Thermochemistry studies the thermal effects of chemical reactions. In many cases, these reactions occur at constant volume or constant pressure. From the first law of thermodynamics it follows 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 - the change in enthalpy:
These equalities, when applied to chemical reactions, constitute the essence Hess's law:
The thermal effect of a chemical reaction occurring 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 occurs 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 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 enthalpies of different reactions, the concept of a “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, having the properties of an infinitely rarefied gas. Enthalpy of reaction between substances in standard states at temperature T, denote ( r means "reaction"). Thermochemical equations indicate not only the formulas of substances, 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.
Corollary 1.
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 elements, which are 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 concept of “enthalpy of formation” is 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 zero: 
Corollary 2. Standard enthalpy of a chemical reaction
equal to the difference in 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 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.
Corollary 3. The enthalpy of a chemical reaction is equal to the energy difference between the chemical bonds being broken and those formed.
Energy of communication A-B name the energy required to break a bond and separate the resulting particles over an infinite distance:
AB (g) A (g) + B (g) .
Communication energy is always positive.
Most thermochemical data in reference books are given at a temperature of 298 K. To calculate thermal effects at other temperatures, use Kirchhoff equation:
(differential form) (3.7)
(integral form) (3.8)
Where C p- the difference between the isobaric heat capacities of the reaction products and the starting substances. If the difference T 2 - T 1 is small, then you can accept C p= const. If there is a large temperature difference, it is necessary to use the temperature dependence C p(T) type:
where are the coefficients a, b, c etc. for individual substances they are taken from the reference book, and the sign indicates the difference between the 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.
Solution. The enthalpies of formation correspond to the following reactions:
H 2 (g) + SO 2 (g) = H 2 O (l), H 1 0 = -285.8;
H 2 (g) + SO 2 (g) = H 2 O (g), H 2 0 = -241.8.
The second reaction can be carried out in two stages: first, burn hydrogen to form liquid water according to the first reaction, and then evaporate the water:
H 2 O (l) = H 2 O (g), H 0 isp = ?
Then, according to Hess's law,
H 1 0 + H 0 isp = H 2 0 ,
where H 0 isp = -241.8 - (-285.8) = 44.0 kJ/mol.
Answer. 44.0 kJ/mol.
Example 3-2. Calculate enthalpy of reaction
6C (g) + 6H (g) = C 6 H 6 (g)
a) by enthalpies of formation; b) by binding energies, under the assumption that the double bonds in the C 6 H 6 molecule are fixed.
Solution. a) Enthalpies of formation (in kJ/mol) are found in the reference book (for example, 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 the reaction is:
rH 0 = 82.93 - 6,716.68 - 6,217.97 = -5525 kJ/mol.
b) In this reaction, chemical bonds are not broken, but only formed. In the approximation of fixed double bonds, the 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 the reaction is:
rH 0 = -(6,412 + 3,348 + 3,612) = -5352 kJ/mol.
The difference with the exact result -5525 kJ/mol is due to the fact that in the benzene molecule there are no single C-C bonds and double C=C bonds, but there are 6 aromatic C C bonds.
Answer. a) -5525 kJ/mol; b) -5352 kJ/mol.
Example 3-3. Using reference data, calculate the enthalpy of the reaction
3Cu (tv) + 8HNO 3(aq) = 3Cu(NO 3) 2(aq) + 2NO (g) + 4H 2 O (l)
Solution. The abbreviated ionic equation for the reaction is:
3Cu (s) + 8H + (aq) + 2NO 3 - (aq) = 3Cu 2+ (aq) + 2NO (g) + 4H 2 O (l).
According to Hess's law, the enthalpy of the reaction is equal to:
rH 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 equal, by definition, 0). Substituting the values of enthalpies of formation (P.W.Atkins, Physical Chemistry, 5th edition, pp. C9-C15), we find:
rH 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 enthalpy of formation at 298 K is given: f H 0 (CH 4) = -17.9 kcal/mol, f H 0 (CO 2) = -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 equal to:
C p (CH 4) = 3.422 + 0.0178. T, C p(O2) = 6.095 + 0.0033. T,
C p (CO 2) = 6.396 + 0.0102. T, C p(H 2 O (g)) = 7.188 + 0.0024. T.
Solution. Enthalpy of methane combustion reaction
CH 4 (g) + 2O 2 (g) = CO 2 (g) + 2H 2 O (g)
at 298 K is equal to:
94.1 + 2 (-57.8) - (-17.9) = -191.8 kcal/mol.
Let us find the difference in heat capacities as a function of temperature:
C p = C p(CO2) + 2 C p(H 2 O (g)) - C p(CH 4) - 2 C p(O2) =
= 5.16 - 0.0094T(cal/(mol K)).
The enthalpy of the reaction at 1000 K is calculated using the Kirchhoff equation:
= + 
= -191800 + 5.16
(1000-298) - 0.0094 (1000 2 -298 2)/2 = -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 = 92.4 cal/g), taken at room temperature, into a molten state, if C p(Al TV) = 0.183 + 1.096 10 -4 T cal/(g K)?
