The polarity of the HCl molecule. Polar and non-polar molecules. Polar molecules

We will find out today how to determine the polarity of communication and why it is necessary. We will reveal the physical meaning of the value under consideration.

Chemistry and physics

Once all disciplines devoted to the study of the surrounding world were combined with one definition. And astronomers, and alchemists, and biologists were philosophers. But now there is a strict distribution over the sections of science, and large universities know exactly what you need to know mathematicians, and what - linguists. However, in the case of chemistry and physics, there is no clear boundary. Often they mutually penetrate each other, and it happens that they go parallel courses. In particular, the controversial object is the polarity of communication. How to determine if this area of \u200b\u200bknowledge is related to physics or chemistry? According to the formal sign - to the second science: now schoolchildren study this concept as part of chemistry, but without knowledge of physics, they can not do.

Building atom

In order to understand how to determine the polarity of the communication, you must first remember how an atom is arranged. At the end of the nineteenth century it was known that any atom neutral in general, but contains different charges in different circumstances. The reservation has established that in the center of any atom is a heavy and positively charged kernel. The charge of the atomic nucleus is always integer, that is, it is +1, +2 and so on. Around the kernel there is a corresponding amount of lungs negatively charged which strictly corresponds to the chore charge. That is, if the charge of the core is +32, thirty-two electrons should be located around it. They occupy certain positions around the kernel. Each electron, as it were, "smeared" around the kernel on its orbit. Its shape, position and distance to the nucleus are determined by four

Why polarity occurs

In a neutral atom located far from other particles (for example, in deep SpaceOutside the galaxy), all orbitals are symmetrical about the center. Despite the rather complicated form of some of them, the orbital of any two electrons do not intersect in one atom. But if our separately taken atom in Vacuum will meet another other way (for example, enter the gas cloud), then he wants to interact with it: the orbit of valence external electrons will be pulled out towards the neighboring atom, they will merge with it. There will be a general electronic cloud, new chemical compound And, consequently, the polarity of communication. How to determine which atom will take a major part of the general electronic cloud, let's tell me further.

What are chemical connections

Depending on the type of interacting molecules, the difference in charges of their nuclei and the forces of the emerging attraction, there are the following types of chemical relations:

• single-electron;
• metallic;
• covalent;
• ionic;
• van der Waalsova;
• hydrogen;
• two-electron three-center.

In order to wonder how to determine the polarity of communication in the compound, it must be covalent or ionic (such as NaCl salt). In general, these two types of communication differ only at how much the electronic cloud is shifted towards one of the atoms. If a covalent bond is not formed by two identical atoms (for example, O 2), then it is always slightly polarized. In the ion connection, the displacement is stronger. It is believed that ionic communication leads to the formation of ions, since one of the atoms "takes" the electrons of the other.

But in fact, completely polar compounds do not exist: just one ion is very attracting a common electronic cloud to itself. So much that the remaining piece of equilibrium can be neglected. So, we hope it became clear that the polarity of the covalent bond can be determined, and the polarity of ionic communication does not make sense to determine. Although in this case the difference between these two types of communication is the approximation, model, and not a true physical phenomenon.

Definition of polarity of communication

We hope the reader already realized that polarity chemical bond - This is the deviation of the distribution in the space of a common electronic cloud from the equilibrium. A equilibrium distribution exists in an isolated atom.

Methods for measuring polarity

How to determine the polarity of communication? This question is far from unambiguous. To begin, it is necessary to say that the symmetry of the electronic cloud of the polarized atom differs from a similar neutral, the X-ray spectrum will change. Thus, the displacement of the lines in the spectrum will give an idea of \u200b\u200bwhat the polarity of communication. And if it is necessary to understand how to determine the polarity of communication in the molecule more accurately, then it is necessary to know not only the spectrum of emission or absorption. It is required to find out:

• the dimensions of the atoms involved in the connection;
• charges of their nuclei;
• what connections were created at the atom before the emergence of this;
• what is the structure of the whole substance;
• if the structure is crystalline, which defects exist in it and how they affect all substance.

The polarity of communication is indicated as the upper sign of the following form: 0.17+ or 0.3-. It is also worth remembering that the same type of atoms will have unlike the polarity of communication in conjunction with various substances. For example, in BEO oxide at oxygen, the polarity is 0.35-, and in MGO - 0.42-.

Polarity atom

The reader can ask such a question: "How to determine the polarity of a chemical connection if there are so many factors?" The answer is simultaneously simple, and complicated. Quantitative polarity measures are defined as effective atom charges. This value is a difference between the charge located in a certain area of \u200b\u200bthe electron and the corresponding kernel area. In general, this value shows quite well a certain asymmetricity of the electronic cloud, which occurs in the formation of a chemical bond. The complexity is that it is almost impossible to determine which area of \u200b\u200bfinding an electron (especially in complex molecules) is almost impossible. So, as in the case of the separation of chemical ties on ion and covalent, scientists resort to simplifications and models. At the same time, those factors and values \u200b\u200bthat affect the result are slightly discarded.

