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Who Knows what entropy and enthalpy are. and got the best answer
Answer from Vic [active]
Enthalpy and entropy
The change in free energy (ΔG) of a chemical reaction depends on a number of factors, including the temperature and concentration of the reactants.
A. Heat of reaction and calorimetry
All chemical reactions are accompanied by the release or absorption of heat. Reactions of the first type are called exothermic, reactions of the second type are called endothermic. The measure of the heat of reaction is the change in enthalpy ΔН, which corresponds to heat transfer at constant pressure. In the case of exothermic reactions, the system loses heat and ΔН is a negative value. In the case of endothermic reactions, the system absorbs heat and ΔН is a positive value.
For many chemical reactions, ΔG and ΔH have similar values. This circumstance makes it possible to determine the energy value of food products. In living organisms, food is usually oxidized with oxygen to CO2 and H2O. The maximum chemical work that nutrients can perform in the body, i.e., ΔG of the oxidation reaction of food components, is determined by burning a sample of the corresponding substance taken in a calorimeter in an oxygen atmosphere. The released heat increases the temperature of the water in the calorimeter. The heat of reaction (enthalpy of combustion) is calculated from the temperature difference.
B. Enthalpy and entropy
< 0) несмотря на то, что являются эндотермическими (ΔΗ >
The higher the degree of disorder (disorder) of the system, the higher the entropy of the system. Thus, if the process goes in the direction of increasing the disorder of the system (and everyday experience shows that this is the most probable process), ΔS is a positive value. To increase the degree of order in the system (ΔS\u003e
ΔG \u003d ΔH - T ΔS
Let us explain the dependence of these three quantities using two examples.
The explosion of an explosive mixture (1) is the interaction of two gases - oxygen and hydrogen - with the formation of water. Like many redox reactions. this is a highly exothermic process (i.e., ΔH<< 0). В то же время в результате реакции возрастает степень упорядоченности системы. Газ с его хаотически мигрирующими молекулами перешел в более упорядоченное состояние -- жидкую фазу, при этом число молекул в системе уменьшилось на 1/3. В результате увеличения степени упорядоченности (ΔS < 0) член уравнения -T · ΔS - величина положительная, однако это с избытком компенсируется ростом энтальпии: в итоге происходит высоко экзергоническая реакция (ΔG <<0).
When table salt (2) is dissolved in water, ΔН is a positive value, the temperature in the vessel with the solution, i.e. in the volume of the solution, decreases. Nevertheless, the process proceeds spontaneously, since the degree of order in the system decreases. In the initial state, the Na + and Cl- ions occupied fixed positions in the crystal lattice. In solution, they move independently of each other in arbitrary directions. Decreased ordering (ΔS\u003e 0) means that the term -T · ΔS has a minus sign. This compensates for ΔН and, in general, ΔG is a negative value. Such processes are usually called entropy.
Answer from 2 answers[guru]
Hello! Here is a selection of topics with answers to your question: Who Knows what entropy and enthalpy are.
Answer from \u003d CaT \u003d[guru]
Entropy (from the Greek ἐντροπία - rotation, transformation) is a concept first introduced in thermodynamics to determine the measure of irreversible dissipation of energy. The term is widely used in other fields of knowledge as well: in statistical physics as a measure of the probability of a certain macroscopic state being realized; in information theory as a measure of the uncertainty of any experience (test), which can have different outcomes, in historical science, for the explication of the phenomenon of alternative history (invariance and variability of the historical process).
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The enthalpy of the system (from the Greek enthalpo I heat), an unambiguous function H of the state of a thermodynamic system with independent parameters of entropy S and pressure P, is related to the internal energy U by the relation
H \u003d U + PV
where V is the volume of the system.
Answer from Yovetlana Pustotina[guru]
entropy is a function of the state of a thermodynamic system, the change in which in an equilibrium process is equal to the ratio of the number of bodies imparted to the system or withdrawn from it to the thermodynamic temperature of the system, nonequilibrium processes in an isolated system are accompanied by an increase in entropy, they bring the system closer to the state of equilibrium, in which the entropy is maximum. this is the essence of the second law of thermodynamics, both laws of thermodynamics were reflected by the German physicist Rudolf Clausius - the energy of the world remains constant, the entropy tends to the maximum value. Enthalpy is a single-valued function of the state of a thermodynamic system at independent parameters of entropy and pressure, associated with internal energy, this value is called the heat content of the system At constant pressure, the change in enthalpy is equal to the amount of heat supplied to the system, in a state of thermodynamic equilibrium the enthalpy of the system is minimal.
Answer from Terminator-5[guru]
How clever and difficult they all answer! Why complicate things, we can say simply. Enthalpy is the state of a person during the inflow and outflow of money. And entropy is the degree of inability to return to the state when the money was still there. The less money left until payday. , the higher the more entropy!
Answer from Just Manya[newbie]
Enthalpy and entropy
The change in free energy (ΔG) of a chemical reaction depends on a number of factors, including the temperature and concentration of the reactants (see p. 24). This section discusses two more parameters that are related to structural and energy changes in molecules.
