Total thermal effect. Thermal effect of a chemical reaction. Thermochemical equations. Calculations of the thermal effect of the reaction. Let's find the internal energy under standard conditions
Any chemical reaction proceeds either with the release or absorption of heat.
- exothermic reactions - reactions in which energy is released:
- C + O 2 \u003d CO 2 - coal combustion;
- H 2 SO 4 + 2KOH \u003d K 2 SO 4 + 2H 2 O - reaction of acid with alkali
- Endothermic reactions - reactions during which energy is absorbed (the reactions indicated below proceed only when heated):
- CaCO 3 \u003d CaO + CO 2 - decomposition of calcium carbonate;
- N 2 + O 2 \u003d 2NO - the formation of nitric oxide.
The heat of a reaction is expressed in kilojoules (kJ) and denoted by the letter Q.
- Q>0 - for exothermic reactions;
- Q<0 - для эндотермических реакций.
The essence of the thermal effect of a chemical reaction is directly related to the law of conservation of energy, according to which energy does not arise from nothing and does not disappear without a trace, but only passes from one state to another.
If energy is released during a chemical reaction, then this energy was previously contained in the substance that entered into the reaction, during which this energy was released, which is commonly called the internal energy of matter(denoted U).
And vice versa, if during the reaction the energy was absorbed from the outside, then it accumulated in the product of the endothermic reaction in the form of the internal energy of the newly formed substance.
Thus, if a certain amount of energy was expended during the formation of a substance, then during its decomposition the same amount of energy will be released. From this we can draw a very important conclusion - the more energy is released during the formation of a substance, the same amount of energy must be spent on its decomposition. Therefore, those substances, during the formation of which a lot of energy was released, are difficult to decompose and are very stable (for example, many polymers).
As is known from the course of physics, the energy of a body is made up of potential and kinetic energy. In chemistry, the kinetic energy of a substance is determined by the energy of the movement of its particles (the faster the particles move, the higher the kinetic energy of the substance); potential energy is determined by the forces of attraction and repulsion between particles.
Since any substance consists of a certain number of particles that are in constant motion, therefore, it has a certain amount of internal energy. In the course of chemical reactions, substances are transformed with a change in their internal energy in one direction or another, which is accompanied by the release or absorption of heat.
In the course of a chemical reaction, internal bonds are broken in the molecules of the starting substances, which requires a certain amount of energy (heat absorption), simultaneously with these processes, new bonds are formed in the molecules of the reaction products, which is accompanied by the release of heat. The ratio of absorbed and released heat in the process of a chemical reaction determines the type of the entire reaction as a whole, whether it is exothermic or endothermic.
Schematically, this can be expressed as follows:
(A-A)+(B-B) = 2(A-B) E A + E B = 2E AB
- A-A - chemical bond in the molecule A 2;
- B-B - chemical bond in the B 2 molecule;
- A-B - chemical bond in the formed AB molecule;
- E A is the energy absorbed when the bond is broken in the molecule A 2 ;
- E B is the energy absorbed when the bond is broken in the B 2 molecule;
- E AB is the energy released during the formation of a new bond in the AB molecule;
- If (E A +E B)< 2E AB - реакция экзотермическая;
- If (E A +E B) > 2E AB - the reaction is endothermic;
Let us designate through U 1 the amount of internal energy of the initial state of the system (the sum of the internal energies of the initial components of the reaction), through U 2 we denote the amount of internal energy of the final state of the system (the sum of the internal energies of the reaction products). Let us denote the difference between the internal energies of the final and initial states of the system as ΔU.
According to the law of conservation of energy, if a certain amount of heat Q is supplied to the system, it will be spent on changing the internal energy of the system (ΔU) and on doing work (A):
Q=ΔU+A, where ΔU=U 2 -U 1
Speaking of work (A) in relation to chemical reactions, they mean work against the external pressure of the earth's atmosphere (p). Most chemical reactions proceed under conditions of constant pressure (p=const), therefore, the work can be written as the product of pressure and a change in the volume of the starting substances (V 1) and reaction products (V 2):
A \u003d p (V 2 -V 1) \u003d pΔV
Based on the accepted notation, the heat effect of a chemical reaction that proceeds at constant pressure can be expressed as follows:
Q \u003d (U 2 -U 1) + p (V 2 -V 1)
Expand the parentheses and group:
Q \u003d U 2 -U 1 + pV 2 -pV 1 Q \u003d (U 2 + pV 2) - (U 1 + pV 1)
We denote the sum in brackets by the letter H, then the equality will take the following form:
Q \u003d H 2 -H 1 H 1 \u003d U 1 + pV 1 H 2 \u003d U 2 + pV 2
The sum of the internal energy of a substance and the product of pressure and its volume is called enthalpy(H).
Enthalpy characterizes the heat content of a substance(its internal energy and the energy spent on overcoming the resistance of external pressure).
Based on the derived formula for heat through enthalpy, we can say that the amount of heat released or absorbed during a chemical reaction is equal to the change in the enthalpy of the resulting reaction products compared to the enthalpy of the starting materials.
In relation to enthalpy, the changes that occur during a chemical reaction can be characterized as follows: the enthalpy change (ΔH) of the reaction is equal to the difference between the sums of the enthalpies of the reaction products (ΣH 2) and the sums of the enthalpies of its starting materials (ΣH 1):
ΔH=ΣH 2 -ΣH 1
- ΔH<0 - для экзотермических реакций;
- ΔH>0 - for endothermic reactions;
For the reaction of two substances A and B, discussed above, we can write:
ΔH \u003d H AB - (H A2 + H B2)
- H AB - enthalpy of 1 mol of substance AB;
- H A2 - enthalpy of 1 mol of substance A 2;
- H B2 - enthalpy of 1 mol of substance B 2.
Since the thermal effect (Q) of exothermic reactions is positive, and endothermic reactions are negative, then:
Q=ΔH ΔH=-Q
From the given equalities it is clear that ΔH is also measured in kJ.
- Q>0; ΔH<0 - для экзотермических реакций;
- Q<0; ΔH>0 - for endothermic reactions.
The thermal effect of a chemical reaction (ΔH) depends on the conditions (pressure and temperature) under which the reaction proceeds. Therefore, thermal effects are usually given for normal conditions (n.c.):
- p=1 atmosphere or 101.325 kPa;
- t=25°C or 298 K.
The molecular equation of a chemical reaction, in which the magnitude of its thermal effect is indicated, is called thermochemical equation.
S (t) + O 2 (g) \u003d SO 2 (g) + 297 kJ ΔH ° \u003d -297 kJ
If the value of the thermal effect is indicated with a plus sign, the reaction is exothermic; if "minus" - endothermic.
The thermochemical equations indicate the state of aggregation of the initial substances and reaction products:
- g - gaseous state;
- g - liquid state;
- t - solid state.
