Heat capacity, enthalpy and entropy. Second law of thermodynamics
Internal energy (U) of a substance consists of the kinetic and potential energy of all particles of the substance, except for the kinetic and potential energy of the substance as a whole. Internal energy depends on the nature of the substance, its mass, pressure, temperature. In chemical reactions, the difference in the internal energy of substances before and after the reaction results in the thermal effect of the chemical reaction. A distinction is made between the thermal effect of a chemical reaction carried out at a constant volume Q v (isochoric thermal effect) and the thermal effect of a reaction at constant pressure Q p (isobaric thermal effect).
The thermal effect at constant pressure, taken with the opposite sign, is called the change in the enthalpy of the reaction (ΔH = -Q p).
Enthalpy is related to internal energy H = U + pv, where p is pressure and v is volume.
Entropy (S)– a measure of disorder in a system. The entropy of a gas is greater than the entropy of a liquid and a solid. Entropy is the logarithm of the probability of the system’s existence (Boltzmann 1896): S = R ln W, where R is the universal gas constant, and W is the probability of the system’s existence (the number of microstates that can create a given macrostate). Entropy is measured in J/molּK and entropy units (1e.u. =1J/molּK).
Gibbs potential (G) or isobaric-isothermal potential. This function of the state of the system is called the driving force of a chemical reaction. Gibbs potential is related to enthalpy and entropy by the relation:
∆G = ∆H – T ∆S, where T is the temperature in K.
6.4 Laws of thermochemistry. Thermochemical calculations.
Hess's law(Herman Ivanovich Hess 1840): the thermal effect of a chemical reaction does not depend on the path along which the process occurs, but depends on the initial and final state of the system.
Lavoisier-Laplace law: the thermal effect of the forward reaction is equal to the thermal effect of the reverse reaction with the opposite sign.
Hess's law and its consequences are used to calculate changes in enthalpy, entropy, and Gibbs potential during chemical reactions:
∆H = ∑∆H 0 298 (cont.) - ∑∆H 0 298 (original)
∆S = ∑S 0 298 (cont.) - ∑S 0 298 (original)
∆G = ∑∆G 0 298 (cont.) - ∑∆G 0 298 (original)
Formulation of the corollary from Hess's law for calculating the change in enthalpy of a reaction: the change in enthalpy of a reaction is equal to the sum of the enthalpies of formation of the reaction products minus the sum of the enthalpies of formation of the starting substances, taking into account stoichiometry.
∆H 0 298 – standard enthalpy of formation (the amount of heat that is released or absorbed during the formation of 1 mole of a substance from simple substances under standard conditions). Standard conditions: pressure 101.3 kPa and temperature 25 0 C.
Berthelot-Thomsen principle: all spontaneous chemical reactions occur with a decrease in enthalpy. This principle works when low temperatures Oh. At high temperatures, reactions can occur with an increase in enthalpy.
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 energy level, preserved in its molecular structure. This means that although a substance may have energy based on , not all of it can be converted into heat. Part of internal energy always remains in the substance and maintains its molecular structure. Some of the substance is inaccessible when its temperature approaches the ambient temperature. Hence, enthalpy is the amount of energy that is available to be converted into heat at a certain temperature and pressure. Enthalpy units- British thermal unit or joule for energy and Btu/lbm or J/kg for specific energy.
Enthalpy quantity
Quantity enthalpy of matter based on its given temperature. This temperature- this 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 -150 °C, and methane- and ethane-based refrigerants -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 beneficial effect of the process.
Enthalpy often defined as total energy of matter, 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, instead of 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
Who knows what entropy and enthalpy are. and got the best answer
Answer from Vika[active]
Enthalpy and entropy
The change in free energy (ΔG) of a chemical reaction depends on a number of factors, including temperature and concentration of 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. A measure of the heat of reaction is the change in enthalpy ΔH, which corresponds to heat transfer at constant pressure. In the case of exothermic reactions, the system loses heat and ΔH is a negative value. In the case of endothermic reactions, the system absorbs heat and ΔH is a positive value.
