All weak electrolytes. Strong and weak electrolytes
Instruction
The essence of this theory is that when melted (dissolved in water), almost all electrolytes decompose into ions, which are both positively and negatively charged (which is called electrolytic dissociation). Under the influence of an electric current, negative (“-”) towards the anode (+), and positively charged (cations, “+”), move towards the cathode (-). Electrolytic dissociation is a reversible process (the reverse process is called "molarization").
The degree (a) of electrolytic dissociation is dependent on the electrolyte itself, the solvent, and their concentration. This is the ratio of the number of molecules (n) that have decayed 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 decompose into ions when dissolved in water. Strong electrolytes, as a rule, are substances with highly polar or bonds: these are salts that are highly soluble (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 ( ); there are practically no molecules that are undissociated.
Weak electrolytes are substances that only partially dissociate into ions. Weak electrolytes, along with ions in solution, contain undissociated molecules. Weak electrolytes do not give a strong concentration of ions in solution.
The weak ones are:
- organic acids (almost all) (C2H5COOH, CH3COOH, etc.);
- some of the acids (H2S, H2CO3, etc.);
- almost all salts, 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.
note
Although pure water conducts electricity very poorly, it still has a measurable electrical conductivity, due to the fact that water dissociates slightly into hydroxide ions and hydrogen ions.
Most electrolytes are corrosive substances, so when working with them, be extremely careful and follow safety regulations.
A strong base is an inorganic chemical compound formed by a hydroxyl group -OH and an alkali (group I elements of the periodic system: Li, K, Na, RB, Cs) or alkaline earth metal (group II elements Ba, Ca). They are written as formulas LiOH, KOH, NaOH, RbOH, CsOH, Ca(OH) ₂, Ba(OH) ₂.
You will need
- evaporating cup
- burner
- indicators
- metal rod
- H₃RO₄
Instruction
Strong bases exhibit, characteristic of all. The presence in the solution is determined by the change in color of the indicator. Add phenolphthalein to the sample with the test solution or omit litmus paper. Methyl orange is yellow, phenolphthalein is purple, and litmus paper is blue. The stronger the base, the more intense the color of the indicator.
If you need to find out which alkalis are presented to you, then spend qualitative analysis solutions. The most common strong bases are lithium, potassium, sodium, barium, and calcium. Bases react with acids (neutralization reactions) to form salt and water. In this case, Ca(OH) ₂, Ba(OH) ₂ and LiOH can be distinguished. When with acid, insoluble ones are formed. The remaining hydroxides will not give precipitation, tk. all K and Na salts are soluble.
3 Ca(OH) ₂ + 2 H₃RO₄ --→ Ca₃(PO₄)₂↓+ 6 H₂O
3 Va(OH) ₂ +2 H₃RO₄ --→ Va₃(PO₄)₂↓+ 6 H₂О
3 LiOH + Н₃РО₄ --→ Li₃РО₄↓ + 3 H₂О
Strain them and dry them. Inject the dried sediments into the flame of the burner. Lithium, calcium and barium ions can be qualitatively determined by changing the color of the flame. Accordingly, you will determine where which hydroxide is. Lithium salts color the burner flame carmine red. Barium salts - in green, and calcium salts - in raspberry.
The remaining alkalis form soluble orthophosphates.
3 NaOH + Н₃РО₄--→ Na₃РО₄ + 3 H₂О
3 KOH + H₃PO₄--→ K₃PO₄ + 3 H₂O
Evaporate the water to a dry residue. Evaporated salts on a metal rod alternately bring into the burner flame. There, sodium salt - the flame will turn bright yellow, and potassium - pink-purple. Thus, having a minimum set of equipment and reagents, you have determined all the strong reasons given to you.
An electrolyte is a substance that in the solid state is a dielectric, that is, does not conduct electric current, however, in a dissolved or molten form it becomes a conductor. Why is there such a drastic change in properties? The fact is that electrolyte molecules in solutions or melts dissociate into positively charged and negatively charged ions, due to which these substances in such a state of aggregation are able to conduct electric current. Most salts, acids, bases have electrolytic properties.
