Abstract on the topic:
Chemical formula
Chemical formula- reflection of information about the composition and structure of substances using chemical symbols, numbers and dividing marks - brackets.
The composition of the molecules of complex substances is expressed using chemical formulas.
Based on the chemical formula, the name of the substance can be given.
The chemical formula stands for:
- 1 molecule or 1 mole of a substance;
- qualitative composition (what chemical elements the substance consists of);
- quantitative composition (how many atoms of each element a molecule of a substance contains).
- The formula HNO3 stands for:
- nitric acid;
- 1 molecule of nitric acid or 1 mole of nitric acid;
- qualitative composition: the nitric acid molecule consists of hydrogen, nitrogen and oxygen;
- quantitative composition: a nitric acid molecule contains one atom of the element hydrogen, one atom of the element nitrogen, three atoms of the element oxygen.
Kinds
Currently, the following types of chemical formulas are distinguished:
- The simplest formula. It can be obtained experimentally by determining the ratio of chemical elements in a substance using the atomic mass values of the elements. So, the simplest formula of water will be H 2 O, and the simplest formula of benzene CH (unlike C 6 H 6 - true, see below). Atoms in formulas are indicated by the signs of chemical elements, and their relative quantities are indicated by numbers in subscript format.
- True Formula. Can be obtained if the molecular weight of the substance is known. The true formula of water is H 2 O, which coincides with the simplest one. The true formula of benzene is C 6 H 6, which differs from the simplest one. True formulas are also called gross formulas or empirical. They reflect the composition, but not the structure, of the molecules of a substance. The true formula shows the exact number of atoms of each element in one molecule. This quantity corresponds to an index - a small number after the symbol of the corresponding element. If the index is 1, that is, there is only one atom of a given element in the molecule, then such an index is not indicated.
- Rational formula. Rational formulas highlight groups of atoms characteristic of classes of chemical compounds. For example, for alcohols the -OH group is allocated. When writing a rational formula, such groups of atoms are enclosed in parentheses (OH). The number of repeating groups is indicated by numbers in subscript format, which are placed immediately after the closing bracket. Square brackets are used to reflect the structure of complex compounds. For example, K 4 is potassium hexacyanocobaltoate. Rational formulas are often found in a semi-expanded form, when some of the same atoms are shown separately to better reflect the structure of the molecule of a substance.
- Structural formula. Graphically shows the relative arrangement of atoms in a molecule. Chemical bonds between atoms are indicated by lines. There are two-dimensional (2D) and three-dimensional (3D) formulas. Two-dimensional ones are a reflection of the structure of matter on a plane. Three-dimensional ones make it possible to represent its composition, relative position, connections and distances between atoms most closely to theoretical models of the structure of matter.
- Ethanol
- The simplest formula is C 2 H 6 O
- True, empirical, or gross formula: C 2 H 6 O
- Rational formula: C 2 H 5 OH
- Rational formula in semi-expanded form: CH 3 CH 2 OH
- Structural formula (2D):
There are other ways to write chemical formulas. New methods appeared in the late 1980s with the development of personal computer technology (SMILES, WLN, ROSDAL, SLN, etc.). Personal computers also use special software called molecular editors to work with chemical formulas.
Notes
- 1 2 3 Basic concepts of chemistry - de.gubkin.ru/chemistry/ch1-th/node6.html
This abstract is based on an article from the Russian Wikipedia. Synchronization completed 07/10/11 17:38:37
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Currently, the following types of chemical formulas are distinguished:
- The simplest formula. It can be obtained experimentally by determining the ratio of chemical elements in a substance using the atomic mass values of the elements. So, the simplest formula of water will be H 2 O, and the simplest formula of benzene CH (unlike C 6 H 6 - true, see below). Atoms in formulas are indicated by the signs of chemical elements, and their relative quantities are indicated by numbers in subscript format.
