Definitions of enthalpy and entropy

Tch, what a way to deceive people; go read your own book on physical chemistry instead.

Enthalpy: The sum of a system’s thermodynamic energy and the product of pressure and volume. Entropy: the ratio of the reversible heat exchanged between a system and its environment to the temperature of the system.

I’m sorry; I really didn’t know, that’s why I asked. I brought up this issue during some operations a few days ago, and that’s why I inquired. If you know the answer, could you please tell me in detail? Thank you

For professionals, this is like knowing what 1+1 equals. If it’s from another field, it can be easily found through searching

In the Tianjin University edition of \"Physical Chemistry\", on page P46, the enthalpy of a system is equal to the sum of the system’s thermodynamic energy U and the product of the system’s pressure and volume. Definition: H = U + PV. Definition of entropy: dS = δQ/r/T. S is a state function and is referred to as entropy. δQr is the reversible heat exchanged between the system and the environment, and T is the temperature of the system.

Enthalpy: H=U+PV. Entropy: dS=δQ/dT. Both are state functions, and this is explained in great detail in physical chemistry; you can refer to any textbook on physical chemistry of a given phase to study it carefully.

Definition of enthalpy: H = U + PV. Definition of entropy: dS = δQ/r/T. Enthalpy is a function introduced for the convenience of calculation, while entropy represents the degree of disorder in a system.

Key concepts for understanding enthalpy and entropy: 1. Under certain conditions, such as specific temperature and pressure conditions, a reaction can proceed automatically on its own without the need for external forces, such as the influence of energy sources like light or electricity; such reactions are known as spontaneous reactions； Just because a reaction can proceed spontaneously does not mean it will necessarily occur. 2. Enthalpy and entropy are two factors that influence the direction of a reaction; enthalpy is considered from an energy perspective, while entropy is considered in terms of disorder and order. 3. The combination of the entropy factor and the enthalpy factor gives rise to free energy, which is a measure of the direction in which chemical changes occur in a substance. 4. From the perspective of middle school teaching, it is better to consider reversibility and irreversibility as a matter of direction. A reversible reaction is one that can proceed in both the forward and reverse directions ； An irreversible reaction is one that can only proceed in one direction. Enthalpy is a state function that has the dimension of energy, but it does not possess a precise physical meaning. Its definition is given by H = U + pV. We cannot determine the exact enthalpy value of a substance; instead, we can only know the change in enthalpy during a physical or chemical process. The enthalpy change is an inevitable product of chemical reactions. Entropy, as mentioned above; the definition of entropy change is given by the formula ΔS=ΔQ/T. In simple terms, the value of entropy indicates the degree of disorder in a system, and entropy increase is the driving force behind chemical reactions. Enthalpy: fire + contain, the energy contained within. Entropy: fire + quotient, the quotient of energy and temperature. Chinese people’s mastery of Chinese characters is truly as if blessed by the gods. :D

Generally, what we use are the differences in enthalpy or entropy. With different benchmarks for such things, the values also vary.

Enthalpy is energy, while entropy represents the degree of disorder and indirectly indicates energy

Simply put: Enthalpy represents the latent heat of a substance, while entropy indicates the degree of disorder in the state of that substance

