soluble substances. Solubility of solids in water
A solution is a homogeneous system consisting of two or more substances, the content of which can be changed within certain limits without violating homogeneity.
Aquatic solutions are made up of water(solvent) and solute. The state of substances in an aqueous solution, if necessary, is indicated by a subscript (p), for example, KNO 3 in solution - KNO 3 (p) .
Solutions that contain a small amount of solute are often referred to as diluted and solutions with high content solute - concentrated. A solution in which further dissolution of a substance is possible is called unsaturated and a solution in which a substance ceases to dissolve under given conditions is saturated. The last solution is always in contact (in heterogeneous equilibrium) with the undissolved substance (one or more crystals).
Under special conditions, such as gentle (without stirring) cooling of a hot unsaturated solution solid substances can form supersaturated solution. When a crystal of a substance is introduced, such a solution is separated into a saturated solution and a precipitate of the substance.
In accordance with chemical theory of solutions D. I. Mendeleev, the dissolution of a substance in water is accompanied, firstly, destruction chemical bonds between molecules (intermolecular bonds in covalent substances) or between ions (in ionic substances), and thus the particles of the substance mix with water (in which some of the hydrogen bonds between molecules are also destroyed). Chemical bonds are broken due to the thermal energy of the movement of water molecules, and in this case cost energy in the form of heat.
Secondly, once in the water, the particles (molecules or ions) of the substance are subjected to hydration. As a result, hydrates- compounds of indeterminate composition between particles of a substance and water molecules (the internal composition of the particles of a substance itself does not change when dissolved). This process is accompanied highlighting energy in the form of heat due to the formation of new chemical bonds in hydrates.
In general, a solution cools down(if the cost of heat exceeds its release), or heats up (otherwise); sometimes - if the cost of heat and its release are equal - the temperature of the solution remains unchanged.
Many hydrates are so stable that they do not break down even when the solution is completely evaporated. So, solid crystal hydrates of salts CuSO 4 5H 2 O, Na 2 CO 3 10H 2 O, KAl (SO 4) 2 12H 2 O, etc. are known.
The content of a substance in a saturated solution at T= const quantifies solubility this substance. Solubility is usually expressed as the mass of solute per 100 g of water, for example 65.2 g KBr/100 g H 2 O at 20 °C. Therefore, if 70 g of solid potassium bromide is introduced into 100 g of water at 20 °C, then 65.2 g of salt will go into solution (which will be saturated), and 4.8 g of solid KBr (excess) will remain at the bottom of the beaker.
It should be remembered that the solute content in rich solution equals, in unsaturated solution less and in supersaturated solution more its solubility at a given temperature. So, a solution prepared at 20 ° C from 100 g of water and sodium sulfate Na 2 SO 4 (solubility 19.2 g / 100 g H 2 O), with a content
15.7 g of salt - unsaturated;
19.2 g salt - saturated;
2O.3 g of salt is supersaturated.
The solubility of solids (Table 14) usually increases with increasing temperature (KBr, NaCl), and only for some substances (CaSO 4 , Li 2 CO 3) is the opposite observed.
The solubility of gases decreases with increasing temperature, and increases with increasing pressure; for example, at a pressure of 1 atm, the solubility of ammonia is 52.6 (20 ° C) and 15.4 g / 100 g H 2 O (80 ° C), and at 20 ° C and 9 atm it is 93.5 g / 100 g H 2 O.
In accordance with the solubility values, substances are distinguished:
– well soluble, the mass of which in a saturated solution is commensurate with the mass of water (for example, KBr - at 20 ° C the solubility is 65.2 g / 100 g H 2 O; 4.6 M solution), they form saturated solutions with a molarity of more than 0.1 M;
– sparingly soluble, the mass of which in a saturated solution is much less than the mass of water (for example, CaSO 4 - at 20 ° C, the solubility is 0.206 g / 100 g H 2 O; 0.015 M solution), they form saturated solutions with a molarity of 0.1–0.001 M;
– practically insoluble the mass of which in a saturated solution is negligible compared to the mass of the solvent (for example, AgCl - at 20 ° C, the solubility is 0.00019 g per 100 g H 2 O; 0.0000134 M solution), they form saturated solutions with a molarity of less than 0.001 M.
Compiled according to reference data solubility table common acids, bases and salts (Table 15), in which the type of solubility is indicated, substances are noted that are not known to science (not obtained) or completely decomposed by water.
Conventions used in the table:
"r" is a highly soluble substance
"m" - poorly soluble substance
"n" - practically insoluble substance
"-" - the substance is not received (does not exist)
"" - the substance is mixed with water indefinitely
Note. This table corresponds to the preparation of a saturated solution at room temperature by introducing a substance (in the appropriate state of aggregation) into water. It should be noted that it is not always possible to obtain precipitates of poorly soluble substances using ion exchange reactions (for details, see 13.4).
