Internal energy (U) of a substance is made up 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 values ​​of 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 a 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 reaction (ΔH = -Q p).

Enthalpy is related to internal energy H = U + pv, where p is pressure and v is volume.

Entropy (S) is a measure of disorder in the 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 existence of the system (Boltzmann 1896): S = R ln W, where R is the universal gas constant, and W is the probability of the existence of the system (the number of microstates that can be used to realize 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 The laws of thermochemistry. thermochemical calculations.

Hess' law(German Ivanovich Hess 1840): the thermal effect of a chemical reaction does not depend on the path along which the process takes place, 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, 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)

The formulation of the consequence of the Hess law for calculating the change in the enthalpy of reaction: the change in the enthalpy of the 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 at low temperatures. At high temperatures, reactions can proceed with an increase in enthalpy.

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. The 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 ΔН is a positive value.
For many chemical reactions, ΔG and ΔН have similar values. This circumstance allows you 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 do 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 raises 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 entropy of the system is the higher, the greater the degree of disorder (disorder) 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 likely process), ΔS is a positive value. To increase the degree of order in the system (ΔS >
∆G = ∆H - T ∆S
Let us explain the dependence of these three quantities on 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. it 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 sodium chloride (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, as 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 order (ΔS > 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]

Hey! 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 dissipation of energy. The term is widely used in other areas of knowledge: in statistical physics as a measure of the probability of the implementation 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, 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), a single-valued function H of the state of a thermodynamic system with independent entropy parameters S and pressure P, is related to 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 in which in an equilibrium process is equal to the ratio of the amount of body supplied 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 entropy, they bring the system closer to the equilibrium state in which 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 its maximum value. Enthalpy is a single-valued function of the state of a thermodynamic system with independent entropy parameters and pressure, is 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, you 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 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 energy changes in molecules.
B. Enthalpy and entropy
The heat of reaction ΔН and the change in free energy ΔG are not always comparable. In fact, spontaneous reactions are known (ΔG< 0) несмотря на то, что являются эндотермическими (ΔΗ >0). This is because the reaction is affected by a change in the degree of order in the system. The change in the entropy ΔS serves as a measure of the change in the order of the system.
The entropy of the system is the higher, the greater the degree of disorder (disorder) 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 likely 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 the fundamental law of nature - the second law of thermodynamics. Quantitatively, the dependence between changes in enthalpy, entropy and free energy is described by the Gibbs-Helmholtz equation:
∆G = ∆H - T ∆S

Answer from 2 answers[guru]

Hey! Here are other threads with similar questions.

When working with any calculations, calculations and forecasting various phenomena related to heat engineering, everyone is faced with the concept of enthalpy. But for people whose specialty does not concern thermal power engineering or who only superficially encounter such terms, the word "enthalpy" will inspire fear and horror. So, let's see if everything is really so scary and incomprehensible?

If we try to say it quite simply, the term enthalpy refers to the energy that is available for conversion into heat at a certain constant pressure. The term enthalpy in Greek means "I heat". That is, the formula containing the elementary sum of internal energy and the work done is called enthalpy. This value is denoted by the letter i.

If we write the above in physical quantities, transform and derive the formula, then we get i = u + pv (where u is the internal energy; p, u are the pressure and specific volume of the working fluid in the same state for which the value of internal energy is taken). Enthalpy is an additive function, that is, the enthalpy of the entire system is equal to the sum of all its constituent parts.

The term "enthalpy" is complex and multifaceted.

But if you try to understand it, then everything will go very simply and clearly.

• First, in order to understand what enthalpy is, it is worth knowing the general definition, which we did.
• Secondly, it is worth finding the mechanism for the appearance of this physical unit, to understand where it came from.
• Thirdly, you need to find a connection with other physical units that are inextricably interconnected with them.
• And finally, fourthly, you need to look at the examples and the formula.

Well, well, the mechanism of work is clear. You just need to carefully read and understand. We have already dealt with the term "Enthalpy", we have also given its formula. But another question immediately arises: where did this formula come from and why is entropy associated, for example, with internal energy and pressure?

Essence and meaning

In order to try to figure out the physical meaning of the concept of "enthalpy" you need to know the first law of thermodynamics:

energy does not disappear into nowhere and does not arise from nothing, but only passes from one form to another in equal quantities. Such an example is the transition of heat (thermal energy) into mechanical energy, and vice versa.

We need to transform the equation of the first law of thermodynamics into the form dq = du + pdv = du + pdv + vdp - vdp = d(u + pv) - vdp. From here we see the expression (u + pv). It is this expression that is called enthalpy (the full formula was given above).