IMPORTANT! The thermal effect of the reaction indicated in the thermochemical equation refers to molar number of starting materials and reaction products.
Recall that the stoichiometric coefficients in front of the formulas of substances in the equation show the number of their moles. Since it is customary to calculate Q for 1 mole of the reaction product, fractional coefficients are allowed in thermochemical equations.
1. Thermochemistry. thermal effects. Hess' law.
2. Application of the Hess law to calculate the thermal effects of chemical reactions. Standard heats of formation and combustion.
3. Heat capacity
4. Dependence of the thermal effect of the reaction on temperature.
Thermochemistry is a branch of chemical thermodynamics that studies the thermal effects of chemical reactions.
At the heart of the calculation The thermal effects of reactions underlie the first law of thermodynamics.
Chemical reactions usually proceed at constant pressure (for example, in an open flask) or at constant volume (in an autoclave), i.e. are isobaric or isochoric processes.
At chemical transformations, a part of the energy contained in substances is released. According to the law of conservation and transformation of energy, this part of the internal energy of the system during a chemical reaction goes to doing work and releasing or absorbing heat. The work is usually small. It can be calculated or it can be neglected.
The heat of reaction has a significant value, and in many cases can be directly measured, for which there are calorimetric methods.
The thermal effect of a chemical reaction is the heat released or absorbed as a result of a chemical reaction in an irreversible process at a constant volume or pressure and provided that the reaction products and starting materials have the same temperature and there are no other types of work except expansion.
The importance of thermochemistry in practice is very high, since the heat balance is calculated in many processes in technological practice, medicine and biochemistry.
The basis of thermochemistry is the Hess law, which was experimentally established by the Russian scientist Acad. G.I. Hess in 1836-40. This law is also called the law of constancy of the sum of the heats of reaction:
The thermal effect of a chemical reaction is determined only by the nature and state of the initial substances and products, but does not depend on intermediate chemical reactions, i.e. from the way of transition from the initial state to the final one.
Hess' law is a special case of the law of conservation of energy.
To clarify Hess's law, consider an example of obtaining an aqueous solution of NH 4 Cl from NH 3 (g) and HCl (g) and water. The process can be done in two ways:
1) NH 3 (g) + HCl (g) = NH 4 Cl (g) (41.85 kcal / mol is released)
2). NH 4 Cl (g) + aq \u003d NH 4 Cl aq (absorbed 3.92 kcal / mol) (-3.92)
Result: 37.93 kcal/mol is released
2nd way:
one). NH 3 (g) + aq \u003d NH 3 aq 8.35 kcal / mol is released (+8.35)
2). HCl(g) +aq = HCl aq released 17.32 kcal/mol (+17.32)
3). HCl aq + NH 3 aq \u003d NH 4 Cl aq 12, 27 kcal / mol is released
Result: 37.94 kcal/mol is released
(There are two ways to write the heats of reactions and, accordingly, two systems of signs: thermodynamic and thermochemical. In thermodynamic heat, it is considered positive if it is received by the system. In thermochemical positive, if it is released. Therefore, when exothermic reactions, when the internal energy of the system decreases, the enthalpy decreases, ΔU and ΔΗ has a negative sign.
At endothermic reactions, when energy is absorbed by the system, on the contrary - ΔU and ΔΗ have positive values.
Thermochemical equations in this regard are written as follows:
C 6 H 6 + 7.5 O 2 \u003d 6CO 2 + 3H 2 O + 780.98 kcal
In a thermodynamic system, the reaction equation is written and next to it is the value of the difference between the internal energies (or enthalpies) of the reaction products and the starting materials:
C 6 H 6 + 7.5 O 2 \u003d 6CO 2 + 3H 2 O; Q p \u003d ΔH 0 298 -780, 98 kcal
Chemical equations that indicate the amount of heat released or absorbed are called thermochemical equations.
Hess's law makes it possible to calculate the thermal effects of a reaction in those cases where their direct measurement is not feasible for any reason.
Typically, chemical reactions are carried out either at constant volume or at constant pressure. Wherein:
Q V =ΔU and Q p = ΔU +pdV =ΔH
from these equations it follows that
Q p - Q V = pdV,
those. the difference in thermal effects at constant pressure and constant volume is equal to the work of expansion.
Since pV = nRT, then pΔV = ΔnRT,
where Δn is the change in the number of moles of gaseous reaction participants.
Further substitution gives an equation expressing the relationship between isobaric and isochoric heat effects:
Q p - Q V = ∆nRT,
or ΔH = ΔU + ΔnRT
if Δn =0, then ΔН = ΔU
If the reaction involves solid and liquid substances, then when calculating Δn they not taken into account. During the course of chemical reactions, the change in the number of moles is equal to the difference between the stoichiometric coefficients in the reaction equation.
2. Application of Hess's law to calculate the thermal effects of chemical reactions. Standard heats of formation and combustion.
For the convenience of comparing thermal effects, the concept of the thermal effect of a reaction under standard conditions is introduced.
The thermal effect under standard conditions (ΔH) is such a thermal effect that accompanies the reaction at standard pressure (p 0 \u003d 1.013 10 5 Pa) and at standard temperature (298 K)
The thermal effect under standard conditions is calculated from the standard heats of formation and combustion.
The standard heat of formation is the heat effect of the reaction of formation of 1 mole of a given substance from simple substances (or elements) at a pressure of 1.013 10 5 Pa and provided that all participants in the reaction are in stable states of aggregation.
Standard heats of formation are denoted as follows: ΔН 0 f 298
(formation), determined for about 4 thousand substances and summarized in tables.
For solid and liquid substances, their stable form at an external pressure of 1 atm or 1.013 10 5 Pa is taken as the standard state. For gases, the state of an ideal gas at the same pressure of 1.013 10 5 Pa is taken as standard.
Standard heats of formation of simple substances (elements)
(eg N 2 , O 2 , S rhombus, C gr) are taken equal to 0.
The standard heat of combustion is the heat released during combustion in an oxygen atmosphere of 1 mole of a substance at a standard pressure of 1.013 10 5 Pa to the simplest oxides. In this case, all participants in the reaction must be in stable states of aggregation. Standard heats of formation are denoted as follows: ΔН 0 s298 (combustion).
The standard heats of the higher oxides are naturally taken to be 0.
Using tabular data for ΔН 0 f 298 and ΔН 0 s298, it is possible to calculate the thermal effect of the reaction under standard conditions.
In this case, the consequences of Hess's law are applied:
1. The thermal effect of the reaction under standard conditions is equal to the difference between the sum of the heats of formation of the reaction products and the sum of the heats of formation of the starting substances, multiplied by the corresponding stoichiometric coefficients:
2. The thermal effect of the reaction under standard conditions is equal to the difference between the sum of the heats of combustion of the starting materials and the sum of the heats of combustion of the reaction products, multiplied by the corresponding stoichiometric coefficients:
Example1.
