Grounds - complex substances consisting of a metal atom and one or more hydroxyl groups. General formula of bases Me(OH) n . Bases (from the point of view of the theory of electrolytic dissociation) are electrolytes that dissociate when dissolved in water to form metal cations and hydroxide ions OH -.
Classification. Based on their solubility in water, bases are divided into alkalis(water soluble bases) and water-insoluble bases . Alkalis form alkali and alkaline earth metals, as well as some other metal elements. Based on acidity (the number of ОН– ions formed during complete dissociation, or the number of dissociation steps), bases are divided into monoacid (with complete dissociation, one O H – ion is obtained; one dissociation step) and polyacid (with complete dissociation, more than one OH – ion is obtained; more than one dissociation step). Among polyacid bases there are diacid(for example, Sn(OH) 2 ), triacid(Fe(OH) 3) and tetra-acid (Th(OH) 4). For example, the base KOH is a monoacid base.
There is a group of hydroxides that exhibit chemical duality. They interact with both bases and acids. This amphoteric hydroxides ( cm. table 1).
Table 1 - Amphoteric hydroxides
|
Amphoteric hydroxide (basic and acidic form) |
Acid residue and its valence |
Complex ion |
|
Zn(OH) 2 / H 2 ZnO 2 |
ZnO2(II) |
2– |
|
Al(OH) 3 / HAlO 2 |
AlO2(I) |
– , 3– |
|
Be(OH)2/H2BeO2 |
BeO2(II) |
2– |
|
Sn(OH) 2 / H 2 SnO 2 |
SnO2(II) |
2– |
|
Pb(OH) 2 / H 2 PbO 2 |
PbO2(II) |
2– |
|
Fe(OH) 3 / HFeO 2 |
FeO2(I) |
– , 3– |
|
Cr(OH)3/HCrO2 |
CrO2(I) |
– , 3– |
Physical properties. Bases are solids of various colors and varying solubility in water.
Chemical properties of bases
1) Dissociation: CON + n H 2 O K + × m H 2 O + OH – × d H 2 O or abbreviated: KOH K + + OH – .
Polyacid bases dissociate in several steps (mostly dissociation occurs in the first step). For example, the diacid base Fe(OH) 2 dissociates in two steps:
Fe(OH) 2 FeOH + + OH – (1st stage);
FeOH + Fe 2+ + OH – (2nd stage).
2) Interaction with indicators(alkalis turn violet litmus blue, methyl orange yellow, and phenolphthalein crimson):
indicator + OH – ( alkali)colored compound.
3 ) Decomposition with the formation of oxide and water (see. table 2). Hydroxides alkali metals are resistant to heat (melt without decomposition). Alkaline earth and heavy metal hydroxides usually decompose easily. The exception is Ba(OH) 2, for which t the difference is quite high (approx. 1000° C).
Zn(OH) 2 ZnO + H 2 O.
Table 2 - Decomposition temperatures of some metal hydroxides
| Hydroxide | t razl, °C | Hydroxide | t razl, °C | Hydroxide | t razl, °C |
| LiOH | 925 | Cd(OH)2 | 130 | Au(OH)3 | 150 |
| Be(OH)2 | 130 | Pb(OH)2 | 145 | Al(OH)3 | >300 |
| Ca(OH)2 | 580 | Fe(OH)2 | 150 | Fe(OH) 3 | 500 |
| Sr(OH)2 | 535 | Zn(OH)2 | 125 | Bi(OH)3 | 100 |
| Ba(OH)2 | 1000 | Ni(OH)2 | 230 | In(OH)3 | 150 |
4 ) Interaction of alkalis with some metals(for example Al and Zn):
In solution: 2Al + 2NaOH + 6H 2 O ® 2Na + 3H 2
2Al + 2OH – + 6H 2 O ® 2 – + 3H 2.
When fused: 2Al + 2NaOH + 2H 2 O 2NaAl O 2 + 3H 2.
5 ) Interaction of alkalis with non-metals:
6 NaOH + 3Cl 2 5Na Cl + NaClO 3 + 3H 2 O.
6) Interaction of alkalis with acidic and amphoteric oxides:
2NaOH + CO 2 ® Na 2 CO 3 + H 2 O 2OH – + CO 2 ® CO 3 2– + H 2 O.
In solution: 2NaOH + ZnO + H 2 O ® Na 2 2OH – + ZnO + H 2 O ® 2– .
When fused with amphoteric oxide: 2NaOH + ZnO Na 2 ZnO 2 + H 2 O.