Physical meaning of polarity connection

What is the physical meaning of the polarity of the communication? Consider one example. The hydrogen atom H is part of both fluoride hydrochloric acid (HF) and hydrochloric acid (HCl). Its polarity in HF is 0.40+, in HCl - 0.18+. This means that the general electronic cloud is much stronger towards Fluoride than in the direction of chlorine. And it means that the electronegability of the fluorine atom is much stronger than the electric negatsis of the chlorine atom.

The polarity of the atom in the molecule

But a thoughtful reader will remember that, in addition to simple compounds, in which there are two atoms, there are more complex. For example, in order to form one sulfuric acid molecule (H 2 SO 4), two hydrogen atoms are required, one sulfur, and four oxygen. Then another question arises: how to determine the greatest polarity of communication in the molecule? First you need to remember that any compound has some structure. I.e sulfuric acid - This is not a pile of all atoms in one big bunch, but some structure. The central atom of sulfur is joined by four oxygen atoms, forming the semblance of a cross. From two opposite sides, oxygen atoms join the gray double connections. From the other remaining sides, oxygen atoms join the sulfur single bonds and "hold" on the other side of hydrogen. Thus, the following links exist in the sulfuric acid molecule:

By defining the polarity of each of these connections on the reference book, you can find the largest. However, it is necessary to remember that if at the end of the long chain of atoms is a strong electronegative element, it can "drag" the electronic clouds of neighboring ties, increasing their polarity. In more complex than the chain, other effects are quite possible.

What is the polarity of the molecule differ from the polarity of communication?

How to determine the polarity of communication, we told. What is the physical meaning of the concept, we revealed. But these words are found in other phrases that belong to this section of chemistry. Surely readers are interested in how chemical bonds and the polarity of molecules interact. We answer: these concepts mutually complement each other and are impossible separately. We will demonstrate on the classical example of water.

In the molecule H 2 O two identical connection h-o. Between them angle of 104.45 degrees. So the structure of the water molecule is something like a two-suck fork with hydrogen at the ends. Oxygen is a more electronegative atom, it densites the electronic clouds of two hydrogen. Thus, with overall e-reference, the tongs are a little more positive, and the base is a bit more negative. Simplification leads to the fact that the water molecule has a pole. This is called the polarity of the molecule. Therefore, water is such a good solvent, this difference in charges allows molecules to be slightly delayed by electronic clouds of other substances, separating the crystals to the molecules, and the molecules to atoms.

To understand why the molecules in the absence of charge exists a polarity, it is necessary to remember: it is important not only chemical formula Substances, but also the structure of the molecule, types and types of links, which arise in it, the difference in the electronegativity of atoms included in it.

Induced or forced polarity

In addition to its own polarity, there is also induced or caused by factors from the outside. If an external electromagnetic field is acting on the molecule, which is more significant inside the forces inside the molecule, it is capable of changing the configuration of electronic clouds. That is, if the oxygen molecule pulls the hydrogen clouds in H 2 O, and the external field is coated with this action, the polarization is enhanced. If the field seems to interfere with oxygen, the polarity of the communication slightly decreases. It should be noted that it is necessary to attach a sufficiently much effort to somehow affect the polarity of molecules, and even more - to affect the polarity of the chemical bond. This effect is achieved only in laboratories and cosmic processes. The usual microwave only enhances the amplitude of oscillations of water and fats. But this does not affect the polarity of communication.

In which case, the direction of polarity makes sense

In connection with the term, which is considered by us, it is impossible not to mention and reverse polarity. If we are talking about molecules, the polarity has a "plus" or "minus" sign. This means that an atom or gives its electronic cloud and thus becomes a little more positive, or, on the contrary, pulls the cloud on itself and acquires a negative charge. And the direction of polarity makes sense only when the charge moves, that is, when the conductor goes the current. As you know, electrons move from their source (negatively charged) to the place of attraction (positively charged). It is worth recalling that there is a theory according to which electrons actually move in the opposite direction: from a positive source to negative. But in general it does not matter, only the fact of their movement is important. So, in some processes, for example, when welding metal parts, it is important where exactly what poles are attached. Consequently, it is important to know how polarity is connected: directly or in the opposite direction. In some devices, even household, it also matters.

In homoother molecules (H 2, F 2, etc.), an electron pair, which forms a bond, equally belongs to each atom, so the centers of positive and negative charges in the molecule coincide. Such molecules are notolar.

However, in heteronuclear molecules, contribution to the relationship of the wave functions of different atomic atoms. Near one of the atoms, excess electronic density appears, therefore, an excess negative charge, and near the other - positive. In this case, they talk about the displacement of the electronic pair from one atom to another, but it is not literally understood that it is not literally, but only as an increase in the likelihood of finding an electronic pair near one of the cores of the molecule.

To determine the direction of such a displacement and semicolone assessment of its magnitude, the concept of electronegability was introduced.

