Even at the dawn of the study of electrical phenomena, scientists noticed that not only metals, but also solutions can conduct current. But not everyone. Yes, aqueous solutions table salt and other salts, solutions of strong acids and alkalis conduct electricity well. Solutions of acetic acid, carbon dioxide and sulfur dioxide conduct it much worse. But solutions of alcohol, sugar and most other organic compounds do not conduct electricity at all.
Electric current is the directed movement of free charged particles . In metals, such movement is carried out due to relatively free electrons, electron gas. But not only metals are able to conduct electricity.
Electrolytes are substances whose solutions or melts conduct electricity.
Non-electrolytes are substances whose solutions or melts do not conduct electricity.
To describe the electrical conductivity of some solutions, it is necessary to understand what a solution is. By the end of the 19th century, there were 2 main theories of solutions
· Physical. According to this theory, a solution is a purely mechanical mixture of components and there is no interaction between particles in it. She described the properties of electrolytes well, but had certain difficulties in describing electrolyte solutions.
· Chemical. According to this theory, when dissolved, a chemical reaction occurs between the solute and the solvent. This is confirmed by the presence of a thermal effect during dissolution, as well as a change in color. For example, when white anhydrous copper sulfate is dissolved, a saturated blue solution is formed.
The truth is between these two. extreme points. Namely , both chemical and physical processes take place in solutions.
In 1887, the Swedish physical chemist Svante Arrhenius, investigating the electrical conductivity of aqueous solutions, suggested that in such solutions substances break up into charged particles - ions that can move to the electrodes - a negatively charged cathode and a positively charged anode.
This is the reason electric current in solutions. This process is called electrolytic dissociation (literal translation - splitting, decomposition under the influence of electricity). This name also suggests that dissociation occurs under the action of an electric current. Further research has shown that this is not the case: ions are onlycharge carriers in solution and exist in it regardless of whether it passes throughsolution current or not. With the active participation of Svante Arrhenius, the theory of electrolytic dissociation was formulated, which is often named after this scientist. The main idea of this theory is that electrolytes under the action of a solvent spontaneously decompose into ions. And it is these ions that are charge carriers and are responsible for the electrical conductivity of the solution.
1. Electrolytes in solutions under the action of a solvent spontaneously decompose into ions. Such a process is called electrolytic dissociation. Dissociation can also take place during the melting of solid electrolytes.
2. Ions differ from atoms in composition and properties. IN aqueous solutions ions are in a hydrated state. Ions in the hydrated state differ in properties from ions in gaseous state substances. This is explained as follows: ionic compounds already initially contain cations and anions. When dissolved, the water molecule begins to approach the charged ions - the positive pole to the negative ion, the negative pole to the positive. Ions are called hydrated.
3. In solutions or melts of electrolytes, ions move randomly, but when an electric current is passed, the ions move in a direction: cations - to the cathode, anions - to the anode.
In light of the theory of electrolytic dissociation, bases, acids and salts can be defined as electrolytes.
Foundations- these are electrolytes, as a result of the dissociation of which in aqueous solutions, only one type of anion is formed: hydroxide anion: OH -
NaOH ↔ Na + + OH −
The dissociation of bases containing several hydroxyl groups occurs in steps.
Ba(OH) 2 ↔ Ba(OH) + + OH − First step
Ba(OH) + ↔ Ba 2+ + 2OH − Second stage
Ba(OH) 2 ↔ Ba 2+ + 2 OH − Summary equation
acids- these are electrolytes, as a result of the dissociation of which in aqueous solutions, only one type of cation is formed: H +, It is the hydrated proton that is called the hydrogen ion and denoted H 3 O +, but for simplicity we write H +
HNO 3 ↔ H + + NO 3 -
Polybasic acids dissociate in steps
H 3 PO 4 ↔ H + + H 2 PO 4 - First stage:
H 2 PO 4 - ↔ H + + HPO 4 2- Second step:
HPO 4 2- ↔ H + + PO 4 3- Third stage:
H 3 PO 4 ↔ 3H + + PO 4 3- Summary equation
salt- These are electrolytes that dislocate in aqueous solutions into metal cations and anions of the acid residue.
Na 2 SO 4 ↔ 2Na + + SO 4 2−
Medium salts these are electrolytes that dissociate in aqueous solutions into metal cations or ammonium cations, and acid residue anions.
Basic salts- These are electrolytes that dissociate in aqueous solutions into metal cations, hydroxide anions and anions of the acid residue.
Acid salts these are electrolytes that dissociate in aqueous solutions into metal cations, hydrogen cations and anions of the acid residue.
double salts- These are electrolytes that dissociate in aqueous solutions into cations of several metals and anions of the acid residue.
KAl (SO 4) 2 ↔ K + + Al 3+ + 2SO 4 2
mixed salts are electrolytes that dissociate in aqueous solutions into metal cations and anions of several acidic residues
Electrolytic dissociation is to some extent a reversible process. But when some compounds are dissolved, the dissociation equilibrium is largely shifted towards the dissociated form. In solutions of such electrolytes, dissociation proceeds almost irreversibly. Therefore, when writing the equations of dissociation of such substances, either an equal sign or a straight arrow is written, indicating that the reaction occurs almost irreversibly. Such substances are called strong electrolytes.
Weak called electrolytes, in which dissociation occurs insignificantly. When writing, the sign of reversibility is used. Tab.1.
For a quantitative assessment of the strength of the electrolyte, the concept is introduced degree of electrolytic dissociation .
The strength of an electrolyte can also be expressed using constants chemical equilibrium dissociation. It is called the dissociation constant.
Factors affecting the degree of electrolytic dissociation:
The nature of the electrolyte
The concentration of the electrolyte in the solution
· Temperature
As the temperature increases and the solution is diluted, the degree of electrolytic dissociation increases. Therefore, the strength of the electrolyte can only be assessed by comparing them under the same conditions. The standard is t=18 0 C and c=0.1 mol/l.
