obtained from a given base is used (the so-called " conjugate" acid), since these constants are related by the simple relation
K o (A) = TO(H 2 O)/ K to(ON THE).
In other words, The weaker the conjugate acid, the stronger the base. And vice versa, the stronger the acid, the weaker the conjugate base .
Acidity and basicity constants are usually determined experimentally. The values of the acidity constants of various acids are given in Appendix 13, and the values of the basicity constants of bases are given in Appendix 14.
To estimate what fraction of the molecules of an acid or base in a state of equilibrium has undergone a reaction with water, a value similar (and homogeneous) to the mole fraction is used and is called degree of protolysis(). For acid HA
.
Here, the value with the subscript “pr” (in the numerator) characterizes the reacted part of the acid molecules NA, and the value with the subscript “out” (in the denominator) characterizes the initial portion of the acid.
According to the reaction equation
n pr (HA) = n(H3O) = n(A) c pr(HA) = c(H3O) = c(A);
==a · With ref(NA);
= (1 – a) · With ref(NA).
Substituting these expressions into the acidity constant equation, we obtain
Thus, knowing the acidity constant and the total concentration of the acid, it is possible to determine the degree of protolysis of this acid in a given solution. Similarly, the base basicity constant can be expressed through the degree of protolysis, therefore, in general form
This equation is a mathematical expression Ostwald's dilution law. If the solutions are diluted, that is, the initial concentration does not exceed 0.01 mol/l, then the approximate ratio can be used
K= 2 · c ref.
To roughly estimate the degree of protolysis, this equation can also be used at concentrations up to 0.1 mol/l.
Acid-base reactions are reversible processes, but not always. Let us consider the behavior of hydrogen chloride and hydrogen fluoride molecules in water:
A hydrogen chloride molecule gives up a proton to a water molecule and becomes a chloride ion. Therefore, in water, hydrogen chloride exhibits properties of an acid, and water itself - properties of a base. The same thing happens with the hydrogen fluoride molecule, and, therefore, hydrogen fluoride also exhibits the properties of an acid. Therefore, an aqueous solution of hydrogen chloride is called hydrochloric (or hydrochloric) acid, and an aqueous solution of hydrogen fluoride is called hydrofluoric (or hydrofluoric) acid. But there is a significant difference between these acids: hydrochloric acid reacts with excess water irreversibly (completely), and hydrofluoric acid reacts reversibly and slightly. Therefore, a hydrogen chloride molecule easily donates a proton to a water molecule, but a hydrogen fluoride molecule does this with difficulty. Therefore, hydrochloric acid is classified as strong acids, and hydrofluoric - to weak.
Strong acids: HCl, HBr, HI, HClO 4, HClO 3, H 2 SO 4, H 2 SeO 4, HNO 3 and some others.
Now let's turn our attention to the right-hand sides of the equations for the reactions of hydrogen chloride and hydrogen fluoride with water. The fluoride ion can accept a proton (by removing it from the oxonium ion) and turn into a hydrogen fluoride molecule, but the chloride ion cannot. Consequently, the fluoride ion exhibits the properties of a base, while the chloride ion does not exhibit such properties (but only in dilute solutions).
Like acids, there are strong And weak grounds.
Strong base substances include all highly soluble ionic hydroxides (they are also called " alkalis"), since when they are dissolved in water, hydroxide ions completely go into solution.
Weak bases include NH 3 ( K O= 1.74 10 -5) and some other substances. These also include practically insoluble hydroxides of elements that form metals ("metal hydroxides") because when these substances interact with water, only an insignificant amount of hydroxide ions passes into solution.
Weak particle bases (also called " anionic bases"): F, NO 2, SO 3 2, S 2, CO 3 2, PO 4 3 and other anions formed from weak acids.
The anions Cl, Br, I, HSO 4, NO 3 and other anions formed from strong acids do not have basic properties
The cations Li, Na, K, Ca 2, Ba 2 and other cations that are part of strong bases do not have acidic properties.
In addition to acid and base particles, there are also particles that exhibit both acidic and basic properties. These properties of the water molecule are already known to you. In addition to water, these are hydrosulfite ion, hydrosulfide ion and other similar ions. For example, HSO 3 exhibits both the properties of an acid
HSO 3 + H 2 O SO 3 + H 3 O, and the properties of the base
HSO 3 + H 2 O H 2 SO 3 + OH.
Such particles are called ampholytes.
