Ba oh 2 is a strong or weak base. Examples of strong and weak acids and bases. Strong acid and strong base

After reading the article, you will be able to separate substances into salts, acids and bases. The article describes what the pH of a solution is, what general properties of acids and bases have.

In simple terms, acid is everything with H, and base is with OH. BUT! Not always. To distinguish acid from base, you must ... remember them! I'm sorry. In order to somehow make life easier, our three friends, Arrhenius and Bronsted and Lowry, came up with two theories that are called by their name.

Like metals and non-metals, acids and bases are the separation of substances according to similar properties. The first theory of acids and bases belonged to the Swedish scientist Arrhenius. Arrhenius acid is a class of substances that, in reaction with water, dissociate (disintegrate), forming the hydrogen cation H +. Arrhenius bases in aqueous solution form OH - anions. The following theory was proposed in 1923 by scientists Bronsted and Lowry. The Bronsted-Lowry theory defines by acids substances capable of giving up a proton in a reaction (a hydrogen cation is called a proton in reactions). Bases, respectively, are substances that can accept a proton in a reaction. The current theory is the Lewis theory. Lewis's theory defines acids as molecules or ions capable of accepting electron pairs, thereby forming Lewis adducts (an adduct is a compound formed by combining two reagents without the formation of by-products).

In inorganic chemistry, as a rule, by acid they mean the Bronsted-Lowry acid, that is, substances capable of donating a proton. If we mean the definition of acid according to Lewis, then in the text such an acid is called Lewis acid. These rules are valid for acids and bases.

Dissociation

Dissociation is the process of decomposition of a substance into ions in solutions or melts. For example, the dissociation of hydrochloric acid is the decomposition of HCl into H + and Cl -.

Properties of acids and bases

The bases are usually soapy to the touch; acids, for the most part, have a sour taste.

When the base reacts with many cations, a precipitate is formed. When an acid reacts with anions, gas is usually produced.

Frequently used acids:
H 2 O, H 3 O +, CH 3 CO 2 H, H 2 SO 4, HSO 4 -, HCl, CH 3 OH, NH 3

Commonly used bases:
OH -, H 2 O, CH 3 CO 2 -, HSO 4 -, SO 4 2−, Cl -

Strong and weak acids and bases

Strong acids

Such acids that dissociate completely in water, producing hydrogen cations H + and anions. An example of a strong acid is hydrochloric acid HCl:

HCl (solution) + H 2 O (g) → H 3 O + (solution) + Cl - (solution)

Examples of strong acids: HCl, HBr, HF, HNO 3, H 2 SO 4, HClO 4

List of strong acids

• HCl - hydrochloric acid
• HBr - hydrogen bromide
• HI - hydrogen iodide
• HNO 3 - nitric acid
• HClO 4 - perchloric acid
• H 2 SO 4 - sulfuric acid

Weak acids

Only partially soluble in water, e.g. HF:

HF (p-p) + H2O (l) → H3O + (p-p) + F - (p-p) - in such a reaction more than 90% of the acid does not dissociate:
= < 0,01M для вещества 0,1М

Strong and weak acids can be distinguished by measuring the conductivity of solutions: the conductivity depends on the number of ions, the stronger the acid, the more it is dissociated, therefore, the stronger the acid, the higher the conductivity.

List of weak acids

• HF hydrogen fluoride
• H 3 PO 4 phosphoric
• H 2 SO 3 sulphurous
• H 2 S hydrogen sulfide
• H 2 CO 3 coal
• H 2 SiO 3 silicon

Strong bases

Strong bases completely dissociate in water:

NaOH (solution) + H 2 O ↔ NH 4

Strong bases include metal hydroxides of the first (alkalines, alkali metals) and the second (alkalineterrenes, alkaline earth metals) groups.