B. Enthalpy and entropy
The heat of reaction ΔН and the change in free energy ΔG do not always have comparable values. In fact, reactions are known that occur spontaneously (ΔG< 0) несмотря на то, что являются эндотермическими (ΔΗ > 0). This is because the progress of the reaction is influenced by a change in the degree of ordering of the system. A measure of the change in the ordering of the system is the change in entropy ΔS.
The higher the degree of disorder (disorder) of the system, the higher the entropy of the system. Thus, if the process goes in the direction of increasing the disorder of the system (and everyday experience shows that this is the most probable process), ΔS is a positive value. To increase the degree of order in the system (ΔS\u003e 0), it is necessary to expend energy. Both of these provisions follow from the fundamental law of nature - the second law of thermodynamics. Quantitatively, the relationship between changes in enthalpy, entropy and free energy is described by the Gibbs-Helmholtz equation:
ΔG \u003d ΔH - T ΔS
Answer from 2 answers[guru]
Hello! Here are more threads with similar questions.
Heat capacity and its types. Specific heat from call the amount of heat d, which is required to change the temperature of a unit of amount of a substance by one degree: from = d / bT, s = dg / dT.
Depending on the method of measuring the unit of the amount of a substance, the nature of the thermodynamic process and the value of the temperature interval, several types of heat capacities are distinguished.
1. Depending on the unit amount of a substance (1 kg, 1 m 3, 1 mol), the heat capacity is mass from [J / (kg-K)], volume from" [J / (m 3 - K)] and molar c d [JDmol - K)].
The relationship between them is expressed by the following relationship:
where p n is the density under normal physical conditions.
The amount of heat is respectively determined by the formula
where m - gas mass, kg; U n - the volume of gas reduced to normal physical conditions; p is the number of moles of gas.
2. The heat capacity depends on the nature of the process and the properties of the gas. Depending on the method of heat supply, heat capacity is distinguished at constant pressure (isobaric) with p and heat capacity at constant volume (isochoric) c v. The concepts of "heat capacity at constant temperature" and "adiabatic heat" are rarely used, since at T \u003d const from \u003d d ^ / O \u003d oo, and for dg \u003d 0 c \u003d O / d / \u003d 0.
Back in 1842, one of the founders of the law of conservation and transformation of energy, R. Yu. Mayer, established that
The physical meaning of this relationship is easy to understand. If to heat 1 mole (or 1 kg) of gas in the cylinder above the piston by one degree at a constant volume, i.e. with a fixed piston, it is necessary to expend the amount of heat with then at constant pressure, the work of c /? will be added to this amount of heat? (or I), which the expanding gas will make, pushing the released piston.
Attitude to = c p / c v called the adiabatic exponent. Note that knowing to and using equations (1.5), one can determine
3. Since the heat capacity changes with temperature, depending on the temperature range, the true (s) and average (c t) specific heat capacities are distinguished. The heat capacity is called true, corresponding to an infinitely small temperature range: c \u003d dq / dT, and the average is the heat capacity corresponding to the finite range of temperature variation: with t = q / (T 2 - D)).
The dependence of heat capacity on temperature can be expressed as a numerical series, in which the first two terms are of primary importance:
where a, b, d - constants, depending on the nature of the gas.
It has been experimentally established that the heat capacity of real gases also depends on pressure, the effect of which is insignificant at high temperatures characteristic of combustion products in heat engines (1000 ... 2000 ° C). When calculating steam engines, turbines, heat converters, the influence of pressure on the heat capacity cannot be neglected.
In practical calculations, tabular data of average heat capacities in the temperature range from 0 to I.In this case, the amount of heat required to heat 1 kg of the working fluid from 0 to /, or to / 2, will be
Here c ^ 0 and with? 0 - tabular values \u200b\u200bof heat capacities in the temperature ranges (0 ... /]) and (0 ... / 2).
The amount of heat required to heat 1 kg of body from t x up to / 2, is defined as the difference:
Enthalpy. In some cases, it turns out to be expedient to combine the parameters and and pv into a total caloric parameter called enthalpy:
Enthalpy is a thermodynamic function that has the meaning of the total (internal and external) energy of the system. It is made up of internal energy and and elastic energy pv, due to the presence of external ambient pressure r, those. pv there is work that needs to be spent to introduce a working fluid with a volume v in a pressurized environment r.
For an ideal gas, the following relations are true:
When r \u003d const you can get:
Differentiating i - and + pv and substituting into the differential equation of the first law of thermodynamics for the flow of the working fluid, one can obtain
Enthalpy is measured in the same units as heat, work and internal energy (J / kg). Since enthalpy, like internal energy, is a function of state, its absolute value can only be determined with an accuracy up to a certain constant, conventionally chosen for the origin.
According to the international agreement, the so-called enthalpy for water and steam is taken as a reference point triple point (T \u003d 273.16 K and p \u003d 0.0006 Pa), in which the simultaneous existence of three phases is possible: ice, liquid and vapor. The temperature can be taken as the reference point for the enthalpy for gases T- 0 K.