Most ampholyte particles are molecules of weak acids that have lost some protons (HS, HSO 3, HCO 3, H 2 PO 4, HPO 4 2 and some others). The HSO 4 anion does not show basic properties and is a fairly strong acid ( TO K = 1.12. 10–2), and therefore does not apply to ampholytes. Salts containing such anions are called acid salts.
Examples of acid salts and their names:
As you've probably noticed, acid-base and redox reactions have a lot in common. The diagram shown in Figure 12.3 will help you trace the common features and find the differences between these types of reactions.
ACID STRENGTH, BASE STRENGTH, ACIDITY CONSTANT, BASICITY CONSTANT, CONJUGATED ACID, CONJUGATE BASE, DEGREE OF PROTOLYSIS, OSTWALD'S LAW OF DILUTION, STRONG ACID, WEAK ACID, STRONG BASE, WEAK BASE, ALKALI, ANIONIC BASE, AMPHOLYTES, ACID SALTS
1.Which acid is more inclined to donate a proton in an aqueous solution: a) nitric or nitrogenous, b) sulfuric or sulfurous, c) sulfuric or hydrochloric, d) hydrogen sulfide or sulfurous? Write down reaction equations. In the case of reversible reactions, write down the expression for the acidity constants.
2. Compare the atomization energy of HF and HCl molecules. Are these data consistent with the strength of hydrofluoric and hydrochloric acids?
3.Which particle is a stronger acid: a) a carbonic acid molecule or a bicarbonate ion, b) a phosphoric acid molecule, a dihydrogen phosphate ion or a hydrogen phosphate ion, c) a hydrogen sulfide molecule or a hydrosulfide ion?
4. Why don’t you find acidity constants for sulfuric, hydrochloric, nitric and some other acids in Appendix 13?
5.Prove the validity of the relationship connecting the basicity constant and the acidity constant of conjugate acids and bases.
6. Write down the equations for the reactions with water of a) hydrogen bromide and nitrous acid, b) sulfuric and sulfurous acids, c) nitric acid and hydrogen sulfide. What are the differences between these processes?
7. For the following ampholytes: HS, HSO 3, HCO 3, H 2 PO 4, HPO 4 2, H 2 O - create equations for the reactions of these particles with water, write down expressions for the acidity and basicity constants, write down the values of these constants from Appendix 13 and 14. Determine which properties, acidic or basic, predominate in these particles?
8.What processes can occur when phosphoric acid is dissolved in water?
Comparison of the reactivity of strong and weak acids.
12.5. Acid-base reactions of oxonium ions
Both acids and bases differ in strength, solubility, stability, and some other characteristics. The most important of these characteristics is strength. The most characteristic properties of acids are manifested in strong acids. In solutions of strong acids, the acid particles are oxonium ions. Therefore, in this section we will consider reactions in solutions that occur during the interaction of oxonium ions with various substances containing base particles. Let's start with the strongest foundations.
a) Reactions of oxonium ions with oxide ions
Among the very strong bases, the most important is the oxide ion, which is part of the basic oxides, which, as you remember, are ionic substances. This ion is one of the strongest bases. Therefore, basic oxides (for example, composition MO), even those that do not react with water, easily react with acids. Reaction mechanism:
In these reactions, the oxide ion does not have time to go into solution, but immediately reacts with the oxonium ion. Consequently, the reaction occurs on the surface of the oxide. Such reactions go to completion, since a very weak ampholyte (water) is formed from a strong acid and a strong base.
Example. Reaction of nitric acid with magnesium oxide:
MgO + 2HNO 3p = Mg(NO 3) 2p + H 2 O.
All basic and amphoteric oxides react in this way with strong acids, but if an insoluble salt is formed, the reaction in some cases slows down very much, since a layer of insoluble salt prevents the penetration of the acid to the surface of the oxide (for example, the reaction of barium oxide with sulfuric acid).
b) Reactions of oxonium ions with hydroxide ions
Of all the base species that exist in aqueous solutions, the hydroxide ion is the strongest base. Its basicity constant (55.5) is many times higher than the basicity constants of other base particles. Hydroxide ions are part of alkalis and, when dissolved, go into solution. The mechanism of reaction of oxonium ions with hydroxide ions:
.
Example 1. Reaction of hydrochloric acid with sodium hydroxide solution:
HCl p + NaOH p = NaCl p + H 2 O.
Like reactions with basic oxides, such reactions go to completion (irreversible) because as a result of the transfer of a proton by an oxonium ion (a strong acid, K K = 55.5) hydroxide ion (strong base, K O = 55.5) water molecules (a very weak ampholyte, K K= K O = 1.8·10 -16).
