Electrolytes are substances, alloys of substances or solutions that have the ability to electrolytically conduct galvanic current. It is possible to determine which electrolytes a substance belongs to using the theory of electrolytic dissociation.
Instructions
1. The essence of this theory is that when melted (dissolved in water), virtually all electrolytes are decomposed into ions, which are both positively and negatively charged (which is called electrolytic dissociation). Under the influence of electric current, negative ones (anions, “-”) move towards the anode (+), and positively charged ones (cations, “+”) move towards the cathode (-). Electrolytic dissociation is a reversible process (the reverse process is called “molarization”).
2. The degree of (a) electrolytic dissociation depends on the nature of the electrolyte itself, the solvent, and their concentration. This is the ratio of the number of molecules (n) that broke up into ions to the total number of molecules introduced into the solution (N). You get: a = n / N
3. Thus, powerful electrolytes are substances that completely disintegrate into ions when dissolved in water. Strong electrolytes, as usual, include substances with highly polar or ionic bonds: these are salts that are highly soluble, strong acids (HCl, HI, HBr, HClO4, HNO3, H2SO4), as well as powerful bases (KOH, NaOH, RbOH, Ba (OH)2, CsOH, Sr(OH)2, LiOH, Ca(OH)2). In a strong electrolyte, the substance dissolved in it is mostly in the form of ions (anions and cations); There are actually no molecules that are undissociated.
4. Weak electrolytes are substances that dissociate into ions only partially. Weak electrolytes, together with ions in solution, contain undissociated molecules. Weak electrolytes do not give a strong concentration of ions in solution. Weak ones include: - organic acids (approximately all) (C2H5COOH, CH3COOH, etc.); - some of the inorganic acids (H2S, H2CO3, etc.); - virtually all salts, sparingly soluble in water, ammonium hydroxide, as well as all bases (Ca3(PO4)2; Cu(OH)2; Al(OH)3; NH4OH); - water. They actually do not conduct electric current, or they conduct, but poorly.
A strong base is an inorganic chemical compound formed by the hydroxyl group -OH and an alkaline (elements of group I of the periodic table: Li, K, Na, RB, Cs) or alkaline earth metal (elements of group II Ba, Ca). Written in the form of the formulas LiOH, KOH, NaOH, RbOH, CsOH, Ca(OH)?, Ba(OH)?.
You will need
- evaporation cup
- burner
- indicators
- metal rod
- N?RO?
Instructions
1. Powerful bases exhibit chemical properties characteristic of all hydroxides. The presence of alkalis in a solution is determined by a change in the color of the indicator. Add methyl orange, phenolphthalein or omit the litmus paper to the sample with the test solution. Methyl orange produces a yellow color, phenolphthalein produces a purple color, and litmus paper turns blue. The stronger the base, the more saturated the color of the indicator.
2. If you need to find out which alkalis are presented to you, then conduct a good review of the solutions. Particularly common powerful bases are lithium, potassium, sodium, barium and calcium hydroxides. Bases react with acids (neutralization reactions) to form salt and water. In this case, it is possible to isolate Ca(OH)?, Ba(OH)? and LiOH. When interacting with orthophosphoric acid, insoluble precipitates are formed. The remaining hydroxides will not produce precipitation, because all K and Na salts are soluble.3 Ca(OH) ? + 2 N?RO? –? Ca?(PO?)??+ 6 H?O3 Ba(OH) ? +2 N?RO? –? Ba?(PO?)??+ 6 H?O3 LiOH + H?PO? –? Li?PO?? + 3 H?О Strain them and dry them. Add the dried sediment to the burner flame. By changing the color of the flame, it is possible to accurately determine the ions of lithium, calcium and barium. Accordingly, you will determine which hydroxide is which. Lithium salts color the burner flame a carmine-scarlet color. Barium salts are green, and calcium salts are red.
3. The remaining alkalis form soluble orthophosphates.3 NaOH + H?PO?–? Na?PO? + 3 H?O3 KOH + H?PO?–? K?RO? + 3 H?ОIt is necessary to evaporate the water to a dry residue. Place the evaporated salts on a metal rod one by one into the burner flame. Where the sodium salt is located, the flame will turn clear yellow, and potassium orthophosphate will turn pink-violet. Thus, having the smallest set of equipment and reagents, you have identified all the powerful bases given to you.
An electrolyte is a substance that in its solid state is a dielectric, that is, it does not conduct electric current, but when dissolved or molten it becomes a conductor. Why does such a sharp change in properties occur? The fact is that electrolyte molecules in solutions or melts dissociate into positively charged and negatively charged ions, as a result of which these substances in this state of aggregation are capable of conducting electric current. Many salts, acids, and bases have electrolytic properties.
Instructions
1. Is everything electrolytes identical in strength, that is, they are excellent conductors of current? No, because many substances in solutions or melts dissociate only to a small extent. Consequently electrolytes are divided into strong, medium strength and weak.