3-2. The standard enthalpy of the reaction CaCO 3 (s) = CaO (s) + CO 2 (g) occurring in an open vessel at a temperature of 1000 K is 169 kJ/mol. What is the heat of this reaction, occurring 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) = 2NO 2 (g), H 1 0 = -114.2 kJ/mol,
4NO 2 (g) + O 2 (g) = 2N 2 O 5 (g), H 2 0 = -110.2 kJ/mol,
N 2 (g) + O 2 (g) = 2NO (g), H 3 0 = 182.6 kJ/mol.
3-5. The enthalpies of combustion of -glucose, -fructose and sucrose at 25 o C are equal to -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) = B 2 O 3 (tv) + 3H 2 O (g), H 1 0 = -2035.6 kJ/mol,
2B(tv) + 3/2 O 2 (g) = B 2 O 3 (tv), H 2 0 = -1273.5 kJ/mol,
H 2 (g) + 1/2 O 2 (g) = H 2 O (g), H 3 0 = -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.
The thermal effect of a chemical reaction or the change in enthalpy of a system due to the occurrence of a chemical reaction is the amount of heat attributed to the change in a chemical variable received by the system in which the chemical reaction took place and the reaction products took on the temperature of the reactants.
For the thermal effect to be a quantity that depends only on the nature of the ongoing chemical reaction, the following conditions must be met:
· The reaction must proceed either at constant volume Q v (isochoric process) or at constant pressure Q p (isobaric process).
· No work is performed in the system, except for the expansion work possible at P = const.
If the reaction is carried out under standard conditions at T = 298.15 K = 25 ˚C and P = 1 atm = 101325 Pa, the thermal effect is called the standard thermal effect of the reaction or the standard enthalpy of the reaction ΔH r O. In thermochemistry, the standard heat of reaction is calculated using standard enthalpies of formation.
Standard enthalpy of formation (standard heat of formation)
The standard heat of formation is understood as the thermal effect of the reaction of the formation of one mole of a substance from simple substances and its components that are in stable standard states.
For example, the standard enthalpy of formation of 1 mole of methane from carbon and hydrogen is equal to the thermal effect of the reaction:
C(tv) + 2H 2 (g) = CH 4 (g) + 76 kJ/mol.
The standard enthalpy of formation is denoted ΔHfO. Here the index f means formation, and the crossed out circle, reminiscent of a Plimsol disk, means that the value refers to the standard state of matter. In the literature, another designation for standard enthalpy is often found - ΔH 298.15 0, where 0 indicates equality of pressure to one atmosphere (or, somewhat more precisely, standard conditions), and 298.15 - temperature. Sometimes the index 0 is used for quantities related to a pure substance, stipulating that it can be used to designate standard thermodynamic quantities only when a pure substance is chosen as the standard state. For example, the state of a substance in an extremely dilute solution can also be accepted as standard. “Plimsoll disk” in this case means the actual standard state of matter, regardless of its choice.
The enthalpy of formation of simple substances is taken equal to zero, and the zero value of the enthalpy of formation refers to the state of aggregation, stable at T = 298 K. For example, for iodine in the crystalline state ΔH I2(s) 0 = 0 kJ/mol, and for liquid iodine ΔH I2 (g) 0 = 22 kJ/mol. The enthalpies of formation of simple substances under standard conditions are their main energy characteristics.
The thermal effect of any reaction is found as the difference between the sum of the heats of formation of all products and the sum of the heats of formation of all reactants in a given reaction (a consequence of Hess’s law):
ΔH reaction O = ΣΔH f O (products) - ΣΔH f O (reagents)
Thermochemical effects can be incorporated into chemical reactions. Chemical equations that indicate the amount of heat released or absorbed are called thermochemical equations. Reactions accompanied by the release of heat into the environment have a negative thermal effect and are called exothermic. Reactions accompanied by the absorption of heat have a positive thermal effect and are called endothermic. The thermal effect usually refers to one mole of reacted starting material whose stoichiometric coefficient is maximum.
Temperature dependence of the thermal effect (enthalpy) of the reaction
To calculate the temperature dependence of the enthalpy of a reaction, it is necessary to know the molar heat capacities of the substances participating in the reaction. The change in the enthalpy of the reaction with increasing temperature from T 1 to T 2 is calculated according to Kirchhoff’s law (it is assumed that in this temperature range the molar heat capacities do not depend on temperature and there are no phase transformations):
If phase transformations occur in a given temperature range, then in the calculation it is necessary to take into account the heats of the corresponding transformations, as well as the change in the temperature dependence of the heat capacity of substances that have undergone such transformations:
where ΔC p (T 1 ,T f) is the change in heat capacity in the temperature range from T 1 to the phase transition temperature; ΔC p (T f ,T 2) is the change in heat capacity in the temperature range from the phase transition temperature to the final temperature, and T f is the phase transition temperature.
The standard enthalpy of combustion is ΔH horo, the thermal effect of the combustion reaction of one mole of a substance in oxygen to the formation of oxides in the highest oxidation state. The heat of combustion of non-combustible substances is assumed to be zero.
