5.27. Chemical Properties of Acids, Bases, and Salts in Light of the Theory of Electrolytic Dissociation

Lecture

The theory of electrolytic dissociation provides a unified approach to understanding the processes occurring in solutions involving electrolytes — acids, bases, and salts. This approach is based on the fact that when electrolytes dissolve in water, a solution is obtained containing cations and anions. It is these ions that take part in chemical reactions.

Chemical reactions in electrolyte solutions are reactions involving ions formed as a result of the dissociation of electrolytes.

Reactions between ions in solutions that occur without a change in the oxidation states of atoms are called ion exchange reactions.

Conditions for the irreversible course of ion exchange reactions in electrolyte solutions

Formation
of a white precipitate
of CaF2 upon
the interaction
of Ca2+ and F– ions

When solutions of different calcium salts are mixed with solutions of fluorides of different metals, the same precipitate is obtained — calcium fluoride CaF2↓. This happens because in all cases the calcium ions Ca2+ present in the solutions react with fluoride ions F– to form a sparingly soluble substance:

Ca2+ + 2F– = CaF2↓.

In this connection, consider why mineral waters rich in soluble calcium salts contain almost no fluoride anions.

When hydrochloric, sulfuric, or nitric acid acts on the carbonates of calcium, sodium, and other metals, carbon dioxide gas is evolved:

Na2CO3 + 2HCl = 2NaCl + CO2↑ + H2O.

In all these cases, the interaction of carbonate ions 5.27. Chemical Properties of Acids, Bases, and Salts in Light of the Theory of Electrolytic Dissociation and hydrogen ions H+ forms the weak acid H2CO3:

5.27. Chemical Properties of Acids, Bases, and Salts in Light of the Theory of Electrolytic Dissociation ,

which decomposes into carbon dioxide and water. The reaction proceeds with vigorous evolution of CO2, despite the very low solubility of CaCO3. No interaction occurs if no gas is evolved, no precipitate forms, and no poorly dissociating substance is formed. For example, if solutions of sodium sulfate Na2SO4 and potassium nitrate KNO3 are mixed, the resulting solution will contain the cations Na+, K+, and the anions 5.27. Chemical Properties of Acids, Bases, and Salts in Light of the Theory of Electrolytic Dissociation and

These examples allow us to conclude that ion exchange reactions proceed irreversibly when gaseous substances, precipitates of sparingly soluble substances, or poorly dissociating compounds — weak electrolytes — are formed.

According to Le Chatelier's principle, the evolution of a gas or the formation of a precipitate removes the reaction product from the reaction mixture — the solution — which ensures that the reaction proceeds to completion.

Reactions in solutions are described by equations in three forms: molecular, full ionic, and net ionic. In all equations, weak electrolytes, gases, and sparingly soluble substances are represented by molecular formulas, for example H2O, H2, Fe(OH)2, regardless of their structure (molecular or non-molecular).

The essence of the processes taking place is most clearly expressed when the equations of electrolyte reactions are written in ionic form. To do this, we first write the reaction equation in molecular form:

H2SO4 + 2NaOH = Na2SO4 + 2H2O,

and then the equation in full ionic form, showing strong electrolytes as ions, and the resulting gaseous and poorly dissociating compounds — in this case water — as molecules:

5.27. Chemical Properties of Acids, Bases, and Salts in Light of the Theory of Electrolytic Dissociation ,

and insoluble compounds of non-molecular structure as formula units.

To write the equation in net ionic form, we exclude from both sides of the equation the ions that do not take part in the reaction:

2H+ + 2OH– = 2H2O or H+ + OH– = H2O.

The reaction equation in net ionic form fully expresses the chemical essence of the interaction taking place.

Obviously, whichever alkali and strong acid we take, the interaction between them with the formation of a soluble salt will be expressed by the same equation: H+ + OH– = H2O. In all these cases the same chemical reaction will occur — the neutralization reaction — with the release of about 57 kJ of energy per mole of water formed.

Equations in net ionic form relate not only to one specific reaction between particular substances, but also cover a group of similar reactions. This is their main value and generalizing significance.

Let us examine the chemical properties of acids, bases, and salts from the standpoint of the theory of electrolytic dissociation.