Recall that reactions of acids with bases (including alkalis) are called neutralization reactions.
You already know that pure water contains oxonium and hydroxide ions (due to autoprotolysis of water), but their concentrations are equal and extremely insignificant: With(H 3 O) = With(OH) = 10 -7 mol/l. Therefore, their presence in water is practically invisible.
The same is observed in solutions of substances that are neither acids nor bases. Such solutions are called neutral.
But if you add an acid or base substance to water, an excess of one of these ions will appear in the solution. The solution will become sour or alkaline.
Hydroxide ions are part of not only alkalis, but also practically insoluble bases, as well as amphoteric hydroxides (amphoteric hydroxides in this regard can be considered as ionic compounds). Oxonium ions also react with all these substances, and, as in the case of basic oxides, the reaction occurs on the surface of the solid. Reaction mechanism for hydroxide composition M(OH) 2:
.
Example 2. Reaction of a solution of sulfuric acid with copper hydroxide. Since the hydrogen sulfate ion is a rather strong acid ( K K 0.01), the reversibility of its protolysis can be neglected and the equations of this reaction can be written as follows:
Cu(OH) 2 + 2H 3 O = Cu 2 + 4H 2 O
Cu(OH) 2 + H 2 SO 4р = CuSO 4 + 2H 2 O.
c) Reactions of oxonium ions with weak bases
As in solutions of alkalis, solutions of weak bases also contain hydroxide ions, but their concentration is many times lower than the concentration of the base particles themselves (this ratio is equal to the degree of protolysis of the base). Therefore, the rate of the neutralization reaction of hydroxide ions is many times less than the rate of the neutralization reaction of the base particles themselves. Consequently, the reaction between oxonium ions and base particles will be predominant.
Example 1. Reaction of neutralization of hydrochloric acid with ammonia solution:
.
The reaction produces ammonium ions (a weak acid, K K = 6·10 -10) and water molecules, but since one of the initial reagents (ammonia) the base is weak ( K O = 2·10 -5), then the reaction is reversible
But the equilibrium in it is very strongly shifted to the right (towards the reaction products), so much so that reversibility is often neglected by writing the molecular equation of this reaction with an equal sign:
HCl p + NH 3p = NH 4 Cl p + H 2 O.
Example 2. Reaction of hydrobromic acid with a solution of sodium bicarbonate. Being an ampholyte, the bicarbonate ion behaves like a weak base in the presence of oxonium ions:
The resulting carbonic acid can be contained in aqueous solutions only in very small concentrations. As the concentration increases, it decomposes. The decomposition mechanism can be imagined as follows:
Summary chemical equations:
H 3 O + HCO 3 = CO 2 + 2H 2 O
HBr р + NaHCO 3р = NaBr р + CO 2 + H 2 O.
Example 3. Reactions that occur when solutions of perchloric acid and potassium carbonate are combined. The carbonate ion is also a weak base, although stronger than the bicarbonate ion. The reactions between these ions and the oxonium ion are completely analogous. Depending on the conditions, the reaction may stop at the stage of formation of a bicarbonate ion, or may lead to the formation of carbon dioxide:
a) H 3 O + CO 3 = HCO 3 + H 2 O
HClO 4p + K 2 CO 3p = KClO 4p + KHCO 3p;
b) 2H 3 O + CO 3 = CO 2 + 3H 2 O
2HClO 4p + K 2 CO 3p = 2KClO 4p + CO 2 + H 2 O.
Similar reactions occur even when salts containing base particles are insoluble in water. As in the case of basic oxides or insoluble bases, in this case the reaction also occurs on the surface of the insoluble salt.
Example 4. Reaction between hydrochloric acid and calcium carbonate:
CaCO 3 + 2H 3 O = Ca 2 + CO 2 + 3H 2 O
CaCO 3p + 2HCl p = CaCl 2p + CO 2 + H 2 O.
An obstacle to such reactions may be the formation of an insoluble salt, a layer of which will impede the penetration of oxonium ions to the surface of the reagent (for example, in the case of the interaction of calcium carbonate with sulfuric acid).
NEUTRAL SOLUTION, ACIDIC SOLUTION, ALKALINE SOLUTION, NEUTRALIZATION REACTION.
1.Draw up diagrams of the mechanisms of reactions of oxonium ions with the following substances and particles: FeO, Ag 2 O, Fe(OH) 3, HSO 3, PO 4 3 and Cu 2 (OH) 2 CO 3. Using the diagrams, create ionic reaction equations.