The standard enthalpy of solution is ΔH solution, the thermal effect of the process of dissolving 1 mole of a substance in an infinitely large amount of solvent. It is composed of the heat of destruction of the crystal lattice and the heat of hydration (or the heat of solvation for non-aqueous solutions), released as a result of the interaction of solvent molecules with molecules or ions of the solute with the formation of compounds of variable composition - hydrates (solvates). Destruction of the crystal lattice is, as a rule, an endothermic process - ΔH resh > 0, and hydration of ions is exothermic, ΔH hydr< 0. В зависимости от соотношения значений ΔH реш и ΔH гидр энтальпия растворения может иметь как положительное, так и отрицательное значение. Так растворение кристаллического гидроксида калия сопровождается выделением тепла:
ΔH solutionKOH o = ΔH solve o + ΔH hydrK +o + ΔH hydroOH −o = −59 KJ/mol
The enthalpy of hydration - ΔH hydr, refers to the heat that is released when 1 mole of ions passes from vacuum to solution.
Standard enthalpy of neutralization - ΔH neutron enthalpy of the reaction of strong acids and bases with the formation of 1 mole of water under standard conditions:
HCl + NaOH = NaCl + H 2 O
H + + OH − = H 2 O, ΔH neutr ° = −55.9 kJ/mol
The standard enthalpy of neutralization for concentrated solutions of strong electrolytes depends on the ion concentration, due to the change in the ΔH value of hydration ° of ions upon dilution
Enthalpy is a property of a substance that indicates the amount of energy that can be converted into heat.
Enthalpy is a thermodynamic property of a substance that indicates the level of energy stored in its molecular structure. This means that although a substance may have energy based on temperature and pressure, not all of it can be converted into heat. Part of the internal energy always remains in the substance and maintains its molecular structure. Some of the kinetic energy of a substance is unavailable when its temperature approaches the temperature environment. Therefore, enthalpy is the amount of energy that is available to be converted into heat at a certain temperature and pressure. The units of enthalpy are British thermal unit or joule for energy and Btu/lbm or J/kg for specific energy.
Enthalpy quantity
The amount of enthalpy of a substance is based on its given temperature. This temperature is the value that is chosen by scientists and engineers as the basis for calculations. It is the temperature at which the enthalpy of a substance is zero J. In other words, the substance has no available energy that can be converted into heat. This temperature is different for different substances. For example, this temperature of water is the triple point (0 °C), nitrogen is -150 °C, and refrigerants based on methane and ethane are -40 °C.
If the temperature of a substance is higher than its given temperature or changes state to gaseous state at a given temperature, enthalpy is expressed as a positive number. Conversely, at a temperature below this, the enthalpy of a substance is expressed as a negative number. Enthalpy is used in calculations to determine the difference in energy levels between two states. This is necessary to set up the equipment and determine the efficiency of the process.
Enthalpy is often defined as the total energy of a substance, since it is equal to the sum of its internal energy (u) in a given state along with its ability to do work (pv). But in reality, enthalpy does not indicate the total energy of a substance at a given temperature above absolute zero (-273°C). Therefore, rather than defining enthalpy as the total heat of a substance, it is more accurately defined as the total amount of available energy of a substance that can be converted into heat.
H = U + pV
HESS'S LAW: thermal effect of chemistry. r-tion depends only on the initial and final states of the system and does not depend on its intervals. states. G. z. is an expression of the law of conservation of energy for systems in which chemical reactions occur. r-tion, and a consequence of the first law of thermodynamics, however, it was formulated earlier than the first law. Valid for processes flowing at constant volume or constant pressure; for the former, the thermal effect is equal to the change in internal energy of the system due to chemical r-tion, for the second - the change in enthalpy. To calculate the thermal effects of districts, incl. practically impossible, constitute a thermochemical system. equations, which represent the equations of districts, recorded together with the corresponding thermal effects at a given temperature. In this case, it is important to indicate the state of aggregation of the reacting substances, because The magnitude of the thermal effect of the district depends on this.
Thermochemical system The equation can be solved by operating with formulas in identical states, as with ordinary terms of math. ur.
MINISTRY OF EDUCATION OF THE RUSSIAN FEDERATION
Voronezh State Technical University
COURSE PROJECT
in the discipline “Theoretical Foundations of Progressive Technology”
Topic: “The thermal effect of a chemical reaction and its practical application.”
Voronezh 2004
| Introduction……………………………………………………………………………… | 3 |
| 1. Thermal effect of a chemical reaction………………………………... | 4 |
| 1.1. Equations of chemical reactions……………………………... | 8 |
| 1.2. Basic laws of thermochemistry………………………………. | 10 |
| 2. Application of the thermal effect in practice…………………………. | 12 |
| 2.1. Heat-resistant coatings……………………………………. | 1 |
| 2.2. Thermochemical method of diamond processing………………... | 14 |
| 2.3.Technogenic raw materials for cement production……………… | 15 |
| 2.4. Biosensors……………………………………………………. | 16 |
| Conclusion…………………………………………………………………. | 17 |
| List of references……………………………………………………… | 18 |
Introduction
The thermal effects of chemical reactions are necessary for many technical calculations. They find wide application in many industries, as well as in military developments.