For many chemical reactions, ΔG and ΔH have close values. This circumstance makes it possible to determine the energy value of food products. In living organisms, food is usually oxidized by 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 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 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 >
ΔG = 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, ΔH 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, 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 order (ΔS > 0) means that the -T · ΔS term in the equation has a minus sign. This compensates for ΔH and, in general, ΔG is a negative value. Such processes are usually called entropic.
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 =CaT=[guru]
Entropy (from the Greek ἐντροπία - rotation, transformation) is a concept first introduced in thermodynamics to determine the measure of irreversible energy dissipation. The term is widely used in other fields of knowledge: in statistical physics as a measure of the probability of the occurrence of any macroscopic state; in information theory as a measure of the uncertainty of any experience (test), which may have different outcomes, in historical science, to explicate the phenomenon of alternative history (invariance and variability of the historical process).
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Enthalpy of the system (from the Greek enthalpo I heat), a single-valued 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=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 of which in an equilibrium process is equal to the ratio of the amount of body imparted to the system or removed from it to the thermodynamic temperature of the system; non-equilibrium processes in an isolated system are accompanied by an increase in zntropy, they bring the system closer to an equilibrium state in which zntropy 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, entropy tends to its maximum value. Enthalpy is a single-valued function of the state of a thermodynamic system with independent parameters of entropy and pressure; it is associated with internal energy; this quantity 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 complex their answers are! Why complicate it, it can be said 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 there was still money. The less money is left before payday. , the higher, the greater the 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 temperature and concentration of reactants (see p. 24). This section discusses two more parameters that are associated with structural and energetic changes in molecules.
B. Enthalpy and entropy
The heat of reaction ΔH and the change in free energy ΔG do not always have comparable values. In fact, reactions are known to occur spontaneously (ΔG< 0) несмотря на то, что являются эндотермическими (ΔΗ >0). This happens because the reaction is influenced by a change in the degree of order of the system. A measure of the change in the order 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 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 > 0), it is necessary to expend energy. Both of these provisions follow from a 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 = ΔH - T ΔS
Answer from 2 answers[guru]
Hello! Here are other threads with similar questions.
Enthalpy vs 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 a daily phenomenon that we encounter in our lives, but those people who are not inquisitive enough never try to find the answers why such phenomena exist. Biologists, chemists and physicists are just a few people trying to find answers. Our modern world today integrated with such laws of science as thermodynamics. "Thermodynamics" is a branch of natural science that involves the study of the internal movements of body systems. This is a study of the relationship of heat to various forms of energy and work. Applications of thermodynamics are seen in the flow of electricity and from the simple turning and turning of screws and other simple machines. As long as heat and friction are involved, thermodynamics exists. 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. To create a thermodynamic system requires internal energy. This energy serves as the impetus or trigger for the creation of the system. The units of enthalpy are the joule (International System of Units) and the calorie (British thermal unit). "Enthalpy" is the Greek word "enthalpos" (to pour in heat). Heike Kamerlingh Onnes was the person who coined the word, while Alfred W. Porter was the one who designated the symbol "H" for "enthalpy". In biological, chemical, and physical measurements, enthalpy is the most preferred expression for changes in the energy of a system because it has the ability to simplify specific definitions of energy transfer. It is not possible to reach a value for the total enthalpy because the total enthalpy of the system cannot be directly measured. Only enthalpy change is the preferred measurement of quantity rather than the absolute value of enthalpy. In endothermic reactions there is a positive change in enthalpy, while in exothermic reactions there is a negative change in enthalpy. Simply put, the enthalpy of a system is equivalent to the sum of the non-mechanical work and the heat supplied. At constant pressure, enthalpy is equivalent to the change in the internal energy of the system and the work that the system exerted on its surroundings. In other words, heat can be absorbed or released by a certain chemical reaction under such conditions.
