Dilute and concentrated sulfuric acid are such important chemicals that the world produces more of them than any other substance. The economic we alth of a country can be judged by the amount of sulfuric acid it produces.
Dissociation process
Sulfuric acid is used in the form of aqueous solutions of various concentrations. It undergoes a dissociation reaction in two steps, producing H+ ions in solution.
H2SO4 =H+ + HSO4 -;
HSO4- =H + + SO4 -2.
Sulfuric acid is strong, and the first stage of its dissociation is so intense that almost all of the original molecules decompose into H+-ions and HSO4-1 -ions (hydrosulfate) in solution. The latter partially decay further, releasing another H+-ion and leaving a sulfate ion (SO4-2 ) in solution. However, hydrogen sulfate, being a weak acid, still prevails.in solution over H+ and SO4-2. Its complete dissociation occurs only when the density of the sulfuric acid solution approaches the density of water, that is, with strong dilution.
Properties of sulfuric acid
It is special in that it can act as a normal acid or as a strong oxidizing agent, depending on its temperature and concentration. A cold dilute solution of sulfuric acid reacts with active metals to form a s alt (sulfate) and release hydrogen gas. For example, the reaction between cold dilute H2SO4 (assuming its complete two-stage dissociation) and metallic zinc looks like this:
Zn + H2SO4 = ZnSO4+ H2.
Hot concentrated sulfuric acid, with a density of about 1.8 g/cm3, can act as an oxidizing agent, reacting with materials that are normally inert to acids, such as like metallic copper. During the reaction, copper is oxidized, and the mass of the acid decreases, a solution of copper (II) sulfate in water and gaseous sulfur dioxide (SO2) instead of hydrogen is formed, which would be expected when the acid reacts with metal.
Cu + 2H2SO4 =CuSO4 + SO 2 + 2H2 O.
How is the concentration of solutions generally expressed
Actually, the concentration of any solution can be expressed in differentways, but the most widely used weight concentration. It shows the number of grams of a solute in a given mass or volume of a solution or solvent (usually 1000 g, 1000 cm3, 100 cm3 and 1 dm 3). Instead of the mass of a substance in grams, you can take its amount expressed in moles - then you get the molar concentration per 1000 g or 1 dm3 solution.
If the molar concentration is defined in relation not to the amount of the solution, but only to the solvent, then it is called the molality of the solution. It is characterized by independence from temperature.
Often, the weight concentration is indicated in grams per 100 g of solvent. Multiplying this figure by 100%, you get it in weight percent (percentage concentration). It is this method that is most often used in application to sulfuric acid solutions.
Each value of the concentration of a solution determined at a given temperature corresponds to its very specific density (for example, the density of a solution of sulfuric acid). Therefore, sometimes the solution is characterized precisely by it. For example, a solution of H2SO4, characterized by a percentage concentration of 95.72%, has a density of 1.835 g/cm3 at t=20 °С. How to determine the concentration of such a solution, if only the density of sulfuric acid is given? A table giving such a correspondence is an integral part of any textbook on general or analytical chemistry.
Example of concentration conversion
Let's try to move from one way of expressing concentrationsolution to another. Suppose we have a solution of H2SO4 in water with a percentage concentration of 60%. First, we determine the corresponding density of sulfuric acid. A table containing percentage concentrations (first column) and their corresponding densities of an aqueous solution of H2SO4 (fourth column) is shown below.
From it we determine the desired value, which is equal to 1, 4987 g/cm3. Let us now calculate the molarity of this solution. To do this, it is necessary to determine the mass of H2SO4 in 1 liter of solution and the corresponding number of moles of acid.
Volume occupied by 100 g of stock solution:
100 / 1, 4987=66.7 ml.
Since 66.7 milliliters of a 60% solution contains 60 g of acid, 1 liter of it will contain:
(60 / 66, 7) x 1000=899.55
The molar weight of sulfuric acid is 98. Hence, the number of moles contained in 899.55 grams of its grams will be:
899, 55 / 98=9, 18 mol.
The dependence of the density of sulfuric acid on the concentration is shown in fig. below.
Using sulfuric acid
It is applied in various industries. In the production of iron and steel, it is used to clean the surface of the metal before it is coated with another substance, it is involved in the creation of synthetic dyes, as well as other types of acids, such as hydrochloric and nitric. She alsoused in the production of pharmaceuticals, fertilizers and explosives, and is also an important reagent in the removal of impurities from oil in the oil refining industry.
This chemical is incredibly useful in the home, and is readily available as a sulfuric acid solution used in lead-acid batteries (like those found in cars). Such an acid typically has a concentration of about 30% to 35% H2SO 4 by weight, with the rest being water.
For many home applications, 30% H2SO4 will be more than enough to meet your needs. However, industry also requires a much higher concentration of sulfuric acid. Usually, during the production process, it first turns out to be quite diluted and contaminated with organic impurities. The concentrated acid is obtained in two stages: first it is brought to 70%, and then - in the second stage - it is raised to 96-98%, which is the limit for economically viable production.
Density of sulfuric acid and its grades
Although almost 99% sulfuric acid can be obtained briefly by boiling, the subsequent loss of SO3 at the boiling point reduces the concentration to 98.3%. In general, the 98% variety is more stable in storage.
Commercial grades of acid differ in its percentage concentration, and for them those values are chosen at which crystallization temperatures are minimal. This is done to reduce the precipitation of sulfuric acid crystals.sediment during transportation and storage. The main varieties are:
- Tower (nitrous) - 75%. The density of sulfuric acid of this grade is 1670 kg/m3. Get it so-called. nitrous method, in which the roasting gas obtained during the roasting of primary raw materials, containing sulfur dioxide SO2, in lined towers (hence the name of the variety) is treated with nitrous (this is also H2 SO4, but with nitrogen oxides dissolved in it). As a result, acid and nitrogen oxides are released, which are not consumed in the process, but are returned to the production cycle.