Enthalpy is referred to in English as “enthalpy”. Before introducing enthalpy, we need to understand molecular thermal motion, thermal energy, and the first law of thermodynamics. In 1827, the British botanist Brown placed extremely fine pollen particles on the surface of water and observed them under a microscope; he found that the pollen particles were constantly moving on the water’s surface, with highly irregular movement patterns. At first, people thought it was caused by external factors such as vibration or fluid convection, but experiments later proved that the cause of this movement lies within the liquid itself, not in the external environment. It turns out that the movement of pollen on the water surface is caused by collisions with water molecules from all directions. Thus, this type of motion is called Brownian motion, and Brownian motion shows that liquid molecules are in constant random motion. It can be observed from the experiments that Brownian motion becomes more intense as the temperature rises. This means that the random motion of molecules is related to temperature; the higher the temperature, the more intense the random motion of the molecules. It is precisely because the random motion of molecules is related to temperature that this kind of motion of molecules is commonly referred to as thermal motion of molecules. 　　In thermology, molecules, atoms, and ions follow the same laws when undergoing thermal motion; therefore, they are collectively referred to as molecules. 　　Since the molecules that make up an object are in constant random motion, then, like all moving objects, the molecules undergoing thermal motion also possess kinetic energy. The motion of individual molecules (their speed and direction) is random, but as a whole, under certain conditions, a large number of molecules follow certain statistical laws. The macroscopic quantity related to thermal motion – temperature – is the statistical average of the thermal motion of these many molecules. Molecular kinetic energy is related to temperature; the higher the temperature, the greater the average kinetic energy of the molecules, and vice versa. Therefore, from the perspective of molecular kinetic theory, temperature is a measure of the average kinetic energy of the thermal motion of the molecules in a substance (i.e., its microscopic meaning; on a macroscopic level, it indicates the degree of hotness or coldness of the substance). 　　There are intermolecular forces at play, namely what is referred to in chemistry as intermolecular forces (van der Waals forces). Intermolecular forces are the resultant of molecular attraction and molecular repulsion; there exists a distance r0 at which the attraction equals the repulsion, and at this point the intermolecular force is zero. Both molecular attraction and molecular repulsion increase as the distance between molecules decreases, but the repulsion changes more significantly; therefore, when the distance between molecules is greater than r0, attraction occurs, while when it is less than r0, repulsion occurs. Because of the intermolecular forces, there is a potential energy between molecules that is determined by their relative positions; this is known as molecular potential energy. The change in molecular potential energy is similar to that in spring elastic potential energy. When the volume of an object changes, the distance between molecules also changes; therefore, the molecular potential energy is related to the volume of the object. 　　The sum of the kinetic energy of thermal motion of all molecules in an object and their potential energy is called the thermodynamic energy of that object, also known as internal energy. Thermal energy, like kinetic energy and potential energy, is a state quantity of an object. 　　In junior high school, we learned that there are two ways to change the internal energy of an object: doing work and heat transfer. 　　If an object does not exchange heat with its surroundings, that is, it neither absorbs nor releases heat, then the work done on it by the surroundings is equal to the increase in its thermal energy: ΔU1 = W. If the object does work on its surroundings, then W is negative, and the increase in thermal energy ΔU1 is also negative, indicating a decrease in thermal energy. 　　If no work is done on the object by the external environment, and the object does no work on the external environment either, then the heat absorbed by the object is equal to the increase in its thermal energy: ΔU2 = Q. If the object releases heat, then Q is negative, and the increase in thermal energy ΔU2 is also negative, indicating a decrease in thermal energy. 　　Under normal circumstances, if an object undergoes both work and heat transfer with its surroundings, then the increase in the object’s thermal energy is equal to the work done on the object by the surroundings plus the heat absorbed by the object from those surroundings. That is: ΔU = ΔU1 + ΔU2 = Q + W. Since thermal energy U is a state function, it follows that: ΔU = U_final – U_initial = Q + W. This equation represents the first law of thermodynamics. 　　Chemical reactions all take place under certain conditions, among which constant volume and constant pressure are the most common and important. 　　A chemical reaction in a closed container is a constant-volume process. Since the volume of the system remains constant and only volume work is done (that is, work is done on the system by changing its volume, thereby altering its internal energy; for example, if a match head is placed in a syringe, the tip of the syringe is blocked and the piston is compressed, the match head will burn), W=0. Substituting this into the expression for the law of heat conservation gives: ΔU=Q. This shows that the heat absorbed in a constant-volume process equals the change in the system’s thermal energy. In other words, as long as it is known that the process takes place at constant volume and only volume work is done, Q depends solely on the initial and final states of the system. 　　A chemical reaction carried out in an open container is a constant-pressure process. The so-called transverse pressure means that the pressure p in the system is equal to the external pressure p_ext, and remains constant, that is, p = p_ext = constant. Since the process is at constant pressure and only volume work is done, we have:
W = W_volume = -p_ext (V2 – V1) = -(p2V2 – p1V1)
Here, W represents the work done on the system by the external environment; therefore, the work done by the system on the outside world is negative. The product of pressure and the change in volume is the work done by the system on its surroundings; this can be understood using the formulas p=F/S and V=Sh, hence Fh=pV. 　　Substituting this into the expression for the law of heat conservation gives: Q = ΔU – W = U2 – U1 + (p2V2 – p1V1) = (U2 + p2V2) – (U1 + p1V1). Since U + pV is a combination of state functions (that is, state quantities; in other words, for a given state there is only one value for thermal energy U, along with external pressure p and volume V), it is defined as a new state function – enthalpy – denoted by the symbol H. Thus, the above equation can be rewritten as: Q = H2 – H1 = ΔH. This shows that the heat exchanged during a constant-pressure process equals the change in the system’s enthalpy; in other words, as long as it is known that the process occurs at constant pressure and only volume work is done, Q depends solely on the initial and final states of the system. 　　The physical meaning of enthalpy can be understood as, under the special conditions of constant pressure and only volume work being done, Q=ΔH, that is, the change in heat during a reaction. Because only under these conditions does enthalpy exhibit its properties. For example, when a substance is heated at constant pressure, it absorbs heat and its temperature rises; ΔH>0, so the enthalpy of the substance at high temperatures is greater than its enthalpy at low temperatures. Again, for an exothermic chemical reaction at constant pressure, ΔH