Instruction
What substances are strong? Such substances, in solutions or melts of which almost 100% of the molecules are exposed, and regardless of the concentration of the solution. The list includes the vast majority of soluble alkalis, salts and some acids, such as hydrochloric, bromine, iodine, nitric, etc.
And how do the weak ones 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, they go the worse and slower, the higher the concentration of the solution. Such electrolytes include, for example, (ammonium hydroxide), most organic and inorganic acids (including hydrofluoric - HF) and, of course, the familiar water to all of us. Since only a negligible fraction of its molecules decomposes into hydrogen ions and hydroxyl ions.
Remember that the degree of dissociation and, accordingly, the strength of the electrolyte depend on factors: the nature of the electrolyte itself, the solvent, and the temperature. Therefore, this division itself is to a certain extent conditional. After all, the same substance can, under different conditions, be both a strong 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; strong electrolytes they do not obey the law of the acting masses.
Sources:
- strong electrolytes list
salt- this is 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: normal, acidic, basic, double, mixed, hydrated, complex. It depends on the compositions of the cation and anion. How can you determine base salt?
Electrolytes as chemicals have been known since ancient times. However, they have conquered most of their areas of application relatively recently. We will discuss the highest priority areas for the industry to use these substances and figure out what the latter are and how they differ from each other. But let's start with an excursion into history.
Story
The oldest known electrolytes are salts and acids, discovered back in ancient world. However, ideas about the structure and properties of electrolytes have evolved over time. Theories of these processes have evolved since the 1880s, when a number of discoveries were made related to theories of the properties of electrolytes. There have been several qualitative leaps in theories describing the mechanisms of interaction of electrolytes with water (after all, only in solution do they acquire the properties due to which they are used in industry).
Now we will analyze in detail several theories that have had the greatest influence on the development of ideas about electrolytes and their properties. And let's start with the most common and simple theory that each of us passed at school.
Arrhenius theory of electrolytic dissociation
in 1887, the Swedish chemist and Wilhelm Ostwald created the theory of electrolytic dissociation. However, everything is not so simple here either. Arrhenius himself was a supporter of the so-called physical theory of solutions, which did not take into account the interaction of the constituent substances with water and argued that there are free charged particles (ions) in the solution. By the way, it is from such positions that electrolytic dissociation is considered at school today.
Let's talk about what this theory gives and how it explains to us the mechanism of interaction of substances with water. Like any other, she has several postulates that she uses:
1. When interacting with water, the substance decomposes into ions (positive - cation and negative - anion). These particles undergo hydration: they attract water molecules, which, by the way, are positively charged on one side and negatively charged on the other (form a dipole), as a result, they form into aqua complexes (solvates).
2. The process of dissociation is reversible - that is, if the substance has broken up into ions, then under the influence of any factors it can again turn into the original one.
3. If you connect electrodes to the solution and turn on the current, then the cations will begin to move to the negative electrode - the cathode, and the anions to the positively charged - the anode. That is why substances that are highly soluble in water conduct electricity better than water itself. For the same reason they are called electrolytes.
4. electrolyte characterizes the percentage of a substance that has undergone dissolution. This indicator depends on the properties of the solvent and the solute itself, on the concentration of the latter and on the external temperature.
Here, in fact, are all the main postulates of this simple theory. We will use them in this article to describe what happens in an electrolyte solution. We will analyze examples of these compounds a little later, but now we will consider another theory.
Lewis acid and base theory
According to the theory of electrolytic dissociation, an acid is a substance in which a hydrogen cation is present, and a base is a compound that decomposes into a hydroxide anion in solution. There is another theory named after the famous chemist Gilbert Lewis. It allows you to somewhat expand the concept of acid and base. According to Lewis theory, acids are molecules of a substance that have free electron orbitals and are able to accept an electron from another molecule. It is easy to guess that the bases will be such particles that are able to donate one or more of their electrons to the "use" of the acid. It is very interesting here that not only an electrolyte, but also any substance, even insoluble in water, can be an acid or base.