STRONG ELECTROLYTES | WEAK ELECTROLYTES |
The degree of dissociation at 18 0 C in solutions with an electrolyte concentration of 0.1 mol/l is close to 100%. They dissociate almost irreversibly. | The degree of dissociation at 18 0 C in solutions with an electrolyte concentration of 0.1 mol/l is much less than 100%. Dissociation is irreversible. |
Some inorganic acids (HNO 3, HClO 4, HI, HCl, HBr, H 2 SO 4) | Metal hydroxides, except for IA and IIA groups, ammonia solution Many inorganic acids (H 2 S, HCN, HClO, HNO 2) Organic acids (HCOOH, CH 3 COOH) |
The essence of the reaction in electrolyte solutions is expressed by the ionic equation. It takes into account the fact that in one solution electrolytes are present in the form of ions. And weak electrolytes and non-dissociable substances are written in a form that can be dissociated into ions. The solubility of an electrolyte in water cannot be used as a measure of its strength. Many water-insoluble salts are strong electrolytes, but the concentration of ions in solution is very low precisely because of their low solubility. That is why, when writing reaction equations with the participation of such substances, it is customary to write them in the undissociated form .
Reactions in electrolyte solutions proceed in the direction of ion binding.
There are several forms of ion binding.
1. Precipitation
2. Gas evolution
3. Formation of a weak electrolyte.
· one. Sediment formation:
BaCl 2 + Na 2 CO 3 → BaCO 3 ↓ + 2NaCl.
Ba 2+ +2Cl - + 2Na + + CO 3 2- →BaCO 3 ↓ + 2Na + +2Cl - complete ionic equation
Ba 2+ + CO 3 2- → BaCO 3 ↓ abbreviated ionic equation.
The abbreviated ionic equation shows that the interaction of any soluble compound containing the Ba 2+ ion with a compound containing the carbonate anion CO 3 2 will result in an insoluble precipitate of BaCO 3 ↓.
2. Gas release.
Na 2 CO 3 + H 2 SO 4 → Na 2 SO 4 + H 2 O + CO 2
2Na + + CO 3 2- +2H + + SO 4 2 - → 2Na + + SO 4 2 - + H 2 O + CO 2 complete ionic equation
2H + + CO 3 2- → H 2 O + CO 2 abbreviated ionic equation.
3. Formation of a weak electrolyte
KOH + HBr → KBr + H2O
K + + OH - + H + + Br - → K + + Br - + H 2 O complete ionic equation
OH - + H + → H 2 O abbreviated ionic equation.
Considering these examples, we have seen that all reactions in electrolyte solutions occur in the direction of ion binding.
During the lesson, we will study the topic “Electrolytic dissociation. Ion exchange reactions. Consider the theory of electrolytic dissociation and get acquainted with the definition of electrolytes. Let's get acquainted with the physical and chemical theory of solutions. Consider, in the light of the theory of electrolytic dissociation, the definition of bases, acids and salts, and also learn how to write equations for ion exchange reactions and learn about the conditions for their irreversibility.
Topic: Solutions and their concentration, disperse systems, electrolytic dissociation
Lesson: Electrolytic dissociation. Ion exchange reactions
1. Physical and chemical theory of solutions
Even at the dawn of the study of electrical phenomena, scientists noticed that not only metals, but also solutions can conduct current. But not everyone. So, aqueous solutions of sodium chloride and other salts, solutions of strong acids and alkalis conduct current well. Solutions of acetic acid, carbon dioxide and sulfur dioxide conduct it much worse. But solutions of alcohol, sugar and most other organic compounds do not conduct electricity at all.
Electric current is the directed movement of free charged particles. In metals, such movement is carried out due to relatively free electrons, electron gas. But not only metals are able to conduct electricity.
electrolytes - These are substances whose solutions or melts conduct an electric current.
Non-electrolytes - These are substances whose solutions or melts do not conduct electricity.
To describe the electrical conductivity of some solutions, it is necessary to understand what a solution is. By the end of the 19th century, there were 2 main theories of solutions:
· Physical. According to this theory, the solution - it is a purely mechanical mixture of components, and there is no interaction between particles in it. She described the properties of electrolytes well, but had certain difficulties in describing electrolyte solutions.
· Chemical. According to this theory, when dissolved, a chemical reaction occurs between the solute and the solvent. This is confirmed by the presence of a thermal effect during dissolution, as well as a change in color. For example, when white anhydrous copper sulfate is dissolved, a saturated blue solution is formed.
The truth is between these two extremes. Namely, both chemical and physical processes take place in solutions.
Rice. 1. Svante Arrhenius
In 1887, the Swedish physical chemist Svante Arrhenius (Fig. 1), while studying the electrical conductivity of aqueous solutions, suggested that in such solutions substances decompose into charged particles - ions that can move to the electrodes - a negatively charged cathode and a positively charged anode.
This is the reason for the electric current in solutions. This process is called electrolytic dissociation (literal translation - splitting, decomposition under the influence of electricity). This name also suggests that dissociation occurs under the action of an electric current. Further studies showed that this is not so: ions are only charge carriers in solution and exist in it regardless of whether current passes through the solution or not. With the active participation of Svante Arrhenius, the theory of electrolytic dissociation was formulated, which is often named after this scientist. The main idea of this theory is that electrolytes under the action of a solvent spontaneously decompose into ions. And it is these ions that are charge carriers and are responsible for the electrical conductivity of the solution.
2. Main provisions of the theory of electrolytic dissociation
1. Electrolytes in solutions under the action of a solvent spontaneously decompose into ions. Such a process is called electrolytic dissociation. Dissociation can also take place during the melting of solid electrolytes.
2. Ions differ from atoms in composition and properties. In aqueous solutions, ions are in a hydrated state. Ions in the hydrated state differ in properties from ions in the gaseous state of matter. This is explained as follows: cations and anions are already initially present in ionic compounds. When dissolved, the water molecule begins to approach the charged ions: the positive pole - to negative ion, negative pole - to the positive. Ions are called hydrated (Fig. 2).
3. In solutions or melts of electrolytes, ions move randomly, but when an electric current is passed, the ions move in a direction: cations - towards the cathode, anions - to the anode.
3. Bases, acids, salts in the light of the theory of electrolytic dissociation
In the light of the theory of electrolytic dissociation, bases, acids and salts can be defined as electrolytes.
Foundations- these are electrolytes, as a result of the dissociation of which in aqueous solutions only one type of anion is formed: hydroxide anion: OH-.