List of strong bases

• NaOH sodium hydroxide (caustic soda)
• KOH potassium hydroxide (caustic potassium)
• LiOH lithium hydroxide
• Ba (OH) 2 barium hydroxide
• Ca (OH) 2 calcium hydroxide (slaked lime)

Weak bases

In a reversible reaction in the presence of water, forms OH - ions:

NH 3 (solution) + H 2 O ↔ NH + 4 (solution) + OH - (solution)

Most of the weak bases are anions:

F - (p-p) + H 2 O ↔ HF (p-p) + OH - (p-p)

Weak bases list

• Mg (OH) 2 magnesium hydroxide
• Fe (OH) 2 iron (II) hydroxide
• Zn (OH) 2 zinc hydroxide
• NH 4 OH ammonium hydroxide
• Fe (OH) 3 iron (III) hydroxide

Reactions of acids and bases

Strong acid and strong base

This reaction is called neutralization: when the amount of reagents is sufficient for the complete dissociation of the acid and base, the resulting solution will be neutral.

Example:
H 3 O + + OH - ↔ 2H 2 O

Weak base and weak acid

General view of the reaction:
Weak base (solution) + H 2 O ↔ Weak acid (solution) + OH - (solution)

Strong base and weak acid

The base completely dissociates, the acid partially dissociates, the resulting solution has weak base properties:

HX (p-p) + OH - (p-p) ↔ H 2 O + X - (p-p)

Strong acid and weak base

The acid completely dissociates, the base does not completely dissociate:

Dissociation of water

Dissociation is the breakdown of a substance into its constituent molecules. The properties of an acid or base depend on the equilibrium that is present in the water:

H 2 O + H 2 O ↔ H 3 O + (solution) + OH - (solution)
K c \u003d / 2
Equilibrium constant of water at t \u003d 25 °: K c \u003d 1.83⋅10 -6, the following equality also takes place: \u003d 10 -14, which is called the constant of water dissociation. For pure water \u003d \u003d 10 -7, whence -lg \u003d 7.0.

This value (-lg) is called pH - hydrogen potential. If pH< 7, то вещество имеет кислотные свойства, если pH > 7, then the substance has basic properties.

Methods for determining pH

Instrumental method

A special device is a pH meter - a device that transforms the concentration of protons in a solution into an electrical signal.

Indicators

A substance that changes color in a certain range of pH values \u200b\u200bdepending on the acidity of the solution, using several indicators, a fairly accurate result can be achieved.

Salt

Salt is an ionic compound formed by a cation other than H + and an anion other than O 2-. In a weak aqueous solution, salts are completely dissociated.

To determine the acid-base properties of a salt solution, it is necessary to determine which ions are present in the solution and consider their properties: neutral ions formed from strong acids and bases do not affect the pH: they don’t give up ions either H + or OH - in water. For example, Cl -, NO - 3, SO 2 - 4, Li +, Na +, K +.

Anions formed from weak acids exhibit alkaline properties (F -, CH 3 COO -, CO 2 - 3), there are no alkaline cations.

All cations, except for metals of the first and second groups, have acidic properties.

Buffer solution

Solutions that maintain the pH level with the addition of a small amount of a strong acid or strong base mainly consist of:

• A mixture of a weak acid, corresponding salt and a weak base
• Weak base, corresponding salt and strong acid

To prepare a buffer solution of a certain acidity, it is necessary to mix a weak acid or base with the corresponding salt, while taking into account:

• The pH range over which the buffer solution will be effective
• Solution capacity - the amount of strong acid or strong base that can be added without affecting the pH of the solution
• There should be no unwanted reactions that could change the composition of the solution

Test:

To understand how the hydrolysis of salts in their aqueous solutions proceeds, we first give the definition of this process.

Definition and features of hydrolysis

This process involves the chemical action of water ions with salt ions, as a result, a weak base (or acid) is formed, and the reaction of the medium changes. Any salt can be represented as a product of the chemical interaction between a base and an acid. Depending on what their strength is, there are several options for the course of the process.

Hydrolysis types

In chemistry, three types of reaction between salt and water cations are considered. Each process is carried out with a change in the pH of the medium, therefore, it is assumed that different types of indicators are used to determine the pH. For example, violet litmus is used for an acidic environment, phenolphthalein is suitable for an alkaline reaction. Let us analyze in more detail the features of each hydrolysis option. Strong and weak bases can be determined from the solubility table, and the strength of acids can be determined from the table.