There are several electronegability scales. However, elements are located in a number of electronegability in the same manner, so the differences are insignificant, and electronegativity scales are completely comparable.

Electricity by R. Malcienne has half an ionization energies and an electron affinity (see Section 2.10.3):

Valence electronic pair shifts to a more electronegative atom.

It is more convenient to use the absolute values \u200b\u200bof electronegativity, but relative. The unit has adopted electronenence lithium 3 Li. The relative electronegability of any element is equal to:

The smallest electronegility has heavy alkaline metal (X FR \u003d 0.7). The electronegative element is fluorine (x f \u003d 4.0). Under the periods there is a general tendency to grow electronegability, and according to subgroups - its decrease (Table 3.4).

With the practical use of these tables (as well as the data of other electronegability scales), it should be borne in mind that in molecules consisting of three or more atoms, the value of electronegativity under the influence of neighboring atoms can change significantly. Strictly speaking, the element cannot be attributed to constant electronegativity. It depends on the valence state of the element, the type of compound, etc. Nevertheless, this concept is useful for a qualitative explanation of the properties of chemical bonds and compounds.

Table 3.4.

Electricity of S- and P-elements by Pauling

The polarity of the communication is determined by the displacement of the valence electron pair in ductomic molecules and is quantitatively characterized dipole moment or electric moment dipolemolecules. It is equal to the product of the distance between the cores g. In the molecule and efficient charge 5, corresponding to this distance:

Insofar as g. They are considered a vector directed from a positive to negative charge, the dipole moment is also a vector and has the same direction. The unit of measurement of the dipole moment is Debu D (1D \u003d 3.33 Yu -30 KL M).

The dipole moment of the complex molecule is defined as the vector amount of dipole moments of all connections. Therefore, if the molecule is symmetrical about the line of each connection, the total dipole moment of such a molecule, despite the polar

connections a-B is zero: d \u003d ^ d; \u003d 0. Examples may

living previously considered symmetric molecules, which are formed by hybrid orbital: BEF 2, BF 3, CH 4, SF 6, etc.

Molecules, bonds in which are formed by non-libid orbital or hybrid orbital with the participation of vapor electrons, are asymmetrical relative to the lines of relationships. The dipole moments of such molecules are not equal to zero. Examples of such polar molecules: H 2 S, NH 3, H 2 0, etc. in fig. 3.18 shows a graphical interpretation of summation of polar verses in the symmetric molecule BEF 2 (FL) and an asymmetrical molecule H 2 S (b).

Fig. 3.18.Dipoles Molecules BEF 2 (a) and H 2 S (b)

As already noted, the higher the difference in the electronegateness of atoms forming the connection, the stronger the valence electron pair is shifted, the more the polar communication and, therefore, more effective charge B, which illustrates Table. 3.5.

Table 3.5

Changing the nature of communication in a number of compounds of elements II period with fluorine

IN polar communication Two components can be selected conditionally: ionic, due to electrostatic attraction, and covalent, due to overlapping orbital. As the difference in electronegate OH The valence electron steam is increasingly shifted to the fluorine atom, which is becoming more and more negative effective charge. The contribution to the ion component is increasing, the proportion of the covalent component decreases. Quantitative changes are moving to high-quality: in the UF molecule, the electronic pair practically fully belongs to Fectour, and its effective charge is approaching one, i.e. To the charge of the electron. We can assume that two ions were formed: Li + Cation and Anion F ~ And the connection is due only to their electrostatic attraction (the covalent component can be neglected). Such a connection is called ionic. It can be viewed as extreme case of covalent polar communication.

The electrostatic field does not have preferential directions. therefore ion connection Unlike covalent not peculiar. Ion interacts with any number of opposite charge ions. This caused another distinguishing property of ionic communication - lack of saturation.

For ionic molecules, you can calculate communication energy. If we consider ions as non-deformable balls with charges ± e, That the strength of attraction between them depending on the distance between the ions centers g. It can be expressed by the Coulomb equation:

The energy of attraction is determined by the ratio

When rapprocheted, repulsion force appears due to interaction electronic shells. It is inversely proportional to the distance to the degree p:

where IN - Some permanent. Exponent p A much greater unit and for different ion configurations lies in the range from 5 to 12. Given that the power is a derivative of energy in a distance, from equation (3.6) we obtain:

With change g. change F NP. and F QTT. At some distance g 0 These forces are equalized, which corresponds to a minimum of the resulting interaction energy U Q. After transformation, you can get

This equation is known as the Born equation.

Minimum on curve dependence U \u003d F (R) correspond to the equilibrium distance g 0 and energy U Q. This is the energy of communication between ions. Even p It is unknown, then you can estimate the magnitude of the communication energy, accepting 1 /P equal to zero:

The error does not exceed 20%.

For ions with charges z L. and Z 2 equations (3.7) and (3.8) take the form:

Since in molecules of such a type, the existence of communication approaching the purely ionic, problematic, the latest equations should be considered a very rough approximation.