NaOH ↔ Na+ + OH−
The dissociation of bases containing several hydroxyl groups occurs in steps:
Ba(OH)2↔ Ba(OH)+ + OH− First step
Ba(OH)+ ↔ Ba2+ + 2OH− Second step
Ba(OH)2↔ Ba2+ + 2 OH− Summary equation
acids - these are electrolytes, as a result of the dissociation of which in aqueous solutions only one type of cations is formed: H +. It is the hydrated proton that is called the hydrogen ion and is designated H3O+, but for simplicity it is written H+.
HNO3↔ H+ + NO3−
Polybasic acids dissociate in steps:
H3PO4↔ H+ + H2PO4- First stage
H2PO4- ↔ H+ + HPO42- Second stage
HPO42-↔ H+ + PO43- Third stage
H3PO4↔ 3H+ + PO43-Summary Equation
salt - these are electrolytes that dissociate in aqueous solutions into metal cations and anions of the acid residue.
Na2SO4 ↔ 2Na+ + SO42−
Medium salts - these are electrolytes that dissociate in aqueous solutions into metal cations or ammonium cations and anions of the acid residue.
Basic salts - these are electrolytes that dissociate in aqueous solutions into metal cations, hydroxide anions and anions of the acid residue.
Acid salts - these are electrolytes that dissociate in aqueous solutions into metal cations, hydrogen cations and anions of the acid residue.
double salts - these are electrolytes that dissociate in aqueous solutions into cations of several metals and anions of the acid residue.
KAl(SO4)2↔ K+ + Al3+ + 2SO42
mixed salts - these are electrolytes that dissociate in aqueous solutions into metal cations and anions of several acidic residues
4. Strong and weak electrolytes
Electrolytic dissociation to varying degrees - the process is reversible. But when some compounds are dissolved, the dissociation equilibrium is largely shifted towards the dissociated form. In solutions of such electrolytes, dissociation proceeds almost irreversibly. Therefore, when writing the equations of dissociation of such substances, either an equal sign or a straight arrow is written, indicating that the reaction occurs almost irreversibly. Such substances are called strong electrolytes.
Weak called electrolytes, in which dissociation occurs insignificantly. When writing, use the sign of reversibility. Tab. one.
For a quantitative assessment of the strength of the electrolyte, the concept is introduced degree of electrolyticdissociation.
The strength of an electrolyte can also be expressed using chemical equilibrium constants dissociation. It is called the dissociation constant.
Factors affecting the degree of electrolytic dissociation:
The nature of the electrolyte
The concentration of the electrolyte in the solution
· Temperature
As the temperature increases and the solution is diluted, the degree of electrolytic dissociation increases. Therefore, it is possible to evaluate the strength of an electrolyte only by comparing them under the same conditions. The standard is t = 180C and c = 0.1 mol/l.
5. Ion exchange reactions
The essence of the reaction in electrolyte solutions is expressed by the ionic equation. It takes into account the fact that in one solution electrolytes are present in the form of ions. And weak electrolytes and non-dissociable substances are written in a form that can be dissociated into ions. The solubility of an electrolyte in water cannot be used as a measure of its strength. Many water-insoluble salts are strong electrolytes, but the concentration of ions in solution is very low precisely because of their low solubility. That is why, when writing reaction equations with the participation of such substances, it is customary to write them in the undissociated form .
Reactions in electrolyte solutions proceed in the direction of ion binding.
There are several forms of ion binding:
1. Precipitation
2. Gas evolution
3. Formation of a weak electrolyte.
1. Sedimentation:
BaCl2 + Na2CO3 → BaCO3↓ + 2NaCl.
Ba2++2Cl - + 2Na++CO32-→ BaCO3↓ + 2Na++2Cl- complete ionic equation
Ba2+ + CO32-→ BaCO3↓ abbreviated ionic equation.
The abbreviated ionic equation shows that when any soluble compound containing the Ba2+ ion reacts with a compound containing the carbonate anion CO32-, the result is an insoluble precipitate of BaCO3↓.
2. Gas evolution:
Na2CO3 +H2SO4 → Na2SO4 + H2O + CO2&
The abbreviated ionic equation H + + OH - \u003d H 2 O corresponds to the interaction of nitric acid with:
1) sodium oxide
2) copper hydroxide
3) sodium hydroxide
Answer: 3
Explanation:
Nitric acid is a strong acid, therefore, almost all of its molecules dissociate into H + cations and NO 3 - anions. Strong water-soluble bases dissociate into hydroxide ions OH −, i.e. alkali. Of all the answers presented in the task, sodium hydroxide is suitable, which decomposes into Na + and OH - in an aqueous solution.
The complete ionic equation for the reaction of NaOH and HNO 3: Na + + OH − + H + + NO 3 − = Na + + NO 3 − + H 2 O. Reducing the same ions on the left and right in the equation, we obtain the reduced ionic equation presented in the task . This reaction proceeds due to the formation of a low-dissociating substance - water.
Sodium oxide does not dissociate in water, but reacts with it to form alkali:
Na 2 O + H 2 O \u003d 2 NaOH.
Copper hydroxide is insoluble base, so it does not dissociate in water.
Full ionic equation Cu(OH) 2 + 2H + + 2NO 3 − = Cu 2+ + 2NO 3 − + 2H 2 O
Abbreviated ionic equation: Cu(OH) 2 + 2H + = Cu 2+ + 2H 2 O
The water-soluble KNO 3 salt does not give hydroxide ions upon dissociation. Being a strong electrolyte, it decomposes into K + cations and NO 3 anions -
A precipitate forms when sulfuric acid is added to a solution containing ions:
1) NH 4 + and NO 3 -
2) K + and SiO 3 2−
Answer: 2
Explanation:
Sulphuric acid is a strong electrolyte and dissociates in water into ions: H + and SO 4 2-. When H + cations interact with SiO 3 2− anions, water-insoluble silicic acid H 2 SiO 3 is formed.
The acid residue of sulfuric acid SO 4 2- does not form precipitates with the proposed cations, as can be checked from the table of the solubility of acids, bases and salts in water.
The H + cation, except with SiO 3 2− , also does not form precipitates with the proposed anions.
The abbreviated ionic equation Cu 2+ + 2OH - = Cu(OH) 2 corresponds to the interaction between:
1) CuSO 4 (p-p) and Fe (OH) 3
2) CuS and Ba (OH) 2 (p-p)
3) CuCl 2 (p-p) and NaOH (p-p)
Answer: 3
Explanation:
In the first case, the reaction between copper sulfate CuSO 4 and iron (III) hydroxide Fe (OH) 3 does not proceed, since iron hydroxide is an insoluble base and does not dissociate in an aqueous solution.
