When any strong acid is neutralized by any strong base, for each mole of water formed, about the heat is released:
This suggests that such reactions are reduced to one process. We will obtain the equation for this process if we consider in more detail one of the given reactions, for example, the first. Let's rewrite its equation, writing strong electrolytes in ionic form, since they exist in solution in the form of ions, and weak electrolytes in molecular form, since they are in solution mainly in the form of molecules (water is a very weak electrolyte, see § 90):
Considering the resulting equation, we see that the ions did not undergo changes during the reaction. Therefore, we will rewrite the equation again, eliminating these ions from both sides of the equation. We get:
Thus, the reactions of neutralization of any strong acid with any strong base come down to the same process - the formation of water molecules from hydrogen ions and hydroxide ions. It is clear that the thermal effects of these reactions must also be the same.
Strictly speaking, the reaction of the formation of water from ions is reversible, which can be expressed by the equation
However, as we will see below, water is a very weak electrolyte and dissociates only to a negligible extent. In other words, the equilibrium between water molecules and ions is strongly shifted towards the formation of molecules. Therefore, in practice, the reaction of neutralization of a strong acid with a strong base proceeds to completion.
When mixing a solution of any silver salt with hydrochloric acid or with a solution of any of its salts, a characteristic white cheesy precipitate of silver chloride is always formed:
Such reactions also come down to one process. In order to obtain its ionic-molecular equation, we rewrite, for example, the equation of the first reaction, writing strong electrolytes, as in the previous example, in ionic form, and the substance in the sediment in molecular form:
As can be seen, the ions do not undergo changes during the reaction. Therefore, we exclude them and rewrite the equation again:
This is the ion-molecular equation of the process under consideration.
Here we must also keep in mind that the silver chloride precipitate is in equilibrium with the ions in solution, so that the process expressed by the last equation is reversible:
However, due to the low solubility of silver chloride, this equilibrium is very strongly shifted to the right. Therefore, we can assume that the reaction of formation from ions is almost completed.
The formation of a precipitate will always be observed when there are significant concentrations of and ions in one solution. Therefore, with the help of silver ions it is possible to detect the presence of ions in a solution and, conversely, with the help of chloride ions - the presence of silver ions; An ion can serve as a reactant on an ion, and an ion can serve as a reactant on an ion.
In the future, we will widely use the ionic-molecular form of writing equations for reactions involving electrolytes.
To draw up ion-molecular equations, you need to know which salts are soluble in water and which are practically insoluble. general characteristics The solubility of the most important salts in water is given in Table. 15.
Table 15. Solubility of the most important salts in water
Ionic-molecular equations help to understand the characteristics of reactions between electrolytes. Let us consider, as an example, several reactions that occur with the participation of weak acids and bases.
As already mentioned, the neutralization of any strong acid by any strong base is accompanied by the same thermal effect, since it comes down to the same process - the formation of water molecules from hydrogen ions and hydroxide ions.
However, when neutralizing a strong acid with a weak base, or a weak acid with a strong or weak base, the thermal effects are different. Let's write ion-molecular equations for such reactions.
Neutralization of a weak acid (acetic acid) with a strong base (sodium hydroxide):
Here, the strong electrolytes are sodium hydroxide and the resulting salt, and the weak electrolytes are acid and water:
As can be seen, only sodium ions do not undergo changes during the reaction. Therefore, the ion-molecular equation has the form:
Neutralization of a strong acid (nitrogen) with a weak base (ammonium hydroxide):
Here we must write the acid and the resulting salt in the form of ions, and ammonium hydroxide and water in the form of molecules:
The ions do not undergo changes. Omitting them, we obtain the ionic-molecular equation:
Neutralization of a weak acid (acetic acid) with a weak base (ammonium hydroxide):
In this reaction, all substances except the one formed weak electrolytes. Therefore, the ion-molecular form of the equation looks like:
Comparing the obtained ion-molecular equations with each other, we see that they are all different. Therefore, it is clear that the heats of the reactions considered are also different.