2. What substances are considered powerful electrolytes? Such substances in solutions or melts of which virtually 100% of the molecules undergo dissociation, regardless of the concentration of the solution. The list of strong electrolytes includes an absolute variety of soluble alkalis, salts and some acids, such as hydrochloric, bromide, iodide, nitric, etc.
3. How are they different from them? electrolytes medium strength? The fact that they dissociate to a much lesser extent (from 3% to 30% of molecules disintegrate into ions). Typical representatives of such electrolytes are sulfuric and phosphoric acids.
4. How do weak compounds behave in solutions or melts? electrolytes? Firstly, they dissociate to a very small extent (no more than 3% of the total number of molecules), and secondly, their dissociation is the more clumsy and leisurely, the higher the saturation of the solution. Such electrolytes include, say, ammonia (ammonium hydroxide), many organic and inorganic acids (including hydrofluoric acid - HF) and, of course, familiar water to all of us. Because only a pitifully small fraction of its molecules breaks down into hydrogen ions and hydroxyl ions.
5. Remember that the degree of dissociation and, accordingly, the strength of the electrolyte depend on many factors: the nature of the electrolyte itself, the solvent, and temperature. Consequently, this distribution itself is to a certain extent arbitrary. In tea, the same substance can, under different conditions, be both a powerful electrolyte and a weak one. To assess the strength of the electrolyte, a special value was introduced - the dissociation constant, determined on the basis of the law of mass action. But it is applicable only to weak electrolytes; powerful electrolytes do not obey the law of mass action.
Salts- these are chemical substances consisting of a cation, that is, a positively charged ion, a metal and a negatively charged anion - an acid residue. There are many types of salts: typical, acidic, basic, double, mixed, hydrated, complex. This depends on the cation and anion compositions. How is it possible to determine base salt?
Instructions
1. Let's imagine you have four identical containers with burning solutions. You know that these are solutions of lithium carbonate, sodium carbonate, potassium carbonate and barium carbonate. Your task: determine what salt is contained in the entire container.
2. Recall the physical and chemical properties of compounds of these metals. Lithium, sodium, potassium are alkali metals of the first group, their properties are very similar, activity increases from lithium to potassium. Barium is a group 2 alkaline earth metal. Its carbonic salt dissolves perfectly in hot water, but dissolves poorly in cold water. Stop! This is the first chance to immediately determine which container contains barium carbonate.
3. Cool the containers, say by placing them in a container with ice. Three solutions will remain clear, but the fourth will quickly become cloudy and a white precipitate will begin to form. This is where the barium salt is found. Set this container aside.
4. You can quickly determine barium carbonate using another method. Alternately, pour a little of the solution into another container with a solution of some sulfate salt (say, sodium sulfate). Only barium ions, binding with sulfate ions, instantly form a dense white precipitate.
5. It turns out that you have identified barium carbonate. But how do you differentiate between the 3 alkali metal salts? This is quite easy to do, you will need porcelain evaporation cups and an alcohol lamp.
6. Pour a small amount of the entire solution into a separate porcelain cup and evaporate the water over the fire of a spirit lamp. Small crystals form. Place them in the flame of an alcohol lamp or a Bunsen burner - supported by steel tweezers or a porcelain spoon. Your task is to notice the color of the blazing “tongue” of flame. If it is a lithium salt, the color will be clear red. Sodium will color the flame intense yellow, and potassium will color the flame purple-violet. By the way, if barium salt had been tested in the same way, the color of the flame should have been green.
Useful advice
One famous chemist in his youth exposed the greedy hostess of a boarding house in much the same way. He sprinkled the remains of the half-eaten dish with lithium chloride, a substance that is certainly harmless in small quantities. The next day, at lunch, a slice of meat from the dish served to the table was burned in front of a spectroscope - and the residents of the boarding house saw a clear red stripe. The hostess was preparing food from yesterday's leftovers.
Pay attention!
True, pure water conducts electricity very poorly, it still has measurable electrical conductivity, explained by the fact that water dissociates slightly into hydroxide ions and hydrogen ions.
Useful advice
Many electrolytes are hostile substances, so when working with them, be extremely careful and follow safety regulations.
Electrolytes are substances, alloys of substances or solutions that have the ability to electrolytically conduct galvanic current. You can determine which electrolytes a substance belongs to using the theory of electrolytic dissociation.
Instructions
- The essence of this theory is that when melted (dissolved in water), almost all electrolytes are decomposed into ions, which are both positively and negatively charged (which is called electrolytic dissociation). Under the influence of electric current, negative ones (anions, “-”) move towards the anode (+), and positively charged ones (cations, “+”) move towards the cathode (-). Electrolytic dissociation is a reversible process (the reverse process is called “molarization”).