In the second case, the reaction also does not proceed due to the insolubility of copper sulfide CuS.
In the third variant, the exchange reaction between copper chloride (II) and NaOH proceeds due to precipitation of Cu(OH) 2 .
The reaction equation in molecular form is as follows:
CuCl 2 + 2NaOH \u003d Cu (OH) 2 ↓ + 2NaCl.
The equation for this reaction in full ionic form is:
Cu 2+ + 2Cl − + 2Na + + 2OH − = Cu(OH) 2 ↓ + 2Na + + 2Cl − .
Reducing the same ions Na + and Cl - in the left and right parts of the full ionic equation, we obtain the reduced ionic equation:
Cu 2+ + 2OH - \u003d Cu (OH) 2 ↓
Copper oxide CuO (II), being an oxide transition metal(IA group) does not interact with water, as it does not form a soluble base.
The interaction of solutions of copper(II) chloride and potassium hydroxide corresponds to the reduced ionic equation:
1) Cl - + K + = KCl
2) CuCl 2 + 2OH - \u003d Cu (OH) 2 + 2Cl -
3) Cu 2+ + 2KOH = Cu(OH) 2 + 2K +
Answer: 4
Explanation:
The exchange reaction between solutions of copper (II) chloride and potassium hydroxide in molecular form is written as follows:
CuCl 2 + 2KOH = Cu(OH) 2 ↓ + 2KCl
The reaction takes place due to the precipitation of a blue precipitate of Cu(OH) 2 .
CuCl 2 and KOH are soluble compounds, therefore, in solution they decompose into ions.
We write the reaction in full ionic form:
Cu 2+ + 2Cl − + 2K + + 2OH − = Cu(OH) 2 ↓ + 2Cl − + 2K +
We reduce identical ions 2Cl − and 2K +
left and right of the full ionic equation and we get the reduced ionic equation:
Cu 2+ + 2OH - \u003d Cu (OH) 2 ↓
KCl, CuCl 2 and KOH are soluble substances and in an aqueous solution dissociate into cations and anions almost completely. In other proposed answers, these compounds appear in an undissociated form, so options 1, 2 and 3 are not correct.
Which abbreviated ionic equation corresponds to the interaction of sodium silicate with nitric acid?
1) K + + NO 3 - = KNO 3
2) H + + NO 3 - = HNO 3
3) 2H + + SiO 3 2- = H 2 SiO 3
Answer: 3
Explanation:
The reaction of interaction of sodium silicate with nitric acid (exchange reaction) in molecular form is written as follows:
Na 2 SiO 3 + 2HNO 3 \u003d H 2 SiO 3 ↓ + 2NaNO 3
Since sodium silicate is a soluble salt and nitric acid is strong, both substances in solution dissociate into ions. We write the reaction in full ionic form:
2Na + + SiO 3 2− + 2H + + 2NO 3 − = H 2 SiO 3 ↓ + 2Na + + 2NO 3 −
SiO 3 2- + 2H + = H 2 SiO 3 ↓
The rest of the proposed options do not reflect the sign of the reaction - precipitation. In addition, in the presented answer options, the soluble salts of KNO 3 and K 2 SiO 3 and the strong acid HNO 3 are presented in an undissociated form, which, of course, is not true, since these substances are strong electrolytes.
The abbreviated ionic equation Ba 2+ + SO 4 2− =BaSO 4 corresponds to the interaction
1) Ba(NO 3) 2 and Na 2 SO 4
2) Ba (OH) 2 and CuSO 4
3) BaO and H 2 SO 4
Answer: 1
Explanation:
The reaction of interaction of barium nitrate with sodium sulfate (exchange reaction) in molecular form is written as follows:
Ba(NO 3) 2 + Na 2 SO 4 = BaSO 4 ↓ + 2NaNO 3
Since barium nitrate and sodium sulfate are soluble salts, both substances in solution dissociate into ions. We write the reaction in full ionic form:
Ba 2+ + 2NO 3 − + 2Na + + SO 4 2− = BaSO 4 ↓ + 2Na + + 2NO 3 −
Having reduced the Na + and NO 3 − ions in the left and right parts of the equation, we obtain the reduced ionic equation:
Ba 2+ + SO 4 2− = BaSO 4 ↓
The reaction of interaction of barium hydroxide with copper sulfate (exchange reaction) in molecular form is written as follows:
Ba(OH) 2 + CuSO 4 = BaSO 4 ↓ + Cu(OH) 2 ↓
Two precipitates are formed. Since barium hydroxide and copper sulfate are soluble substances, both dissociate into ions in solution. We write the reaction in full ionic form:
Ba 2+ + 2OH − + Cu 2+ + SO 4 2− = BaSO 4 ↓ + Cu(OH) 2 ↓
The reaction of interaction of barium oxide with sulfuric acid (exchange reaction) in molecular form is written as follows:
BaO + H 2 SO 4 \u003d BaSO 4 ↓ + H 2 O
Since BaO is an oxide, it does not dissociate in water (BaO interacts with water to form alkali), we write the BaO formula in undissociated form. Sulfuric acid is strong, therefore, in solution it dissociates into H + cations and SO 4 2− anions. The reaction proceeds due to the precipitation of barium sulfate and the formation of a low-dissociating substance. We write the reaction in full ionic form:
BaO + 2H + + SO 4 2− = BaSO 4 ↓ + 2H 2 O
Here, too, there are no identical ions in the left and right parts of the equation and it is impossible to reduce anything, then the reduced ionic equation looks the same as the complete one.
The reaction of interaction of barium carbonate with sulfuric acid (exchange reaction) in molecular form is written as follows:
BaCO 3 + H 2 SO 4 = BaSO 4 ↓ + CO 2 + H 2 O
The reaction proceeds due to the formation of a precipitate, gas evolution and the formation of a low-dissociating compound - water. Since BaCO 3 is an insoluble salt, therefore, it does not decompose into ions in solution, we write the formula BaCO 3 in molecular form. Sulfuric acid is strong, therefore, in solution it dissociates into H + cations and SO 4 2− anions. We write the reaction in full ionic form:
BaCO 3 + 2H + + SO 4 2− = BaSO 4 ↓ + CO 2 + H 2 O
The full ionic equation coincides with the reduced one, since there are no identical ions on the left and right sides of the equation.