As already indicated, the reactions of neutralization of strong acids with strong bases, during which hydrogen ions and hydroxide ions combine to form a water molecule, proceed almost to completion. Neutralization reactions in which at least one of the starting substances is a weak electrolyte and in which molecules of weakly associated substances are present not only on the right side, but also on the left side ion-molecular equation, do not proceed completely.
They reach a state of equilibrium in which the salt coexists with the acid and base from which it was formed. Therefore, it is more correct to write the equations of such reactions as reversible reactions.
Topic: Chemical bond. Electrolytic dissociation
Lesson: Writing Equations for Ion Exchange Reactions
Let's create an equation for the reaction between iron (III) hydroxide and nitric acid.
Fe(OH) 3 + 3HNO 3 = Fe(NO 3) 3 + 3H 2 O
(Iron (III) hydroxide is an insoluble base, therefore it is not subjected to. Water is a poorly dissociated substance; it is practically not dissociated into ions in solution.)
Fe(OH) 3 + 3H + + 3NO 3 - = Fe 3+ + 3NO 3 - + 3H 2 O
Cross out the same number of nitrate anions on the left and right and write the abbreviated ionic equation:
Fe(OH) 3 + 3H + = Fe 3+ + 3H 2 O
This reaction proceeds to completion, because a slightly dissociable substance is formed - water.
Let's write an equation for the reaction between sodium carbonate and magnesium nitrate.
Na 2 CO 3 + Mg(NO 3) 2 = 2NaNO 3 + MgCO 3 ↓
Let's write this equation in ionic form:
(Magnesium carbonate is insoluble in water and therefore does not break down into ions.)
2Na + + CO 3 2- + Mg 2+ + 2NO 3 - = 2Na + + 2NO 3 - + MgCO 3 ↓
Let's cross out the same number of nitrate anions and sodium cations on the left and right, and write the abbreviated ionic equation:
CO 3 2- + Mg 2+ = MgCO 3 ↓
This reaction proceeds to completion, because a precipitate is formed - magnesium carbonate.
Let's write an equation for the reaction between sodium carbonate and nitric acid.
Na 2 CO 3 + 2HNO 3 = 2NaNO 3 + CO 2 + H 2 O
(Carbon dioxide and water are products of the decomposition of the resulting weak carbonic acid.)
2Na + + CO 3 2- + 2H + + 2NO 3 - = 2Na + + 2NO 3 - + CO 2 + H 2 O
CO 3 2- + 2H + = CO 2 + H 2 O
This reaction proceeds to completion, because As a result, gas is released and water is formed.
Let's create two molecular reaction equations, which correspond to the following abbreviated ionic equation: Ca 2+ + CO 3 2- = CaCO 3 .
The abbreviated ionic equation shows the essence of the ion exchange reaction. IN in this case we can say that to obtain calcium carbonate, it is necessary that the composition of the first substance include calcium cations, and the composition of the second - carbonate anions. Let's create molecular equations for reactions that satisfy this condition:
CaCl 2 + K 2 CO 3 = CaCO 3 ↓ + 2KCl
Ca(NO 3) 2 + Na 2 CO 3 = CaCO 3 ↓ + 2NaNO 3
1. Orzhekovsky P.A. Chemistry: 9th grade: textbook. for general education establishment / P.A. Orzhekovsky, L.M. Meshcheryakova, L.S. Pontak. - M.: AST: Astrel, 2007. (§17)
2. Orzhekovsky P.A. Chemistry: 9th grade: general education. establishment / P.A. Orzhekovsky, L.M. Meshcheryakova, M.M. Shalashova. - M.: Astrel, 2013. (§9)
3. Rudzitis G.E. Chemistry: inorganic. chemistry. Organ. chemistry: textbook. for 9th grade. / G.E. Rudzitis, F.G. Feldman. - M.: Education, OJSC “Moscow Textbooks”, 2009.
4. Khomchenko I.D. Collection of problems and exercises in chemistry for high school. - M.: RIA “New Wave”: Publisher Umerenkov, 2008.
5. Encyclopedia for children. Volume 17. Chemistry / Chapter. ed. V.A. Volodin, Ved. scientific ed. I. Leenson. - M.: Avanta+, 2003.