13.2. Electrolytic dissociation
The dissolution of any substance in water is accompanied by the formation of hydrates. If at the same time there are no formula changes in the particles of the dissolved substance in the solution, then such substances are classified as non-electrolytes. They are, for example, gas nitrogen N 2 liquid chloroform CHCl 3 , solid sucrose C 12 H 22 O 11, which exist in an aqueous solution in the form of hydrates of their molecules.
Many substances are known general view MA), which, after dissolution in water and the formation of hydrates of the MA nH 2 O molecules, undergo significant formula changes. As a result, hydrated ions appear in the solution - M + nH 2 O cations and A nH 2 O anions:
Such substances are electrolytes.
The process of the appearance of hydrated ions in an aqueous solution called electrolytic dissociation(S. Arrhenius, 1887).
Electrolytic dissociation ionic crystalline substances (M +) (A -) in water is irreversible reaction:
Such substances are strong electrolytes, they are many bases and salts, for example:
Electrolytic dissociation of MA substances consisting of polar covalent molecules, is reversible reaction:
Such substances are classified as weak electrolytes, they are many acids and some bases, for example:
In dilute aqueous solutions of weak electrolytes, we will always find both the original molecules and the products of their dissociation - hydrated ions.
The quantitative characteristic of the dissociation of electrolytes is called degree of dissociation and marked? , always? > 0.
For strong electrolytes? = 1 by definition (the dissociation of such electrolytes is complete).
For weak electrolytes, the degree of dissociation is the ratio of the molar concentration of the dissociated substance (s d) to the total concentration of the substance in solution (s):
The degree of dissociation is a fraction of unity or 100%. For weak electrolytes? « From 1 (100%).
For weak acids H n A, the degree of dissociation for each next step decreases sharply compared to the previous one:
The degree of dissociation depends on the nature and concentration of the electrolyte, as well as on the temperature of the solution; it grows with decrease the concentration of a substance in a solution (i.e., when the solution is diluted) and when heating.
AT diluted solutions strong acids H n A their hydroanions H n-1 A do not exist, for example:
B concentrated solutions, the content of hydroanions (and even initial molecules) becomes noticeable:
(it is impossible to sum the equations of the stages of reversible dissociation!). When heated value? 1 and? 2 increase, which promotes reactions involving concentrated acids.
Acids are electrolytes that, when dissociated, supply hydrogen cations to an aqueous solution and do not form any other positive ions:
Common strong acids:
In a dilute aqueous solution (conditionally up to 10% or 0.1 molar), these acids dissociate completely. For strong acids H n A, the list includes them hydroanions(anions of acid salts), which also dissociate completely under these conditions.
Common weak acids:
Bases are electrolytes that, when dissociated, supply hydroxide ions to an aqueous solution and do not form any other negative ions:
Dissociation sparingly soluble bases Mg (OH) 2, Cu (OH) 2, Mn (OH) 2, Fe (OH) 2 and others practical value does not have.
To strong grounds ( alkalis) include NaOH, KOH, Ba(OH) 2 and some others. The best known weak base is ammonia hydrate NH 3 H 2 O.
Medium salts are electrolytes that, upon dissociation, supply any cations, except H +, and any anions, except OH -, to an aqueous solution:
We are talking only about highly soluble salts. Dissociation sparingly soluble and practically insoluble salt doesn't matter.
Dissociate similarly double salts:
Acid salts(most of them are soluble in water) dissociate completely according to the type of medium salts:
The resulting hydroanions are, in turn, exposed to water:
a) if the hydroanion belongs strong acid, then it itself also dissociates completely:
and the full dissociation equation can be written as:
(solutions of such salts will necessarily be acidic, as well as solutions of the corresponding acids);
b) if the hydroanion belongs weak acid, then its behavior in water is dual - either incomplete dissociation as a weak acid:
or interaction with water (called reversible hydrolysis):
At? 1 > ? 2 dissociation predominates (and the salt solution will be acidic), and when? 1 > ? 2 - hydrolysis (and the salt solution will be alkaline). So, solutions of salts with anions HSO 3 -, H 2 PO 4 -, H 2 AsO 4 - and HSeO 3 - will be acidic, solutions of salts with other anions (most of them) will be alkaline. In other words, the name "acidic" for salts with the majority of hydroanions does not imply that these anions will behave like acids in solution (hydrolysis of hydroanions and the calculation of the ratio between α 1 and a 2 are studied only in higher education).
Basic salts of MgCl(OH), Cu 2 CO 3 (OH) 2 and others are mostly practically insoluble in water, and it is impossible to discuss their behavior in an aqueous solution.
13.3. dissociation of water. Solution medium
The water itself is very weak electrolyte:
The concentrations of the H + cation and the OH anion - in pure water are very small and amount to 1 10 -7 mol / l at 25 °C.
The hydrogen cation H + is the simplest nucleus - the proton p+ (electron shell cation H + - empty, 1s 0). A free proton has high mobility and penetrating power; surrounded by polar H 2 O molecules, it cannot remain free. The proton immediately attaches to the water molecule:
In the future, for simplicity, the notation H + is left (but H 3 O + is implied).