At the same time, the problems of polarity and ionization of communication can be approached from the opposite position - from the point of view of polarization of ions. It is assumed that the complete transmission of electrons occurs, and the molecule consists of separate ions. Then there is a shift of electronic clouds under the action of an electric field created by ions - polarization ions.

Polarization - the process of the dual, in which combines polarizing action ions with them polarizability. Polarizability is the ability of an electronic cloud of ion, molecules or an atom to deformation under the action of an electrostatic field of another ion. The intensity of this field defines the polarizing action of the ion. From equation (3.10) it follows that the polarizing action of the ion is the greater, the more his charge and less radius. The radii of cations, as a rule, is significantly less than the radii of anions, therefore, it is almost necessary to face the polarization of anions under the action of cations, and not vice versa. The polarizability of ions also depends on their charge and radius. Big and charge ions are easier than polarizes. The polarizing action of the ion comes down to pulling out the electronic cloud of the opposite charge ion. As a result, the ionic communication is reduced, i.e. Communication becomes polar covalent. Thus, the polarization of ions reduces the degree of ionic communication and in its effect is the opposite of the polarization of communication.

Polarization of ions in the molecule, i.e. An increase in the shares of covalent bond increases, increases the strength of its decay to ions. In a series of compounds of this cation with anions of the same type, the degree of dissociation in solutions decreases with an increase in the polarizability of the anions. For example, in a number of lead halides RS1 2 - RVG 2 - PH 2 grows the radius of halide anions, their polarizability is enhanced, the disintegration of ions is weakened, which is expressed in decreasing solubility.

When comparing the properties of salts with the same anion and sufficiently large cations, the polarization of the cations should be taken into account. For example, the radius of the HG 2+ ion is greater than the radius of the ion of Ca 2+, therefore the HG 2+ polarizes stronger than Ca 2+. As a result, CAC1 2 is strong electrolyte. dissociated in the solution aimed, and HgCl 2 is a weak electrolyte, i.e. It practically does not dissociate in solutions.

The polarization of ions in the molecule reduces its strength during decay to atoms or molecules. For example, in a series of CAC1 2 - Savg 2 - CA1 2 increases the radius of halide ions, their polarization is enhanced by the ion of Ca 2+, therefore the temperature of the thermal dissociation on calcium and halogen decreases: San1 2 \u003d Ca + B1 2.

If the ion polarizes easily, then its excitement requires a small energy, which corresponds to the absorption of visible light quanta. This is the cause of painting solutions of such compounds. The amplification of polarizability leads to an increase in color, for example, in a row of NiCl 2 - NIBR 2 - NIL 2 (amplification of the polarizability of anion) or in a row of KS1 - SIS1 2 (enhancing the polarizability of the cation).

The boundary between the covalent polar and ionic relationship is very conditional. For molecules in a gaseous condition, they believe that with the difference of electronegate Ah\u003e 2.5 ion connection. In solvent solutions, as well as in crystalline, the solvent molecules and the adjacent particles in the nodes of the crystal lattice are strongly influenced. Therefore, the ionic nature of communication is manifested with a significantly smaller difference in the debris. It can practically be assumed that the relationship between typical metals and non-metals in solutions and ionic crystals.

Polarity of chemical ties - Characteristics of a chemical bond showing the change in the distribution of electron density in the space around the nuclei in comparison with the distribution of electron density in the neutral atoms forming this link. As a quantitative connection polarity measure, the so-called effective charges on atoms are used. Effective charge is defined as the difference between the charge of electrons located in a certain area of \u200b\u200bspace near the kernel and the charge of the kernel. However, this measure has only a conditional and approximate meaning, since it is impossible to unambiguously allocate a region in the molecule relating exclusively to a separate atom, and with several connections to a specific connection. The presence of an effective charge can be indicated by the symbols of charges at atoms (for example, H + δ is CL -Δ, where δ is some share of elementary charge). Almost all chemical bonds, the exceptions of bonds in the dioxide homoomuclear molecules - to one degree or another polar. Covalent bonds are usually weakly polar. Ion connections are strongly polar. Polarity of the molecule Determined by the difference in the electronegateness of atoms forming a two-centered bond, the geometry of the molecule, as well as the presence of vapor electronic pairs, since part of the electron density in the molecule can be localized not in the direction of connections. The polarity of communication is expressed through its ion component, that is, through the shift of the electronic pair to a more electronegative atom. Polarity of communication can be expressed through it dipole moment μ, equal to the product of an elementary charge on the dipole length μ \u003d E ∙ L. The polarity of the molecule is expressed through its dipole moment, which is equal to the vector sum of all dipole moments of the molecule bonds. The dipole is a system of two equal, but opposite to the sign of charges located at a single distance from each other. The dipole moment is measured in pendant meters (CL ∙ M) or in debaby (D); 1d \u003d 0.333 ∙ 10 -29 CL ∙ m.

