Electrolytes as chemicals have been known since ancient times. However, they have conquered most of their applications relatively recently. We will discuss the most priority areas for the use of these substances for industry and we will understand what the latter are and how they differ from each other. But let's start with a digression into 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 over time. Theories of these processes have evolved since the 1880s, when a series of discoveries were made related to theories of the properties of electrolytes. There were several qualitative leaps in theories that describe 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 went through in school.
Arrhenius Theory of Electrolytic Dissociation
in 1887, the Swedish chemist Svante Arrhenius and the Russian-German chemist Wilhelm Ostwald created the theory of electrolytic dissociation. However, here, too, is 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 the constituent substances with water and argued that free charged particles (ions) exist 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 the one hand and negatively (on the other hand (form a dipole), and as a result they form into aquacomplexes (solvates).
2. The process of dissociation is reversible - that is, if a substance decays 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 apply current, then the cations will begin to move to the negative electrode - the cathode, and the anions to the positively charged - to the anode. That is why substances that are readily soluble in water conduct 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 underwent dissolution. This indicator depends on the properties of the solvent and the dissolved substance itself, on the concentration of the latter and on the external temperature.
Here, in fact, all the basic tenets 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.
Theory of Acids and Lewis Bases
According to the theory of electrolytic dissociation, an acid is a substance in the solution of which there is a hydrogen cation, and the base is a compound that decomposes in solution into a hydroxide anion. 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 the Lewis theory, acids are ions or molecules of a substance that have free electronic orbitals and are able to receive an electron from another molecule. It is easy to guess that the particles will be those that are capable of donating one or more of their electrons to the "use" of the acid. It is very interesting here that the acid or base can be not only an electrolyte, but also any substance, even insoluble in water.
Brandsted-Lowry Protolytic 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 well explains the existence of substances exhibiting properties of both acid and base. Such compounds are called amphoteric. In the Bronsted-Lowry theory, the term ampholytes is also used for them, while acid or bases are commonly called protoliths.
We come to the next part of the article. Here we will tell how the strong and weak electrolytes differ from each other and discuss the influence of external factors on their properties. And then we proceed 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 at the bottom of the solution. This means that they have a very low degree of dissociation and a high binding energy, which, under normal conditions, does not allow the molecule to decay into its constituent ions. Dissociation of weak electrolytes occurs either very slowly, or with increasing temperature and concentration of this substance in solution.
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 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 Electrolyte Behavior
Now let's see how the change in the external environment affects the properties of substances. Concentration directly affects the degree of electrolyte dissociation. Moreover, this ratio can be expressed mathematically. The law describing this relationship is called the Ostwald 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 concentration of the electrolyte in the solution. Using this formula, you 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 increases solubility and chemical activity. This can explain the course of some reactions only at elevated temperatures. Under normal conditions, they go either very slowly or in both directions (such a process is called reversible).
We examined the factors that determine the behavior of such a system as an electrolyte solution. We now turn to the practical application of these, no doubt, very important chemicals.
Industrial use
Of course, everyone heard the word "electrolyte" in relation to batteries. The car uses lead-acid batteries, in which 40 percent sulfuric acid plays the role of an electrolyte. To understand why this substance is needed at all, it is worth understanding the features of the battery.
So what is the principle of operation of any battery? In them there is a reversible reaction of the conversion of one substance into another, as a result of which electrons are released. When the battery is charged, an interaction of substances occurs, which does not work under normal conditions. This can be represented as the accumulation of electricity in a substance as a result of a chemical reaction. During a discharge, 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. As you might guess, this current source consists of an element containing lead (as well as lead dioxide PbO 2 ) and an acid. Any battery consists of electrodes and the space between them, filled just with electrolyte. As the latter, as we have already found out, in our example, sulfuric acid with a concentration of 40 percent is used. The cathode of such a battery is made of lead dioxide, and the anode consists of pure lead. All this because different reversible reactions take place on these two electrodes with the participation of ions to which the acid is dissociated:
- PbO 2 + SO 4 2- + 4H + + 2e - = PbSO 4 + 2H 2 O (reaction occurring on the negative electrode - cathode).
- Pb + SO 4 2- - 2e - = PbSO 4 (The reaction proceeding on 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 chemical current source, these reactions are different, but the mechanism of their course is generally described in the same way: two processes occur, in one of which the electrons are "absorbed", and in the other, on the contrary, "exit". The most important thing is that the number of absorbed electrons is equal to the number of emitted.
In fact, in addition to batteries, there are many uses for these substances. In general, electrolytes, the examples of which we have cited, are just a grain of the variety of substances that are combined under this term. They surround us everywhere, everywhere. For example, the human body. You think these substances are not there? Very wrong. They are everywhere in us, and the largest number are 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 the 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, wherever they are used. These are not only batteries in cars and batteries. This is any chemical and food production, military factories, clothing factories and so on.
The composition of the electrolyte, by the way, is different. So, it is possible to distinguish acid and alkaline electrolyte. They fundamentally differ in their properties: as we have said, acids are proton donors, and alkalis are acceptors. But over time, the composition of the electrolyte changes due to the loss of part of the substance, the concentration either decreases or increases (it all depends on what is lost, water or electrolyte).
Every day we come across them, but few people know for sure the definition of such a term as electrolytes. We have analyzed examples of specific substances, so we will 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. The decisive role in this is played by ions. These charged particles can transfer charge from one part of the solution to another. So, anions always tend to a positive electrode, and cations - to a negative one. Thus, acting on the solution with electric current, we separate the charges on different sides of the system.
Such a physical characteristic as density is very interesting. Many properties of the compounds we are discussing depend on it. And often the question pops up: "How to increase the density of the electrolyte?" In fact, the answer is simple: you need to lower the water content in the solution. Since the density of the electrolyte is largely determined by the density of sulfuric acid, it mainly depends on the concentration of the latter. There are two ways to implement your 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 a hundred degrees Celsius. If this method does not help, do not worry, there is one more thing: simply replace the old electrolyte with a new one. To do this, you need to drain the old solution, clean the insides from the remains of sulfuric acid with distilled water, and then fill in a new portion. As a rule, high-quality electrolyte solutions immediately have the desired concentration value. After the replacement, you can forget about how to increase the density of the electrolyte for a long time.
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. There is a separate question about how much electrolyte in the battery can be. 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 produce.
Where is this useful?
If you are a car enthusiast or just interested in 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 knowledge of the properties of these substances, their use 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, then 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 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 would be Varypaev’s Chemical Sources of Current. It sets out in detail the whole theory of the operation of batteries, various batteries and hydrogen elements.
Conclusion
We have come to an end. To summarize. Above, we have analyzed everything related to the concept of 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 blood electrolytes? Thanks to them we live. What about our cars? With the help of this knowledge, we can fix any problem associated with 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 far from all that can be said about these amazing substances.