Parameter name	Meaning
Article subject:	Ionic equations
Rubric (thematic category)	Metals and Welding

Most chemical reactions take place in solutions. Electrolyte solutions contain ions, in connection with this, reactions in electrolyte solutions are actually reduced to reactions between ions. Reactions between ions are called ionic reactions, and the equations of such reactions are called ionic equations. 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.

white matter precipitates, then an arrow pointing down is placed next to its formula, and if a gaseous substance is released during the reaction, then an arrow pointing up is placed next to its formula.

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, that is, those that do not participate in the reaction in the left and right often equations), then we obtain a reduced ionic reaction equation:

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

Ion exchange reactions proceed to completion when either a precipitate or a low-dissociating substance, such as water, is formed. By adding an excess of nitric acid solution to a solution of sodium hydroxide dyed crimson with phenolphthalein, the solution will become colorless, which will serve as a signal for a chemical reaction:

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

The indicated reaction of the interaction of a strong acid with an alkali is commonly called the neutralization reaction. This is a special case of an exchange reaction.

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:

and then divide the resulting precipitate into three parts and add a solution of sulfuric acid to the precipitate in the first test tube, a solution of 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. This would mean that in all cases chemical reaction, the essence of which is reflected using the same ionic equation.

To make sure of this, write down the molecular, full and abbreviated ionic equations of the given 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. Next, pour a solution of hydrochloric acid into the first, and nitric acid into the second. In both cases, we will notice a characteristic "boiling" due to the released carbon dioxide. We write the reaction equations for the first case:

Reactions occurring in electrolyte solutions are described using ionic equations. These reactions are called ion exchange reactions, since electrolytes exchange their ions in solutions. Τᴀᴋᴎᴍ ᴏϬᴩᴀᴈᴏᴍ, two conclusions can be drawn. 1. Reactions in aqueous electrolyte solutions are reactions between ions, and therefore are depicted as ionic equations. Οʜᴎ are simpler than molecular ones and are more general.

2. Ion exchange reactions in electrolyte solutions proceed almost irreversibly only if, as a result, a precipitate, a gas, or a low-dissociating substance is formed.

7. Complex compounds

Ionic equations - concept and types. Classification and features of the category "Ionic Equations" 2017, 2018.

Subject: chemical bond. Electrolytic dissociation

Lesson: Writing Equations for Ion Exchange Reactions

Let us compose the reaction equation 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. A single collection of 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.

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 sides of the equation, i.e. ions that do not participate in the reaction (2Na + and 2Cl - in the left and right parts of the equation), then we get the reduced ionic reaction equation:

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

Rice. 133.
Reaction between 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, then the solution will discolor, which will serve as a signal for a chemical reaction:

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 a chemical reaction has taken place, 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.

Work with computer

1. Talk to electronic application. Study the material of the lesson and complete the suggested tasks.
2. Search the Internet for email addresses that can serve as additional sources, revealing the content of the keywords and phrases of the paragraph. Offer the teacher your help in preparing a new lesson - make a report on the key words and phrases of the next paragraph.

Questions and tasks

Quite often, schoolchildren and students have to make up the so-called. ionic reaction equations. In particular, problem 31, proposed at the Unified State Examination in Chemistry, is devoted to this topic. In this article, we will discuss in detail the algorithm for writing short and complete ionic equations, we will analyze many examples different levels difficulties.

Why ionic equations are needed

Let me remind you that when many substances are dissolved in water (and not only in water!) A process of dissociation occurs - substances break up into ions. For example, HCl molecules in an aqueous medium dissociate into hydrogen cations (H + , more precisely, H 3 O +) and chlorine anions (Cl -). Sodium bromide (NaBr) is in an aqueous solution not in the form of molecules, but in the form of hydrated Na + and Br - ions (by the way, ions are also present in solid sodium bromide).