Types media of aqueous solutions:
For water at room temperature we have:
Therefore, in pure water:
This equality is also valid for aqueous solutions:
The practical pH scale corresponds to the interval 1-13 (dilute solutions of acids and bases):
In a practically neutral medium with pH = 6–7 and pH = 7–8, the concentration of H + and OH - is very low (1 10 -6 - 1 10 -7 mol / l) and is almost equal to the concentration of these ions in pure water. Such solutions of acids and bases are considered extremely diluted (contain very little substance).
For the practical establishment of the type of medium of aqueous solutions, indicators Substances that give a characteristic color to neutral, acidic and/or alkaline solutions.
Common indicators in the laboratory are litmus, methyl orange, and phenolphthalein.
Methyl orange (an indicator for an acidic environment) becomes pink in a strongly acidic solution (Table 16), phenolphthalein (an indicator for an alkaline environment) - raspberry in a strongly alkaline solution, and litmus is used in all environments.
13.4. Ion exchange reactions
In dilute solutions of electrolytes (acids, bases, salts) chemical reactions usually occur with the participation ions. In this case, all elements of the reagents can retain their oxidation states ( exchange reactions) or change them redox reactions). The examples given below refer to exchange reactions (for the occurrence of redox reactions, see Section 14).
In accordance with Berthollet's ruleionic reactions proceed almost irreversibly if poorly soluble solid substances are formed(they fall out) volatile substances(they are released as gases) or soluble substances are weak electrolytes(including water). Ionic reactions are represented by a system of equations - molecular, full and short ionic. The full ionic equations are omitted below (the reader is invited to make up their own).
When writing the equations of ionic reactions, it is necessary to be guided by the solubility table (see Table 8).
Examples precipitation reactions:
Attention! The slightly soluble (“m”) and practically insoluble (“n”) salts indicated in the solubility table (see Table 15) precipitate exactly as they are presented in the table (СаF 2 v, PbI 2 v, Ag 2 SO 4 v, AlPO 4 v, etc.).
In table. 15 not listed carbonates- medium salts with the anion CO 3 2-. It should be borne in mind that:
1) K 2 CO 3, (NH 4) 2 CO 3 and Na 2 CO 3 are soluble in water;
2) Ag 2 CO 3, BaCO 3 and CaCO 3 are practically insoluble in water and precipitate as such, for example:
3) salts of other cations, such as MgCO 3 , CuCO 3 , FeCO 3 , ZnCO 3 and others, although insoluble in water, do not precipitate from an aqueous solution during ionic reactions (i.e., they cannot be obtained by this method).
For example, iron (II) carbonate FeCO 3 obtained "dry" or taken in the form of a mineral siderite, when introduced into water, it precipitates without visible interaction. However, when trying to obtain it by an exchange reaction in a solution between FeSO 4 and K 2 CO 3, a precipitate of the basic salt precipitates (a conditional composition is given, in practice the composition is more complex) and carbon dioxide is released:
Similar to FeCO 3 , sulfide chromium (III) Cr 2 S 3 (insoluble in water) does not precipitate from solution:
In table. 15 also does not indicate the salts that decompose water - sulfide aluminum Al 2 S 3 (as well as BeS) and acetate chromium (III) Cr (CH 3 COO) 3:
Consequently, these salts also cannot be obtained by the exchange reaction in solution:
(in the last reaction, the composition of the precipitate is more complex; such reactions are studied in more detail in higher education).
Examples reactions with evolution of gases:
Examples reactions with the formation of weak electrolytes:
If the reactants and products of the exchange reaction are not strong electrolytes, there is no ionic form of the equation, for example:
13.5. Salt hydrolysis
Salt hydrolysis is the interaction of its ions with water, leading to the appearance of an acidic or alkaline environment, but not accompanied by the formation of a precipitate or gas (we are talking about medium salts below).
The hydrolysis process proceeds only with the participation soluble salt and consists of two stages:
1) dissociation salt in solution irreversible reaction (degree of dissociation? = 1, or 100%);
2) actually hydrolysis, i.e. the interaction of salt ions with water, - reversible reaction (degree of hydrolysis?< 1, или 100 %).
The equations of the 1st and 2nd stages - the first of them is irreversible, the second is reversible - cannot be added!
Note that salts formed by cations alkalis and anions strong acids do not undergo hydrolysis, they only dissociate when dissolved in water. In solutions of KCl, NaNO 3, Na 2 SO 4 and BaI 2 salts, the medium neutral.
In case of interaction anion salt hydrolysis at the anion.
The dissociation of the KNO 2 salt proceeds completely, the hydrolysis of the NO 2 anion - to a very small extent (for a 0.1 M solution - by 0.0014%), but this is enough for the solution to become alkaline(among the hydrolysis products there is an OH - ion), it has pH = 8.14.