Hydrolysis by cation

As an example of such a salt, consider ferric chloride (2). Iron (2) hydroxide is a weak base and hydrochloric acid is strong. In the process of interaction with water (hydrolysis), a basic salt (iron hydroxychloride 2) is formed, and hydrochloric acid is also formed. An acidic environment appears in the solution, it can be determined using blue litmus (pH less than 7). In this case, the hydrolysis itself proceeds along the cation, since a weak base is used.

Let us give one more example of the course of hydrolysis for the described case. Consider the magnesium chloride salt. Magnesium hydroxide is a weak base and hydrochloric acid is a strong base. In the process of interaction with water molecules, magnesium chloride is converted into a basic salt (hydroxychloride). Magnesium hydroxide, the formula of which is generally presented as M (OH) 2, is slightly soluble in water, but strong hydrochloric acid makes the solution acidic.

Anion hydrolysis

The next variant of hydrolysis is typical for a salt that is formed by a strong base (alkali) and a weak acid. As an example for this case, consider sodium carbonate.

This salt contains a strong sodium base as well as a weak carbonic acid. Interaction with water molecules proceeds with the formation of an acidic salt - sodium bicarbonate, that is, hydrolysis by anion occurs. It also forms in the solution which gives the solution an alkaline environment.

Let's give one more example for this case. Potassium sulfite is a salt that is formed by a strong base - caustic potassium, as well as a weak one.In the process of interaction with water (during hydrolysis), potassium hydrosulfite (acid salt) and potassium hydroxide (alkali) are formed. The medium in the solution will be alkaline, it can be confirmed with phenolphthalein.

Complete hydrolysis

The salt of a weak acid and a weak base undergoes complete hydrolysis. Let's try to find out what its peculiarity is, and what products will be formed as a result of this chemical reaction.

Let us analyze the hydrolysis of a weak base and a weak acid using the example of aluminum sulfide. This salt is formed by aluminum hydroxide, which is a weak base, as well as a weak hydrosulfuric acid. When interacting with water, complete hydrolysis is observed, as a result of which gaseous hydrogen sulfide is formed, as well as aluminum hydroxide in the form of a precipitate. This interaction proceeds both along the cation and the anion; therefore, this variant of hydrolysis is considered complete.

Also, magnesium sulfide can be cited as an example of the interaction of this type of salt with water. This salt contains magnesium hydroxide, its formula is Mg (OH) 2. It is a weak base, insoluble in water. In addition, there is hydrogen sulphide acid inside the magnesium sulfide, which is weak. When interacting with water, complete hydrolysis occurs (by cation and anion), as a result of which magnesium hydroxide is formed in the form of a precipitate, and hydrogen sulfide is released in the form of a gas.

If we consider the hydrolysis of a salt that is formed by a strong acid and a strong base, then it should be noted that it does not proceed. The medium in solutions of salts such as potassium chloride remains neutral.

Conclusion

Strong and weak bases, acids with which salts are formed, affect the result of hydrolysis, the reaction of the medium in the resulting solution. Such processes are widespread in nature.

Hydrolysis is of particular importance in the chemical transformation of the earth's crust. It contains metal sulfides, which are poorly soluble in water. In the course of their hydrolysis, hydrogen sulfide is formed, its release during volcanic activity to the surface of the earth.

When silicate rocks transform into hydroxides, they cause gradual destruction of rocks. For example, a mineral such as malachite is a hydrolysis product of copper carbonates.

An intensive process of hydrolysis also takes place in the World Ocean. and calcium, which are carried away by water, have a slightly alkaline environment. In such conditions, the process of photosynthesis in marine plants is excellent, and marine organisms develop more intensively.

The oil contains impurities of water and calcium and magnesium salts. In the process of heating oil, they interact with water vapor. In the course of hydrolysis, hydrogen chloride is formed, when it interacts with the metal, the equipment is destroyed.

ELECTROLYTES- Substances, solutions or melts of which conduct electric current.

NON-ELECTROLYTES- Substances, solutions or melts of which do not conduct electric current.

Dissociation - decomposition of compounds into ions.

Dissociation degree - the ratio of the number of molecules dissociated into ions to the total number of molecules in solution.