Determine the heat of formation of HI (g) from the reaction:
½ H 2 + 1/2 I 2 \u003d HI (g)
Decision. The thermal effect here is equal to the thermal effect of the formation of HI, since ΔΗ (H 2) and ΔΗ (I 2) are equal to 0. According to the tables, we find the heat of formation of HI, it is equal to 26.04 kJ / mol.
Example 2
Calculate the thermal effect of the reaction
CH 4 (g) + CO 2 (g) \u003d 2CO (g) + 2H 2
according to the standard heats of formation.
Solution From the tables find ΔΗ 0 f for the reaction participants:
ΔΗ 0 f 298 CH4 (g) \u003d -74.85, ΔΗ 0 f 298 CO 2 (g) \u003d -393.51, ΔΗ 0 f 298 CO \u003d -110.5, ΔΗ 0 f H 2 \u003d 0
According to the corollary of Hess's law, we have:
ΔΗ r \u003d 2 ΔΗ 0 f 298 CO + ΔΗ 0 f H 2 - ΔΗ 0 f 298 CH4 (g) - ΔΗ 0 f 298 CO 2 (g) \u003d
2(-110.5,) + 0 - (-74.85 -393.51) = + 247.39 kJ|mol
reaction endothermic
by heat of combustion:
ΔΗ r \u003d ΔΗ 0 f 298 CH4 (g) + ΔΗ 0 f 298 CO 2 (g) - (2 ΔΗ 0 f 298 CO + ΔΗ 0 f H 2) \u003d
ΔΗ 0 s 298 CH4 (g) = -802.32 kJ/mol, ΔΗ 0 s298 CO 2 (g) = 0, ΔΗ 0 s298 CO = -283.0.
ΔΗ 0 сН2 = -241.84.
ΔΗ r \u003d -802.32 +0 - (2 -283.0 -2 -241.84) \u003d + 247.36 kJ / mol
Hess' law is of exceptional practical importance. With it, you can find out the thermal effect of any reaction without making direct measurements for this. This is especially valuable in cases where reactions are not feasible or are distorted by side effects.
For example, (example 3) the heat of formation of glucose cannot be experimentally found, since the reaction proceeding according to the equation:
6C gr + 6H 2 (g) + 3O 2 (g) \u003d C 6 H 12 O 6 (tv) is not feasible.
But using Hess's law, one can combine thermochemical equations from which this effect can be calculated. For example like this:
1. 6C g + 6O 2 (g) \u003d 6CO 2 (g) ΔΗ 1 \u003d 6 (-94.0) \u003d - 564 kcal | mol
2. 6H 2 (g) + 3O 2 (g) \u003d 6H 2 O (l) ΔΗ 2 \u003d 6 (-68.3) \u003d - 410 kcal / mol
3. C 6 H 12 O 6 (tv) + 6O 2 (g) \u003d 6CO 2 (g) + 6H 2 O (l) ΔΗ 3 \u003d -670 kcal / mol
A similar combination of enthalpies gives the enthalpy of formation of glucose:
ΔΗ 1 + ΔΗ 2 - ΔΗ 3 = ΔΗ f C 6 H 12 O 6
ΔΗ f C 6 H 12 O 6 \u003d - 304 kcal | mol
Here, the heat of formation is calculated from the heats of combustion.
Example 4
Determine the heat (enthalpy) of the phase transition:
Na(k) = Na(g)
ΔΗ 0 ex = 108.3 -0 = 108.3 kJ / mol
SO 3 (g) \u003d SO 3 (g)
-439,0 -396.1
ΔΗ 0 test \u003d -396.1 - (-439) \u003d 42.9 kJ / mol
Example 5
Determine the dissociation energy of a diatomic molecule into atoms (chemical bond energy):
Cl 2 (g) \u003d 2Cl (g)
0 2(121,3)
ΔΗ 0 diss = 2(121.3) -0 = 242.6 kJ/mol
Example6
Determine the energy of transformation of an atom into an ion (ionization energy):
H(g) = H + (g) + e
217,98 1536,2
ΔΗ 0 ionization \u003d 1536.2 - 217.98 \u003d 1318, 22 kJ / mol
With the help of thermochemical calculations, it is possible to determine the energy of chemical bonds, the energy of the crystal lattice, the energy of intermolecular interaction, the enthalpy of dissolution (hydration), and the effects of phase transformations.
1. Heat capacity.
3. Significance of the first law of thermodynamics..
Heat capacity is the ratio of the amount of heat imparted to the system to the temperature increase observed in this case (in the absence of a chemical reaction, the transition of a substance from one state of aggregation to another and at A "= 0.
Heat capacity is usually calculated per 1 g of mass, then it is called specific (J / g * K), or per 1 mol (J / mol * K), then it is called molar.
Distinguish average and true heat capacity.
Middle heat capacity is the heat capacity in the temperature range, i.e. the ratio of the heat imparted to the body to the increment in its temperature by ΔТ
True The heat capacity of a body is the ratio of an infinitesimal amount of heat received by the body to the corresponding increase in its temperature.
It is easy to establish a connection between the average and true heat capacity:
substituting the values of Q into the expression for the average heat capacity, we have:
The true heat capacity depends on the nature of the substance, the temperature and the conditions under which the heat transfer to the system occurs.
So, if the system is enclosed in a constant volume, i.e. for isochoric process we have:
If the system expands or contracts while the pressure remains constant, i.e. for isobaric process we have:
But õQ V = dU and õQ P =dH therefore
C V = (õU/õT) V a
C P = (õH/õT) P
(if one or more variables are held constant while others change, then the derivatives are said to be partial with respect to the changing variable).
Both ratios are valid for any substances and any states of aggregation. To show the relationship between С V and С P, it is necessary to differentiate the expression for the enthalpy H \u003d U + pV by temperature / For an ideal gas
for one mole
The difference R is the work of the isobaric expansion of 1 mole of an ideal gas as the temperature rises by one unit.
For liquids and solids, due to a small change in volume when heated, С P = С V
2. Dependence of the thermal effect on temperature. Kirchhoff equation.
Using Hess's law, one can calculate the thermal effect of a reaction at the temperature (usually 298K) at which the standard heats of formation or combustion of all participants in the reaction are measured.
But more often it is necessary to know the thermal effect of a reaction at different temperatures.