- Empirical formula. Different authors may use this term to mean the simplest, true or rational formulas
- True Formula. Can be obtained if the molecular weight of the substance is known. The true formula of water is H 2 O, which coincides with the simplest one. The true formula of benzene is C 6 H 6, which differs from the simplest one. True formulas are also called gross formulas. They reflect the composition, but not the structure, of the molecules of a substance. The true formula shows the exact number of atoms of each element in one molecule. This quantity corresponds to an index - a small number after the symbol of the corresponding element. If the index is 1, that is, there is only one atom of a given element in the molecule, then such an index is not indicated.
- Rational formula. Rational formulas highlight groups of atoms characteristic of classes of chemical compounds. For example, for alcohols the -OH group is allocated. When writing a rational formula, such groups of atoms are enclosed in parentheses (OH). The number of repeating groups is indicated by numbers in subscript format, which are placed immediately after the closing bracket. Square brackets are used to reflect the structure of complex compounds. For example, K 4 is potassium hexacyanocobaltate. Rational formulas are often found in a semi-expanded form, when some of the same atoms are shown separately to better reflect the structure of the molecule of a substance.
- Structural formula. Graphically shows the relative arrangement of atoms in a molecule. Chemical bonds between atoms are indicated by lines. There are two-dimensional (2D) and three-dimensional (3D) formulas. Two-dimensional ones are a reflection of the structure of matter on a plane. Three-dimensional ones make it possible to represent its composition, relative position, connections and distances between atoms most closely to theoretical models of the structure of matter.
- Ethanol
- The simplest formula is C 2 H 6 O
- True, empirical, or gross formula: C 2 H 6 O
- Rational formula: C 2 H 5 OH
- Rational formula in semi-expanded form: CH 3 CH 2 OH
- Structural formula (2D):
There are other ways to write chemical formulas. New methods appeared in the late 1980s with the development of personal computer technology (SMILES, WLN, ROSDAL, SLN, etc.). Personal computers also use special software called molecular editors to work with chemical formulas.
Gross, structural and electronic formulas of compounds
Vutlerov's second postulate. The chemical reactivity of certain groups of atoms depends significantly on their chemical environment, that is, on which atoms or groups of atoms a certain group is adjacent to.
The formulas of compounds that we used in the study of inorganic chemistry reflect only the number of atoms of a particular element in the molecule. Such formulas are called “gross formulas” or “molecular formulas”.
As follows from Vutlerov’s first postulate, in organic chemistry it is not only the number of certain atoms in a molecule that is important, but also the order of their binding, that is, it is not always advisable to use gross formulas for organic compounds. For example, for clarity, when considering the structure of the methane molecule, we used structural formulas - a schematic representation of the order of bonding of atoms into a molecule. When depicting structural formulas, a chemical bond is denoted by a dash, a double bond by two dashes, etc.
The electronic formula (or Lewis formula) is very similar to the structural formula, but in this case it is not the formed bonds that are represented, but the electrons, both those that form a bond and those that do not.
For example, sulfate acid, already discussed, can be written using the following formulas. The gross formula is H 2 80 4, the structural and electronic formulas are as follows:
Structural formulas organic compounds
Almost all organic substances consist of molecules, the composition of which is expressed by chemical formulas, for example CH 4, C 4 H 10, C 2 H 4 O 2. What structure do molecules of organic substances have? The founders of organic chemistry, F. Kekule and A. M. Vutlerov, asked themselves this question in the mid-19th century. Studying the composition and properties of various organic substances, they came to the following conclusions:
Atoms in molecules of organic substances are connected by chemical bonds in a certain sequence, according to their valency. This sequence is usually called the chemical structure;
Carbon atoms in all organic compounds are chotivalent, and other elements exhibit their characteristic valences.
This position is the basis of the theory of the structure of organic compounds, formulated by O. M. Butlerov in 1861.
The chemical structure of organic compounds is visually represented by structural formulas, in which chemical bonds between atoms are indicated by dashes. The total number of lines extending from the symbol of each element is equal to its atomic valence. Multiple bonds are represented by two or three dashes.
