Acids- electrolytes, upon dissociation of which only H + ions are formed from positive ions:
HNO 3 ↔ H + + NO 3 - ;
CH 3 COOH↔ H + +CH 3 COO — .
All acids are classified into inorganic and organic (carboxylic), which also have their own (internal) classifications.
Under normal conditions, significant amounts of inorganic acids exist in liquid state, some are in the solid state (H 3 PO 4, H 3 BO 3).
Organic acids with up to 3 carbon atoms are highly mobile, colorless liquids with a characteristic pungent odor; acids with 4-9 carbon atoms are oily liquids with an unpleasant odor, and acids with a large number carbon atoms are solids that are insoluble in water.
Chemical formulas of acids
Let us consider the chemical formulas of acids using the example of several representatives (both inorganic and organic): hydrochloric acid - HCl, sulfuric acid - H 2 SO 4, phosphoric acid - H 3 PO 4, acetic acid - CH 3 COOH and benzoic acid - C 6 H5COOH. The chemical formula shows the quality and quantitative composition molecules (how many and which atoms are included in a particular compound) Using the chemical formula, you can calculate the molecular mass of acids (Ar(H) = 1 amu, Ar(Cl) = 35.5 amu, Ar( P) = 31 amu, Ar(O) = 16 amu, Ar(S) = 32 amu, Ar(C) = 12 amu) :
Mr(HCl) = Ar(H) + Ar(Cl);
Mr(HCl) = 1 + 35.5 = 36.5.
Mr(H 2 SO 4) = 2×Ar(H) + Ar(S) + 4×Ar(O);
Mr(H 2 SO 4) = 2×1 + 32 + 4×16 = 2 + 32 + 64 = 98.
Mr(H 3 PO 4) = 3×Ar(H) + Ar(P) + 4×Ar(O);
Mr(H 3 PO 4) = 3×1 + 31 + 4×16 = 3 + 31 + 64 = 98.
Mr(CH 3 COOH) = 3×Ar(C) + 4×Ar(H) + 2×Ar(O);
Mr(CH 3 COOH) = 3×12 + 4×1 + 2×16 = 36 + 4 + 32 = 72.
Mr(C 6 H 5 COOH) = 7×Ar(C) + 6×Ar(H) + 2×Ar(O);
Mr(C 6 H 5 COOH) = 7 × 12 + 6 × 1 + 2 × 16 = 84 + 6 + 32 = 122.
Structural (graphic) formulas of acids
The structural (graphic) formula of a substance is more clear. It shows how atoms are connected to each other within a molecule. Let us indicate the structural formulas of each of the above compounds:
Rice. 1. Structural formula of hydrochloric acid.
Rice. 2. Structural formula of sulfuric acid.
Rice. 3. Structural formula of phosphoric acid.
Rice. 4. Structural formula of acetic acid.
Rice. 5. Structural formula of benzoic acid.
Ionic formulas
All inorganic acids are electrolytes, i.e. capable of dissociating in an aqueous solution into ions:
HCl ↔ H + + Cl - ;
H 2 SO 4 ↔ 2H + + SO 4 2- ;
H 3 PO 4 ↔ 3H + + PO 4 3- .
Examples of problem solving
EXAMPLE 1
| Exercise | With complete combustion of 6 g of organic matter, 8.8 g of carbon monoxide (IV) and 3.6 g of water were formed. Define molecular formula burnt substance, if it is known that its molar mass is 180 g/mol. |
| Solution | Let’s draw up a diagram of the combustion reaction of an organic compound, designating the number of carbon, hydrogen and oxygen atoms as “x”, “y” and “z”, respectively: C x H y O z + O z →CO 2 + H 2 O. Let us determine the masses of the elements that make up this substance. Relative atomic mass values taken from periodic table DI. Mendeleev, round to whole numbers: Ar(C) = 12 amu, Ar(H) = 1 amu, Ar(O) = 16 amu. m(C) = n(C)×M(C) = n(CO 2)×M(C) = ×M(C); m(H) = n(H)×M(H) = 2×n(H 2 O)×M(H) = ×M(H); Let's calculate the molar masses of carbon dioxide and water. As is known, the molar mass of a molecule is equal to the sum of the relative atomic masses of the atoms that make up the molecule (M = Mr): M(CO 2) = Ar(C) + 2×Ar(O) = 12+ 2×16 = 12 + 32 = 44 g/mol; M(H 2 O) = 2×Ar(H) + Ar(O) = 2×1+ 16 = 2 + 16 = 18 g/mol. m(C) = ×12 = 2.4 g; m(H) = 2 × 3.6 / 18 × 1 = 0.4 g. m(O) = m(C x H y O z) - m(C) - m(H) = 6 - 2.4 - 0.4 = 3.2 g. Let's define chemical formula connections: x:y:z = m(C)/Ar(C) : m(H)/Ar(H) : m(O)/Ar(O); x:y:z= 2.4/12:0.4/1:3.2/16; x:y:z= 0.2: 0.4: 0.2 = 1: 2: 1. This means the simplest formula for the compound CH 2 Oi molar mass 30 g/mol. To find the true formula of an organic compound, we find the ratio of the true and resulting molar masses: M substance / M(CH 2 O) = 180 / 30 = 6. This means that the indices of carbon, hydrogen and oxygen atoms should be 6 times higher, i.e. the formula of the substance will be C 6 H 12 O 6. This is glucose or fructose. |