The second law of thermodynamics. The second law of thermodynamics, like the first, is an experimental law based on centuries-old observations of scientists, but it was established only in the middle of the 19th century.
Observations of natural phenomena show that the emergence and development of spontaneously occurring natural processes in it, the work of which can be used for human needs, is possible only if there is no balance between the thermodynamic system participating in the process and the environment. These processes are always characterized by their one-way flow from a higher potential to a lower one (from a higher temperature to a lower one or from a higher pressure to a lower one). In the course of these processes, the thermodynamic system tends to come into equilibrium with the environment, characterized by the equality of pressure and temperature of the system and the environment.
It also follows from observations of natural phenomena that in order to force the process to proceed in the direction opposite to the direction of the spontaneous process, it is necessary to spend energy borrowed from the external environment.
The second law of thermodynamics is a generalization of the stated provisions and is as follows.
- 1. The spontaneous course of natural processes arises and develops in the absence of equilibrium between the thermodynamic system participating in the process and the environment.
- 2. Natural processes spontaneously occurring in nature, the work of which can be used by man, always proceed in only one direction from a higher potential to a lower one.
- 3. The course of spontaneously proceeding processes occurs in the direction leading to the establishment of equilibrium of the thermodynamic system with the environment, and upon reaching this equilibrium, the processes stop.
- 4. The process can proceed in the opposite direction to the spontaneous process, if the energy for this is borrowed from the external environment.
The formulations of the second law of thermodynamics, given by various scientists, took the form of postulates obtained as a result of the development of the provisions expressed by the French scientist Sadi Carnot.
In particular, the postulate of the German scientist R. Clausius is that heat cannot pass from a cold body to a warm one without compensation. The essence of the postulate of the English scientist W. Thomson is that it is impossible to cycle a heat engine without transferring a certain amount of heat from a heat source with a higher temperature to a source with a lower temperature.
This wording should be understood as follows: in order for a periodically operating machine to work, it is necessary that there are at least two sources of heat of different temperatures; in this case, only part of the heat taken from the high-temperature source can be converted into work, while the other part of the heat must be transferred to the low-temperature source. A high temperature source is sometimes called a heat sink or upper heat source, and a low temperature source is sometimes referred to as a heat sink, bottom heat source, or refrigerator.
Entropy. In thermodynamics, one more parameter of the working fluid state is used - entropy, establishing a connection between the amount of heat and temperature (R. Clausius, 1850). Let us explain this parameter based on the following considerations.
The equation of the first law of thermodynamics can be written in the form
In this equation d q is not a complete differential, since the term d / enters the right-hand side of the equation, which is not a total differential, since work is not a parameter of the gas state, but a function of the process. As a consequence, the equation cannot be integrated in the interval of two arbitrarily chosen gas states.
It is known from mathematics that any binomial can be represented as a total differential if it is multiplied by the so-called integrating factor.
When multiplied by an integrating factor 1 / T (Where T - absolute temperature), the above equation takes the form
Equation (1.6) can be represented in a slightly different form, namely:
Expression (1.7) says that d q / T is the total differential of some function s (i.e. d q / T \u003d ds), which is a parameter of the state of the gas, since it depends only on two parameters of the state of the gas and therefore does not depend on how the gas has passed from one state to another. This parameter of the gas state is generally called the entropy of the gas and is denoted by the letter S (J / K). The entropy per kg of gas is called specific entropy gas and denote by the letter s [J / (kg-K)).
The above equation d q = di - vdp is also an incomplete differential equation, since d q is not a complete differential. However, this equation, when multiplied by an integrating factor 1/7 '', can be reduced to the form of a complete differential equation
Consequently,
Considering that for ideal gas pv = RT and therefore 
equation (1.8) for an ideal gas can be transformed as follows:
After integration, it will take the form
The change in entropy in the interval between two gas states (7 and 2) expressed by the equation
From equation (1.9) it follows that the amount of heat involved in one or another thermodynamic process when the working fluid changes from state 7 to state 2, can be expressed as follows:
This integral can be calculated if the functional relationship between Tns is known. Using this dependence, curves are plotted in the coordinate system s- T, reflecting certain thermodynamic processes.
Based on expression (1.10), we can conclude that for the process 1-2 (fig. 1.5) area 7- 2-s 2 -s b lying under the curve representing this process expresses the amount of heat involved in this process.
Figure: 1.5.
To determine the numerical values \u200b\u200bof entropy, use the reference point at T \u003d 0 K, for which i 0 \u003d 0.
The physical meaning of entropy. Entropy cannot be measured, its meaning is difficult to demonstrate with the help of visual aids, but it can be understood from the following interpretations.
1. Entropy is a measure of the value of heat, its efficiency and technological efficiency. We can say that for an isolated system (heater - working fluid) As \u003d 0, when receiving the amount of heat from the heater q u 3 | \u003d (f / G,) and the less s it those. the higher T and the more work done by the system.
Everyday experience shows that the higher the temperature of the coolant with the same amount of heat q, those. the less entropy s = (q / T), the more valuable the heat, since it can be used not only for work, but also for technological needs - metal smelting, heating, etc.