Entropy Introduction to entropy: A physical term representing the quotient of heat divided by temperature; it indicates the degree to which heat is converted into work. Physical meaning: It signifies the level of disorder in the microscopic thermal motion of matter. 　　One of the parameters in thermodynamics that characterize the state of a substance, usually denoted by the symbol S. In classical thermodynamics, it can be defined as an increment: dS = (dQ/T), where T is the thermodynamic temperature of the substance ； dQ is the heat added to the substance during the entropy increase process. The subscript “reversible” indicates that the change process induced by the heating process is reversible. If the process is irreversible, then dS > (dQ/T), indicating irreversibility. The entropy per unit mass of a substance is called specific entropy, denoted as s. Entropy was initially introduced as a parameter of the state of matter that reflects the irreversibility of spontaneous processes, based on the second law of thermodynamics. The second law of thermodynamics is a principle derived from numerous observations, and it can be expressed as follows: ① Heat always flows from hotter objects to colder objects; it is impossible for heat to flow in the opposite direction without causing some other change ； ②Work can be completely converted into heat, but no heat engine can convert all the heat it receives into work in a complete and continuous manner (that is, a second kind of perpetual motion machine cannot be created) ； ③In an isolated system, the actual processes that take place always result in an increase in the entropy of the entire system; this is known as the principle of entropy increase. Friction causes a portion of mechanical energy to be irreversibly converted into heat, increasing entropy. The heat dQ is transferred from the object at higher temperature (T1) to the object at lower temperature (T2). The entropy of the object at higher temperature decreases by dS1 = dQ/T1, while the entropy of the object at lower temperature increases by dS2 = dQ/T2. When these two objects are considered as a single system, the change in entropy is dS = dS2 – dS1, which is greater than 0; thus, entropy increases. 　　◎ In physics, it refers to the quotient of thermal energy divided by temperature, indicating the degree to which heat is converted into work. 　　◎ In science and technology, it generally refers to a measure of the state of certain material systems, or the extent to which certain states of such systems may occur. It is also used in social sciences as a metaphor for certain states of human society. 　　◎ In information theory, entropy represents a measure of uncertainty. 　　Energy can be converted into work only when the energy density varies within that specific system; in such cases, energy tends to flow from areas of higher density to those of lower density until everything becomes uniform. It is through this flow of energy that you can obtain work from it. 　　The water level at the sources of rivers is relatively high, and the potential energy of the water there is also greater than that of the water at the river mouths. For this reason, water flows down rivers into the ocean. If it didn’t rain, all the water on the continent would flow into the oceans, causing the sea level to rise slightly. The total potential energy remains constant at this point. But it is distributed fairly evenly. 　　It is when the water flows downward that it can make the water wheel rotate, thereby allowing the water to do work. Water at the same level cannot do work; even if it is located on a very high plateau and thus possesses abnormally high potential energy, it still cannot do work. What plays a decisive role here is the difference in energy density and the flow toward homogenization. 　　Entropy is a measure of chaos and disorder. The higher the entropy value, the greater the degree of chaos and disorder. Our universe is a universe of increasing entropy. The second law of thermodynamics embodies this characteristic. Life is a high degree of order, and wisdom is also a high degree of order. Why does life exist in a universe where entropy is increasing? Will intelligence evolve? (Negative entropy). The second law of thermodynamics also reveals that local order is possible, but only at the expense of greater disorder elsewhere. To survive, life needs energy and food, at the cost of the death of plants and animals (entropy increase). All things grow thanks to the sun. The order in animals and plants is also achieved at the cost of the depletion of nuclear reactions in the sun (entropy increase) or other forms of entropy increase. A person trapped in a completely sealed lead box is unable to maintain their negative entropy through the entropy increase in other places. In this relatively closed system, the law of entropy increase disrupts the order of life. Entropy is the arrow of time, which is irreversible in this universe. Entropy is closely related to time. If time stops “flowing,” entropy increase becomes meaningless. “Anything that any known substance can ‘contain’ is nothing other than ‘time’. What is trapped by low temperatures is also “time”. Life is an ordered “structure” of matter. “\"Structure\" and specific materials are concepts at different levels. Just as the building materials of a building and the design of the building are not concepts at the same level. Biology has proven that in anyone who is older, not a single atom in their body is the same one they had at birth. But you are still you, I am still me, and life goes on. On the other hand, in dead people, since metabolism ceases, the molecules in their bodies can remain intact for a long time. Consciousness is a higher level of order than life, and it can be transmitted between lives. Having said this, I think the hierarchical relationship between matter and consciousness should be fairly clear now. (Excerpt from People’s Net BBS forum) This is true for any type of energy. In a steam engine, there is a heat reservoir that turns water into steam, and a cold reservoir that condenses the steam back into water. It is this temperature difference that plays a decisive role. It is impossible to obtain any work at any single, uniform temperature—no matter how high that temperature may be. 　　