Consider the reaction:
ν A A+ν B B= ν C С+ν D D
Let us denote by H the enthalpy of the participant in the reaction per 1 mole. The total change in the enthalpy ΔΗ (T) of the reaction is expressed by the equation:
ΔΗr = (ν C H C + ν D H D) (ν A H A + ν B H B)
If the reaction proceeds at constant pressure, then the change in enthalpy will be equal to the heat effect of the reaction. And if we differentiate this equation with respect to temperature, we get:
As
4. Significance of the first law of thermodynamics..
The first law of thermodynamics is a universal law of nature. It is completely true for living organisms. The flow of processes in a living organism requires the expenditure of energy. It is necessary for muscle activity and, in particular, for the work of the heart and maintaining a constant body temperature. Even at rest, a person weighing 80 kg gives the environment 1200 kcal per day. For normal life, flows of substances from one part of the body to others are necessary. The transport of these substances also requires energy. In the body, electrical work is also performed, which is necessary for the transmission of nerve impulses. Thermochemistry allows you to create a balance of energy in a living organism. (The experiments of Lavoisier, Laplace -1780, on guinea pigs - by measuring the amount of CO 2 and the heat emitted by it, showed that oxidation in the body and direct combustion of nutrients give close thermal effects. Later, W. Atwater, 1904. showed in an experiment with person in the calorimeter, his daily energy balance).
The presence of an energy balance for a living organism shows that the organism is not a source of new energy, but obeys the first law of thermodynamics.
Microcalorimetry has recently been successfully used to study the thermal effects of protein denaturation processes and their interaction with metal ions and hydronium ions in the body. Since the thermal effects of these processes are very small (for example, the heat of DNA denaturation (helix-coil transition) is only 4 kcal per mole), and the concentration of this biopolymer in the test volume is very low, the thermal effect can only be measured with an ultrasensitive microcalorimeter. Such devices - differential scanning microcalorimeters - were designed by our scientists in the Pushchino academic campus, under the guidance of Academician Privalov.
Any chemical processes, as well as a number of physical transformations of substances (evaporation, condensation, melting, polymorphic transformations, etc.) are always accompanied by a change in the internal energy of systems. Thermochemistry - This is a branch of chemistry that studies the change in the amount of heat during the course of a process. One of the founders of thermochemistry is the Russian scientist G. I. Hess.
Thermal effect of a chemical reaction is the heat released or absorbed during a chemical reaction. The standard thermal effect of a chemical reaction is the heat released or absorbed during a chemical reaction under standard conditions. All chemical processes can be divided into two groups: exothermic and endothermic.
exothermic are reactions in which heat is released into the environment. In this case, the stock of internal energy of the initial substances (U 1) is greater than the resulting products (U 2). Therefore, ∆U< 0, а это приводит к образованию термодинамически устойчивых веществ.
Endothermic These are reactions in which heat is absorbed from the environment. In this case, the stock of internal energy of the initial substances (U 1) is less than that of the resulting products (U 2). Consequently, ∆U > 0, and this leads to the formation of thermodynamically unstable substances. In contrast to thermodynamics, in thermochemistry, the released heat is considered positive, and the absorbed heat is considered negative. Heat in thermochemistry is denoted by Q. The unit of heat is J/mol or kJ/mol. Depending on the conditions of the process, there are isochoric and isobaric thermal effects.
Isochoric (Q V) the thermal effect is the amount of heat that is released or absorbed during a given process at a constant volume (V \u003d const) and equal temperatures of the final and initial states (T 1 \u003d T 2).
Isobaric (Q p) the thermal effect is the amount of heat that is released or absorbed during a given process at constant pressure (p \u003d const) and equal temperatures of the final and initial states (T 1 \u003d T 2).
For liquid and solid systems, the change in volume is small and it can be assumed that Q p » Q V . For gaseous systems
Q р = Q V – ∆nRT, (4.3)
where ∆n is the change in the number of moles of gaseous participants in the reaction
∆n = ån cont. reactions – ån ref. substances. (4.4)
In all cases, the transformation of a part of the internal (chemical) energy into thermal (or other types) and vice versa, thermal into chemical occurs in strict accordance with the law of conservation of energy and the first law of thermodynamics.
In thermochemistry, it is customary to use thermochemical equations these are equations of chemical reactions, in which the initial substances are given on the left side of the equality, and the reaction products plus (or minus), the thermal effect are given on the right side, and the aggregate state of substances and their crystalline forms are also shown. For example,
C graphite + O 2 \u003d CO 2 (g) + 393.77 kJ
H 2 + 1 / 2O 2 \u003d H 2 O (l) + 289.95 kJ
C (diamond) + 2S (rhombus) \u003d CS 2 (g) - 87.9 kJ
With thermochemical equations, you can perform all algebraic operations: add, subtract, multiply, transfer terms, etc.
The thermal effects of many chemical and physical processes are determined empirically (calorimetry) or calculated theoretically using the heats of formation (decomposition) and heats of combustion of certain chemical compounds.
The heat of education of a given compound is the amount of heat released or absorbed during the formation of 1 mole of it from simple substances in kJ. The heats of formation of simple substances that are in a stable state under standard conditions are taken as zero. In reactions
K (tv) + 1/2Cl (g) = KS1 (tv) + 442.13 kJ
C (tv) + 1 / 2H 2 (g) + 1 / 2N (g) = HCN (g) - 125.60 kJ
thermal effects 442.13 kJ and -125.60 kJ are the heats of formation of KCl and HCN, respectively. Heats of decomposition of these compounds into simple substances, according to the law of conservation of energy, are equal in absolute value, but opposite in sign, i.e. for KCl, the heat of decomposition is -442.13 kJ, and for HCN it is +125.60 kJ.
The more heat is released during the formation of a compound, the more heat must be expended to decompose it, and the stronger the given compound under normal conditions. Chemically stable and durable substances are: SiO 2, A1 2 O 3, P 2 O 5, KCl, NaCl, etc. Substances formed with heat absorption are not very stable (for example, NO, CS 2, C 2 H 2 , HCN and all explosives). The heats of formation of organic compounds cannot be determined experimentally. They are calculated theoretically from the values of the calorific values of these compounds, found empirically.
Heat of combustion The heat released during the complete combustion of 1 mole of a substance in a stream of oxygen is called. The heats of combustion are determined on a calorimeter installation, the main parts of which are: an oxygen cylinder, a calorimetric bomb, a calorimeter with a weighed amount of water and a stirrer, and an electrical ignition device.
The magnitude of the thermal effects of chemical reactions depend on many factors: the nature of the reacting substances, the state of aggregation of the initial and final substances, the reaction conditions (temperature, pressure, system volume, concentration).
Thermochemistry studies the thermal effects of chemical reactions. In many cases, these reactions proceed at constant volume or constant pressure. It follows from the first law of thermodynamics that, under these conditions, heat is a function of state. At constant volume, heat is equal to the change in internal energy:
and at constant pressure - a change in enthalpy:
These equalities, when applied to chemical reactions, are the essence of Hess' law:
The thermal effect of a chemical reaction proceeding at constant pressure or constant volume does not depend on the reaction path, but is determined only by the state of the reactants and reaction products.
In other words, the thermal effect of a chemical reaction is equal to the change in the state function.
In thermochemistry, unlike other applications of thermodynamics, heat is considered positive if it is released into the environment, i.e. if H < 0 или U
< 0. Под тепловым эффектом химической реакции
понимают значение H(which is simply called the "enthalpy of reaction") or U reactions.
