Electrolytes: examples. Composition and properties of electrolytes. Strong and weak electrolytes

Electrolytes as chemicals have been known since ancient times. However, most of the areas of their application they have gained relatively recently. We will discuss the industry's most priority areas for the use of these substances and understand what the latter represent and how they differ from each other. But let's start with a tour of history.

History

The oldest known electrolytes are salts and acids, discovered in the Ancient World. However, ideas about the structure and properties of electrolytes developed with time. The theories of these processes evolved, beginning in the 1880s, when a number of discoveries were made, related to the theories of electrolyte properties. Several qualitative leaps were observed in the theories describing the mechanisms of interaction of electrolytes with water (in fact, only in solution they acquire those properties that make them used in industry).

Now we will discuss in detail several theories that have had the greatest impact on the development of ideas about electrolytes and their properties. And start with the most common and simple theory that each of us passed at school.

The theory of electrolytic dissociation Arrhenius

In 1887 the Swedish chemist Svante Arrhenius and the Russian-German chemist Wilhelm Ostwald created the theory of electrolytic dissociation. However, here, too, it's not so simple. Arrhenius himself was a supporter of the so-called physical theory of solutions, which did not take into account the interaction of constituent substances with water and claimed that in the solution there are free charged particles (ions). By the way, it is from such positions that electrolytic dissociation in school is being considered today.

Let's talk all the same about what this theory gives and how it explains 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, incidentally, are positively charged on one side, and on the other - negatively (form a dipole), as a result are formed into aquacomplexes (solvates).

2. The process of dissociation is reversible - that is, if the substance has broken up into ions, then under the influence of some factors it can again become the initial one.

3. If you connect electrodes to the solution and start up the current, the cations will start moving to the negative electrode - the cathode, and the anions to the positively charged - the anode. That is why substances that are very soluble in water conduct an electric current better than water itself. For the same reason they were called electrolytes.

4. The degree of dissociation of the electrolyte characterizes the percentage of the substance that has undergone dissolution. This indicator depends on the properties of the solvent and the most dissolved substance, on the concentration of the latter and on the external temperature.

Here, in fact, and all the basic postulates of this simple theory. We will use them in this article to describe what happens in the electrolyte solution. Examples of these compounds will be discussed later, but now we will consider another theory.

Theory of acids and bases of Lewis

According to the theory of electrolytic dissociation, acid is a substance in the solution of which there is a hydrogen cation, and the base is a compound decomposing into a hydroxide anion in solution. There is another theory, named after the famous chemist Gilbert Lewis. It allows us to expand the concept of acid and base somewhat. According to Lewis theory, acids are ions or molecules of matter that have free electron orbitals and are capable of taking an electron from another molecule. It is easy to guess that the bases will be those particles that are capable of giving one or more of their electrons to "use" the acid. It is very interesting here that acid or base can be not only electrolyte, but any substance, even insoluble in water.

The prototypical theory of Brandsted-Lowry

In 1923, independently of each other, two scientists - J. Bronsted and T. Lowry - proposed a theory, which 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 the proton from the acid to the 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 exhibiting properties and acids and bases. Such compounds are called amphoteric. In the Bronsted-Lowry theory, the term ampholytes is also used for them, whereas acid or bases are commonly called protoliths.

We have come to the next part of the article. Here we describe how different strong and weak electrolytes differ from each other and discuss the influence of external factors on their properties. And then we will begin to describe their practical application.

Strong and weak electrolytes

Each substance interacts with water individually. Some dissolve in it well (for example, table salt), and 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 on the bottom of the solution. This means that they have a very low degree of dissociation and a high binding energy, which does not allow the molecule to decompose into its constituent ions under normal conditions. The dissociation of weak electrolytes occurs either very slowly, or with an increase in the temperature and concentration of this substance in the solution.

Let's talk about strong electrolytes. These include all soluble salts, as well as strong acids and alkalis. They easily decay into ions and it is very difficult to collect them in the precipitate. By the way, the current in electrolytes is carried out precisely by the ions contained in the solution. Therefore, the best electrolyte conducts the current. Examples of the latter: strong acids, alkalis, soluble salts.

Factors affecting the behavior of electrolytes

Now let's see how the change in the external environment affects the properties of substances. Concentration directly affects the degree of dissociation of the electrolyte. Moreover, this relationship can be expressed mathematically. The law describing this connection is called the Ostwald's dilution law and is written as: a = (K / c) ^1/2 . Here, a is the degree of dissociation (taken in fractions), K is the dissociation constant, different for each substance, and c is the electrolyte concentration in the solution. By this formula, one can learn a lot about the substance and its behavior in solution.

But we deviated from the topic. In addition to concentration, the degree of dissociation is also affected by the temperature of the electrolyte. For most substances, its increase raises solubility and chemical activity. It is this that can explain the course of certain reactions only at elevated temperature. Under normal conditions, they go either very slowly or in both directions (this process is called reversible).

We have analyzed the factors that determine the behavior of such a system as the electrolyte solution. Now let's turn to the practical application of these, undoubtedly, very important chemicals.

Industrial use

Of course, everyone has heard the word "electrolyte" applied to batteries. In the car use lead-acid batteries, the role of electrolyte in which performs 40 percent sulfuric acid. To understand why there is any need for this substance, it is necessary to understand the features of the battery.

