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. The general characteristics of the solubility of the most important salts in water are 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 those formed are 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, 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.
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 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 of the journal “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.
11. Electrolytic dissociation. Ionic reaction equations
11.5. Ionic reaction equations
Since electrolytes in aqueous solutions break down into ions, it can be argued that reactions in aqueous solutions of electrolytes are reactions between ions. Such reactions can occur with a change in the oxidation state of atoms:
Fe 0 + 2 H + 1 Cl = Fe + 2 Cl 2 + H 0 2
and without change:
NaOH + HCl = NaCl + H2O
In general, reactions between ions in solutions are called ionic, and if they are exchange reactions, then ion exchange reactions. Ion exchange reactions occur 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) sparingly soluble substance (precipitate). The formulas of sparingly soluble substances are determined from the solubility table (AgCl, BaSO 4, H 2 SiO 3, Mg(OH) 2, Cu(OH) 2, etc.). The formulas of gases and weak electrolytes need to 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 is reflected ionic reaction equations, which are obtained from molecular equations following the following rules:
1) the formulas of weak electrolytes, insoluble and slightly soluble substances, gases, oxides, hydroanions of weak acids (HS − , HSO 3 − , HCO 3 − , H 2 PO 4 − , HPO 4 2 − ; exception - HSO ion) are not written in the form of ions 4 – in a dilute solution); hydroxocations of weak bases (MgOH +, CuOH +); complex ions ( 3− , 2− , 2− );
2) the formulas of strong acids, alkalis, and water-soluble salts are represented in the form of ions. The formula Ca(OH) 2 is written as ions if lime water is used, but is not written as ions in the case of lime milk containing insoluble Ca(OH) 2 particles.
There are full ionic and abbreviated (short) ionic reaction equations. The abbreviated ionic equation is missing the ions present on both sides of the full ionic equation. Examples of writing molecular, full ionic and abbreviated ionic equations:
- NaHCO 3 + HCl = NaCl + H 2 O + CO 2 - molecular,
Na + + HCO 3 − + H + + Cl − = Na + + Cl − + H 2 O + CO 2 - complete 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 − - complete ionic,
Ba 2 + + SO 4 2 − = BaSO 4 ↓ - abbreviated ionic.
Sometimes the full ionic equation and the abbreviated ionic equation 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 compiled 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 the abbreviated ion-molecular equation
Ca 2 + + SO 4 2 − = 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 is 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 − (reaction does not occur);
4) Ca 2+ + 2Cl − + PbSO 4 (reaction does not occur).
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 ion pairs that do not exist together in significant quantities in aqueous solution
Example 11.6. Indicate in this row: HSO 3 − , Na + , Cl − , CH 3 COO − , Zn 2+ - formulas of ions that cannot be present in significant quantities: a) in an acidic environment; b) in an alkaline environment.
Solution. a) In an acidic environment, i.e. together with H + ions, the anions HSO 3 − and CH 3 COO − 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 medium, 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 an acidic or an alkaline medium, 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 − = Cu(OH) 2 ↓
Instructions
Consider an example of the formation of a sparingly soluble compound.
Na2SO4 + BaCl2 = BaSO4 + 2NaCl
Or an ionic version:
2Na+ +SO42- +Ba2++ 2Cl- = BaSO4 + 2Na+ + 2Cl-
When solving ionic equations, the following rules must be observed:
Identical ions from both parts are excluded;
It should be remembered that the sum of the electric charges on the left side of the equation must be equal to the sum of the electric charges on the right side of the equation.
Write ionic equations for the interaction between aqueous solutions of the following substances: a) HCl and NaOH; b) AgNO3 and NaCl; c) K2CO3 and H2SO4; d) CH3COOH and NaOH.
Solution. Write down the equations of interaction of these substances in molecular form:
a) HCl + NaOH = NaCl + H2O
b) AgNO3 + NaCl = AgCl + NaNO3
c) K2CO3 + H2SO4 = K2SO4 + CO2 + H2O
d) CH3COOH + NaOH = CH3COONa + H2O
Note that the interaction of these substances is possible, because the result is the binding of ions with the formation of either weak (H2O), or sparingly soluble substance (AgCl), or gas (CO2).
By excluding identical ions from the left and right sides of the equality (in the case of option a) - ions and , in case b) - sodium ions and -ions, in case c) - potassium ions and sulfate ions), d) - sodium ions, you get solving these ionic equations:
a) H+ + OH- = H2O
b) Ag+ + Cl- = AgCl
c) CO32- + 2H+ = CO2 + H2O
d) CH3COOH + OH- = CH3COO- + H2O
Quite often in independent and test work there are tasks that involve solving reaction equations. However, without some knowledge, skills and abilities, even the simplest chemical equations don't write.
