Acids are made up of. Inorganic acids. Interaction of acids with bases and amphoteric hydroxides

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 depends largely on the energy of tearing connections A-N, 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 k(NA).

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 NA

Here, the value with the index “pr” (in the numerator) characterizes the reacted part of the acid molecules NA, and the value with the index “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 fluorescent – 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 base particles (they are 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
Do not possess acidic properties cations Li, Na, K, Ca 2, Ba 2 and other cations that are part of strong bases.

In addition to acid and base particles, there are also particles that exhibit both acidic and basic properties. You already know such properties of the water molecule. In addition to water, these are hydrosulfite ion, hydrosulfide ion and other similar ions. For example, HSO 3 exhibits the properties of an acid
HSO 3 + H 2 O SO 3 + H 3 O and base properties
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 exhibit basic properties and is a rather strong acid ( TO K = 1.12. 10–2), and therefore does not belong 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. Follow common features and the diagram shown in Figure 12.3 will help you find the differences between these types of reactions.

ACID STRENGTH, BASE STRENGTH, ACIDITY CONSTANT, BASICITY CONSTANT, CONJUGATED ACID, CONJUGATED BASE, DEGREE OF PROTOLYSIS, OSTWALD'S LAW OF DILUTION, STRONG ACID, WEAK ACID, STRONG BASE, WEAK BASE, ALKALI, A NIONE 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 molecule of carbonic acid or a bicarbonate ion, b) a molecule phosphoric acid, dihydrogen phosphate ion or hydrogen phosphate ion, c) hydrogen sulfide molecule or 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: 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 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, KО = 55.5) water molecules (a very weak ampholyte, K K= K O = 1.8·10 -16).
Let us remember 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:

Emerging 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 merging solutions of perchloric acid and potassium carbonate. 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, carbonic, etc.) in reaction with low-active basic and amphoteric oxides(CuO, FeO, Fe 2 O 3, Al 3 O 3, ZnO, Cr 2 O 3, etc.) do not enter.

Example. The reaction occurring between manganese(II) oxide and solution 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. According to general rule balance in such reversible reactions biased 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?

DEFINITION

Acids– electrolytes, upon dissociation of which only H + ions (H 3 O +) are formed from positive ions:

HNO 3 ↔ H + + NO 3 - ;

H 2 S ↔ H + + HS — ↔ 2H + + S 2- .

There are several classifications of acids, so, according to the number of hydrogen atoms capable of heating in an aqueous solution, acids are divided into monobasic (HF, HNO 2), dibasic (H 2 CO 3) and tribasic (H 3 PO 4). Depending on the content of oxygen atoms in the acid, acids are divided into oxygen-free (HCl, HF) and oxygen-containing (H 2 SO 4, H 2 SO 3).

Chemical properties of acids

The chemical properties of inorganic acids include:

— the ability to change the color of indicators, for example, when litmus gets into an acid solution, it becomes red (this is due to the dissociation of acids);

— interaction with active metals, standing in the activity series up to hydrogen

Fe + H 2 SO 4 (p - p) = FeSO 4 + H 2;

— interaction with basic and amphoteric oxides

2HCl + FeO = FeCl 2 + H 2 O;

6HNO 3 + Al 2 O 3 = 2Al(NO 3) 3 + 3H 2 O;

- interaction with bases (in the case of interaction of acids with alkalis, a neutralization reaction occurs during which salt and water are formed; only water-soluble acids react with bases insoluble in water)

H 2 SO 4 + 2NaOH = Na 2 SO 4 + H 2 O;

H 2 SO 4 + Cu(OH) 2 ↓ = CuSO 4 + 2H 2 O;

- interaction with salts (only if during the reaction the formation of a slightly or insoluble compound, water, or the release of a gaseous substance occurs)

H 2 SO 4 + BaCl 2 = BaSO 4 ↓ + 2HCl;

2HNO 3 + Na 2 CO 3 = 2NaNO 3 + CO 2 + H 2 O;

— strong acids capable of displacing weaker ones from solutions of their salts

K 3 PO 4 + 3HCl = 3KCl + H 3 PO 4 ;

Na 2 CO 3 + 2HCl = 2NaCl + CO 2 + H 2 O;

— redox reactions associated with the properties of acid anions:

H 2 SO 3 + Cl 2 + H 2 O = H 2 SO 4 + 2HCl;

Pb + 4HNO 3(conc) = Pb(NO 3) 2 + 2NO 2 + 2H 2 O.

