When compiling ionic equations, one should be guided by the fact that the formulas of low-dissociating, insoluble and gaseous substances are written in molecular form. If a substance precipitates, then, as you already know, an arrow pointing down (↓) is placed next to its formula, and if a gaseous substance is released during the reaction, then an upward arrow () is placed next to its formula.

For example, if a solution of barium chloride BaCl 2 is added to a solution of sodium sulfate Na 2 SO 4 (Fig. 132), then a white precipitate of barium sulfate BaSO 4 is formed as a result of the reaction. We write the molecular reaction equation:

Rice. 132.
Reaction between sodium sulfate and barium chloride

We rewrite this equation, depicting strong electrolytes as ions, and those leaving the reaction sphere as molecules:

We have thus written down the complete ionic reaction equation. If we exclude identical ions from both parts of the equation, i.e. ions that do not participate in the reaction (2Na + and 2Cl - in the left and right parts equation), then we get the reduced ionic equation of the reaction:

This equation shows that the essence of the reaction is reduced to the interaction of barium ions Ba 2+ and sulfate ions, as a result of which a BaSO 4 precipitate is formed. In this case, it does not matter at all which electrolytes included these ions before the reaction. A similar interaction can also be observed between K 2 SO 4 and Ba(NO 3) 2 , H 2 SO 4 and BaCl 2 .

Laboratory experiment No. 17
Interaction of solutions of sodium chloride and silver nitrate

To 1 ml of sodium chloride solution in a test tube, add a few drops of silver nitrate solution with a pipette. What are you watching? Write down the molecular and ionic equations of the reaction. According to the abbreviated ionic equation, offer several options for carrying out such a reaction with other electrolytes. Write down the molecular equations of the reactions performed.

Thus, the abbreviated ionic equations are equations in general view, which characterize the essence of a chemical reaction and show which ions react and which substance is formed as a result.

Rice. 133.
Interaction nitric acid and sodium hydroxide

If an excess of nitric acid solution (Fig. 133) is added to a solution of sodium hydroxide, colored crimson by phenolphthalein, the solution will become colorless, which will serve as a signal for a chemical reaction to occur:

NaOH + HNO 3 \u003d NaNO 3 + H 2 O.

The full ionic equation for this reaction is:

Na + + OH - + H + + NO 3 = Na + + NO - 3 + H 2 O.

But since the Na + and NO - 3 ions in the solution remain unchanged, they can not be written, and ultimately the abbreviated ionic reaction equation is written as follows:

H + + OH - \u003d H 2 O.

It shows that the interaction of a strong acid and an alkali is reduced to the interaction of H + ions and OH - ions, as a result of which a low-dissociating substance is formed - water.

Such an exchange reaction can occur not only between acids and alkalis, but also between acids and insoluble bases. For example, if you get a blue precipitate of insoluble copper (II) hydroxide by reacting copper (II) sulfate with alkali (Fig. 134):

and then divide the resulting precipitate into three parts and add a solution of sulfuric acid to the precipitate in the first test tube, hydrochloric acid to the precipitate in the second test tube, and a solution of nitric acid to the precipitate in the third test tube, then the precipitate will dissolve in all three test tubes (Fig. 135) .

Rice. 135.
The interaction of copper (II) hydroxide with acids:
a - sulfuric; b - salt; in - nitrogen

This will mean that in all cases chemical reaction, the essence of which is reflected using the same ionic equation.

Cu(OH) 2 + 2H + = Cu 2+ + 2H 2 O.

To verify this, write down the molecular, full and abbreviated ionic equations of the above reactions.

Laboratory experiment No. 18
Obtaining insoluble hydroxide and its interaction with acids

Pour 1 ml of iron (III) chloride or sulfate solution into three test tubes. Pour 1 ml of alkali solution into each test tube. What are you watching? Then add solutions of sulfuric, nitric and hydrochloric acids to the test tubes, respectively, until the precipitate disappears. Write down the molecular and ionic equations of the reaction.

Suggest several options for carrying out such a reaction with other electrolytes. Write down the molecular equations for the proposed reactions.

Consider ionic reactions that proceed with the formation of gas.

Pour 2 ml of sodium carbonate and potassium carbonate solutions into two test tubes. Then pour hydrochloric acid into the first, and a solution of nitric acid into the second (Fig. 136). In both cases, we will notice a characteristic "boiling" due to the carbon dioxide released.