7) Interaction of bases with acids:
H 2 SO 4 + Ca(OH) 2 ® CaSO 4 ¯ + 2H 2 O 2H + + SO 4 2– + Ca 2+ +2OH – ® CaSO 4 ¯ + 2H 2 O
H 2 SO 4 + Zn(OH) 2 ® ZnSO 4 + 2H 2 O 2H + + Zn(OH) 2 ® Zn 2+ + 2H 2 O.
8) Interaction of alkalis with amphoteric hydroxides(cm. table 1):
In solution: 2NaOH + Zn(OH) 2 ® Na 2 2OH – + Zn(OH) 2 ® 2–
For fusion: 2NaOH + Zn(OH) 2 Na 2 ZnO 2 + 2H 2 O.
9 ) Interaction of alkalis with salts. The reaction involves salts that correspond to a base that is insoluble in water :
CuS O 4 + 2NaOH ® Na 2 SO 4 + Cu(OH) 2 ¯ Cu 2+ + 2OH – ® Cu(OH) 2 ¯ .
Receipt. Water-insoluble bases obtained by reacting the corresponding salt with an alkali:
2NaOH + ZnS O 4 ® Na 2 SO 4 + Zn(OH) 2 ¯ Zn 2+ + 2OH – ® Zn(OH) 2 ¯ .
Alkalis receive:
1) Interaction of metal oxide with water:
Na 2 O + H 2 O ® 2NaOH CaO + H 2 O ® Ca(OH) 2.
2) Interaction of alkali and alkaline earth metals with water:
2Na + H 2 O ® 2NaOH + H 2 Ca + 2H 2 O ® Ca(OH) 2 + H 2 .
3) Electrolysis of salt solutions:
2NaCl + 2H2OH2 + 2NaOH + Cl2.
4 ) Exchange interaction of alkaline earth metal hydroxides with certain salts. The reaction must necessarily produce an insoluble salt. .
Ba(OH) 2 + Na 2 CO 3 ® 2NaOH + BaCO 3 ¯ Ba 2 + + CO 3 2 – ® BaCO 3 ¯ .
L.A. Yakovishin
Metal and hydroxyl group (OH). For example, sodium hydroxide - NaOH, calcium hydroxide - Ca(OH) 2 , barium hydroxide - Ba(OH) 2, etc.
Preparation of hydroxides.
1. Exchange reaction:
CaSO 4 + 2NaOH = Ca(OH) 2 + Na 2 SO 4,
2. Electrolysis of aqueous salt solutions:
2KCl + 2H 2 O = 2KOH + H 2 + Cl 2,
3. Interaction of alkali and alkaline earth metals or their oxides with water:
K+2H 2 O = 2 KOH + H 2 ,
Chemical properties of hydroxides.
1. Hydroxides are alkaline in nature.
2. Hydroxides dissolves in water (alkali) and is insoluble. For example, KOH- dissolves in water, and Ca(OH) 2 - slightly soluble, white solution. Metals of group 1 of the periodic table D.I. Mendeleev gives soluble bases (hydroxides).
3. Hydroxides decompose when heated:
Cu(OH) 2 = CuO + H 2 O.
4. Alkalis react with acidic and amphoteric oxides:
2KOH + CO 2 = K 2 CO 3 + H 2 O.
5. Alkalis can react with some non-metals in different ways at different temperatures:
NaOH + Cl 2 = NaCl + NaOCl + H 2 O(cold),
NaOH + 3 Cl 2 = 5 NaCl + NaClO 3 + 3 H 2 O(heat).
6. Interact with acids:
KOH + HNO3 = KNO 3 + H 2 O.
Bases, amphoteric hydroxides
Bases are complex substances consisting of metal atoms and one or more hydroxyl groups (-OH). The general formula is Me +y (OH) y, where y is the number of hydroxo groups equal to the oxidation state of the metal Me. The table shows the classification of bases.
Properties of alkalis, hydroxides of alkali and alkaline earth metals
1. Aqueous solutions of alkalis are soapy to the touch and change the color of indicators: litmus - blue, phenolphthalein - crimson.
2. Aqueous solutions dissociate:
3. Interact with acids, entering into an exchange reaction:
Polyacid bases can give medium and basic salts:
4. React with acidic oxides, forming medium and acidic salts depending on the basicity of the acid corresponding to this oxide:
5. Interact with amphoteric oxides and hydroxides:
a) fusion:
b) in solutions:
6. Interact with water-soluble salts if a precipitate or gas is formed:
Insoluble bases (Cr(OH) 2, Mn(OH) 2, etc.) interact with acids and decompose when heated:
Amphoteric hydroxides
Amphoteric compounds are compounds that, depending on conditions, can be both donors of hydrogen cations and exhibit acidic properties, and their acceptors, i.e., exhibit basic properties.
