Bases (hydroxides)- complex substances, the molecules of which have one or more hydroxy groups OH in their composition. 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 by dissolving ammonia in water (reactions of addition of water to ammonia):
NH 3 + H 2 O = NH 4 OH (ammonium hydroxide).
The valence of the hydroxyl 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 grounds - solids that have different colors. Some bases are highly soluble in water (NaOH, KOH, etc.). However, most of them do not dissolve in water.
Water-soluble bases are called alkalis. Alkali solutions are "soapy", slippery to the touch and quite 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 bases- these are amphoteric hydroxides, which, when interacting with acids, act as bases, 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 according to the feature.
Strong bases easily donate their hydroxyl groups in aqueous solutions, 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 their color depending on the interaction with different chemicals. In neutral solutions - they have one color, in acid solutions - another. When interacting with bases, they change their color: the methyl orange indicator turns yellow, the litmus indicator turns blue, and phenolphthalein turns fuchsia.
2. React with acidic oxides 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 the 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 a new salt and base:
2NaOH + CuSO 4 → Cu(OH) 2 + Na 2 SO 4.
5. Able to decompose into water and basic oxide when heated:
Cu (OH) 2 \u003d CuO + H 2 O.
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Bases are complex compounds that include two main structural components:
- Hydroxo group (one or more). Hence, by the way, the second name of these substances is “hydroxides”.
- Metal atom or ammonium ion (NH4+).
The name of the base comes from the combination of the names of both of its components: for example, calcium hydroxide, copper hydroxide, silver hydroxide, etc.
The only exception to the general rule for the formation of bases should be considered when the hydroxo group is attached not to the metal, but to the ammonium cation (NH4 +). This substance is formed when ammonia dissolves in water.
If we talk about the properties of bases, then it should immediately be noted that the valency of the hydroxo group is equal to one, respectively, the number of these groups in the molecule will directly depend on what valency the metals that enter into the reaction have. Examples in this case are the formulas of such substances as NaOH, Al(OH)3, Ca(OH)2.
The chemical properties of bases are manifested in their reactions with acids, salts, other bases, as well as in their action on indicators. In particular, alkalis can be determined by exposing a certain indicator to their solution. In this case, it will noticeably change its color: for example, it will become blue from white, and phenolphthalein will become crimson.
The chemical properties of bases, manifested in their interaction with acids, lead to the famous neutralization reactions. The essence of such a reaction is that the metal atoms, joining the acid residue, form a salt, and the hydroxo group and the hydrogen ion, when combined, turn into water. This reaction is called a neutralization reaction because no alkali or acid remains after it.
The characteristic chemical properties of bases are also manifested in their reaction with salts. It should be noted that only alkalis react with soluble salts. The structural features of these substances lead to the fact that as a result of the reaction a new salt and a new, most often insoluble, base are formed.
Finally, the chemical properties of the bases perfectly manifest themselves during thermal exposure to them - heating. Here, when carrying out certain experiments, it should be borne in mind that almost all bases, with the exception of alkalis, behave extremely unstable when heated. The vast majority of them almost instantly decomposes into the corresponding oxide and water. And if we take the bases of such metals as silver and mercury, then under normal conditions they cannot be obtained, since they begin to decompose already at room temperature.
Bases, amphoteric hydroxides
Bases are complex substances consisting of metal atoms and one or more hydroxo 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 alkali hydroxides of alkali and alkaline earth metals
1. Aqueous solutions of alkalis are soapy to the touch, change the color of indicators: litmus - blue, phenolphthalein - raspberry.
2. Aqueous solutions dissociate:
3. Interact with acids, entering into an exchange reaction:
Polyacid bases can give intermediate and basic salts:
4. Interact with acid oxides, forming medium and acid 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. React 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
Compounds are called amphoteric, which, depending on the 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 reveal the main properties:
Zn(OH) 2 + 2HCl = ZnCl 2 + 2H 2 O
2. Interacting with alkalis - strong bases, they exhibit acidic properties:
Zn (OH) 2 + 2NaOH \u003d Na 2 ( complex salt)
Al (OH) 3 + NaOH \u003d Na ( complex salt)
Compounds are called complex in which at least one covalent bond was formed by the donor-acceptor mechanism.
The general method for obtaining bases is based on exchange reactions, by which both insoluble and soluble bases can be obtained.
CuSO 4 + 2KOH \u003d Cu (OH) 2 ↓ + K 2 SO 4
K 2 CO 3 + Ba (OH) 2 \u003d 2 KOH + BaCO 3 ↓
When soluble bases are obtained by this method, an insoluble salt precipitates.