The reduced ionic equation Ba 2+ + CO 3 2− = BaCO 3 corresponds to the interaction
1) barium sulfate and potassium carbonate
2) barium hydroxide and carbon dioxide
3) barium chloride and sodium carbonate
4) barium nitrate and carbon dioxide
Answer: 3
Explanation:
The reaction between barium sulfate BaSO 4 and potassium carbonate K 2 CO 3 does not proceed, since barium sulfate is an insoluble salt. A necessary condition for the exchange of two salts is the solubility of both salts.
The reaction between barium hydroxide Ba(OH) 2 and carbon dioxide CO 2 ( acid oxide) occurs due to the formation of an insoluble salt BaCO 3 . This is the reaction of alkali with acid oxide to form salt and water. Let's write the reaction in molecular form:
Ba(OH) 2 + CO 2 = BaCO 3 ↓ + H 2 O
Since barium hydroxide is a soluble base, in solution it dissociates into Ba 2+ cations and OH − hydroxide ions. Carbon monoxide does not dissociate in water, therefore, in ionic equations, its formula should be written in molecular form. Barium carbonate is an insoluble salt, therefore, in the ionic reaction equation, it is also written in molecular form. Thus, the reaction of interaction of barium hydroxide and carbon dioxide in full ionic form is as follows:
Ba 2+ + 2OH − + CO 2 = BaCO 3 ↓ + H 2 O
Since there are no identical ions on the left and right sides of the equation and it is impossible to reduce anything, the reduced ionic equation looks the same as the complete one.
The reaction of interaction of barium chloride with sodium carbonate (exchange reaction) in molecular form is written as follows:
BaCl 2 + Na 2 CO 3 \u003d BaCO 3 ↓ + 2NaCl
Since barium chloride and sodium carbonate are soluble salts, both substances in solution dissociate into ions. We write the reaction in full ionic form:
Ba 2+ + 2Cl − + 2Na + + CO 3 2- = BaCO 3 ↓ + 2Na + + 2Cl −
Reducing the Na + and Cl − ions in the left and right parts of the equation, we obtain the reduced ionic equation:
Ba 2+ + CO 3 2- \u003d BaCO 3 ↓
The reaction between barium nitrate Ba (NO 3) 2 and carbon dioxide CO 2 (acidic oxide) in an aqueous solution does not proceed. Carbon dioxide CO 2 in an aqueous solution forms a weak unstable carbonic acid H 2 CO 3 , which is not able to displace strong HNO 3 from a Ba(NO 3) 2 salt solution.
Electrolytes and non-electrolytes
It is known from the lessons of physics that solutions of some substances are capable of conducting electric current, while others are not.
Substances whose solutions conduct electricity are called electrolytes.
Substances whose solutions do not conduct electricity are called non-electrolytes. For example, solutions of sugar, alcohol, glucose and some other substances do not conduct electricity.
Electrolytic dissociation and association
Why do electrolyte solutions conduct electricity?
Swedish scientist S. Arrhenius, studying electrical conductivity various substances, came in 1877 to the conclusion that the cause of electrical conductivity is the presence in solution ions formed when an electrolyte is dissolved in water.
The process by which an electrolyte breaks down into ions is called electrolytic dissociation.
S. Arrhenius, who adhered to the physical theory of solutions, did not take into account the interaction of electrolyte with water and believed that free ions were present in solutions. In contrast to him, the Russian chemists I. A. Kablukov and V. A. Kistyakovsky applied the chemical theory of D. I. Mendeleev to the explanation of electrolytic dissociation and proved that when an electrolyte is dissolved, chemical interaction solute with water, which leads to the formation of hydrates, and then they dissociate into ions. They believed that in solutions there are not free, not "naked" ions, but hydrated ones, that is, "dressed in a fur coat" of water molecules.
Water molecules are dipoles(two poles), since the hydrogen atoms are located at an angle of 104.5 °, due to which the molecule has an angular shape. The water molecule is shown schematically below.
As a rule, substances dissociate most easily with ionic bond and, accordingly, with the ionic crystal lattice, since they already consist of ready-made ions. When they dissolve, the dipoles of water are oriented with oppositely charged ends around the positive and negative ions of the electrolyte.
Forces of mutual attraction arise between electrolyte ions and water dipoles. As a result, the bond between the ions weakens, and the transition of ions from the crystal to the solution occurs. Obviously, the sequence of processes occurring during the dissociation of substances with an ionic bond (salts and alkalis) will be as follows:
1) orientation of water molecules (dipoles) near crystal ions;
2) hydration (interaction) of water molecules with ions of the surface layer of the crystal;
3) dissociation (decay) of the electrolyte crystal into hydrated ions.
Simplified, the ongoing processes can be reflected using the following equation:
Similarly, electrolytes dissociate, in the molecules of which there is a covalent bond (for example, molecules of hydrogen chloride HCl, see below); only in this case, under the influence of water dipoles, does the covalent polar bond transform into an ionic one; the sequence of processes occurring in this case will be as follows:
1) orientation of water molecules around the poles of electrolyte molecules;
2) hydration (interaction) of water molecules with electrolyte molecules;
3) ionization of electrolyte molecules (transformation of a covalent polar bond into an ionic one);
4) dissociation (decay) of electrolyte molecules into hydrated ions.
Simplified, the process of dissociation of hydrochloric acid can be reflected using the following equation:
It should be taken into account that in electrolyte solutions randomly moving hydrated ions can collide and reunite with each other. This reverse process is called association. Association in solutions occurs in parallel with dissociation, therefore, the sign of reversibility is put in the reaction equations.
The properties of hydrated ions differ from those of non-hydrated ones. For example, the unhydrated copper ion Cu 2+ is white in anhydrous copper(II) sulfate crystals and is blue when hydrated, i.e. bound to water molecules Cu 2+ nH 2 O. Hydrated ions have both constant and variable the number of water molecules.
Degree of electrolytic dissociation
In electrolyte solutions, along with ions, molecules are also present. Therefore, electrolyte solutions are characterized degree of dissociation, which is denoted by the Greek letter a ("alpha").