Protolithic Brendsted-Lowry theory
In 1923, independently of each other, two scientists - J. Bronsted and T. Lowry - proposed a theory that is now actively used by scientists to describe chemical processes. The essence of this theory is that the meaning of dissociation is reduced to the transfer of a proton from an acid to a base. Thus, the latter is understood here as a proton acceptor. Then the acid is their donor. The theory also explains well the existence of substances that exhibit the properties of both acids and bases. Such compounds are called amphoteric. In the Bronsted-Lowry theory, the term ampholytes is also used for them, while acids or bases are usually called protoliths.
We have come to the next part of the article. Here we will tell you how strong and weak electrolytes differ from each other and discuss the influence external factors on their properties. And then we will proceed to the description of their practical application.
Strong and weak electrolytes
Each substance interacts with water individually. Some dissolve well in it (for example, table salt), while some do not dissolve at all (for example, chalk). Thus, all substances are divided into strong and weak electrolytes. The latter are substances that interact poorly with water and settle at the bottom of the solution. This means that they have a very low degree of dissociation and a high bond energy, which under normal conditions does not allow the molecule to decompose into its constituent ions. The dissociation of weak electrolytes occurs either very slowly or with an increase in temperature and concentration of this substance in solution.
Let's talk about strong electrolytes. These include all soluble salts, as well as strong acids and alkalis. They easily break up into ions and it is very difficult to collect them in precipitation. The current in electrolytes, by the way, is carried out precisely thanks to the ions contained in the solution. Therefore, strong electrolytes conduct current best of all. Examples of the latter: strong acids, alkalis, soluble salts.
Factors affecting the behavior of electrolytes
Now let's figure out how a change in the external environment affects the concentration directly affects the degree of electrolyte dissociation. Moreover, this ratio can be expressed mathematically. The law describing this connection is called the Ostwald dilution law and is written as follows: a = (K / c) 1/2. Here a is the degree of dissociation (taken in fractions), K is the dissociation constant, which is different for each substance, and c is the concentration of the electrolyte in the solution. By this formula, you can learn a lot about the substance and its behavior in solution.
But we digress from the topic. In addition to concentration, the degree of dissociation is also affected by the temperature of the electrolyte. For most substances, increasing it increases solubility and reactivity. This can explain the occurrence of some reactions only when elevated temperature. Under normal conditions, they go either very slowly or in both directions (such a process is called reversible).
We have analyzed the factors that determine the behavior of a system such as an electrolyte solution. Now let's move on to practical application these, no doubt, very important chemicals.
Industrial use
Of course, everyone has heard the word "electrolyte" in relation to batteries. The car uses lead-acid batteries, the electrolyte in which is 40% sulfuric acid. To understand why this substance is needed there at all, it is worth understanding the features of the operation of batteries.
So what is the principle of operation of any battery? In them, a reversible reaction of the transformation of one substance into another occurs, as a result of which electrons are released. When the battery is charged, an interaction of substances takes place, which is not obtained under normal conditions. This can be represented as the accumulation of electricity in a substance as a result of a chemical reaction. When the discharge begins, the reverse transformation begins, leading the system to the initial state. These two processes together make up one charge-discharge cycle.
Consider the above process on a specific example - a lead-acid battery. As you might guess, this current source consists of an element containing lead (as well as lead dioxide PbO 2) and acid. Any battery consists of electrodes and the space between them, filled just with electrolyte. As the last, as we have already found out, in our example, sulfuric acid is used at a concentration of 40 percent. The cathode of such a battery is made of lead dioxide, and the anode is made of pure lead. All this is because different reversible reactions occur on these two electrodes with the participation of ions into which the acid has dissociated:
- PbO 2 + SO 4 2- + 4H + + 2e - \u003d PbSO 4 + 2H 2 O (reaction occurring at the negative electrode - cathode).
- Pb + SO 4 2- - 2e - \u003d PbSO 4 (Reaction occurring at the positive electrode - anode).
If we read the reactions from left to right - we get the processes that occur when the battery is discharged, and if from right to left - when charging. In each of these reactions are different, but the mechanism of their occurrence is generally described in the same way: two processes occur, in one of which electrons are "absorbed", and in the other, on the contrary, they "leave". The most important thing is that the number of absorbed electrons is equal to the number of emitted ones.