Anions undergo hydrolysis only weak acids (in this example- nitrite ion NO 2 - , corresponding to a weak nitrous acid HNO 2). The anion of a weak acid attracts the hydrogen cation present in water to itself and forms a molecule of this acid, while the hydroxide ion remains free:
List of hydrolyzable anions:
Please note that in examples (c - e) it is impossible to increase the number of water molecules and instead of hydroanions (HCO 3 -, HPO 4 2-, HS -) write the formulas of the corresponding acids (H 2 CO 3, H 3 PO 4, H 2 S ). Hydrolysis is a reversible reaction, and it cannot proceed “to the end” (until the formation of acid H n A).
If such an unstable acid as H 2 CO 3 were formed in a solution of its salt Na 2 CO 3, then CO 2 gas would be released from the solution (H 2 CO 3 \u003d CO 2 v + H 2 O). However, when soda is dissolved in water, a transparent solution is formed without gas evolution, which is evidence of the incompleteness of the hydrolysis of the CO| anion. with the appearance in the solution of only the hydroanion of carbonic acid HCOg.
The degree of salt hydrolysis by anion depends on the degree of dissociation of the hydrolysis product - acid (HNO 2, HClO, HCN) or its hydroanion (HCO 3 -, HPO 4 2-, HS -); the weaker the acid, the higher the degree of hydrolysis. For example, ions CO 3 2-, PO 4 3- and S 2- undergo hydrolysis to a greater extent (in 0.1 M solutions ~ 5%, 37% and 58%, respectively) than the NO 2 ion, since the dissociation of H 2 CO 3 and H 2 S in the 2nd stage, and H 3 PO 4 in the 3rd stage (i.e., the dissociation of HCO 3 -, HS - and HPO 4 2- ions) proceeds much less than the dissociation of acid HNO 2 . Therefore, solutions, for example, Na 2 CO 3, K 3 PO 4 and BaS will highly alkaline(which is easy to verify by the soapiness of the soda solution to the touch). An excess of OH ions in a solution is easy to detect with an indicator or measure with special instruments (pH meters).
If aluminum is introduced into a concentrated solution of a salt that is highly hydrolyzed by anion, for example Na 2 CO 3 , then the latter (due to amphotericity) will react with OH -
and hydrogen evolution will be observed. This is additional evidence of the hydrolysis of the CO 3 2- ion (after all, we did not add NaOH alkali to the Na 2 CO 3 solution!).
In case of interaction cation dissolved salt with water the process is called salt hydrolysis by cation:
The dissociation of the Ni(NO 3) 2 salt proceeds completely, the hydrolysis of the Ni 2+ cation proceeds to a very small extent (for a 0.1 M solution, by 0.001%), but this is enough for the solution to become sour(among the hydrolysis products there is an H + ion), it has pH = 5.96.
Only cations of poorly soluble basic and amphoteric hydroxides and the ammonium cation NH 4 + undergo hydrolysis. The hydrolysable cation attracts the OH - anion present in the water and forms the corresponding hydroxocation, while the H + cation remains free:
The ammonium cation in this case forms a weak base - ammonia hydrate:
List of hydrolyzable cations:
Examples:
Please note that in examples (a - c) it is impossible to increase the number of water molecules and instead of hydroxocations FeOH 2+, CrOH 2+, ZnOH + write the formulas of FeO (OH), Cr (OH) 3, Zn (OH) 2 hydroxides. If hydroxides were formed, then precipitates would fall out of solutions of FeCl 3, Cr 2 (SO 4) 3 and ZnBr 2 salts, which is not observed (these salts form transparent solutions).
An excess of H + cations is easy to detect with an indicator or measure with special instruments. You can also
do such an experience. In a concentrated solution of a salt that is highly hydrolyzed by cation, for example AlCl 3:
magnesium or zinc is added. The latter will react with H +:
and hydrogen evolution will be observed. This experiment is additional evidence of the hydrolysis of the Al 3+ cation (because we did not add acid to the AlCl 3 solution!).