"Entropy" is the second law of thermodynamics. This is one of the most fundamental laws in the field of physics. This is important for understanding life and cognition. This is considered as the Law of Disorder. In the middle of the last century, "entropy" had already been formulated with extensive efforts by Clausius and Thomson. Clausius and Thomson were inspired by Carnot's observation of the flow that turns a mill wheel. 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 "S", which states that a world is said to be 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 symbol "H" and entropy takes the symbol "S".
Heike Kamerlingh Onnes coined the term "enthalpy" and Clausius coined the term "entropy".
Heat capacity and its types. Specific heat capacity With call the amount of heat d, which is required to change the temperature of a unit amount of a substance by one degree: With = d/bT, s = dg/dT.
Depending on the method of measuring the unit amount of a substance, the nature of the thermodynamic process and the size of the temperature interval, several types of heat capacities are distinguished.
1. Depending on the unit amount of the substance (1 kg, 1 m 3, 1 mol), the heat capacity is mass With[J/(kg-K)], volumetric With"[J/(m 3 - K)] and molar s [JDmol-K)].
The connection between them is expressed by the following relationship:
where pH is the density under normal physical conditions.
The amount of heat is accordingly determined by the formula
Where m- mass of gas, kg; U n- volume of gas reduced to normal physical conditions; P- number of moles of gas.
2. Heat capacity depends on the nature of the process and the properties of the gas. Depending on the method of heat supply, heat capacity at constant pressure (isobaric) is distinguished. with p and heat capacity at constant volume (isochoric) c v . The concepts of “heat capacity at constant temperature” and “adiabatic heat capacity” are rarely used, since when T= const With= d^/O = oo, and when dg= 0 s = O/d/ = 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 dependence is easy to understand. If to heat 1 mole (or 1 kg) of gas in a cylinder above the piston by one degree at a constant volume, i.e. with the piston fixed motionless, it is necessary to expend the amount of heat with you then, at constant pressure, work c/? will be added to this amount of heat? (or I), which will be performed by the expanding gas, 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 heat capacity changes with temperature, depending on the temperature range, true (c) and average (c t) specific heats are distinguished. The true heat capacity is the one corresponding to an infinitesimal temperature range: c = dq/dT, and the average is the heat capacity corresponding to the final temperature range: with t = q/(T 2- G)).
The dependence of heat capacity on temperature can be expressed by 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 influence of which at high temperatures characteristic of combustion products in heat engines (1000... 2000 °C) is insignificant. When calculating steam engines, turbines, and heat converters, the influence of pressure on 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 working fluid from 0 to /, or to / 2, will be
Here s^0 and with? 0 - tabular values of heat capacities in the temperature ranges (0.../]) and (0.../ 2).
The amount of heat required to heat 1 kg of body from t x to / 2, is defined as the difference:
Enthalpy. In some cases it turns out to be advisable to combine parameters And And pv into a common 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 p.v. caused by the presence of external environmental pressure R, those. pv there is work that must be expended to introduce a working fluid of volume v into a pressurized environment R.
For an ideal gas the following relations are valid:
At R= const can be obtained:
Having differentiated i - and + pv and substituting into the differential equation of the first law of thermodynamics for the flow of the working fluid, we 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 to within a certain constant, arbitrarily chosen for the reference point.
According to international agreement, the so-called enthalpy is taken as the starting point for water and water vapor. triple point (T = 273.16 K and p = 0.0006 Pa), in which the simultaneous existence of three phases is possible: ice, liquid and vapor. Temperature can be taken as the starting point for enthalpy for gases T- 0 K.
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 natural processes occurring spontaneously in it, the work of which can be used for human needs, is possible only in the absence of equilibrium between the thermodynamic system involved 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). When these processes occur, the thermodynamic system tends to come into equilibrium with the environment, characterized by equality of pressure and temperature of the system and the environment.
From observations of natural phenomena it also follows that in order to force the process to proceed in the direction opposite to the direction of the spontaneous process, it is necessary to expend 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 involved in the process and the environment.
- 2. Natural processes occurring spontaneously in nature, the work of which can be used by man, always flow in only one direction from a higher potential to a lower one.