If the reaction proceeds in solution or in the solid phase, where the change in volume is negligible, then
H = U + (pV) U. (3.3)
If ideal gases participate in the reaction, then at a constant temperature
H = U + (pV) = U+n. RT, (3.4)
where n is the change in the number of moles of gases in the reaction.
In order to facilitate comparison of the enthalpies of different reactions, the concept of "standard state" is used. The standard state is the state of a pure substance at a pressure of 1 bar (= 10 5 Pa) and a given temperature. For gases, this is a hypothetical state at a pressure of 1 bar, which has the properties of an infinitely rarefied gas. The enthalpy of reaction between substances in standard states at a temperature T, denote ( r means "reaction"). In thermochemical equations, not only the formulas of substances are indicated, but also their aggregate states or crystalline modifications.
Important consequences follow from Hess's law, which make it possible to calculate the enthalpies of chemical reactions.
Consequence 1.
is equal to the difference between the standard enthalpies of formation of reaction products and reagents (taking into account stoichiometric coefficients):
Standard enthalpy (heat) of formation of a substance (f means "formation") at a given temperature is the enthalpy of the reaction of formation of one mole of this substance from the elements in the most stable standard state. According to this definition, the enthalpy of formation of the most stable simple substances in the standard state is 0 at any temperature. Standard enthalpies of formation of substances at a temperature of 298 K are given in reference books.
The concepts of "enthalpy of formation" are used not only for ordinary substances, but also for ions in solution. In this case, the H + ion is taken as the reference point, for which the standard enthalpy of formation in an aqueous solution is assumed to be equal to zero: 
Consequence 2. Standard enthalpy of a chemical reaction
is equal to the difference between the enthalpies of combustion of the reactants and reaction products (taking into account stoichiometric coefficients):
(c means "combustion"). The standard enthalpy (heat) of combustion of a substance is called the enthalpy of the reaction of complete oxidation of one mole of a substance. This consequence is usually used to calculate the thermal effects of organic reactions.
Consequence 3. The enthalpy of a chemical reaction is equal to the difference between the energies of broken and formed chemical bonds.
By bond energy A-B name the energy required to break the bond and dilute the resulting particles to an infinite distance:
AB (r) A (r) + B (r) .
The bond energy is always positive.
Most of the thermochemical data in handbooks are given at a temperature of 298 K. To calculate the thermal effects at other temperatures, use Kirchhoff equation:
(differential form) (3.7)
(integral form) (3.8)
where Cp is the difference between the isobaric heat capacities of the reaction products and starting materials. If the difference T 2 - T 1 is small, then you can accept Cp= const. With a large temperature difference, it is necessary to use the temperature dependence Cp(T) type:
where coefficients a, b, c etc. for individual substances, they are taken from the reference book, and the sign denotes the difference between products and reagents (taking into account the coefficients).
EXAMPLES
Example 3-1. The standard enthalpies of formation of liquid and gaseous water at 298 K are -285.8 and -241.8 kJ/mol, respectively. Calculate the enthalpy of vaporization of water at this temperature.
Decision. The enthalpies of formation correspond to the following reactions:
H 2 (g) + SO 2 (g) \u003d H 2 O (g), H 1 0 = -285.8;
H 2 (g) + SO 2 (g) \u003d H 2 O (g), H 2 0 = -241.8.
The second reaction can be carried out in two stages: first, burn the hydrogen to form liquid water according to the first reaction, and then evaporate the water:
H 2 O (g) \u003d H 2 O (g), H 0 Spanish = ?
Then, according to Hess's law,
H 1 0 + H 0 Spanish = H 2 0 ,
where H 0 Spanish \u003d -241.8 - (-285.8) \u003d 44.0 kJ / mol.
Answer. 44.0 kJ/mol.
Example 3-2. Calculate the enthalpy of reaction
6C (g) + 6H (g) \u003d C 6 H 6 (g)
a) according to the enthalpies of formation; b) by binding energies, assuming that the double bonds in the C 6 H 6 molecule are fixed.
Decision. a) The enthalpies of formation (in kJ/mol) are found in the handbook (e.g. P.W. Atkins, Physical Chemistry, 5th edition, pp. C9-C15): f H 0 (C 6 H 6 (g)) = 82.93, f H 0 (C (g)) = 716.68, f H 0 (H (g)) = 217.97. The enthalpy of reaction is:
r H 0 \u003d 82.93 - 6 716.68 - 6 217.97 \u003d -5525 kJ / mol.
b) In this reaction, chemical bonds are not broken, but only formed. In the fixed double bond approximation, a C 6 H 6 molecule contains 6 C-H bonds, 3 C-C bonds, and 3 C=C bonds. Bond energies (in kJ/mol) (P.W.Atkins, Physical Chemistry, 5th edition, p. C7): E(C-H) = 412, E(C-C) = 348, E(C=C) = 612. The enthalpy of reaction is:
r H 0 \u003d - (6 412 + 3 348 + 3 612) \u003d -5352 kJ / mol.
The difference with the exact result of -5525 kJ / mol is due to the fact that in the benzene molecule there are no C-C single bonds and C=C double bonds, but there are 6 C C aromatic bonds.
Answer. a) -5525 kJ/mol; b) -5352 kJ/mol.
Example 3-3. Using the reference data, calculate the enthalpy of reaction
3Cu (tv) + 8HNO 3(aq) = 3Cu(NO 3) 2(aq) + 2NO (g) + 4H 2 O (l)
Decision. The abbreviated ionic reaction equation is:
3Cu (tv) + 8H + (aq) + 2NO 3 - (aq) \u003d 3Cu 2+ (aq) + 2NO (g) + 4H 2 O (l).
According to Hess' law, the enthalpy of reaction is:
r H 0 = 4f H 0 (H 2 O (l)) + 2 f H 0 (NO(g)) + 3 f H 0 (Cu 2+ (aq)) - 2 f H 0 (NO 3 - (aq))
(the enthalpies of formation of copper and the H + ion are, by definition, 0). Substituting the enthalpies of formation (P.W. Atkins, Physical Chemistry, 5th edition, pp. C9-C15), we find:
r H 0 = 4 (-285.8) + 2 90.25 + 3 64.77 - 2 (-205.0) = -358.4 kJ
(based on three moles of copper).
Answer. -358.4 kJ.
Example 3-4. Calculate the enthalpy of combustion of methane at 1000 K if the enthalpies of formation at 298 K are given: f H 0 (CH 4) \u003d -17.9 kcal / mol, f H 0 (CO 2) \u003d -94.1 kcal / mol, f H 0 (H 2 O (g)) = -57.8 kcal / mol. The heat capacities of gases (in cal/(mol. K)) in the range from 298 to 1000 K are:
C p (CH 4) = 3.422 + 0.0178. T, Cp(O 2) = 6.095 + 0.0033. T,
C p (CO 2) \u003d 6.396 + 0.0102. T, Cp(H 2 O (g)) = 7.188 + 0.0024. T.