Examples of tasks of parts A, B1. A strong electrolyte is
1) C 6 H 5 OH
2) CH 3 COOH
3) C 2 H 4 (OH) 2
2. Weak electrolyte is
1) hydrogen iodide
2) hydrogen fluoride
3) ammonium sulfate
4) barium hydroxide
3. In an aqueous solution of every 100 molecules, 100 hydrogen cations are formed for an acid
1) coal
2) nitrogenous
3) nitrogen
4-7. In the equation for the dissociation of a weak acid over all possible steps
the sum of the coefficients is
8-11. For the equations of dissociation in a solution of two alkalis of the set
8. NaOH, Ba (OH) 2
9. Sr (OH) 2, Ca (OH) 2
10. KOH, LiOH
11. CsOH, Ca (OH) 2
the total sum of the coefficients is
12. Lime water contains a set of particles
1) CaOH +, Ca 2+, OH -
2) Ca 2+, OH -, H 2 O
3) Ca 2+, H 2 O, O 2-
4) CaOH +, O 2-, H +
13-16. With the dissociation of one formula unit of salt
14. K 2 Cr 2 O 7
16. Cr 2 (SO 4) 3
the number of ions formed is
17. Greatest the amount of PO 4 -3 ion can be found in a solution containing 0.1 mol
18. Precipitation reaction is
1) MgSO 4 + H 2 SO 4 >…
2) AgF + HNO 3 >…
3) Na 2 HPO 4 + NaOH >…
4) Na 2 SiO 3 + HCl >…
19. The reaction with the release of gas is
1) NaOH + CH 3 COOH >…
2) FeSO 4 + KOH >…
3) NaHCO 3 + HBr >…
4) Pl(NO 3) 2 + Na 2 S>…
20. Brief ionic equation OH - + H + = H 2 O corresponds to the interaction
1) Fe(OH) 2 + HCl >…
2) NaOH + HNO 2 >…
3) NaOH + HNO 3 >…
4) Ba (OH) 2 + KHSO 4 > ...
21. In the ionic reaction equation
SO 2 + 2OH = SO 3 2- + H 2 O
OH ion - can respond to the reagent
4) C 6 H 5 OH
22-23. Ionic equation
22. ZCa 2+ + 2PO 4 3- \u003d Ca 3 (PO 4) 2 v
23. Ca 2+ + HPO 4 2- \u003d CaHRO 4 v
corresponds to the reaction between
1) Ca (OH) 2 and K 3 PO 4
2) CaCl 2 and NaH 2 PO 4
3) Ca (OH) 2 and H 3 RO 4
4) CaCl and K 2 HPO 4
24-27. In the molecular reaction equation
24. Na 3 PO 4 + AgNO 3 >…
25. Na 2 S + Cu (NO 3) 2 > ...
26. Ca(HSO 3) 2 >…
27. K 2 SO 3 + 2HBr >… the sum of the coefficients is
28-29. For a complete neutralization reaction
28. Fe(OH) 2 + HI >…
29. Ba (OH) 2 + H 2 S > ...
the sum of the coefficients in the full ionic equation is
30-33. In the short ionic reaction equation
30. NaF + AlCl 3 >…
31. K 2 CO 3 + Sr (NO 3) 2 > ...
32. Mgl 2 + K 3 PO 4 > ...
33. Na 2 S + H 2 SO 4 > ...
the sum of the coefficients is
34-36. In an aqueous solution of salt
34. Ca(ClO 4) 2
36. Fe 2 (SO 4) 3
environment is formed
1) acidic
2) neutral
3) alkaline
37. The concentration of hydroxide ion increases after salt is dissolved in water.
38. The neutral medium will be in the final solution after mixing the solutions of the initial salts in the sets
1) BaCl 2, Fe (NO 3) 3
2) Na 2 CO 3, SrS
4) MgCl 2 , RbNO 3
39. Establish a correspondence between salt and its ability to hydrolyze.
40. Establish a correspondence between salt and solution medium.
41. Establish a correspondence between salt and the concentration of the hydrogen cation after the salt is dissolved in water.
Today we will talk about the substance - water!
Have any of you seen water?
Did the question seem ridiculous to you? But it refers to completely pure water, in which there are no impurities. To be honest and accurate in the answer, you will have to admit that neither I nor you have seen such water yet. That is why on a glass of water after the inscription "H 2 O" there is a question mark. So, there is not pure water in the glass, but what then?
Gases dissolved in this water: N 2, O 2, CO 2, Ar, salts from the soil, iron cations from water pipes. In addition, the smallest particles of dust are suspended in it. That's what we call h and s t o y water! Many scientists are working on solving a difficult problem - to get absolutely clean water. But so far it has not been possible to obtain such ultrapure water. However, you may object that there is distilled water. By the way, what is she?
In fact, we get such water when we sterilize the jars before canning. Turn the jar upside down and place it over boiling water. Droplets appear on the bottom of the jar, this is distilled water. But as soon as we turn the jar over, gases from the air enter it, and again there is a solution in the jar. Therefore, competent housewives try to fill the jars with the necessary contents immediately after sterilization. They say that the products in this case will be stored longer. Perhaps they are right. Feel free to experiment! Precisely because water is capable of dissolving various substances in itself, scientists still cannot obtain ideally pure water in large volumes. And it would be so useful, for example, in medicine for the preparation of medicines.
By the way, being in a glass, water "dissolves" the glass. Therefore, the thicker the glass, the longer the glasses will last. What is sea water?
This is a solution that contains many substances. For example, table salt. How can you single out table salt from sea water?
Evaporation. By the way, this is exactly what our ancestors did. There were salt pans in Onega, where salt was evaporated from sea water. Salt was sold to Novgorod merchants, they bought expensive jewelry and chic fabrics for their brides and wives. Even the Moscow fashionistas did not have such outfits as the Pomoroks. And all only thanks to the knowledge of the properties of solutions! So, today we are talking about solutions and solubility. Write down the definition of the solution in your notebook.