The purpose of this course work is to study the practical application of the thermal effect. We will look at some options for its use, and find out how important it is to use the thermal effects of chemical reactions in the context of the development of modern technologies.
1. Thermal effect of a chemical reaction
Each substance stores a certain amount of energy. We encounter this property of substances already at breakfast, lunch or dinner, since food allows our body to use the energy of a wide variety of chemical compounds contained in food. In the body, this energy is converted into movement, work, and is used to maintain a constant (and quite high!) body temperature.
One of the most famous scientists working in the field of thermochemistry is Berthelot. Berthelot - professor of chemistry at the Higher Pharmaceutical School in Paris (1859). Minister of Education and Foreign Affairs.
Beginning in 1865, Berthelot was actively involved in thermochemistry and conducted extensive calorimetric research, which led, in particular, to the invention of the “calorimetric bomb” (1881); he owns the concepts of “exothermic” and “endothermic” reactions. Berthelot obtained extensive data on the thermal effects of a huge number of reactions, on the heat of decomposition and formation of many substances.
Berthelot studied the effect of explosives: explosion temperature, combustion rate and blast wave propagation, etc.
The energy of chemical compounds is concentrated mainly in chemical bonds. It takes energy to break a bond between two atoms. When a chemical bond is formed, energy is released.
Any chemical reaction consists of breaking some chemical bonds and forming others.
When, as a result of a chemical reaction during the formation of new bonds, more energy is released than was required to destroy the “old” bonds in the starting substances, the excess energy is released in the form of heat. An example is combustion reactions. For example, natural gas (methane CH 4) burns in oxygen in the air, releasing a large amount of heat (Fig. 1a). Such reactions are exothermic.
Reactions that occur with the release of heat exhibit a positive thermal effect (Q>0, DH<0) и называются экзотермическими.
In other cases, the destruction of bonds in the original substances requires more energy than can be released during the formation of new bonds. Such reactions occur only when energy is supplied from outside and are called endothermic.
Reactions that occur with the absorption of heat from the environment (Q<0, DH>0), i.e. with a negative thermal effect, are endothermic.
An example is the formation of carbon monoxide (II) CO and hydrogen H2 from coal and water, which occurs only when heated (Fig. 1b).
Rice. 1a
Rice. 1b
Rice. 1a,b. Depiction of chemical reactions using molecular models: a) exothermic reaction, b) endothermic reaction. The models clearly show how, with a constant number of atoms between them, old chemical bonds are destroyed and new chemical bonds arise.
Thus, any chemical reaction is accompanied by the release or absorption of energy. Most often, energy is released or absorbed in the form of heat (less often in the form of light or mechanical energy). This heat can be measured. The measurement result is expressed in kilojoules (kJ) for one mole of reactant or (less commonly) for one mole of reaction product. This quantity is called the thermal effect of the reaction.
- Thermal effect is the amount of heat released or absorbed by a chemical system when a chemical reaction occurs in it.
Thermal effect is indicated by the symbols Q or DH (Q = -DH). Its value corresponds to the difference between the energies of the initial and final states of the reaction:
DH = H end - H ref. = E con. - E ref.
Icons (d), (g) indicate the gaseous and liquid states of substances. There are also designations (tv) or (k) - solid, crystalline substance, (aq) - substance dissolved in water, etc.
The designation of the state of aggregation of a substance is important. For example, in the combustion reaction of hydrogen, water is initially formed in the form of steam (gaseous state), upon condensation of which some more energy can be released. Consequently, for the formation of water in the form of a liquid, the measured thermal effect of the reaction will be slightly greater than for the formation of only steam, since when the steam condenses, another portion of heat will be released.
A special case of the thermal effect of the reaction is also used - the heat of combustion. From the name itself it is clear that the heat of combustion serves to characterize the substance used as fuel. The heat of combustion is referred to 1 mole of a substance that is a fuel (a reducing agent in an oxidation reaction), for example:
The energy (E) stored in molecules can be plotted on the energy scale. In this case, the thermal effect of the reaction (D E) can be shown graphically (Fig. 2).
Fig.2. Graphic representation of the thermal effect (Q = D E): A) exothermic reaction of hydrogen combustion; b) endothermic reaction of water decomposition under the influence of electric current. The reaction coordinate (horizontal axis of the graph) can be considered, for example, as the degree of conversion of substances (100% is the complete conversion of the starting substances).
1.1. Chemical Reaction Equations
· Equations of chemical reactions in which the thermal effect of the reaction is written along with the reagents and products are called thermochemical equations.
The peculiarity of thermochemical equations is that when working with them, you can transfer the formulas of substances and the magnitude of thermal effects from one part of the equation to another. As a rule, this cannot be done with ordinary equations of chemical reactions.
Term-by-term addition and subtraction of thermochemical equations is also allowed. This may be necessary to determine the thermal effects of reactions that are difficult or impossible to measure experimentally.
Let's give an example. In the laboratory, it is extremely difficult to carry out the “pure form” reaction of producing CH4 methane by direct combination of carbon with hydrogen:
C + 2 H 2 = CH 4
But you can learn a lot about this reaction through calculations. For example, find out whether this reaction will be exo - or endothermic, and even quantify the magnitude of the thermal effect.