- 2. Entropy is a measure of the loss of work due to the irreversibility of real processes. The greater the irreversibility of the process in an isolated system, the more the entropy increases s 2 " 5, and the greater the share of energy is not converted into work, dissipating into the environment.
- 3. Entropy is a measure of disorder. If we establish some measure of the disorder of the macrosystem - the disorder of the arrangement and movement of particles D, then we can write s \u003d k InZ).
Consequently, an increase in disorder means an increase in entropy, a dissipation of energy. With the supply of heat, the chaos of the thermal motion of particles increases, and the entropy increases. Otherwise, the cooling of the system at a constant volume is the extraction of heat from it and, consequently, of entropy. In this case, the orderliness of the system increases, and the entropy decreases. When a gas condenses into a liquid, the molecules occupy more definite positions, the ordering of their arrangement increases abruptly, which corresponds to an abrupt decrease in entropy. With a further decrease in temperature, the thermal motion becomes less and less intense, the disorder becomes less and less, which means that the entropy becomes less and less. When the liquid turns into a solid, the molecules (ions) form regular crystal lattices, that is, the disorder will again decrease, and with it the entropy will also decrease, etc. This pattern allows us to assume that at an absolute temperature equal to zero, the thermal motion will completely stop and the maximum order will be established in the system, i.e. disorder and entropy will be zero. This assumption is consistent with experience, but cannot be verified experimentally (since absolute temperature equal to zero is unattainable) and is called third law of thermodynamics.
Consequently
Reversible and irreversible thermodynamic processes. For
studies of thermodynamic processes introduce the concept of equilibrium (reversible) processes.
The state of the working fluid, in which the pressure and temperature, and, consequently, the specific volume at all its points do not change without external energy impact in time, is called equilibrium state.
A sequential change in the state of the working fluid, resulting from the energetic interaction of the working fluid with the environment, is called thermodynamic process. The process, during the implementation of which the body sequentially passes through a continuous series of equilibrium states, is called equilibrium.
Reversible process such a thermodynamic process is called that allows it to flow through the same equilibrium states both in forward and backward directions, and no changes remain in the environment.
If the specified condition is not met, then the process turns out irreversible. An example of an irreversible process is the transfer of heat in a steam boiler from gases with a temperature of 600 ... 1000 ° C to steam with a temperature of 400 ... 500 ° C, since the return transfer of heat from steam to gases without changing their temperatures is impossible.
In its pure form, reversible processes are not observed in nature and technology. However, their study is of great importance, since many real processes are close to reversible.
Enthalpy versus entropy
Curiosity is one aspect of a person that helps him discover various phenomena in the world. One person looks up at the sky and wonders how rain is formed. One person looks at the ground and wonders how plants can grow. This is an everyday phenomenon that we encounter in our life, but those people who are not curious enough never try to find answers to why such phenomena exist. Biologists, chemists, and physicists are just a few people trying to find answers. Our modern world today is integrated with such laws of science as thermodynamics. "Thermodynamics" is a branch of natural science that includes the study of the internal movements of body systems. This is a study examining the relationship between heat and various forms of energy and work. The applications of thermodynamics are manifested in the flow of electricity and from the simple turning and turning of a screw and other simple machines. As long as heat and friction are involved, there is thermodynamics. The two most common principles of thermodynamics are enthalpy and entropy. In this article, you will learn more about the differences between enthalpy and entropy.
In a thermodynamic system, the measure of its total energy is called enthalpy. Internal energy is required to create a thermodynamic system. This energy serves as an impetus or trigger to create a system. The units of enthalpy are joule (International System of Units) and calorie (British thermal unit). Enthalpy is the Greek word for enthalpos (to infuse warmth). Heike Kamerlingh Onnes was the person who coined the word, while Alfred W. Porter was the one who coined the H for "enthalpy." In biological, chemical, and physical measurements, enthalpy is the preferred expression for changes in the energy of a system because it has the ability to simplify specific definitions of energy transfer. The value for the total enthalpy cannot be reached because the total enthalpy of the system cannot be directly measured. Only the change in enthalpy is the preferred measurement of quantity and not the absolute value of enthalpy. In endothermic reactions, a positive change in enthalpy is observed, and in exothermic reactions, a negative change in enthalpy occurs. In simple terms, the enthalpy of a system is equivalent to the sum of non-mechanical work and supplied heat. At constant pressure, the enthalpy is equivalent to the change in the internal energy of the system and the work that the system exerted towards its environment. In other words, heat can be absorbed or released by a specific chemical reaction under such conditions.
"Entropy" is the second law of thermodynamics. This is one of the most fundamental laws in physics. This is important for understanding life and learning. This is considered the Law of Confusion. In the middle of the last century, "entropy" had already been formulated with the extensive efforts of Clausius and Thomson. Clausius and Thomson were inspired by Carnot's observation of the stream that turns the millwheel. Carnot stated that thermodynamics is the flow of heat from higher to lower temperatures that makes a steam engine work. Clausius was the one who coined the term "entropy". The symbol for entropy is the "S," which states that the world is considered inherently active when it acts spontaneously to dissipate or minimize the presence of thermodynamic force.