| Answer | C6H12O6 |
EXAMPLE 2
| Exercise | Derive the simplest formula of a compound in which the mass fraction of phosphorus is 43.66%, and the mass fraction of oxygen is 56.34%. |
| Solution | Mass fraction element X in a molecule of composition HX is calculated using the following formula: ω (X) = n × Ar (X) / M (HX) × 100%. Let us denote the number of phosphorus atoms in the molecule by “x”, and the number of oxygen atoms by “y” Let's find the corresponding relative atomic masses elements of phosphorus and oxygen (relative atomic mass values taken from D.I. Mendeleev’s Periodic Table, rounded to whole numbers). Ar(P) = 31; Ar(O) = 16. We divide the percentage content of elements into the corresponding relative atomic masses. Thus we will find the relationship between the number of atoms in the molecule of the compound: x:y = ω(P)/Ar(P) : ω (O)/Ar(O); x:y = 43.66/31: 56.34/16; x:y: = 1.4: 3.5 = 1: 2.5 = 2: 5. This means that the simplest formula for combining phosphorus and oxygen is P 2 O 5 . It is phosphorus(V) oxide. |
| Answer | P2O5 |
2. Bases react with acids to form salt and water (neutralization reaction). For example:
KOH + HC1 = KS1 + H 2 O;
Fe(OH) 2 + 2HNO 3 = Fe(NO 3) 2 + 2H 2 O
3. Alkalis react with acidic oxides to form salt and water:
Ca(OH) 2 + CO 2 = CaCO 2 + H 2 O.
4. Alkali solutions react with salt solutions if the result is the formation of an insoluble base or an insoluble salt. For example:
2NaOH + CuSO 4 = Cu(OH) 2 ↓ + Na 2 SO 4;
Ba(OH) 2 + Na 2 SO 4 = 2NaOH + BaSO 4 ↓
5. When heated, insoluble bases decompose into basic oxide and water.
2Fe(OH) 3 Fe 2 O 3 + ZH 2 O.
6. Alkali solutions interact with metals that form amphoteric oxides and hydroxides (Zn, Al, etc.).
2AI + 2KOH + 6H 2 O = 2K + 3H 2.
Getting grounds
Receipt soluble bases:
a) interaction of alkali and alkaline earth metals with water:
2Na + 2H 2 O = 2NaOH + H 2;
b) interaction of oxides of alkali and alkaline earth metals with water:
Na 2 O + H 2 O = 2NaOH.
2. Receipt insoluble bases the action of alkalis on soluble metal salts:
2NaOH + FeSO 4 = Fe(OH) 2 ↓ + Na 2 SO 4.
Acids - complex substances, when dissociated in water, hydrogen ions H + and no other cations are formed.
Chemical properties
The general properties of acids in aqueous solutions are determined by the presence of H + ions (or rather H 3 O +), which are formed as a result electrolytic dissociation acid molecules:
1. Acids change the color of indicators equally (Table 6).
2. Acids interact with bases.
For example:
H 3 PO 4 + 3NaOH = Na 3 PO 4 + ZH 2 O;
H 3 PO 4 + 2NaOH = Na 2 HPO 4 + 2H 2 O;
H 3 PO 4 + NaOH = NaH 2 PO 4 + H 2 O;
3. Acids interact with basic oxides:
2HCl + CaO = CaC1 2 + H 2 O;
H 2 SO 4 + Fe 2 O 3 = Fe 2 (SO 4) 3 + ZN 2 O.
4. Acids interact with amphoteric oxides:
2HNO 3 + ZnO = Zn(NO 3) 2 + H 2 O.
5. Acids react with some intermediate salts to form a new salt and a new acid, reactions are possible in in that case, if the result is an insoluble salt or a weaker (or more volatile) acid than the original. For example:
2HC1+Na2CO3 = 2NaCl+H2O +CO2;
2NaCl + H 2 SO 4 = 2HCl + Na 2 SO 4.
6. Acids interact with metals. The nature of the products of these reactions depends on the nature and concentration of the acid and on the activity of the metal. For example, diluted sulfuric acid, hydrochloric acid and other non-oxidizing acids interact with metals that are in the series of standard electrode potentials (see Chapter 7.) to the left of hydrogen. As a result of the reaction, salt and hydrogen gas are formed:
H 2 SO 4 (dil)) + Zn = ZnSO 4 + H 2;
2HC1 + Mg = MgCl 2 + H 2.
Oxidizing acids (concentrated sulfuric acid, nitric acid HNO 3 of any concentration) also interact with metals that are among the standard electrode potentials after hydrogen to form a salt and acid reduction product. For example:
2H 2 SO 4 (conc) + Zn = ZnSO 4 + SO 2 + 2H 2 O;
Obtaining acids
1. Oxygen-free acids are obtained by synthesis from simple substances and subsequent dissolution of the product in water.
