Tables of grounds. Grounds

DEFINITION

Reasons are called electrolytes, upon dissociation of which only OH - ions are formed from negative ions:

Fe(OH) 2 ↔ Fe 2+ + 2OH - ;

NH 3 + H 2 O ↔ NH 4 OH ↔ NH 4 + + OH - .

All inorganic bases are classified into water-soluble (alkalis) - NaOH, KOH and water-insoluble (Ba(OH) 2, Ca(OH) 2). Depending on the manifestations chemical properties Among the bases, amphoteric hydroxides are distinguished.

Chemical properties of bases

When indicators act on solutions of inorganic bases, their color changes, so when a base gets into a solution, litmus becomes blue, methyl orange becomes yellow, and phenolphthalein becomes crimson.

Inorganic bases are able to react with acids to form salt and water, and water-insoluble bases react only with water-soluble acids:

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

NaOH + HCl = NaCl + H 2 O.

Bases that are insoluble in water are thermally unstable, i.e. when heated, they undergo decomposition to form oxides:

2Fe(OH) 3 = Fe 2 O 3 + 3 H 2 O;

Mg(OH) 2 = MgO + H 2 O.

Alkalis (water-soluble bases) react with acidic oxides to form salts:

NaOH + CO 2 = NaHCO 3.

Alkalis are also capable of entering into interaction reactions (ORR) with some non-metals:

2NaOH + Si + H 2 O → Na 2 SiO 3 +H 2.

Some bases enter into exchange reactions with salts:

Ba(OH) 2 + Na 2 SO 4 = 2NaOH + BaSO 4 ↓.

Amphoteric hydroxides (bases) also exhibit the properties of weak acids and react with alkalis:

Al(OH) 3 + NaOH = Na.

Amphoteric bases include aluminum and zinc hydroxides. chromium (III), etc.

Physical properties of bases

Most bases are solids that vary in solubility in water. Alkalis are water-soluble bases that are most often white solids. Water-insoluble bases can have different colors, for example, iron (III) hydroxide is a brown solid, aluminum hydroxide is a white solid, and copper (II) hydroxide is a blue solid.

Getting grounds

Bases are prepared in different ways, for example, by the reaction:

- exchange

CuSO 4 + 2KOH → Cu(OH) 2 ↓ + K 2 SO 4 ;

K 2 CO 3 + Ba(OH) 2 → 2KOH + BaCO 3 ↓;

— interactions active metals or their oxides with water

2Li + 2H 2 O→ 2LiOH +H 2;

BaO + H 2 O → Ba(OH) 2 ↓;

— electrolysis of aqueous salt solutions

2NaCl + 2H 2 O = 2NaOH + H 2 + Cl 2.

Examples of problem solving

EXAMPLE 1

Exercise

Calculate the practical mass of aluminum oxide (the yield of the target product is 92%) from the decomposition reaction of aluminum hydroxide weighing 23.4 g.

Solution

Let's write the reaction equation:

2Al(OH) 3 = Al 2 O 3 + 3H 2 O.

Molar mass of aluminum hydroxide calculated using table chemical elements DI. Mendeleev – 78 g/mol. Let's find the amount of aluminum hydroxide:

v(Al(OH) 3) = m(Al(OH) 3)/M(Al(OH) 3);

v(Al(OH) 3) = 23.4/78 = 0.3 mol.

According to the reaction equation v(Al(OH) 3): v(Al 2 O 3) = 2:1, therefore, the amount of aluminum oxide substance will be:

v(Al 2 O 3) = 0.5 × v(Al(OH) 3);

v(Al 2 O 3) = 0.5 × 0.3 = 0.15 mol.

Molar mass of aluminum oxide, calculated using the table of chemical elements by D.I. Mendeleev – 102 g/mol. Let's find the theoretical mass of aluminum oxide:

m(Al 2 O 3) th = 0.15×102 = 15.3 g.

Then, the practical mass of aluminum oxide is:

m(Al 2 O 3) pr = m(Al 2 O 3) th × 92/100;

m(Al 2 O 3) pr = 15.3 × 0.92 = 14 g.

Answer

Weight of aluminum oxide - 14 g.

EXAMPLE 2

Exercise

Carry out a series of transformations:

Fe→ FeCl 2 → Fe(OH) 2 →Fe(OH) 3 →Fe(NO 3) 3

Bases (hydroxides) – complex substances, the molecules of which 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 valency 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, which 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, litmus indicator - in Blue colour, 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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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.

NH 3 + H 2 O = NH 4 OH (ammonium hydroxide).

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.

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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The division of bases into groups according to various characteristics is presented in Table 11.