Using the example of the saturated hydrocarbon propane C 3 H 8, let's consider how to compose the structural formula of an organic substance.
1. Draw a carbon skeleton. In this case, the chain consists of three Carbon atoms:
S-S- WITH
2. Carbon is tetravalent, so we depict insufficient features from each carbon atom in such a way that there are four features next to each atom:
3. Add the symbols of Hydrogen atoms:
Often structural formulas are written in abbreviated form, without depicting the C - H bond. Abbreviated structural formulas are much more compact than expanded ones:
CH 3 - CH 2 - CH 3.
Structural formulas show only the sequence of connections of atoms, but do not reflect the spatial structure of molecules, in particular bond angles. It is known, for example, that the angle between C bonds in propane is 109.5°. However, the structural formula of propane looks like this angle is 180°. Therefore, it would be more correct to write the structural formula of propane in a less convenient, but more true form:
Professional chemists use the following structural formulas, in which neither Carbon atoms nor Hydrogen atoms are shown at all, but only the carbon skeleton is depicted in the form of interconnected C-C bonds, as well as functional groups. To ensure that the backbone does not look like one continuous line, chemical bonds are depicted at an angle to each other. So, in the propane molecule C 3 H 8 there are only two C-C bonds, so propane is represented by two dashes.
Homologous series of organic compounds
Let's consider the structural formulas of two compounds of the same class, for example alcohols:
The molecules of methyl CH 3 OH and ethyl C 2 H 5 OH alcohols have the same functional group OH, common to the entire class of alcohols, but differ in the length of the carbon skeleton: in ethanol there is one more Carbon atom. Comparing the structural formulas, one can notice that when the carbon chain is increased by one Carbon atom, the composition of the substance changes to a CH 2 group, when the carbon chain is lengthened by two atoms - to two CH 2 groups, etc.
Compounds of the same class, having a similar structure, but differing in composition by one or more CH 2 groups, are called homologues.
The CH 2 group is called a homologous difference. The totality of all homologues forms a homological series. Methanol and ethanol belong to the homologous series of alcohols. All substances of the same series have similar chemical properties, and their composition can be expressed by a general formula. For example, the general formula of the homologous series of alcohols is C n H 2 n +1 VON, where n - natural number.
|
Connection class |
General formula |
General formula highlighting the functional group |
|
Alkanes |
C n H 2 n + 2 |
|
|
Cycloalkani |
C n H 2 n |
|
|
Alkenes |
C n H 2 n |
|
|
Alkadieni |
C n H 2 n-2 |
|
|
Alkini |
C n H 2 n-2 |
|
|
Mononuclear arenes (homologous series to benzene) |
C n H 2 n-6 |
|
|
Monohydric alcohols |
C n H 2 n + 2 V |
C n H 2 n +1 V H |
|
Polyhydric alcohols |
C n H 2 n + 2 O x |
C n H 2 n + 2-x (B H) x |
|
Aldehydes |
C n H 2 n B |
C n H 2 n +1 CHO |
|
Monobasic carboxylic acids |
C n H 2 n O 2 |
C n H 2 n +1 COOH |
|
esters |
C n H 2 n B |
C n H 2 n +1 COOC n H 2n+1 |
|
Carbohydrates |
C n (H 2 O) m |
|
|
Primary amines |
C n H 2 n + 3 N |
C n H 2 n +1 NH 2 |
|
Amino acids |
C n H 2 n +1 NO |
H 2 NC n H 2n COOH |
The gross formula of the substance and its transformation into toluene indicate that it is methylcyclohexadiene. It is capable of adding oleic anhydride, which is typical for conjugated dienes.
The gross formula of a substance is reliably determined only by a combination of elemental analysis with the determination of molecular weight.
Determining the gross formula of a substance thus requires analysis of homologous series of fragment ions and characteristic differences.