"Enthalpy" is the transfer of energy, and "entropy" is the Law of Disorder.
Enthalpy takes the character "H" and entropy takes the character "S".
Heike Kamerlingh Onnes coined the term "enthalpy" and Clausius coined the term "entropy".
Heat effect of a chemical reaction or the change in the enthalpy of the system due to the occurrence of a chemical reaction - the amount of heat, referred to the change in the chemical variable, received by the system in which the chemical reaction took place and the reaction products took the temperature of the reactants.
Enthalpy, thermal function and heat content - thermodynamic potential characterizing the state of the system in thermodynamic equilibrium when choosing as independent variables pressure, entropy and number of particles.
The enthalpy change does not depend on the process path, being determined only by the initial and final state of the system. If the system in some way returns to its original state (circular process), then the change in any of its parameters, which is a function of the state, is equal to zero, hence D H = 0
For the thermal effect to be a value that depends only on the nature of the ongoing chemical reaction, the following conditions must be met:
The reaction must proceed either at a constant volume Q v (isochoric process), or at constant pressure Q p ( isobaric process).
The molar heat capacity at constant pressure is denoted as C p ... In an ideal gas, it is related to the heat capacity at a constant volume mayer's relation C p = C v + R.
Molecular kinetic theory allows you to calculate the approximate values \u200b\u200bof the molar heat capacity for various gases through the value universal gas constant:
· For monatomic gases, that is, about 20.8 J / (mol · K);
· For diatomic gases, that is, about 29.1 J / (mol · K);
For polyatomic gases C p = 4R, that is, about 33.3 J / (mol · K).
where the heat capacity at constant pressure is denoted as C p
No work is done in the system, except for the possible expansion work at P \u003d const.
If the reaction is carried out under standard conditions at T \u003d 298 K \u003d 25 ° C and P \u003d 1 atm \u003d 101325 Pa, the heat effect is called the standard heat effect of the reaction or the standard enthalpy of reaction D H r O. In thermochemistry, the standard heat of reaction is calculated using the standard enthalpies of formation.
To calculate the temperature dependence of the enthalpy of reaction, it is necessary to know the molar heat capacity substances participating in the reaction. The change in the enthalpy of reaction with an increase in temperature from T 1 to T 2 is calculated according to the Kirchhoff law (it is assumed that in this temperature range the molar heat capacities do not depend on temperature and do not 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 DC p (T 1, T f) is the change in heat capacity in the temperature range from T 1 to the phase transition temperature; DC 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. Combustion standard enthalpy
Combustion standard enthalpy - D H gor, the thermal effect of the combustion reaction of one mole of a substance in oxygen to form oxides in the highest oxidation state. The heat of combustion of non-combustible substances is taken to be zero.
Standard enthalpy of dissolution - D H solution, the thermal effect of the dissolution process of 1 mole of a substance in an infinitely large amount of solvent. Consists of the heat of destruction crystal lattice and warmth hydration (or warmth solvation for non-aqueous solutions), released as a result of the interaction of solvent molecules with molecules or ions of the solute to form compounds of variable composition - hydrates (solvates). The destruction of the crystal lattice, as a rule, is an endothermic process - D H res\u003e 0, and ion hydration is exothermic, D H hydras< 0. В зависимости от соотношения значений ДH Resh and D H hydr enthalpy of dissolution can have both positive and negative values. So dissolving crystalline potassium hydroxide accompanied by the release of heat:
D H solKOH o \u003d D H res o + D H hydrK + o + D H hydroOH -o \u003d? 59 kJ / mol
Under the enthalpy of hydration - D H hydr, it is understood the heat that is released during the transition of 1 mole of ions from vacuum to solution.
Heat capacityfrom P , c V [J. mol -1. K -1, cal. mol -1. K -1]
True molar heat capacity:
at V \u003d const c V \u003d; P \u003d const c P =.
The average molar heat capacity is numerically equal to the heat that must be reported to one mole of a substance in order to heat it by 1 K:.
Heat capacities at constant pressure or volume are related by the equality
forideal gas ;
forchrist. substances (, T - thermal coefficients).
The temperature dependence of the heat capacity of many monatomic crystals at T< q D /12 описывается законом кубов Дебая (q D - характеристическая температура Дебая) c V = aT 3 , при T c V 3R. В области средних температур применяют различные степенные полиномы (см., напр., закон Кирхгофа).
Dulong and Petit rule: atomic heat capacity at V \u003d const for any simple crystalline substance is approximately equal to V 3R (ie 25 J. mol -1. K -1).
Additivity rule: (with P, i is the heat capacity of the structural fragments constituting the compound, for example, atoms or groups of atoms).
Heat[J. mol -1, cal. mol -1] Q - a form of energy transfer from a more heated body to a less heated one, not associated with the transfer of matter and the performance of work.