When obtaining water-insoluble bases with amphoteric properties, an excess of alkali should be avoided, since dissolution of the amphoteric base may occur, for example:
AlCl 3 + 4KOH \u003d K [Al (OH) 4] + 3KSl
In such cases, ammonium hydroxide is used to obtain hydroxides, in which amphoteric hydroxides do not dissolve:
AlCl 3 + 3NH 3 + ZH 2 O \u003d Al (OH) 3 ↓ + 3NH 4 Cl
Hydroxides of silver and mercury decompose so easily that when you try to obtain them by an exchange reaction, instead of hydroxides, oxides precipitate:
2AgNO 3 + 2KOH \u003d 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 \u003d 2LiOH + H 2
SrO + H 2 O \u003d Sr (OH) 2
acids
Acids are called complex substances, the molecules of which consist of hydrogen atoms that can be replaced by metal atoms, and acid residues. Under normal conditions, acids can be solid (phosphoric H 3 PO 4; silicon H 2 SiO 3) and liquid (sulfuric acid H 2 SO 4 will be a pure 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 Bronsted and the English chemist Lowry, an acid is a substance splitting off with this reaction protons, A basis- a substance capable of receive protons.
acid → base + H +
Based on these 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
A more general definition of acids and bases proposed by the American chemist G. Lewis. He suggested that acid-base interactions are quite do not necessarily occur with protone transfer. In the determination of acids and bases according to Lewis, the main role in chemical reactions is given to electronic steam.
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).
The Lewis definition covers all acid-base processes that have been considered by the previously proposed theories. The table compares the definitions of acids and bases currently in use.
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 splitting off 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 RO 4).
According to the composition of the acid is divided into anoxic(HCl, H 2 S) and oxygen-containing(HClO 4 , HNO 3).
Usually names of oxygenated acids derived from the name of a non-metal with the addition of the endings -kai, -way, 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 metal oxidation state): - oval, ististaya, - ovate:
If we consider the polarity of the hydrogen-non-metal 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 that 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 system, boron, aluminum, carbon, silicon form covalent, weakly polar bonds with hydrogen atoms that are not prone to dissociation. For elements of groups V-VII of the Periodic system, within a period, the polarity of the non-metal-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. Atoms of non-metals, in which several electrons are needed to complete the electron shell, pull towards themselves (polarize) a pair of bond electrons the stronger, the greater the charge of the nucleus. 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, and the hydrogen atom in the dipole of the element-hydrogen bond becomes more electropositive. If polar molecules are in a polar solvent, the 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 O-E bond affects the polarity of the H-O bond. Therefore, these acids dissociate, as a rule, more easily than water.
H 2 SO 3 + H 2 O ⇆ H s O + + HSO 3
HNO 3 + H 2 O ⇆ H s O + + NO 3
Let's look at a few examples properties of oxygenated acids, formed by elements that are capable of exhibiting different oxidation states. It is known that hypochlorous acid HClO very weak hydrochloric acid HClO 2 also weak but stronger than hypochlorous, hypochlorous acid HclO 3 strong. Perchloric acid HClO 4 is one of the the strongest inorganic acids.
Dissociation according to the acidic type (with the elimination of the H ion) requires breaking the O-H bond. How can one 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 bond of oxygen with chlorine is formed, an electron density is drawn away from the chlorine atom, and hence from the single O-Cl bond. As a result, the electron density partially leaves the О-Н bond, which is weakened because of this.
Such a pattern - enhancement of acidic properties with an increase in the degree of oxidation of the central atom - characteristic not only for chlorine, but also for other elements. For example, nitric acid HNO 3 , in which the nitrogen oxidation state is +5, is stronger than nitrous acid HNO 2 (nitrogen oxidation state is +3); sulfuric acid H 2 SO 4 (S +6) is stronger than sulfurous acid H 2 SO 3 (S +4).
