Unit of measure for standard enthalpy. Observation of enthalpy during the experiment

Atmospheric pressure usually acts as a constant pressure. Enthalpy, like internal energy, is a state function. Internal energy is the sum of the kinetic and potential energies of the entire system. It is the basis for the enthalpy equation. Enthalpy is the sum multiplied by the volume of the system and is equal to: H=U+pV, where p is the pressure in the system, V is the volume of the system. The above is used to calculate the enthalpy when all three quantities are given: pressure, volume and internal energy. However, enthalpy is not always calculated in this way. In addition to it, there are several more ways to calculate enthalpy.

Knowing the free energy and entropy, we can calculate enthalpy. Free energy, or Gibbs energy, is part of the enthalpy of the system, for the transformation into work, and is equal to the difference between enthalpy and temperature, multiplied by entropy: ΔG \u003d ΔH-TΔS (ΔH, ΔG, ΔS - increments) disorder of the particles of the system. It increases with increasing temperature T and pressure. At ΔG<0 процесс идет самопроизвольно, при ΔG>0 - does not work.

In addition, enthalpy is also calculated from the chemical equation. Given the equation chemical reaction of the form A+B=C, then enthalpy can be determined by the formula: dH \u003d dU + ΔnRT, where Δn \u003d nk-nн (nk and nн are the number of moles of reaction products and starting materials) isobaric process entropy is equal to the change in heat in the system: dq=dH. When constant pressure enthalpy is equal to: H=∫СpdTIf enthalpy and entropy balance each other, the enthalpy increment is equal to the product of temperature and entropy increment: ΔH=TΔS

Sources:

how to calculate entropy change in a reaction

To amount heat, received or given away by a substance, it is necessary to find its mass, as well as the change in temperature. According to the table of specific heat capacities, find this value for this material, and then calculate the amount of heat using the formula. To determine the amount of heat released during the combustion of fuel, you can find out its mass and specific heat combustion. The same situation with melting and evaporation.

You will need

To determine the amount of heat, take a calorimeter, thermometer, scales, tables of thermal properties of substances.

Instruction

Calculation of the amount given or received by the body. Measure the body weight on the scale in kilograms, then measure the temperature and heat it, limiting contact as much as possible external environment again measuring the temperature. To do this, use a thermally insulated vessel (calorimeter). In practice, this can be done as follows: take any body with room temperature, this will be its initial value. Then pour into the calorimeter hot water and put the body in there. After a while (not immediately, the body should warm up), measure the temperature of the water, it will be equal to the body temperature. In the table of specific heat, find this value for the material from which the body under study is made. Then the amount of heat that it is will be the product of the specific heat capacity by the mass of the body and its temperature (Q \u003d c m (t2-t1)). The result will be in joules. The temperature can be in degrees Celsius. If the amount of heat turned out to be positive, the body heats up, if it cools.

Calculation of the amount of heat during fuel combustion. Measure the mass of fuel that burns. If liquid, measure its volume and multiply by the density taken in a special table. Then, in the reference table, find the specific heat of combustion of this fuel and multiply by its mass. The result will be the amount of heat released during the combustion of fuel.

Calculation of the amount of heat during melting and vaporization. Measure the mass of the melting body, and specific heat melting for given substance from a special table. Multiply these values and get the amount absorbed by the body during melting. The same amount of heat is released by the body during crystallization.
To measure the amount of heat absorbed by a liquid, find its mass, as well as the specific heat of vaporization. The product of these quantities will give the amount of heat absorbed by a given liquid during evaporation. During condensation, exactly the same amount of heat is released that was absorbed during evaporation.

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 ( full formula given above).

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

AT technical thermodynamics enthalpy values are used, which are calculated from the conventionally accepted zero. All absolute values These quantities are very difficult to determine, 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 is the specific enthalpy, for the entire mass of the body, the total enthalpy is denoted by the letter I, according to world system 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, great amount 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) is 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 found important practical use when liquefying gases. Thus, enthalpy is the total energy of the expanded system, which is the sum of internal energy and external - potential energy 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.
AT general case the change in the energy of a thermodynamic system is not necessary condition to change 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.

Enthalpy is the energy inherent in a particular system that is in thermodynamic equilibrium with constant parameters (pressure and entropy).

Entropy is a characteristic of the orderliness of a thermodynamic system.