*Reactions in solutions involving ions

Recall that, besides ion exchange reactions, ions may also take part in redox reactions:

2FeCl3 + Fe = 3FeCl2 or 2Fe3+ + Fe = 3Fe2+;

2NaI + Br2 = 2NaBr + I2 or 2I– + Br2 = 2Br– + I2;

2CH3COOAg + Cu = (CH3COO)2Cu + 2Ag↓ or 2Ag+ + Cu = 2Ag↓ + Cu2+.

Ion exchange reactions between salts with the formation of precipitates occur in all cases where the solubility of the reactants is higher than the solubility of one of the products. When soluble reactants react, both insoluble and sparingly soluble products will precipitate.

Let us give several examples of writing equations for reactions involving ions.

Let us consider a reaction with the formation of a sparingly soluble substance.

Example 1. Write the equations for the exchange reaction occurring in solution between iron(III) chloride FeCl3 and potassium hydroxide KOH in molecular, full ionic, and net ionic forms.

Solution

Let us write the reaction equation in molecular form:

FeCl3 + 3KOH = Fe(OH)3↓ + 3KCl.

Let us write the reaction equation in full ionic form:

Fe3+ + 3Cl– + 3K+ + 3OH– = Fe(OH)3↓ + 3K+ + 3Cl–.

Let us write the reaction equation in net ionic form:

Fe3+ + 3OH– = Fe(OH)3↓.

As a result of the interaction of Fe3+ cations and ОН– anions, a reddish-brown precipitate of the insoluble base — iron(III) hydroxide Fe(OH)3 — is formed.

Let us consider a reaction with the formation of a gaseous product.

Example 2. Write the equations for the reaction between sodium hydrogen carbonate NaHCO3 and hydrochloric acid HCl in molecular, full ionic, and net ionic forms.

Solution

Let us write the equations:

NaHCO3 + HCl = NaCl + H2O + CO2↑.

Na+ + 5.27. Chemical Properties of Acids, Bases, and Salts in Light of the Theory of Electrolytic Dissociation + H+ + Cl– = Na+ + Cl– + H2O + CO2↑.

5.27. Chemical Properties of Acids, Bases, and Salts in Light of the Theory of Electrolytic Dissociation + H+ = H2O + CO2↑.

The result of the interaction of H+ cations and 5.27. Chemical Properties of Acids, Bases, and Salts in Light of the Theory of Electrolytic Dissociation anions is the formation of liquid and gaseous reaction products — H2O and CO2.

The reactions of intermediate formation and immediate decomposition of molecules of the weak and unstable carbonic acid H2CO3 are not shown here. Write them yourself.

Let us consider a reaction with the formation of a poorly dissociating substance.

Example 3. Write the equations for the reaction between copper(II) oxide CuO and sulfuric acid H2SO4 in molecular, full ionic, and net ionic forms.

Solution

Let us write the equations:

CuO + H2SO4 = CuSO4 + H2O.

CuO + 2H+ + 5.27. Chemical Properties of Acids, Bases, and Salts in Light of the Theory of Electrolytic Dissociation = Cu2+ + H2O + .

CuO + 2H+ = Cu2+ + H2O.

The dissolution of the black precipitate of copper(II) oxide with the formation of a blue solution is a sign of a chemical reaction, which occurs as a result of the action of the hydrogen cations H+ of sulfuric acid on the CuO precipitate, forming a weak electrolyte — water molecules — and Cu2+ cations as part of the soluble copper(II) salt CuSO4.

Let us determine whether a reaction is possible.

Example 4. Determine whether sodium chloride NaCl and potassium acetate CH3COOK interact in solution:

NaCl + CH3COOK = CH3COONa + KCl.

Solution

Let us write the reaction equation in full ionic form:

Na+ + Cl– + K+ + CH3COO– = Na+ + CH3COO– + K+ + Cl–.

From the full ionic equation it can be seen that the solution contains only free ions: Na+, K+, Cl–, and CH3COO–. Thus, when solutions of NaCl and CH3COOK are mixed, the ions do not combine, and consequently no ion exchange reaction occurs. This conclusion is confirmed in practice by the fact that when solutions of sodium chloride and potassium acetate are mixed, no precipitate forms, no heat is released or absorbed, and no other signs of a chemical reaction are observed, which indicates its absence.

General chemical properties of acids

Acids are electrolytes whose dissociation produces only hydrogen cations as cations.