- Contact - 92, 5-98, 0%. The density of 98% sulfuric acid of this grade is 1836.5 kg/m3. It is also obtained from roasting gas containing SO2, and the process includes the oxidation of dioxide to anhydride SO3 when it comes into contact (hence the name of the variety) with several layers of solid vanadium catalyst.
- Oleum - 104.5%. Its density is 1896.8 kg/m3. This is a solution of SO3 in H2SO4, in which the first component contains 20%, and acids - exactly 104.5%.
- High percentage oleum - 114.6%. Its density is 2002 kg/m3.
- Battery - 92-94%.
How a car battery works
The operation of this one of the most massive electrical devices is completely based on electrochemical processes occurring in the presence of an aqueous solution of sulfuric acid.
The car battery contains dilute sulfuric acid electrolyte andpositive and negative electrodes in the form of several plates. The positive plates are made of a reddish-brown material - lead dioxide (PbO2), and the negative plates are made of grayish "spongy" lead (Pb).
Because the electrodes are made of lead or lead-containing material, this type of battery is often referred to as a lead-acid battery. Its performance, i.e., the magnitude of the output voltage, is directly determined by the current density of sulfuric acid (kg/m3 or g/cm3) filled into the battery as an electrolyte.
What happens to the electrolyte when the battery is discharged
The lead-acid battery electrolyte is a solution of battery sulfuric acid in chemically pure distilled water at a concentration of 30% when fully charged. A pure acid has a density of 1.835 g/cm3, an electrolyte is about 1.300 g/cm3. When the battery is discharged, electrochemical reactions take place in it, as a result of which sulfuric acid is taken from the electrolyte. The density of the solution concentration depends almost proportionally, so it should decrease due to a decrease in the electrolyte concentration.
As long as the discharge current flows through the battery, the acid near its electrodes is actively used, and the electrolyte becomes more and more dilute. Diffusion of acid from the volume of the entire electrolyte and to the electrode plates maintains an approximately constant intensity of chemical reactions and, as a result, the outputvoltage.
At the beginning of the discharge process, diffusion of acid from the electrolyte into the plates occurs quickly because the resulting sulfate has not yet clogged the pores in the active material of the electrodes. As sulfate begins to form and fill the pores of the electrodes, diffusion occurs more slowly.
Theoretically, you can continue the discharge until all the acid is used up and the electrolyte is pure water. However, experience shows that discharges should not continue after the density of the electrolyte has dropped to 1.150 g/cm3.
When the density drops from 1, 300 to 1, 150, this means that so much sulfate was formed during the reactions, and it fills all the pores in the active materials on the plates, i.e. almost all sulfuric acid. The density depends on the concentration proportionally, and in the same way the battery charge depends on the density. On fig. The dependence of battery charge on electrolyte density is shown below.
Changing the density of the electrolyte is the best means of determining the state of discharge of a battery, provided it is used properly.
Degrees of discharge of a car battery depending on the density of the electrolyte
Its density should be measured every two weeks and the readings should be continuously recorded for future reference.
The denser the electrolyte, the more acid it contains, and the more charged the battery. Density in 1.300-1.280g/cm3indicates full charge. As a rule, the following degrees of battery discharge are distinguished depending on the density of the electrolyte:
- 1, 300-1, 280 - fully charged:
- 1, 280-1, 200 - more than half empty;
- 1, 200-1, 150 - less than half full;
- 1, 150 - almost empty.
A fully charged battery has a voltage of 2.5 to 2.7 volts per cell before being connected to its car mains. As soon as a load is connected, the voltage rapidly drops to about 2.1 volts within three or four minutes. This is due to the formation of a thin layer of lead sulfate on the surface of the negative electrode plates and between the lead peroxide layer and the metal of the positive plates. The final value of the cell voltage after connecting to the car network is about 2.15-2.18 volts.
When current begins to flow through the battery during the first hour of operation, there is a voltage drop to 2 V, due to an increase in the internal resistance of the cells due to the formation of more sulfate, which fills the pores of the plates, and the removal of acid from the electrolyte. Shortly before the start of current flow, the density of the electrolyte is maximum and equal to 1, 300 g/cm3. At first, its rarefaction occurs quickly, but then a balanced state is established between the density of the acid near the plates and in the main volume of the electrolyte, the removal of acid by the electrodes is supported by the supply of new parts of the acid from the main part of the electrolyte. In this case, the average density of the electrolytecontinues to steadily decrease according to the dependence shown in Fig. higher. After the initial drop, the voltage decreases more slowly, the rate of decrease depending on the load on the battery. The time graph of the discharge process is shown in fig. below.
Monitoring the state of the electrolyte in the battery
A hydrometer is used to determine the density. It consists of a small sealed glass tube with an expansion at the lower end filled with shot or mercury and a graduated scale at the upper end. This scale is labeled from 1.100 to 1.300 with various values in between, as shown in Fig. below. If this hydrometer is placed in an electrolyte, it will sink to a certain depth. In doing so, it will displace a certain volume of electrolyte, and when an equilibrium position is reached, the weight of the displaced volume will simply be equal to the weight of the hydrometer. Since the density of the electrolyte is equal to the ratio of its weight to volume, and the weight of the hydrometer is known, each level of its immersion in the solution corresponds to a certain density.
Some hydrometers do not have a scale with density values, but are marked with the inscriptions: "Charged", "Half discharge", "Full discharge" or similar.