How is the gross formula of a substance determined?
In addition to the PMR spectrum and the gross formula of a substance, to establish the structural formula, there is data on its nature or origin, without which an unambiguous interpretation of the spectrum would be impossible.
At the beginning of each article, the gross formula of the substance, its name and the structural formula are given. The search for the required substance in the directory is carried out using a known gross formula and formula index or by a well-known name and alphabetical index located at the end of the directory.
The first column of all tables gives the gross formula of the substance, the next column shows its chemical formula. Then the temperature at which the measurements were taken is indicated. For halogens (except iodine), only data obtained at the standard NQR temperature of liquid nitrogen (77 K) are given - Data for other temperatures are given in the absence of measurements at 77 K, which is specified in the notes.
Mass spectrometry methods are used to identify substances, determine the gross formulas of substances and their chemical structure. Important for chemistry are such physical characteristics as ionization potential and energy of breaking of chemical bonds.
To find any compound in the formula index, you must first calculate the gross formula of the substance and arrange the elements according to the Hill system: for inorganic substances in alphabetical order, for example H3O4P (phosphoric acid), CuO4S (copper sulfate), O7P2Zn2 (zinc pyrophosphate), etc. .
To find any compound in the formula index, you must first calculate the gross formula of the substance and arrange the elements according to the Hill system: for inorganic substances in alphabetical order, for example H3O4P (phosphoric acid), CuO4S (copper sulfate), O7P2Zn2 (zinc pyrophosphate), etc. .
The capabilities of low-resolution mass spectrometry do not allow separating the second and third stages of group identification, and the determination of the gross formula of a substance is carried out simultaneously with limiting the number of possible options for assigning it to specific homologous series. By definition, a homologous group unites a series of compounds whose mass numbers are comparable modulo 14, including isobaric ones. In some cases, isobaric compounds of different series have similar patterns of fragmentation, which is manifested in the similarity of their low-resolution mass spectra.
The mass of the molecular ion (180 1616) is measured with high accuracy, which allows you to immediately determine the gross formula of the substance.
Based on the above, in the elemental analysis of organic compounds, weight-free methods have been proposed for determining the stoichiometry of molecules characterizing the gross formula of a substance. Basically, these methods are intended to determine the stoichiometry of organogenic elements: carbon, hydrogen and nitrogen. They are based on comparison of analytical signals of the mineralization products of a substance sample. Such signals include, for example, the areas of chromatographic peaks, volumes of titrant common to two elements, etc. Thus, it is possible to work without balances with micro- and ultra-microquantities.
Quantitative analysis of polymers includes the following questions: 1) quantitative elemental analysis, which makes it possible to determine the gross formula of a substance; 2) determination of the number of functional and terminal groups in polymer chains; 3) definition of mol.
Accurate molecular weight values can be obtained from mass spectra and serve as the basis for certain alternative assumptions about the gross formula of a substance, its qualitative and quantitative composition. So, in particular, an odd molecular weight can serve as evidence of the presence in a molecule of one (three, five, generally an odd number) nitrogen atom: nitrogen is the only organogen element with an odd valence with an even atom. In contrast, an even molecular weight indicates the absence of nitrogen or the possibility of an even number of nitrogen atoms. Thus, for example, an organic substance with M 68 can have only three gross formulas: CsHs, 4 6 or C3H, and taking them into account will significantly facilitate the interpretation of spectral data and the final choice of structure.
An even more valuable source of the necessary additional information is the data of quantitative (elemental) analysis, which, in combination with the determination of molecular weight, makes it possible to establish the gross formula of a substance.
An even more valuable source of the necessary additional information is the data of quantitative (elemental) analysis, which, in combination with the determination of molecular weight, makes it possible to establish the gross formula of a substance. Classical (chemical) methods for establishing the gross formula are now increasingly being replaced by mass spectrometric methods, based on accurate measurement of the intensity of isotopic lines of molecular ions or very accurate measurement of mass numbers on high-resolution spectrometers.