ENTHALPY(from 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. At constant p, the change in enthalpy is equal to the amount of heat supplied to the system, so enthalpy is often called the heat function or heat content. In the state of thermodynamic equilibrium (at constant p and S), the enthalpy of the system is minimal.

Entropy is a measure of disorder, a measure of homogeneity, a measure of confusion, and a measure of symmetry.

Few scientists understood this concept..........Usually, as it was figuratively said, this is a measure of the chaos of the system.....That is, it turns out that chaos can be ordered. That is, it allows you to distinguish reversible processes from irreversible ones ....... For reversible processes, the entropy is maximum and constant ...... and for irreversible ones it increases. I will give you one article ...... Thermodynamics is based on the difference between two types of processes - reversible and irreversible. A reversible process is a process that can go both in the forward and in the opposite direction, and when the system returns to its original state, no changes occur. Any other process is called irreversible. The laws of the classical mechanistic research program are reversible. With the advent of thermodynamics, the notion of the irreversibility of processes enters physics, which indicates the limits of applicability of the dynamic description of phenomena.

Entropy (Greek in and turn, transformation) is one of the main. concepts of classical physics, introduced into science by R. Clausius. With macroscopic t. sp. E. expresses the ability of energy to transform: the more E. of the system, the less the energy contained in it is capable of transformations. With the help of the concept of E., one of the fundamentals is formulated. physical laws - the law of increasing E., or the second law of thermodynamics, which determines the direction of energy transformations: in closed system E. cannot decrease. The achievement of a maximum E. characterizes the offensive equilibrium state, in which further energy transformations are no longer possible - all energy has turned into heat and the state has come thermal equilibrium.

Short review

Zero law

First law

It can also be defined as: the amount of heat supplied to an isolated system is expended in doing work and changing internal energy

Second law

third law

In short, entropy is postulated to be "temperature dependent" and leads to the formulation of the idea of absolute zero.

Fourth Law (provisional)

Any non-equilibrium system has such properties, called kinetic, which determine the features of the flow of non-equilibrium processes in the direction indicated by the second law of thermodynamics, and on which the thermodynamic forces driving these non-equilibrium processes do not depend.

Principles of thermodynamics

Zero start of thermodynamics

The zero law of thermodynamics is so named because it was formulated after the first and second laws were among the established scientific concepts. It states that an isolated thermodynamic system spontaneously enters a state of thermodynamic equilibrium over time and remains in it for an arbitrarily long time if the external conditions remain unchanged. It is also called common beginning. Thermodynamic equilibrium implies the presence of mechanical, thermal and chemical equilibrium in the system, as well as phase equilibrium. Classical thermodynamics postulates only the existence of a state of thermodynamic equilibrium, but says nothing about the time it takes to reach it.

In literature in zero start also often include statements about thermal equilibrium properties. Thermal equilibrium can exist between systems separated by an immovable heat-permeable partition, that is, a partition that allows systems to exchange internal energy, but does not let matter through. The postulate of transitivity of thermal equilibrium states that if two bodies separated by such a partition (diathermic) are in thermal equilibrium with each other, then any third body that is in thermal equilibrium with one of these bodies will also be in thermal equilibrium with the other body.

In other words, if two closed systems A and B brought into thermal contact with each other, then after reaching thermodynamic equilibrium by the complete system A+B systems A and B will be in thermal equilibrium with each other. However, each of the systems A and B itself is also in thermodynamic equilibrium. Then if the systems B and C are in thermal equilibrium, then the systems A and C are also in thermal equilibrium with each other.

In foreign and translated literature, the postulate itself about the transitivity of thermal equilibrium is often called the zero start, and the position on reaching thermodynamic equilibrium can be called the “minus first” start. The importance of the postulate of transitivity lies in the fact that it allows us to introduce some function of the state of the system, which has the properties empirical temperature, that is, to create devices for measuring temperature. The equality of empirical temperatures measured using such an instrument, a thermometer, is a condition for the thermal equilibrium of systems (or parts of the same system).

First law of thermodynamics

The first law of thermodynamics expresses the universal law of conservation of energy in relation to the problems of thermodynamics and excludes the possibility of creating a perpetual motion machine of the first kind, that is, a device capable of doing work without the corresponding expenditure of energy.

internal energy U A thermodynamic system can be changed in two ways, by doing work on it or by exchanging heat with the environment. The first law of thermodynamics states that the heat received by the system goes to increase the internal energy of the system and to perform work by this system, which can be written as δQ = δA + dU. Here dU- total differential internal energy of the system, δQ is the elementary amount of heat transferred to the system, and δA- infinitely small or elementary work done by the system. Since work and heat are not state functions, but depend on the way the system transitions from one state to another, the notation with the symbol is used δ to emphasize that δQ and δA are infinitesimal quantities that cannot be considered differentials of any function.