Table 11
Classification of bases

All bases, except a solution of ammonia in water, are solid substances of different colors. For example, calcium hydroxide Ca(OH) 2 is white, copper (II) hydroxide Cu(OH) 2 is blue, nickel (II) hydroxide Ni(OH) 2 is green, iron (III) hydroxide Fe(OH) 3 is red- brown, etc.

An aqueous solution of ammonia NH 3 H 2 O, unlike other bases, does not contain metal cations, but a complex single-charge ammonium cation NH - 4 and exists only in solution (you know this solution as ammonia). It easily decomposes into ammonia and water:

However, no matter how different the bases are, they all consist of metal ions and hydroxo groups, the number of which is equal to the oxidation state of the metal.

All bases, and primarily alkalis (strong electrolytes), form upon dissociation hydroxide ions OH - , which determine the series general properties: soapiness to the touch, change in color of indicators (litmus, methyl orange and phenolphthalein), interaction with other substances.

Typical base reactions

The first reaction (universal) was considered in § 38.

Laboratory experiment No. 23
Interaction of alkalis with acids

H + + OH - = H 2 O.

Carry out the reactions for which you have created equations. Remember what substances (except acid and alkali) are needed to observe these chemical reactions.

The second reaction occurs between alkalis and non-metal oxides, which correspond to acids, for example,

Compliant

etc.

When oxides interact with bases, salts of the corresponding acids and water are formed:

Rice. 141.
Interaction of alkali with non-metal oxide

Laboratory experiment No. 24
Interaction of alkalis with non-metal oxides

Repeat the experiment you did before. Pour 2-3 ml of a clear solution of lime water into a test tube.

Place a juice straw in it, which acts as a gas outlet tube. Gently pass exhaled air through the solution. What are you observing?

Write down the molecular and ionic equations for the reaction.

Rice. 142.
Interaction of alkalis with salts:
a - with the formation of sediment; b - with gas formation

The third reaction is a typical ion exchange reaction and only occurs if it results in a precipitate or gas being released, for example:

Laboratory experiment No. 25
Interaction of alkalis with salts

Heat the contents of the 1st test tube and identify one of the reaction products by smell.

Formulate a conclusion about the possibility of interaction of alkalis with salts.

Insoluble bases decompose when heated into metal oxide and water, which is not typical for alkalis, for example:

Fe(OH) 2 = FeO + H 2 O.

Laboratory experiment No. 26
Preparation and properties of insoluble bases

Pour 1 ml of copper (II) sulfate or chloride solution into two test tubes. Add 3-4 drops of sodium hydroxide solution to each test tube. Describe the copper(II) hydroxide formed.

Note. Leave the test tubes with the resulting copper (II) hydroxide for the next experiments.

Write down the molecular and ionic equations for the reaction. Indicate the type of reaction based on the “number and composition of starting substances and reaction products.”

Add 1-2 ml of copper (II) hydroxide obtained in the previous experiment to one of the test tubes of hydrochloric acid. What are you observing?

Using a pipette, place 1-2 drops of the resulting solution on a glass or porcelain plate and, using crucible tongs, carefully evaporate it. Examine the crystals that form. Note their color.

Write down the molecular and ionic equations for the reaction. Indicate the type of reaction based on “the number and composition of starting materials and reaction products,” “participation of a catalyst,” and “reversibility of a chemical reaction.”

Heat one of the test tubes with copper hydroxide () obtained earlier or given by the teacher (Fig. 143). What are you observing?

Rice. 143.
Decomposition of copper(II) hydroxide when heated

Draw up an equation for the reaction carried out, indicate the condition for its occurrence and the type of reaction based on the characteristics “number and composition of starting substances and reaction products”, “release or absorption of heat” and “reversibility of a chemical reaction”.

Key words and phrases

Classification of bases.
Typical properties of bases: their interaction with acids, non-metal oxides, salts.
A typical property of insoluble bases is decomposition when heated.
Conditions for typical base reactions.

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Questions and tasks

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 that, depending on 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

From the standpoint of the theory of electrolytic dissociation, bases (basic hydroxides) are substances that dissociate in solutions to form OH hydroxide ions - .

According to modern nomenclature, they are usually called hydroxides of elements, indicating, if necessary, the valence of the element (in 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

Tables of grounds. Grounds

Chemical properties of bases

Physical properties of bases

Getting grounds

Examples of problem solving

Laboratory experiment No. 23 Interaction of alkalis with acids

Laboratory experiment No. 24 Interaction of alkalis with non-metal oxides

Laboratory experiment No. 25 Interaction of alkalis with salts

Laboratory experiment No. 26 Preparation and properties of insoluble bases

Laboratory experiment No. 23
Interaction of alkalis with acids

Laboratory experiment No. 24
Interaction of alkalis with non-metal oxides

Laboratory experiment No. 25
Interaction of alkalis with salts

Laboratory experiment No. 26
Preparation and properties of insoluble bases