1.3.2. Acids: Classification, Chemical Properties, Nomenclature, Methods of Obtaining Acids

Lecture

Acids are complex substances whose molecules contain hydrogen atoms capable of being replaced by metal atoms, and acid residues.

From the standpoint of the theory of electrolytic dissociation, the basics of which you studied in the 9th grade, acids are electrolytes whose dissociation produces only hydrogen cations H+ as cations.

It is the hydrogen ions that account for the general properties of acids, while the ions of the acid residues determine their specific properties.

Classification of acids

Various criteria may be used to classify acids (Table 1.3).

Table 1.3. Classification of acids

Classification criterion	Classification groups	Examples
1. By origin	Mineral (inorganic)	НCl, H2SO4, HNO3
Organic (carboxylic)	HCOOH, CH3COOH, C17H35COOH
2. By the presence of oxygen atoms	Oxygen-containing (hydroxides)	H3PO4, H2SO4, H2CO3
Oxygen-free	HCl, H2S, HF
3. By the number of hydrogen atoms capable of being replaced by metal atoms	Monobasic	HNO3, HF, НCl, CH3COOH
Polybasic (dibasic, tribasic)	H2SO4, H2SO3, H2CO3; H3PO4
4. By strength (ability to dissociate into ions in aqueous solution)	Strong	H2SO4, HNO3, НCl, HClO4
Weak	H2CO3, H2S
HF, H3PO4

Thus, dividing acids into monobasic and polybasic allows us to say that in polybasic acids one or several hydrogen atoms can be replaced by a metal. This means that polybasic acids can form not only normal salts but also acid salts, for example: Na2CO3 and NaHCO3, K2S and KHS.

The division of acids into strong and weak is based on the fact that strong acids dissociate completely into ions in solution:

1.3.2. Acids: Classification, Chemical Properties, Nomenclature, Methods of Obtaining Acids

Weak acids dissociate into ions only to a small extent, and their dissociation process is reversible:

1.3.2. Acids: Classification, Chemical Properties, Nomenclature, Methods of Obtaining Acids

You will become more closely acquainted with the dissociation processes of acids while studying the material of Chapter IV.

Nomenclature of acids

When studying the nomenclature of acids, it is first necessary to distinguish between oxygen-containing and oxygen-free acids. Thus, when forming the name of an oxygen-free acid, the name of the element that is part of the acid residue is added before the words "hydro... acid," for example: HCl — hydrochloric acid, H2S — hydrosulfuric acid.

The name of an oxygen-containing acid is derived from the name of the element, to which the corresponding suffix is added. If the atoms of the element forming the oxygen-containing acid are in the highest oxidation state, the oxygen-containing acid has the suffix -ic, for example, 1.3.2. Acids: Classification, Chemical Properties, Nomenclature, Methods of Obtaining Acids — sulfuric acid, — nitric acid, — perchloric acid.

1.3.2. Acids: Classification, Chemical Properties, Nomenclature, Methods of Obtaining Acids

When the atoms of an element can exist in several positive oxidation states, the suffixes change in the following order as the oxidation state decreases: -ic, -ic (with "-ate" root, lower), -ous, -ous (hypo-). For example, 1.3.2. Acids: Classification, Chemical Properties, Nomenclature, Methods of Obtaining Acids — perchloric, — chloric, — chlorous, — hypochlorous acid.

Chemical properties of acids

The general properties of acids include those caused by the presence of hydrogen ions in their solutions: interaction with indicators, as well as the formation of salts upon reaction with metals, basic and amphoteric oxides, bases, and salts.

Reagent	Equation of the salt-forming reaction
1. Metal	Мg + 2HCl = MgCl2 + H2↑
2. Oxide: a) basic b) amphoteric	МgO + 2HNO3 = Mg(NO3)2 + H2O ZnO + 2HCl = ZnCl2 + H2O
3. Base	NaOH + HCl = NaCl + H2O
4. Salt	2HNO3 + Na2CO3 = 2NaNO3 + H2O + CO2↑

Acid solutions change the color of such indicators as litmus and methyl orange to red.

Besides their general properties, acids can display oxidation-reduction properties owing to their acid residues. You are already familiar, for example, with some of these reactions:

1.3.2. Acids: Classification, Chemical Properties, Nomenclature, Methods of Obtaining Acids

Methods of obtaining acids

Classifying acids by the presence or absence of oxygen in their composition makes it possible to compare certain methods of obtaining acids.

Thus, many oxygen-free acids can be obtained by the reaction of hydrogen combining with a nonmetal, followed by dissolving the product in water:

H2 + Cl2 = 2HCl;

H2 + S = H2S.