Chemical properties of amphoteric compounds
1. Interacting with strong acids, they exhibit basic properties:
Zn(OH) 2 + 2HCl = ZnCl 2 + 2H 2 O
2. Interacting with alkalis - strong bases, they exhibit acidic properties:
Zn(OH) 2 + 2NaOH = Na 2 ( complex salt)
Al(OH) 3 + NaOH = Na ( complex salt)
Complex compounds are those in which at least one covalent bond is formed by a donor-acceptor mechanism.
The general method for preparing bases is based on exchange reactions, with the help of which both insoluble and soluble bases can be obtained.
CuSO 4 + 2KOH = Cu(OH) 2 ↓ + K 2 SO 4
K 2 CO 3 + Ba(OH) 2 = 2 KOH + BaCO 3 ↓
When soluble bases are obtained by this method, an insoluble salt precipitates.
When preparing water-insoluble bases with amphoteric properties, excess alkali should be avoided, since dissolution of the amphoteric base may occur, for example:
AlCl 3 + 4KOH = K[Al(OH) 4 ] + 3KCl
In such cases, ammonium hydroxide is used to obtain hydroxides, in which amphoteric hydroxides do not dissolve:
AlCl 3 + 3NH 3 + ZH 2 O = Al(OH) 3 ↓ + 3NH 4 Cl
Silver and mercury hydroxides decompose so easily that when trying to obtain them by exchange reaction, instead of hydroxides, oxides precipitate:
2AgNO 3 + 2KOH = Ag 2 O↓ + H 2 O + 2KNO 3
In industry, alkalis are usually obtained by electrolysis of aqueous solutions of chlorides.
2NaCl + 2H 2 O → ϟ → 2NaOH + H 2 + Cl 2
Alkalis can also be obtained by reacting alkali and alkaline earth metals or their oxides with water.
2Li + 2H 2 O = 2LiOH + H 2
SrO + H 2 O = Sr(OH) 2
Acids
Acids are complex substances whose molecules consist of hydrogen atoms that can be replaced by metal atoms and acidic residues. Under normal conditions, acids can be solid (phosphoric H 3 PO 4; silicon H 2 SiO 3) and liquid (in its pure form, sulfuric acid H 2 SO 4 will be a liquid).
Gases such as hydrogen chloride HCl, hydrogen bromide HBr, hydrogen sulfide H 2 S form the corresponding acids in aqueous solutions. The number of hydrogen ions formed by each acid molecule during dissociation determines the charge of the acid residue (anion) and the basicity of the acid.
According to protolytic theory of acids and bases, proposed simultaneously by the Danish chemist Brønsted and the English chemist Lowry, an acid is a substance splitting off with this reaction protons, A basis- a substance that can accept protons.
acid → base + H +
Based on such ideas, it is clear basic properties of ammonia, which, due to the presence of a lone electron pair at the nitrogen atom, effectively accepts a proton when interacting with acids, forming an ammonium ion through a donor-acceptor bond.
HNO 3 + NH 3 ⇆ NH 4 + + NO 3 —
acid base acid base
More general definition of acids and bases proposed by the American chemist G. Lewis. He suggested that acid-base interactions are completely do not necessarily occur with the transfer of protones. In the determination of acids and bases according to Lewis, the main role in chemical reactions is given to electron pairs
Cations, anions, or neutral molecules that can accept one or more pairs of electrons are called Lewis acids.
For example, aluminum fluoride AlF 3 is an acid, since it is able to accept an electron pair when interacting with ammonia.
AlF 3 + :NH 3 ⇆ :
Cations, anions, or neutral molecules capable of donating electron pairs are called Lewis bases (ammonia is a base).
Lewis's definition covers all acid-base processes that were considered by previously proposed theories. The table compares the definitions of acids and bases currently used.
Nomenclature of acids
Since there are different definitions of acids, their classification and nomenclature are rather arbitrary.
According to the number of hydrogen atoms capable of elimination in an aqueous solution, acids are divided into monobasic(e.g. HF, HNO 2), dibasic(H 2 CO 3, H 2 SO 4) and tribasic(H 3 PO 4).
According to the composition of the acid, they are divided into oxygen-free(HCl, H 2 S) and oxygen-containing(HClO 4, HNO 3).