Signs at δQ and δA in the above ratio, they express an agreement that the work done by the system and the heat received by the system, accepted in the majority, are considered positive contemporary works on thermodynamics.

If the system performs only mechanical work due to a change in its volume, then elementary work is written as δA = P dV, where dV- increase in volume. In quasi-static processes, this work is equal to the work external forces over the system taken with the opposite sign: δA internal = –δA external, but for non-quasistatic processes this relation is not satisfied. In general, elementary work is written as the sum δA = A 1 da 1 + A 2 da 2 + ... , where A 1 ,A 2 , ... - functions of parameters a 1 ,a 2 , ... and temperature T, called generalized forces .

Work associated with a change in the amount of a substance in a system (chemical work) can be isolated from general expression to work in a separate term .

Second law of thermodynamics

The second law of thermodynamics sets limits on the direction of processes that can occur in thermodynamic systems, and excludes the possibility of creating a perpetual motion machine of the second kind. In fact, this result was already reached by Sadi Carnot in the essay “On driving force fire and about machines capable of developing this force. However, Carnot relied on the ideas of the theory of caloric and did not give a clear formulation of the second law of thermodynamics. This was done in 1850-1851 independently by Clausius and Kelvin. There are several different, but at the same time equivalent formulations of this law.

Kelvin's postulate: "A circular process is impossible, the only result of which would be the production of work by cooling the heat reservoir." Such a circular process is called the Thomson-Planck process, and it is postulated that such a process is impossible.

Postulate of Clausius: “Heat cannot spontaneously transfer from a body that is less heated to a body that is hotter.” The process in which no other change occurs, except for the transfer of heat from a cold body to a hot one, is called the Clausius process. The postulate states that such a process is impossible. Heat can transfer spontaneously in only one direction, from a more heated body to a less heated one, and such a process is irreversible.

Taking as a postulate the impossibility of the Thomson-Planck process, it can be proved that the Clausius process is impossible, and vice versa, from the impossibility of the Clausius process it follows that the Thomson-Planck process is also impossible.

The consequence of the second law of thermodynamics, postulated in these formulations, allows us to introduce for thermodynamic systems one more function of the thermodynamic state S, called entropy, such that its total differential for quasi-static processes is written as dS=δQ/T. In combination with temperature and internal energy, introduced in the zero and first principles, entropy constitutes a complete set of quantities necessary for the mathematical description of thermodynamic processes. Only two of the three quantities mentioned, with which thermodynamics adds to the list of variables used in physics, are independent.

Third law of thermodynamics

The third law of thermodynamics or the Nernst theorem states that the entropy of any equilibrium system, as the temperature approaches absolute zero, ceases to depend on any state parameters and tends to a certain limit. In fact, the content of the Nernst theorem includes two provisions. The first of them postulates the existence of an entropy limit as it tends to absolute zero. Numerical value this limit is assumed zero, therefore, in the literature it is sometimes said that the entropy of the system tends to zero as the temperature tends to 0 K. The second statement of the Nernst theorem states that all processes near absolute zero, transferring the system from one equilibrium state to another, occur without a change in entropy.

Zero values of temperature and entropy at absolute zero are accepted as convenient conventions for eliminating ambiguity in constructing a scale for thermodynamic quantities. The zero temperature value serves as a reference point for constructing a thermodynamic temperature scale. The entropy that vanishes at absolute zero temperature is called absolute entropy. In handbooks of thermodynamic quantities, absolute entropy values at a temperature of 298.15 K are often given, which correspond to an increase in entropy when a substance is heated from 0 K to 298.15 K.

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

Enthalpy- this is thermodynamic property substance that indicates energy level stored in his 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. A part of a substance is inaccessible when its temperature approaches the temperature environment. Consequently, 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 quantity

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 at various substances different. 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 positive number. Conversely, at a temperature below a given enthalpy of a substance is expressed 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 useful action process.

enthalpy often defined as the total energy of matter, since it is equal to the sum of its internal energy (u) in given state along with his ability to get the job done (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

What is the enthalpy of formation of substances? How to use this quantity in thermochemistry? In order to find answers to these questions, let us consider the basic terms associated with the thermal effect of a chemical interaction.