S + H 2 = H 2 S.
2. Oxoacids are obtained by interaction acid oxides with water.
SO 3 + H 2 O = H 2 SO 4.
3. Most acids can be obtained by reacting salts with acids.
Na 2 SiO 3 + H 2 SO 4 = H 2 SiO 3 + Na 2 SO 4.
Amphoteric hydroxides
1. B neutral environment(pure water) amphoteric hydroxides are practically insoluble and do not dissociate into ions. They dissolve in acids and alkalis. Dissociation amphoteric hydroxides in acidic and alkaline environments can be expressed by the following equations:
Zn+ OH - Zn(OH)H + + ZnO
A1 3+ + ZON - Al(OH) 3 H + + AlO+ H 2 O
2. Amphoteric hydroxides react with both acids and alkalis, forming salt and water.
Interaction of amphoteric hydroxides with acids:
Zn(OH) 2 + 2HCl + ZnCl 2 + 2H 2 O;
Sn(OH) 2 + H 2 SO 4 = SnSO 4 + 2H 2 O.
Interaction of amphoteric hydroxides with alkalis:
Zn(OH) 2 + 2NaOH Na 2 ZnO 2 + 2H 2 O;
Zn(OH) 2 + 2NaOH Na 2 ;
Pb(OH) 2 + 2NaOHNa 2 .
Salts – products of the replacement of hydrogen atoms in an acid molecule with metal atoms or the replacement of a hydroxide ion in a base molecule with acidic residues.
General chemical properties salts
1. Salts in aqueous solutions dissociate into ions:
a) medium salts dissociate into metal cations and anions of acidic residues:
NaCN =Na + +СN - ;
6) acid salts dissociate into metal cations and complex anions:
KHSO 3 = K + + HSO 3 -;
c) basic salts dissociate into complex cations and anions of acidic residues:
AlOH(CH 3 COO) 2 = AlOH 2+ + 2CH 3 COO - .
2. Salts react with metals to form a new salt and a new metal. A given metal can displace from salt solutions only those metals that are located to the right of it in electrochemical series voltage:
CuSO 4 + Fe = FeSO 4 + Cu.
Soluble salts react with alkalis to form a new salt and a new base. The reaction is possible if the resulting base or salt precipitates.
For example:
FeCl 3 +3KOH = Fe(OH) 3 ↓+3KS1;
K 2 CO 3 + Ba(OH) 2 = BaCO 3 ↓+ 2KOH.
4. Salts react with acids to form new more weak acid or new insoluble salt:
Na 2 CO 3 + 2HC1 = 2NaCl + CO 2 + H 2 O.
When a salt reacts with an acid that forms a given salt, an acidic salt is obtained (this is possible if the salt is formed by a polybasic acid).
For example:
Na 2 S + H 2 S = 2NaHS;
CaCO 3 + CO 2 + H 2 O = Ca(HCO 3) 2.
5. Salts can interact with each other to form new salts if one of the salts precipitates:
AgNO 3 + KC1 = AgCl↓ + KNO 3.
6. Many salts decompose when heated:
MgCO 3 MgO+ CO 2;
2NaNO 3 2NaNO 2 + O 2 .
7. Basic salts react with acids to form medium salts and water:
Fe(OH) 2 NO 3 +HNO 3 = FeOH(NO 3) 2 +H 2 O;
FeOH(NO 3) 2 + HNO 3 = Fe(NO 3) 3 + H 2 O.
8. Acidic salts react with alkalis to form medium salts and water:
NaHSO 4 + NaOH = Na 2 SO 3 + H 2 O;
KN 2 RO 4 + KON = K 2 NRO 4 + H 2 O.
Obtaining salts
All methods for obtaining salts are based on the chemical properties of the most important classes of non-salts. organic compounds. Ten classical methods for obtaining salts are presented in the table. 7.
In addition to general methods for obtaining salts, some private methods are also possible:
1. Interaction of metals whose oxides and hydroxides are amphoteric with alkalis.
2. Fusion of salts with certain acid oxides.
K 2 CO 3 + SiO 2 K 2 SiO 3 + CO 2 .
3. Interaction of alkalis with halogens:
2KOH + Cl 2 KCl + KClO + H 2 O.
4. Interaction of halides with halogens:
2KVg + Cl 2 = 2KS1 + Br 2.
Tartaric acid: general description of the substance, location in nature, physical and chemical characteristics. Properties of tartaric acid salts. Its production...
Tartaric acid: structural formula, properties, preparation and application
From Masterweb
04.12.2018 15:00Tartaric acid belongs to the class of carboxylic acids. This substance received its name due to the fact that the main source of its production is grape juice. During fermentation of the latter, acid is released in the form of a poorly soluble potassium salt. The main area of application of this substance is the production of food industry products.