Usually names of oxygen-containing acids are derived from the name of the non-metal with the addition of the endings -kai, -vaya, if the oxidation state of the non-metal is equal to the group number. As the oxidation state decreases, the suffixes change (in order of decreasing oxidation state of the metal): -opaque, rusty, -ovate:
If we consider the polarity of the hydrogen-nonmetal bond within a period, we can easily relate the polarity of this bond to the position of the element in the Periodic Table. From metal atoms, which easily lose valence electrons, hydrogen atoms accept these electrons, forming a stable two-electron shell like the shell of a helium atom, and give ionic metal hydrides.
In hydrogen compounds of elements of groups III-IV of the Periodic Table, boron, aluminum, carbon, and silicon form covalent, weakly polar bonds with hydrogen atoms that are not prone to dissociation. For elements of groups V-VII of the Periodic Table, within a period, the polarity of the nonmetal-hydrogen bond increases with the charge of the atom, but the distribution of charges in the resulting dipole is different than in hydrogen compounds of elements that tend to donate electrons. Non-metal atoms, which require several electrons to complete the electron shell, attract (polarize) a pair of bonding electrons the more strongly, the greater the nuclear charge. Therefore, in the series CH 4 - NH 3 - H 2 O - HF or SiH 4 - PH 3 - H 2 S - HCl, bonds with hydrogen atoms, while remaining covalent, become more polar in nature, and the hydrogen atom in the element-hydrogen bond dipole becomes more electropositive. If polar molecules find themselves in a polar solvent, a process of electrolytic dissociation can occur.
Let us discuss the behavior of oxygen-containing acids in aqueous solutions. These acids have an H-O-E bond and, naturally, the polarity of the H-O bond is influenced by the O-E bond. Therefore, these acids, as a rule, dissociate more easily than water.
H 2 SO 3 + H 2 O ⇆ H 3 O + + HSO 3
HNO 3 + H 2 O ⇆ H 3 O + + NO 3
Let's look at a few examples properties of oxygen-containing acids, formed by elements that are capable of exhibiting different degrees of oxidation. It is known that hypochlorous acid HClO very weak chlorous acid HClO 2 also weak, but stronger than hypochlorous, hypochlorous acid HClO 3 strong. Perchloric acid HClO 4 is one of the strongest inorganic acids.
For acidic dissociation (with the elimination of the H ion), the cleavage of the O-H bond is necessary. How can we explain the decrease in the strength of this bond in the series HClO - HClO 2 - HClO 3 - HClO 4? In this series, the number of oxygen atoms associated with the central chlorine atom increases. Each time a new oxygen-chlorine bond is formed, electron density is drawn from the chlorine atom, and therefore from the O-Cl single bond. As a result, the electron density partially leaves the O-H bond, which is weakened as a result.
This pattern - strengthening of acidic properties with increasing degree of oxidation of the central atom - characteristic not only of chlorine, but also of other elements. For example, nitric acid HNO 3, in which the oxidation state of nitrogen is +5, is stronger than nitrous acid HNO 2 (the oxidation state of nitrogen is +3); sulfuric acid H 2 SO 4 (S +6) is stronger than sulfurous acid H 2 SO 3 (S +4).
Obtaining acids
1. Oxygen-free acids can be obtained by direct combination of non-metals with hydrogen.
H 2 + Cl 2 → 2HCl,
H 2 + S ⇆ H 2 S
2. Some oxygen-containing acids can be obtained interaction of acid oxides with water.
3. Both oxygen-free and oxygen-containing acids can be obtained according to exchange reactions between salts and other acids.
BaBr 2 + H 2 SO 4 \u003d BaSO 4 ↓ + 2HBr
CuSO 4 + H 2 S \u003d H 2 SO 4 + CuS ↓
FeS + H 2 SO 4 (pa zb) = H 2 S + FeSO 4
NaCl (T) + H 2 SO 4 (conc) = HCl + NaHSO 4
AgNO 3 + HCl = AgCl↓ + HNO 3
CaCO 3 + 2HBr = CaBr 2 + CO 2 + H 2 O
4. Some acids can be obtained using redox reactions.
H 2 O 2 + SO 2 = H 2 SO 4
3P + 5HNO 3 + 2H 2 O = ZN 3 PO 4 + 5NO 2
Sour taste, effect on indicators, electrical conductivity, interaction with metals, basic and amphoteric oxides, bases and salts, formation of esters with alcohols - these properties are common to inorganic and organic acids.
can be divided into two types of reactions:
1) are common For acids reactions are associated with the formation of hydronium ion H 3 O + in aqueous solutions;
2) specific(i.e. characteristic) reactions specific acids.