Decision. Enthalpy of combustion reaction of methane
CH 4 (g) + 2O 2 (g) \u003d CO 2 (g) + 2H 2 O (g)
at 298 K is:
94.1 + 2 (-57.8) - (-17.9) = -191.8 kcal/mol.
Let us find the difference in heat capacities as a function of temperature:
Cp = Cp(CO2) + 2 Cp(H 2 O (g)) - Cp(CH 4) - 2 Cp(O2) =
= 5.16 - 0.0094T(cal/(mol. K)).
We calculate the reaction enthalpy at 1000 K using the Kirchhoff equation:
= + 
= -191800 + 5.16
(1000-298) - 0.0094 (1000 2 -298 2) / 2 \u003d -192500 cal / mol.
Answer. -192.5 kcal/mol.
TASKS
3-1. How much heat is required to transfer 500 g of Al (mp. 658 o C, H 0 pl \u003d 92.4 cal / g), taken at room temperature, into a molten state, if Cp(Al TV) \u003d 0.183 + 1.096 10 -4 T cal/(g K)?
3-2. The standard enthalpy of the reaction CaCO 3 (tv) \u003d CaO (tv) + CO 2 (g), proceeding in an open vessel at a temperature of 1000 K, is 169 kJ / mol. What is the heat of this reaction, proceeding at the same temperature, but in a closed vessel?
3-3. Calculate the standard internal energy of formation of liquid benzene at 298 K if the standard enthalpy of its formation is 49.0 kJ/mol.
3-4. Calculate the enthalpy of formation of N 2 O 5 (g) at T= 298 K based on the following data:
2NO (g) + O 2 (g) \u003d 2NO 2 (g), H 1 0 \u003d -114.2 kJ / mol,
4NO 2 (g) + O 2 (g) \u003d 2N 2 O 5 (g), H 2 0 \u003d -110.2 kJ / mol,
N 2 (g) + O 2 (g) \u003d 2NO (g), H 3 0 = 182.6 kJ/mol.
3-5. The enthalpies of combustion of -glucose, -fructose and sucrose at 25 ° C are -2802,
-2810 and -5644 kJ/mol, respectively. Calculate the heat of hydrolysis of sucrose.
3-6. Determine the enthalpy of formation of diborane B 2 H 6 (g) at T= 298 K from the following data:
B 2 H 6 (g) + 3O 2 (g) \u003d B 2 O 3 (tv) + 3H 2 O (g), H 1 0 \u003d -2035.6 kJ / mol,
2B (tv) + 3/2 O 2 (g) \u003d B 2 O 3 (tv), H 2 0 \u003d -1273.5 kJ / mol,
H 2 (g) + 1/2 O 2 (g) \u003d H 2 O (g), H 3 0 \u003d -241.8 kJ / mol.
3-7. Calculate the heat of formation of zinc sulfate from simple substances at T= 298 K based on the following data.
It is known from the theory of chemical bonding that the formation of bonds accompanied by the release of energy, therefore, if reactions proceeded between free atoms, then all reactions would be accompanied by the release of energy. But chemical reactions, as a rule, proceed between the molecules of substances.
Compare the amount of energy released during the formation of a molecule HCl from hydrogen atoms ( H) and chlorine ( Cl):
H + Cl = HCl + 432 kJ/mol
with the amount of energy released during the formation of the HCl molecule from simple substances ( H 2 and Cl2):
1/2H 2 + 1/2Cl 2 \u003d HCl + 92.31 kJ / mol.
The reaction energy from simple substances is less than from free atoms, because. part of the energy is spent on breaking bonds in hydrogen (H-H) and chlorine (Cl-Cl) molecules.
Hence, essence of chemical reactions is reduced to the breaking of bonds in the molecules of the starting substances and the emergence of new bonds in the molecules of the reaction products. Depending on the ratio of the breaking energies and the formation of the corresponding bonds, energy is released or absorbed. Energy is usually released or absorbed in the form of heat.
The reactions that flow with the released heat, are called exothermic . For example:
H 2 + Cl 2 = 2HCl + 184.6 kJ
or H 2 + Cl 2 = 2HCl; DH = -184.6 kJ.
H 2(= 435.9 kJ/mol) and Cl2( = 242.3 kJ / mol) less energy is spent, and when bonds are formed in molecules HCl(E HCl \u003d 431.4 kJ / mol) - more is released, i.e.
2 ´ 431.4 > (435.9 + 242.3).
The reactions that flow with heat absorption, are called endothermic . For example:
N 2 + O 2 \u003d 2NO - 180.8 kJ
or N 2 + O 2 = 2NO; DH = 180.8 kJ.
It follows from the example that for breaking bonds in molecules N 2( = =945.43 kJ/mol) and O2( = 498.38 kJ / mol) more energy is spent, and when bonds are formed in molecules NO- less is allocated, i.е.
2 ´631.5< (945,43 + 498,38).
Thermal effect of the reaction is the amount of heat released or absorbed during a reaction. It is symbolized Q and is expressed in kJ. For exothermic reactions Q > 0 (+Q), for endothermic - Q < 0 (–Q). Currently, for consistency with thermodynamics, the heat effect of a reaction is denoted D.H.(enthalpy change).
Enthalpy (H) is a value that characterizes the amount of energy in a substance. For exothermic reactions the energy reserve in the reaction products is less than in the starting materials, so the change in enthalpy D.H.< 0 (–DH). For endothermic reactions the energy reserve in the reaction products is greater than in the starting materials, so the change in enthalpy DH > 0(+DH). Therefore, the relationship between D.H. and Q is expressed by the equation:
The thermal effect of the reaction depends on temperature and pressure, so we agreed to determine it at pressure ( R) 1 atm or 101.3 kPa and a temperature of 25 ° C or 298 K. These conditions are called standard .
At constant pressure thermal effect of the reaction defined as enthalpy change, and when constant volume- as change in internal energy.
Thermochemical equations
Chemical equations in which the thermal effects of reactions are indicated are called thermochemical .
In thermochemical equations, the state of aggregation of the initial substances and reaction products must be indicated: g - gaseous, g - liquid, to - crystalline or tv - solid. Depending on the designation of the thermal effect ( Q or D.H.) the thermochemical equation of the exothermic reaction of the formation of water (H 2 O (g)) from simple substances H 2 and O 2 is written as follows:
2H 2 (g) + O 2 (g) \u003d 2H 2 O (g) + 571.6 kJ
2H 2 (g) + O 2 (g) \u003d 2H 2 O (g); DH = - 571.66 kJ.