“\"Entropy\" is a term coined in 1850 by the German physicist Rudolf Clausius (1822–1888) to denote the degree of uniformity with which any form of energy is distributed in space. The more uniform the energy distribution, the greater the entropy. If, for the system we are considering, energy is distributed perfectly evenly, then the entropy of this system reaches its maximum value. 　　According to Clausius, in a system, if it is allowed to develop naturally, the energy difference always tends to be eliminated. When a hot object is brought into contact with a cold object, heat flows in the manner described below: the hot object cools down, while the cold object warms up, until both objects reach the same temperature. If two reservoirs are connected, and the water level in one of them is higher than that in the other, gravity will cause the water level in one reservoir to drop while raising the water level in the other, until the water levels in both reservoirs are equal and their potential energies are also equal. 　　Therefore, Clausius said that a universal law in nature is that differences in energy density tend to even out. In other words, “entropy increases over time.” 　　In past research on the flow of energy from areas with higher density to those with lower density, focus has mainly been on heat as a form of energy. Therefore, the science concerning energy flow and work-energy conversion is called “thermodynamics,” derived from the Greek word for “heat motion.” 　　It has long been established that energy cannot be created nor destroyed. This is one of the most fundamental laws ； That’s why people call it the “first law of thermodynamics”. 　　The statement put forward by Clausius that entropy increases over time seems to be another fundamental universal law, which is why it is known as the \"second law of thermodynamics\". 　　Describes one of the important state functions of a thermodynamic system. The magnitude of entropy reflects the stability of the system’s state, while changes in entropy indicate the direction in which a thermodynamic process proceeds. Entropy provides a quantitative expression for the second law of thermodynamics. 　　To quantitatively express the second law of thermodynamics, a state function that remains constant in reversible processes and changes monotonically in irreversible processes should be sought. While studying the Carnot heat engine, Clausius derived a formula that applies to any cyclic process, based on the Carnot theorem. In this formula, Q represents the small amount of heat absorbed by the system from a heat source at temperature T; the equal sign and the inequality sign correspond respectively to reversible and irreversible processes. The reversibility of a cycle indicates the existence of a state function, entropy, which can be defined by another equation (see relevant literature). 　　For an adiabatic process, Q=0; hence S≥0. That is, the entropy of a system remains constant during a reversible adiabatic process, and increases monotonically during an irreversible adiabatic process. This is the principle of entropy increase. Since all changes within an isolated system are independent of the outside world, it must be an adiabatic process; therefore, the principle of entropy increase can also be stated as follows: the entropy of an isolated system never decreases. It shows that as an isolated system moves from a non-equilibrium state to an equilibrium state, its entropy increases monotonically, and when the system reaches equilibrium, the entropy attains its maximum value. The change and maximum value of entropy determine the direction and limits of the processes occurring in an isolated system; the principle of entropy increase is nothing other than the second law of thermodynamics. 　　Energy is a measure of the motion of matter; it comes in various forms and can be converted into one another. The greater the amount of a certain form of energy, such as internal energy, the greater the potential for conversion. The original meaning of entropy is transformation; it describes the direction in which internal energy converts spontaneously into other forms of energy, as well as the extent to which this conversion takes place. As the conversion proceeds, the system approaches an equilibrium state, and the entropy value increases. This indicates that although the total amount of energy remains constant throughout this process, the amount of energy that is available for use or conversion decreases over time. Internal energy, entropy, and the first and second laws of thermodynamics provide a comprehensive understanding of the fundamental characteristics of energy conversion processes associated with thermal motion. 　　On a microscopic level, entropy is a measure of the disorder of the numerous microscopic particles that make up a system; the more disordered and chaotic the system, the greater its entropy. The microscopic essence and statistical meaning of the irreversibility of thermodynamic processes is that the system moves from order to disorder, from states with lower probabilities to states with higher probabilities. 　　The reason for this phenomenon is also simple: in nature, there are far more ways to move toward disorder than to move toward order. For example, it takes some effort to get a group of students to line up properly on the playground, but it’s easy for them to run around randomly there. 　　