The hydrogen ion can enter into redox reaction, reducing to hydrogen, as well as in a compound reaction with negatively charged or neutral particles having lone pairs of electrons, i.e. acid-base reactions.
The general properties of acids include reactions of acids with metals in the voltage series up to hydrogen, for example:
Zn + 2Н + = Zn 2+ + Н 2
Acid-base reactions include reactions with basic oxides and bases, as well as with intermediate, basic, and sometimes acidic salts.
2 CO 3 + 4HBr = 2CuBr 2 + CO 2 + 3H 2 O
Mg(HCO 3) 2 + 2HCl = MgCl 2 + 2CO 2 + 2H 2 O
2KHSO 3 + H 2 SO 4 = K 2 SO 4 + 2SO 2 + 2H 2 O
Note that polybasic acids dissociate stepwise, and at each subsequent step the dissociation is more difficult, therefore, with an excess of acid, acidic salts are most often formed, rather than average ones.
Ca 3 (PO 4) 2 + 4H 3 PO 4 = 3Ca (H 2 PO 4) 2
Na 2 S + H 3 PO 4 = Na 2 HPO 4 + H 2 S
NaOH + H 3 PO 4 = NaH 2 PO 4 + H 2 O
KOH + H 2 S = KHS + H 2 O
At first glance, the formation of acid salts may seem surprising monobasic hydrofluoric acid. However, this fact can be explained. Unlike all other hydrohalic acids, hydrofluoric acid in solutions is partially polymerized (due to the formation of hydrogen bonds) and various particles (HF) X may be present in it, namely H 2 F 2, H 3 F 3, etc.
A special case of acid-base equilibrium - reactions of acids and bases with indicators that change their color depending on the acidity of the solution. Indicators are used in qualitative analysis to detect acids and bases in solutions.
The most commonly used indicators are litmus(V neutral environment purple, V sour - red, V alkaline - blue), methyl orange(V sour environment red, V neutral - orange, V alkaline - yellow), phenolphthalein(V strongly alkaline environment crimson red, V neutral and acidic - colorless).
Specific Properties different acids can be of two types: firstly, reactions leading to the formation insoluble salts, and secondly, redox transformations. If the reactions associated with the presence of the H + ion are common to all acids (qualitative reactions for detecting acids), specific reactions are used as qualitative reactions for individual acids:
Ag + + Cl - = AgCl (white precipitate)
Ba 2+ + SO 4 2- = BaSO 4 (white precipitate)
3Ag + + PO 4 3 - = Ag 3 PO 4 (yellow precipitate)
Some specific reactions of acids are due to their redox properties.
Anoxic acids in an aqueous solution can only be oxidized.
2KMnO 4 + 16HCl = 5Сl 2 + 2КСl + 2МnСl 2 + 8Н 2 O
H 2 S + Br 2 \u003d S + 2HBg
Oxygen-containing acids can be oxidized only if the central atom in them is in a lower or intermediate oxidation state, as, for example, in sulfurous acid:
H 2 SO 3 + Cl 2 + H 2 O \u003d H 2 SO 4 + 2HCl
Many oxygen-containing acids, in which the central atom has the maximum oxidation state (S +6, N +5, Cr +6), exhibit the properties of strong oxidizing agents. Concentrated H 2 SO 4 is a strong oxidizing agent.
Cu + 2H 2 SO 4 (conc) = CuSO 4 + SO 2 + 2H 2 O
Pb + 4HNO 3 = Pb(NO 3) 2 + 2NO 2 + 2H 2 O
C + 2H 2 SO 4 (conc) = CO 2 + 2SO 2 + 2H 2 O
It should be remembered that:
- Acid solutions react with metals that are to the left of hydrogen in the electrochemical voltage series, subject to a number of conditions, the most important of which is the formation of a soluble salt as a result of the reaction. The interaction of HNO 3 and H 2 SO 4 (conc.) with metals proceeds differently.
Concentrated sulfuric acid in the cold passivates aluminum, iron, and chromium.
- In water, acids dissociate into hydrogen cations and anions of acid residues, for example:
- Inorganic and organic acids react with basic and amphoteric oxides, provided that a soluble salt is formed:
- Both acids react with bases. Polybasic acids can form both intermediate and acid salts (these are neutralization reactions):
- The reaction between acids and salts occurs only if a precipitate or gas is formed:
The interaction of H 3 PO 4 with limestone will stop due to the formation of the last insoluble precipitate Ca 3 (PO 4) 2 on the surface.