Actually, in addition to batteries, there are many applications of these substances. In general, electrolytes, examples of which we have given, are just a grain of the variety of substances that are combined under this term. They surround us everywhere, everywhere. Take, for example, the human body. Do you think these substances are not there? You are very mistaken. They are everywhere in us, and the most a large number of make up blood electrolytes. These include, for example, iron ions, which are part of hemoglobin and help transport oxygen to the tissues of our body. Blood electrolytes also play a key role in the regulation of water-salt balance and heart function. This function is performed by potassium and sodium ions (there is even a process that occurs in cells, which is called the potassium-sodium pump).
Any substances that you can dissolve at least a little are electrolytes. And there is no such industry and our life with you, wherever they are applied. This is not only batteries in cars and batteries. This is any chemical and food production, military plants, clothing factories and so on.
The composition of the electrolyte, by the way, is different. So, it is possible to distinguish acidic and alkaline electrolyte. They fundamentally differ in their properties: as we have already said, acids are proton donors, and alkalis are acceptors. But over time, the composition of the electrolyte changes due to the loss of a part of the substance, the concentration either decreases or increases (it all depends on what is lost, water or electrolyte).
We encounter them every day, but few people know exactly the definition of such a term as electrolytes. We have analyzed examples of specific substances, so let's move on to slightly more complex concepts.
Physical properties of electrolytes
Now about physics. The most important thing to understand when studying this topic is how current is transmitted in electrolytes. Ions play a decisive role in this. These charged particles can transfer charge from one part of the solution to another. So, anions always tend to the positive electrode, and cations - to the negative. Thus, acting on the solution with an electric current, we separate the charges on different sides of the system.
Such a very interesting physical characteristic like density. Many properties of the compounds we are discussing depend on it. And the question often pops up: "How to raise the density of the electrolyte?" In fact, the answer is simple: you need to lower the water content of the solution. Since the density of the electrolyte is mostly determined, it mostly depends on the concentration of the latter. There are two ways to carry out the plan. The first is quite simple: boil the electrolyte contained in the battery. To do this, you need to charge it so that the temperature inside rises slightly above one hundred degrees Celsius. If this method does not help, do not worry, there is another one: simply replace the old electrolyte with a new one. To do this, drain the old solution, clean the insides of sulfuric acid residues with distilled water, and then pour in a new portion. As a rule, high-quality electrolyte solutions immediately have the desired concentration. After replacement, you can forget for a long time how to raise the density of the electrolyte.
The composition of the electrolyte largely determines its properties. Characteristics such as electrical conductivity and density, for example, are highly dependent on the nature of the solute and its concentration. Exists separate issue how much electrolyte is in the battery. In fact, its volume is directly related to the declared power of the product. The more sulfuric acid inside the battery, the more powerful it is, that is, the more voltage it can deliver.
Where is it useful?
If you are a car enthusiast or just fond of cars, then you yourself understand everything. Surely you even know how to determine how much electrolyte is in the battery now. And if you are far from cars, then knowing the properties of these substances, their applications and how they interact with each other will not be superfluous at all. Knowing this, you will not be at a loss if you are asked to say which electrolyte is in the battery. Although even if you are not a car enthusiast, but you have a car, then knowing the battery device will not be superfluous at all and will help you with repairs. It will be much easier and cheaper to do everything yourself than to go to the auto center.
And in order to better study this topic, we recommend reading a chemistry textbook for schools and universities. If you know this science well and have read enough textbooks, the best option will be "Chemical Current Sources" by Varypaev. It outlines in detail the whole theory of the operation of batteries, various batteries and hydrogen cells.
Conclusion
We've come to the end. Let's summarize. Above, we have analyzed everything related to such a concept as electrolytes: examples, theory of structure and properties, functions and applications. Once again it is worth saying that these compounds are part of our life, without which our bodies and all areas of industry could not exist. Do you remember blood electrolytes? Thanks to them we live. What about our cars? With this knowledge, we will be able to fix any problem related to the battery, as we now understand how to increase the density of the electrolyte in it.
It is impossible to tell everything, and we did not set such a goal. After all, this is not all that can be said about these amazing substances.