This thermochemical equation shows that when two moles of hydrogen and one mole of oxygen interact, two moles of water are formed and 571.66 kJ of heat are released. To show the thermal effect during the formation of 1 mol of a substance, fractional coefficients are used in thermochemical equations:
H 2 (g) + 1 / 2O 2 (g) \u003d H 2 O (g) + 285.83 kJ
or H 2 (g) + 1/2O 2 (g) \u003d H 2 O (g); DH = -285.83 kJ.
According to the thermochemical equations of reactions, various calculations can be carried out.
Hess' law
The most important law on which most thermochemical calculations are based is Hess's law.
Hess' law: the thermal effect of a chemical reaction depends only on the nature and physical state of the initial substances and final products, but does not depend on the path of transition from the initial state to the final state.
For example, the thermal effect of the reaction of carbon oxidation to carbon monoxide (IV) does not depend on whether this oxidation is carried out directly by burning coal to CO 2:
C (tv) + O 2 (g) \u003d CO 2 (g) (DH 1)
or in two stages, receiving CO at the first stage, and then burning CO to CO 2:
first stage: C (tv) + 1 / 2O 2 (g) \u003d CO (g) (DH 2),
second stage: CO (g) + 1/2O 2 (g) \u003d CO 2 (g) (DH 3).
This can be illustrated visually in the form of a diagram, Fig. 4.
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Figure 4 - Scheme for determining the thermal effect (DH) during the formation of CO 2
According to Hess's law, thermal effects are interconnected by the relation DH 1 = DH 2 + DH 3, using which one of them can be determined if the other two are known. Thus, on the basis of the Hess law, it is possible to calculate the thermal effects of such reactions for which it is impossible to measure experimentally. For example, it is practically impossible to measure the heat of combustion of carbon to carbon monoxide (II), because the reaction product will always consist of a mixture of CO and CO 2 . But experimentally it is possible to measure the heat of complete combustion of carbon to CO 2 (DH 1 = -393 kJ/mol) and the heat of combustion of CO to CO 2 (DH 3 = -283 kJ/mol). With these data, according to the Hess law, it is easy to calculate the heat of combustion of carbon to CO, i.e. D.H.2:
DH 2 \u003d DH 1 - DH 3 \u003d -393 - (-283) \u003d -110 kJ / mol
Thermochemical calculations
used in thermochemical calculations. corollary of Hess' law : the heat effect of a chemical reaction is equal to the sum of the heats of formation of the reaction products minus the sum of the heats of formation of the starting materials, taking into account the stoichiometric coefficients in the reaction equation.
For the reaction: aA + bB = cC + dD
Of particular importance in calculating the thermal effects of reactions are the heats (enthalpies) of formation of compounds. Standard heat (enthalpy) of compound formation- this is the amount of heat that is released or absorbed during the formation of one mole of a chemical compound from simple substances under standard conditions (temperature 298 K, pressure 101.3 kPa). It is measured in kJ / mol and is designated DH 0 298 (sometimes the indices are omitted and denoted DH).
Standard heat (enthalpy) of formationsimple matter is equal to zero.
Example 1. Calculate the Thermal Effect of a Chemical Reaction
2H 2 + CO ® CH 3 OH (l)
at 298 K and determine how much DH and DU differ at this temperature.
Decision
Thermal effect of the reaction equals the difference between the sum of the heats of formation of the final and the sum of the heats of formation of the initial substances. Since the standard heats of formation are related to 1 mole of a substance, they are multiplied by the corresponding stoichiometric coefficients n reaction equations.
2H 2 + CO ® CH 3 OH (l)
KJ/mol 2 ´0 –110.53 –238.57
= -238.57 - (-110.53) = -128.04 kJ.
Thermal effect of reaction at constant volume, or isochoric thermal effect , can be found from the well-known equation relating the isobaric and isochoric thermal effects:
where: Dn- change in the number of moles gaseous substances as a result of the reaction, calculated from its stoichiometric equation.
Dn \u003d - 2 - 1 \u003d - 3 mol.
Example 2. Calculate DH o, DU o, DG o (Gibbs energy), DF o (Helmholtz energy) for a chemical reaction:
2H 2 + CO \u003d CH 3 OH (g).
Determine in which direction the reaction will go at standard pressure and 298 K.
Decision
The Gibbs energy will be calculated according to the equation:
DG 0 298 = DH 0 298 - TDS 0 298,
where DH 0 298 is the thermal effect of the reaction under standard conditions and temperature T = 298 K.
DS 0 298 - entropy change as a result of the reaction under standard conditions and temperature T \u003d 298 K. To calculate DS 0 298, use the equation
where n i is the number of moles of the i-th substance, corresponding to the stoichiometric coefficient in front of this substance in the reaction equation.
The Helmholtz energy will be calculated according to the equation:
DF 0 298 = DG 0 298 - DnRT,
where: Dn is the change in the number of moles of gaseous substances as a result of the reaction.
We start solving the problem by writing out reference data:
2H 2 + CO ® CH 3 OH (g) (Dn =1–2–1= –2)
KJ/mol 0 –110.53 –201.00
2 ´130.52 197.15 239.76
= -201.00 - 0 - (-110.53) = -90.47 kJ.
239.76 – 2 ´130.52 – 197.15 = –218.43 J/K.
DG 0 298 \u003d -90470 - 298´ (-218.43) \u003d -25377.86 J.
DF 0 298 = -25377.86 - (-2) ´298´8.314 = -20422.66 J.
DG 0 298< 0 и DF 0 298 < 0, следовательно реакция протекает в прямом направлении.
CHEMICAL KINETICS
Chemical kinetics studies the mechanism and rate of reactions.
Average rate of a homogeneous chemical reaction(w) is determined by the change in the amount of any of the substances involved in the reaction per unit of time (t) per unit volume (or the change in the concentration of any substance per unit of time):
 .
| (13) |
Factors affecting the rate of a chemical reaction
The dependence of the rate of a chemical reaction on concentration obeys the law of mass action. The law was discovered by Guldberg and Waage (1876). According to this law, instantaneous (true) reaction rate is proportional to the product of the concentration of the reactants raised to powers equal to the stoichiometric coefficients in the equation of the rate-limiting reaction step. Particles interact in a collision, and the number of collisions of molecules is proportional to the product of the concentrations of the reactants.
In the reaction A + B \u003d AB, proceeding in a closed vessel, the rate of interaction of substances in accordance with the law is expressed by the equation:
where k is the proportionality factor, called reaction rate constant, [A] and [B] are the equilibrium concentrations of substances A and B.
Rate constant reactions depends on temperature, the nature of the substance and does not depend on concentration, that is, it is a measure of the reactivity of substances. At a concentration of reactants equal to 1 mol / dm 3, w \u003d k, therefore physical meaning of the reaction rate constant is the rate of a chemical reaction at reagent concentrations of 1 mol/dm 3 .