12. The donor-acceptor mechanism of COV. Comprehensive compounds.

Donor-acceptor mechanism (Other coordination mechanism) - A method for the formation of a covalent chemical bond between two atoms or a group of atoms, carried out by a vapor pair of electrons of an atom donor and the free orbit of an atom acceptor. If one of the two molecules has an atom with free orbital, and the other atom with a priest of uncertain electrons, then the D-A interaction arises with them.

Comprehensive connection - Complex compounds that have covalent bonds formed by the ladies. Consider an example of SO4. Cu-complexing agent, 4-coordination number. () - the inner sphere, is the most sphere, NH3 ligands.

The coordination number for the complex compound has the same meaning that valence in conventional connections. Takes values \u200b\u200bfrom 1-12 (except 10 and 11).

13. Intermolecular interaction. Hydrogen bond.

Hydrogen communications - type of chemical bond between the electronegative atom and the hydrogen atom H.associated covalently with another electronegative atom (as part of the same molecule or in another molecule). Usually depicted by points or dotted line on the structural schemes. Hydrogen bonds in strength exceeds Vanderwalso interaction, and its energy is 8-40 kJ / mol. However, it is usually an order of magnitude less than a covalent bond. Hydrogen bond is characteristic of hydrogen compounds with the most electronegative elements: fluorine, oxygen, nitrogen, chlorine and sulfur. Hydrogen bond is very common and plays an important role in the association of molecules, in crystallization processes, dissolution, crystalline formation, electrolytic dissociation and other important physicochemical processes. The water molecule forms four hydrogen bonds than the features of the structure of water and ice are explained, as well as many abnormal properties of water: 1) Max. Density at a temperature of +42) water has the highest heat capacity from known liquids. When water heated, a significant part of the energy is spent on the tip of the relationship, hence the increased heat capacity. Intermolecular interaction - The interaction of molecules among themselves, not leading to the discontinuity or the formation of new chemical bonds. Based on them, as is the basis of chemical communications, electrical interactions lie. Distinguish orientation, induction and dispersion interaction. Orientational forces, dipole-dipole attraction. It is carried out between molecules that are permanent dipoles. As a result of the random thermal motion of molecules when they are conversible with each other, the same name, the charged ends of dipoles is mutually repelled, and the oppositely charged attracts. The more polarons of the molecule, the stronger they are attracted and thus more orientation interaction. Energity of the interaction is inversely proportional to the duration between dipoles. Dispersion attraction (London forces). Interaction between instantaneous and induced dipolem. When molecules occlude molecules, the orientation of microdipole ceases to be independent and their appearance and disappearance in different molecules occurs in the tact of each other. The synchronous appearance and disappearance of microdipoles of different molecules is accompanied by their attraction. The energy of such interaction is inversely proportional to the sixth degree of distance between dipoles. Induction attraction. The interaction between the constant dipole and induced (induced). There are polar and non-polar molecules. Under the action of the polar molecule, the non-polar molecule is deformed and it occurs (induced) dipole. The dipole induced is attracted to a constant dipol of the polar molecule and in turn enhances the electric moment of the dipole of the polar molecule. The energy of such interaction is inversely proportional to the sixth degree of distance between dipoles.

14. System. Phase. Component. Parameters. Status functions: internal energy and enthalpy. Standard conditions.

System - this is a body or group of bodies in the interaction that mentally highlighted from ambient. There are homogeneous (homogeneous) and heterogeneous (inhomogeneous). Isolated The system does not have metabolism and energies with the environment. Closed - It does not have only mass transfer (irreversible transfer of the mass of the mixture component within one or several phases). Open - It has energy and mass exchange. Phase - A combination of all homogeneous parts of the system, the same in composition and to all the physical. and chemical. Properties that do not depend on the amount of substance. The phases are separated from each other by the surfaces of the section, on which all properties of the phase dramatically change the jump. Components - components of the system, chemically individual substances that make up this system and capable of independent existence, being isolated from other parts of the system. The system transmission is determined by a set of variable variables - parameters. Intensive and extensive parameters are distinguished. Intense - do not depend on the mass or number of particles in-BA. (P, T), and extensive - depend (V, E). The state function is thermodynamic functions, the values \u200b\u200bof which depend only on the state of the system and do not depend on the path to which the system came to this state. Changing the status function The most important functions are internal energy U I. systems enthalpyH (heat generation) Internal energy - Total energy supply: Energy of translational and rotational motion, energy of oscillations, internalized energy, with the exception of the kinetic energy of the system as a whole and the potential energy of the system position. Enthalpy - This property of a substance indicating the amount of energy that can be converted into heat. Enthalpy - This is a thermodynamic property of a substance that indicates the level of energy stored in its molecular structure. This means that, although the substance can have energy on the basis of temperature and pressure, not all of it can be converted to heat. Part of the internal energy always remains in the substance and supports its molecular structure. standard pressure for gases, liquids, and solid bodies, equal to 10 5 Pa (750mm Hg. Art.); standard temperature for gases, equal to 273.15 K (0 ° C); standard molarity For solutions equal to 1 mol l -1. In these conditions of the constant dissociarationsDistillated solvenifies 1.0 × 10 -14.