Rice. 136.
Interaction of soluble carbonates:
a - c hydrochloric acid; b - with nitric acid

Let us write the molecular and ionic reaction equations for the first case:

Reactions occurring in electrolyte solutions are written using ionic equations. These reactions are called ion exchange reactions, since electrolytes exchange their ions in solution. Thus, two conclusions can be drawn.

Keywords and phrases

1. Molecular and ionic equations of reactions.
2. Ion exchange reactions.
3. Neutralization reactions.

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Questions and tasks

Since electrolytes in solution are in the form of ions, the reactions between solutions of salts, bases and acids are reactions between ions, i.e. ionic reactions. Some of the ions, participating in the reaction, lead to the formation of new substances (slightly dissociating substances, precipitation, gases, water), while other ions, being present in the solution, do not give new substances, but remain in the solution. In order to show the interaction of which ions leads to the formation of new substances, molecular, complete and brief ionic equations are composed.

AT molecular equations All substances are represented as molecules. Complete ionic equations show the entire list of ions present in 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 (slightly dissociating substances, precipitation, gases, water).

When compiling ionic reactions, it should be remembered that substances are slightly dissociated (weak electrolytes), slightly - and sparingly soluble (precipitating - “ H”, “M”, see appendix‚ table 4) and gaseous 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 the removal of a substance in the form of a gas.

The procedure for compiling ionic equations from known molecular equations consider 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, while well-dissociating substances are written in the form of ions, and poorly dissociating substances (including water), gases or hardly soluble substances are written in the form of molecules. The coefficient before the formula of a substance in the molecular equation equally applies to each of the ions that make up the substance, and therefore it is taken out in the ionic equation before the ion:

2 Na + + CO 3 2- + 2H + + 2Cl -<=>2Na + + 2Cl - + CO 2 + H 2 O

3. From both parts of the equality, ions that occur in the left and right parts are excluded (reduced) (underlined by the corresponding dashes):

2 Na++ 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 in the course of the reaction are formed and / or slightly dissociated, and / or sparingly soluble, and / or gaseous substances, and / or water, and such compounds are absent in the starting materials, then the reaction will be practically irreversible (→), and a molecular, complete and short ionic equation can be compiled for it. If such substances exist both in the reactants‚ and in the products, then the reaction will be reversible (<=>):

molecular equation: CaCO 3 + 2HCl<=>CaCl 2 + H 2 O + CO 2

Full ionic equation: CaCO 3 + 2H + + 2Cl -<=>Ca 2+ + 2Cl - + H 2 O + CO 2

When any strong acid is neutralized with any strong base, about heat is released for each mole of water formed:

This suggests that such reactions are reduced to one process. We will obtain the equation of this process if we consider in more detail one of the above reactions, for example, the first one. We 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 during the reaction, the ions and did not change. Therefore, we rewrite the equation again, excluding these ions from both sides of the equation. We get:

Thus, the reactions of neutralization of any strong acid with any strong base are reduced to the same process - to 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 formation of water from ions is reversible, which can be expressed by the equation

However, as we shall see below, water is a very weak electrolyte and dissociates only to a negligible degree. 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 the end.

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:

Similar reactions are also reduced 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 precipitate in molecular form:

As can be seen, the ions and do not undergo changes during the reaction. Therefore, we eliminate them and rewrite the equation again:

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 ions and 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 practically comes to an end.

The formation of a precipitate will always be observed when ions and are in a significant concentration 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 for an ion, and an ion as a reactant for an ion.

In the future, we will widely use the ion-molecular form of writing the equations of 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 solubility in water of the most important salts is given in table. fifteen.

Table 15. Solubility of the most important salts in water

Ionic-molecular equations help to understand the features of reactions between electrolytes. Consider, as an example, several reactions involving 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 is reduced to the same process - the formation of water molecules from hydrogen ions and hydroxide ions.

However, when a strong acid is neutralized with a weak base, a weak acid with a strong or weak base, the thermal effects are different. Let us write the 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 ones 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 (nitric acid) with a weak base (ammonium hydroxide):

Here, in the form of ions, we must write the acid and the resulting salt, and in the form of molecules, ammonium hydroxide and water:

Ions do not undergo changes. Omitting them, we obtain the ion-molecular equation:

Neutralization of a weak acid (acetic acid) with a weak base (ammonium hydroxide):

In this reaction, all substances, except for the resulting weak electrolytes. Therefore, the ion-molecular form of the equation has the form:

Comparing the obtained ion-molecular equations, we see that they are all different. Therefore, it is clear that the heats of the considered reactions are not the same.