2.Which of the following oxides will oxonium ions react with: CaO, CO, ZnO, SO 2, B 2 O 3, La 2 O 3? Write ionic equations for these reactions.
3.Which of the following hydroxides will oxonium ions react with: Mg(OH)2, B(OH)3, Te(OH)6, Al(OH)3? Write ionic equations for these reactions.
4. Make up ionic and molecular equations for the reactions of hydrobromic acid with solutions of the following substances: Na 2 CO 3, K 2 SO 3, Na 2 SiO 3, KHCO 3.
5. Make up ionic and molecular equations for the reactions of a solution of nitric acid with the following substances: Cr(OH) 3, MgCO 3, PbO.
Reactions of solutions of strong acids with bases, basic oxides and salts.
12.6. Acid-base reactions of weak acids
Unlike solutions of strong acids, solutions of weak acids contain not only oxonium ions as acid particles, but also molecules of the acid itself, and there are many times more acid molecules than oxonium ions. Therefore, in these solutions, the predominant reaction will be the reaction of the acid particles themselves with the base particles, and not the reactions of oxonium ions. The rate of reactions involving weak acids is always lower than the rate of similar reactions involving strong acids. Some of these reactions are reversible, and the more, the weaker the acid involved in the reaction.
a) Reactions of weak acids with oxide ions
This is the only group of reactions of weak acids that proceed irreversibly. The speed of the reaction depends on the strength of the acid. Some weak acids (hydrogen sulfide, carbon, etc.) do not react with low-active basic and amphoteric oxides (CuO, FeO, Fe 2 O 3, Al 3 O 3, ZnO, Cr 2 O 3, etc.).
Example. The reaction that occurs between manganese(II) oxide and a solution of acetic acid. The mechanism of this reaction:
Reaction equations:
MnO + 2CH 3 COOH = Mn 2 + 2CH 3 COO + H 2 O
MnO + 2CH 3 COOH p = Mn(CH 3 COO) 2p + H 2 O. (Salts of acetic acid are called acetates)
b) Reactions of weak acids with hydroxide ions
As an example, consider how phosphoric (orthophosphoric) acid molecules react with hydroxide ions:
As a result of the reaction, water molecules and dihydrogen phosphate ions are obtained.
If after completion of this reaction hydroxide ions remain in the solution, then dihydrogen phosphate ions, being ampholytes, will react with them:
Hydrophosphate ions are formed, which, also being ampholytes, can react with an excess of hydroxide ions:
.
Ionic equations for these reactions
H 3 PO 4 + OH H 2 PO 4 + H 2 O;
H 2 PO 4 + OH HPO 4 2 + H 2 O;
HPO 4 + OH PO 4 3 + H 2 O.
The equilibria of these reversible reactions are shifted to the right. In an excess of alkali solution (for example, NaOH), all these reactions proceed almost irreversibly, so their molecular equations are usually written as follows:
H 3 PO 4р + NaOH р = NaH 2 PO 4р + H 2 O;
NaH 2 PO 4р + NaOH р = Na 2 HPO 4р;
Na 2 HPO 4р + NaOH р = Na 3 PO 4р + H 2 O.
If the target product of these reactions is sodium phosphate, then the overall equation can be written:
H 3 PO 4 + 3NaOH = Na 3 PO 4 + 3H 2 O.
Thus, a molecule of phosphoric acid, entering into acid-base interactions, can sequentially donate one, two or three protons. In a similar process, a molecule of hydrosulfide acid (H 2 S) can donate one or two protons, and a molecule of nitrous acid (HNO 2) can donate only one proton. Accordingly, these acids are classified as tribasic, dibasic and monobasic.
The corresponding characteristic of the base is called acidity.
Examples of one-acid bases are NaOH, KOH; examples of diacid bases are Ca(OH) 2, Ba(OH) 2.
The strongest of the weak acids can also react with hydroxide ions that are part of insoluble bases and even amphoteric hydroxides.
c) Reactions of weak acids with weak bases
Almost all of these reactions are reversible. In accordance with the general rule, the equilibrium in such reversible reactions is shifted towards weaker acids and weaker bases.
BASICITY OF ACID, ACIDITY OF BASE.
1.Draw up diagrams of the mechanisms of reactions occurring in an aqueous solution between formic acid and the following substances: Fe 2 O 3, KOH and Fe(OH) 3. Using the diagrams, create ionic and molecular equations for these reactions. (tetraaquazinc ion) and 3aq aq+ H 3 O .