General description
Tartaric acid belongs to the category of acyclic dibasic hydroacids, which contain both hydroxyl and carboxyl groups. Such compounds are also considered as hydroxyl derivatives of carboxylic acids. This substance has other names:
- dioxysuccinic;
- tartare;
- 2, 3-dihydroxybutanedioic acid.
Chemical formula of tartaric acid: C4H6O6.
This compound is characterized by stereoisometry and can exist in 3 forms. The structural formulas of tartaric acids are presented in the figure below.
The third form (mesotartaric acid) is the most stable. D- and L-acids are optically active, but a mixture of these isomers, taken in equivalent quantities, is optically inactive. This acid is also called r- or i-tartaric (racemic, grape). In appearance, this substance is colorless crystals or white powder.
Location in nature
L-tartaric (RR-tartaric) and grape acids are found in large quantities in grapes, their processed products, as well as in the acidic juices of many fruits. For the first time this connection was isolated from tartar - the sediment that falls during the production of wine. It is a mixture of potassium tartrate and calcium.
Mesotartaric acid does not occur in nature. It can only be obtained artificially– when boiling D- and L-isomers in caustic alkalis, as well as during the oxidation of maleic acid or phenol.
Physical characteristics
Main physical properties tartaric acid are:
- Molecular weight – 150 a. e.m.
- Melting point: o D- or L-isomer – 170 °C; o grape acid – 260 °C; o mesotartaric acid – 140 °C.
- Density – 1.66-1.76 g/cm3.
- Solubility – 135 g of anhydrous substance per 100 g of water (at a temperature of 20 ° C).
- Heat of combustion – 1096.7 kJ/(g∙mol).
- Specific heat capacity – 1.26 kJ/(mol∙°С).
- Molar heat capacity – 0.189 kJ/(mol∙°С).
The acid dissolves well in water, and heat absorption and a decrease in the temperature of the solution are observed.
Crystallization from aqueous solutions occurs in the hydrate form (2C4H6O6)∙H2O. The crystals have the shape of rhombic prisms. In mesotartaric acid they are prismatic or scaly. When heated above 73 °C, the anhydrous form crystallizes from alcohol.
Chemical properties
Tartaric acid, like other hydroxy acids, has all the properties of alcohols and acids. The functional groups –COOH and –OH can react with other compounds both independently and have a mutual influence on each other, which determines chemical features of this substance:
- Electrolytic dissociation. Tartaric acid is more strong electrolyte than the original ones carboxylic acids. D- or L-isomers have the highest degree of dissociation, mesotartaric acid has the least.
- Formation of acidic and medium salts (tartrates). The most common of them are: sour tartrate and potassium tartrate, calcium tartrate.
- Formation of chelate complexes with metals having different structures. The composition of these compounds depends on the acidity of the medium.
- Education esters when replacing –OH in the carboxyl group.
When L-tartaric acid is heated to 165 °C, the product is dominated by mesotartaric and grape acids, in the range of 165-175 °C by grape, and above 175 °C by metatartaric acid, which is a yellowish resinous substance.
Grape acid when heated to 130 ° C mixed with hydrochloric acid partially turns into meso-wine.
Properties of salts
Among the characteristics of tartaric acid salts are the following:
- Acid potassium salt KHC4H4O6 (potassium hydrogen tartrate, cream of tartar): o poorly soluble in water and alcohol; o precipitates during prolonged exposure; o has the appearance of colorless small crystals, the shape of which can be rhombic, square, hexagonal or rectangular; o relative density – 1.973.
- Calcium tartrate CaC4H4O6: o appearance– rhombic crystals; o poorly soluble in water.
- Average potassium salt K2C4H4∙0.5 H2O, acid calcium salt CaH2 (C4H4O6)2 – good solubility in water.
Synthesis
There are 2 types of raw materials for producing tartaric acid:
- tartrate lime (a product of processing marc, sedimentary yeast, waste from the production of cognac alcohol from wine materials);
- potassium hydrogen tartrate (formed in young wine when it is cooled, as well as when concentrating grape juice).
The accumulation of tartaric acid in grapes depends on its variety and the climatic conditions in which it was grown (less of it is formed in cold years).
Tartaric lime is first purified from impurities by washing with water, filtration, and centrifugation. Potassium hydrothorate is ground in ball mills or crushers to a particle size of 0.1-0.3 mm, and then processed into lime in an exchange precipitation reaction with calcium chloride and calcium carbonate.
Tartaric acid is produced in reactors. First, water is poured into it after washing the gypsum sludge, then cream of tartar is loaded at the rate of 80-90 kg/m3. This mass is heated to 70-80 °C, calcium chloride and milk of lime are added to it. The decomposition of the tartar lasts 3-3.5 hours, after which the suspension is filtered and washed.
The acid is isolated from tartrate of lime by decomposition of H2SO4 in an acid-resistant steel reactor. The mass is heated to 85-90 °C. Excess acid is neutralized at the end of the process using chalk. The acidity of the solution should be no more than 1.5. The tartaric acid solution is then evaporated and crystallized. Dissolved gypsum precipitates.