Entropy in information theory: a unit of measurement for information. Shannon, the founder of information theory, proposed a measure of information based on probabilistic statistical models in his book \"A Mathematical Theory of Communication\". He defined information as “something used to eliminate uncertainty.” 　　Shannon formula: I(A) = -log P(A). I(A) measures the amount of information provided by the occurrence of event A; it is known as the entropy of event A, while P(A) is the probability of event A occurring. If a random experiment has N possible outcomes or a random message has N possible values, and if their probabilities are p1, p2, …, pN respectively, then the sum of the self-information of these events: H = -SUM(pi*log(pi)), i=1,2…N is called entropy. 　　In information theory, entropy can be used as a measure of the uncertainty of an event. The greater the amount of information, the more regular the architecture, and the more complete the functions, the lower the entropy. By utilizing the concept of entropy, it is possible to theoretically study the measurement, transmission, transformation, and storage of information. Furthermore, entropy also has certain applications in fields such as cybernetics, probability theory, number theory, astrophysics, and life sciences. 　　In physics, Boltzmann said: “When energy is reduced, atoms become in a more disordered state.” ”Entropy is a measure of disorder: it is a profoundly meaningful concept that stems from Boltzmann’s new interpretation. Surprisingly, it is possible to develop a way to measure disorder, and that is the probability of a particular state—which is defined as the number of ways in which atoms can be arranged. He expressed it very precisely as: S = K log W. Here, S is entropy, which is proportional to the logarithm of the probability W of a given state; K is a constant of proportionality, now known as the Boltzmann constant. 　　If it weren’t for Boltzmann, our progress would have regressed by several decades, perhaps a hundred years. 　　His immortal formula S=KlogW is engraved on his tombstone. 　　Entropy was originally a term in thermodynamics, representing the degree to which heat is evenly distributed within a system. Later, this concept was borrowed by many other disciplines, giving rise to further concepts. But no matter how it varies across different disciplines, the concept it expresses remains the same: the degree of homogenization of the material distribution within a system. Entropy has now become a generalized concept rather than one exclusive to physics. 　　Entropy is a physical concept; in everyday language, it is often referred to as disorder. However, entropy is quite different from the notion of disorder as understood in common sense. The second law of thermodynamics states that the entropy of a closed system can never decrease. A closed system is one in which neither matter nor energy can enter or leave freely. -----------------------Entropy (information theory) The concept of entropy was first introduced in 1864 by Rudolf Clausius, and it was applied in thermodynamics. It was later introduced into information theory for the first time in 1948 by Claude Elwood Shannon. 　　Definition The entropy in message theory is defined as follows: If there are multiple events within a system S, denoted as S = {E1,...,En}, with each event having a probability distribution P = {p1, ..., pn}, then the information content of each event is given by
Ie = − log2pi (with the logarithm taken base 2, and the unit being bits)
or
Ie = − lnpi (with the logarithm taken base e, and the unit being nats).
If there are 26 letters in the English language, and each letter appears on average the same number of times in a text, then the information content per letter is
I_e = -\log_2 {1\over 26} = 4.7 　　 ； Among the Chinese characters, 2500 are commonly used. Assuming that each character appears an average number of times in a text, the information content of each character is given by I_e = -\log_2 {1/2500} = 11.3. The average information content of the entire system is H_s = \sum_{i=1}^n p_i I_e = -\sum_{i=1}^n p_i \log_2 p_i. This average information content is what is known as message entropy. Because it has the same form as Boltzmann’s formula used in thermodynamics to describe thermodynamic entropy, it is also called “entropy”. 　　If two systems have the same amount of data, such as the same article written in different languages, then since it is the sum of the data volume of all elements, a Chinese article will use fewer characters than an English article uses letters. Therefore, articles printed in Chinese characters are shorter than those printed in letters, which have a higher overall volume across other applications. Even though one Chinese character takes up the space of two letters, articles printed in Chinese characters use less paper than those printed in English letters. 　　In fact, the frequency of each letter and each Chinese character appearing in a text is not uniform; therefore, the actual values are different from those mentioned above. However, the calculation presented is a general concept. The more writing units are used in a text, the greater the amount of information contained in each unit. 　　Properties of entropy 1. Entropy is always greater than or equal to zero, that is, H_s ≥ 0. 　　2. Let N be the total number of events in system S, then entropy H_s ≤ log_2N. The equality holds if and only if p1=p2=...=pn, at which point the entropy of system S is maximized. 　　3. Joint entropy: H(X,Y) ≤ H(X) + H(Y), with equality holding if and only if X and Y are statistically independent. 　　4. Conditional entropy: H(X|Y) = H(X,Y) - H(Y) ≤ H(X), with equality holding if and only if X and Y are statistically independent