Obtaining acids
1. Anoxic acids can be obtained in the direct combination of non-metals with hydrogen.
H 2 + Cl 2 → 2HCl,
H 2 + S ⇆ H 2 S
2. Some oxygenated acids can be obtained interaction of acid oxides with water.
3. Both anoxic and oxygenated 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) \u003d H 2 S + FeSO 4
NaCl (T) + H 2 SO 4 (conc) = HCl + NaHSO 4
AgNO 3 + HCl = AgCl↓ + HNO 3
CaCO 3 + 2HBr \u003d CaBr 2 + CO 2 + H 2 O
4. Some acids can be obtained using redox reactions.
H 2 O 2 + SO 2 \u003d H 2 SO 4
3P + 5HNO 3 + 2H 2 O \u003d ZH 3 PO 4 + 5NO 2
Sour taste, action 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 the 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 reactions, reducing to hydrogen, as well as in a compound reaction with negatively charged or neutral particles having lone pairs of electrons, i.e. in acid-base reactions.
The general properties of acids include the reactions of acids with metals in the series of voltages 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 medium, basic, and sometimes acidic salts.
2 CO 3 + 4HBr \u003d 2CuBr 2 + CO 2 + 3H 2 O
Mg (HCO 3) 2 + 2HCl \u003d MgCl 2 + 2CO 2 + 2H 2 O
2KHSO 3 + H 2 SO 4 \u003d K 2 SO 4 + 2SO 2 + 2H 2 O
Note that polybasic acids dissociate stepwise, and at each next step, dissociation is more difficult, therefore, with an excess of acid, acidic salts are most often formed, rather than medium ones.
Ca 3 (PO 4) 2 + 4H 3 PO 4 \u003d 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 \u003d KHS + H 2 O
At first glance, the formation of acidic salts may seem surprising. monobasic hydrofluoric (hydrofluoric) acid. However, this fact can be explained. Unlike all other hydrohalic acids, hydrofluoric acid is partially polymerized in solutions (due to the formation of hydrogen bonds) and different particles (HF) X can be present in it, namely H 2 F 2, H 3 F 3, etc.
A special case of acid-base balance - reactions of acids and bases with indicators that change 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 highly alkaline environment crimson red, V neutral and acidic - colorless).
Specific Properties different acids can be of two types: first, the reactions leading to the formation insoluble salts, and, secondly, redox transformations. If the reactions associated with the presence of an H + ion in them 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- \u003d 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 aqueous solution can only oxidize.
2KMnO 4 + 16HCl = 5Cl 2 + 2KSl + 2MnCl 2 + 8H 2 O
H 2 S + Br 2 \u003d S + 2HBg
Oxygen-containing acids can only be oxidized 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 \u003d 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 in the electrochemical series of voltages to the left of hydrogen, 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, chromium.
- In water, acids dissociate into hydrogen cations and anions of acid residues, for example:
- Inorganic and organic acids interact with basic and amphoteric oxides, provided that a soluble salt is formed:
- Both those and other acids react with bases. Polybasic acids can form both medium and acidic 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, metal fineness, 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. The volatility of unstable acids cannot be judged. 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 passing the test:
Mendeleev table
Solubility table
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 common properties acids and bases have.
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. An Arrhenius acid is a class of substances that, in reaction with water, dissociate (decompose), forming a hydrogen cation H +. Arrhenius bases in aqueous solution form OH - anions. The following theory was proposed in 1923 by the scientists Brönsted and Lowry. The Bronsted-Lowry theory defines acids as substances capable of donating a proton in a reaction (a hydrogen cation is called a proton in reactions). Bases, respectively, are substances capable of accepting a proton in a reaction. The current theory is the Lewis theory. Lewis theory defines acids as molecules or ions capable of accepting electron pairs, thereby forming Lewis adducts (an adduct is a compound formed by combining two reactants without forming by-products).
In inorganic chemistry, as a rule, by acid they mean Bronsted-Lowry acid, that is, substances capable of donating a proton. If they mean the definition of a Lewis acid, then in the text such an acid is called a Lewis acid. These rules are valid for acids and bases.
Dissociation
Dissociation is the process of disintegration of a substance into ions in solutions or melts. For example, the dissociation of hydrochloric acid is the breakdown of HCl into H + and Cl - .
Properties of acids and bases
Bases tend to be soapy to the touch, while acids tend to taste sour.
When a base reacts with many cations, a precipitate is formed. When an acid reacts with anions, gas is usually released.