STRONG ELECTROLYTES when dissolved in water, they almost completely dissociate into ions.

When writing the equations for the dissociation of strong electrolytes, an equal sign is put.

Strong electrolytes include:

Soluble salts ( see solubility table);

Many inorganic acids: HNO 3, H 2 SO 4, HClO 3, HClO 4, HMnO 4, HCl, HBr, HI ( look acids-strong electrolytes in the solubility table);

Bases of alkaline (LiOH, NaOH, KOH) and alkaline earth (Ca (OH) 2, Sr (OH) 2, Ba (OH) 2) metals ( see bases-strong electrolytes in the solubility table).

WEAK ELECTROLYTES in aqueous solutions only partially (reversibly) dissociate into ions.

When writing the equations for the dissociation of weak electrolytes, a reversibility sign is put.

Weak electrolytes include:

· Almost all organic acids and water (H 2 O);

Some inorganic acids: H 2 S, H 3 PO 4, HClO 4, H 2 CO 3, HNO 2, H 2 SiO 3 ( look acids are weak electrolytes in the solubility table);

Insoluble metal hydroxides (Mg (OH) 2, Fe (OH) 2, Zn (OH) 2) ( see reasons-cpoor electrolytes in the solubility table).

Several factors affect the degree of electrolytic dissociation:

the nature of the solvent and electrolyte: strong electrolytes are substances with ionic and covalent strongly polar bonds; good ionizing ability, i.e. the ability to cause the dissociation of substances is possessed by solvents with a high dielectric constant, the molecules of which are polar (for example, water);

temperature: since dissociation is an endothermic process, an increase in temperature increases the value of α;

concentration: when the solution is diluted, the degree of dissociation increases, and with an increase in concentration, it decreases;

dissociation stage: each subsequent stage is less effective than the previous one, about 1000-10,000 times; for example, for phosphoric acid α 1\u003e α 2\u003e α 3:

H3PО4⇄Н ++ H2PО − 4 (first stage, α 1),

H2PО − 4⇄Н ++ HPО2−4 (second stage, α 2),

НPО2−4⇄Н ++ PО3−4 (third stage, α 3).

For this reason, the concentration of hydrogen ions in a solution of this acid is the highest, and the concentration of phosphate ions PO3−4 is the lowest.

1. The solubility and the degree of dissociation of a substance are not related to each other. For example, acetic acid, which is readily (unlimitedly) water-soluble, is a weak electrolyte.

2. The solution of a weak electrolyte contains less than others those ions that are formed at the last stage of electrolytic dissociation

The degree of electrolytic dissociation is also affected by adding other electrolytes: for example, the degree of dissociation of formic acid

HCOOH ⇄ HCOO - + H +

decreases if a little sodium formate is added to the solution. This salt dissociates with the formation of formate ions HCOO -:

HCOONa → HCOO - + Na +

As a result, the concentration of НСОО– ions in the solution increases, and according to Le Chatelier's principle, an increase in the concentration of formate ions shifts the equilibrium of the dissociation of formic acid to the left; the degree of dissociation decreases.

Ostwald's dilution law - the ratio expressing the dependence of the equivalent electrical conductivity of a diluted solution of a binary weak electrolyte on the concentration of the solution:

Here is the dissociation constant of the electrolyte, is the concentration, and are the values \u200b\u200bof the equivalent electrical conductivity at concentration and at infinite dilution, respectively. The ratio is a consequence of the law of mass action and equality

where is the degree of dissociation.

Ostwald's dilution law was derived by W. Ostwald in 1888 and he also confirmed empirically. The experimental establishment of the correctness of the Ostwald dilution law was of great importance for substantiating the theory of electrolytic dissociation.