Applications
The use of tartaric acid is mainly associated with the food industry. Its use helps to increase appetite, enhance the secretory function of the stomach and pancreas, and improve the digestive process. Previously, tartaric acid was widely used as an acidifier, but now it has been replaced by citric acid (including in winemaking when processing very ripe grapes).
Diacetyl tartrate ester is used to improve the quality of bread. Thanks to its use, the porosity and volume of bread crumb, as well as its shelf life, increases.
The main areas of application of tartaric acid are due to its physicochemical properties:
- acidifier and acidity regulator;
- antioxidant;
- preservative;
- catalyst for solveolysis with water in organic synthesis and analytical chemistry.
In the food industry, the substance is used as an additive E334 in such food products as:
- confectionery, cookies;
- canned vegetables and fruits;
- jellies and jams;
- low alcohol drinks, lemonade.
Metatartaric acid is used as a stabilizer and additive to prevent cloudiness in wine, champagne and the appearance of tartar.
Winemaking and brewing
Tartaric acid is added to the must if its level is below 0.65% for red wines and 0.7-0.8% for white wines. Adjustments are made before fermentation begins. First, this is done on a prototype, then the substance is added to the wort in small portions. If tartaric acid is in excess, then cold stabilization is carried out. Otherwise, crystals will precipitate in bottles of commercial wine.
In beer production, acid is used to wash cultivated yeast from wild yeast. Contamination of beer by the latter is the cause of its cloudiness and defects. The addition of even a small amount of tartaric acid (0.5-1.0%) neutralizes these microorganisms.
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Acids- these are complex substances whose molecules consist of hydrogen atoms that can be replaced and acidic residues.
The acid residue has a negative charge.
Oxygen-free acids: HCl, HBr, H 2 S, etc.
An element that, together with hydrogen and oxygen atoms, forms an oxygen-containing acid molecule is called acid-forming.
According to the number of hydrogen atoms in the molecule, acids are divided into monobasic And polybasic.
Monobasic acids contain one hydrogen atom: HCl, HNO 3, HBr, etc.
Polybasic acids contain two or more hydrogen atoms: H 2 SO 4 (dibasic), H 3 PO 4 (tribasic).
In oxygen-free acids, to the name of the element that forms the acid, add the connecting vowel “o” and the words “... hydrogen acid" For example: HF – hydrofluoric acid.
If the acid-forming element exhibits the maximum oxidation state (it corresponds to the group number), then add “...naya acid". Butexample:
HNO 3 – nitrogen oh acid (because the nitrogen atom has a maximum oxidation state of +5)
If the oxidation state of the element is below the maximum, then add "...tired acid":
1+3-2
HNO 2 – nitrogen exhausted acid (since the acid-forming element N has a minimum oxidation state).
H3PO4 – ortho phosphoric acid.
HPO 3 – meta phosphoric acid.
Structural formulas of acids.
In a molecule of an oxygen-containing acid, a hydrogen atom is bonded to an atom of the acid-forming element through an oxygen atom. Therefore, when compiling a structural formula, all hydroxide ions must first be attached to the atom of the acid-forming element.
Then connect the remaining oxygen atoms with two dashes directly to the atoms of the acid-forming element (Fig. 2).
Well, to complete the acquaintance with alcohols, I will also give the formula of another well-known substance - cholesterol. Not everyone knows that it is a monohydric alcohol!
|`/`\\`|<`|w>`\`/|<`/w$color(red)HO$color()>\/`|0/`|/\<`|w>|_q_q_q<-dH>:a_q|0<|dH>`/<`|wH>`\|dH; #a_(A-72)<_(A-120,d+)>-/-/<->`\
I marked the hydroxyl group in it in red.
Carboxylic acids
Any winemaker knows that wine should be stored without access to air. Otherwise it will turn sour. But chemists know the reason - if you add another oxygen atom to an alcohol, you get an acid.Let's look at the formulas of acids that are obtained from alcohols already familiar to us:
| Substance | Skeletal formula | Gross formula | ||
|---|---|---|---|---|
| Methane acid (formic acid) |
H/C`|O|\OH | HCOOH | O//\OH | |
| Ethanoic acid (acetic acid) |
H-C-C\O-H; H|#C|H | CH3-COOH | /`|O|\OH | |
| Propanic acid (methylacetic acid) |
H-C-C-C\O-H; H|#2|H; H|#3|H | CH3-CH2-COOH | \/`|O|\OH | |
| Butanoic acid (butyric acid) |
H-C-C-C-C\O-H; H|#2|H; H|#3|H; H|#4|H | CH3-CH2-CH2-COOH | /\/`|O|\OH | |
| Generalized formula | (R)-C\O-H | (R)-COOH or (R)-CO2H | (R)/`|O|\OH |
A distinctive feature of organic acids is the presence of a carboxyl group (COOH), which gives such substances acidic properties.
Anyone who has tried vinegar knows that it is very sour. The reason for this is the presence of acetic acid in it. Typically table vinegar contains between 3 and 15% acetic acid, with the rest (mostly) water. Consumption of acetic acid in undiluted form poses a danger to life.