A solution is a homogeneous system consisting of solvent and solute molecules, between which physical and chemical interactions occur.
Consider schemes 1–2 and analyze what solutions are.
Which solution would you prefer when making soup? Why?
Determine where is the dilute solution, where is the concentrated solution of copper sulphate?
If a certain volume of a solution contains little solute, then such a solution is called diluted, if a lot - concentrated
.
Determine which solution is where?
Do not confuse the concepts of "saturated" and "concentrated" solution, "unsaturated" and "dilute" solution.
Some substances dissolve well in water, others little, and still others do not dissolve at all. Watch the video "SOLUBILITY OF SOLIDS IN WATER"
Complete the task in the notebook: Distribute the proposed substances -CO 2, H 2, O 2 , H 2 SO 4 , Vinegar, NaCl, Chalk, Rust, Vegetable oil, Alcoholinto the empty columns of table 1, using your life experience.
Table 1
|
Dissolved |
Substance examples |
|
|
Soluble |
Slightly soluble |
|
|
Gas |
||
|
Liquid |
||
|
Solid |
||
Can you tell me about the solubility FeSO4?
How to be?
In order to determine the solubility of substances in water, we will use the table of the solubility of salts, acids and bases in water. It is in the attachments to the lesson.
In the top row of the table are cations, in the left column are anions; we are looking for an intersection point, we look at the letter - this is solubility.
Let's determine the solubility of salts: AgNO 3 , AgCl, CaSO 4 .
Solubility increases with increasing temperature (there are exceptions). You know perfectly well that it is more convenient and faster to dissolve sugar in hot, and not in cold water. See "Thermal Phenomena in Dissolution"
Try it yourself, using the table, to determine the solubility of substances.
Exercise. Determine the solubility of the following substances: AgNO 3 , Fe (OH) 2 , Ag 2 SO 3 , Ca (OH) 2 , CaCO 3 , MgCO 3 , KOH.
DEFINITIONS on the topic "Solutions"
Solution- a homogeneous system consisting of solvent and solute molecules, between which physical and chemical interactions occur.
saturated solution A solution in which a given substance no longer dissolves at a given temperature.
unsaturated solution A solution in which a substance can still dissolve at a given temperature.
suspensioncalled a suspension in which small particles of solid matter are evenly distributed among water molecules.
emulsioncalled a suspension in which small droplets of a liquid are distributed among the molecules of another liquid.
dilute solutions - solutions with a small content of dissolved substance.
concentrated solutions - solutions with a high content of solute.
ADDITIONALLY:
According to the ratio of the predominance of the number of particles passing into the solution or removed from the solution, solutions are distinguished saturated, unsaturated and supersaturated. According to the relative amounts of solute and solvent, solutions are divided into diluted and concentrated.
A solution in which a given substance at a given temperature no longer dissolves, i.e. a solution in equilibrium with a solute is called rich, and a solution in which an additional amount of a given substance can still be dissolved, - unsaturated.
A saturated solution contains the maximum possible (for given conditions) amount of solute. Therefore, a saturated solution is one that is in equilibrium with an excess of solute. The concentration of a saturated solution (solubility) for a given substance under strictly certain conditions(temperature, solvent) is a constant value.
A solution containing more solute than it should be under the given conditions in a saturated solution is called supersaturated. Supersaturated solutions are unstable, non-equilibrium systems in which a spontaneous transition to an equilibrium state is observed. In this case, an excess of the solute is released, and the solution becomes saturated.
Saturated and unsaturated solutions should not be confused with dilute and concentrated solutions. dilute solutions- solutions with a small content of a dissolved substance; concentrated solutions- solutions with a high content of solute. It must be emphasized that the concepts of dilute and concentrated solutions are relative, expressing only the ratio of the amounts of a solute and solvent in a solution.Solubility is the property of a substance to form homogeneous mixtures with various solvents. As we already mentioned, the amount of solute required to obtain a saturated solution determines this substance. In this regard, solubility has the same measure as composition, for example, the mass fraction of a solute in its saturated solution or the amount of a solute in its saturated solution.
All substances in terms of their solubility can be classified into:
- Highly soluble - more than 10 g of the substance can dissolve in 100 g of water.
- Slightly soluble - less than 1 g of the substance can dissolve in 100 g of water.
- Insoluble - less than 0.01 g of the substance can dissolve in 100 g of water.
It is known that if polarity solute is similar to the polarity of the solvent, it is more likely to dissolve. If the polarities are different, then with a high degree of probability the solution will not work. Why is this happening?
polar solvent is a polar solute.