The thermal effects of the combustion reactions of methane, carbon and hydrogen are known (these reactions occur easily):
a) CH 4 (g) + 2 O 2 (g) = CO 2 (g) + 2 H 2 O (l) + 890 kJ
b) C(tv) + O 2 (g) = CO 2 (g) + 394 kJ
c) 2 H 2 (g) + O 2 (g) = 2 H 2 O (l) + 572 kJ
Let us subtract the last two equations (b) and (c) from equation (a). We will subtract the left sides of the equations from the left, and the right sides from the right. In this case, all molecules O 2, CO 2 and H 2 O will contract. We get:
CH 4 (g) - C (tv) - 2 H 2 (g) = (890 - 394 - 572) kJ = -76 kJ
This equation looks somewhat unusual. Let's multiply both sides of the equation by (-1) and move CH 4 to the right side with the opposite sign. We get the equation we need for the formation of methane from coal and hydrogen:
C(tv) + 2 H 2 (g) = CH 4 (g) + 76 kJ/mol
So, our calculations showed that the thermal effect of the formation of methane from carbon and hydrogen is 76 kJ (per mole of methane), and this process must be exothermic (energy will be released in this reaction).
It is important to pay attention to the fact that term-by-term addition, subtraction and reduction in thermochemical equations can only be substances that are in identical states of aggregation, otherwise we will make a mistake in determining the thermal effect on the value of the heat of transition from one state of aggregation to another.
1.2. Basic laws of thermochemistry
· The branch of chemistry that studies the transformation of energy in chemical reactions is called thermochemistry.
There are two most important laws of thermochemistry. The first of them, the Lavoisier–Laplace law, is formulated as follows:
· The thermal effect of a forward reaction is always equal to the thermal effect of a reverse reaction with the opposite sign.
This means that during the formation of any compound, the same amount of energy is released (absorbed) as is absorbed (released) during its decomposition into the original substances. For example:
2 H 2 (g) + O 2 (g) 2 H 2 O (l) + 572 kJ (combustion of hydrogen in oxygen)
2 H 2 O (l) + 572 kJ = 2 H 2 (g) + O 2 (g) (decomposition of water by electric current)
Lavoisier–Laplace's law is a consequence of the law of conservation of energy.
The second law of thermochemistry was formulated in 1840 by Russian academician G. I. Hess:
· The thermal effect of a reaction depends only on the initial and final states of the substances and does not depend on the intermediate stages of the process.
This means that the total thermal effect of a series of successive reactions will be the same as that of any other series of reactions if the starting and ending substances are the same at the beginning and at the end of these series. These two basic laws of thermochemistry give thermochemical equations some similarity to mathematical ones, when in reaction equations it is possible to transfer terms from one part to another, to add, subtract and reduce formulas of chemical compounds term by term. In this case, it is necessary to take into account the coefficients in the reaction equations and not to forget that the substances being added, subtracted or reduced by moles must be in the same state of aggregation.
2. Application of the thermal effect in practice
The thermal effects of chemical reactions are needed for many technical calculations. For example, consider the powerful Russian Energia rocket, capable of launching spacecraft and other payloads into orbit. The engines of one of its stages operate on liquefied gases - hydrogen and oxygen.
Suppose we know the work (in kJ) that will have to be spent to deliver a rocket with cargo from the surface of the Earth to orbit, we also know the work to overcome air resistance and other energy costs during the flight. How to calculate the required supply of hydrogen and oxygen, which (in a liquefied state) are used in this rocket as fuel and oxidizer?
Without the help of the thermal effect of the reaction of the formation of water from hydrogen and oxygen, this is difficult to do. After all, the thermal effect is the very energy that should put the rocket into orbit. In the combustion chambers of a rocket, this heat is converted into the kinetic energy of molecules of hot gas (steam), which escapes from the nozzles and creates jet thrust.
In the chemical industry, thermal effects are needed to calculate the amount of heat to heat reactors in which endothermic reactions occur. In the energy sector, thermal energy production is calculated using the heat of combustion of fuel.
Dietitians use the thermal effects of food oxidation in the body to create proper diets not only for patients, but also for healthy people - athletes, workers in various professions. Traditionally, calculations here use not joules, but other energy units - calories (1 cal = 4.1868 J). The energy content of food is referred to any mass of food products: 1 g, 100 g, or even standard packaging of the product. For example, on the label of a jar of condensed milk you can read the following inscription: “calorie content 320 kcal/100 g.”
The thermal effect is calculated when producing monomethylaniline, which belongs to the class of substituted aromatic amines. The main area of application of monomethylaniline is as an anti-knock additive for gasoline. It is possible to use monomethylaniline in the production of dyes. Commercial monomethylaniline (N-methylaniline) is isolated from the catalyzate by periodic or continuous rectification. Thermal effect of the reaction ∆Н= -14±5 kJ/mol.