The presence of acids in solutions is detected using indicators. Thus, in an acidic medium (pH < 7) litmus and methyl orange turn red (see Fig. 59). The general chemical properties of strong acids are due to reactions in which only the hydrogen cations H+ take part. These include the formation of salts upon interaction with metals, basic and amphoteric oxides, bases, and salts (Table 17).

Table 17. Chemical properties of acids in light of the theory of electrolytic dissociation

Reagent	Reaction equation in molecular and full ionic forms	Reaction equation in net ionic form
1. Indicator	As a rule, the dissociation equation: HBr → H+ + Br–
2. Metal	Мg + 2HCl = MgCl2 + H2↑ Mg0 + 2Н+ + 2Сl– = Mg2+ + 2Сl– + H2↑	Mg0 + 2H+ = Mg2+ + H2↑
3. Oxide a) basic	МgO + 2HNO3 = Mg(NO3)2 + H2O MgO + 2Н+ + = Mg2+ + + H2O	MgO + 2H+ = Mg2+ + H2O
b) amphoteric	ZnO + 2HCl = ZnCl2 + H2O ZnO + 2Н+ + 2Сl– = Zn2+ + 2Сl– + H2O	ZnO + 2H+ = Zn2+ + H2O
4. Base a) alkali	NaOH + HCl = NaCl + H2O Na+ + OH– + H+ + Сl– = Na+ + Сl– + H2O	H+ + OH– = H2O
b) insoluble base	H2SO4 + Fe(OH)2 = FeSO4 + 2H2O 2H+ + + Fe(OH)2 = Fe2++ + 2H2O	2H+ + Fe(OH)2 = Fe2+ + 2H2O
5. Salt	2HNO3 + Na2CO3 = 2NaNO3 + H2O + CO2↑ 2H+ + + 2Na+ + = 2Na+ + + CO2↑ + H2O	2H+ + = CO2↑ + H2O

Thus, the hydrogen cation Н+ in acid solutions is responsible for the general reactions of this class of compounds. This conclusion is confirmed by the absence of acid-residue anions in the net ionic equations of all ion exchange reactions involving hydrogen ions. Acid-residue anions are responsible only for the specific properties of particular acids: oxidizing, reducing, participation in exchange reactions with the precipitation of insoluble salts of these anions, and so on.

Acids form an excess of H+ cations (hydronium cations H3O+) in aqueous solutions and create an acidic environment.

In a separate column of the solubility table — the acids column — pairs are marked: the H+ cation and the anions 5.27. Chemical Properties of Acids, Bases, and Salts in Light of the Theory of Electrolytic Dissociation , S2– or , which interact to form acids — weak electrolytes: H2CO3, H2S, and H2SiO3. What other anions in the solubility table react with the H+ cation to form molecules of weak acids?

After the reaction of the H+ cation with the anions 5.27. Chemical Properties of Acids, Bases, and Salts in Light of the Theory of Electrolytic Dissociation and S2– , the sparingly soluble gaseous substances CO2 (after the decomposition of the unstable acid H2CO3) and H2S are formed with a characteristic "effervescence" of the solution and the release of gas bubbles. The water-insoluble silicic acid H2SiO3 precipitates.

Note that the violet color of litmus in a neutral medium is a mixture of blue and red colors, and the orange color of methyl orange is a mixture of the yellow and red colors of the indicators. Such a superposition of colors indicates that both colored forms of the indicators are present in equal measure in a neutral medium, since the concentrations of hydrogen ions Н+ and hydroxide anions ОН– in pure water are equal and, at 25 °С, both equal 10–7 mol/dm3.

General chemical properties of bases

Among bases, only alkalis are readily soluble. The presence of alkalis in solutions is detected using indicators, in which case pH > 7.

Alkalis are electrolytes whose dissociation produces only hydroxide ions as anions. Their general reactions are presented in Table 18.