Additional web resources
1. A unified collection of digital educational resources (video experiences on the topic): ().
2. Electronic version magazine "Chemistry and Life": ().
Homework
1. In the table, mark with a plus sign the pairs of substances between which ion exchange reactions are possible and proceed to completion. Write reaction equations in molecular, full and reduced ionic form.
|
Reacting substances |
K2 CO3 |
AgNO3 |
FeCl3 |
HNO3 |
|
|
CuCl2 |
|||||
2. p. 67 No. 10,13 from the textbook P.A. Orzhekovsky “Chemistry: 9th grade” / P.A. Orzhekovsky, L.M. Meshcheryakova, M.M. Shalashova. - M.: Astrel, 2013.
2.6 Ionic-molecular equations
When any strong acid is neutralized by any strong base, about 57.6 kJ of heat is released for each mole of water formed:
HCl + NaOH = NaCl + H 2 O + 57.53 kJ
HNO 3 + KOH = KNO 3 + H 2 O +57.61 kJ
This suggests that such reactions are reduced to one process. We will obtain the equation for this process if we consider in more detail one of the given reactions, for example, the first. Let's rewrite its equation, writing strong electrolytes in ionic form, since they exist in solution in the form of ions, and weak electrolytes in molecular form, since they are in solution mainly in the form of molecules (water is a very weak electrolyte):
H + + Cl - + Na + + OH - = Na + + Cl - + H 2 O
Considering the resulting equation, we see that during the reaction the Na + and Cl - ions did not undergo changes. Therefore, we will rewrite the equation again, eliminating these ions from both sides of the equation. We get:
H + + OH - = H 2 O
Thus, the reactions of neutralization of any strong acid with any strong base come down to the same process - the formation of water molecules from hydrogen ions and hydroxide ions. It is clear that the thermal effects of these reactions must also be the same.
Strictly speaking, the reaction of the formation of water from ions is reversible, which can be expressed by the equation
H + + OH - ↔ H 2 O
However, as we will see below, water is a very weak electrolyte and dissociates only to a negligible extent. In other words, the equilibrium between water molecules and ions is strongly shifted towards the formation of molecules. Therefore, in practice, the reaction of neutralization of a strong acid with a strong base proceeds to completion.
When mixing a solution of any silver salt with hydrochloric acid or with a solution of any of its salts, a characteristic white cheesy precipitate of silver chloride is always formed:
AgNO 3 + HC1 = AgCl↓ + HNO 3
Ag 2 SO 4 + CuCl 2 = 2AgCl↓ + CuSO 4
Such reactions also come down to one process. In order to obtain its ionic-molecular equation, we rewrite, for example, the equation of the first reaction, writing strong electrolytes, as in the previous example, in ionic form, and the substance in the sediment in molecular form:
Ag + + NO 3 - + H + + C1 - = AgCl↓+ H + + NO 3 -
As can be seen, the H + and NO 3 - ions do not undergo changes during the reaction. Therefore, we exclude them and rewrite the equation again:
Ag + + С1 - = AgCl↓
This is the ion-molecular equation of the process under consideration.
Here it must also be borne in mind that the silver chloride precipitate is in equilibrium with the Ag + and C1 - ions in solution, so that the process expressed by the last equation is reversible:
Ag + + C1 - ↔ AgCl↓
However, due to the low solubility of silver chloride, this equilibrium is very strongly shifted to the right. Therefore, we can assume that the reaction of the formation of AgCl from ions is almost complete.
The formation of an AgCl precipitate will always be observed when there are significant concentrations of Ag + and C1 - ions in the same solution. Therefore, using silver ions, you can detect the presence of C1 - ions in a solution and, conversely, using chloride ions - the presence of silver ions; the C1 - ion can serve as a reagent for the Ag + ion, and the Ag + ion can serve as a reagent for the C1 ion.
In the future, we will widely use the ionic-molecular form of writing equations for reactions involving electrolytes.
To draw up ion-molecular equations, you need to know which salts are soluble in water and which are practically insoluble. The general characteristics of the solubility of the most important salts in water are given in Table 2.