2.1. Heat-resistant coatings
The development of high-temperature technology necessitates the creation of particularly heat-resistant materials. This problem can be solved by using refractory and heat-resistant metals. Intermetallic coatings are attracting increasing attention because they have many valuable qualities: resistance to oxidation, aggressive melts, heat resistance, etc. Of interest is also the significant exothermicity of the formation of these compounds from their constituent elements. There are two possible ways to use the exothermicity of the reaction of the formation of intermetallic compounds. The first is the production of composite, two-layer powders. When heated, the components of the powder interact, and the heat of the exothermic reaction compensates for the cooling of the particles, reaching the protected surface in a completely molten state and forming a low-porosity coating firmly adhered to the base. Another option would be to apply a mechanical mixture of powders. When the particles are heated sufficiently, they interact already in the coating layer. If the magnitude of the thermal effect is significant, then this can lead to self-melting of the coating layer, the formation of an intermediate diffusion layer that increases the adhesion strength, and obtaining a dense, low-porosity coating structure. When choosing a composition that forms an intermetallic coating with a great thermal effect and has many valuable qualities - corrosion resistance, sufficient heat resistance and wear resistance, nickel aluminides, in particular NiAl and Ni 3 Al, attract attention. The formation of NiAl is accompanied by a maximum thermal effect.
2.2.Thermochemical method of diamond processing
The “thermochemical” method got its name due to the fact that it occurs at elevated temperatures, and is based on the use of the chemical properties of diamond. The method is carried out as follows: the diamond is brought into contact with a metal capable of dissolving carbon, and in order for the dissolution or processing process to proceed continuously, it is carried out in a gas atmosphere that interacts with carbon dissolved in the metal, but does not react directly with the diamond. During the process, the magnitude of the thermal effect takes on a high value.
To determine the optimal conditions for thermochemical processing of diamond and identify the capabilities of the method, it was necessary to study the mechanisms of certain chemical processes, which, as shown by an analysis of the literature, have not been studied at all. A more specific study of the thermochemical processing of diamond was hampered, first of all, by insufficient knowledge of the properties of the diamond itself. They were afraid of ruining it with heat. Research on the thermal stability of diamond has only been carried out in recent decades. It has been established that diamonds that do not contain inclusions can be heated to 1850 “C” in a neutral atmosphere or in a vacuum without any harm to them, and only higher.
Diamond is the best blade material due to its unique hardness, elasticity and low friction against biological tissue. Operating with diamond knives facilitates operations and reduces the healing time of incisions by 2-3 times. According to the microsurgeons of the MNTK for eye microsurgery, knives sharpened by thermochemical method are not only not inferior, but also superior in quality to the best foreign samples. Thousands of operations have already been performed with thermochemically sharpened knives. Diamond knives of different configurations and sizes can be used in other areas of medicine and biology. Thus, microtomes are used to make preparations in electron microscopy. The high resolution of the electron microscope places special demands on the thickness and quality of the section of specimens. Diamond microtomes, sharpened by thermochemical method, make it possible to produce sections of the required quality.
2.3. Technogenic raw materials for cement production
Further intensification of cement production involves the widespread introduction of energy and resource-saving technologies using waste from various industries.
When processing skarn-magnetite ores, dry magnetic separation (DMS) tailings are released, which are crushed stone material with a grain size of up to 25 mm. SMS tailings have a fairly stable chemical composition, wt.%: SiO 2 40...45, Al 2 O 3 10...12, Fe 2 O 3 15...17, CaO 12...13, MgO 5...6, S 2...3, R 2 O 2…4. The possibility of using SMS tailings in the production of Portland cement clinker has been proven. The resulting cements are characterized by high strength properties.
The thermal effect of clinker formation (TEC) is defined as the algebraic sum of the heats of endothermic processes (decarbonization of limestone, dehydration of clay minerals, formation of a liquid phase) and exothermic reactions (oxidation of pyrite introduced by CMS tailings, formation of clinker phases).
The main advantages of using skarn-magnetite ore enrichment waste in cement production are:
Expansion of the raw material base due to man-made sources;
Saving natural raw materials while maintaining cement quality;
Reducing fuel and energy costs for clinker firing;
Possibility of producing low-energy active low-basic clinkers;
Solving environmental problems through rational waste disposal and reducing gas emissions into the atmosphere during clinker firing.
2.4. Biosensors
Biosensors are sensors based on immobilized enzymes. They allow you to quickly and efficiently analyze complex, multicomponent mixtures of substances. Currently, they are increasingly used in a number of branches of science, industry, agriculture and healthcare. The basis for the creation of automatic enzymatic analysis systems was the latest advances in the field of enzymology and engineering enzymology. The unique qualities of enzymes - specificity of action and high catalytic activity - contribute to the simplicity and high sensitivity of this analytical method, and the large number of enzymes known and studied to date makes it possible to constantly expand the list of analyzed substances.
Enzyme microcalorimetric sensors - use the thermal effect of an enzymatic reaction. It consists of two columns (measuring and control), filled with a carrier with an immobilized enzyme and equipped with thermistors. When the analyzed sample is passed through the measuring column, a chemical reaction occurs, which is accompanied by a recorded thermal effect. This type of sensor is interesting for its versatility.
Conclusion.
So, after analyzing the practical application of the thermal effect of chemical reactions, we can conclude: the thermal effect is closely related to our everyday life, it is subject to constant research and finds new applications in practice.