Commonly used acids:
H 2 O, H 3 O +, CH 3 CO 2 H, H 2 SO 4, HSO 4 -, HCl, CH 3 OH, NH 3
Commonly used bases:
OH - , H 2 O, CH 3 CO 2 - , HSO 4 - , SO 4 2 - , Cl -
Strong and weak acids and bases
Strong acids
Such acids that completely dissociate in water, producing hydrogen cations H + and anions. An example of a strong acid is hydrochloric acid HCl:
HCl (solution) + H 2 O (l) → H 3 O + (solution) + Cl - (solution)
Examples of strong acids: HCl, HBr, HF, HNO 3 , H 2 SO 4 , HClO 4
List of strong acids
- HCl - hydrochloric acid
- HBr - hydrogen bromide
- HI - hydrogen iodide
- HNO 3 - nitric acid
- HClO 4 - perchloric acid
- H 2 SO 4 - sulfuric acid
Weak acids
Dissolve in water only partially, for example, HF:
HF (solution) + H2O (l) → H3O + (solution) + F - (solution) - in such a reaction, more than 90% of the acid does not dissociate:
= < 0,01M для вещества 0,1М
Strong and weak acids can be distinguished by measuring the conductivity of solutions: the conductivity depends on the number of ions, the stronger the acid, the more dissociated it is, therefore, the stronger the acid, the higher the conductivity.
List of weak acids
- HF hydrofluoric
- H 3 PO 4 phosphoric
- H 2 SO 3 sulfurous
- H 2 S hydrogen sulfide
- H 2 CO 3 coal
- H 2 SiO 3 silicon
Strong bases
Strong bases completely dissociate in water:
NaOH (solution) + H 2 O ↔ NH 4
Strong bases include hydroxides of metals of the first (alkalins, alkali metals) and the second (alcaline terrenes, alkaline earth metals) groups.
List of strong bases
- NaOH sodium hydroxide (caustic soda)
- KOH potassium hydroxide (caustic potash)
- LiOH lithium hydroxide
- Ba(OH) 2 barium hydroxide
- Ca(OH) 2 calcium hydroxide (slaked lime)
Weak bases
In a reversible reaction in the presence of water, it forms OH - ions:
NH 3 (solution) + H 2 O ↔ NH + 4 (solution) + OH - (solution)
Most weak bases are anions:
F - (solution) + H 2 O ↔ HF (solution) + OH - (solution)
List of weak bases
- Mg(OH) 2 magnesium hydroxide
- Fe (OH) 2 iron (II) hydroxide
- Zn(OH) 2 zinc hydroxide
- NH 4 OH ammonium hydroxide
- Fe (OH) 3 iron (III) hydroxide
Reactions of acids and bases
Strong acid and strong base
Such a reaction is called neutralization: if the amount of reagents is sufficient to completely dissociate the acid and base, the resulting solution will be neutral.
Example:
H 3 O + + OH - ↔ 2H 2 O
Weak base and weak acid
General 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 (solution) + OH - (solution) ↔ H 2 O + X - (solution)
Strong acid and weak base
The acid completely dissociates, the base does not completely dissociate:
Water dissociation
Dissociation is the breakdown of a substance into its constituent molecules. The properties of an acid or base depend on the equilibrium that is present in water:
H 2 O + H 2 O ↔ H 3 O + (solution) + OH - (solution)
K c = / 2
The equilibrium constant of water at t=25°: K c = 1.83⋅10 -6 , the following equality also takes place: = 10 -14 , which is called the dissociation constant of water. For pure water = = 10 -7 , whence -lg = 7.0.
This value (-lg) is called pH - the potential of hydrogen. If pH< 7, то вещество имеет кислотные свойства, если pH >7, then the substance has basic properties.
Methods for determining pH
instrumental method
A special device pH meter is a device that transforms the concentration of protons in a solution into an electrical signal.
Indicators
A substance that changes color in a certain range of pH values depending on the acidity of the solution, using several indicators, you can achieve a fairly accurate result.
Salt
A salt is an ionic compound formed by a cation other than H + and an anion other than O 2- . In a weak aqueous solution, salts completely dissociate.
To determine the acid-base properties of a salt solution, it is necessary to determine which ions are present in the solution and consider their properties: neutral ions formed from strong acids and bases do not affect pH: neither H + nor OH - ions are released in water. For example, Cl - , NO - 3 , SO 2- 4 , Li + , Na + , K + .
Anions formed from weak acids exhibit alkaline properties (F - , CH 3 COO - , CO 2- 3), cations with alkaline properties do not exist.
All cations, except for metals of the first and second groups, have acidic properties.
buffer solution
Solutions that maintain their pH when a small amount of a strong acid or strong base is added generally consist of:
- A mixture of a weak acid, the corresponding salt and a weak base
- Weak base, corresponding salt and strong acid
To prepare a buffer solution of a certain acidity, it is necessary to mix a weak acid or base with the corresponding salt, while taking into account:
- pH range in which the buffer solution will be effective
- The capacity of a solution is the amount of strong acid or strong base that can be added without affecting the pH of the solution.