If gaseous or liquid substances react with solids, then the reaction rate depends on the concentration of substances in the gaseous or liquid state, but does not depend on the concentration of substances in the solid state, for example, for the reaction
H 2 (g) + S (tv) \u003d H 2 S (g) w \u003d k ´ [H 2].
The rate of chemical reactions occurring with the participation of gaseous substances depends on pressure. If the pressure in the system is increased by compression, then the volume of the system will decrease, the concentration of interacting substances will increase, and the reaction rate will increase.
Effect of temperature on the reaction rate. The rate of a chemical reaction depends on temperature. With an increase in temperature by 10 ° C, the rate of most reactions increases by a factor of 2-4 (empirical van't Hoff rule).
The value showing how many times the reaction rate increases with an increase in temperature by 10 ° C is called temperature coefficient reaction rates, denoted by γ (gamma). The value of γ varies from 2 to 4.
Mathematical expression of the Van't Hoff rule:
 ,
| (15) |
where w 2 and w 1 are the reaction rates at temperatures t 2 and t 1, respectively;
∆t \u003d t 2 - t 1.
An increase in the rate of reactions with an increase in temperature is associated with an increase in the speed of particles and the number of collisions between them. However, calculations show that when the reaction system is heated from 273 K to 373 K (from 0 to 100 ° C), the number of collisions will increase by a factor of = 1.2, and the reaction rate at γ = 2 increases by a factor of 2 10 = 1024 times. Therefore, the main reason for the strong influence of temperature on speed lies elsewhere.
Not every collision leads to a chemical interaction. Only particles with a certain energy react. The transformation of some substances into others occurs through the stage of formation of some activated complex. The energy required to transfer molecules to the state of an activated complex is called activation energy(E act). In a collision, only particles with an energy greater than or equal to the activation energy interact. For most reactions, Eact = 0 – 500, kJ/mol. When heated, the number of active particles with E ³ E act increases, the number of effective collisions and the reaction rate increase.
The dependence of the reaction rate constant k on the activation energy E act and temperature T is expressed by the Arrhenius equation (1889):
 ,
| (16) |
where Z is the number of collisions per second per unit volume,
R is the universal gas constant (8.314 J/mol´K),
e is the base of the natural logarithm (e = 2.718),
T - temperature on the Kelvin scale, K,
P is the steric factor.
With decrease in activation energy and with temperature rises rate constant reactions, and therefore speed reaction.
The phenomenon of a change in the rate of a process in the presence of certain substances (catalysts) is called catalysis.
Catalyst- a substance that changes the rate of a reaction, actively participates in it, remaining chemically unchanged after the reaction.
Catalysts or increase speed reactions (they are called activators or positive catalysts), or slow down reactions (they are called inhibitors or negative catalysts).
For example, in the presence of MnO 2 (catalyst), rapid decomposition of hydrogen peroxide is observed: 2H 2 O 2 2H 2 O + O 2 .
If the catalyst is in the same phase as the reactants, catalysis is called homogeneous. If the catalyst and reactants are in different phases, then heterogeneous catalysis.
In the presence of a catalyst, a different activated complex is formed with a different activation energy, which leads to a change in the reaction rate.
The increase in the reaction rate in the presence of a catalyst is associated with lower activation energy of the new process path.
With heterogeneous catalysis, the process is more complicated, because intermediate surface compounds are formed at the active sites (active sites) of the catalyst, so solid catalysts must have a large (developed) surface.
The basic law of chemical kinetics is discovered in 1864-1867. Guldberg and Waage (Norway) law of mass action, Whereby the rate of an elementary reaction is proportional to the product of the concentrations of the reacting substances in powers equal to the stoichiometric coefficients. This dependence of the reaction rate on concentration is due to the fact that the probability of collision of molecules and, consequently, their interaction, is proportional to the product of the concentrations of the reagents.
Let us consider in general terms a one-stage reversible reaction occurring in a homogeneous medium
A (d) +2B (d) Û AB 2(d)
Let us assume that substances A and B are brought into contact in a closed vessel. The rate of interaction of these substances, according to the law of mass action, is expressed by the ratio:
where is the coefficient of proportionality - rate constant direct reaction
[A] and [B] are the equilibrium molar concentrations of A and B.
If the reaction proceeds in a heterogeneous system, then its rate does not depend on the concentration of the solid, since its concentration is constant, therefore the solid is not included in the equation of the law of mass action.
In general, concentration is denoted by the letter With. is the concentration of any reagent (since they are all related by stoichiometric coefficients). For an ideal gas (conditionally, under normal conditions, all gases are equated to ideal ones), the Clapeyron-Mendeleev equation is applicable:
SOLUTIONS
Solutions are homogeneous systems consisting of two or more components and products of their interaction. The dissolution of substances in water is a physicochemical process in which, under the influence of solvent molecules in a solute, bonds between particles are broken and chemical compounds of the solute and solvent are formed (solvates and hydrates, if the solvent is water). Then the hydrated particles are evenly distributed throughout the volume of the solution.
Dissolution can be both endothermic and exothermic, since the destruction of the structure of the solute occurs with the absorption of a certain amount of heat (+ H), and the interaction of the solvent with the particles of the solute is accompanied by the release of heat (- H). Depending on which processes prevail during dissolution, the thermal effect of the process is positive or negative.
The ability of a substance to dissolve in a given solvent is characterized by solubility. Solubility is a number showing how many grams of a solute can be dissolved in 100 g of a solvent at a given temperature. The solubility of a substance depends on the nature of the substance, temperature, pressure.
One of the most important characteristics of solutions is their concentration.
Ways of expressing concentrations:
1. Molar concentration- the number of moles of a solute in 1 liter of solution:
2. Normal concentration- the number of equivalents of a solute in 1 liter of solution
3.Molar concentration - shows how many moles of a solute are contained in 1 kilogram of solvent.
4. Mass fraction - the number of grams of a substance contained in 100 g of a solution.
ELECTROLYTE SOLUTIONS
electrolytes- These are substances whose solutions and melts conduct an electric current.
When dissolved in water or other solvents consisting of polar molecules, electrolytes undergo electrolytic dissociation , i.e., to a greater or lesser extent, they decompose into positively and negatively charged ions - cations and anions. The idea of this process was put forward by the Swedish chemist S. Arrhenius. He also owns the first concept of acids and bases. According to the theory of electrolytic dissociation by S. Arrhenius:
acids- these are substances, during the dissociation of which hydrogen ions H + are formed in an aqueous solution;
grounds- these are substances, during the dissociation of which hydroxide ions OH - are formed in an aqueous solution;
salt- these are substances, during the dissociation of which in an aqueous solution, base cations and acid anions are formed.
The dissociation of two or more basic acids and two or more acidic bases proceeds stepwise. For example:
H 3 RO 4 H + + H 2 RO 4 -
H 2 RO 4 ¯ H + + HPO 4 2-
HRO 4 2 ¯ H + + RO 4 3-
Ba(OH) 2 BaOH + + OH -