Carboxylic acids can have multiple carboxyl groups. In this case they are called: dibasic, tribasic etc...
Food products contain many other organic acids. Here are just a few of them:
The name of these acids corresponds to the food products in which they are contained. By the way, please note that here there are acids that also have a hydroxyl group, characteristic of alcohols. Such substances are called hydroxycarboxylic acids(or hydroxy acids).
Below, under each of the acids, there is a sign specifying the name of the group of organic substances to which it belongs.
Radicals
Radicals are another concept that has influenced chemical formulas. The word itself is probably known to everyone, but in chemistry radicals have nothing in common with politicians, rebels and other citizens with an active position.
Here these are just fragments of molecules. And now we will figure out what makes them special and get acquainted with a new way of writing chemical formulas.
Generalized formulas have already been mentioned several times in the text: alcohols - (R)-OH and carboxylic acids - (R)-COOH. Let me remind you that -OH and -COOH are functional groups. But R is a radical. It’s not for nothing that he is depicted as the letter R.
To be more specific, a monovalent radical is a part of a molecule lacking one hydrogen atom. Well, if you subtract two hydrogen atoms, you get a divalent radical.
Radicals in chemistry received their own names. Some of them even received Latin designations similar to the designations of the elements. And besides, sometimes in formulas radicals can be indicated in abbreviated form, more reminiscent of gross formulas.
All this is demonstrated in the following table.
| Name | Structural formula | Designation | Brief formula | Example of alcohol | ||
|---|---|---|---|---|---|---|
| Methyl | CH3-() | Me | CH3 | (Me)-OH | CH3OH | |
| Ethyl | CH3-CH2-() | Et | C2H5 | (Et)-OH | C2H5OH | |
| I cut through | CH3-CH2-CH2-() | Pr | C3H7 | (Pr)-OH | C3H7OH | |
| Isopropyl | H3C\CH(*`/H3C*)-() | i-Pr | C3H7 | (i-Pr)-OH | (CH3)2CHOH | |
| Phenyl | `/`=`\//-\\-{} | Ph | C6H5 | (Ph)-OH | C6H5OH | |
I think everything is clear here. I just want to draw your attention to the column where examples of alcohols are given. Some radicals are written in a form that resembles the gross formula, but the functional group is written separately. For example, CH3-CH2-OH turns into C2H5OH.
And for branched chains like isopropyl, structures with brackets are used.
There is also such a phenomenon as free radicals. These are radicals that, for some reason, have separated from functional groups. In this case, one of the rules with which we began studying the formulas is violated: the number of chemical bonds no longer corresponds to the valency of one of the atoms. Well, or we can say that one of the connections becomes open at one end. Free radicals usually live for a short time as the molecules tend to return to a stable state.
Introduction to nitrogen. Amines
I propose to get acquainted with another element that is part of many organic compounds. This nitrogen.
It is denoted by the Latin letter N and has a valency of three.
Let's see what substances are obtained if nitrogen is added to the familiar hydrocarbons:
| Substance | Expanded structural formula | Simplified structural formula | Skeletal formula | Gross formula |
|---|---|---|---|---|
| Aminomethane (methylamine) |
H-C-N\H;H|#C|H | CH3-NH2 | \NH2 | |
| Aminoethane (ethylamine) |
H-C-C-N\H;H|#C|H;H|#3|H | CH3-CH2-NH2 | /\NH2 | |
| Dimethylamine | H-C-N<`|H>-C-H; H|#-3|H; H|#2|H | $L(1.3)H/N<_(A80,w+)CH3>\dCH3 | /N<_(y-.5)H>\ | |
| Aminobenzene (Aniline) |
H\N|C\\C|C<\H>`//C<|H>`\C<`/H>`||C<`\H>/ | NH2|C\\CH|CH`//C<_(y.5)H>`\HC`||HC/ | NH2|\|`/`\`|/_o | |
| Triethylamine | $slope(45)H-C-C/N\C-C-H;H|#2|H; H|#3|H; H|#5|H;H|#6|H; #N`|C<`-H><-H>`|C<`-H><-H>`|H | CH3-CH2-N<`|CH2-CH3>-CH2-CH3 | \/N<`|/>\| |
As you probably already guessed from the names, all these substances are united under the general name amines. The functional group ()-NH2 is called amino group. Here are some general formulas of amines:
In general, there are no special innovations here. If these formulas are clear to you, then you can safely engage in further study of organic chemistry using a textbook or the Internet.
But I would also like to talk about the formulas in inorganic chemistry. You will see how easy it will be to understand them after studying the structure of organic molecules.
Rational formulas
It should not be concluded that inorganic chemistry is easier than organic chemistry. Of course, inorganic molecules usually look much simpler because they don't tend to form such complex structures like hydrocarbons. But then we have to study more than a hundred elements that make up the periodic table. And these elements tend to combine according to their chemical properties, but with numerous exceptions.
So, I won’t tell you any of this. The topic of my article is chemical formulas. And with them everything is relatively simple.
Most often used in inorganic chemistry rational formulas. And now we’ll figure out how they differ from those already familiar to us.
