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
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 a closed system, E. cannot decrease. The achievement of the maximum E. characterizes the onset of an equilibrium state, in which further energy transformations are no longer possible - all energy has turned into heat and a state of thermal equilibrium has come.
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 the literature, the zeroth beginning also often includes statements about the properties of thermal equilibrium. 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 is the total differential of the 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 relation, they express an agreement that the work done by the system and the heat received by the system, accepted in most modern works on thermodynamics, are considered positive.
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 of external forces on 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 separated from the general expression for work into 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 his essay “On the driving force of fire and on 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. The numerical value of this limit is usually assumed to be equal to 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 proposition of the Nernst theorem states that all processes near absolute zero that transfer the system from one equilibrium state to the other, 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.
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
This is a physical quantity that determines the ratio of a small amount of heat to a change in temperature. The heat capacity units are J/K.
The specific heat capacity is the amount of heat energy required to increase the temperature by one degree Celsius for a body having a mass of one kilogram.
Thermochemical effect
For almost any chemical reaction, you can 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, joining processes are characterized by a positive effect.
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 the free energy of the interacting system. As a condition for achieving thermodynamic equilibrium, the minimum value of the thermodynamic potential is considered.
Only under the condition of maintaining constant external conditions in time, we can talk about the invariance 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 chemicals. The quantitative characteristic of an equilibrium process is the equilibrium constant expressed in terms of partial pressures, equilibrium concentrations, and mole fractions of interacting substances.
For any 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 the initial moment of time, there are only initial substances A and B in the system. The rate of the forward reaction has a maximum value, and the reverse process does not proceed. 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. The physical meaning of 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 a thermodynamic potential that characterizes the state of a system in equilibrium when selected as independent variables of entropy, pressure, and the number of particles.
Enthalpy characterizes the level of energy that is stored in its molecular structure. Therefore, if a substance has energy, it is not fully 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 the total amount of 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 a 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 of spontaneous occurrence of a chemical process and to calculate its thermal effect.
Which I wrote about in this article, heat power engineering rarely comes across, then the term enthalpy, which will be discussed in the article, is much more often used in practice.
So what is enthalpy? To put it simply, enthalpy is the energy that is available for conversion into heat at a certain constant pressure. When I studied at the university, I remember a teacher telling us that enthalpy can be conditionally called heat content, since at constant pressure the change in enthalpy is equal to the amount of heat supplied to the system.
And in general, the term enthalpy itself is made up of the ancient Greek words - heat and prefixes - c. This combination of words can be understood as "heat". And for the first time this term was introduced into thermodynamics by the scientist D. Gibbs. Well, this is to make it clearer, since enthalpy, by the way, as well as entropy, cannot be measured directly, such as pressure or temperature. Enthalpy is determined only by calculation. That is, figuratively speaking, it cannot be “touched”, “touched”.
Let's consider in more detail. The value of the enthalpy of a substance is determined from the expression:
i = u + pu,
where u is the internal energy; p, u - pressure and specific volume of the working fluid in the same state, for which the value of internal energy is taken.
That is, we can say that the enthalpy of any thermodynamic system is the sum of the internal energy of the system and the potential energy of the source of external pressure.
Enthalpy is found as the sum of quantities that are determined by the state of the substance, is a function of the state and is measured in J / kg. More often, enthalpy in an off-system measurement system is measured in kcal/kg. Enthalpy is one of the auxiliary functions, the use of which makes it possible to significantly simplify thermodynamic calculations. For example, a huge number of heat supply processes in thermal power engineering (in steam boilers, combustion chambers of gas turbines and jet engines, heat exchangers) are carried out at constant pressure. For this reason, enthalpy values are usually given in tables of thermodynamic properties.
In technical thermodynamics, enthalpy values are used, which are counted from the conventionally accepted zero. The absolute values of 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 0 K. In tables and diagrams, i and s values \u200b\u200bare often given, which are measured from 0 ° C.
In conclusion, we can say that enthalpy, like internal energy and other thermodynamic parameters, has a well-defined value for each state, that is, it is a function of the state of the working fluid.