Electrolytic dissociation of water. PH pH Water is a weak amphoteric electrolyte: Н2О Н + + ОН- or, more precisely: 2Н2О \u003d Н3О + + ОН- The dissociation constant of water at 25оС is: This constant value corresponds to the dissociation of one out of a hundred million water molecules, therefore the concentration of water can be considered constant and equal to 55.55 mol / l (water density 1000 g / l, mass 1 l 1000 g, amount of water substance 1000 g: 18 g / mol \u003d 55.55 mol, C \u003d 55.55 mol: 1 l \u003d 55 , 55 mol / L). Then This value is constant at a given temperature (25 ° C), it is called the ionic product of water KW: Water dissociation is an endothermic process, therefore, with an increase in temperature in accordance with Le Chatelier's principle, dissociation increases, the ionic product increases and reaches 10-13 at 100 ° C. In pure water at 25оС, the concentrations of hydrogen and hydroxyl ions are equal to each other: \u003d \u003d 10-7 mol / l Solutions in which the concentrations of hydrogen and hydroxyl ions are equal to each other are called neutral. If acid is added to pure water, the concentration of hydrogen ions will increase and become more than 10-7 mol / l, the medium will become acidic, while the concentration of hydroxyl ions will instantly change so that the ionic product of water retains its value of 10-14. The same will happen when alkali is added to clean water. The concentrations of hydrogen and hydroxyl ions are related to each other through the ionic product, therefore, knowing the concentration of one of the ions, it is easy to calculate the concentration of the other. For example, if \u003d 10-3 mol / l, then \u003d KW / \u003d 10-14 / 10-3 \u003d 10-11 mol / l, or, if \u003d 10-2 mol / l, then \u003d KW / \u003d 10-14 / 10-2 \u003d 10-12 mol / l. Thus, the concentration of hydrogen or hydroxyl ions can serve as a quantitative characteristic of the acidity or alkalinity of the medium. In practice, they use not the concentrations of hydrogen or hydroxyl ions, but the hydrogen pH or hydroxyl pOH indicators. The pH is equal to the negative decimal logarithm of the concentration of hydrogen ions: pH \u003d - lg The hydroxyl exponent pOH is equal to the negative decimal logarithm of the concentration of hydroxyl ions: pOH \u003d - lg It is easy to show by taking the logarithm of the ionic product of water that pH + pOH \u003d 14 If the pH of the medium is 7 - the medium is neutral, if less than 7 it is acidic, and the lower the pH, the higher the concentration of hydrogen ions. pH is more than 7 - the medium is alkaline, the higher the pH, the higher the concentration of hydroxyl ions.

Bases (hydroxides) - complex substances, the molecules of which contain one or more hydroxy OH groups. Most often, the bases are composed 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 by dissolving ammonia in water (the reaction of adding water to ammonia):

NH 3 + H 2 O \u003d NH 4 OH (ammonium hydroxide).

The valence of the hydroxy group is 1. The number of hydroxyl groups in the base molecule depends on the valence of the metal and is equal to it. For example, NaOH, LiOH, Al (OH) 3, Ca (OH) 2, Fe (OH) 3, etc.

All the reasons - solids that have different colors. Some bases are readily soluble in water (NaOH, KOH, etc.). However, most of them do not dissolve in water.

Bases that are soluble in water are called alkalis. Alkali solutions are “soapy”, slippery to the touch and rather caustic. Alkalis 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 basesAre amphoteric hydroxides, which act as bases when interacting with acids, and behave like acids with alkali.

Different bases differ in their ability to split off hydroxy groups, so they are divided into strong and weak bases.

Strong bases in aqueous solutions easily give up their hydroxy groups, while weak ones do not.

Chemical properties of bases

The chemical properties of bases are characterized by their ratio to acids, acid anhydrides and salts.

1. Affect indicators... Indicators change their color depending on interaction with different chemicals. In neutral solutions - they have one color, in acid solutions - another. When interacting with bases, they change their color: the indicator methyl orange turns yellow, the litmus indicator turns blue, and phenolphthalein becomes fuchsia.

2. Interact with acidic oxides with the formation of salt and water:

2NaOH + SiO 2 → Na 2 SiO 3 + H 2 O.

3. Reacts with acids, forming salt and water. The reaction of interaction 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. React with salts, forming new salt and base:

2NaOH + CuSO 4 → Cu (OH) 2 + Na 2 SO 4.

5. Able to decompose when heated into water and basic oxide:

Cu (OH) 2 \u003d CuO + H 2 O.

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