Table 18. Chemical properties of alkalis in light of the theory of electrolytic dissociation

Reagent	Reaction equation in molecular form	Reaction equation in net ionic form
1. Indicator	As a rule, the dissociation equation: NaOH → Na+ + OH–
2. Acid	2HCl + Ca(OH)2 = CaCl2 + 2H2O	H+ + OH– = H2O
3. Salt a) salt of a water-insoluble base	Mg(NO3)2 + 2NaOH = 2NaNO3 + Mg(OH)2↓	Mg2+ + 2OH– = Mg(OH)2↓
b) ammonium salt	NH4Cl + KOH = KCl + NH3↑ + H2O	+ OH– = NH3↑ + H2O
c) acid salt	NaHCO3 + NaOH = Na2CO3 + H2O	+ OH– = + H2O
4. Oxide a) acidic oxide	CO2 + KOH = KHCO3 CO2 + 2KOH = K2CO3 + Н2О	CO2 + OH– = CO2 + 2OH– = + H2O
b) amphoteric oxide	ZnO + 2NaOH + H2O = Na2[Zn(OH)4]	ZnO + 2OH– + H2O = [Zn(OH)4]2–
5. Amphoteric hydroxide	Zn(OH)2 + 2NaOH = Na2[Zn(OH)4]	Zn(OH)2 + 2OH– = [Zn(OH)4]2–

Water-insoluble bases can dissolve in acids, for example:

Fe(OH)3 + 3HCl = FeCl3 + 3H2O.

Alkalis react not only with strong acids, but also with weak acids and acids that are insoluble in water:

H2SiO3 + 2NaOH = Na2SiO3 + 2H2O.

Thus, the OН– anion in alkali solutions is responsible for the general reactions of this class of compounds. This conclusion is confirmed by the absence of metal cations in the net ionic equations of all ion exchange reactions involving alkalis. Metal cations are responsible only for the specific properties of particular alkalis: solubility, participation in exchange reactions with the precipitation of insoluble salts of these cations, and so on.

General chemical properties of salts

The chemical properties and reactions of salts in solutions are due mainly to reactions of their ions. As a result of the interaction of salts of weak acids with strong acids, weak acids are formed — an ion exchange reaction takes place, in which the acid is displaced from its salt by a stronger acid.

Reactions of salts with alkalis lead to the precipitation of water-insoluble bases, the release of ammonia from ammonium salts, and the formation of normal salts from acid salts.

In aqueous solutions, salts undergo ion exchange reactions with other salts, as well as displacement reactions with more active metals. Typical reactions involving salts in solutions are given in Table 19.

Table 19. Chemical properties of salts in light of the theory of electrolytic dissociation

Reagent	Reaction equation in molecular form	Reaction equation in net ionic form
1. Acid	Na2CO3 + 2HCl = 2NaCl + CO2↑ + H2O	+ 2H+ = CO2↑ + H2O
2. Alkali	CuCl2 + 2KOH = Cu(OH)2↓ + 2KCl	Cu2+ + 2OH– = Cu(OH)2↓
3. Salt	BaCl2 + К2SO4 = BaSO4↓ + 2КCl	Ba2+ + = BaSO4↓
4. Metal	CuSO4 + Fe = FeSO4 + Cu↓	Cu2+ + Fe = Fe2+ + Cu↓

From the standpoint of the theory of electrolytic dissociation, the chemical properties of salt solutions are due to reactions involving metal (or ammonium) cations and acid-residue anions.

In addition to metal and ammonium cations, salts may also contain phosphonium cations 5.27. Chemical Properties of Acids, Bases, and Salts in Light of the Theory of Electrolytic Dissociation , organic ammonium derivatives, such as the tetramethylammonium cation , as well as complex cations — and so on. The anions in salts are often complex anions, such as , , , [Zn(OH)4]2–, and others.

The chemical properties of electrolyte solutions (acids, bases, salts) are due to reactions of the ions formed upon their dissociation.

Ion exchange reactions proceed irreversibly when gaseous substances, precipitates of sparingly soluble substances, or poorly dissociating compounds — weak electrolytes — are formed.

Questions, assignments, problems

1. Using the solubility table, name four anions that form insoluble salts with the Ca2+ cation.

2. Which of the following compounds and elementary substances does dilute sulfuric acid react with: HCl, Na2SO4, NaHSO3, Ba(OH)2, NH3, LiOH, CO2, SO2? Write equations for the possible chemical reactions in molecular, full ionic, and net ionic forms.

3. Write, in molecular form, three equations for neutralization reactions corresponding to the equation: Н+ + ОН– = Н2О.

4. Write, in molecular and full ionic forms, the chemical reaction equations corresponding to the equations:

a) Cu2+ + S2– = CuS↓;
b) Fe2O3 + 6H+ = 2Fe3+ + 3H2O;
c) BaCO3 + 2H+ = Ba2+ + CO2↑ + H2O;
d) .