- 3. The course of spontaneously occurring 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 direction opposite 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 resulted in 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 move 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 carry out a heat engine cycle without transferring a certain amount of heat from a heat source with a higher temperature to a source with a lower temperature.
This formulation should be understood as follows: in order for a periodically operating machine to work, it is necessary that there are at least two heat sources of different temperatures; in this case, only part of the heat taken from a high-temperature source can be converted into work, while the other part of the heat must be transferred to a low-temperature source. The high-temperature source is sometimes called a heat sink or top heat source, and the low-temperature source is sometimes called a heat sink, bottom heat source, or refrigerator.
Entropy. In thermodynamics, another parameter of the state of the working fluid 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 as
In this equation d q is not a complete differential, since the right side of the equation includes the term d/, which is not a complete differential, since work is not a parameter of the state of the gas, but a function of the process. As a result, the equation cannot be integrated over the interval of two arbitrarily chosen states of the gas.
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 integrating factor 1 /T(Where T - absolute temperature), the resulting equation will take the form
Equation (1.6) can be presented in a slightly different form, namely:
Expression (1.7) indicates that d q/T represents the total differential of some function s(i.e. d q/T= 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 the way the gas passed from one state to another. This gas state parameter is generally called gas entropy and is denoted by the letter S(J/C). Entropy per 1 kg of gas is called specific entropy gas and is designated by the letter s[J/(kg-K)].
The equation given earlier d q = di - vdp is also an incomplete differential equation, since d q is not a total differential. However, this equation, when multiplied by the integrating factor 1/7’, can be reduced to the form of a complete differential equation
Hence,
Considering that for an 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 a particular 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 constructed 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.
Rice. 1.5.
To determine the numerical values of entropy, use the origin at T = 0 K, for which i 0 = 0.
Physical meaning of entropy. Entropy cannot be measured, its meaning is difficult to demonstrate with the help of visual aids, but 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 = 0, when receiving the amount of heat from the heater q u3| = (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 lower the entropy s = (q/T), the more valuable the heat is, since it can be used not only to perform work, but also for technological needs - metal smelting, heating, etc.
- 2. Entropy is a measure of work loss due to the irreversibility of real processes. The more irreversible the process in an isolated system, the more 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 a certain measure of the disorder of the macrosystem - the disorder of the location and movement of particles D, then we can write s = k InZ).
Consequently, an increase in disorder means an increase in entropy, the dissipation of energy. When heat is supplied, the randomness of the thermal motion of particles increases, and entropy increases. Otherwise, cooling a system at a constant volume means extracting heat from it, and therefore entropy. In this case, the order of the system increases, and entropy decreases. When a gas condenses into a liquid, the molecules occupy more specific positions, the order of their arrangement increases abruptly, which corresponds to an abrupt decrease in entropy. With a further decrease in temperature, thermal motion becomes less and less intense, disorder becomes less and less, and therefore entropy becomes less and less. When the liquid turns into a solid, the molecules (ions) form regular crystal lattices, i.e., the disorder will decrease again, and with it the entropy will decrease, etc. This pattern allows us to assume that at an absolute temperature equal to zero, thermal motion will completely stop and maximum order will be established in the system, i.e. disorder and entropy will become zero. This assumption is consistent with experience, but cannot be verified experimentally (since an absolute temperature equal to zero is unattainable) and is called third law of thermodynamics.
Hence
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 therefore the specific volume at all its points do not change without external energy influence in time, is called equilibrium state.
A consistent change in the state of the working fluid that occurs as a result of the energy interaction of the working fluid with the environment is called thermodynamic process. The process during which a body sequentially passes through a continuous series of equilibrium states is called equilibrium.
Reversible process is called a thermodynamic process that allows it to flow through the same equilibrium states in both forward and reverse directions, and no changes remain in the environment.
If the specified condition is not met, then the process ends up 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 having a temperature of 400...500°C, since the reverse transfer of heat from steam to gases without changing their temperatures is impossible.
Reversible processes are not observed in their pure form in nature and technology. However, their study is of great importance, since many real processes are close to reversible.