Physical properties of acids

At no. Most inorganic acids exist in a liquid state, some exist in a solid state (H 3 PO 4, H 3 BO 3). Almost all acids are highly soluble in water, except silicic acid (H 2 SiO 3)

Obtaining acids

The main methods for producing acids:

— reactions of interaction of acid oxides with water

SO 3 + H 2 O = H 2 SO 4;

- reactions of combining non-metals with hydrogen (oxygen-free acids)

H 2 + S ↔ H 2 S;

- exchange reactions between salts and other acids

K 2 SiO 3 + 2HCl → H 2 SiO 3 ↓ + 2KCl.

Application of acids

Of all the inorganic acids, the most widely used are hydrochloric, sulfuric, orthophosphoric and nitric acids. They are used as raw materials for the production of a different range of substances - other acids, salts, fertilizers, dyes, explosives, varnishes and paints, etc. Dilute hydrochloric, phosphoric and boric acids are used in medicine. Acids are also widely used in everyday life.

Examples of problem solving

EXAMPLE 1

EXAMPLE 2

Exercise

calculate the mass of silicic acid (assuming its composition H 2 SiO 3) obtained by acting on a 400 ml sodium silicate solution with mass fraction salt 20% (solution density 1.1 g/ml) excess hydrochloric acid.

Solution

Let's write the equation for the reaction for producing silicic acid:

2HCl + Na 2 SiO 3 = 2NaCl + H 2 SiO 3 ↓.

Let's find the mass of sodium silicate knowing the volume of the solution, its density and the content of the main substance in the solution (see the problem statement):

m(Na 2 SiO 3) = V(Na 2 SiO 3)×ρ×ω/100%;

m(Na 2 SiO 3) = 400×1.1×20/100% = 88 g.

Then, the amount of sodium silicate substance:

v(Na 2 SiO 3) = m(Na 2 SiO 3)/M(Na 2 SiO 3);

v(Na 2 SiO 3) = 88/122 = 0.72 mol.

According to the reaction equation, the amount of silicic acid substance is v(H 2 SiO 3) = v(Na 2 SiO 3) = 0.72 mol. Therefore, the mass of silicic acid will be equal to:

m(H 2 SiO 3) = 0.72 × 78 = 56.2 g.

Answer

The mass of silicic acid is 56.2 g.

Acids are chemicals, which supply hydrogen ions or protons when mixed in solutions. The number of protons released by a particular acid actually determines the strength of the acid - whether it is a strong acid or a weak acid. To understand the power of acids, you need to compare their tendency to donate protons to a similar base (mostly water). The strength is indicated by the pKA number.

What is a strong acid?

An acid is said to be strong if it dissociates or ionizes completely in solution. This means that he can give greatest number H+ ions or protons when mixed in solution. These ions are charged particles. Since a strong acid suppresses more ions when it breaks down or ionizes, this means that the strong acid is a conductor of electricity.

When an acid is mixed in H2O, a proton (H+ ion) is transferred to H2O to form H3O+ (Hydroxonium ion) and an a- ion, which is where the acid starts.

In general,

Such chemical reactions can be respected, but in a few cases the acid releases H+ ion quite easily and the reaction appears to be one-sided. And the acid is completely dissociated.

For example, when hydrogen chloride is dissolved in H2O to make HCl, there is so little reverse reaction that we can write:

Someday there will be a 100% virtual reaction in which hydrogen chloride will exhibit a reaction with H3O + (hydroxide ion) and Cl – ions. Here the strong acid is hydrogen chloride.

What is a weak acid?

An acid is said to be weak if it partially or partially ionizes, releasing only some of its hydrogen atoms into solution. Therefore, it is less efficient than a strong acid in releasing protons. Weak acids have a higher pKa than strong acids.

Ethanoic acid is good example weak acid. It shows reaction with H2O to produce H3O+ (hydroxide ions) and CH3COOH (ethanoate ions), but the reverse reaction shows more success than the forward one. The molecules react quite easily to improve the acid, and H2O.

At any one time, only about one percent of CH 3 COOH molecules show conversion to ions. All that remains is a simple molecule of acetic acid (systematically called ethanoic acid).