Distinguish between strong and weak electrolytes. Strong electrolytes in solutions are almost completely dissociated. This group of electrolytes includes most salts, alkalis and strong acids. Weak electrolytes include weak acids and weak bases and some salts: mercury (II) chloride, mercury (II) cyanide, iron (III) thiocyanate, and cadmium iodide. Solutions of strong electrolytes at high concentrations have a significant electrical conductivity, and it increases slightly with dilution of the solutions.
Solutions of weak electrolytes at high concentrations are characterized by insignificant electrical conductivity, which increases greatly with dilution of the solutions.
When a substance is dissolved in any solvent, simple (non-solvated) ions are formed, neutral molecules of the solute, solvated (hydrated in aqueous solutions) ions (for example, etc.), ion pairs (or ion twins), which are electrostatically associated groups of oppositely charged ions (for example,), the formation of which is observed in the vast majority of non-aqueous electrolyte solutions, complex ions (for example,), solvated molecules, etc.
In aqueous solutions of strong electrolytes, only simple or solvated cations and anions exist. There are no solute molecules in their solutions. Therefore, it is incorrect to assume the presence of molecules or the presence of long-term bonds between or and in an aqueous solution of sodium chloride.
In aqueous solutions of weak electrolytes, the solute can exist in the form of simple and solvated (-hydrated) ions and undissociated molecules.
In non-aqueous solutions, some strong electrolytes (for example, ) are not completely dissociated even at moderately high concentrations. In most organic solvents, the formation of ion pairs of oppositely charged ions is observed (for more details, see Book 2).
In some cases, it is impossible to draw a sharp line between strong and weak electrolytes.
Interionic forces. Under the action of interionic forces around each freely moving ion, other ions are grouped symmetrically, charged with the opposite sign, forming the so-called ionic atmosphere, or ionic cloud, which slows down the movement of the ion in solution.
For example, in a solution, chloride ions cluster around moving potassium ions, and an atmosphere of potassium ions is created near moving chloride ions.
Ions, the mobility of which is weakened by the forces of interionic extension, exhibit a reduced chemical activity in solutions. This causes deviations in the behavior of strong electrolytes from the classical form of the law of mass action.
Foreign ions present in a solution of a given electrolyte also have a strong influence on the mobility of its ions. The higher the concentration, the more significant the interionic interaction and the stronger the foreign ions affect the ion mobility.
Weak acids and bases have a hydrogen or hydroxyl bond in their molecules that is largely covalent rather than ionic; therefore, when weak electrolytes are dissolved in solvents that differ by a very high dielectric constant, most of their molecules do not decompose into ions.
Solutions of strong electrolytes differ from solutions of weak electrolytes in that they do not contain undissociated molecules. This is confirmed by modern physical and physico-chemical studies. For example, the study of crystals of strong electrolytes of the type by X-ray diffraction confirms the fact that the crystal lattices of salts are built from ions.
When dissolved in a solvent with a high dielectric constant, solvate (hydrated in water) shells are formed around the ions, preventing their combination into molecules. Thus, since strong electrolytes, even in the crystalline state, do not contain molecules, they do not contain molecules in solution even more so.
However, it has been experimentally found that the electrical conductivity of aqueous solutions of strong electrolytes is not equivalent to the electrical conductivity that could be expected during the dissociation of molecules of dissolved electrolytes into ions.
Using the theory of electrolytic dissociation proposed by Arrhenius, it turned out to be impossible to explain this and a number of other facts. To explain them, new scientific provisions were put forward.
At present, the discrepancy between the properties of strong electrolytes and the classical form of the law of mass action can be explained using the theory of strong electrolytes proposed by Debye and Hückel. The main idea of this theory is that forces of mutual attraction arise between ions of strong electrolytes in solutions. These interionic forces cause the behavior of strong electrolytes to deviate from the laws of ideal solutions. The presence of these interactions causes mutual deceleration of cations and anions.
Influence of dilution on interionic attraction. Interionic attraction causes deviations in the behavior of real solutions in the same way as intermolecular attraction in real gases entails deviations in their behavior from the laws of ideal gases. The greater the concentration of the solution, the denser the ionic atmosphere and the lower the mobility of the ions, and hence the electrical conductivity of the electrolytes.