First, let's get acquainted with another element - calcium. This is also a very common element.
It is designated Ca and has a valency of two. Let's see what compounds it forms with the carbon, oxygen and hydrogen we know.
| Substance | Structural formula | Rational formula | Gross formula |
|---|---|---|---|
| Calcium oxide | Ca=O | CaO | |
| Calcium hydroxide | H-O-Ca-O-H | Ca(OH)2 | |
| Calcium carbonate | $slope(45)Ca`/O\C|O`|/O`\#1 | CaCO3 | |
| Calcium bicarbonate | HO/`|O|\O/Ca\O/`|O|\OH | Ca(HCO3)2 | |
| Carbonic acid | H|O\C|O`|/O`|H | H2CO3 |
At first glance, you can see that the rational formula is something between a structural and a gross formula. But it is not yet very clear how they are obtained. To understand the meaning of these formulas, you need to consider the chemical reactions in which substances participate.
Calcium in its pure form is a soft white metal. It does not occur in nature. But it is quite possible to buy it at a chemical store. It is usually stored in special jars without access to air. Because in air it reacts with oxygen. Actually, that’s why it doesn’t occur in nature.
So, the reaction of calcium with oxygen:
2Ca + O2 -> 2CaO
The number 2 before the formula of a substance means that 2 molecules are involved in the reaction.
Calcium and oxygen produce calcium oxide. This substance also does not occur in nature because it reacts with water:
CaO + H2O -> Ca(OH2)
The result is calcium hydroxide. If you look closely at its structural formula (in the previous table), you can see that it is formed by one calcium atom and two hydroxyl groups, with which we are already familiar.
These are the laws of chemistry: if a hydroxyl group attaches to organic matter, it turns out alcohol, and if it is applied to a metal, it turns out to be hydroxide.
But calcium hydroxide does not occur in nature due to the presence of carbon dioxide in the air. I think everyone has heard about this gas. It is formed during the respiration of people and animals, the combustion of coal and petroleum products, during fires and volcanic eruptions. Therefore, it is always present in the air. But it also dissolves quite well in water, forming carbonic acid:
CO2 + H2O<=>H2CO3
Sign<=>indicates that the reaction can proceed in both directions under the same conditions.
Thus, calcium hydroxide, dissolved in water, reacts with carbonic acid and turns into slightly soluble calcium carbonate:
Ca(OH)2 + H2CO3 -> CaCO3"|v" + 2H2O
A down arrow means that as a result of the reaction, the substance precipitates.
With further contact of calcium carbonate with carbon dioxide in the presence of water, a reversible reaction occurs to form an acidic salt - calcium bicarbonate, which is highly soluble in water
CaCO3 + CO2 + H2O<=>Ca(HCO3)2
This process affects the hardness of the water. When the temperature rises, bicarbonate turns back into carbonate. Therefore, in regions with hard water, scale forms in kettles.
Chalk, limestone, marble, tuff and many other minerals are largely composed of calcium carbonate. It is also found in corals, mollusk shells, animal bones, etc...
But if calcium carbonate is heated over very high heat, it will turn into calcium oxide and carbon dioxide.
This short story about the calcium cycle in nature should explain why rational formulas are needed. So, rational formulas are written so that the functional groups are visible. In our case it is:
In addition, individual elements - Ca, H, O (in oxides) - are also independent groups.Ions
I think it's time to get acquainted with ions. This word is probably familiar to everyone. And after studying the functional groups, it doesn’t cost us anything to figure out what these ions are.
In general, the nature of chemical bonds is usually that some elements give up electrons while others gain them. Electrons are particles with a negative charge. An element with a full complement of electrons has zero charge. If he gave away an electron, then its charge becomes positive, and if he accepted it, then it becomes negative. For example, hydrogen has only one electron, which it gives up quite easily, turning into a positive ion. There is a special entry for this in chemical formulas:
H2O<=>H^+ + OH^-
Here we see that as a result electrolytic dissociation water breaks down into a positively charged hydrogen ion and a negatively charged OH group. The OH^- ion is called hydroxide ion. It should not be confused with the hydroxyl group, which is not an ion, but part of some kind of molecule. The + or - sign in the upper right corner shows the charge of the ion.
But carbonic acid never exists as an independent substance. In fact, it is a mixture of hydrogen ions and carbonate ions (or bicarbonate ions):
H2CO3 = H^+ + HCO3^-<=>2H^+ + CO3^2-
The carbonate ion has a charge of 2-. This means that two electrons have been added to it.
Negatively charged ions are called anions. Typically these include acidic residues.
Positively charged ions - cations. Most often these are hydrogen and metals.
And here you can probably fully understand the meaning of rational formulas. The cation is written in them first, followed by the anion. Even if the formula does not contain any charges.
You probably already guess that ions can be described not only by rational formulas. Here is the skeletal formula of the bicarbonate anion:
Here the charge is indicated directly next to the oxygen atom, which received an extra electron and therefore lost one line. Simply put, each extra electron reduces the number of chemical bonds depicted in the structural formula. On the other hand, if some node of the structural formula has a + sign, then it has an additional stick. As always, this fact needs to be demonstrated with an example. But among the substances familiar to us there is not a single cation that consists of several atoms.