Instructions
First of all, you need to study the basic organic and inorganic compounds. As a last resort, you can have a suitable cheat sheet in front of you that can help during the task. After training, the necessary knowledge and skills will still be stored in your memory.
The basic material is covering, as well as methods for obtaining each compound. They are usually presented in the form of general diagrams, for example: 1. + base = salt + water
2. acid oxide + base = salt + water
3. basic oxide + acid = salt + water
4. metal + (diluted) acid = salt + hydrogen
5. soluble salt + soluble salt = insoluble salt + soluble salt
6. soluble salt + = insoluble base + soluble salt
Having before your eyes a table of salt solubility, and, as well as cheat sheets, you can decide on them equations reactions. It is only important to have a complete list of such schemes, as well as information about the formulas and names of various classes of organic and inorganic compounds.
After the equation itself is completed, it is necessary to check the correct spelling of the chemical formulas. Acids, salts and bases are easily checked using the solubility table, which shows the charges of the acidic residues and metal ions. It is important to remember that any one must be generally electrically neutral, that is, the number of positive charges must coincide with the number of negative ones. In this case, it is necessary to take into account the indices, which are multiplied by the corresponding charges.
If this stage has been passed and you are confident in the correctness of the spelling equations chemical reactions, then you can now safely set the coefficients. The chemical equation is represented by the conventional notation reactions using chemical symbols, indices and coefficients. At this stage of the task, you must adhere to the rules: The coefficient is placed in front of the chemical formula and applies to all elements that make up the substance.
The index is placed after the chemical element a little lower, and refers only to the chemical element to the left of it.
If a group (for example, an acid residue or a hydroxyl group) is in brackets, then you need to understand that two adjacent indices (before and after the bracket) are multiplied.
When counting the atoms of a chemical element, the coefficient is multiplied (not added!) by the index.
Next, the amount of each chemical element is calculated so that the total number of elements included in the starting substances coincides with the number of atoms included in the compounds formed in the products reactions. By analyzing and applying the above rules, you can learn to solve equations reactions included in chains of substances.
Most chemical reactions take place in solutions. Electrolyte solutions contain ions, so reactions in electrolyte solutions actually come down to reactions between ions.
Reactions between ions are called ionic reactions, and the equations for such reactions are called ionic equations.
When drawing up ionic equations, one should be guided by the fact that the formulas of slightly dissociating, insoluble and gaseous substances are written in molecular form.
A white substance precipitates, then an arrow pointing downward is placed next to its formula, and if a gaseous substance is released during the reaction, an arrow pointing upward is placed next to its formula.
Let's rewrite this equation, depicting strong electrolytes in the form of ions, and reactions leaving the sphere as molecules:
We have thus written down the complete ionic equation of the reaction.
If we exclude identical ions from both sides of the equation, that is, those not participating in the reaction in the left and right equations), we obtain a shortened ionic reaction equation: 
Thus, abbreviated ionic equations are equations in general form that characterize the essence of a chemical reaction, show which ions react and what substance is formed as a result.
Ion exchange reactions proceed to completion in cases where either a precipitate or a slightly dissociating substance, such as water, is formed. When adding an excess of nitric acid solution to a solution of sodium hydroxide colored crimson with phenolphthalein, the solution will become discolored, which will serve as a signal for a chemical reaction to occur: 
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 low-dissociation substance is formed - water.
This reaction between a strong acid and an alkali is called a 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 obtain 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 will mean that in all cases a chemical reaction took place, the essence of which is reflected using the same ionic equation.
To verify this, write down the molecular, complete and abbreviated ionic equations of the given reactions.
Let's consider ionic reactions that occur with the formation of gas. Pour 2 ml of solutions of sodium carbonate and potassium carbonate into two test tubes. Then 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. Let us write down the reaction equations for the first case: 
Reactions occurring in electrolyte solutions are described using ionic equations. These reactions were called ion exchange reactions, since in solutions electrolytes exchange their ions. Thus, two conclusions can be drawn.
1. Reactions in aqueous solutions of electrolytes are reactions between ions, and therefore are depicted in the form of ionic equations.
They are simpler than molecular ones and are more general in nature.
2. Ion exchange reactions in electrolyte solutions proceed practically irreversibly only if the result is the formation of a precipitate, gas or slightly dissociating substance.
7. Complex connections