Thermal effect of the reaction

This is a value that characterizes the amount of heat released or absorbed during the interaction of substances.

If the process is carried out under standard conditions, the thermal effect is called the standard effect of the reaction. This is the standard enthalpy of formation of the reaction products.

Heat capacity of the process

it physical quantity, which determines the ratio of a small amount of heat to a change in temperature. The heat capacity units are J/K.

Specific heat is the amount of thermal energy required to raise the temperature by one degree Celsius for a body weighing one kilogram.

Thermochemical effect

For almost any chemical reaction, it is possible to calculate the amount of energy that is absorbed or released during the interaction of chemical components.

Exothermic transformations are such transformations, as a result of which a certain amount of heat is released into the atmosphere. For example, positive effect connection processes are characterized.

The enthalpy of the reaction is calculated taking into account the composition of the substance, as well as stereochemical coefficients. Endothermic interactions involve the absorption of some amount of heat in order to start a chemical reaction.

The standard enthalpy is a quantity used in thermochemistry.

Spontaneous flow of the process

In a thermodynamic system, a process proceeds spontaneously when there is a decrease in free energy interacting system. As a condition for achieving thermodynamic equilibrium, the minimum value of the thermodynamic potential is considered.

Only if the constants are kept in time external conditions, we can talk about the immutability of the interaction.

One of the sections of thermodynamics studies precisely the equilibrium states in which enthalpy is a value calculated for each individual process.

Chemical processes are reversible in those cases when they proceed simultaneously in two mutually reverse directions: reverse and forward. If a reverse process is observed in a closed system, then after a certain time interval the system will reach an equilibrium state. It is characterized by the cessation of changes in the concentration of all substances over time. Such a state does not mean a complete cessation of the reaction between the initial substances, since equilibrium is a dynamic process.

Enthalpy is a physical quantity that can be calculated for different chemical substances. Quantitative characteristic equilibrium process is the equilibrium constant, expressed through partial pressures, equilibrium concentrations, mole fractions interacting substances.

For anyone reversible process the equilibrium constant can be calculated. It depends on the temperature, as well as on the nature of the interacting components.

Consider an example of the emergence of an equilibrium state in the system. AT initial moment there is only time in the system starting materials A and B. The rate of the forward reaction has maximum value, and the reverse process does not occur. As the concentration of the initial components decreases, an increase in the rate of the reverse process is observed.

Considering that enthalpy is a physical quantity that can be calculated for the reactants, as well as for the products of the process, certain conclusions can be drawn.

After a certain time interval, the rate of the direct process is equal to the rate of the reverse interaction. The equilibrium constant is the ratio of the rate constants of the forward and reverse processes. physical meaning This value shows how many times the rate of the direct process exceeds the value of the reverse interaction at a certain concentration and temperature.

The influence of external factors on the kinetics of the process

Since enthalpy is a quantity that is used for thermodynamic calculations, there is a relationship between it and the process conditions. For example, the thermodynamic interaction is affected by concentration, pressure, temperature. When one of these values changes, the equilibrium shifts.

Enthalpy is thermodynamic potential, which characterizes the state of the system in equilibrium when choosing entropy, pressure, and the number of particles as independent variables.

Enthalpy characterizes the level of energy that is stored in its molecular structure. Therefore, if a substance has energy, it is not in in full is converted into heat. Part of it is stored directly in the substance, it is necessary for the functioning of the substance at a certain pressure and temperature.

Conclusion

Enthalpy change is a measure of the heat of a chemical reaction. It characterizes the amount of energy that is necessary for heat transfer at a constant pressure. This value is used in situations where pressure and temperature will be constant values in the process.

Enthalpy is often characterized in terms of the total energy of a substance, since it is defined as the sum of the internal energy and the work done by the system.

In reality, this value acts as total energy, which characterizes the energy indicators of a substance that is converted into heat.

This term was proposed by H. Kamerling-Onnes. When carrying out thermodynamic calculations in inorganic chemistry, the amount of substance must be taken into account. Calculations are carried out at a temperature corresponding to 298 K, a pressure of 101 kPa.

Hess's law, which is the main parameter for modern thermochemistry, makes it possible to determine the possibility spontaneous flow chemical process, calculate its thermal effect.

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