- No undesired reactions should occur that could change the composition of the solution
Test:
The general properties of the bases are due to the presence in their solutions of the OH - ion, which creates an alkaline environment in the solution (phenolphthalein turns crimson, methyl orange - yellow, litmus - blue).
1. Chemical properties of alkalis:
1) interaction with acid oxides:
2KOH+CO 2 ®K 2 CO 3 +H 2 O;
2) reaction with acids (neutralization reaction):
2NaOH+ H 2 SO 4 ®Na 2 SO 4 + 2H 2 O;
3) interaction with soluble salts (only if, under the action of alkali on a soluble salt, a precipitate precipitates or gas is released):
2NaOH + CuSO 4 ®Cu (OH) 2 ¯ + Na 2 SO 4,
Ba(OH) 2 +Na 2 SO 4 ®BaSO 4 ¯+2NaOH, KOH(conc.)+NH 4 Cl(crystal)®NH 3 +KCl+H 2 O.
2. Chemical properties of insoluble bases:
1) interaction of bases with acids:
Fe (OH) 2 + H 2 SO 4 ® FeSO 4 + 2H 2 O;
2) decomposition upon heating. Insoluble bases, when heated, decompose into a basic oxide and water:
Cu(OH) 2 ®CuO+H 2 O
End of work -
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Getting the grounds
1. Preparation of alkalis: 1) interaction of alkali or alkaline earth metals or their oxides with water: Сa+2H2O®Ca(OH)2+H
Nomenclature of acids
The names of acids are derived from the element from which the acid is derived. At the same time, the name of oxygen-free acids usually has the ending -hydrogen: HCl - hydrochloric, HBr - bromine
Chemical properties of acids
The general properties of acids in aqueous solutions are due to the presence of H + ions formed during the dissociation of acid molecules, thus, acids are proton donors: HxAn«xH +
Obtaining acids
1) interaction of acid oxides with water: SO3+H2O®H2SO4, P2O5+3H2O®2H3PO4;
Chemical properties of acid salts
1) acid salts contain hydrogen atoms that can take part in the neutralization reaction, so they can react with alkalis, turning into medium or other acid salts - with a smaller number
Obtaining acid salts
An acid salt can be obtained: 1) by the reaction of incomplete neutralization of a polybasic acid with a base: 2H2SO4+Cu(OH)2®Cu(HSO4)2+2H
Basic salts.
Basic (hydroxosalts) are salts that are formed as a result of incomplete replacement of the hydroxide ions of the base with acid anions. Single acid bases, e.g. NaOH, KOH,
Chemical properties of basic salts
1) basic salts contain hydroxo groups that can take part in the neutralization reaction, so they can react with acids, turning into medium salts or into basic salts with less
Obtaining basic salts
The basic salt can be obtained: 1) by the reaction of incomplete neutralization of the base with an acid: 2Cu(OH)2+H2SO4®(CuOH)2SO4+2H2
Medium salts.
Medium salts are products of complete replacement of H + acid ions with metal ions; they can also be considered as products of complete substitution of the OH ions of the anion base
Nomenclature of intermediate salts
In Russian nomenclature (used in technological practice), there is the following order of naming medium salts: the word is added to the root of the name of the oxygen-containing acid
Chemical properties of medium salts
1) Almost all salts are ionic compounds, therefore, in a melt and in an aqueous solution, they dissociate into ions (when current is passed through solutions or salt melts, the process of electrolysis occurs).
Obtaining medium salts
Most of the methods for obtaining salts are based on the interaction of substances of the opposite nature - metals with non-metals, acid oxides with basic ones, bases with acids (see table 2).
The structure of the atom.
An atom is an electrically neutral particle consisting of a positively charged nucleus and negatively charged electrons. The ordinal number of an element in the Periodic Table of Elements is equal to the charge of the nucleus
Composition of atomic nuclei
The nucleus is made up of protons and neutrons. The number of protons is equal to the atomic number of the element. The number of neutrons in the nucleus is equal to the difference between the mass number of the isotope and
Electron
Electrons revolve around the nucleus in certain stationary orbits. Moving along its orbit, the electron does not emit or absorb electromagnetic energy. Emission or absorption of energy
The rule for filling electronic levels, sublevels of elements
The number of electrons that can be in one energy level is determined by the formula 2n2, where n is the number of the level. Maximum filling of the first four energy levels: for the first
Ionization energy, electron affinity, electronegativity.