When writing the "ordinary" (molecular) equations, we do not take into account that not molecules enter into the reaction, but ions. Here, for example, is the equation for the reaction between hydrochloric acid and sodium hydroxide:

HCl + NaOH = NaCl + H 2 O. (1)

Of course, this diagram does not quite correctly describe the process. As we have already said, there are practically no HCl molecules in an aqueous solution, but there are H + and Cl - ions. The same is true for NaOH. It would be better to write the following:

H + + Cl - + Na + + OH - = Na + + Cl - + H 2 O. (2)

That's what it is complete ionic equation. Instead of "virtual" molecules, we see particles that are actually present in the solution (cations and anions). We will not dwell on the question why we have written H 2 O in molecular form. This will be explained a little later. As you can see, there is nothing complicated: we have replaced the molecules with ions, which are formed during their dissociation.

However, even the complete ionic equation is not perfect. Indeed, take a closer look: both in the left and in the right parts of equation (2) there are identical particles - Na + cations and Cl - anions. These ions do not change during the reaction. Why then are they needed at all? Let's remove them and get short ionic equation:

H + + OH - = H 2 O. (3)

As you can see, it all comes down to the interaction of H + and OH - ions with the formation of water (neutralization reaction).

All complete and short ionic equations are written down. If we solved problem 31 at the exam in chemistry, we would get the maximum mark for it - 2 points.

So, once again about the terminology:

• HCl + NaOH = NaCl + H 2 O - molecular equation ("usual" equation, schematically reflecting the essence of the reaction);
• H + + Cl - + Na + + OH - = Na + + Cl - + H 2 O - complete ionic equation (real particles in solution are visible);
• H + + OH - = H 2 O - a short ionic equation (we removed all the "garbage" - particles that do not participate in the process).

Algorithm for writing ionic equations

1. We compose the molecular equation of the reaction.
2. All particles that dissociate in solution to a noticeable degree are written as ions; substances that are not prone to dissociation, we leave "in the form of molecules."
3. We remove from the two parts of the equation the so-called. observer ions, i.e., particles that do not participate in the process.
4. We check the coefficients and get the final answer - a short ionic equation.

Example 1. Write a complete and short ionic equation describing the interaction of aqueous solutions of barium chloride and sodium sulfate.

Solution. We will act in accordance with the proposed algorithm. Let's set up the molecular equation first. Barium chloride and sodium sulfate are two salts. Let's look at the section of the reference book "Properties of inorganic compounds". We see that salts can interact with each other if a precipitate forms during the reaction. Let's check:

Exercise 2. Complete the equations for the following reactions:

1. KOH + H 2 SO 4 \u003d
2. H 3 PO 4 + Na 2 O \u003d
3. Ba(OH) 2 + CO 2 =
4. NaOH + CuBr 2 =
5. K 2 S + Hg (NO 3) 2 \u003d
6. Zn + FeCl 2 =

Exercise 3. Write the molecular equations for the reactions (in aqueous solution) between: a) sodium carbonate and nitric acid, b) nickel (II) chloride and sodium hydroxide, c) orthophosphoric acid and calcium hydroxide, d) silver nitrate and potassium chloride, e) phosphorus oxide (V) and potassium hydroxide.

I sincerely hope that you had no problems completing these three tasks. If this is not the case, you need to return to the topic " Chemical properties main classes of inorganic compounds".

How to turn a molecular equation into a complete ionic equation

The most interesting begins. We must understand which substances should be written as ions and which should be left in "molecular form". You have to remember the following.

In the form of ions write:

• soluble salts (I emphasize that only salts are highly soluble in water);
• alkalis (let me remind you that water-soluble bases are called alkalis, but not NH 4 OH);
• strong acids (H 2 SO 4 , HNO 3 , HCl, HBr, HI, HClO 4 , HClO 3 , H 2 SeO 4 , ...).

As you can see, this list is easy to remember: it includes strong acids and bases and all soluble salts. By the way, to especially vigilant young chemists who may be outraged by the fact that strong electrolytes (insoluble salts) are not included in this list, I can tell you the following: NOT including insoluble salts in this list does not at all reject the fact that they are strong electrolytes.