Ionic-molecular equations help to understand the characteristics of reactions between electrolytes. Let us consider, as an example, several reactions occurring with the participation of weak acids and bases.
Table 2. Solubility of the most important salts in water
As already mentioned, the neutralization of any strong acid by any strong base is accompanied by the same thermal effect, since it comes down to the same process - the formation of water molecules from hydrogen ions and hydroxide ions. However, when neutralizing a strong acid with a weak base, or a weak acid with a strong or weak base, the thermal effects are different. Let's write ion-molecular equations for such reactions.
Neutralization of a weak acid (acetic acid) with a strong base (sodium hydroxide):
CH 3 COOH + NaOH = CH 3 COONa + H 2 O
Here the strong electrolytes are sodium hydroxide and the resulting salt, and the weak ones are acid and water:
CH 3 COOH + Na + + OH - = CH 3 COO - + Na + + H 2 O
As can be seen, only sodium ions do not undergo changes during the reaction. Therefore, the ion-molecular equation has the form:
CH 3 COOH + OH - = CH 3 COO - + H 2 O
Neutralization of a strong acid (nitrogen) with a weak base (ammonium hydroxide):
HNO 3 + NH 4 OH = NH 4 NO 3 + H 2 O
Here we must write the acid and the resulting salt in the form of ions, and ammonium hydroxide and water in the form of molecules:
H + + NO 3 - + NH 4 OH = NH 4 - + NH 3 - + H 2 O
NO 3 - ions do not undergo changes. Omitting them, we obtain the ionic-molecular equation:
H + + NH 4 OH= NH 4 + + H 2 O
Neutralization of a weak acid (acetic acid) with a weak base (ammonium hydroxide):
CH 3 COOH + NH 4 OH = CH 3 COONH 4 + H 2 O
In this reaction, all substances, except the salt formed, are weak electrolytes. Therefore, the ion-molecular form of the equation looks like:
CH 3 COOH + NH 4 OH = CH 3 COO - + NH 4 + + H 2 O
Comparing the obtained ion-molecular equations with each other, we see that they are all different. Therefore, it is clear that the heats of the reactions considered are also different.
Reactions of neutralization of strong acids with strong bases, during which hydrogen ions and hydroxide ions combine to form a water molecule, proceed almost to completion. Neutralization reactions, in which at least one of the starting substances is a weak electrolyte and in which molecules of weakly dissociating substances are present not only on the right, but also on the left side of the ion-molecular equation, do not proceed to completion. They reach a state of equilibrium in which the salt coexists with the acid and base from which it was formed. Therefore, it is more correct to write the equations of such reactions as reversible reactions:
CH 3 COOH + OH - ↔ CH 3 COO - + H 2 O
H + + NH 4 OH↔ NH 4 + + H 2 O
CH 3 COOH + NH 4 OH ↔ CH 3 COO - + NH 4 + + H 2 O
With other solvents, the considered patterns remain the same, but there are also deviations from them, for example, a minimum (anomalous electrical conductivity) is often observed on the λ-c curves. 2. Ion mobility Let us relate the electrical conductivity of the electrolyte to the speed of movement of its ions in electric field. To calculate electrical conductivity, it is enough to count the number of ions...
When studying the synthesis of new materials and processes of ion transport in them. IN pure form Such patterns are most clearly visible in the study of single-crystal solid electrolytes. At the same time, when using solid electrolytes as working media for functional elements, it is necessary to take into account that materials of a given type and shape are needed, for example in the form of dense ceramics...
17-25 kg/t aluminum, which is ~ 10-15 kg/t higher compared to the results for sandy alumina. Alumina used for aluminum production must contain a minimum amount of iron, silicon, heavy metals with a lower release potential at the cathode than aluminum, because they are easily reduced and converted into cathode aluminum. It is also undesirable to be present in...
Instructions
Before you begin ionic equations, you need to understand some rules. Insoluble in water, gaseous and poorly dissociating substances (for example, water) do not disintegrate into ions, which means write them in molecular form. This also includes weak electrolytes such as H2S, H2CO3, H2SO3, NH4OH. The solubility of compounds can be determined from the solubility table, which is an approved reference material for all types of control. All charges that are inherent in cations and anions are also indicated there. To fully complete the task, you must write molecular, complete and ionic abbreviated equations.
