The energy of breaking a chemical bond. chemical bond

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MAIN CHARACTERISTICS OF CHEMICAL BOND

Bond energy is the energy required to break a chemical bond. The energies of breaking and forming a bond are equal in magnitude but opposite in sign. The greater the chemical bond energy, the more stable the molecule. The binding energy is usually measured in kJ/mol.

For polyatomic compounds with bonds of the same type, its average value is taken as the bond energy, calculated by dividing the energy of formation of a compound from atoms by the number of bonds. So, 432.1 kJ / mol is spent on breaking the H–H bond, and 1648 kJ / ∙ mol is spent on breaking four bonds in a methane CH 4 molecule, and in this case E C–H \u003d 1648: 4 \u003d 412 kJ / mol.

The bond length is the distance between the nuclei of interacting atoms in a molecule. It depends on the size of the electron shells and the degree of their overlap.

Bond polarity is the distribution of electrical charge between atoms in a molecule.

If the electronegativity of the atoms involved in the formation of the bond is the same, then the bond will be non-polar, and in the case of different electronegativity - polar. The extreme case of a polar bond, when the shared electron pair is almost completely biased towards the more electronegative element, results in an ionic bond.

For example: H–H is non-polar, H–Cl is polar and Na + –Cl - is ionic.

It is necessary to distinguish between the polarities of individual bonds and the polarity of the molecule as a whole.

Molecule polarity is the vector sum of the dipole moments of all the bonds of the molecule.

For example:

1) The linear CO 2 molecule (O=C=O) is non-polar - the dipole moments of the polar C=O bonds compensate each other.

2) The water molecule is polar– dipole moments of two О-Н bonds do not compensate each other.

Spatial structure of molecules determined by the shape and location in space of electron clouds.

Bond order is the number of chemical bonds between two atoms.

For example, the bond order in H 2 , O 2 and N 2 molecules is 1, 2 and 3, respectively, since the bond in these cases is formed due to the overlap of one, two and three pairs of electron clouds.

4.1. covalent bond is a bond between two atoms through a common electron pair.

The number of chemical bonds is determined by the valencies of the elements.

The valency of an element is the number of orbitals that take part in the formation of bonds.

Covalent non-polar bond - this bond is carried out due to the formation of electron pairs between atoms with equal electronegativity. For example, H 2, O 2, N 2, Cl 2, etc.

A covalent polar bond is a bond between atoms with different electronegativity.

For example, HCl, H 2 S, PH 3, etc.

A covalent bond has the following properties:

1) Saturation- the ability of an atom to form as many bonds as it has valences.

2) Orientation– the electron clouds overlap in the direction that provides the maximum overlap density.

4.2. An ionic bond is a bond between oppositely charged ions.

This is an extreme case of a covalent polar bond and occurs when there is a large difference in the electronegativity of the interacting atoms. The ionic bond does not have directionality and saturation.

The oxidation state is the conditional charge of an atom in a compound, based on the assumption that the bonds are completely ionized.

Lecture for teachers

A chemical bond (hereinafter referred to as a bond) can be defined as the interaction of two or more atoms, as a result of which a chemically stable polyatomic microsystem (molecule, crystal, complex, etc.) is formed.

The doctrine of bonding occupies a central place in modern chemistry, since chemistry as such begins where an isolated atom ends and a molecule begins. In essence, all the properties of substances are due to the peculiarities of the bonds in them. The main difference between a chemical bond and other types of interaction between atoms is that its formation is determined by a change in the state of electrons in a molecule compared to the initial atoms.

Communication theory should provide answers to a number of questions. Why are molecules formed? Why do some atoms interact and others don't? Why do atoms combine in certain ratios? Why are atoms arranged in space in a certain way? And finally, it is necessary to calculate the bond energy, its length and other quantitative characteristics. The correspondence of theoretical ideas to experimental data should be considered as a criterion for the truth of a theory.