Just as the properties of a real gas at low pressures approach the properties of an ideal gas, so the properties of strong electrolyte solutions at high dilution approach the properties of ideal solutions.
In other words, in dilute solutions, the distances between the ions are so large that the mutual attraction or repulsion experienced by the ions is extremely small and practically reduces to zero.
Thus, the observed increase in the electrical conductivity of strong electrolytes upon dilution of their solutions is explained by the weakening of the interionic forces of attraction and repulsion, which causes an increase in the speed of ion movement.
The less dissociated the electrolyte and the more diluted the solution, the less the interionic electric influence and the less deviations from the law of mass action are observed, and, conversely, the greater the concentration of the solution, the greater the interionic electric influence and the more deviations from the law of mass action are observed.
For the above reasons, the law of mass action in its classical form cannot be applied to aqueous solutions of strong electrolytes, as well as to concentrated aqueous solutions of weak electrolytes.
Strong and weak electrolytes
Acids, bases and salts in aqueous solutions dissociate - break down into ions. This process may be reversible or irreversible.
With irreversible dissociation in solutions, the entire substance or almost everything decomposes into ions. This is typical for strong electrolytes (Fig. 10.1, a, p. 56). Strong electrolytes include some acids and all water-soluble salts and bases (hydroxides of alkaline and alkaline earth elements) (Scheme 5, p. 56).
Rice. 10.1. Comparison of the number of ions in solutions with the same initial amount of electrolyte: a - chloride acid (strong electrolyte); b - nitrite acid
(weak electrolyte)
Scheme 5. Classification of electrolytes by strength
With reversible dissociation, two opposite processes take place: simultaneously with the decay of a substance into ions (dissociation), the reverse process of combining ions into molecules of a substance (association) occurs. Due to this, part of the substance in solution exists in the form of ions, and part - in the form of molecules (Fig. 10.1, b). electrolytes,
which, when dissolved in water, decompose into ions only partially, are called weak electrolytes. These include water, many acids, as well as insoluble hydroxides and salts (Scheme 5).
In the dissociation equations for weak electrolytes, instead of the usual arrow, a bidirectional arrow is written (the sign of reversibility):
The strength of electrolytes can be explained by the polarity of the chemical bond, which is broken upon dissociation. The more polar the bond, the easier it becomes ionic under the action of water molecules, therefore, the stronger the electrolyte. In salts and hydroxides, the polarity of the bond is the highest, since there is an ionic bond between metal ions, acid residues and hydroxide ions, so all soluble salts and bases are strong electrolytes. In oxygen-containing acids, upon dissociation, the O-H bond, the polarity of which depends on the qualitative and quantitative composition of the acid residue. The strength of most oxygenated acids can be determined by writing the usual acid formula as E(OH) m O n . If this formula contains n< 2 — кислота слабая, если n >2 - strong.
The dependence of the strength of acids on the composition of the acid residue
Degree of dissociation
The strength of electrolytes is quantitatively characterized by the degree of electrolytic dissociation a, showing the proportion of substance molecules that have decomposed into ions in solution.
The degree of dissociation a is equal to the ratio of the number of molecules N or the amount of substance n decomposed into ions to the total number of molecules N 0 or the amount of solute n 0:
The degree of dissociation can be expressed not only in fractions of a unit, but also as a percentage:
The value of a can vary from 0 (no dissociation) to 1, or 100% (complete dissociation). The better the electrolyte decomposes, the greater the value of the degree of dissociation.
By the value of the degree of electrolytic dissociation, electrolytes are often divided not into two, but into three groups: strong, weak, and electrolytes of medium strength. Strong electrolytes are considered those with a degree of dissociation of more than 30%, and weak - with a degree of less than 3%. Electrolytes with intermediate values \u200b\u200bof a - from 3% to 30% - are called electrolytes of medium strength. According to this classification, acids are considered as such: HF, HNO 2, H 3 PO 4, H 2 SO 3 and some others. The last two acids are electrolytes of medium strength only in the first stage of dissociation, while in others they are weak electrolytes.