The heat of a chemical reaction at a constant volume or pressure (i.e., the heat effect of a chemical reaction) does not depend on the path of the process, but is determined only by the initial and final state of the system (Hess's law):
\u003d U, \u003d H.
The difference between the thermal effects at P \u003d const (Q P) and V \u003d const (Q V) is equal to the work performed by the system (V\u003e 0) or on the system (V<0) за счет изменения ее объема при завершении изобарно-изотермической реакции:
- \u003d n RT.
The standard heat of reaction can be calculated using the standard heats of formation () or combustion () of substances:
where n i, j are stoichiometric coefficients in the chemical reaction equation.
For ideal gases at T, P \u003d const: r H \u003d r U + n RT.
The temperature dependence of the thermal effect of a chemical reaction is determined s by Akon Kirchhoff .
= = , = = ,
those. The influence of temperature on the heat effect of the reaction is due to the difference in the heat capacities of the reaction products and the initial substances, taking into account the stoichiometric coefficients:
With P \u003d const:
enthalpy thermodynamic entropy pressure
If the temperature dependence of c P is approximated by the equation
\u003d a + b . T + c . then
H (T 2 ) \u003d H (T 1 ) + a . .
Heat of adsorption - heat per mole of a substance, which is released during its adsorption. Adsorption is always an exothermic process (Q\u003e 0). With constant adsorption (Г, q \u003d const):
The Q value is an indirect criterion for determining the type of adsorption: if Q< 30 40 кДж/моль) - физическая адсорбция, Q > 40 kJ / mol - chemisorption.
The heat of education - isobaric thermal effect of a chemical reaction of the formation of a given chemical compound from simple substances, referred to one mole of this compound. It is believed that simple substances react in that modification and that state of aggregation that are stable at a given temperature and pressure of 1 atm.
Heat of combustion (ts) -the thermal effect of combustion of 1 mole of the substance and cooling of the reaction products to the initial temperature of the mixture. TS, unless otherwise stated, corresponds to the combustion of C to CO 2, H 2 to H 2 O (f), for other substances in each case indicate the products of their oxidation.
Phase transition heat - heat absorbed (released) as a result of the equilibrium transition of a substance from one phase to another (see phase transition).
Thermodynamic variables (t.p.)- quantities that quantitatively express thermodynamic properties. T.P. divided into independent variables (measured in experience) and functions. Note: pressure, temperature, elemental chemical composition are independent so on, entropy, energy are functions. The set of values \u200b\u200bof the independent variables sets the thermodynamic state of the system (see also the equation of state). Variables that are fixed by the conditions for the existence of the system and, therefore, cannot change within the problem under consideration, are called thermodynamic parameters.
Extensive - so on, proportional to the amount of matter or the mass of the system. Note.: volume, entropy, internal energy, enthalpy, Gibbs and Helmholtz energies, charge, surface area.
Intensive - so on, independent of the amount of substance or mass of the system. Approx.: pressure, thermodynamic temperature, concentrations, molar and specific thermodynamic quantities, electric potential, surface tension. Extensive ones add up, intensive ones level out.
When chemical reactions occur, energy levels are rearranged. Some bonds in molecules are destroyed and others are formed. All this requires certain energy costs. The transformation of some types of energy and work into others, as well as the direction and limits of the spontaneous course of chemical processes, is studied by chemical thermodynamics. The object of study of chemical thermodynamics is the system.
A system is a collection of interacting substances, mentally or actually isolated from the environment (test tube, autoclave).
Systems are: homogeneous - consisting of one phase (a homogeneous solution of table salt) and heterogeneous - consisting of several phases (water with ice).
A phase is a part of a system that is homogeneous in composition and properties and is separated from other parts of the system by an interface.
In chemical thermodynamics the following systems are considered: isolated - not exchanging matter and energy with the environment; closed - exchanging energy with the environment and not exchanging matter. There are open systems that exchange matter and energy with the environment; these are living organisms. But they are not considered in chemical thermodynamics.
The state of the system can be characterized by thermodynamic parameters, which include: temperature, pressure, concentration, density, volume, mass.
If the state of the system is characterized by constant and unchanged values \u200b\u200bof thermodynamic parameters in time at all points of the system, then it is in a state of equilibrium. When one of the state parameters changes, the system goes into a state of new equilibrium. Chemical thermodynamics considers transitions from one state to another, while some parameters may change or remain constant:
isobaric - at constant pressure;
isochoric - at constant volume;
isothermal - at constant temperature;
isobaric - isothermal - at constant pressure and temperature, etc.
The thermodynamic properties of the system can be expressed using several functions of the state of the system, called characteristic functions: internal energy U, enthalpy H, entropy S, Gibbs energy G, Helmholtz energy F. Characteristic functions have one feature: they do not depend on the way (path) of achieving a given system state. Their value is determined by the parameters of the system (pressure, temperature, etc.) and depends on the amount or mass of the substance, therefore it is customary to refer them to one mole of the substance.
Enthalpy and entropy
The heat of reaction of the DN and the change in the free energy DG do not always have comparable values. In fact, reactions are known that occur spontaneously (DG< 0) несмотря на то, что являются эндотермическими (ДЗ > 0). This is because the progress of the reaction is influenced by a change in the degree of ordering of the system. The measure of the change in the ordering of the system is the change in the entropy ДS.