And such a substance is ammonia. His aqueous solution often called ammonia and is included in any first aid kit. Ammonia is a compound of hydrogen and nitrogen and has the rational formula NH3. Let's consider chemical reaction which occurs when ammonia is dissolved in water:
NH3 + H2O<=>NH4^+ + OH^-
The same thing, but using structural formulas:
H|N<`/H>\H + H-O-H<=>H|N^+<_(A75,w+)H><_(A15,d+)H>`/H + O`^-# -H
On the right side we see two ions. They were formed as a result of one hydrogen atom moving from a water molecule to an ammonia molecule. But this atom moved without its electron. The anion is already familiar to us - it is a hydroxide ion. And the cation is called ammonium. It exhibits properties similar to metals. For example, it may combine with an acidic residue. The substance formed by combining ammonium with a carbonate anion is called ammonium carbonate: (NH4)2CO3.
Here is the reaction equation for the interaction of ammonium with a carbonate anion, written in the form of structural formulas:
2H|N^+<`/H><_(A75,w+)H>_(A15,d+)H + O^-\C|O`|/O^-<=>H|N^+<`/H><_(A75,w+)H>_(A15,d+)H`|0O^-\C|O`|/O^-|0H_(A-15,d-)N^+<_(A105,w+)H><\H>`|H
But in this form the reaction equation is given for demonstration purposes. Typically equations use rational formulas:
2NH4^+ + CO3^2-<=>(NH4)2CO3
Hill system
So, we can assume that we have already studied structural and rational formulas. But there is another issue that is worth considering in more detail. How do gross formulas differ from rational ones?
We know why the rational formula of carbonic acid is written H2CO3, and not some other way. (The two hydrogen cations come first, followed by the carbonate anion.) But why is the gross formula written CH2O3?
In principle, the rational formula of carbonic acid may well be considered a true formula, because it has no repeating elements. Unlike NH4OH or Ca(OH)2.
But an additional rule is very often applied to gross formulas, which determines the order of elements. The rule is quite simple: carbon is placed first, then hydrogen, and then the remaining elements in alphabetical order.
So CH2O3 comes out - carbon, hydrogen, oxygen. This is called the Hill system. It is used in almost all chemical reference books. And in this article too.
A little about the easyChem system
Instead of a conclusion, I would like to talk about the easyChem system. It is designed so that all the formulas that we discussed here can be easily inserted into the text. Actually, all the formulas in this article are drawn using easyChem.
Why do we even need some kind of system for deriving formulas? The thing is that the standard way to display information in Internet browsers is hypertext markup language (HTML). It is focused on processing text information.
Rational and gross formulas can be depicted using text. Even some simplified structural formulas can also be written in text, for example alcohol CH3-CH2-OH. Although for this you would have to use the following entry in HTML: CH 3-CH 2-OH.
This of course creates some difficulties, but you can live with them. But how to depict the structural formula? In principle, you can use a monospace font:
H H | | H-C-C-O-H | | H H Of course it doesn’t look very nice, but it’s also doable.
The real problem comes when trying to draw benzene rings and when using skeletal formulas. There is no other way left except connecting a raster image. Rasters are stored in separate files. Browsers can include images in gif, png or jpeg format.
To create such files, a graphic editor is required. For example, Photoshop. But I have been familiar with Photoshop for more than 10 years and I can say for sure that it is very poorly suited for depicting chemical formulas.
Molecular editors cope with this task much better. But with a large number of formulas, each of which is stored in a separate file, it is quite easy to get confused in them.
For example, the number of formulas in this article is . They are displayed in the form of graphic images (the rest using HTML tools).
The easyChem system allows you to store all formulas directly in an HTML document in text form. In my opinion, this is very convenient.
In addition, the gross formulas in this article are calculated automatically. Because easyChem works in two stages: first the text description is converted into an information structure (graph), and then various actions can be performed on this structure. Among them, the following functions can be noted: calculation of molecular weight, conversion to a gross formula, checking for the possibility of output as text, graphic and text rendering.
Thus, to prepare this article, I only used a text editor. Moreover, I didn’t have to think about which of the formulas would be graphic and which would be text.
Here are a few examples that reveal the secret of preparing the text of an article: Descriptions from the left column are automatically turned into formulas in the second column.
In the first line, the description of the rational formula is very similar to the displayed result. The only difference is that the numerical coefficients are displayed interlinearly.
In the second line, the expanded formula is given in the form of three separate chains separated by a symbol; I think it is easy to see that the textual description is in many ways reminiscent of the actions that would be required to depict the formula with a pencil on paper.
The third line demonstrates the use of slanted lines using the \ and / symbols. The ` (backtick) sign means the line is drawn from right to left (or bottom to top).
There is much more detailed documentation on using the easyChem system here.
Let me finish this article and wish you good luck in studying chemistry.