4. In what direction will the equilibrium in this solution shift a) when it is diluted with water, b) when a solution of a strong acid is added to it?
We have given a definition hydrolysis, remembered some facts about salts. Now we will discuss strong and weak acids and find out that the “scenario” of hydrolysis depends on which acid and which base formed the given salt.
← Hydrolysis of salts. Part I
Strong and weak electrolytes
Let me remind you that all acids and bases can be divided into strong And weak. Strong acids (and, in general, strong electrolytes) dissociate almost completely in an aqueous solution. Weak electrolytes disintegrate into ions to a small extent.
Strong acids include:
- H 2 SO 4 (sulfuric acid),
- HClO 4 (perchloric acid),
- HClO 3 (chloric acid),
- HNO 3 (nitric acid),
- HCl (hydrochloric acid),
- HBr (hydrobromic acid),
- HI (hydriodic acid).
Below is a list of weak acids:
- H 2 SO 3 (sulfurous acid),
- H 2 CO 3 (carbonic acid),
- H 2 SiO 3 (silicic acid),
- H 3 PO 3 (phosphorous acid),
- H 3 PO 4 (orthophosphoric acid),
- HClO 2 (chlorous acid),
- HClO (hypochlorous acid),
- HNO 2 (nitrous acid),
- HF (hydrofluoric acid),
- H 2 S (hydrogen sulfide acid),
- most organic acids, eg acetic acid (CH 3 COOH).
Naturally, it is impossible to list all the acids existing in nature. Only the most “popular” ones are given. It should also be understood that the division of acids into strong and weak is quite arbitrary.
The situation is much simpler with strong and weak bases. You can use the solubility table. Strong reasons include all soluble in water bases other than NH 4 OH. These substances are called alkalis (NaOH, KOH, Ca(OH) 2, etc.)
Weak grounds are:
- all water-insoluble hydroxides (e.g. Fe(OH) 3, Cu(OH) 2, etc.),
- NH 4 OH (ammonium hydroxide).
Salt hydrolysis. Key facts
It may seem to those reading this article that we have already forgotten about the main topic of conversation and have gone somewhere aside. This is wrong! Our conversation about acids and bases, about strong and weak electrolytes is directly related to the hydrolysis of salts. Now you will see this.
So let me give you the basic facts:
- Not all salts undergo hydrolysis. Exist hydrolytically stable compounds, such as sodium chloride.
- Hydrolysis of salts can be complete (irreversible) and partial (reversible).
- During the hydrolysis reaction, an acid or base is formed and the acidity of the medium changes.
- The fundamental possibility of hydrolysis, the direction of the corresponding reaction, its reversibility or irreversibility are determined acid strength And foundation force, which form this salt.
- Depending on the strength of the respective acid and resp. bases, all salts can be divided into 4 groups. Each of these groups is characterized by its own “scenario” of hydrolysis.
Example 4. The salt NaNO 3 is formed by a strong acid (HNO 3) and a strong base (NaOH). Hydrolysis does not occur, no new compounds are formed, and the acidity of the medium does not change.
Example 5. The salt NiSO 4 is formed by a strong acid (H 2 SO 4) and a weak base (Ni(OH) 2). Hydrolysis of the cation occurs, during the reaction an acid and a basic salt are formed.
Example 6. Potassium carbonate is formed by a weak acid (H 2 CO 3) and a strong base (KOH). Hydrolysis by anion, formation of alkali and acid salt. Alkaline solution.
Example 7. Aluminum sulfide is formed by a weak acid (H 2 S) and a weak base (Al(OH) 3). Hydrolysis occurs at both the cation and the anion. Irreversible reaction. During the process, H 2 S and aluminum hydroxide are formed. The acidity of the medium changes slightly.
Try it yourself:
Exercise 2. What type of salts are the following: FeCl 3, Na 3 PO 3, KBr, NH 4 NO 2? Are these salts subject to hydrolysis? By cation or by anion? What is formed during the reaction? How does the acidity of the environment change? You don’t have to write down the reaction equations for now.
All we have to do is discuss 4 groups of salts sequentially and give a specific “scenario” of hydrolysis for each of them. In the next part, we'll start with salts formed by a weak base and a strong acid.
Before discussing the chemical properties of bases and amphoteric hydroxides, let's clearly define what they are?