- The degree of (a) electrolytic dissociation depends on the nature of the electrolyte itself, the solvent, and their concentration. This is the ratio of the number of molecules (n) that broke up into ions to the total number of molecules introduced into the solution (N). You get: a = n / N
- Thus, strong electrolytes are substances that completely disintegrate into ions when dissolved in water. Strong electrolytes, as a rule, include substances with highly polar or ionic bonds: these are salts that are highly soluble, strong acids (HCl, HI, HBr, HClO4, HNO3, H2SO4), as well as strong bases (KOH, NaOH, RbOH, Ba (OH)2, CsOH, Sr(OH)2, LiOH, Ca(OH)2). In a strong electrolyte, the substance dissolved in it is mostly in the form of ions (anions and cations); There are practically no molecules that are undissociated.
- Weak electrolytes are substances that dissociate into ions only partially. Weak electrolytes, together with ions in solution, contain undissociated molecules. Weak electrolytes do not produce a strong concentration of ions in solution. Weak electrolytes include:
- organic acids (almost all) (C2H5COOH, CH3COOH, etc.);
- some of the inorganic acids (H2S, H2CO3, etc.);
- almost all salts that are slightly soluble in water, ammonium hydroxide, as well as all bases (Ca3(PO4)2; Cu(OH)2; Al(OH)3; NH4OH);
- water. They practically do not conduct electric current, or conduct, but poorly.
Depending on the degree of dissociation, electrolytes are distinguished between strong and weak. K is the dissociation constant, which depends on the temperature and nature of the electrolyte and solvent, but does not depend on the concentration of the electrolyte. Reactions between ions in electrolyte solutions proceed almost completely towards the formation of precipitation, gases and weak electrolytes.
An electrolyte is a substance that conducts electric current due to dissociation into ions, which occurs in solutions and melts, or the movement of ions in the crystal lattices of solid electrolytes. Examples of electrolytes include aqueous solutions of acids, salts and bases and some crystals (for example, silver iodide, zirconium dioxide).
How to determine strong and weak electrolytes
At the same time, processes of association of ions into molecules occur in the electrolyte. To quantitatively characterize electrolytic dissociation, the concept of the degree of dissociation was introduced. Most often they mean an aqueous solution containing certain ions (for example, “absorption of electrolytes” in the intestines). Multicomponent solution for electrodeposition of metals, as well as etching, etc. (technical term, for example, gilding electrolyte).
The main object of research and development in electroplating is electrolytes for surface treatment and coating. When chemically etching metals, the names of electrolytes are determined by the name of the main acids or alkalis that promote the dissolution of the metal. This is how the group name of electrolytes is formed. Sometimes the difference (especially in the polarizability) between electrolytes of different groups is leveled out by additives contained in the electrolytes.
Electrolytes and electrolytic dissociation
Therefore, such a name cannot be a classification name (that is, a group name), but must serve as an additional subgroup name of the electrolyte. If the electrolyte density in all battery cells is normal or close to normal (1.25-1.28 g/cm3), and the NRC is not lower than 12.5 V, then it is necessary to check for an open circuit inside the battery. If the electrolyte density is low in all cells, the battery should be charged until the density stabilizes.
In technology[edit edit wiki text]
When transitioning from one state to another, the voltage and density of the electrolyte change linearly within certain limits (Fig. 4 and Table 1). The deeper the battery discharges, the lower the density of the electrolyte. Accordingly, the volume of the electrolyte contains the amount of sulfuric acid necessary for full use of the active substance of the plates in the reaction.
Ionic conductivity is inherent in many chemical compounds that have an ionic structure, such as salts in solid or molten states, as well as many aqueous and non-aqueous solutions. Electrolytic dissociation refers to the disintegration of electrolyte molecules in solution with the formation of positively and negatively charged ions - cations and anions. The degree of dissociation is often expressed as a percentage. This is explained by the fact that the concentrations of metallic copper and silver are introduced into the equilibrium constant.
This is explained by the fact that the concentration of water during reactions in aqueous solutions changes very slightly. Therefore, it is assumed that the concentration remains constant and is introduced into the equilibrium constant. Since electrolytes form ions in solutions, so-called ionic reaction equations are often used to reflect the essence of reactions.
The term electrolyte is widely used in biology and medicine. The process of decomposition of molecules in a solution or melt of an electrolyte into ions is called electrolytic dissociation. Therefore, in electrolytes a certain proportion of the molecules of the substance are dissociated. There is no clear boundary between these two groups; the same substance can exhibit the properties of a strong electrolyte in one solvent, and a weak electrolyte in another.
Electrolytes are classified into two groups depending on the degree of dissociation - strong and weak electrolytes. Strong electrolytes have a dissociation degree greater than one or more than 30%, weak electrolytes less than one or less than 3%.
Process of dissociation
Electrolytic dissociation is the process of decomposition of molecules into ions - positively charged cations and negatively charged anions. Charged particles carry electric current. Electrolytic dissociation is possible only in solutions and melts.