So what is the principle of any battery? In them, a reversible reaction of the transformation of one substance into another takes place, as a result of which electrons are released. When the battery is charged, there is interaction of substances, which is not obtained under normal conditions. This can be thought of 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 its initial state. These two processes together constitute one charge-discharge cycle.

Consider the above process on a specific example - a lead-acid battery. It is not difficult to guess that 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 explained, in our example sulfuric acid is used with a concentration of 40 percent. The cathode of such a battery is made from lead dioxide, and the anode consists of pure lead. All this because on these two electrodes there are different reversible reactions involving ions, to which the acid is dissociated:

1. PbO ₂ + SO ₄ ^2- + 4H ⁺ + 2e ^- = PbSO ₄ + 2H ₂ O (reaction occurring at the negative electrode - cathode).
2. Pb + SO ₄ ^2- - 2e ^- = PbSO ₄ (Reaction flowing on the positive electrode - anode).

If we read the reactions from left to right, we get the processes that take place when the battery is discharged, and if from right to left - during the charge. In each chemical source of current, these reactions are different, but the mechanism of their flow is generally described in the same way: two processes occur, in one of which the electrons are "absorbed", and in the other, "go out." The most important thing is that the number of absorbed electrons is equal to the number of released electrons.

Actually, in addition to batteries, there are a lot of applications of these substances. In general, electrolytes, the examples of which we have given, are only a grain of the variety of substances that are united under this term. They surround us everywhere, everywhere. Here, for example, is the human body. Do you think these substances are not there? Very mistaken. They are everywhere in us, and the greatest number is formed by electrolytes of blood. These include, for example, iron ions that are part of hemoglobin and help transport oxygen to the tissues of our body. Electrolytes of blood also play a key role in regulating the water-salt balance and the work of the heart. This function is performed by potassium and sodium ions (there is even a process that takes place in the cells, which is called a potassium-sodium pump).

Any substances that you can dissolve at least a little - electrolytes. And there is no such branch of industry and our life with you, wherever they are applied. It's not just batteries in cars and batteries. This is any chemical and food production, military factories, garment factories and so on.

The composition of the electrolyte, by the way, is different. Thus, it is possible to isolate the acidic and alkaline electrolyte. They basically differ in their properties: as we have already said, acids are donors of protons, and alkalies - acceptors. But with time the composition of the electrolyte changes due to the loss of part of the substance, the concentration either decreases or increases (everything depends on what is lost, water or electrolyte).

We face them every day, but very few people know exactly the definition of such a term as electrolytes. Examples of specific substances, we dismantled, so let's move on to a slightly more complex concepts.

Physical properties of electrolytes

Now about physics. The most important thing to understand when studying this topic is how the current is transferred in electrolytes. The decisive role in this is played by ions. These charged particles can carry a charge from one part of the solution to another. Thus, the anions tend always to the positive electrode, and the cations to the negative electrode. Thus, acting on the solution with an electric current, we divide the charges on different sides of the system.

Very interesting is a physical characteristic, such as density. Many properties of the compounds under discussion depend on it. And often a question pops up: "How to raise the density of electrolyte?" In fact, the answer is simple: you need to lower the water content of the solution. Since the density of the electrolyte is mainly determined by the density of sulfuric acid, it largely depends on the concentration of the latter. There are two ways to accomplish this. The first is simple enough: boil the electrolyte contained in the battery. To do this, you need to charge it so that the temperature inside rises slightly above a hundred degrees Celsius. If this method does not help, do not worry, there is one more: simply replace the old electrolyte with a new one. To do this, drain the old solution, clean the inside of the remains of sulfuric acid with distilled water, and then pour in a new portion. As a rule, qualitative solutions of electrolyte immediately have the necessary value of concentration. After a replacement, you can forget about how to raise the density of electrolyte for a long time.

The composition of the electrolyte largely determines its properties. Such characteristics as electrical conductivity and density, for example, strongly depend on the nature of the dissolved substance and its concentration. There is a separate question about how much electrolyte in the battery can be. In fact, its volume is directly related to the declared capacity of the product. The more sulfuric acid inside the battery, the more powerful it is, ie, the greater the voltage is able to produce.

Where is it useful?

If you are a car enthusiast or just fond of cars, then you understand everything yourself. Surely you even know how to determine how much electrolyte in the battery is now. And if you are far from cars, then the knowledge of the properties of these substances, their applications and how they interact with each other will not be superfluous. 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, the knowledge of the battery device will not be superfluous and will help you in repair. It will be much easier and cheaper to do everything yourself, rather than go to the car center.

And to better study this topic, we recommend reading the chemistry textbook for schools and universities. If you know this science well and have read enough textbooks, the best option will be "Chemical sources of current" Varypaev. There, the entire theory of the operation of accumulators, various batteries and hydrogen elements is described in detail.

Conclusion

We have come to an end. Let's sum up. Above we have analyzed everything that concerns 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 spheres of industry could not exist. Do you remember about blood electrolytes? Thanks to them we live. What about our machines? With the help of this knowledge, we can fix any problem related to the battery, since now we understand how to raise the density of electrolyte in it.

Everything can not be told, and we did not set such a goal. After all, this is not all that can be told about these amazing substances.