With the development of modern technologies, the warm effect has found its application in various industries. Chemical, military, construction, food, mining and many other industries use the thermal effect in their developments. It is used in internal combustion engines, refrigeration units and various combustion devices, as well as in the production of surgical instruments, heat-resistant coatings, new types of building materials and so on.
In modern conditions of constantly developing science, we are seeing the emergence of more and more new developments and discoveries in the field of production. This entails more and more new areas of application of the thermal effect of chemical reactions.
References
1. Musabekov Yu. S., Marcelin Berthelot, M., 1965; Centenaire de Marcelin Berthelot, 1827-1927, P., 1929.
2. Patent 852586 Russian Federation. MKI V 28 D 5/00. Method of dimensional processing of diamond / A.P.Grigoriev, S.H.Lifshits, P.P.Shamaev (Russian Federation). - 2 s.
3. Klassen V.K. . Material balance. Thermal engineering calculations of thermal units. – Belgorod: BTISM, 1978. –114 p.
4. Peregudov V.V., Rogovoy M.I. Thermal processes and installations in the technology of construction products and parts. – M.: Stroyizdat, 1983.-416 p.
5. E-mail: [email protected]
6. "Biotechnologies" (http://www.ictc.ru/R_42.htm).
7. S.D. Varfolomeev, Yu.M. Evdokimov, M.A. Ostrovsky. "BULLETIN OF THE RUSSIAN ACADEMY OF SCIENCES".
Although most people become familiar with the term “thermal effect of a chemical reaction” in chemistry lessons, it is nevertheless used more widely. It is difficult to imagine any area of activity where this phenomenon would not be used.
Let us give an example of just some of them, where knowledge of what the thermal effect of a reaction is is necessary. Currently, the automotive industry is developing at a fantastic pace: the number of cars increases several times every year. At the same time, the main source of energy for them is gasoline (alternative developments are so far embodied only in a few prototypes). To adjust the force of fuel flaring, special additives are used to reduce the intensity of detonation. A striking example is monomethylaniline. When it is obtained, the thermal effect of the reaction is calculated, which in this case is -11-19 kJ/mol.
Another area of application is the food industry. Without a doubt, any person paid attention to the calorie content of a particular product. In this case, the calorie content and the thermal effect of the reaction are directly related, since heat is released during the oxidation of food. By adjusting your diet based on these data, you can achieve a significant reduction in body weight. Despite the fact that the thermal effect of a reaction is measured in joules, there is a direct relationship between them and calories: 4 J = 1 kcal. In relation to food products, the calculated quantity (weight) is usually indicated.
Let's now turn to the theory and give a definition. So, the thermal effect indicates what is released or absorbed by the system as it flows through it. It is worth considering that in addition to heat, radiation can be generated. The thermal effect of a chemical reaction is numerically equal to the difference between the energy levels of the system: initial and residual. If during the reaction process heat is absorbed from the surrounding space, then we speak of an endothermic process. Accordingly, the release of thermal energy is characteristic of an exothermic process. It is quite simple to distinguish them: if the value of the total energy released as a result of the reaction is greater than that expended to start it (for example, the thermal energy of burning fuel), then this is exothermy. But for the decomposition of water and coal into hydrogen, it is necessary to expend additional energy on heating, so its absorption takes place (endothermy).
The thermal effect of a reaction can be calculated using known formulas. In calculations, the thermal effect is denoted by the letter Q (or DH). The difference is in the type of process (endo or exo), so Q = - DH. Thermochemical equations require the indication of the thermal effect and reagents (the reverse calculation is also correct). The peculiarity of such equations is the possibility of transferring the magnitude of thermal effects and the substances themselves to different parts. It is possible to carry out term-by-term subtraction or addition of the formulas themselves, but taking into account
Let's give an example of the reactions of carbon and hydrogen:
1) CH4 + 2O2 = CO2 + 2H2O + 890 kJ
2) C + O2 = CO2 + 394 kJ
3) 2H2 + O2 = 2H2O + 572 kJ
Now subtract 2 and 3 from 1 (right parts from right parts, left parts from left parts).
As a result we get:
CH4 - C - 2 H4 = 890 - 394 - 572 = - 76 kJ.
If we multiply all parts by - 1 (remove the negative value), we get:
C + 2H2 = CH4 + 76 kJ/mol.
How can you interpret the result? The thermal effect occurring during the formation of methane from hydrogen and carbon will be 76 J for each mole of gas produced. It also follows from the formulas that it will be released, that is, we are talking about an exothermic process. Such calculations avoid the need for direct laboratory experiments, which are often difficult.
Or a change in the enthalpy of a system due to the occurrence of a chemical reaction - the amount of heat attributed to the change in a chemical variable received by the system in which the chemical reaction took place and the reaction products took on the temperature of the reactants.
For the thermal effect to be a quantity that depends only on the nature of the ongoing chemical reaction, the following conditions must be met:
- The reaction must proceed either at constant volume Q v (isochoric process), or at constant pressure Q p (isobaric process).
- No work is performed in the system, except for the expansion work possible at P = const.