The driving force for dissociation is the disintegration of polar covalent bonds under the action of water molecules. Polar molecules are attracted by water molecules. In solids, ionic bonds are broken during heating. High temperatures cause vibrations of ions at the nodes of the crystal lattice.
Rice. 1. The process of dissociation.
Substances that easily disintegrate into ions in solutions or melts and, therefore, conduct electric current are called electrolytes. Non-electrolytes do not conduct electricity because do not break down into cations and anions.
Depending on the degree of dissociation, strong and weak electrolytes are distinguished. Strong ones dissolve in water, i.e. completely, without the possibility of recovery, disintegrate into ions. Weak electrolytes partially break down into cations and anions. The degree of their dissociation is less than that of strong electrolytes.
The degree of dissociation shows the proportion of disintegrated molecules in the total concentration of substances. It is expressed by the formula α = n/N.
Rice. 2. Degree of dissociation.
Weak electrolytes
List of weak electrolytes:
- dilute and weak inorganic acids - H 2 S, H 2 SO 3, H 2 CO 3, H 2 SiO 3, H 3 BO 3;
- some organic acids (most organic acids are non-electrolytes) - CH 3 COOH, C 2 H 5 COOH;
- insoluble bases - Al(OH) 3, Cu(OH) 2, Fe(OH) 2, Zn(OH) 2;
- Ammonium hydroxide - NH 4 OH.
Rice. 3. Solubility table.
The dissociation reaction is written using the ionic equation:
- HNO 2 ↔ H + + NO 2 – ;
- H 2 S ↔ H + + HS – ;
- NH 4 OH ↔ NH 4 + + OH – .
Polybasic acids dissociate stepwise:
- H 2 CO 3 ↔ H + + HCO 3 – ;
- HCO 3 – ↔ H + + CO 3 2- .
Insoluble bases also decompose in stages:
- Fe(OH) 3 ↔ Fe(OH) 2 + + OH – ;
- Fe(OH) 2 + ↔ FeOH 2+ + OH – ;
- FeOH 2+ ↔ Fe 3+ + OH – .
Water is classified as a weak electrolyte. Water practically does not conduct electric current, because... weakly decomposes into hydrogen cations and hydroxide ion anions. The resulting ions are reassembled into water molecules:
H 2 O ↔ H + + OH – .
If water easily conducts electricity, it means there are impurities in it. Distilled water is non-conductive.
The dissociation of weak electrolytes is reversible. The resulting ions reassemble into molecules.
What have we learned?
Weak electrolytes include substances that partially disintegrate into ions - positive cations and negative anions. Therefore, such substances do not conduct electricity well. These include weak and dilute acids, insoluble bases, and slightly soluble salts. The weakest electrolyte is water. Dissociation of weak electrolytes is a reversible reaction.
The value of a is expressed in fractions of a unit or in % and depends on the nature of the electrolyte, solvent, temperature, concentration and composition of the solution.
The solvent plays a special role: in some cases, when moving from aqueous solutions to organic solvents, the degree of dissociation of electrolytes can sharply increase or decrease. In the following, in the absence of special instructions, we will assume that the solvent is water.
According to the degree of dissociation, electrolytes are conventionally divided into strong(a > 30%), average (3% < a < 30%) и weak(a< 3%).
Strong electrolytes include:
1) some inorganic acids (HCl, HBr, HI, HNO 3, H 2 SO 4, HClO 4 and a number of others);
2) hydroxides of alkali (Li, Na, K, Rb, Cs) and alkaline earth (Ca, Sr, Ba) metals;
3) almost all soluble salts.
Electrolytes of medium strength include Mg(OH) 2, H 3 PO 4, HCOOH, H 2 SO 3, HF and some others.
All carboxylic acids (except HCOOH) and hydrated forms of aliphatic and aromatic amines are considered weak electrolytes. Many inorganic acids (HCN, H 2 S, H 2 CO 3, etc.) and bases (NH 3 ∙H 2 O) are also weak electrolytes.
Despite some similarities, in general one should not equate the solubility of a substance with its degree of dissociation. Thus, acetic acid and ethyl alcohol are unlimitedly soluble in water, but at the same time, the first substance is a weak electrolyte, and the second is a non-electrolyte.
Acids and bases
Despite the fact that the concepts “acid” and “base” are widely used to describe chemical processes, there is no single approach to the classification of substances in terms of classifying them as acids or bases. Currently existing theories ( ionic theory S. Arrhenius, protolytic theory I. Brønsted and T. Lowry And electronic theory G. Lewis) have certain limitations and are therefore only applicable in special cases. Let's take a closer look at each of these theories.
Arrhenius theory.
In Arrhenius's ionic theory, the concepts of "acid" and "base" are closely related to the process of electrolytic dissociation:
An acid is an electrolyte that dissociates in solutions to form H + ions;
The base is an electrolyte that dissociates in solutions to form OH - ions;
An ampholyte (amphoteric electrolyte) is an electrolyte that dissociates in solutions to form both H + ions and OH - ions.