Difference Between Strong Acid and Weak Acid

Definition

Strong acid

A strong acid is an acid that ionizes completely in aqueous solution. A strong acid always loses a proton (AH+) when it dissolves in H2O. In other words, a strong acid is always on its tiptoes and is quite efficient at supplying protons.

Weak acid

A weak acid is one that is partially ionized in solution. He only highlights large number hydrogen atoms into solution. Therefore, it is less capable than a strong acid.

Electrical conductivity

Strong acid

Strong acids always exhibit strong conductivity. Strong acids generally carry more current than weak acids at the same voltage and concentration.

Weak acid

Weak acids have low conductivity. They are poor conductors and show a low value for the current passage

Reaction speed

Strong acid

The reaction rate is faster in strong acids

Weak acid

The reaction rate is slower in weak acids

Examples

Strong acid

Hydrochloric acid (HCl), nitric acid (HNO 3), perchloric acid (HClO 4), sulfuric acid (H 2 SO 4), hydroxide acid (HI), hydrobromic acid (HBr), perchloric acid (HClO 3).

The differences between strong and weak acids are given below: Comparison table

Acids

Acids are called complex substances, whose molecules consist of hydrogen cations (protons) and anions of acid residues.

Acids can be inorganic oxygen-free, inorganic oxygen-containing, organic and complex. Examples:

HCl, HBr, H 2 S – inorganic not containing oxygen (oxygen-free);

HNO 3, HClO 4, H 2 SO 4, H 3 PO 4 – inorganic oxygen-containing;

Organic acids;

Complex acids.

Classification of acids

Acids are classified according to many criteria, in particular, by basicity, by the strength of the acid, by the type of bond it breaks with hydrogen, and by whether the acid is organic or inorganic.

Classification by basicity

According to their basicity, acids are divided into:

1) monobasic 2) dibasic 3) tribasic 4) polybasic 5) polybasic

Monobasic acids

These primarily include acids, the molecules of which contain only one hydrogen atom, which is split off in water or polar solvents in the form of a proton and can be replaced by a metal atom, for example:

HJ, HBr, HCl, HNO 2, HNO 3, HClO, HClO 2, HClO 3, HClO 4, HMnO 4, H − C ≡ N. The names of these acids are respectively: hydroiodic, hydrobromic, hydrochloric or hydrochloric, nitrous, nitric, hypochlorous , chloride, chloric acid, chlorine, manganese and hydrocyanic. During dissociation, all of them eliminate one hydrogen cation:

HClO 2 Н + + СlO 2 ˉ ; HNO 3 H + + NO 3 ˉ

Along with such monobasic acids, there are acids in which there are several hydrogen atoms, but only one of them is capable of dissociation or replacement with a metal, for example, phosphinic acid:

For example, when interacting with an excess of alkali, only this hydrogen atom is replaced and an average salt is obtained, in which the second hydrogen atom can no longer be replaced:

Na(PH 2 O 2) - medium salt. Hydrogen atoms incapable of substitution are placed after the central atom in the acid residue, and the residue itself is placed in parentheses.

There are also many organic matter, in which only one hydrogen atom is capable of dissociation, although there can be many hydrogen atoms.

For example, propionic and benzoic acids each have six hydrogen atoms, but only one of them is capable of dissociation or replacement by a metal.

Dibasic acids

Dibasic acids primarily include those whose molecules contain two hydrogen atoms and both of them are capable of dissociation, which occurs in stages:

H 2 S, H 2 Se, H 2 Te, H 2 C 2 O 4, H 2 SO 3, H 2 SO 4, H 2 SiO 3, H 2 CO 3, H 2 CrO 4, H 2 Cr 2 O 7 . The names of these acids are respectively: hydrogen sulfide, hydrogen selenide, hydrogen telluride, oxalic, sulfurous, sulfuric, silicon, carbonic, chromic, dichromic.