The higher the degree of disorder (disorder) of the system, the higher the entropy of the system. Thus, if the process goes in the direction of increasing the disorder of the system (and everyday experience shows that this is the most probable process), DS is a positive value. To increase the degree of order in the system (DS\u003e 0), it is necessary to expend energy. Both of these provisions follow from the fundamental law of nature - the second law of thermodynamics. Quantitatively, the relationship between changes in enthalpy, entropy and free energy is described by the Gibbs-Helmholtz equation:
ДG \u003d ДH - T * ДS
Let us explain the dependence of these three quantities using two examples.
The explosion of an explosive mixture (1) is the interaction of two gases - oxygen and hydrogen - with the formation of water. Like many redox reactions, this is a highly exothermic process (i.e., DN<<0). В то же время в результате реакции возрастает степень упорядоченности системы. Газ с его хаотически мигрирующими молекулами перешел в более упорядоченное состояние - жидкую фазу, при этом число молекул в системе уменьшилось на 1/3. В результате увеличения степени упорядоченности (ДS<0) член уравнения - T · ДS - величина положительная, однако это с избытком компенсируется ростом энтальпии: в итоге происходит высоко экзергоническая реакция (ДG <<0).
When table salt (2) is dissolved in water, DN is a positive value, the temperature in the vessel with the solution, i.e. in the volume of the solution decreases. Nevertheless, the process proceeds spontaneously, since the degree of order in the system decreases. In the initial state, the Na + and Cl - ions occupied fixed positions in the crystal lattice. In solution, they move independently of each other in arbitrary directions. A decrease in the ordering (DS\u003e 0) means that the term in the equation - T · DS has a minus sign. This compensates for DN and in general DG is a negative value. Such processes are usually called entropy.
Gibbs energy. Helmholtz energy. Direction of chemical reactions
If the process proceeds spontaneously, then the internal energy (enthalpy) should decrease, and the entropy should increase. To compare these values, they must be expressed in the same units, and for this, DS should be multiplied by T. In this case, we have DN - enthalpy factor and TDS - entropy factor.
In the course of the reaction, the particles tend to unite, which leads to a decrease in enthalpy (ДН< 0), с другой стороны - должна возрастать энтропия, т.е. увеличиваться число частиц в системе (ТДS > 0). The "driving force" of the reaction is determined by the difference between these values \u200b\u200band is denoted as DG.
ДGp, T \u003d ДH - TДS
and is called the change in Gibbs energy (isobaric-isothermal potential).
Gibbs energy is part of the energy effect of a reaction that can be turned into work, which is why it is called free energy. This is also a thermodynamic function of state and, therefore, for the reaction
bB + dD \u003d lL + mM
the Gibbs energy of a chemical reaction can be calculated as the sum of the Gibbs energies of the formation of the reaction products minus the Gibbs energies of the formation of the initial substances, taking into account the stoichiometric coefficients by the formula:
ДG \u003d lДfGL + mДfGM - dДfGD - bДfGB
where ДfG is the Gibbs energy of the formation of substances.
The Gibbs energy of the formation of substances is the change in the Gibbs energy of the system during the formation of 1 mol of a substance from simple substances that are stable at 298 K.
The Gibbs energy of the formation of simple substances DfG is taken to be zero. If the resulting substance and the initial simple substances are in standard states, then the Gibbs energy of formation is called the standard Gibbs energy of the formation of the substance DfG0. Its values \u200b\u200bare given in reference books.
The obtained value of DG is a criterion for the spontaneous course of the reaction in the forward direction, if DG< 0. Химическая реакция не может протекать самопроизвольно в прямом направлении, если энергия Гиббса системы возрастает, т.е. ДG > 0. If ДG \u003d 0, then the reaction can proceed both in the forward and in the opposite directions, i.e. the reaction is reversible.
The direction of chemical reactions depends on their nature. Thus, the condition ДG< 0 соблюдается при любой температуре для экзотермических реакций (ДН < 0), у которых в ходе реакции возрастает число молей газообразных веществ, и, следовательно, энтропия (ДS > 0). In such reactions, both driving forces (MD) and (TDS) are directed towards the direct reaction and ДG< 0 при любых температурах. Такие реакции являются необратимыми.
On the contrary, an endothermic reaction (DN\u003e 0), as a result of which the number of moles of gaseous substances decreases (DS< 0) не могут протекать самопроизвольно в прямом направлении при любой температуре, т.к. всегда ДG > 0.
If, as a result of an exothermic reaction (DN< 0) уменьшается число молей газообразных веществ и, соответственно, энтропия (ДS < 0), то при невысокой температуре ДН > TDS and reaction is possible in the forward direction (DG< 0). При высоких температурах ДH < TДS и прямая реакция самопроизвольно протекать не может (ДG > 0), and the opposite reaction is possible.
To determine the equilibrium temperature, you can use the condition:
where Tr is the temperature at which equilibrium is established, i.e. the possibility of direct and reverse reactions.