For example:
HA ⇄ H + + A - nH + + MeO n n - ⇄ Me(OH) n ⇄ Me n + + nOH -According to the ionic theory, acids can be either neutral molecules or ions, for example:
HF ⇄ H + + F -
H 2 PO 4 - ⇄ H + + HPO 4 2 -
NH 4 + ⇄ H + + NH 3
Similar examples can be given for grounds:
KOH K + + OH -
- ⇄ Al(OH) 3 + OH -
+ ⇄ Fe 2+ + OH -
Ampholytes include hydroxides of zinc, aluminum, chromium and some others, as well as amino acids, proteins, and nucleic acids.
In general, acid-base interaction in a solution comes down to a neutralization reaction:
H + + OH - H 2 O
However, a number of experimental data show the limitations of the ionic theory. So, ammonia, organic amines, metal oxides such as Na 2 O, CaO, anions of weak acids, etc. in the absence of water they exhibit the properties of typical bases, although they do not contain hydroxide ions.
On the other hand, many oxides (SO 2 , SO 3 , P 2 O 5 , etc.), halides, acid halides, without containing hydrogen ions, exhibit acidic properties even in the absence of water, i.e. neutralize bases.
In addition, the behavior of an electrolyte in an aqueous solution and in a non-aqueous medium may be opposite.
So, CH 3 COOH in water is a weak acid:
CH 3 COOH ⇄ CH 3 COO - + H + ,
and in liquid hydrogen fluoride it exhibits the properties of a base:
HF + CH 3 COOH ⇄ CH 3 COOH 2 + + F -
Studies of these types of reactions, and especially reactions occurring in non-aqueous solvents, have led to the development of more general theories of acids and bases.
The theory of Bronsted and Lowry.
A further development of the theory of acids and bases was the protolytic (proton) theory proposed by I. Brønsted and T. Lowry. According to this theory:
An acid is any substance whose molecules (or ions) are capable of donating a proton, i.e. be a proton donor;
A base is any substance whose molecules (or ions) are capable of attaching a proton, i.e. be a proton acceptor;
Thus, the concept of foundation is significantly expanded, which is confirmed by the following reactions:
OH - + H + H 2 O
NH 3 + H + NH 4 +
H 2 N-NH 3 + + H + H 3 N + -NH 3 +
According to the theory of I. Brønsted and T. Lowry, an acid and a base form a conjugate pair and are related by equilibrium:
ACID ⇄ PROTON + BASE
Since the proton transfer reaction (protolytic reaction) is reversible, and a proton is also transferred in the reverse process, the reaction products are acids and bases in relation to each other. This can be written as an equilibrium process:
NA + B ⇄ VN + + A - ,
where HA is an acid, B is a base, BH + is an acid conjugate to base B, A - is a base conjugate to acid HA.
Examples.
1) in the reaction:
HCl + OH - ⇄ Cl - + H 2 O,
HCl and H 2 O are acids, Cl - and OH - are the corresponding bases conjugate to them;
2) in the reaction:
HSO 4 - + H 2 O ⇄ SO 4 2 - + H 3 O +,
HSO 4 - and H 3 O + are acids, SO 4 2 - and H 2 O are bases;
3) in the reaction:
NH 4 + + NH 2 - ⇄ 2NH 3,
NH 4 + is an acid, NH 2 - is a base, and NH 3 acts as both an acid (one molecule) and a base (another molecule), i.e. demonstrates signs of amphotericity - the ability to exhibit the properties of an acid and a base.
Water also has this ability:
2H 2 O ⇄ H 3 O + + OH -
Here, one molecule H 2 O attaches a proton (base), forming a conjugate acid - hydronium ion H 3 O +, the other gives up a proton (acid), forming a conjugate base OH -. This process is called autoprotolysis.
From the above examples it is clear that, in contrast to the ideas of Arrhenius, in the theory of Brønsted and Lowry, reactions of acids with bases do not lead to mutual neutralization, but are accompanied by the formation of new acids and bases.
It should also be noted that the protolytic theory considers the concepts of “acid” and “base” not as a property, but as a function that the compound in question performs in a protolytic reaction. The same compound can react as an acid under some conditions and as a base under others. Thus, in an aqueous solution, CH 3 COOH exhibits the properties of an acid, and in 100% H 2 SO 4 it exhibits the properties of a base.
However, despite its advantages, the protolytic theory, like the Arrhenius theory, is not applicable to substances that do not contain hydrogen atoms, but at the same time exhibit the function of an acid: boron, aluminum, silicon, tin halides.
Lewis' theory.
Another approach to the classification of substances from the point of view of classifying them as acids and bases was the Lewis electron theory. Within the framework of electronic theory:
an acid is a particle (molecule or ion) capable of attaching an electron pair (electron acceptor);
A base is a particle (molecule or ion) capable of donating an electron pair (electron donor).