If the reaction is carried out under standard conditions at T = 298.15 K = 25 ˚C and P = 1 atm = 101325 Pa, the thermal effect is called the standard thermal effect of the reaction or the standard enthalpy of reaction Δ H rO. In thermochemistry, the standard heat of reaction is calculated using standard enthalpies of formation.
Standard enthalpy of formation (standard heat of formation)
The standard heat of formation is understood as the thermal effect of the reaction of the formation of one mole of a substance from simple substances and its components that are in stable standard states.
For example, the standard enthalpy of formation of 1 mole of methane from carbon and hydrogen is equal to the thermal effect of the reaction:
C(tv) + 2H 2 (g) = CH 4 (g) + 76 kJ/mol.
The standard enthalpy of formation is denoted by Δ H fO. Here the index f means formation, and the crossed out circle, reminiscent of a Plimsol disk, means that the value refers to the standard state of matter. Another designation for standard enthalpy is often found in the literature - ΔH 298.15 0, where 0 indicates pressure equal to one atmosphere (or, somewhat more precisely, standard conditions), and 298.15 is temperature. Sometimes index 0 is used for quantities related to pure substance, stipulating that it is possible to designate standard thermodynamic quantities with it only when a pure substance is chosen as the standard state. For example, the state of a substance in an extremely dilute solution can also be accepted as standard. “Plimsoll disk” in this case means the actual standard state of matter, regardless of its choice.
The enthalpy of formation of simple substances is taken equal to zero, and the zero value of the enthalpy of formation refers to the state of aggregation, stable at T = 298 K. For example, for iodine in the crystalline state Δ H I 2 (tv) 0 = 0 kJ/mol, and for liquid iodine Δ H I 2 (g) 0 = 22 kJ/mol. The enthalpies of formation of simple substances under standard conditions are their main energy characteristics.
The thermal effect of any reaction is found as the difference between the sum of the heats of formation of all products and the sum of the heats of formation of all reactants in a given reaction (a consequence of Hess’s law):
Δ H reaction O = ΣΔ H f O (products) - ΣΔ H f O (reagents)Thermochemical effects can be incorporated into chemical reactions. Chemical equations that indicate the amount of heat released or absorbed are called thermochemical equations. Reactions accompanied by the release of heat into the environment have a negative thermal effect and are called exothermic. Reactions accompanied by the absorption of heat have a positive thermal effect and are called endothermic. The thermal effect usually refers to one mole of reacted starting material whose stoichiometric coefficient is maximum.
Temperature dependence of the thermal effect (enthalpy) of the reaction
To calculate the temperature dependence of the enthalpy of a reaction, it is necessary to know the molar heat capacities of the substances participating in the reaction. The change in the enthalpy of the reaction with increasing temperature from T 1 to T 2 is calculated according to Kirchhoff’s law (it is assumed that in this temperature range the molar heat capacities do not depend on temperature and there are no phase transformations):
If phase transformations occur in a given temperature range, then in the calculation it is necessary to take into account the heats of the corresponding transformations, as well as the change in the temperature dependence of the heat capacity of substances that have undergone such transformations:
where ΔC p (T 1 ,T f) is the change in heat capacity in the temperature range from T 1 to the phase transition temperature; ΔC p (T f ,T 2) is the change in heat capacity in the temperature range from the phase transition temperature to the final temperature, and T f is the phase transition temperature.
Standard enthalpy of combustion - Δ H hor o, the thermal effect of the combustion reaction of one mole of a substance in oxygen to the formation of oxides in the highest oxidation state. The heat of combustion of non-combustible substances is assumed to be zero.
Standard enthalpy of solution - Δ H solution, the thermal effect of the process of dissolving 1 mole of a substance in an infinitely large amount of solvent. It is composed of the heat of destruction of the crystal lattice and the heat of hydration (or the heat of solvation for non-aqueous solutions), released as a result of the interaction of solvent molecules with molecules or ions of the solute with the formation of compounds of variable composition - hydrates (solvates). Destruction of the crystal lattice is usually an endothermic process - Δ H resh > 0, and ion hydration is exothermic, Δ H hydr< 0. В зависимости от соотношения значений ΔH resh and Δ H hydr enthalpy of dissolution can have both positive and negative values. Thus, the dissolution of crystalline potassium hydroxide is accompanied by the release of heat:
Δ H dissolveKOH o = Δ H decide + Δ H hydrK + o + Δ H hydroOH − o = −59 KJ/molUnder the enthalpy of hydration - Δ H hydr, refers to the heat that is released when 1 mole of ions passes from vacuum to solution.
Standard enthalpy of neutralization - Δ H neutro enthalpy of the reaction of strong acids and bases to form 1 mole of water under standard conditions:
HCl + NaOH = NaCl + H 2 O H + + OH − = H 2 O, ΔH neutr ° = −55.9 kJ/mol
The standard enthalpy of neutralization for concentrated solutions of strong electrolytes depends on the ion concentration, due to the change in the ΔH value of hydration ° of the ions upon dilution.
Notes
Literature
- Knorre D. G., Krylova L. F., Muzykantov V. S. Physical chemistry. - M.: Higher School, 1990
- Atkins P. Physical chemistry. - Moscow. : World, 1980
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