There are two main methods of describing the relationship that allow you to answer the questions posed. These are the methods of valence bonds (BC) and molecular orbitals (MO). The first one is more clear and simple. The second is more strict and universal. For reasons of greater clarity, the focus here will be on the VS method.

Quantum mechanics makes it possible to describe communication based on the most general laws. Although there are five types of bonds (covalent, ionic, metallic, hydrogen and intermolecular bonds), the bond is one in nature, and the differences between its types are relative. The essence of communication is in the Coulomb interaction, in the unity of opposites - attraction and repulsion. The division of communication into types and the difference in the methods of its description indicates rather than the diversity of communication, but the lack of knowledge about it at the present stage of development of science.

This lecture will cover material related to topics such as chemical bond energy, quantum mechanical model of a covalent bond, exchange and donor-acceptor mechanisms for the formation of a covalent bond, excitation of atoms, bond multiplicity, hybridization of atomic orbitals, electronegativity of elements and polarity of a covalent bond , the concept of the method of molecular orbitals, chemical bonding in crystals.

Chemical bond energy

According to the principle of least energy, the internal energy of a molecule, compared with the sum of the internal energies of its constituent atoms, must decrease. The internal energy of a molecule includes the sum of the interaction energies of each electron with each nucleus, each electron with each other electron, each nucleus with each other nucleus. Attraction must prevail over repulsion.

The most important characteristic of a bond is the energy that determines its strength. The measure of bond strength can be both the amount of energy expended on breaking it (bond dissociation energy) and the value that, when summed over all bonds, gives the energy of formation of a molecule from elementary atoms. The bond breaking energy is always positive. The bond formation energy is the same in magnitude, but has a negative sign.

For a diatomic molecule, the binding energy is numerically equal to the energy of dissociation of the molecule into atoms and the energy of formation of the molecule from atoms. For example, the binding energy in the HBr molecule is equal to the amount of energy released in the process H + Br = HBr. Obviously, the binding energy of HBr is greater than the amount of energy released during the formation of HBr from gaseous molecular hydrogen and liquid bromine:

1 / 2H 2 (g.) + 1 / 2Br 2 (l.) \u003d HBr (g.),

to the value of the evaporation energy of 1/2 mol Br 2 and to the values ​​of the decomposition energies of 1/2 mol H 2 and 1/2 mol Br 2 into free atoms.

Quantum-mechanical model of a covalent bond by the method of valence bonds on the example of a hydrogen molecule

In 1927, the Schrödinger equation was solved for the hydrogen molecule by the German physicists W. Heitler and F. London. This was the first successful attempt to apply quantum mechanics to solving communication problems. Their work laid the foundations for the method of valence bonds, or valence schemes (VS).

The calculation results can be represented graphically as dependences of the forces of interaction between atoms (Fig. 1, a) and the energy of the system (Fig. 1, b) on the distance between the nuclei of hydrogen atoms. The nucleus of one of the hydrogen atoms will be placed at the origin of coordinates, and the nucleus of the second will be brought closer to the nucleus of the first hydrogen atom along the abscissa axis. If the electron spins are antiparallel, the forces of attraction (see Fig. 1, a, curve I) and repulsive forces (curve II) will increase. The resultant of these forces is represented by curve III. At first, attractive forces prevail, then repulsive ones. When the distance between the nuclei becomes equal to r 0 = 0.074 nm, the attractive force is balanced by the repulsive force. The balance of forces corresponds to the minimum energy of the system (see Fig. 1b, curve IV) and, consequently, the most stable state. The depth of the “potential well” represents the binding energy E 0 H–H in the H 2 molecule at absolute zero. It is 458 kJ/mol. However, at real temperatures, bond breaking requires a slightly lower energy E H–H, which at 298 K (25 °C) is 435 kJ/mol. The difference between these energies in the H2 molecule is the energy of vibrations of hydrogen atoms (E col = E 0 H–H – E H–H = 458 – 435 = 23 kJ/mol).