Example of dissociation of a dibasic acid:

H 2 S Н + + HS − first stage; HS - Н + + S 2− second stage

Examples of interaction with alkali

H 2 S + KOH → KHS + H 2 O H 2 S + 2 KOH → K 2 S + 2 H 2 O

There are also acids that have more than two hydrogen atoms, but only two of them are capable of dissociation, for example:

In malonic, succinic, adipic and phthalic acids, of the hydrogen atoms present in their molecules, only two underlined ones are replaced in aqueous solutions by metal or dissociated:

Tribasic acids

These are acids containing three hydrogen atoms, all of which are capable of dissociation. For example, in orthophosphoric acid:

N 3 RO 4 N + + N 2 RO 4 − N 2 RO 4 − N + + NRO 4 2− NRO 4 2− N + + RO 4 3−

The three stages of dissociation correspond to two acidic and one intermediate salts:

NaH 2 PO 4 – sodium dihydrogen orthophosphate – acid salt;

Na 2 HPO 4 – sodium hydrogen orthophosphate – acid salt;

Na 3 PO 4 – sodium orthophosphate – medium salt.

For comparison: Na 2 (PHO 3) - disodium salt of phosphonic acid - the average salt.

Polybasic acids

An example of a polybasic acid is the RNA molecule. Below in its fragment, a repeatedly repeating elementary unit is highlighted - a nucleotide residue, where the nitrogenous base can be one of four residues: adenine, guanine, cytosine or uracil. Each nucleotide contains a fragment of orthophosphoric acid, where the hydrogen atom is underlined, capable of dissociation and replacement with a metal (see formula on page 4).

Classification of acids by strength

According to their strength, acids are divided into strong acids, medium-strength acids, weak acids, and some authors also distinguish very weak acids. A measure of the strength of acids is the value – pK a.

Derivation of the formula pK a. Any acid is capable of dissociation into ions HA H + + A − . For any equilibrium process, we can write an equation for the equilibrium constant:

If we take the logarithm of this expression using decimal logarithms, then we get equation (1):

If we reverse the signs and use the properties of logarithms, we obtain equation (2):

It is customary to denote the value - logK a as pK a, and the value - log as pH.

As a result, equation (2) is transformed into equation (3):

From equation (3) it follows that pK a = pH in the case when

and this in turn is possible if

Thus, pK is the pH value of the medium at which the concentration of an undissociated acid is equal to the concentration of its anion or, in other words, when the acid is half dissociated. For each acid, the pKa value can be determined. If the pK a value is negative, then the acid is strong; if the pK a value is positive, but less than 3.5, then the acid is of medium strength, and if more than 3.5, then the acid is weak.

Knowing the pKa makes it easy to predict whether a given acid will displace another acid from its salt. Mathematical calculation shows that an acid having a pKa less by one displaces another acid from its salt by 90%, for example:

If the pKa of the displacing acid is less than the pKa of the displaced acid by 2 pH units or more, then the acid is displaced by 99% or more. For example:

Any strong acid will displace any weak acid from its salt almost completely.

According to the type of bond being broken with the “H” atom

According to this type, acids are divided into element(E) - H, O - H, N - H, C - H And S−H acids.

TO E–N include, for example: HF, HCl, HBr, HI, H 2 Se, H 2 Te.

O − N – acids. IN O − N In acids, hydrogen is separated upon dissociation from oxygen, although these acids in the vast majority of cases also contain other atoms, for example:

In some HE acids contain hydrogen atoms that are not connected to oxygen, but they, as a rule, are not capable of dissociation, for example:

Phosphinic acid is monobasic HE acid. Hydrogen atoms associated with phosphorus are not capable of dissociation and are not replaced by the metal, even with a large excess of concentrated alkali.

Phosphonic acid is dibasic HE acid, and the hydrogen atom associated with phosphorus is also not capable of dissociation and replacement with a metal.

N–H acids. These include ammonia, primary and secondary amines. For example, in ammonia you can replace the hydrogen atom bonded to nitrogen with sodium:

Acetanilide or acetic acid anilide reacts even more easily with metals:

2, 4, 6, 2 ’ , 4 ’ , 6 ’ – hexanitrodiphenylamine simply dissociates in water since its pK a = 5.4 and it is an acid not much weaker than acetic acid:

There is also an acid, which is both N–H And S–H acid. This is thiocyanic acid:

Salts of this acid are called thiocyanates or thiocyanates: KNCS - potassium thiocyanate. Residues of this acid in different complex compounds are coordinated to the central atoms either by their nitrogen atom or by their sulfur atom. For example, the anion - NCS in potassium hexarodanoferrate (III) - K 3 is coordinated by a nitrogen atom to the iron (III) cation, and in potassium tetrarodane mercuryate (II) - K 2 by a sulfur atom to the mercury (II) cation:

S–H - acids

TO S–H- acids refers to hydrogen sulfide, which is dibasic S–H- acid:

H 2 S H + + SH − pK a = 7.00 SH − H + + S 2− pK a = 12.60

TO S–H- acids applies the same endlessly large number mercaptans - compounds with the general formula R – S – H, where R is a hydrocarbon radical, for example: ethyl mercaptan, thiophenol (or phenyl mercaptan) and 2-furyl mercaptan (2-mercaptofuran).