The features of the properties of nitric HNO 3 and concentrated sulfuric H 2 SO 4 (conc.) acids are due to the fact that when they interact with simple substances (metals and non-metals), not H + cations, but nitrate and sulfate ions will act as oxidizing agents. It is logical to expect that as a result of such reactions, not hydrogen H 2 is formed, but other substances are obtained: necessarily salt and water, as well as one of the products of the reduction of nitrate or sulfate ions, depending on the concentration of acids, the position of the metal in a series of voltages and reaction conditions (temperature, degree of metal grinding, etc.).
These features of the chemical behavior of HNO 3 and H 2 SO 4 (conc.) clearly illustrate the thesis of the theory of chemical structure about the mutual influence of atoms in the molecules of substances.
The concepts of volatility and stability (stability) are often confused. Volatile acids are called acids, the molecules of which easily pass into a gaseous state, that is, they evaporate. For example, hydrochloric acid is a volatile but persistent, stable acid. It is impossible to judge the volatility of unstable acids. For example, non-volatile, insoluble silicic acid decomposes into water and SiO 2 . Aqueous solutions of hydrochloric, nitric, sulfuric, phosphoric and a number of other acids are colorless. An aqueous solution of chromic acid H 2 CrO 4 is yellow, permanganic acid HMnO 4 is raspberry.
Reference material for taking the test:
Mendeleev table
Solubility table
3. Hydroxides
Among multielement compounds, an important group is hydroxides. Some of them exhibit the properties of bases (basic hydroxides) - NaOH, Ba(OH ) 2, etc.; others exhibit the properties of acids (acid hydroxides) - HNO3, H3PO4 and others. There are also amphoteric hydroxides, which, depending on the conditions, can exhibit both the properties of bases and the properties of acids - Zn (OH) 2, Al (OH) 3, etc.
3.1. Classification, preparation and properties of bases
Bases (basic hydroxides), from the standpoint of the theory of electrolytic dissociation, are substances that dissociate in solutions with the formation of OH hydroxide ions - .
According to modern nomenclature, they are usually called hydroxides of elements, indicating, if necessary, the valence of the element (Roman numerals in brackets): KOH - potassium hydroxide, sodium hydroxide NaOH , calcium hydroxide Ca(OH ) 2, chromium hydroxide ( II)-Cr(OH ) 2, chromium hydroxide ( III) - Cr (OH) 3.
Metal hydroxides usually divided into two groups: water soluble(formed by alkali and alkaline earth metals - Li, Na, K, Cs, Rb, Fr, Ca, Sr, Ba and therefore called alkalis) and insoluble in water. The main difference between them is that the concentration of OH ions - in alkali solutions is quite high, but for insoluble bases it is determined by the solubility of the substance and is usually very small. However, small equilibrium concentrations of the OH ion - even in solutions of insoluble bases, the properties of this class of compounds are determined.
By the number of hydroxyl groups (acidity) , capable of being replaced by an acidic residue, are distinguished:
Mono-acid bases - KOH, NaOH;
Diacid bases - Fe (OH) 2, Ba (OH) 2;
Triacid bases - Al (OH) 3, Fe (OH) 3.
Getting grounds
1. The general method for preparing bases is an exchange reaction, with the help of which both insoluble and soluble bases can be obtained:
CuSO 4 + 2KOH = Cu(OH) 2 ↓ + K 2 SO 4 ,
K 2 SO 4 + Ba(OH) 2 = 2KOH + BaCO 3↓ .
When soluble bases are obtained by this method, an insoluble salt precipitates.
When preparing water-insoluble bases with amphoteric properties, excess alkali should be avoided, since dissolution of the amphoteric base may occur, for example,
AlCl 3 + 3KOH = Al(OH) 3 + 3KCl,
Al(OH) 3 + KOH = K.
In such cases, ammonium hydroxide is used to obtain hydroxides, in which amphoteric oxides do not dissolve:
AlCl 3 + 3NH 4 OH = Al(OH) 3 ↓ + 3NH 4 Cl.
Silver and mercury hydroxides decompose so easily that when trying to obtain them by exchange reaction, instead of hydroxides, oxides precipitate:
2AgNO 3 + 2KOH = Ag 2 O ↓ + H 2 O + 2KNO 3.
2. Alkalis in technology are usually obtained by electrolysis of aqueous solutions of chlorides:
2NaCl + 2H 2 O = 2NaOH + H 2 + Cl 2.