An even more valuable source of necessary additional information is the data of quantitative (elemental) analysis, which, in combination with the determination of molecular weight, allows one to determine the gross formula of a substance.
Please note that this is a rare case when the gross formula corresponds to one substance. Usually, based on this data, we can only indicate the gross formula of a substance, but not the structural formula. And often we cannot even correlate a substance with a certain class. To obtain the structural formula of a substance, additional data on the chemical properties of this substance is required.
Elemental analysis is used for the quantitative determination of organic and organoelement compounds containing nitrogen, halogens, sulfur, as well as arsenic, bismuth, mercury, antimony and other elements. Elemental analysis can also be used to qualitatively confirm the presence of these elements in the composition of the test compound or to establish or confirm the gross formula of a substance.
The last row is less probable, since its sign is the presence in the spectra of intense peaks of the 4th homologous group, which are not present in the case under consideration. Subsequent detailing of the assignment can be unambiguously carried out using the spectra of the ion series (see section 5.5), however, given the high intensity of the peaks of molecular ions in this spectrum, it is advisable to clarify the gross formula of the substance using isotopic signals.
The concept of homology is one of the most important in organic chemistry, and homological series form the basis of the modern classification of organic compounds. Questions of whether compounds belong to different homologous series are very important and are associated, for example, with problems of isomerism in organic chemistry, in particular with the creation of effective algorithms for determining the number of possible isomers based on the gross formula of a substance using a computer.
Collection scheme for quantitative elemental analysis. In elemental analysis, there is a trend towards reducing manual labor and increasing the accuracy of determinations. The development of instrument technology has made it possible in recent years to develop a device for automatic elemental analysis, in which carbon dioxide, water and nitrogen formed during the combustion of a sample are sent by a helium current to a gas chromatograph attached to the device, with the help of which their simultaneous quantitative determination is carried out. On the other hand, the use of a high-resolution mass spectrometer (see section 1.1.9.3) makes it possible to simply determine the gross formula of a substance without quantitative elemental analysis.
An interactive mode of operation of the RASTR system has been developed. The exchange of information between a person and a computer is carried out through an alphanumeric display. The program polls the worker, simultaneously indicating the form of the answer. Information is required on the types of experimental spectra available, their characteristics and spectral parameters. After entering all the spectral information and the gross formula of the substance, the operator indicates the mode for constructing implications - logical relationships between the characteristics of the spectrum and the structure of the compound. The operator has the opportunity to make any changes to them: exclude or add information to library fragments, remove any implications or add new ones. As a result of solving a system of consistent logical equations, sets of fragments that satisfy the spectra and chemical information are displayed on the display.
When processing mass spectra manually, a necessary identification stage is determining the class of the substance. This stage is also included, either explicitly or implicitly, in many complex identification algorithms designed for computers. A similar operation can be performed in the case where the mass spectrum of the substance being determined was not previously known, but the patterns of fragmentation of the class of compounds to which it belongs have been well studied. This is possible on the basis of qualitative and quantitative patterns of fragmentation common to a given class or homologous series. If for an unknown component it was possible to register a peak as important for identification as the peak of a molecular ion, then, in combination with information about the class of the compound, the molecular weight makes it possible to determine the gross formula of the substance. It should be noted that the use of isotopic peaks to determine the gross formula in chromatography-mass spectrometric analysis is of limited importance and is possible only with high intensity of these peaks and the peak of the molecular ion. For certain groups of isomers of aromatic and paraffin hydrocarbons, individual identification algorithms have been developed, built taking into account certain quantitative features of their mass spectra.
Well, to complete our 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 valence 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 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 tend to look much simpler because they don't tend to form 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 is added to an organic substance, an alcohol is obtained, and if it is added to a metal, a hydroxide is obtained.
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, this 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. Its aqueous solution is 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. Consider the chemical reaction that 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.