The degree of dissociation is a variable. It depends not only on the nature of the electrolyte, but also on its concentration in the solution. This dependence was first identified and studied by Wilhelm Ostwald. Today it is called the Ostwald dilution law: when a solution is diluted with water, as well as when the temperature rises, the degree of dissociation increases.
Calculation of the degree of dissociation
Example. Hydrogen fluoride was dissolved in one liter of water with a substance amount of 5 mol. The resulting solution contains 0.06 mol of hydrogen ions. Determine the degree of dissociation of fluoric acid (in percent).
We write the equation for the dissociation of fluoric acid:
Dissociation from one acid molecule produces one hydrogen ion. If the solution contains 0.06 mol of H+ ions, this means that 0.06 mol of hydrogen fluoride molecules have dissociated. Therefore, the degree of dissociation is:
Outstanding German physical chemist, laureate Nobel Prize in chemistry in 1909. Born in Riga, studied at Dorpat University, where he began teaching and research activities. At the age of 35 he moved to Leipzig, where he headed the Institute of Physics and Chemistry. He studied the laws of chemical equilibrium, the properties of solutions, discovered the dilution law named after him, developed the foundations of the theory of acid-base catalysis, and devoted much time to the history of chemistry. He founded the world's first department of physical chemistry and the first physical and chemical journal. In his personal life, he had strange habits: he felt disgusted with a haircut, and communicated with his secretary exclusively with the help of a bicycle bell.
Key Idea
The dissociation of weak electrolytes is a reversible process, and of strong ones
irreversible.
test questions
116. Define strong and weak electrolytes.
117. Give examples of strong and weak electrolytes.
118. What value is used to quantify the strength of the electrolyte? Is it constant in all solutions? How can the degree of electrolyte dissociation be increased?
Tasks for mastering the material
119. Give one example each of salts, acids and bases, which are: a) a strong electrolyte; b) weak electrolyte.
120. Give an example of a substance: a) dibasic acid, which according to the first stage is an electrolyte of medium strength, and according to the second - a weak electrolyte; b) a dibasic acid, which is a weak electrolyte in both stages.
121. In some acid, the degree of dissociation in the first stage is 100%, and in the second - 15%. What kind of acid could it be?
122. What particles are more in a hydrogen sulfide solution: H 2 S molecules, H + ions, S 2- ions or HS - ions?
123. From the given list of substances, separately write down the formulas: a) strong electrolytes; b) weak electrolytes.
NaCl, HCl, NaOH, NaNO 3 , HNO 3 , HNO 2 , H 2 SO 4 , Ba(OH) 2 , H 2 S, K 2 S, Pb(NO 3) 2 .
124. Make the equations of dissociation of strontium nitrate, mercury (11) chloride, calcium carbonate, calcium hydroxide, sulfide acid. When is dissociation reversible?
125. An aqueous solution of sodium sulfate contains 0.3 mol of ions. What mass of this salt was used to prepare such a solution?
126. A 1 liter hydrogen fluoride solution contains 2 g of this acid, and the amount of hydrogen ion substance is 0.008 mol. What is the amount of fluoride ions in this solution?
127. Three test tubes contain the same volumes of solutions of chloride, fluoride and sulfide acids. In all test tubes, the amounts of acid substances are equal. But in the first test tube, the amount of Hydrogen ion substance is 3. 10 -7 mol, in the second - 8. 10 -5 mol, and in the third - 0.001 mol. Which tube contains each acid?
128. The first tube contains an electrolyte solution, the degree of dissociation of which is 89%, the second contains an electrolyte with a degree of dissociation of 8% o, and the third - 0.2% o. Give two examples each of electrolytes of different classes of compounds that can be contained in these test tubes.
129*. In additional sources, find information on the dependence of the strength of electrolytes on the nature of substances. Establish the relationship between the structure of substances, nature chemical elements, which form them, and the strength of electrolytes.
This is textbook material.
ELECTROLYTES Substances whose solutions or melts conduct electricity.
NON-ELECTROLYTES Substances whose solutions or melts do not conduct electricity.
Dissociation- decomposition of compounds into ions.
Degree of dissociation is the ratio of the number of molecules dissociated into ions to the total number of molecules in the solution.