All other substances must be present in the ionic equations in the form of molecules. Those demanding readers who are not satisfied with the vague term "all other substances", and who, following the example of the hero of a famous film, demand "announce full list I give the following information.

In the form of molecules, write:

• all insoluble salts;
• all weak bases (including insoluble hydroxides, NH 4 OH and similar substances);
• all weak acids (H 2 CO 3 , HNO 2 , H 2 S, H 2 SiO 3 , HCN, HClO, almost all organic acids ...);
• in general, all weak electrolytes (including water!!!);
• oxides (all types);
• all gaseous compounds (in particular H 2 , CO 2 , SO 2 , H 2 S, CO);
• simple substances (metals and non-metals);
• almost all organic compounds(exception - water-soluble salts of organic acids).

Phew, I don't think I forgot anything! Although it is easier, in my opinion, to remember list No. 1. Of the fundamentally important in list No. 2, I will once again note the water.

Let's train!

Example 2. Make a complete ionic equation describing the interaction of copper (II) hydroxide and hydrochloric acid.

Solution. Let's start, of course, with the molecular equation. Copper (II) hydroxide is an insoluble base. All insoluble bases react with strong acids to form a salt and water:

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

And now we find out which substances to write in the form of ions, and which - in the form of molecules. The lists above will help us. Copper (II) hydroxide is an insoluble base (see solubility table), a weak electrolyte. Insoluble bases are written in molecular form. HCl- strong acid, in solution almost completely dissociates into ions. CuCl 2 is a soluble salt. We write in ionic form. Water - only in the form of molecules! We get the full ionic equation:

Cu (OH) 2 + 2H + + 2Cl - \u003d Cu 2+ + 2Cl - + 2H 2 O.

Example 3. Write a complete ionic equation for the reaction of carbon dioxide with an aqueous solution of NaOH.

Solution. Carbon dioxide is a typical acidic oxide, NaOH is an alkali. When acidic oxides interact with aqueous solutions of alkalis, salt and water are formed. We compose the molecular reaction equation (do not forget, by the way, about the coefficients):

CO 2 + 2NaOH \u003d Na 2 CO 3 + H 2 O.

CO 2 - oxide, gaseous compound; keep the molecular shape. NaOH - strong base (alkali); written in the form of ions. Na 2 CO 3 - soluble salt; write in the form of ions. Water is a weak electrolyte, practically does not dissociate; leave it in molecular form. We get the following:

CO 2 + 2Na + + 2OH - \u003d Na 2+ + CO 3 2- + H 2 O.

Example 4. Sodium sulfide in aqueous solution reacts with zinc chloride to form a precipitate. Write the complete ionic equation for this reaction.

Solution. Sodium sulfide and zinc chloride are salts. When these salts interact, zinc sulfide precipitates:

Na 2 S + ZnCl 2 \u003d ZnS ↓ + 2NaCl.

I will immediately write down the full ionic equation, and you will analyze it yourself:

2Na + + S 2- + Zn 2+ + 2Cl - = ZnS↓ + 2Na + + 2Cl - .

Here are some tasks for you to independent work and a little test.

Exercise 4. Write the molecular and full ionic equations for the following reactions:

1. NaOH + HNO3 =
2. H 2 SO 4 + MgO =
3. Ca(NO 3) 2 + Na 3 PO 4 =
4. CoBr 2 + Ca(OH) 2 =

Exercise 5. Write complete ionic equations describing the interaction of: a) nitric oxide (V) with an aqueous solution of barium hydroxide, b) a solution of cesium hydroxide with hydroiodic acid, c) aqueous solutions of copper sulfate and potassium sulfide, d) calcium hydroxide and aqueous solution iron(III) nitrate.

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 (low-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.

IN 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 low-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 and / or slightly dissociated, and / or hardly 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, full and short ionic equation. 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