If, as a result of an endothermic reaction (DN\u003e 0), the number of moles of gaseous substances and the entropy of the system (DS\u003e 0) increase, then at low temperatures, when DN\u003e TDS, a spontaneously direct reaction cannot proceed (ДG\u003e 0), and at high temperatures when DN< TДS, прямая реакция может протекать самопроизвольно (ДG < 0).
The relationship between DG and DG0 is expressed by the Van't Hoff isotherm equation, which for the reaction
bB + dD \u003d lL + mM
In isochoric-isothermal conditions, free energy is called Helmholtz energy or isochoric-isothermal potential and is equal to It characterizes the direction and limit of the spontaneous flow of a chemical reaction under isochoric-isothermal conditions, which is possible with DF< 0.
Thermodynamic potentials, functions of parameters of the state of a macroscopic system (t-ry T, pressure p, volume V, entropy S, number of moles of components ni, chemical potentials of components m, etc.), used in Ch. arr. to describe thermodynamic equilibrium. Each thermodynamic potential corresponds to a set of state parameters, called. natural variables.
The most important thermodynamic potentials: internal energy U (natural variables S, V, ni); enthalpy H \u003d U - (- pV) (natural variables S, p, ni); Helmholtz energy (Helmholtz free energy, Helmholtz f-tion) F \u003d \u003d U - TS (natural variables V, T, ni); Gibbs energy (free Gibbs energy, Gibbs f-tion) G \u003d U - - TS - (- pV) (natural variables p, T, ni); great thermodynamic. potential (natural variables V, T, mi) Thermodynamic potentials can be represented by the general formula
where Lk are intensive parameters that do not depend on the system mass (such are T, p, mi), Xk are extensive parameters proportional to the system mass (V, S, ni). Index l \u003d 0 for the internal energy U, 1 for H and F, 2 for G and W. Thermodynamic potentials are f-tions of the state of a thermodynamic system, i.e. their change in any transition process between two states is determined only by the initial and final states and does not depend on the transition path. The total differentials of thermodynamic potentials are as follows:
Ur-nie (2) called. fundamental ur-ni Gibbs in energetic. expression. All thermodynamic potentials have the dimension of energy.
Equilibrium conditions thermodynamic. systems are formulated as the equality to zero of the total differentials of the thermodynamic potentials with the constancy of the corresponding natural variables:
thermodynamic enthalpy reaction entropy
Thermodynamic. the stability of the system is expressed by the inequalities:
Thermodynamic potentials, taken as functions of their natural variables, are characteristic functions of the system. This means that any thermodynamic. St.-in (compressibility, heat capacity, etc.) m. b. expressed by a relationship that includes only a given thermodynamic potential, its natural variables and derivatives of thermodynamic potentials of different orders with respect to natural variables. In particular, using thermodynamic potentials, one can obtain the equations of state of the system.
Derivatives of thermodynamic potentials have important properties. The first partial derivatives with respect to natural extensive variables are equal to intensive variables, for example:
[in general: (9Yl / 9Xi) \u003d Li]. Conversely, derivatives with respect to natural intensive variables are equal to extensive variables, e.g .:
[in general: (9Yl / 9Li) \u003d Xi]. The second partial derivatives with respect to natural variables define fur. and thermal. system properties, for example:
Because differentials of thermodynamic potentials are complete, cross second partial derivatives of thermodynamic potentials are equal, for example. for G (T, p, ni):
Relations of this type are called Maxwell's relations.
Thermodynamic potentials can also be represented as functions of variables other than natural ones, for example. G (T, V, ni), however, in this case, the Holy Islands of thermodynamic potentials as characteristic. functions will be lost. In addition to thermodynamic potentials, characteristic. f-tions are entropy S (natural variables U, V, ni), Massier function F1 \u003d (natural variables 1 / T, V, ni), Planck function (natural variables 1 / T, p / T, ni ). Thermodynamic potentials are related to each other by the Gibbs-Helmholtz equations. E.g. for H and G
In general
Thermodynamic potentials are homogeneous f-tions of the first degree of their natural extensive variables. For example, with an increase in the entropy S or the number of moles ni, the enthalpy H also increases proportionally. According to Euler's theorem, the homogeneity of thermodynamic potentials leads to relations of the type:
In statistical thermodynamics, analogs of Helmholtz energy and large thermodynamics are used. potential, to-eye correspond respectively canonical. and macrocanonical. Gibbs distributions. This makes it possible to calculate thermodynamic potentials for model systems (ideal gas, ideal solution) based on the molecular constants characterizing the equilibrium nuclear configuration (internuclear distances, bond and torsion angles, vibration frequencies, etc.), which are m b. obtained from spectroscopic. and other data. It is possible to calculate thermodynamic potentials through the sum over states Z (integral over states). This approach makes it possible to establish a relationship between thermodynamic potentials and molecular constants of a substance. Calculating the sum (integral) Z for real systems is a very difficult task, usually statistical calculations are used to determine the thermodynamic potentials of ideal gases.