Thiophenol has a pK a = 9.43, that is, approximately 6 times stronger acid than phenol (pK a = 10)

C – H - acids

TO WITH - H- acids refers to acetylene, in which both hydrogen atoms can be replaced by an alkali metal, for example sodium. Acetylene is a weak acid, its pK a = 22.

It also includes an infinitely large number of terminal alkynes, in which the hydrogen atom connected to the carbon atom at the triple bond can be replaced either with Na or by the action of the Tollens reagent on silver:

CH 3 – C ≡ C – H + OH → CH 3 – C ≡ C – Ag↓ + H 2 O + 2 NH 3

The most powerful known S–N acids is trinitromethane, which almost completely dissociates into ions in water, as its pK a = 0.16, that is, it is an acid of medium strength, but very close to strong acids.

Methods for producing acids

Some acids can be obtained by direct interaction of simple substances:

H 2 + F 2 → 2 HF liquid hydrofluoric acid (hydrofluoric acid)

H 2 + 2 C + N 2 ―-→ 2 HCN hydrocyanic acid

Acids, which are solutions of acid gases in water, can be obtained in two technological stages:

1) interaction of hydrogen with simple substance;

2) dissolving this acidic gas in water, for example:

We also get sulfurous and carbonic acids:

S + O 2 → SO 2 SO 2 + H 2 O H 2 SO 3 C + O 2 → CO 2 CO 2 + H 2 O H 2 CO 3

Many acids can be obtained by reacting acid oxides with water. Some reactions are reversible (with CO 2 and SO 2), others are not reversible: SO 3 + H 2 O → H 2 SO 4,

N 2 O 5 + H 2 O → 2 HNO 3, Cl 2 O 7 + H 2 O → 2 HClO 4.

When some acid oxides react with water, depending on the reaction conditions, they can produce different acids, For example:

When some acidic oxides react with water, two different acids are formed as a result of a disproportionation reaction:

Acids can be obtained by displacing them from salts with stronger acids:

From salts of stronger, but volatile acids, they can be isolated under the influence of weaker, but not volatile acids:

If the acid room temperature is not volatile, but its boiling point is not too high, it can be isolated when heated:

Acids can be obtained by electrolysis of salts in which the cation is discharged at the cathode, but the anion cannot be discharged at the anode:

Acids can be obtained by exchange reactions of acids with acid oxides:

4 HClO 4 + P 4 O 10 → 4 HPO 4 + 2 Cl 2 O 7 4 HNO 3 + P 4 O 10 → 4 HPO 3 + 2 N 2 O 5

Organic acids also enter into the same reactions, forming anhydrides carboxylic acids:

Some acids can be obtained by oxidizing other acids with atmospheric oxygen:

2 H 2 SO 3 + O 2 → 2 H 2 SO 4 2 HNO 2 + O 2 → 2 HNO 3

Some by additional oxidation of oxides in an aqueous solution with atmospheric oxygen, for example, when industrial method obtaining nitric acid:

4 NO 2 + 2 H 2 O + O 2 → 4 HNO 3

Some acids are obtained by disproportionation of other acids:

Some acids, for example, sulfuric and phosphoric, can be converted into other acids by reacting with the corresponding oxides:

H 2 SO 4 + SO 3 → H 2 S 2 O 7 (disulfur) H 2 S 2 O 7 + SO 3 → H 2 S 3 O 10 (disulfur)

H 2 S 3 O 10 + SO 3 → H 2 S 4 O 13 (tetrasulfur) 8 H 3 PO 4 + P 4 O 10 → 6 H 4 P 2 O 7 (pyrophosphoric)

When oxidizing non-metals with oxidizing acids:

As + 5 HNO 3 → H 3 AsO 4 + 5 NO 2 + H 2 O

Complex acids can be obtained by oxidizing noble metals with “regia vodka”:

3 Pt + 18 HCl conc. + 4 HNO 3 conc. → 3 H 2 + 4 NO + 8 H 2 O

Au + HNO 3 conc. + 4 HCl conc. → H + NO + 2 H 2 O

Or oxidation of silicon with a mixture of hydrofluoric and nitric acids:

3 Si + 18 HF + 4 HNO 3 → 3 H 2 + 4 NO + 8 H 2 O

Organic acids can be obtained by oxidation under different conditions of many classes organic compounds, in particular alkanes, alkenes, alkadienes, alkynes, alkylarenes, primary alcohols, aldehydes, ketones, esters. Monocarboxylic acids can be obtained from dicarboxylic acids by decarboxylation (the material will be presented in the relevant sections of organic chemistry).

Related information.

A little theory

Acids

Acids - these are complex substances formed by hydrogen atoms that can be replaced by metal atoms and acidic leftovers.

Acids- these are electrolytes, upon dissociation of which only hydrogen cations and anions of acid residues are formed.

Classification of acids

Classification of acids by composition

Classification of acids according to the number of hydrogen atoms

Classification of acids into strong and weak acids.

Chemical properties of acids

Interaction with basic oxides to form salt and water:

Interaction with amphoteric oxides to form salt and water:

Reaction with alkalis to form salt and water (neutralization reaction):

Interaction with salts, if precipitation occurs or gas is released:

Strong acids displace weaker ones from their salts:

(in this case, unstable carbonic acid is formed, which immediately breaks down into water and carbon dioxide)

- litmus turns red

Methyl orange turns red.

Obtaining acids

1. hydrogen + non-metal
H 2 + S → H 2 S
2. acid oxide + water
P 2 O 5 + 3H 2 O →2H 3 PO 4
Exception:
2NO 2 + H 2 O →HNO 2 + HNO 3
SiO 2 + H 2 O - does not react
3. acid + salt
The reaction product should form a precipitate, gas or water. Typically, stronger acids displace less strong acids from salts. If the salt is insoluble in water, then it reacts with the acid to form a gas.
Na 2 CO 3 + 2HCl →2NaCl + H 2 O + CO 2
K 2 SiO 3 + H 2 SO 4 → K 2 SO 4 + H 2 SiO 3 ↓

Reasons

Reasons(basic hydroxides) are complex substances that consist of metal atoms or ammonium ions and a hydroxyl group (-OH). In an aqueous solution they dissociate to form OH− cations and anions. The name of the base usually consists of two words: “metal/ammonium hydroxide.” Bases that are highly soluble in water are called alkalis.

Classification of bases

1. By solubility in water.
Soluble bases
(alkalis): sodium hydroxide NaOH, potassium hydroxide KOH, barium hydroxide Ba(OH)2, strontium hydroxide Sr(OH)2, cesium hydroxide CsOH, rubidium hydroxide RbOH.
Practically insoluble bases
: Mg(OH)2, Ca(OH)2, Zn(OH)2, Cu(OH)2
The division into soluble and insoluble bases almost completely coincides with the division into strong and weak bases, or hydroxides of metals and transition elements
2. By the number of hydroxyl groups in the molecule.
- Mono-acid(sodium hydroxide NaOH)
- Diacid(copper(II) hydroxide Cu(OH) 2 )
- Triacid(iron(III) hydroxide In(OH) 3 )
3. By volatility.
- Volatile: NH3
- Non-volatile: alkalis, insoluble bases.
4. In terms of stability.
- Stable: sodium hydroxide NaOH, barium hydroxide Ba(OH)2
- Unstable: ammonium hydroxide NH3·H2O (ammonia hydrate).
5. According to the degree of electrolytic dissociation.
- Strong (α > 30%): alkalis.

Weak (α< 3 %): нерастворимые основания.

Receipt

The interaction of a strong base oxide with water produces a strong base or alkali.

Weak base and amphoteric oxidesThey do not react with water, so the corresponding hydroxides cannot be obtained in this way.

Hydroxides of low-active metals are obtained by adding alkali to solutions of the corresponding salts. Since the solubility of weakly basic hydroxides in water is very low, the hydroxide precipitates from solution in the form of a gelatinous mass.