Let's take a solution of common salt in water as an example. As we already know, water molecules are polar in nature with a partial positive charge on each hydrogen atom and a partial negative charge on the oxygen atom. And ionic solids, like sodium chloride, contain cations and anions. So when table salt is placed in water, the partial positive charge on the hydrogen atoms of the water molecules is attracted to the negatively charged chloride ion in NaCl. Similarly, the partial negative charge on the oxygen atoms of water molecules is attracted by the positively charged sodium ion in NaCl. And, since the attraction of water molecules for sodium and chlorine ions is stronger than the interaction that holds them together, the salt dissolves.
Non-polar solvent is a non-polar solute.
Let's try to dissolve a piece of carbon tetrabromide in carbon tetrachloride. In the solid state, carbon tetrabromide molecules are held together by a very weak dispersion interaction. When placed in carbon tetrachloride, its molecules will be arranged more randomly, i.e. the entropy of the system increases and the compound dissolves.
Equilibria in dissolution
Consider a solution of a poorly soluble compound. In order for equilibrium to be established between a solid and its solution, the solution must be saturated and in contact with the undissolved portion of the solid.
For example, let the equilibrium be established in a saturated solution of silver chloride:
AgCl (tv) \u003d Ag + (aq.) + Cl - (aq.)
The compound in question is ionic and is present in dissolved form as ions. We already know that in heterogeneous reactions the concentration of a solid remains constant, which allows us to include it in the equilibrium constant. So the expression for will look like this:
K = [ Cl - ]
Such a constant is called solubility product PR, provided that the concentrations are expressed in mol/L.
PR \u003d [ Cl - ]
Solubility product is equal to the product of the molar concentrations of the ions participating in the equilibrium, in powers equal to the corresponding stoichiometric coefficients in the equilibrium equation.
It is necessary to distinguish between the concept of solubility and the product of solubility. The solubility of a substance can change when another substance is added to the solution, and the solubility product does not depend on the presence of additional substances in the solution. Although these two values are interconnected, which allows knowing one value to calculate the other.
Solubility as a function of temperature and pressure
Water plays an important role in our life, it is able to dissolve a large number of substances that have great importance for us. Therefore, we will focus on aqueous solutions.
Solubility gases increases with rising pressure gas above the solvent, and the solubility of solid and liquid substances depends on pressure insignificantly.
William Henry first came to the conclusion that the amount of gas that dissolves at a constant temperature in a given volume of liquid is directly proportional to its pressure. This statement is known as Henry's law and is expressed as follows:
C \u003d k P,
where C is the solubility of the gas in the liquid phase
P - gas pressure over the solution
k is Henry's constant
The following figure shows the solubility curves of some gases in water temperature at constant gas pressure over the solution (1 atm)
As can be seen, the solubility of gases decreases with increasing temperature, in contrast to most ionic compounds, the solubility of which increases with increasing temperature.
Effect of temperature on solubility depends on the change in enthalpy that occurs during the dissolution process. When an endothermic process occurs, the solubility increases with increasing temperature. This follows from what we already know : if you change one of the conditions under which the system is in equilibrium - concentration, pressure or temperature - then the equilibrium will shift in the direction of the reaction that counteracts this change.
Imagine that we are dealing with a solution in equilibrium with a partially dissolved substance. And this process is endothermic, i.e. goes with the absorption of heat from outside, then:
Substance + solvent + heat = solution
According to principle of Le Chatelier, at endothermic process, the equilibrium shifts in the direction that reduces the heat input, i.e. to the right. Thus, the solubility increases. If the process exothermic, then an increase in temperature leads to a decrease in solubility.
dependence of the solubility of ionic compounds on temperature It is known that there are solutions of liquids in liquids. Some of them can dissolve in each other in unlimited quantities, like water and ethyl alcohol, while others can only partially dissolve. So, if you try to dissolve carbon tetrachloride in water, then two layers are formed: the upper one is a saturated solution of water in carbon tetrachloride and the lower one is a saturated solution of carbon tetrachloride in water. As the temperature rises, in general, the mutual solubility of such liquids increases. This happens until the critical temperature is reached, at which both liquids are mixed in any proportions. The solubility of liquids is practically independent of pressure.
When a substance that can be dissolved in either of these two liquids is introduced into a mixture consisting of two immiscible liquids, its distribution between these liquids will be proportional to the solubility in each of them. Those. according to distribution law a substance that can dissolve in two immiscible solvents is distributed between them so that the ratio of its concentrations in these solvents at a constant temperature remains constant, regardless of the total amount of the solute:
C 1 / C 2 \u003d K,
where C 1 and C 2 are the concentrations of a substance in two liquids
K is the distribution coefficient.
Categories ,Some substances dissolve better in a particular solvent, others worse. It is believed that absolutely insoluble substances do not exist. Every substance is capable of solubility, even if in some cases in very small quantities (for example, mercury in water, benzene in water).
Unfortunately, to date, there is no theory by which one could predict and calculate the solubility of any substance in the corresponding solvent. This is due to the complexity and diversity of the interaction of the components of the solution with each other and the lack of a general theory of solutions (especially concentrated ones). In this regard, the necessary data on the solubility of substances are obtained, as a rule, empirically.