Rice. 1. Dependence of the interaction forces of atoms (a) and the energy of the system (b)
on the distance between the nuclei of atoms in the H 2 molecule

When two hydrogen atoms containing electrons with parallel spins approach each other, the energy of the system constantly increases (see Fig. 1b, curve V) and no bond is formed.

Thus, the quantum mechanical calculation gave a quantitative explanation of the relationship. If a pair of electrons has opposite spins, the electrons move in the field of both nuclei. An area with a high density of an electron cloud appears between the nuclei - an excess negative charge that pulls together positively charged nuclei. From the quantum mechanical calculation, the provisions that are the basis of the VS method follow:

1. The reason for the connection is the electrostatic interaction of nuclei and electrons.
2. The bond is formed by an electron pair with antiparallel spins.
3. Bond saturation is due to the formation of electron pairs.
4. The bond strength is proportional to the degree of electron cloud overlap.
5. The directionality of the connection is due to the overlap of electron clouds in the region of maximum electron density.

Exchange mechanism for the formation of a covalent bond by the VS method. Directionality and saturation of a covalent bond

One of the most important concepts of the VS method is valency. The numerical value of valency in the VS method is determined by the number of covalent bonds that an atom forms with other atoms.

The mechanism of formation of a bond by a pair of electrons with antiparallel spins, which belonged to different atoms before the formation of the bond, considered for the H 2 molecule, is called the exchange mechanism. If only the exchange mechanism is taken into account, the valency of an atom is determined by the number of its unpaired electrons.

For molecules more complex than H 2 , the calculation principles remain unchanged. The formation of a bond leads to the interaction of a pair of electrons with opposite spins, but with wave functions of the same sign, which are summed up. The result of this is an increase in the electron density in the region of overlapping electron clouds and contraction of the nuclei. Consider examples.

In the fluorine molecule F 2, the bond is formed by 2p orbitals of fluorine atoms:

The highest density of the electron cloud is near the 2p orbital in the direction of the symmetry axis. If the unpaired electrons of fluorine atoms are in 2p x orbitals, the bond is carried out in the direction of the x axis (Fig. 2). On the 2p y - and 2p z -orbitals there are unshared electron pairs that do not participate in the formation of bonds (shaded in Fig. 2). In what follows, we will not depict such orbitals.

Rice. 2. Formation of the F 2 molecule

In the hydrogen fluoride molecule, the HF bond is formed by the 1s orbital of the hydrogen atom and the 2p x orbital of the fluorine atom:

The direction of the bond in this molecule is determined by the orientation of the 2px orbital of the fluorine atom (Fig. 3). The overlap occurs in the direction of the x axis of symmetry. Any other variant of overlapping is energetically less favorable.

Rice. 3. Formation of the HF molecule

More complex d- and f-orbitals are also characterized by directions of maximum electron density along their symmetry axes.

Thus, directionality is one of the main properties of a covalent bond.

The directionality of the bond is well illustrated by the example of a hydrogen sulfide H 2 S molecule:

Since the symmetry axes of the valence 3p orbitals of the sulfur atom are mutually perpendicular, it should be expected that the H2S molecule should have a corner structure with an angle between the S–H bonds of 90° (Fig. 4). Indeed, the angle is close to the calculated one and is equal to 92°.

Rice. 4. Formation of the H 2 S molecule

Obviously, the number of covalent bonds cannot exceed the number of bonding electron pairs. However, saturation as a property of a covalent bond also means that if an atom has a certain number of unpaired electrons, then all of them must participate in the formation of covalent bonds.

This property is explained by the principle of least energy. With the formation of each additional bond, additional energy is released. Therefore, all valence possibilities are fully realized.

Indeed, the H 2 S molecule is stable, not HS, where there is an unrealized bond (an unpaired electron is denoted by a dot). Particles containing unpaired electrons are called free radicals. They are extremely reactive and react to form compounds containing saturated bonds.