This is the ratio of the number of particles decaying into ions (N g) to total number dissolved particles (N p).
The degree of electrolyte dissociation is determined empirically and is expressed in fractions or percentages. If a \u003d 0, then there is no dissociation, and if a \u003d 1, or 100%, then the electrolyte completely decomposes into ions. Different electrolytes have different degrees of dissociation, i.e., the degree of dissociation depends on the nature of the electrolyte. It also depends on the concentration: with the dilution of the solution, the degree of dissociation increases.
According to the degree of electrolytic dissociation, electrolytes are divided into strong and weak.
Strong electrolytes- these are electrolytes, which, when dissolved in water, almost completely dissociate into ions. For such electrolytes, the value of the degree of dissociation tends to unity.
Strong electrolytes include:
1) all soluble salts;
2) strong acids, for example: H 2 SO 4, HCl, HNO 3;
3) all alkalis, for example: NaOH, KOH.
Weak electrolytes- these are electrolytes that, when dissolved in water, almost do not dissociate into ions. For such electrolytes, the value of the degree of dissociation tends to zero.
Weak electrolytes include:
1) weak acids - H 2 S, H 2 CO 3, HNO 2;
2) an aqueous solution of ammonia NH 3 H 2 O;
4) some salts.
Dissociation constant
In solutions of weak electrolytes, due to their incomplete dissociation, dynamic equilibrium between non-dissociated molecules and ions. For example, for acetic acid:
You can apply the law of mass action to this equilibrium and write the expression for the equilibrium constant:
The equilibrium constant characterizing the process of dissociation of a weak electrolyte is called dissociation constant.
The dissociation constant characterizes the ability of an electrolyte (acid, base, water) dissociate into ions. The larger the constant, the easier the electrolyte decomposes into ions, therefore, the stronger it is. The values of dissociation constants for weak electrolytes are given in reference books.
The main provisions of the theory of electrolytic dissociation
1. When dissolved in water, electrolytes dissociate (decompose) into positive and negative ions.
ions- this is one of the forms of existence of a chemical element. For example, sodium metal atoms Na 0 interact vigorously with water, forming an alkali (NaOH) and hydrogen H 2, while sodium ions Na + do not form such products. Chlorine Cl 2 has a yellow-green color and a pungent odor, poisonous, and chlorine ions Cl are colorless, non-toxic, odorless.
ions are positively or negatively charged particles into which atoms or groups of atoms of one or more atoms are converted chemical elements by donating or gaining electrons.
In solutions, ions move randomly in different directions.
According to their composition, ions are divided into simple- Cl - , Na + and complex- NH 4 +, SO 2 -.
2. The reason for the dissociation of the electrolyte in aqueous solutions is its hydration, i.e., the interaction of the electrolyte with water molecules and rupture chemical bond in him.
As a result of this interaction, hydrated, i.e., associated with water molecules, ions are formed. Therefore, according to the presence of a water shell, ions are divided into hydrated(in solution and crystalline hydrates) and non-hydrated(in anhydrous salts).
3. Under the action of an electric current, positively charged ions move towards the negative pole of the current source - the cathode and therefore are called cations, and negatively charged ions move towards the positive pole of the current source - the anode and therefore are called anions.
Therefore, there is another classification of ions - by the sign of their charge.
The sum of the charges of the cations (H +, Na +, NH 4 +, Cu 2+) is equal to the sum of the charges of the anions (Cl -, OH -, SO 4 2-), as a result of which electrolyte solutions (HCl, (NH 4) 2 SO 4, NaOH, CuSO 4) remain electrically neutral.
4. Electrolytic dissociation is a reversible process for weak electrolytes.
Along with the process of dissociation (decomposition of the electrolyte into ions), the reverse process also proceeds - association(connection of ions). Therefore, in the equations of electrolytic dissociation, instead of the equal sign, the sign of reversibility is put, for example:
5. Not all electrolytes dissociate into ions to the same extent.
Depends on the nature of the electrolyte and its concentration. Chemical properties electrolyte solutions are determined by the properties of the ions that they form during dissociation.
The properties of solutions of weak electrolytes are due to the molecules and ions formed in the process of dissociation, which are in dynamic equilibrium with each other.
The smell of acetic acid is due to the presence of CH 3 COOH molecules, the sour taste and color change of the indicators are associated with the presence of H + ions in the solution.
The properties of solutions of strong electrolytes are determined by the properties of the ions that are formed during their dissociation.
For example, the general properties of acids, such as sour taste, discoloration of indicators, etc., are due to the presence of hydrogen cations in their solutions (more precisely, oxonium ions H 3 O +). General properties alkalis, such as soapiness to the touch, a change in the color of indicators, etc., are associated with the presence of hydroxide ions OH - in their solutions, and the properties of salts - with their decomposition in solution into metal (or ammonium) cations and anions of acid residues.
According to the theory of electrolytic dissociation all reactions in aqueous electrolyte solutions are reactions between ions. This is the reason for the high rate of many chemical reactions in electrolyte solutions.
The reactions that take place between ions are called ionic reactions
, and the equations of these reactions - ionic equations.
Ion exchange reactions in aqueous solutions can proceed:
1. irreversibly, to end.
2. reversible i.e. flow in two opposite directions at the same time. Exchange reactions between strong electrolytes in solutions proceed to the end or are practically irreversible, when ions, combining with each other, form substances:
a) insoluble;
b) low dissociating (weak electrolytes);
c) gaseous.
Here are some examples of molecular and reduced ionic equations:
The reaction is irreversible, since one of its products is an insoluble substance.
The neutralization reaction is irreversible, since a low-dissociating substance is formed - water.
The reaction is irreversible, since CO 2 gas is formed and a low-dissociating substance is water.
If among the starting materials and among the products of the reaction there are weak electrolytes or poorly soluble substances, then such reactions are reversible, that is, they do not proceed to the end.
In reversible reactions, the equilibrium shifts towards the formation of the least soluble or least dissociated substances.
For example:
The equilibrium shifts towards the formation of a weaker electrolyte - H 2 O. However, such a reaction will not proceed to the end: undissociated molecules of acetic acid and hydroxide ions remain in the solution.
If the starting materials are strong electrolytes that, when interacting, do not form insoluble or low-dissociating substances or gases, then such reactions do not proceed: when the solutions are mixed, a mixture of ions is formed.
