The concentration and density of the sulfuric acid.The dependence of the density on the concentration of sulfuric acid in the battery car

dilute and concentrated sulfuric acid - it is so important chemical products that they produced in the world more than any other substance.The economic wealth of a country can be measured by the volume of produced sulfuric acid in it.

dissociation process

Sulfuric acid is used in the form of aqueous solutions of various concentrations.It undergoes a dissociation reaction in two stages, producing H + ions in solution.

H2SO4 = H + + HSO4-;

HSO4- = H + + SO4-2.

Sulfuric acid is a strong, and the first phase of its dissociation occurs so rapidly that nearly all of the original molecules decay into H + -ions and HSO4 -1 -ions (hydrogen) in the solution.Recent partially disintegrate further, releasing other ions and H + ion leaving sulfate (SO4-2) in the solution.However, hydrogensulfate, being a weak acid, still prevails in a solution of H + and SO4-2.Its complete dissociation occurs only when the density of the sulfuric acid solution is close to the density of water, r. S at high dilution.

Properties sulfuric acid

It is special in the sense that it can act as an ordinary acid or a strong oxidizer - depending on the temperature and concentration.A cold dilute solution of sulfuric acid reacts with the active metal to produce a salt (sulfate), and hydrogen gas evolution.For example, the reaction between cold dilute H2SO4 (assuming its full two-dissociation) and zinc metal looks like:

Zn + H2SO4 = ZnSO4 + H2.

hot concentrated sulfuric acid having a density of about 1.8 g / cm3, it may act as an oxidant, reacting with materials that are generally inert to acids, such as for example metallic copper.During the reaction, the copper is oxidized, and the mass of acid is reduced to form a solution of copper sulfate (II) in water and gaseous sulfur dioxide (SO2) instead of hydrogen, which would be expected by reacting the acid with the metal.

Cu + 2N2SO4 = CuSO4 + SO2 + 2H2 O.

How can the concentration of solutions expressed

In fact, the concentration of any solution can be expressed in various ways, but the most widely used weight concentration.It shows the number of grams of the solute in specific weight or volume of the solution or solvent (typically 1000 g 1,000 cm3, 100 cm3, 1 dm3).Instead of the mass in grams of a substance can take the quantity, expressed in moles, - then the obtained the molar concentration of 1000 g or 1 dm3 of solution.

If the molar concentration is determined in relation to the amount of solution does not, but only to the solvent, it is called the molality of the solution.It is characterized by the independence of the temperature.Often

weight concentration indicated in grams per 100 grams of solvent.Multiplying this figure by 100%, it is prepared on a weight percent (per concentration).This is the method most frequently used is applied to the solution of sulfuric acid.

solution concentration of each value determined at a given temperature, it corresponds a specific gravity (density, for example sulfuric acid).So sometimes the solution is characterized by it.For example, a solution of H2SO4, characterized percentage concentration of 95.72%, a density of 1.835 g / cm3 at t = 20 ° C.How to determine the concentration of such a solution is given only if the density of sulfuric acid?The table giving this correspondence, is a fixture in any textbook on general or analytical chemistry.

Conversion example concentration

try to move from one way of expressing the concentration of the solution to the other.Assume that we have a solution of H2SO4 in water to a concentration of 60% interest.We first define the appropriate density of sulfuric acid.Table containing percentages (first column) and the corresponding density of the aqueous solution of H2SO4 (fourth column) listed below.

it determines the desired value, which is equal to 1.4987 g / cm3.We now calculate the molarity of this solution.For this it is necessary to determine the mass of H2SO4 in 1 liter of solution and the corresponding number of moles of acid.

volume, which occupy 100 g of stock solution:

100 / 1.4987 = 66.7 ml.

Since in 66.7 ml of 60% solution contains 60 g of the acid in 1 liter of it will contain:

(60 / 66.7) x 1000 = 899, '55

Molar mass of sulfuric acidis equal to 98. Hence, the number of moles contained in 899.55 g of its grams equals:

899.55 / 98 = 9.18 mole.

density as a function of the concentration of sulfuric acid is shown in Fig.below.

use of sulfuric acid

It is used in various industries.In the production of iron and steel is used for cleaning the surface of the metal before it is covered with another substance involved in the creation of synthetic dyes as well as other types of acids, such as hydrochloric acid and nitric acid.It is also used in the production of pharmaceuticals, fertilizers and explosives, and still is an important reagent in removing impurities from the oil refining industry.

This chemical is extremely useful in daily life, and is readily available as a solution of sulfuric acid used in lead-acid batteries (for example, those that are in cars).Such acid generally has a concentration of from about 30% to 35% H2SO 4 by weight, the balance - water.