Atom excitation

Let us consider the valence possibilities according to the exchange mechanism of some elements of the 2nd and 3rd periods of the periodic system.

The beryllium atom at the outer quantum level contains two paired 2s electrons. There are no unpaired electrons, so beryllium must have zero valence. However, in compounds it is divalent. This can be explained by the excitation of the atom, which consists in the transition of one of the two 2s electrons to the 2p sublevel:

In this case, the excitation energy E* corresponding to the difference between the energies of the 2p and 2s sublevels is expended.

When a boron atom is excited, its valency increases from 1 to 3:

and at the carbon atom - from 2 to 4:

At first glance, it may seem that excitation contradicts the principle of least energy. However, as a result of excitation, new, additional bonds arise, due to which energy is released. If this additional energy released is greater than the energy expended on the excitation, the principle of least energy is ultimately satisfied. For example, in a CH 4 methane molecule, the average C–H bond energy is 413 kJ/mol. The energy expended on excitation is E* = 402 kJ/mol. The energy gain due to the formation of two additional bonds will be:

D E \u003d E additional light - E * \u003d 2 413 - 402 \u003d 424 kJ / mol.

If the principle of least energy is not respected, i.e. E adm.< Е*, то возбуждение не происходит. Так, энергетически невыгодным оказывается возбуждение атомов элементов 2-го периода за счет перехода электронов со второго на третий квантовый уровень.

For example, oxygen is only divalent for this reason. However, the electronic analogue of oxygen - sulfur - has large valence capabilities, since there is a 3d sublevel at the third quantum level, and the energy difference between the 3s-, 3p- and 3d-sublevels is incomparably less than between the second and third quantum levels of the oxygen atom:

For the same reason, the elements of the 3rd period - phosphorus and chlorine - exhibit variable valence, in contrast to their electronic counterparts in the 2nd period - nitrogen and fluorine. Excitation to the corresponding sublevel can explain the formation of chemical compounds of elements of group VIIIa of the 3rd and subsequent periods. In helium and neon (1st and 2nd periods), which have a completed external quantum level, no chemical compounds have been found, and only they are truly inert gases.

Donor-acceptor mechanism of covalent bond formation

A pair of electrons with antiparallel spins that form a bond can be obtained not only by an exchange mechanism involving the participation of electrons from both atoms, but also by another mechanism, called a donor-acceptor mechanism: one atom (donor) provides an unshared pair of electrons for bond formation, and the other (acceptor) – a vacant quantum cell:

The result for both mechanisms is the same. Often, bond formation can be explained by both mechanisms. For example, the HF molecule can be obtained not only in the gas phase from atoms by the exchange mechanism, as shown above (see Fig. 3), but also in an aqueous solution from H + and F ions by the donor-acceptor mechanism:

Without a doubt, molecules produced by different mechanisms are indistinguishable; connections are completely equal. Therefore, it is more correct not to single out the donor-acceptor interaction as a special type of bond, but to consider it only as a special mechanism for the formation of a covalent bond.

When they want to emphasize the mechanism of bond formation precisely according to the donor-acceptor mechanism, it is denoted in the structural formulas by an arrow from the donor to the acceptor (D® BUT). In other cases, such a bond is not distinguished and is indicated by a dash, as in the case of the exchange mechanism: D–A.

Bonds in the ammonium ion formed by the reaction: NH 3 + H + \u003d NH 4 +,

are expressed in the following way:

The structural formula NH 4 + can be represented as

.

The second form of notation is preferable, since it reflects the experimentally established equivalence of all four bonds.

The formation of a chemical bond by the donor-acceptor mechanism expands the valence capabilities of atoms: valency is determined not only by the number of unpaired electrons, but also by the number of unshared electron pairs and vacant quantum cells involved in the formation of bonds. So, in the above example, the valency of nitrogen is four.

The donor-acceptor mechanism has been successfully used to describe the bond in complex compounds by the VS method.