As already mentioned, neutralization reactions strong acids strong bases, during which hydrogen ions and hydroxide ions are combined into a water molecule, flow almost to the end. Neutralization reactions, on the other hand, 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, but also on the left side of the ion-molecular equation, do not proceed to the end.

They reach a state of equilibrium in which the salt coexists with the acid and base from which it is derived. 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 make 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 exposed. Water is a poorly dissociated substance; it is practically undissociated 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, write the abbreviated ionic equation:

Fe(OH) 3 + 3H + = Fe 3+ + 3H 2 O

This reaction proceeds to the end, because a poorly dissociated substance, water, is formed.

Let's write an equation for the reaction between sodium carbonate and magnesium nitrate.

Na 2 CO 3 + Mg (NO 3) 2 \u003d 2NaNO 3 + MgCO 3 ↓

We 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 ↓

We cross out the same number of nitrate anions and sodium cations on the left and right, we write the abbreviated ionic equation:

CO 3 2- + Mg 2+ \u003d MgCO 3 ↓

This reaction proceeds to the end, 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 \u003d 2NaNO 3 + CO 2 + H 2 O

(Carbon dioxide and water are decomposition products 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 the end, because as a result, gas is released and water is formed.

Let's make 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 this case, we can say that in order to obtain calcium carbonate, it is necessary that the composition of the first substance includes calcium cations, and the composition of the second - carbonate anions. Let us compose the molecular equations of reactions that satisfy this condition:

CaCl 2 + K 2 CO 3 \u003d 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 inst. / P.A. Orzhekovsky, L.M. Meshcheryakova, L.S. Pontak. - M.: AST: Astrel, 2007. (§17)

2. Orzhekovsky P.A. Chemistry: 9th grade: textbook for general education. inst. / P.A. Orzhekovsky, L.M. Meshcheryakova, M.M. Shalashova. - M.: Astrel, 2013. (§ 9)

3. Rudzitis G.E. Chemistry: inorgan. chemistry. Organ. chemistry: textbook. for 9 cells. / G.E. Rudzitis, F.G. Feldman. - M .: Education, JSC "Moscow textbooks", 2009.

4. Khomchenko I.D. Collection of tasks 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, leading. scientific ed. I. Leenson. - M.: Avanta +, 2003.

Additional web resources

1. Single collection digital educational resources (video experiences on the topic): ().

2. Electronic version of the journal "Chemistry and Life": ().

Homework

1. Mark in the table with a plus sign pairs of substances between which ion exchange reactions are possible, going to the end. Write reaction equations in molecular, full and reduced ionic form.

Reactive Substances	K2 CO3		AgNO3	FeCl3	HNO3
CuCl2

2. with. 67 Nos. 10,13 from P.A. Orzhekovsky "Chemistry: 9th grade" / P.A. Orzhekovsky, L.M. Meshcheryakova, M.M. Shalashova. - M.: Astrel, 2013.

11. Electrolytic dissociation. Ionic equations reactions

11.5. Ionic reaction equations

Since electrolytes decompose into ions in aqueous solutions, it can be argued that reactions in aqueous electrolyte solutions are reactions between ions. Such reactions can proceed both with a change in the oxidation state of atoms:

Fe 0   + 2 H + 1 Cl \u003d Fe + 2 Cl 2 + H 0 2

and without change:

NaOH + HCl \u003d NaCl + H 2 O

In the general case, reactions between ions in solutions are called ionic, and if they are exchange, then ion exchange reactions. Ion exchange reactions proceed only when substances are formed that leave the reaction sphere in the form of: a) a weak electrolyte (for example, water, acetic acid); b) gas (CO 2, SO 2); c) a sparingly soluble substance (precipitate). The formulas of sparingly soluble substances are determined according to the solubility table (AgCl, BaSO 4, H 2 SiO 3, Mg (OH) 2, Cu (OH) 2 etc.). The formulas for gases and weak electrolytes must be memorized. Note that weak electrolytes can be highly soluble in water: for example, CH 3 COOH, H 3 PO 4 , HNO 2.