Reference material for passing the test:
Mendeleev table
Solubility table
Electrolytic dissociation- the process of decomposition of the electrolyte into ions during its dissolution or melting.
The classical theory of electrolytic dissociation was created by S. Arrhenius and W. Ostwald in 1887. Arrhenius adhered to the physical theory of solutions, did not take into account the interaction of electrolyte with water, and believed that free ions were present in solutions. Russian chemists I. A. Kablukov and V. A. Kistyakovsky used the chemical theory of solutions of D. I. Mendeleev to explain the electrolytic dissociation and proved that when the electrolyte is dissolved, it chemically interacts with water, as a result of which the electrolyte dissociates into ions.
The classical theory of electrolytic dissociation is based on the assumption of incomplete dissociation of a solute, characterized by the degree of dissociation α, i.e., the proportion of decomposed electrolyte molecules. The dynamic equilibrium between non-dissociated molecules and ions is described by the law of mass action.
Substances that break down into ions are called electrolytes. Electrolytes are substances with an ionic or strongly covalent bond: acids, bases, salts. other substances are non-electrolytes; these include substances with non-polar or weakly polar covalent bonds; for example, many organic compounds.
The main provisions of TED (Theory of Electrolytic Dissociation):
Molecules break up into positively and negatively charged ions (simple and complex).
Under the influence of an electric current, cations (positively charged ions move towards the cathode (-), and anions (negatively charged ions) towards the anode (+)
The degree of dissociation depends on the nature of the substance and solvent, concentration, temperature.
If the degree of dissociation depends on the nature of the substance, then it can be judged that there is a distinction between certain groups of substances.
A large degree of dissociation is inherent in strong electrolytes (most bases, salts, many acids). It should be noted that the decay into ions - reversible reaction. It is also worth saying that examples of the dissociation of double and basic salts will not be analyzed in this topic, their dissociation is described in the topic “salt”.
Examples of strong electrolytes:
NaOH, K 2 SO 4 , HClO 4
Dissociation equations:
NaOH⇄Na + +OH -
K 2 SO 4 ⇄2K + +SO 4 2-
HClO 4 ⇄H + +ClO 4 -
A quantitative characteristic of the strength of electrolytes is the degree of dissociation (α) - the ratio molar concentration of the dissociated electrolyte to its total molar concentration in solution.
The degree of dissociation is expressed in fractions of a unit or as a percentage. The range of values is from 0 to 100%.
α = 0% refers to non-electrolytes (no dissociation)
0% <α < 100% относится к слабым электролитам (диссоциация неполная)
α = 100% refers to strong electrolytes (complete dissociation)
It is also worth remembering the number of dissociation steps, for example:
Dissociation of H 2 SO 4 solution
H 2 SO 4 ⇄ H + + HSO 4 -
HSO 4 - ⇄H + +SO 4 2-
Each stage of dissociation has its own degree of dissociation.
For example, the dissociation of salts CuCl 2 , HgCl 2:
CuCl 2 ⇄Cu 2+ + 2Cl - dissociation proceeds completely
And in the case of mercury chloride, the dissociation is incomplete and then not completely.
HgCl 2 ⇄HgCl + +Cl -
Returning to a solution of sulfuric acid, it is worth saying that the degree of dissociation of both stages of a dilute acid is much greater than that of a concentrated one. During the dissociation of a concentrated solution, there are a lot of substance molecules and a high concentration of HSO 4 - hydroanions.
For polybasic acids and polyacid bases, dissociation proceeds in several steps (depending on basicity).
We list the strong and weak acids and proceed to the ion exchange equations:
Strong acids (HCl, HBr, HI, HClO 3 , HBrO 3 , HIO 3 , HClO 4 , H 2 SO 4 , H 2 SeO 4 , HNO 3 , HMnO 4 , H 2 Cr 2 O 7)
Weak acids (HF, H 2 S, H 2 Se, HClO, HBrO, H 2 SeO 3 , HNO 2 , H 3 PO 4 , H 4 SiO 4 , HCN, H 2 CO 3 , CH 3 COOH)
Chemical reactions in solutions and melts of electrolytes proceed with the participation of ions. In such reactions, the oxidation states of the elements do not change, and the reactions themselves are called ion exchange reactions.
Ion exchange reactions will proceed to completion (irreversibly) if poorly soluble or practically insoluble substances are formed (they precipitate), volatile substances (released as gases) or weak electrolytes (for example, water).
Ion exchange reactions are usually written in three stages:
1. Molecular equation
2. Complete ionic equation
3. Reduced ionic equation
When writing, be sure to indicate precipitation and gases, as well as be guided by the solubility table.
Reactions where all the reactants and products are soluble in water do not proceed.
A few examples:
Na 2 CO 3 + H 2 SO 4 → Na 2 SO 4 + CO 2 + H 2 O
2Na + +CO 3 2- +2H + +SO 4 2- →2Na + +SO 4 2- +CO 2 +H 2 O
CO 3 2- + 2H + → CO 2 + H 2 O
The abbreviated ionic equation is obtained by crossing out identical ions from both sides of the full ionic equation.
If an ion exchange reaction occurs between two salts with the formation of a precipitate, then two highly soluble reagents should be taken. That is, the ion exchange reaction will proceed if the solubility of the reactants is higher than that of one of the products.
Ba(NO 3) 2 + Na 2 SO 4 → BaSO 4 ↓ + 2NaNO 3
Sometimes when writing ion exchange reactions, they skip the full ionic equation and immediately write the abbreviated one.
Ba 2+ +SO 4 2- → BaSO 4 ↓
To obtain a precipitate of a poorly soluble substance, it is always necessary to choose highly soluble reagents in their concentrated solutions.
For example:
2KF+FeCl 2 →FeF 2 ↓+2KCl
Fe 2+ +2F - → FeF 2 ↓
These rules for the selection of reagents for the precipitation of products are valid only for salts.