15. The first top of thermodynamics. The GESS law as a result of the first sense of thermodynamics. Thermochemical calculations.

There are many formulations of the first law: In isolated System The total energy supply is maintained constant. Since the work is one of the forms of energy transition, it is, therefore, it is impossible to create an eternal engine of the first kind (machine making work without energy costs). Mathematical formulation: when flowing isobaric Process: when flowing isochoric Process: when flowing isothermal Process: when flowing circular Process:

Thermochemistry - FIZ area. Chemistry engaged in the study of energy. Reaction effects. If its energy effect is specified in the equation - this is thermochemical UR-E.V \u003d const, p \u003d const, the main law of thermochemistryThis law is a direct consequence of the first law of thermodynamics. With the help of the GESS law, it is possible to calculate the heat of various reactions, without conducting the reactions themselves.

For example:

Conclusion: The heat of evaporation of one praying of water is 44 J.

16. Standard enthalpy education. Corollary from the law of hess.

Under standard heat (enthalpy) education understand the thermal effect of the reaction of the formation of one praying substance from simple substancesHis components in sustainable standard states. Standard Education Enthalpy is denoted by ΔH F o.

Russian scientist Hess (1840) gave the wording the main law of thermochemistry: The thermal effect of the reaction flowing at a constant volume or at constant pressure does not depend on the path of the reaction (from its intermediate stages), and is determined only by the nature and state of the source substances and the reaction products. Consequences from the GESSA law:

1. The thermal effect of the reaction is equal to the difference between the heat of the heat of the combustion of the starting materials and the sum of the heat of combustion of the reaction products. The heat of combustion is the thermal effect of the oxidation reaction of this compound with oxygen with the formation of higher oxides. The heat of formation is the thermal effect of the reaction of the formation of this compound from the simple substances corresponding to the most stable state of the elements during data temperature and pressure.

2. The thermal effect of the reaction is equal to the difference between the heat of the formation of all substances specified in the right part of the equation and the heat of the formation of substances in the left part of the equation taken with the coefficients before the formulas of these substances in the equation of the reaction itself. Currently, the warmth of the formation of over 6,000 substances is known. The standard heat of formation is the magnitude of the heat of education to the temperature of 298K and pressure 1ATM.

17. The dependence of the thermal effect of a chemical response from temperature (Kirchhofa law).Differentiate the equations of the IPO T, and in the first case we take a constant V, and in the second - R.

The temperature coefficient of the thermal effect of the process is equal to the change in the heat capacity of the system occurring as a result of the process (Kirchoff rule). Integrating the diffain equations obtained above, we get:

In a small temperature range, you can restrict ourselves to the first member. power Row For C, and then it will be constant.

18. The second beginning of the thermodynamics. The concept of entropy. Thermodynamic probability. Presented heat. The inequality and the identity of Clausius.

It is not possible to spontaneously transition to heat from the less heated body to the more heated. It is impossible to create an eternal engine of the 2nd kind (the machine that periodically turns the heat of the medium with the post. The temperature in the work. Thermodynamic efficiency: for isolated systems a criterion that allows you to judge the direction of processes and about equilibrium conditionsis a function S.-entropy. The processes flow towards the increase in entropy. In equilibrium, entropy reaches a maximum. Inverse processes can not be spontaneous - the cost of work is required. Phys. The meaning of the function of the state of entropy is easiest to illustrate on the example of fluid boiling. When heated: T and U are increasing until the fluid boils. In this case, the heat of evaporation is absorbed, spent on an increase in disorder in the system. Thus, the entropy is a measure of the state of the system. -2E The beginning of thermodynamics For reversible processes. IN isolated System processes spontaneous, proceed in the direction of increasing entropy uninsulated-possibly Thermodynamic probability (or static weight) - The number of methods that the state of the physical system can be implemented. InequalityClausius (1854): The amount of heat obtained by the system with any circular process, divided into the absolute temperature at which it was obtained ( greamed quantity of heat), inability.