Quantitatively, the ability of a substance to dissolve is most often characterized by solubility or solubility coefficient (S).
Solubility (S) shows how many grams of a substance can dissolve as much as possible under given conditions (temperature, pressure) in 100 g of a solvent to form a saturated solution.
If necessary, the solubility coefficient is also determined for a different amount of solvent (for example, for 1000 g, 100 cm 3 , 1000 cm 3 , etc.).
By solubility, all substances, depending on their nature, are divided into 3 groups: 1) highly soluble; 2) slightly soluble; 3) poorly soluble or insoluble.
The solubility coefficient for substances of the first group is more than 1 g (per 100 g of solvent), for substances of the second group lies in the range of 0.01 - 1.0 g and for substances of the third group S< 0,01 г.
The solubility of substances is influenced by many factors, the main of which are the nature of the solvent and the solute, temperature, pressure, and the presence of other substances (especially electrolytes) in the solution.
Influence of the nature of substances on solubility
It has been experimentally established that in a solvent whose molecules are polar, substances formed by ionic or covalent polar bonds dissolve best. And in a solvent whose molecules are non-polar, substances formed by weakly polar or non-polar covalent bonds dissolve better. In another way, this revealed regularity can be formulated as follows: "Like dissolves into like."
The solubility of substances is largely determined by the strength and nature of their interaction with solvent molecules. The more pronounced this interaction, the greater the solubility and vice versa.
It is known that the forces acting between nonpolar and weakly polar molecules are small and nonspecific; in quantitative terms do not significantly depend on the type of substance.
If similar nonpolar molecules A are introduced into a nonpolar liquid B, then the energy of interaction between particles A and B will not differ significantly from the energy of interaction between particles A and A or particles B and B. Therefore, just as any amount of the same substance is mixed , with a high probability will mix with each other indefinitely (i.e., dissolve in each other) and various non-polar liquids.
For the same reason, molecular crystals usually dissolve better in non-polar liquids.
If the interaction energy of molecules A and A or B and B is greater than A and B, then identical molecules of each component will preferentially bind to each other and their solubility in each other will decrease (Table 6).
The polarity of any solvent is often characterized by the value of its permittivity (ε), which is easily determined empirically. The larger it is, the more polar the substance is.
Table 6. Solubility of KI (wt%) in solvents of different polarity
Solutions play a key role in nature, science and technology. Water is the basis of life, always contains dissolved substances. Fresh water of rivers and lakes contains few dissolved substances, while sea water contains about 3.5% of dissolved salts.
The primordial ocean (during the birth of life on Earth) is thought to have contained only 1% dissolved salts.
“It was in this environment that living organisms first developed, from this solution they scooped up the ions and molecules that are necessary for their further growth and development ... Over time, living organisms developed and transformed, so they were able to leave the aquatic environment and move to land and then rise to air. They obtained these abilities by preserving in their organisms an aqueous solution in the form of liquids that contain a vital supply of ions and molecules, ”the famous American chemist, Nobel Prize winner Linus Pauling describes the role of solutions in nature in these words. Inside each of us, in every cell of our body, there are memories of the primordial ocean, the place where life originated, an aqueous solution that provides life itself.
In any living organism, an unusual solution constantly flows through the vessels - arteries, veins and capillaries, which forms the basis of blood, the mass fraction of salts in it is the same as in the primary ocean - 0.9%. Complex physicochemical processes occurring in the human and animal body also interact in solutions. The process of assimilation of food is associated with the transfer of highly nutritious substances into solution. Natural aqueous solutions are directly related to the processes of soil formation, plant supply nutrients. Such technological processes in the chemical and many other industries, such as the production of fertilizers, metals, acids, paper, occur in solutions. modern science studies the properties of solutions. Let's find out what is a solution?
Solutions differ from other mixtures in that the particles constituent parts are located in them evenly, and in any microvolume of such a mixture, the composition will be the same.
That is why solutions were understood as homogeneous mixtures, which consist of two or more homogeneous parts. This idea was based on the physical theory of solutions.
Adherents of the physical theory of solutions, which van't Hoff, Arrhenius and Ostwald were engaged in, believed that the dissolution process is the result of diffusion.
D. I. Mendeleev and supporters of the chemical theory believed that dissolution is the result of the chemical interaction of a solute with water molecules. Thus, it will be more accurate to define a solution as a homogeneous system that consists of particles of a solute, a solvent, and also the products of their interaction.
Due to the chemical interaction of a solute with water, compounds are formed - hydrates. Chemical interaction is usually accompanied by thermal phenomena. For example, the dissolution of sulfuric acid in water takes place with the release of such an enormous amount of heat that the solution can boil, which is why acid is poured into water, and not vice versa. The dissolution of substances such as sodium chloride, ammonium nitrate, accompanied by the absorption of heat.
M. V. Lomonosov proved that solutions turn into ice at a lower temperature than the solvent.
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