According to Lewis's ideas, an acid and a base interact with each other to form a donor-acceptor bond. As a result of the addition of a pair of electrons, an electron-deficient atom has a complete electronic configuration - an octet of electrons. For example:
The reaction between neutral molecules can be imagined in a similar way:
The neutralization reaction in terms of the Lewis theory is considered as the addition of an electron pair of a hydroxide ion to a hydrogen ion, which provides a free orbital to accommodate this pair:
Thus, the proton itself, which easily attaches an electron pair, from the point of view of the Lewis theory, performs the function of an acid. In this regard, Bronsted acids can be considered as reaction products between Lewis acids and bases. Thus, HCl is the product of neutralization of the acid H + with the base Cl -, and the H 3 O + ion is formed as a result of the neutralization of the acid H + with the base H 2 O.
Reactions between Lewis acids and bases are also illustrated by the following examples:
Lewis bases also include halide ions, ammonia, aliphatic and aromatic amines, oxygen-containing organic compounds such as R 2 CO (where R is an organic radical).
Lewis acids include halides of boron, aluminum, silicon, tin and other elements.
Obviously, in Lewis's theory, the concept of “acid” includes a wider range of chemical compounds. This is explained by the fact that, according to Lewis, the classification of a substance as an acid is determined solely by the structure of its molecule, which determines the electron-acceptor properties, and is not necessarily related to the presence of hydrogen atoms. Lewis acids that do not contain hydrogen atoms are called aprotic.
Problem solving standards
1. Write the equation for the electrolytic dissociation of Al 2 (SO 4) 3 in water.
Aluminum sulfate is a strong electrolyte and in aqueous solution undergoes complete decomposition into ions. Dissociation equation:
Al 2 (SO 4) 3 + (2x + 3y)H 2 O 2 3+ + 3 2 - ,
or (without taking into account the process of ion hydration):
Al 2 (SO 4) 3 2Al 3+ + 3SO 4 2 - .
2. What is the HCO 3 ion from the perspective of the Brønsted-Lowry theory?
Depending on the conditions, the HCO 3 ion can donate protons:
HCO 3 - + OH - CO 3 2 - + H 2 O (1),
add protons like this:
HCO 3 - + H 3 O + H 2 CO 3 + H 2 O (2).
Thus, in the first case, the HCO 3 - ion is an acid, in the second, it is a base, i.e., it is an ampholyte.
3. Determine what the Ag + ion is in the reaction from the standpoint of Lewis theory:
Ag + + 2NH 3 +
During the formation of chemical bonds, which proceeds according to the donor-acceptor mechanism, the Ag + ion, having a free orbital, is an acceptor of electron pairs, and thus exhibits the properties of a Lewis acid.
4. Determine the ionic strength of a solution containing 0.1 mol KCl and 0.1 mol Na 2 SO 4 in one liter.
The dissociation of the presented electrolytes proceeds in accordance with the equations:
Na 2 SO 4 2Na + + SO 4 2 -
Hence: C(K +) = C(Cl -) = C(KCl) = 0.1 mol/l;
C(Na +) = 2×C(Na 2 SO 4) = 0.2 mol/l;
C(SO 4 2 -) = C(Na 2 SO 4) = 0.1 mol/l.
The ionic strength of the solution is calculated using the formula:
5. Determine the concentration of CuSO 4 in a solution of this electrolyte with I= 0.6 mol/l.
The dissociation of CuSO 4 proceeds according to the equation:
CuSO 4 Cu 2+ + SO 4 2 -
Let's take C(CuSO 4) as x mol/l, then, in accordance with the reaction equation, C(Cu 2+) = C(SO 4 2 -) = x mol/l. In this case, the expression for calculating the ionic strength will look like:
6. Determine the activity coefficient of the K + ion in an aqueous solution of KCl with C(KCl) = 0.001 mol/l.
which in this case will take the form:
.
We find the ionic strength of the solution using the formula:
7. Determine the activity coefficient of the Fe 2+ ion in an aqueous solution whose ionic strength is 1.
According to the Debye-Hückel law:
hence:
8. Determine the dissociation constant of acid HA if in a solution of this acid with a concentration of 0.1 mol/l a = 24%.
Based on the degree of dissociation, it can be determined that this acid is an electrolyte of medium strength. Therefore, to calculate the acid dissociation constant, we use Ostwald’s dilution law in its full form:
9. Determine the electrolyte concentration if a = 10%, K d = 10 - 4.
From Ostwald's law of dilution:
10. The degree of dissociation of monobasic acid HA does not exceed 1%. (HA) = 6.4×10 - 7. Determine the degree of dissociation of HA in its solution with a concentration of 0.01 mol/L.