The ionization energy of an atom. The energy required to detach an electron from an unexcited atom is called the first ionization energy (potential) I: E + I \u003d E + + e- Ionization energy
covalent bond
In most cases, when a bond is formed, the electrons of the bonded atoms are shared. This type of chemical bond is called a covalent bond (the prefix "co-" in Latin
Sigma and pi bonds.
Sigma (σ)-, pi (π)-bonds - an approximate description of the types of covalent bonds in the molecules of various compounds, σ-bond is characterized by the fact that the density of the electron cloud is maximum
The formation of a covalent bond by the donor-acceptor mechanism.
In addition to the homogeneous mechanism of covalent bond formation described in the previous section, there is a heterogeneous mechanism - the interaction of oppositely charged ions - the H + proton and
Chemical bond and geometry of molecules. BI3, PI3
figure 3.1 Addition of dipole elements in NH3 and NF3 molecules
Polar and non-polar bond
A covalent bond is formed as a result of the socialization of electrons (with the formation of common electron pairs), which occurs during the overlap of electron clouds. In education
Ionic bond
An ionic bond is a chemical bond that occurs due to the electrostatic interaction of oppositely charged ions. Thus, the process of education and
Oxidation state
Valence 1. Valence - the ability of atoms of chemical elements to form a certain number of chemical bonds. 2. Valency values vary from I to VII (rarely VIII). Valens
hydrogen bond
In addition to various heteropolar and homeopolar bonds, there is another special type of bond that has attracted increasing attention of chemists in the last two decades. This so-called hydrogen
Crystal lattices
So, the crystal structure is characterized by the correct (regular) arrangement of particles in strictly defined places in the crystal. When you mentally connect these points with lines, you get space
Solutions
If crystals of table salt, sugar or potassium permanganate (potassium permanganate) are placed in a vessel with water, then we can observe how the amount of solid substance gradually decreases. At the same time, the water
Electrolytic dissociation
Solutions of all substances can be divided into two groups: electrolytes conduct electric current, non-electrolytes are not conductors. This division is conditional, because all
dissociation mechanism.
Water molecules are dipole, i.e. one end of the molecule is negatively charged, the other positively. The molecule with a negative pole approaches the sodium ion, positive - to the chlorine ion; surround io
Ionic product of water
Hydrogen index (pH) is a value that characterizes the activity or concentration of hydrogen ions in solutions. The hydrogen index is denoted by pH. Hydrogen index numerically
Chemical reaction
A chemical reaction is the transformation of one substance into another. However, this definition needs one significant addition. In a nuclear reactor or in an accelerator, some substances are also converted
Methods for arranging coefficients in the OVR
Electronic balance method 1). Write the equation for the chemical reaction KI + KMnO4 → I2 + K2MnO4 2). Finding atoms, changing
Hydrolysis
Hydrolysis is the process of exchange interaction of salt ions with water, leading to the formation of poorly dissociated substances and accompanied by a change in the reaction (pH) of the medium. essence
The rate of chemical reactions
The reaction rate is determined by the change in the molar concentration of one of the reactants: V = ± ((C2 - C1) / (t2 - t
Factors affecting the rate of chemical reactions
1. The nature of the reactants. An important role is played by the nature of chemical bonds and the structure of the molecules of the reagents. Reactions proceed in the direction of the destruction of less strong bonds and the formation of substances with
Activation energy
The collision of chemical particles leads to a chemical interaction only if the colliding particles have an energy exceeding a certain certain value. Consider mutual
catalysis catalyst
Many reactions can be accelerated or slowed down by the introduction of certain substances. Added substances do not participate in the reaction and are not consumed during its course, but have a significant effect on
Chemical equilibrium
Chemical reactions that proceed at comparable rates in both directions are called reversible. In such reactions, equilibrium mixtures of reactants and products are formed, the composition of which is
Le Chatelier's principle
Le Chatelier's principle says that in order to shift the equilibrium to the right, it is necessary, first, to increase the pressure. Indeed, with an increase in pressure, the system will “resist” the increase in con
Factors affecting the rate of a chemical reaction
Factors affecting the rate of a chemical reaction Increase the rate Decrease the rate Presence of chemically active reagents
Hess' law
Using tabular values
thermal effect
During the reaction, bonds are broken in the starting materials and new bonds are formed in the reaction products. Since the formation of a bond occurs with release, and its breaking with the absorption of energy, then x