The essence of ion exchange reactions reflect ionic reaction equations, which are obtained from molecular equations subject to the following rules:

1) in the form of ions, the formulas of weak electrolytes, insoluble and poorly soluble substances, gases, oxides, hydroanions of weak acids (HS - , HSO 3 - , HCO 3 - , H 2 PO 4 - , HPO 4 2 - ) are not written down; the exception is the HSO ion 4 - in a dilute solution); hydroxocations of weak bases (MgOH + , CuOH +); complex ions ( 3− , 2− , 2−);

2) in the form of ions, formulas of strong acids, alkalis, water-soluble salts are represented. The formula Ca(OH) 2 is expressed as ions when lime water is used, but not as ions when milk of lime contains insoluble Ca(OH) 2 particles.

There are full ionic and abbreviated (short) ionic reaction equations. The reduced ionic equation lacks the ions represented in both sides of the full ionic equation. Examples of writing molecular, full ionic and reduced ionic equations:

• NaHCO 3 + HCl \u003d NaCl + H 2 O + CO 2 - molecular,

Na + + HCO 3 - + H + + Cl - \u003d Na + + Cl - + H 2 O + CO 2   - full ionic,

HCO 3 − + H + = H 2 O + CO 2   - abbreviated ionic;

• BaCl 2 + K 2 SO 4 = BaSO 4 ↓ + 2KCl - molecular,

Ba 2 + + 2 Cl - + 2 K + + SO 4 2 - = BaSO 4   ↓ + 2 K + + 2 Cl - - full ionic,

Ba 2 + + SO 4 2 - = BaSO 4   ↓ - abbreviated ionic.

Sometimes the full ionic and reduced ionic equations are the same:

Ba(OH) 2 + H 2 SO 4 = BaSO 4 ↓ + 2H 2 O

Ba 2+ + 2OH - + 2H + + SO 4 2 - = BaSO 4 ↓ + 2H 2 O,

and for some reactions, the ionic equation cannot be written at all:

3Mg(OH) 2 + 3H 3 PO 4 = Mg 3 (PO 4) 2 ↓ + 6H 2 O

Example 11.5. Indicate a pair of ions that can be present in the full ion-molecular equation if it corresponds to a reduced ion-molecular equation

Ca 2 + + SO 4 2 - \u003d CaSO 4.

1) SO 3 2 − and H + ; 3) CO 3 2 - and K +; 2) HCO 3 - and K +; 4) Cl - and Pb 2+.

Solution. The correct answer is 2):

Ca 2 + + 2 HCO 3 - + 2 K + + SO 4 2 - = CaSO 4   ↓ + 2 HCO 3 - + 2 K + (Ca(HCO 3) 2 salt soluble) or Ca 2+ + SO 4 2 - = CaSO4.

For other cases we have:

1) CaSO 3 + 2H + + SO 4 2 - = CaSO 4 ↓ + H 2 O + SO 2;

3) CaCO 3 + 2K + + SO 4 2 - (the reaction does not proceed);

4) Ca 2+ + 2Cl - + PbSO 4 (the reaction does not proceed).

Answer: 2).

Substances (ions) that react with each other in an aqueous solution (i.e., the interaction between them is accompanied by the formation of a precipitate, gas or weak electrolyte) cannot coexist in an aqueous solution in significant quantities

Table 11.2

Examples of pairs of ions that do not exist together in significant amounts in aqueous solution

Example 11.6. Indicate in this series: HSO 3 - , Na + , Cl - , CH 3 COO - , Zn 2+ - formulas of ions that cannot be present in significant quantities: a) in acidic environment; b) in an alkaline environment.

Solution. a) In an acidic environment, i.e. together with H + ions, HSO 3 - and CH 3 COO - anions cannot be present, since they react with hydrogen cations, forming a weak electrolyte or gas:

CH 3 COO − + H + ⇄ CH 3 COOH

HSO 3 - + H + ⇄ H 2 O + SO 2

b) HSO 3 - and Zn 2+ ions cannot be present in an alkaline environment, since they react with hydroxide ions to form either a weak electrolyte or a precipitate:

HSO 3 - + OH - ⇄ H 2 O + SO 3 2 -

Zn 2+ + 2OH– = Zn(OH) 2 ↓.

Answer: a) HSO 3 - and CH 3 COO -; b) HSO 3 − and Zn 2+.

Residues of acid salts of weak acids cannot be present in significant quantities in either acidic or alkaline environments, because in both cases a weak electrolyte is formed.

The same can be said about the residues of basic salts containing a hydroxo group:

CuOH + + OH - \u003d Cu (OH) 2 ↓