Example No. 1. neutralization reaction between sulfuric acid and potassium hydroxide, consider it from the point of view of ED (electrolytic dissociation theory). First, write down the reaction equation in molecular form and .H2SO4 + 2KOH = K2SO4 + 2H2O Analyze the resulting substances for their solubility and dissociation. All compounds are soluble in water, which means they are ions. The only exception is water, which does not disintegrate into ions and therefore remains in molecular form. Write the ionic complete equation, find the same ions on the left and right sides and . To cancel identical ions, cross them out.2H+ +SO4 2- +2K+ +2OH- = 2K+ +SO4 2- + 2H2OThe result is an ionic abbreviation equation:2H+ +2OH- = 2H2OCoefficients in the form of twos can also be abbreviated:H+ +OH- = H2O
Example No. 2. Write the exchange reaction between copper chloride and , consider it from the point of view of TED. Write the reaction equation in molecular form and assign the coefficients. As a result, the resulting copper hydroxide precipitated a color. CuCl2 + 2NaOH = Cu(OH) 2↓ + 2NaCl Analyze all substances for their solubility in water - everything is soluble except copper hydroxide, which will not be ionized. Write down the complete ionic equation, underline and abbreviate the identical ions: Cu2+ +2Cl- + 2Na+ +2OH- = Cu(OH) 2↓+2Na+ +2Cl- The ionic abbreviated equation remains: Cu2+ +2OH- = Cu(OH) 2↓
Example No. 3. Write the exchange reaction between sodium carbonate and hydrochloric acid, consider it from the point of view of TED. Write the reaction equation in molecular form and assign the coefficients. Sodium chloride is formed and CO2 gas (carbon dioxide or carbon monoxide (IV)) is released. It is formed due to the decomposition of weak, breaking up into oxide and water. Na2CO3 + 2HCl = 2NaCl + CO2+H2OAnalyze all substances for their solubility in water and dissociation. Carbon dioxide leaves the system as a gaseous compound, water is a poorly dissociating substance. All other substances disintegrate into ions. Write down the complete ionic equation, underline and abbreviate the identical ions: 2Na+ +CO3 2- +2H+ +2Cl- =2Na+ +2Cl- +CO2+H2O The ionic abbreviated equation remains: CO3 2- +2H+ =CO2+H2O
Video on the topic
note
To correctly determine the number of ions, you need to multiply the coefficient in front of the formula by the index.
Be sure to check the coefficients in reaction equations.
Sources:
- how to write equations for ion exchange reactions
A reaction equation is a conventional notation of a chemical process in which some substances are converted into others with a change in properties. To record chemical reactions, formulas of substances and knowledge of chemical properties connections.
Instructions
Write the formulas correctly according to them. For example, put aluminum oxide Al₂O₃, index 3 from aluminum (corresponding to its oxidation state in this compound) near oxygen, and index 2 (oxidation state of oxygen) near aluminum.
If the oxidation state is +1 or -1, then the index is not given. For example, you need to write down the formula. Nitrate is an acidic residue of nitric acid (-NO₃, d.o. -1), ammonium (-NH₄, d.o. +1). Thus, ammonium nitrate is NH₄ NO₃. Sometimes the oxidation number is indicated in the name of the compound. Sulfur oxide (VI) - SO₃, silicon oxide (II) SiO. Some (gases) are written with index 2: Cl₂, J₂, F₂, O₂, H₂, etc.
It is necessary to know which substances react. Visible reactions: gas evolution, color change and precipitation. Very often reactions pass without visible changes.
Example 1: neutralization reaction
H₂SO₄ + 2 NaOH → Na₂SO₄ + 2 H₂O
Sodium hydroxide reacts with sulfuric acid to form soluble salt sodium sulfate and water. The sodium ion is split off and combines with the acidic one, replacing the hydrogen. The reaction takes place without external signs.