(total electrolysis reaction)
Alkalis can also be obtained by reacting alkali and alkaline earth metals or their oxides with water:
2 Li + 2 H 2 O = 2 LiOH + H 2,
SrO + H 2 O = Sr (OH) 2.
Chemical properties of bases
1. All bases insoluble in water decompose when heated to form oxides:
2 Fe (OH) 3 = Fe 2 O 3 + 3 H 2 O,
Ca (OH) 2 = CaO + H 2 O.
2. The most characteristic reaction of bases is their interaction with acids - the neutralization reaction. Both alkalis and insoluble bases enter into it:
NaOH + HNO 3 = NaNO 3 + H 2 O,
Cu(OH) 2 + H 2 SO 4 = CuSO 4 + 2H 2 O.
3. Alkalis interact with acidic and amphoteric oxides:
2KOH + CO 2 = K 2 CO 3 + H 2 O,
2NaOH + Al 2 O 3 = 2NaAlO 2 + H 2 O.
4. Bases can react with acidic salts:
2NaHSO 3 + 2KOH = Na 2 SO 3 + K 2 SO 3 + 2H 2 O,
Ca(HCO 3) 2 + Ba(OH) 2 = BaCO 3↓ + CaCO 3 + 2H 2 O.
Cu(OH) 2 + 2NaHSO 4 = CuSO 4 + Na 2 SO 4 + 2H 2 O.
5. It is necessary to especially emphasize the ability of alkali solutions to react with some non-metals (halogens, sulfur, white phosphorus, silicon):
2 NaOH + Cl 2 = NaCl + NaOCl + H 2 O (in the cold),
6 KOH + 3 Cl 2 = 5 KCl + KClO 3 + 3 H 2 O (when heated),
6KOH + 3S = K 2 SO 3 + 2K 2 S + 3H 2 O,
3KOH + 4P + 3H 2 O = PH 3 + 3KH 2 PO 2,
2NaOH + Si + H 2 O = Na 2 SiO 3 + 2H 2.
6. In addition, concentrated solutions of alkalis, when heated, are also capable of dissolving some metals (those whose compounds have amphoteric properties):
2Al + 2NaOH + 6H 2 O = 2Na + 3H 2,
Zn + 2KOH + 2H 2 O = K 2 + H 2.
Alkaline solutions have a pH> 7 (alkaline environment), change the color of indicators (litmus - blue, phenolphthalein - purple).
M.V. Andryukhova, L.N. Borodina
In addition to oxides, acids and salts, there is a group of compounds called bases or hydroxides. All of them have a single molecular structure: they necessarily contain one or more hydroxyl groups connected to a metal ion. Basic hydroxides are genetically related to metal oxides and salts; this determines not only their chemical properties, but also the methods of production in the laboratory and industry.
There are several forms of classification of bases, which are based both on the characteristics of the metal that is part of the molecule and on the ability of the substance to dissolve in water. In our article we will look at these features of hydroxides, and also get acquainted with their chemical properties, on which the use of bases in industry and everyday life depends.
Physical properties
All bases formed by active or typical metals are solids having a wide range of melting points. In relation to water, they are divided into highly soluble - alkalis and insoluble in water. For example, basic hydroxides containing Group IA elements as cations are easily soluble in water and are strong electrolytes. They are soapy to the touch, corrode fabric, skin and are called alkalis. When they dissociate, OH - ions are detected in the solution, determined using indicators. For example, colorless phenolphthalein becomes crimson in an alkaline medium. Both solutions and melts of sodium, potassium, barium, and calcium hydroxides are electrolytes, i.e. conduct electricity and are considered conductors of the second kind. The soluble bases most often used in industry include about 11 compounds, for example, such as basic hydroxides of sodium, potassium, ammonium, etc.
The structure of the base molecule
An ionic bond is formed between the metal cation and the anions of hydroxyl groups in the molecule of the substance. It is strong enough for water-insoluble hydroxides, so polar water molecules are not able to destroy the crystal lattice of such a compound. Alkalis are stable substances and practically do not form oxide and water when heated. Thus, the main hydroxides of potassium and sodium boil at temperatures above 1000 ° C, but they do not decompose. In the graphic formulas of all bases, it is clearly visible that the oxygen atom of the hydroxyl group is bonded by one covalent bond to the metal atom, and the other to the hydrogen atom. The structure of the molecule and the type of chemical bond determine not only the physical, but also all the chemical characteristics of substances. Let's dwell on them in more detail.