STRONG ELECTROLYTES when dissolved in water, they almost completely dissociate into ions.
When writing the equations of dissociation of strong electrolytes put an equal sign.
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 alkali (LiOH, NaOH, KOH) and alkaline earth (Ca (OH) 2, Sr (OH) 2, Ba (OH) 2) metals ( see strong electrolyte bases in the solubility table).
WEAK ELECTROLYTES in aqueous solutions only partially (reversibly) dissociate into ions.
When writing the dissociation equations for weak electrolytes, the sign of reversibility 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-weak electrolytes in the solubility table);
Insoluble metal hydroxides (Mg (OH) 2, Fe (OH) 2, Zn (OH) 2) ( see basescweak electrolytes in the solubility table).
The degree of electrolytic dissociation is influenced by a number of factors:
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 dissociation of substances, have 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 increasing concentration, it decreases;
stage of the dissociation process: each subsequent stage is less effective than the previous one, approximately 1000–10,000 times; for example, for phosphoric acid α 1 > α 2 > α 3:
H3PO4⇄Н++H2PO−4 (first stage, α 1),
H2PO−4⇄H++HPO2−4 (second stage, α 2),
НPO2−4⇄Н++PO3−4 (third stage, α 3).
For this reason, in a solution of this acid, the concentration of hydrogen ions is the highest, and the concentration of PO3−4 phosphate ions is the lowest.
1. The solubility and degree of dissociation of a substance are not related to each other. For example, a weak electrolyte is acetic acid, which is highly (unrestrictedly) soluble in water.
2. A 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 addition of other electrolytes: e.g. degree of dissociation of formic acid
HCOOH ⇄ HCOO − + H+
decreases if a little sodium formate is added to the solution. This salt dissociates to form formate ions HCOO − :
HCOONa → HCOO − + Na +
As a result, the concentration of HCOO– ions in the solution increases, and according to the Le Chatelier principle, an increase in the concentration of formate ions shifts the equilibrium of the formic acid dissociation process to the left, i.e. the degree of dissociation decreases.
Ostwald dilution law- ratio expressing the dependence of the equivalent electrical conductivity of a dilute 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 of 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.
The Ostwald dilution law was developed by W. Ostwald in 1888 and confirmed by him experimentally. The experimental establishment of the correctness of the Ostwald dilution law had great importance to substantiate the theory of electrolytic dissociation.
Electrolytic dissociation of water. Hydrogen indicator pH Water is a weak amphoteric electrolyte: H2O H+ + OH- or, more precisely: 2H2O \u003d H3O + + OH- The dissociation constant of water at 25 ° C is: 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 1000g: 18g / 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 ion product of water KW: The dissociation of water is an endothermic process, therefore, with an increase in temperature, in accordance with the Le Chatelier principle, dissociation increases, the ion product increases and reaches a value of 10-13 at 100 ° C. In pure water at 25°C, the concentrations of hydrogen and hydroxyl ions are equal to each other: = = 10-7 mol/l Solutions in which the concentrations of hydrogen and hydroxyl ions are equal to each other are called neutral. If to clean water add acid, 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 ion product of water retains its value of 10-14. The same thing will happen when alkali is added to pure water. The concentrations of hydrogen and hydroxyl ions are related to each other through the ion product, therefore, knowing the concentration of one of the ions, it is easy to calculate the concentration of the other. For example, if = 10-3 mol/l, then = KW/ = 10-14/10-3 = 10-11 mol/l, or if = 10-2 mol/l, then = KW/ = 10-14 /10-2 = 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, it is not the concentrations of hydrogen or hydroxyl ions that are used, but the hydrogen pH or hydroxyl pOH indicators. The hydrogen index pH is equal to the negative decimal logarithm of the concentration of hydrogen ions: pH = - lg The hydroxyl index pOH is equal to the negative decimal logarithm of the concentration of hydroxyl ions: pOH = - lg It is easy to show by pronouncing the ionic product of water that pH + pOH = 14 the medium is neutral, if less than 7 - acidic, and the lower the pH, the higher the concentration of hydrogen ions. pH greater than 7 - alkaline environment, the higher the pH, the higher the concentration of hydroxyl ions.