19.Plight Nernsto Theorem. Planck postulate. Calculation of the absolute value of entropy. The concept of degeneration of the ideal gas. Nernsta Theorem argues that the change in entropy in reversible chemical. R-found between you in Condensier. condition, tends to zero at T0: Based on this, the plank in 1911 was postulated: "At absolute zero, the temperature of entropy is not only the smallest value, but simply equal to zero." Planck postulate Formulated as follows: "Entropy of a correctly formed crystal clean substance With absolute zero temperature is zero. Absolute valueentropyallows you to set the third beginning of the thermodynamics, or Nernsto theorem: when the absolute temperature is struck to zero, the DS difference for any substance tends to zero, regardless of external parameters. Therefore: the entropy of all substances at an absolute zero of temperature can be taken equal to zero (this wording of Nernsto theorem suggested in 1911 M. Planck). Based on it, for the initial point of the entropy reference, S O \u003d 0 is taken at T \u003d 0. Degenerate gas - Gas whose properties differ significantly from the properties of the classical ideal gas due to the quantum-mechanical effect of the same particles on each other. This mutual influence of particles is due to no power interactions that are absent in perfect gas, but identity (indistinguishability) of the same particles in quantum mechanics. As a result of such an influence, the filling of possible energy levels, even in perfect gas depends on the presence of other particles at this level. Therefore, the heat capacity and pressure of this gas are otherwise dependent on the temperature than the perfect classical gas; Entropy is different, free energy, etc. The degeneracy of the gas occurs when it decreases its temperature to a certain value, called the degeneration temperature. Full degeneration corresponds to the absolute zero of temperature. The effect of the identity of particles affects the more significant than the average distance between the particles r. Compared with the wavelength of de Broglychastitz λ \u003d h / MV. (m. - mass of particles, v. - its speed, h. - Planck constant)

20. The first formula of the first and second start of thermodynamics. Free energy of Gibbs and Helmholtz. Conditions of spontaneous flow of chemical reactions. First law. The heat supplied to the system is consumed on the increment of the internal energy of the system and to work the system over the environment. Second law.(Several formulations): In isolated systems, processes that are accompanied by increasing entropy are accompanied by an increase in entropy: entropy is a thermodynamic function, characterizing the measure of non-order of system status. It is used to judge the direction of spontaneous processes. Generalized law. For each isolated thermodynamic system, there is a state of thermodynamic equilibrium, which it is spontaneously achieved with fixed external conditions over time. TDS\u003e DU + PD E nargen Helmholts.The maximum work that the system can perform with the equilibrium process is equal to the change in the energy of the Helmholtz reaction the energy of the Helmholtz is equal to related energy. It characterizes the limit of spontaneous reaction flow, which is possible when Gibbs energy.Enthalpy and entropy factors characterizing processes are combined with the function - Gibbs energy. The propelled energy of Gibbs can be turned into operation, then it is called free energy. Chemical reaction is possible if Gibbs energy decreases (<0).Энергия Гиббса образования вещества – изменение энергии Гиббса системы при образовании 1 моль вещества из простых веществ, устойчивых при 298 К.

In the formation of a covalent bond between the varieve atoms, the binder pair of electrons shifts towards a more electronegative atom. This leads to the polarization of molecules, therefore all ductomic molecules consisting of non-incomplete elements are in one way or another polar. In more complex molecules, polarity depends on the geometry of the molecule. For the appearance of polarity, it is necessary that the distribution centers of positive and negative charges do not coincide.

In the C0 2 molecule, the carbon is oxygen polar, and there is some positive charge on the carbon atom, and on each of the oxygen atoms is the same negative charge. Consequently, the center of a positive charge is concentrated on the carbon atom. Since oxygen atoms are located on one direct but both sides of the carbon atom (linear molecule) at equal distances, then the positive charge is neutralized. Thus, despite the polarity of each connection in accordance with., The whole molecule is generally notolar and the reason for this is

Fig. 434. Examples of structure and polarity is molecular to its linear structure. On the contrary, the molecule s \u003d c \u003d 0 is polar, since the bondage of carbon - sulfur and carbon - oxygen have different lengths and various polarity. In fig. 4.34 shows the structures and polarity of some molecules.

From the above examples, it follows that if atoms or groups of atoms attached to the central atom are the same or arranged symmetrically relative to it (linear, flat triangular, tetrahedral and other structures), then the molecule will be non-polar. If unequal groups are attached to the central atom or there is an asymmetrical arrangement of groups, then the molecules are polar.

Effective charge of atoms in the molecule has important when considering polar bonds. For example, in the HC1 molecule, the binder electron cloud is shifted towards a more electronegative chlorine atom, as a result of which the hydrogen nucleus charge is not compensated, and on the chlorine atom, the electron density becomes excessive compared to the charge of its kernel. Therefore, the hydrogen atom is polarized positively, and the chlorine atom is negative. A positive charge arises on the hydrogen atom, and on the chlorine atom is negative. This charge is 8, called an effective charge, is usually installed experimentally. So, for hydrogen 8 H \u003d +0.18, and for chlorine 5 C, \u003d -0.18 of the absolute charge of an electron, as a result of this, the bond in the Molecule of the National Assembly has 18% ionic character (ie, the degree of ionicity is 0.18 ).

Since the polarity of the communication depends on the degree of displacement of the binder of the electron pair towards the more electronegative element, then it is necessary to consider the following:

• a) electronegativity (EO) is not a strict physical quantity that can be determined directly experimentally;
• b) the value of electronegability is not constantly, and depends on the nature of the other atom, with which this atom is associated;
• c) the same atom in this chemical bond can sometimes function both both the electropositive, and as electronegative.