Oxygen-containing acids are obtained by the reaction of the corresponding oxides combining with water:

Н2O + SO3 = H2SO4;

3Н2O + Р2О5 = 2H3PO4.

Since SiO2 does not react with water, to obtain silicic acid a soluble metal salt is first prepared, and only then is the acid obtained by an exchange reaction with a stronger acid:

SiO2 + 2NaOH(conc. soln.) 1.3.2. Acids: Classification, Chemical Properties, Nomenclature, Methods of Obtaining Acids Na2SiO3 + H2O;

Na2SiO3 + H2SO4 = H2SiO3↓ + Na2SO4.

Both oxygen-containing and oxygen-free acids can often be obtained by an exchange reaction. An exchange reaction can proceed in solution between a salt containing the required acid residue and another acid; the condition for the reaction to proceed is the formation of a weak acid, a volatile acid, or an insoluble salt:

Na2SiO3 + 2HCl = H2SiO3↓ + 2NaCl;

Na2S + 2HCl = 2NaCl + H2S↑;

BaCl2 + H2SO4 = BaSO4↓ + 2HCl.

An exchange reaction is possible when solid salts react with nonvolatile concentrated sulfuric acid upon heating:

NaCl(solid) + H2SO4(conc.) 1.3.2. Acids: Classification, Chemical Properties, Nomenclature, Methods of Obtaining Acids HCl↑ + NaHSO4

or 2NaCl(solid) + H2SO4(conc.) 1.3.2. Acids: Classification, Chemical Properties, Nomenclature, Methods of Obtaining Acids 2HCl↑ + Na2SO4;

NaNO3(solid) + H2SO4(conc.) 1.3.2. Acids: Classification, Chemical Properties, Nomenclature, Methods of Obtaining Acids HNO3↑ + NaHSO4

or 2NaNO3(solid) + H2SO4(conc.) 1.3.2. Acids: Classification, Chemical Properties, Nomenclature, Methods of Obtaining Acids 2HNO3↑ + Na2SO4;

CH3COONa(solid) + H2SO4(1 : 1) 1.3.2. Acids: Classification, Chemical Properties, Nomenclature, Methods of Obtaining Acids CH3COOН↑ + NaHSO4.

It should be noted that in industry the production of acids is often a multi-stage process. For example, one of the industrial methods of obtaining sulfuric acid includes three stages:

FeS2 → SO2 → SO3 → H2SO4.

Acids are complex substances whose molecules contain hydrogen atoms capable of being replaced by metal atoms, and acid residues; upon dissociation of acids, only hydrogen cations H+ are formed as cations.

The general properties of acids are a change in the color of indicators and the formation of salts upon reaction with metals, basic and amphoteric oxides, bases, and other salts.

Acids are obtained by dissolving the corresponding oxides or hydrogen compounds of nonmetals in water, as well as by an exchange reaction between a salt and a stronger acid.

Questions, tasks, problems

1. List:

a) the criteria by which acids are classified;
b) the methods of obtaining acids;
c) the general chemical properties of acids.

2. Indicate the formulas of:

a) strong acids — HF, HCl, H2S, HВr;
b) weak acids — HNO3, H2SO4, H2SO3, H2CO3.

3. Name the acids whose formulas are:

a) HF, HCl, H2S, H2Se, HВr, HI;
b) H2CO3, HNO3, H2SO4, H2SO3, H3PO4.

4. Give a characterization according to all classification criteria (Table 1.3) of:

a) sulfuric acid;
b) hydrosulfuric acid.

5. Write equations for the reactions of:

a) nitric acid with the substances: sodium hydroxide, aluminum oxide, potassium carbonate;
b) hydrochloric acid with the substances: iron, iron(III) oxide, iron(II, III) oxide, calcium carbonate.

6. Suggest two methods for obtaining each of the acids — HCl and H2SO3. Write the equations of the corresponding reactions.

7. The mass fraction of oxygen in a strong acid of composition Н2ЭО4 is equal to 44.14%. Determine the element Э in the composition of the acid.

8. Write equations for the reactions according to the scheme:

a) KCl HCl MgCl2 MgСО3 MgSO4;
b) CH3COOН CH3COONa CH3COOН … …

9. Every day, about 2 dm3 of gastric juice is produced in the stomach of an adult, with a molar concentration of hydrogen chloride of 16 mmol/dm3. Determine the amount (mol) and mass (g) of НСl in this portion of gastric juice.

10. Determine the volume of vinegar needed to "neutralize" half a teaspoon of baking soda. Use the following data: the mass of NaHCO3 is 2.5 g, the mass fraction of acetic acid is 9%, and the density of vinegar is 1.011 g/cm3.