Calcium and magnesium and features of the properties of their compounds
Both elements are typical representatives of active metals and can interact with oxygen and water. The product of the first reaction is a basic oxide. Hydroxide is formed as a result of an exothermic process that occurs with the release of a large amount of heat. Calcium and magnesium bases are slightly soluble white powdery substances. The following names are often used for calcium compounds: milk of lime (if it is a suspension in water) and lime water. Being a typical basic hydroxide, Ca(OH) 2 reacts with acidic and amphoteric oxides, acids and amphoteric bases, such as aluminum and zinc hydroxides. Unlike typical alkalis, which are resistant to heat, magnesium and calcium compounds decompose under the influence of temperature into oxide and water. Both bases, especially Ca(OH) 2, are widely used in industry, agriculture and domestic needs. Let's consider their application below.
Fields of application of calcium and magnesium compounds
It is well known that a chemical material called fluff or slaked lime is used in construction. This is the base of calcium. Most often it is obtained by the reaction of water with basic calcium oxide. The chemical properties of basic hydroxides allow them to be widely used in various sectors of the national economy. For example, for purification of impurities in the production of raw sugar, for the production of bleach, in the bleaching of cotton and linen yarn. Before the invention of ion exchangers - cation exchangers, calcium and magnesium bases were used in water softening technologies, which made it possible to get rid of bicarbonates that deteriorate its quality. To do this, water was boiled with a small amount of soda ash or slaked lime. An aqueous suspension of magnesium hydroxide can be used as a treatment for patients with gastritis to reduce the acidity of gastric juice.
Properties of basic oxides and hydroxides
The most important substances of this group are reactions with acidic oxides, acids, amphoteric bases and salts. Interestingly, insoluble bases such as copper, iron or nickel hydroxides cannot be obtained by direct reaction of the oxide with water. In this case, the laboratory uses the reaction between the corresponding salt and alkali. As a result, bases are formed that precipitate. For example, this is how a blue precipitate of copper hydroxide and a green precipitate of divalent iron base are obtained. Subsequently, they are evaporated to solid powders, which are classified as water-insoluble hydroxides. A distinctive feature of these compounds is that when exposed to high temperatures they decompose into the corresponding oxide and water, which cannot be said about alkalis. After all, water-soluble bases are thermally stable.
Electrolysis ability
Continuing to study the main ones, we will dwell on one more feature by which we can distinguish the bases of alkali and alkaline earth metals from compounds insoluble in water. This is the inability of the latter to dissociate into ions under the influence of an electric current. On the contrary, melts and solutions of potassium, sodium, barium, and strontium hydroxides are easily electrolyzed and are conductors of the second kind.
Getting grounds
Speaking about the properties of this class of inorganic substances, we have partially listed the chemical reactions that underlie their production in laboratory and industrial conditions. The most accessible and cost-effective method can be considered the method of thermal decomposition of natural limestone, as a result of which it is obtained. If the reaction is carried out with water, it forms a basic hydroxide - Ca(OH) 2. A mixture of this substance with sand and water is called mortar. It continues to be used for plastering walls, for binding bricks and in other types of construction work. Alkalis can also be prepared by reacting the corresponding oxides with water. For example: K 2 O + H 2 O = 2 KON. The process is exothermic and releases a large amount of heat.
Interaction of alkalis with acidic and amphoteric oxides
The characteristic chemical properties of water-soluble bases include their ability to form salts in reactions with oxides containing non-metal atoms in their molecules, for example, carbon dioxide, sulfur dioxide or silicon oxide. In particular, calcium hydroxide is used to dry gases, and sodium and potassium hydroxides are used to obtain the corresponding carbonates. Zinc and aluminum oxides, which are amphoteric substances, can interact with both acids and alkalis. In the latter case, complex compounds can be formed, for example, such as sodium hydroxyzincate.
Neutralization reaction
One of the most important properties of bases, both water-insoluble and alkali, is their ability to react with inorganic or organic acids. This reaction comes down to the interaction between two types of ions: hydrogen and hydroxyl groups. It leads to the formation of water molecules: HCI + KOH = KCI + H 2 O. From the point of view of the theory of electrolytic dissociation, the entire reaction comes down to the formation of a weak, slightly dissociated electrolyte - water.
In the example given, an intermediate salt was formed - potassium chloride. If basic hydroxides are taken for the reaction in an amount less than that required to completely neutralize the polybasic acid, then upon evaporation of the resulting product, crystals of the acid salt are detected. The neutralization reaction plays an important role in metabolic processes occurring in living systems - cells and allows them, with the help of their own buffer complexes, to neutralize the excess amount of hydrogen ions that accumulate in dissimilation reactions.