Enthalpy is also a quantity of state, because the components u (voltage) and p (pressure), v (specific volume) have certain values ​​for each quantity. Knowing this, the first law of thermodynamics can be rewritten in the form: dq = di - vdp.

In technical thermodynamics, enthalpy values ​​are used, which are calculated from the conventionally accepted zero. It is very difficult to determine all the absolute values ​​of these quantities, since for this it is necessary to take into account all the components of the internal energy of a substance when its state changes from O to K.

The formula and values ​​​​of enthalpy were given in 1909 by the scientist G. Kamerling-Onnes.

In the expression i - specific enthalpy, for the entire body mass, the total enthalpy is denoted by the letter I, according to the world system of units, enthalpy is measured in Joules per kilogram and is calculated as:

Functions

Enthalpy ("E") is one of the auxiliary functions, thanks to which the thermodynamic calculation can be greatly simplified. For example, a huge number of heat supply processes in thermal power engineering (in steam boilers or the combustion chamber of gas turbines and jet engines, as well as in heat exchangers) are carried out at constant pressure. For this reason, enthalpy values ​​are usually given in tables of thermodynamic properties.

The enthalpy conservation condition underlies, in particular, the Joule-Thomson theory. Or an effect that has found important practical application in the liquefaction of gases. Thus, enthalpy is the total energy of the expanded system, which is the sum of internal energy and external - the potential energy of pressure. Like any state parameter, enthalpy can be defined by any pair of independent state parameters.

Also, based on the above formulas, we can say: "E" of a chemical reaction is equal to the sum of the enthalpies of combustion of the starting substances minus the sum of the enthalpies of combustion of the reaction products.
In the general case, a change in the energy of a thermodynamic system is not a necessary condition for a change in the entropy of this system.

So, here we have analyzed the concept of "enthalpy". It is worth noting that "E" is inextricably linked with entropy, which you can also read about later.

Entropy

Entropy (from the Greek ?nfsprYab - 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 areas of knowledge: in statistical physics as a measure of the probability of the implementation 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, for the explication of the phenomenon of alternative history (invariance and variability of the historical process).

Entropy is a function of the state of the system, equal in an equilibrium process to the amount of heat communicated to the system or removed from the system.

Entropy is the connection between macro and micro states, the only function in physics that shows the direction of processes. A system state function that does not depend on the transition from one state to another, but depends only on the initial and final positions of the system.

Enthalpy

Enthalpy, also heat function and heat content, is a thermodynamic potential that characterizes the state of a system in thermodynamic equilibrium when pressure, entropy, and the number of particles are chosen as independent variables. If a thermomechanical system is considered as consisting of a macrobody (gas) and a piston with a weight P = p S, which balances the gas pressure p inside the vessel, then such a system is called expanded. The enthalpy or energy of the expanded system E is equal to the sum of the internal energy of the gas U and the potential energy of the piston with the load Epot = pSx = pV

H \u003d E \u003d U + pV Thus, the enthalpy in this state is the sum of the internal energy of the body and the work that must be expended in order to introduce a body of volume V into an environment that has pressure p and is in equilibrium with the body. The enthalpy of the system H - similarly to the internal energy and other thermodynamic potentials - has a well-defined value for each state, i.e., it is a function of the state. Therefore, in the process of changing the state, DH = H2 ? H1The change in enthalpy does not depend on the path of the process, being determined only by the initial and final state of the system. If the system somehow returns to its original state (circular process).

Enthalpy is a property of matter that indicates the amount of energy that can be converted into heat.

Enthalpy is a thermodynamic property of a substance that indicates energy level stored in its molecular structure. This means that although matter can have energy based on , not all of it can be converted into heat. Part of internal energy always remains in matter and maintains its molecular structure. Part of the substance is inaccessible when its temperature approaches the ambient temperature. Hence, enthalpy is the amount of energy that is available for conversion into heat at a given temperature and pressure. Enthalpy units- British thermal unit or joule for energy and Btu/lbm or J/kg for specific energy.

Enthalpy amount

Quantity enthalpies of matter based on its given temperature. Given temperature is the value chosen by scientists and engineers as the basis for calculations. This 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 above its given temperature, or changes state to gaseous at a given temperature, the enthalpy is expressed as a positive number. Conversely, at a temperature below a given 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 the 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, more precisely define it as the total amount of available energy of a substance that can be converted into heat.
H=U+pV