Communication multiplicity. s- and p-bonds

The bond between two atoms can be carried out not only by one, but also by several electron pairs. It is the number of these electron pairs that determines the multiplicity in the VS method - one of the properties of a covalent bond. For example, in an ethane molecule C 2 H 6, the bond between carbon atoms is single (single), in an ethylene molecule C 2 H 4 it is double, and in an acetylene molecule C 2 H 2 it is triple. Some characteristics of these molecules are given in Table. one.

Table 1

Changes in bond parameters between C atoms depending on its multiplicity

As the bond multiplicity increases, as expected, its length decreases. The multiplicity of the bond increases discretely, i.e., by an integer number of times, therefore, if all bonds were the same, the energy would also increase by the corresponding number of times. However, as can be seen from Table. 1, the binding energy grows less intensively than the multiplicity. Therefore, the connections are unequal. This can be explained by the difference in the geometric ways in which the orbitals overlap. Let's consider these differences.

The bond formed by the overlapping of electron clouds along an axis passing through the nuclei of atoms is called s-bond.

If an s-orbital is involved in the bond, then only s -connection (Fig. 5, a, b, c). From here it got its name, because the Greek letter s is a synonym for the Latin s.

With the participation of p-orbitals (Fig. 5, b, d, e) and d-orbitals (Fig. 5, c, e, f) in bond formation, the s-type overlap occurs in the direction of the highest density of electron clouds, which is the most energetically favorable. Therefore, when a connection is formed, this method is always implemented first. Therefore, if the bond is single, then it must be s -connection, if multiple, then one of the connections is sure to s-bond.

Rice. 5. Examples of s-bonds

However, it is clear from geometric considerations that there can be only one between two atoms. s -connection. In multiple bonds, the second and third bonds must be formed by a different geometric way of overlapping electron clouds.

The bond formed by the overlapping of electron clouds on either side of an axis passing through the nuclei of atoms is called p-bond. Examples p -connections are shown in fig. 6. Such an overlap is energetically less favorable than according to s -type. It is carried out by peripheral parts of electron clouds with a lower electron density. An increase in the multiplicity of the connection means the formation p bonds that have less energy than s -communication. This is the reason for the non-linear increase in the binding energy in comparison with the increase in the multiplicity.

Rice. 6. Examples of p-bonds

Consider the formation of bonds in the N 2 molecule. As is known, molecular nitrogen is chemically very inert. The reason for this is the formation of a very strong NєN triple bond:

The scheme of overlapping electron clouds is shown in fig. 7. One of the bonds (2px–2px) is formed according to the s-type. The other two (2рz–2рz, 2рy–2рy) are p-type. In order not to clutter up the figure, the image of the overlapping 2py clouds is rendered separately (Fig. 7b). To get a general picture, Fig. 7a and 7b should be combined.

At first glance, it might seem that s -bond, limiting the approach of atoms, does not allow overlapping orbitals in p -type. However, the image of the orbital includes only a certain fraction (90%) of the electron cloud. Overlapping occurs with a peripheral area outside such an image. If we imagine orbitals that include a large fraction of the electron cloud (for example, 95%), then their overlap becomes obvious (see the dashed lines in Fig. 7a).

Rice. 7. Formation of the N 2 molecule

To be continued

V.I. Elfimov,
professor of the Moscow
state open university

In which one mole of a given bond breaks. It is assumed that the initial substance and reaction products are in their standard states of a hypothetical ideal gas at a pressure of 1 atm and a temperature of 25 0 C. Synonyms for the breaking energy of a chemical bond are: bond energy, dissociation energy of diatomic molecules, chemical bond formation energy.

The breaking energy of a chemical bond can be defined in different ways, for example

From mass spectroscopic data (mass spectrometry).

The breaking energy of chemical bonds in various compounds is reflected in the reference book.

The breaking energy of chemical bonds characterizes the strength of a chemical bond.

The energy of breaking the C-C bond.

see also

Notes

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