5. Indicate the reagents and indicators that can be used to distinguish solutions of Ca(OH)2 and H2SO4: litmus, methyl orange, phenolphthalein, universal indicator paper, CO2, BaCl2, CuCl2, Fe (filings), NaHCO3, KF, FeCl3, H3PO4. Indicate the qualitative signs of the reactions and write their equations in molecular, full ionic, and net ionic forms.

6. Determine the mass of copper that can be recovered from 1 dm3 of a CuCl2 solution with a concentration of 0.07 mol/dm3 using an excess of iron filings, if 95% of the metal is extracted in this way.

7. Indicate the mass of a solution with a sodium carbonate mass fraction of 8.48% required for the complete precipitation of Ca2+ cations from a solution of mass 222 g in which the mass fraction of calcium chloride is 0.10.

8. Calculate the mass of baking soda NaHCO3 required to neutralize 1.00 dm3 of 96% sulfuric acid with ρ = 1.835 g/cm3.

9. To a solution of barium iodide BaI2 of mass 391 g with a salt mass fraction of 40%, a 96% H2SO4 solution with ρ = 1.835 g/cm3 was added in an amount sufficient for the complete precipitation of barium as BaSO4. Calculate the mass (g) and volume (cm3) of the sulfuric acid solution added.

10. Using the data from problem 9, calculate the mass of the BaSO4 precipitate, as well as the mass of the resulting hydroiodic acid solution and the mass fraction of HI in it.

*Questions, assignments, problems

1. Given solutions of the substances: NaOH, HCl, NaCl, KOH, HNO3, Ca(OH)2, NH3, CO2, SO2, СH3СOOH, HNO2, HF. Sort the listed solutions into three groups:

a) solutions with pH < 7;
b) pH > 7;
c) pH ≈ 7.

2. Which substances does dilute sulfuric acid react with: HCl, Na2SO4, NaHSO3, Ba(OH)2, NH3, LiOH, CO2, SO2? Write equations for the possible chemical reactions in molecular, full ionic, and net ionic forms.

3. Complete the chemical reaction equations in molecular form and write them in net ionic form:

a) NaF + CaCl2 = …;
b) NH3 ∙ H2O + H2SO4 = …;
c) Na2S + H2SO4 = …;
d) NaOH + HCl = ….

Indicate the conditions under which these reactions proceed.

4. Indicate the reagents and indicators that can be used to distinguish solutions of Ca(OH)2 and H2SO4: litmus, methyl orange, phenolphthalein, universal indicator paper, CO2, BaCl2, CuCl2, Fe (filings), NaHCO3, KF, FeCl3, H3PO4. Indicate the qualitative signs of the reactions and write their equations in molecular, full ionic, and net ionic forms.

5. Using the solubility table, indicate the anions that form insoluble salts with the Mg2+ cation. Write the equations of three reactions forming these salts in net ionic form.

6. Which pairs of ions will react with each other to form precipitates, and which will form poorly dissociating substances of molecular structure:

a) Ba2++ and ;
b) K+ and OH–;
c) H+ and OH–;
d) Ag+ and Cl–;
e) H+ and F–;
f) H+ and CH3COO–;
g) H+ and ;
h) and OH–;
i) Na+ and ?

7. Will 750 cm3 of a KОН solution with a molar concentration of 0.5 mol/dm3 be enough to completely neutralize a solution of mass 200 g in which the mass fraction of HCl is 7.3%? Determine the pH range (greater or less than 7) of the solution after the reaction is complete.

8. In a solution of nitrous acid, the number of undissociated molecules is 2.5 times greater than the number of dissociated ones. Indicate the degree of dissociation α (in percent) and the pH of the solution if the initial concentration of the acid in it was 0.0035 mol/dm3.

9. Determine the amounts (mol) of the starting substances Al2(SO4)3 · 18H2O and NH3 · H2O whose interaction produces 1.0 mol of aluminum hydroxide Al(OH)3.

10. Calculate the mass of Na2CO3 (a component of laundry detergent) required to precipitate all the calcium and magnesium cations from 20 dm3 of water (for laundering) with a molar concentration of the sum of calcium and magnesium cations of 0.0035 mol/dm3 (the maximum permissible hardness standard for tap water). Write the equations of the corresponding reactions in net ionic form.