Based on the degree of dissociation, it can be determined that this acid is a weak electrolyte. This allows us to use the approximate formula of Ostwald's dilution law:
11. The degree of dissociation of the electrolyte in its solution with a concentration of 0.001 mol/l is 0.009. Determine the dissociation constant of this electrolyte.
From the conditions of the problem it is clear that this electrolyte is weak (a = 0.9%). That's why:
12. (HNO 2) = 3.35. Compare the strength of HNO 2 with the strength of monobasic acid HA, the degree of dissociation of which in a solution with C(HA) = 0.15 mol/l is 15%.
Let's calculate (HA) using the full form of the Ostwald equation:
Since (HA)< (HNO 2), то кислота HA является более сильной кислотой по сравнению с HNO 2 .
13. There are two solutions of KCl, which also contain other ions. It is known that the ionic strength of the first solution ( I 1) is equal to 1, and the second ( I 2) is 10 - 2 . Compare activity rates f(K +) in these solutions and conclude how the properties of these solutions differ from the properties of infinitely dilute KCl solutions.
We calculate the activity coefficients of K + ions using the Debye-Hückel law:
Activity factor f is a measure of the deviation in the behavior of an electrolyte solution of a given concentration from its behavior when the solution is infinitely diluted.
Because f 1 = 0.316 deviates more from 1 than f 2 = 0.891, then in a solution with higher ionic strength there is a greater deviation in the behavior of the KCl solution from its behavior at infinite dilution.
Questions for self-control
1. What is electrolytic dissociation?
2. What substances are called electrolytes and non-electrolytes? Give examples.
3. What is the degree of dissociation?
4. On what factors does the degree of dissociation depend?
5. Which electrolytes are considered strong? Which are medium strength? Which ones are weak? Give examples.
6. What is the dissociation constant? What does the dissociation constant depend on and what does it not depend on?
7. How are the constant and the degree of dissociation related to each other in binary solutions of medium and weak electrolytes?
8. Why do solutions of strong electrolytes show deviations from ideality in their behavior?
9. What is the meaning of the term “apparent degree of dissociation”?
10. What is the activity of an ion? What is the activity coefficient?
11. How does the activity coefficient change with dilution (concentration) of a strong electrolyte solution? What is the limiting value of the activity coefficient for an infinite solution dilution?
12. What is the ionic strength of a solution?
13. How is the activity coefficient calculated? Formulate the Debye-Hückel law.
14. What is the essence of the ionic theory of acids and bases (Arrhenius theory)?
15. What is the fundamental difference between the protolytic theory of acids and bases (the theory of Brønsted and Lowry) from the theory of Arrhenius?
16. How does electronic theory (Lewis theory) interpret the concepts of “acid” and “base”? Give examples.
Variants of tasks for independent solution
Option #1
1. Write the equation for the electrolytic dissociation of Fe 2 (SO 4) 3.
HA + H 2 O ⇄ H 3 O + + A - .
Option No. 2
1. Write the equation for the electrolytic dissociation of CuCl 2.
2. Determine what the S 2 - ion is in the reaction from the standpoint of Lewis theory:
2Ag + + S 2 - ⇄ Ag 2 S.
3. Calculate the molar concentration of the electrolyte in the solution if a = 0.75%, a = 10 - 5.
Option #3
1. Write the equation for the electrolytic dissociation of Na 2 SO 4.
2. Determine what the CN - ion is in the reaction from the standpoint of Lewis theory:
Fe 3 + + 6CN - ⇄ 3 - .
3. The ionic strength of the CaCl 2 solution is 0.3 mol/l. Calculate C(CaCl2).
Option No. 4
1. Write the equation for the electrolytic dissociation of Ca(OH) 2.
2. Determine what the H 2 O molecule is in the reaction from the standpoint of the Brønsted theory:
H 3 O + ⇄ H + + H 2 O.
3. The ionic strength of the K 2 SO 4 solution is 1.2 mol/L. Calculate C(K 2 SO 4).
Option #5
1. Write the equation for the electrolytic dissociation of K 2 SO 3.
NH 4 + + H 2 O ⇄ NH 3 + H 3 O + .
3. (CH 3 COOH) = 4.74. Compare the strength of CH 3 COOH with the strength of monobasic acid HA, the degree of dissociation of which in solution with C(HA) = 3.6 × 10 - 5 mol/l is 10%.
Option #6
1. Write the equation for electrolytic dissociation of K 2 S.
2. Determine what the AlBr 3 molecule is in the reaction from the standpoint of Lewis theory:
Br - + AlBr 3 ⇄ - .
Option No. 7
1. Write the equation for the electrolytic dissociation of Fe(NO 3) 2.
2. Determine what the Cl - ion is in the reaction from the standpoint of Lewis theory:
Cl - + AlCl 3 ⇄ - .