Examples of reactions with precipitation:
1.Ba(OH) 2 + H 2 SO 4 → BaSO 4 ↓ + 2H 2 O
Ba 2+ +SO 4 2- → BaSO 4 ↓
2. AgNO 3 +KI→AgI↓+KNO 3
Ag + +I - →AgI↓
3.H 2 S+Pb(NO 3) 2 →PbS↓+2HNO 3
H 2 S+Pb 2+ →PbS↓+2H +
4. 2KOH + FeSO 4 → Fe(OH) 2 ↓ + K 2 SO 4
Fe 2+ +2OH - →Fe(OH) 2 ↓
Examples of reactions with evolution of gases:
1. CaCO 3 + 2HNO 3 → Ca (NO 3) 2 + CO 2 + H 2 O
CaCO 3 + 2H + → Ca 2+ + CO 2 + H 2 O
2. 2NH 4 Cl + Ca (OH) 2 → 2NH 3 + CaCl 2 + 2H 2 O
NH 4 + +OH - →NH 3 +H 2 O
3. ZnS+2HCl→H 2 S+ZnCl 2
ZnS+2H + →H 2 S+Zn 2+
Examples of reactions with the formation of weak electrolytes:
1.Mg (CH 3 COO) 2 + H 2 SO 4 → MgSO 4 + 2CH 3 COOH
CH 3 COO - + H + →CH 3 COOH
2. HI+NaOH→NaI+H 2 O
H + +OH - →H 2 O
Consider the application of the studied material on specific tasks encountered in exams:
№1
.Among the substances: NaCl, Na 2 S, Na 2 SO 4 - reacts with a solution of Cu (NO3) 2 enters (-s)
1) only Na 2 S
2) NaCl and Na 2 S
3) Na 2 S and Na 2 SO 4
4) NaCl and Na 2 SO 4
The word “enter” means “a reaction proceeds”, and as mentioned above, the reaction proceeds if an insoluble or slightly soluble substance is formed, a gas is released, or a weak electrolyte (water) is formed.
Let's go through the options one by one.
1) Cu(NO 3) 2 +Na 2 S→CuS↓+2NaNO 3 precipitate formed.
2) NaCl + Cu (NO 3) 2 ↛CuCl 2 + 2NaNO 3
Only the reaction with Na 2 S proceeds with the formation of a precipitate
3) With Na 2 S there will also be a precipitate formation as in the first two examples.
Na 2 SO 4 +Cu(NO 3) 2 ↛CuSO 4 +2NaNO 3
All products are highly soluble electrolytes, they are not gases, therefore, the reaction does not proceed.
4) With Na 2 SO 4 the reaction does not proceed as in the previous answer
NaCl+Cu(NO 3) 2 ↛CuCl 2 +2NaNO 3
All products are highly soluble electrolytes, they are not gases, therefore, the reaction does not proceed.
Therefore, suitable 1 Possible answer.
№2 . The gas is released during the interaction
1) MgCl 2 and Ba (NO 3) 2
2) Na 2 CO 3 and CaCl 2
3) NH 4 C and NaOH
4) CuSO 4 and KOH
The word "gas" in such tasks means exactly gases and volatile compounds.
In assignments, NH 3 H 2 O, H 2 CO 3 are usually found as such compounds (under normal reaction conditions, it decomposes into CO 2 and H 2 O, it is customary not to write the full formula of carbonic acid, but immediately paint it into gas and water) , H 2 S.
From the substances presented above, we will not be able to get H 2 S, because there is no sulfide ion in all substances. We also cannot get carbon dioxide, because to get it from salt, you need to add acid, and another salt is paired with sodium carbonate.
We can get gas in 3 options.
NH 4 Cl + NaOH → NH 3 + NaCl + H 2 O
A gas with a pungent odor was released.
Therefore, suitable 3 Possible answer.
№3 .Reacts with hydrochloric acid
1) silver nitrate
2) barium nitrate
3) silver
4) silicon oxide
Among the reagents there are two electrolytes, in order for the reaction to take place, it is necessary that a precipitate stand out.
Hydrochloric acid will not react with silicon oxide, and silver will not displace hydrogen from of hydrochloric acid.
Ba (NO 3) 2 + 2HCl → BaCl 2 + 2HNO 3 the reaction will not proceed, since all products are soluble electrolytes
AgNO 3 +HCl→AgCl↓+NaNO 3
A white cheesy precipitate of silver nitrate will fall out
Therefore, suitable 1
Possible answer.
The following example of a task, unlike the first three, is taken from KIM USE 2017.
The first three are taken from KIM OGE 2017
Establish a correspondence between the formulas of substances and a reagent with which you can distinguish their aqueous solutions: for each position indicated by a letter, select the corresponding position indicated by a number.
FORMULA OF SUBSTANCES REAGENT
A) HNO 3 and H 2 O 1) CaCO 3
B) KCl and NaOH 2) KOH
B) NaCl and BaCl 2 3) HCl
D) AlCl 3 and MgCl 2 4) KNO 3
To complete this task, you must first understand that under each letter two substances are indicated that are in the same solution and you need to select the substance so that at least one of them enters into a qualitative reaction with the reagent substance, which is given under the number.
Add calcium carbonate to a solution of nitric acid, carbon dioxide will become a sign of the reaction:
2HNO 3 + CaCO 3 → Ca (NO 3) 2 + CO 2 + H 2 O
Also, logically, calcium carbonate does not dissolve in water, which means that it will not dissolve in all other solutions either, therefore, dissolution of calcium carbonate can be added to the signs of the reaction, in addition to gas evolution.
The solution under the letter B could be distinguished using hydrochloric acid under the number 3, but only if it would be allowed to use an indicator (phenolphthalein), which would discolor after the reaction, because the alkali would be neutralized .
Therefore, we can distinguish in a solution of OH - an ion only with the help of 5 solution (CuSO 4)
2NaOH+CuSO 4 →Cu(OH) 2 ↓+Na 2 SO 4
Blue crystals formed in two solutions.
The solution under the letter B can also be distinguished with the help of reagent number 5, because sulfate ions, combining with barium, immediately precipitate into a white crystalline precipitate, which is insoluble in excess of even the strongest acids.
BaCl 2 + CuSO 4 → CuCl 2 + BaSO 4 ↓
The solution under the letter G is easy to distinguish with the help of any alkali, because the bases of magnesium and aluminum will immediately precipitate during the reaction. Alkali is represented under the number 2
AlCl 3 +3KOH→Al(OH) 3 ↓+3KCl
MgCl 2 +2KOH→Mg(OH) 2 ↓+2KCl
Editor: Kharlamova Galina Nikolaevna