1) Bases or basic hydroxides include metal hydroxides in the oxidation state +1 or +2, i.e. the formulas of which are written either as MeOH or Me(OH) 2. However, there are exceptions. Thus, the hydroxides Zn(OH) 2, Be(OH) 2, Pb(OH) 2, Sn(OH) 2 are not bases.
2) Amphoteric hydroxides include metal hydroxides in the oxidation state +3, +4, as well as, as exceptions, the hydroxides Zn(OH) 2, Be(OH) 2, Pb(OH) 2, Sn(OH) 2. Metal hydroxides in the oxidation state +4 are not found in Unified State Examination tasks, so they will not be considered.
Chemical properties of bases
All grounds are divided into:
Let us remember that beryllium and magnesium are not alkaline earth metals.
In addition to being soluble in water, alkalis also dissociate very well in aqueous solutions, while insoluble bases have a low degree of dissociation.
This difference in solubility and ability to dissociate between alkalis and insoluble hydroxides leads, in turn, to noticeable differences in their chemical properties. So, in particular, alkalis are more chemically active compounds and are often able to enter into reactions that insoluble bases do not.
Interaction of bases with acids
Alkalis react with absolutely all acids, even very weak and insoluble ones. For example:
Insoluble bases react with almost all soluble acids, but do not react with insoluble silicic acid:
It should be noted that both strong and weak bases with the general formula of the form Me(OH) 2 can form basic salts when there is a lack of acid, for example:
Interaction with acid oxides
Alkalis react with all acidic oxides, forming salts and often water:
Insoluble bases are able to react with all higher acidic oxides corresponding to stable acids, for example, P 2 O 5, SO 3, N 2 O 5, to form medium salts:
Insoluble bases of the type Me(OH) 2 react in the presence of water with carbon dioxide exclusively to form basic salts. For example:
Cu(OH) 2 + CO 2 = (CuOH) 2 CO 3 + H 2 O
Due to its exceptional inertness, only the strongest bases, alkalis, react with silicon dioxide. In this case, normal salts are formed. The reaction does not occur with insoluble bases. For example:
Interaction of bases with amphoteric oxides and hydroxides
All alkalis react with amphoteric oxides and hydroxides. If the reaction is carried out by fusing an amphoteric oxide or hydroxide with a solid alkali, this reaction leads to the formation of hydrogen-free salts:
If aqueous solutions of alkalis are used, then hydroxo complex salts are formed:
In the case of aluminum, under the action of an excess of concentrated alkali, instead of Na salt, Na 3 salt is formed:
Interaction of bases with salts
Any base reacts with any salt only if two conditions are met simultaneously:
1) solubility of the starting compounds;
2) the presence of precipitate or gas among the reaction products
For example:
Thermal stability of substrates
All alkalis, except Ca(OH) 2, are resistant to heat and melt without decomposition.
All insoluble bases, as well as slightly soluble Ca(OH) 2, decompose when heated. The highest decomposition temperature of calcium hydroxide is about 1000 o C:
Insoluble hydroxides have much lower decomposition temperatures. For example, copper (II) hydroxide decomposes already at temperatures above 70 o C:
Chemical properties of amphoteric hydroxides
Interaction of amphoteric hydroxides with acids
Amphoteric hydroxides react with strong acids:
Amphoteric metal hydroxides in the oxidation state +3, i.e. type Me(OH) 3, do not react with acids such as H 2 S, H 2 SO 3 and H 2 CO 3 due to the fact that the salts that could be formed as a result of such reactions are subject to irreversible hydrolysis to the original amphoteric hydroxide and corresponding acid:
Interaction of amphoteric hydroxides with acid oxides
Amphoteric hydroxides react with higher oxides, which correspond to stable acids (SO 3, P 2 O 5, N 2 O 5):
Amphoteric metal hydroxides in the oxidation state +3, i.e. type Me(OH) 3, do not react with acidic oxides SO 2 and CO 2.
Interaction of amphoteric hydroxides with bases
Among bases, amphoteric hydroxides react only with alkalis. In this case, if an aqueous solution of alkali is used, then hydroxo complex salts are formed:
And when amphoteric hydroxides are fused with solid alkalis, their anhydrous analogues are obtained:
Interaction of amphoteric hydroxides with basic oxides
Amphoteric hydroxides react when fused with oxides of alkali and alkaline earth metals:
Thermal decomposition of amphoteric hydroxides
All amphoteric hydroxides are insoluble in water and, like any insoluble hydroxides, decompose when heated into the corresponding oxide and water.