Option No. 8
1. Write the equation for the electrolytic dissociation of K 2 MnO 4 .
2. Determine what the HSO 3 - ion is in the reaction from the standpoint of the Brønsted theory:
HSO 3 - + OH – ⇄ SO 3 2 - + H 2 O.
Option No. 9
1. Write the equation for the electrolytic dissociation of Al 2 (SO 4) 3.
2. Determine what the Co 3+ ion is in the reaction from the standpoint of Lewis theory:
Co 3+ + 6NO 2 - ⇄ 3 - .
3. 1 liter of solution contains 0.348 g of K2SO4 and 0.17 g of NaNO3. Determine the ionic strength of this solution.
Option No. 10
1. Write the equation for the electrolytic dissociation of Ca(NO 3) 2.
2. Determine what the H 2 O molecule is in the reaction from the standpoint of the Brønsted theory:
B + H 2 O ⇄ OH - + BH + .
3. Calculate the electrolyte concentration in the solution if a = 5%, a = 10 - 5.
Option No. 11
1. Write the equation for the electrolytic dissociation of KMnO 4.
2. Determine what the Cu 2+ ion is in the reaction from the perspective of Lewis theory:
Cu 2+ + 4NH 3 ⇄ 2 + .
3. Calculate the activity coefficient of the Cu 2+ ion in a solution of CuSO 4 with C(CuSO 4) = 0.016 mol/l.
Option No. 12
1. Write the equation for the electrolytic dissociation of Na 2 CO 3.
2. Determine what the H 2 O molecule is in the reaction from the standpoint of the Brønsted theory:
K + + xH 2 O ⇄ + .
3. There are two NaCl solutions containing other electrolytes. The ionic strengths of these solutions are respectively equal: I 1 = 0.1 mol/l, I 2 = 0.01 mol/l. Compare activity rates f(Na +) in these solutions.
Option No. 13
1. Write the equation for the electrolytic dissociation of Al(NO 3) 3.
2. Determine what the RNH 2 molecule is in the reaction from the standpoint of Lewis theory:
RNH 2 + H 3 O + ⇄ RNH 3 + + H 2 O.
3. Compare the activity coefficients of cations in a solution containing FeSO 4 and KNO 3, provided that the electrolyte concentrations are 0.3 and 0.1 mol/l, respectively.
Option No. 14
1. Write the equation for the electrolytic dissociation of K 3 PO 4.
2. Determine what the H 3 O + ion is in the reaction from the standpoint of the Brønsted theory:
HSO 3 - + H 3 O + ⇄ H 2 SO 3 + H 2 O.
Option No. 15
1. Write the equation for the electrolytic dissociation of K 2 SO 4.
2. Determine what Pb(OH) 2 is in the reaction from the standpoint of Lewis theory:
Pb(OH) 2 + 2OH - ⇄ 2 - .
Option No. 16
1. Write the equation for the electrolytic dissociation of Ni(NO 3) 2.
2. Determine what the hydronium ion (H 3 O +) is in the reaction from the standpoint of the Brønsted theory:
2H 3 O + + S 2 - ⇄ H 2 S + 2H 2 O.
3. The ionic strength of a solution containing only Na 3 PO 4 is 1.2 mol/l. Determine the concentration of Na 3 PO 4.
Option No. 17
1. Write the equation for the electrolytic dissociation of (NH 4) 2 SO 4.
2. Determine what the NH 4 + ion is in the reaction from the standpoint of the Brønsted theory:
NH 4 + + OH - ⇄ NH 3 + H 2 O.
3. The ionic strength of a solution containing both KI and Na 2 SO 4 is 0.4 mol/l. C(KI) = 0.1 mol/l. Determine the concentration of Na 2 SO 4.
Option No. 18
1. Write the equation for the electrolytic dissociation of Cr 2 (SO 4) 3.
2. Determine what the protein molecule in the reaction is from the perspective of the Brønsted theory:
INFORMATION BLOCK
pH scale
Table 3. Relationship between the concentrations of H + and OH - ions.
Problem solving standards
1. The concentration of hydrogen ions in the solution is 10 - 3 mol/l. Calculate the pH, pOH and [OH - ] values in this solution. Determine the solution medium.
Note. The following ratios are used for calculations: lg10 a = a; 10 lg a = A.
A solution environment with pH = 3 is acidic, since the pH< 7.
2. Calculate the pH of a solution of hydrochloric acid with a molar concentration of 0.002 mol/l.
Since in a dilute solution HC1 » 1, and in a solution of a monobasic acid C(s) = C(s), we can write:
3. 90 ml of water was added to 10 ml of acetic acid solution with C(CH 3 COOH) = 0.01 mol/l. Find the difference in pH values of the solution before and after dilution, if (CH 3 COOH) = 1.85 × 10 - 5.
1) In the initial solution of a weak monobasic acid CH 3 COOH:
Hence:
2) Adding 90 ml of water to 10 ml of acid solution corresponds to a 10-fold dilution of the solution. That's why.