Example 2: iodoform test
С₂H₅OH + 4 J₂ + 6 NaOH→CHJ₃↓ + 5 NaJ + HCOONa + 5 H₂O
The reaction occurs in several stages. The end result is the precipitation of iodoform crystals yellow color(qualitative reaction to).
Example 3:
Zn + K₂SO₄ ≠
The reaction is impossible, because In the series of metal stresses, zinc comes after potassium and cannot displace it from compounds.
The law of conservation of mass states: the mass of substances that react is equal to the mass of the substances formed. Correct recording of a chemical reaction is half of it. It is necessary to set the coefficients. Start equalizing with those compounds whose formulas contain large indices.
K₂Cr₂O₇ + 14 HCl → 2 CrCl₃ + 2 KCl + 3 Cl₂ + 7 H₂O
Start setting the coefficients with potassium dichromate, because its formula contains the largest index (7).
Such accuracy in recording is necessary for calculating mass, volume, concentration, released energy and other quantities. Be careful. Memorize the most common formulas and bases, as well as acid residues.
Sources:
- chemistry equation
Working with formulas and equations in the Word office application, included in the Microsoft Office package, is provided by a special “Formula Editor” utility, which is part of the Math Type program.
Instructions
Click the "Start" button to open the main menu of the system and go to "All Programs".
Point to Microsoft Office and launch the Word application.
Call the context menu of the toolbar by right-clicking and select “Settings”.
Since electrolytes in solution are in the form of ions, reactions between solutions of salts, bases and acids are reactions between ions, i.e. ion reactions. Some of the ions, participating in the reaction, lead to the formation of new substances (lowly dissociating substances, precipitation, gases, water), while other ions, present in the solution, do not produce new substances, but remain in the solution. In order to show which ions interaction leads to the formation of new substances, molecular, complete and brief ionic equations are drawn up.
IN molecular equations All substances are presented in the form of molecules. Complete ionic equations show the entire list of ions present in the solution during a given reaction. Brief ionic equations are composed only of those ions, the interaction between which leads to the formation of new substances (lowly dissociating substances, sediments, gases, water).
When composing ionic reactions, it should be remembered that substances are slightly dissociated (weak electrolytes), slightly and poorly soluble (precipitated - “ N”, “M”, see appendix, table 4) and gaseous ones are written in the form of molecules. Strong electrolytes, almost completely dissociated, are in the form of ions. The “↓” sign after the formula of a substance indicates that this substance is removed from the reaction sphere in the form of a precipitate, and the “” sign indicates that the substance is removed in the form of a gas.
The procedure for composing ionic equations using known molecular equations Let's look at the example of the reaction between solutions of Na 2 CO 3 and HCl.
1. The reaction equation is written in molecular form:
Na 2 CO 3 + 2HCl → 2NaCl + H 2 CO 3
2. The equation is rewritten in ionic form, with well-dissociating substances written in the form of ions, and poorly dissociating substances (including water), gases or sparingly soluble substances - in the form of molecules. The coefficient in front of the formula of a substance in a molecular equation applies equally to each of the ions that make up the substance, and therefore it is placed in front of the ion in the ionic equation:
2 Na + + CO 3 2- + 2H + + 2Cl -<=>2Na + + 2Cl - + CO 2 + H 2 O
3. From both sides of the equality, ions found in the left and right parts(underlined with appropriate dashes):
2Na++ CO 3 2- + 2H + + 2Cl -<=> 2Na+ + 2Cl -+ CO 2 + H 2 O
4. The ionic equation is written in its final form (short ionic equation):
2H + + CO 3 2-<=>CO 2 + H 2 O
If during the reaction, and/or slightly dissociated, and/or sparingly soluble, and/or gaseous substances, and/or water are formed, and such compounds are absent in the starting substances, then the reaction will be practically irreversible (→), and for it it is possible to compose a molecular, complete and brief ionic equation. If such substances are present both in the reagents and in the products, then the reaction will be reversible (<=>):
Molecular equation: CaCO 3 + 2HCl<=>CaCl 2 + H 2 O + CO 2
Complete ionic equation: CaCO 3 + 2H + + 2Cl –<=>Ca 2+ + 2Cl – + H 2 O + CO 2