For many consumer applications 30% H2SO4 will be more than enough to meet their needs.However, in the industry and it requires a much higher concentration of sulfuric acid.Typically, in the production process, it first gets enough diluted and polluted by organic inclusions.Concentrated acid is obtained in two stages: first, it was adjusted to 70%, and then - in a second step - is raised to 96-98%, which is the limiting parameter for economically viable production.

density of sulfuric acid and a variety

Although nearly 99% sulfuric acid may be a short time at reflux, but the consequent loss of SO3 at the boiling point lowers the concentration to 98.3%.Generally, an indicator of variation from 98% more stable in storage.

commercial grade acids vary in its concentration of interest, and for which it selected those values ​​at which the low temperature crystallization.This is done to reduce the loss of sulfuric acid crystals in the sediment during transport and storage.The main varieties are:

  • towers (nitrous) - 75%. density of sulfuric acid in this class is 1670 kg / m3.Get his so-called.nitrous method in which the obtained raw materials during firing kiln gas containing sulfur dioxide SO2, in lined towers (hence the name of the variety) are treated nitroso (this is also H2SO4, but with dissolved nitrogen oxides).As a result, makes acid and nitrogen oxides, which are not consumed in the process, and returned to the production cycle.
  • Contact - 92,5-98,0%.Density sulfuric acid of 98% of the class is equal to 1836.5 kg / m3.Get it as the firing of a gas containing SO2, and the process involves the oxidation of dioxide to anhydride SO3 at its contact (hence the name of the variety) with multiple layers of solid vanadium catalyst.
  • Oleum - 104.5%. Its density is 1896.8 kg / m3.This solution of SO3 in H2SO4, wherein the first component comprises 20%, and acid - is 104.5%.
  • high-grade oleum - 114,6% .Its density - 2002 kg / m3.
  • battery - 92-94%.

How does the car battery

work of one of the most popular electrical devices is fully based on the electrochemical processes that take place in the presence of an aqueous solution of sulfuric acid.

automotive battery comprising a dilute sulfuric acid electrolyte, and positive and negative electrodes in the form of several plates.Positive plates are made of a reddish-brown material - of lead dioxide (PbO2), and negative - of gray "spongy" lead (Pb).

Because the electrodes are made of lead or leaded material, this type of battery is often referred to as lead-acid batteries.Its operability, t. E. The output voltage is directly determined by what the currently sulfuric acid density (kg / m3 or g / cm3), poured into the battery as the electrolyte.

What happens with the electrolyte in the battery discharge

electrolyte lead-acid battery is a storage solution of sulfuric acid in a chemically pure distilled water with a concentration of interest on 30% of a full charge.Net acid has a density of 1.835 g / cm3, electrolyte - about 1.300 g / cm3.When the battery is discharged, it electrochemical reactions occur, resulting in an electrolyte selected from sulfuric acid.The density of the concentration of the solution depends almost proportionately, so it should be reduced due to a decrease in electrolyte concentration.

As long as the discharge current flows through the battery acid near its electrodes is widely used, and the electrolyte becomes more dilute.Diffusion of acids from the entire volume of the electrolyte and electrode plates supports approximately constant intensity of chemical reactions and, consequently, the output voltage.

At the beginning of the discharge of the acid diffusion in the electrolyte plate is fast because the resulting sulfate with yet scored pores in the active material of electrodes.When the sulfate begins to form and fill the pores of the electrodes, the diffusion takes place more slowly.

theoretically possible to continue to discharge until all the acid was not used, and the electrolyte will be composed of pure water.However, experience shows that the level should not continue after the electrolyte density has fallen to 1.150 g / cm3.

When the density falls from 1,300 to 1,150, which means that much sulfate was formed during the reaction, and it fills the pores in the active material on the plates, ie. E. From the solution already removed nearly all sulfuric acid.The density is proportional to the concentration dependent, and just depends on the density of the battery charge.Fig.below shows the dependence of the density of battery electrolyte.

change in density of the electrolyte, the best means of determining the state of battery discharge, provided that it is properly used.

car battery discharge rate, depending on the density of the electrolyte

Its density should be measured every two weeks, and should always be recorded readings for future reference.

denser electrolyte, the more acid it contains and the more the battery is charged.The density in the 1,300-1,280 g / cm3 indicates a full charge.As a rule, the following different degree battery charge, 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 - charged less than half;
  • 1,150 - almost empty.

have a fully charged battery before connecting its road network voltage of each cell is from 2.5 to 2.7 V. As soon as the load is connected, the voltage quickly drops to about 2.1 V for three or four minutes.This occurs due to the formation of a thin layer of lead sulfate on the surface of the negative electrode plates and a layer between the peroxide and the metal lead of the positive plates.The final value of the cell voltage after the connection to the road network is about 2,15-2,18 volts.

When current begins to flow through the battery during the first hour of operation, a voltage drop to 2 V due to increased internal resistance of the cell due to the formation of large amounts of sulfate, which fills the pores of the plates and the selection of the acid electrolyte.Shortly before the start of current flow and maximum density of the electrolyte is 1.300 g / cm3.At first, his depression occurs quickly, but then set a balanced state between the density of acid near the plate and in the bulk of the electrolyte, the selection of acid electrodes supported by the arrival of new parts from the main part of the acid electrolyte.The average density of the electrolyte continues to decrease steadily on the relationship shown in Fig.above.After an initial drop voltage decreases more slowly, reducing its speed depends on the load of the battery.Time schedule discharge process shown in Fig.below.

status monitoring of battery electrolyte

used to determine the density of the hydrometer.It is composed of a sealed glass tube with an extension at the lower end, filled with lead shot or mercury and the graded scale of the upper end.This scale is marked from 1,100 to 1,300 with a variety of intermediate values, as shown in Fig.below.If this hydrometer placed in the electrolyte, it will sink to a certain depth.However, he would displace a certain volume of the electrolyte, and when the position of the equilibrium is reached, the weight of the displaced volume is equal to the weight of a hydrometer.Since the specific gravity is the ratio of its weight to volume, and weight of the hydrometer is known, each level of immersion in the solution it corresponds to a specific density.Some hydrometers have scales with the values ​​of density, but are marked by "Charged", "Half of the discharge", "Full discharge" or the like.