Experimental data suggests that elements can be attributed to the relative values \u200b\u200bof electric negativeness (OEO), the use of which allows you to judge the degree of polarity of the connection between atoms in the molecule (see also paragraphs 3.6 and 4.3).

In a molecule consisting of two atoms, the polarity of the covalent bond is the greater, the higher the OEO of one of them, therefore, with an increase in the OEO of the second element, the degree of ionicity of the compound grows.

To characterize the reaction capacity of molecules, not only the nature of the distribution of electron density is important, but also the possibility of its change under the influence of external influence. The measure of this change is the polarizability of communication, i.e. Its ability to become polar or even more polar. Polarization of communication occurs both under the influence of an external electric field and under the influence of another molecule that is a reaction partner. The result of these effects may be polarization of communication, accompanied by its full gap. In this case, the binder pair of electrons remains in a more electronegative atom, which leads to the formation of multi-dimensional ions. This type of breaking of communication is called a tetherolytic. For example:

In the above example of an asymmetric breaking of the bond, hydrogen is cleaved in the form of H + yion, and the binder pair of electrons remains in chlorine, so the latter turns into an anion C1.

In addition to this type of breaking of communication, a symmetrical gap is possible, when not ions are formed, but atoms and radicals. This type of breaking of communication is called homolitical.

Fig. 32. Schemes of polar and non-polar molecules: a - polar molecule; b-non-polar molecule

In any molecule, there are both positively charged particles - the nuclei of atoms and negatively charged - electrons. For each kind of particles (or, or rather, charges) you can find such a point that will be like their "electric center of gravity". These points are called the poles of the molecule. If in the molecule, the electrical centers of gravity of positive and negative charges will coincide, the molecule will be non-polar. Such, for example, H 2, N 2 molecules, formed by the same atoms, in which the general pairs of electrons are equally belonging to both atoms, as well as many symmetrically constructed molecules with atomic bond, for example, methane CH 4, four-chloride SSL 4.

But if the molecule is built asymmetrically, for example, consists of two heterogeneous atoms, as we have already spoken, the overall pair of electrons can be more or less shifted towardone of the atoms. Obviously, in this case, due to the uneven distribution of positive and negative charges inside the molecule, their electrical centers of gravity will not coincide and the polar molecule will be obtained (Fig. 32).

Polar molecules

Polar molecules are dipoles. This term is denoted by any electronic system, that is, a system consisting of positive and negative charges distributed in such a way that their electric centers of gravity do not coincide.

The distance between the electric centers of gravity of those and other charges (between the poles of the dipole) is called a dipole long. The length of the dipole characterizes the degree of polarity of the molecule. It is clear that for various polar molecules, the length of the dipole is different; What it is more, the sharply expressed the polarity of the molecule.

Fig. 33. The schemes of the structure of CO2 and CS2 molecules

In practice, the degree of polarity of those or other molecules is set by measuring the so-called dipole moment of a molecule T, which is defined as a product of the dipole length l. on the charge of his pole e:

t \u003d.l. E.

The magnitudes of the dipole moments are associated with some of its properties and can be determined experimentally. The order of magnitude t. always 10 -18, as the charge

the throne is 4.80 10 -10 electrostatic units, and the length of the dipole is the value of the same order as the diameter of the molecule, i.e. 10 -8 cm.Below are the dipole moments of molecules of some inorganic substances.

Dipole moments of some substances

t. 10 18

. . . .. …….. 0

Water……. 1,85.

. . . ………..0

Hydrogen chloride ....... 1.04.

Carbon dioxide ......00

Bromistic. ...... 0.79

Seroublerod ........... 0

Iodide hydrogen ...... .. 0.38

Hydrogen sulfide ...........1,1

Carbon monoxide ....... 0,11

Sulphur dioxide. . . ...... 1.6

Sinyl Acid ...... ..2.1

The determination of the magnitude of the dipole moments allows you to make many interesting conclusions regarding the structure of various molecules. Consider some of these conclusions.

Fig. 34. The scheme of the water molecule

As it should be expected, the dipole moments of hydrogen molecules and nitrogen are zero; molecules of these substances perfectlysymmetrical and, therefore, electrical charges in them are distributed evenly. The absence of polarity in carbon dioxide and the survo carbon shows that their molecules are also built symmetrically. The structure of the molecules of these substances is schematically shown in Fig. 33.

A somewhat unexpected is the presence of a rather large dipole moment in the water. Since the formula of water is similar to carbon dioxide formulas

and the servo, should expect that its molecules will also be builtsymmetrically, as well as CS 2 and CO 2 molecules.

However, in view of the experimentally installed polarity of water molecules (molecules), this assumption has to be discarded. Currently, the molecule of water is attributed to the asymmetrical structure (Fig. 34): two hydrogen atoms are connected to an oxygen atom so that their bonds form an angle of about 105 °. A similar arrangement of atomic nuclei is also available in other molecules of the same type (H 2 S, SO 2) with dipole moments.

The polarity of water molecules is explained by many of its physical properties.