Enthalpy: Enthalpy is a state function; in other words, once the state of the system is determined, the value of enthalpy is also fixed. The definition of enthalpy is as follows: H = U + pV, where U represents thermal energy, also known as internal energy, which is the total energy within a system; p is the pressure of the system, and V is its volume. As a state function that describes the state of a system, enthalpy does not have a specific physical meaning. Enthalpy is a thermodynamic state function of a substance, and the enthalpy change refers to the amount of change in the enthalpy of that substance. ΔH (enthalpy change) represents the increase in the enthalpy of a system during a certain process; ΔH = ΔU + Δ(pV). Under constant pressure conditions, ΔH can represent the change in the system’s thermodynamic energy. This definition comes from the first law of thermodynamics, which is the application in thermodynamics of the law of conservation and transformation of energy. Entropy: Entropy is one of the important state functions that describe thermodynamic systems. The magnitude of entropy reflects the stability of the system’s state, while changes in entropy indicate the direction in which a thermodynamic process proceeds. Entropy provides a quantitative expression for the second law of thermodynamics.

I remember it this way: enthalpy is internal energy, which is inherent in a substance. Entropy is the degree of change in internal energy, that is, the level of disorder.

Enthalpy: Enthalpy is a state function; in other words, once the state of the system is determined, the value of enthalpy is also fixed. The definition of enthalpy is as follows: H = U + pV, where U represents thermal energy, also known as internal energy, which is the total energy within a system; p is the pressure of the system, and V is its volume. As a state function that describes the state of a system, enthalpy does not have a specific physical meaning. Enthalpy is a thermodynamic state function of a substance, and the enthalpy change refers to the amount of change in the enthalpy of that substance. Entropy is a physical term; it is the quotient of temperature divided by heat, and it indicates the degree to which heat is converted into work. The value of entropy reflects the stability of the system’s state

Enthalpy: Enthalpy is a state function; in other words, once the state of the system is determined, the value of enthalpy is also fixed. The definition of enthalpy is as follows: H = U + W, where U represents thermodynamic energy, also known as internal energy, which is the total energy within the system, and W is the work done on the system by the environment. As a state function that describes the state of a system, enthalpy does not have a clear physical meaning and holds little practical value. Personally, it can be considered approximately as the energy change of a system; in practical applications, definitions under various constraints are more commonly used, such as standard molar enthalpy of formation and standard molar enthalpy of combustion. The magnitude of entropy reflects the degree of disorder in a system.

Take a look at \"Thermodynamics of Chemical Engineering\" and \"Physical Chemistry\"; they explain it in great detail

Entropy is the quotient of thermal energy divided by temperature, and it indicates the degree to which heat is converted into work. Enthalpy represents the total energy possessed by a substance in a given state; it is the sum of internal energy U and pressure potential energy (flow energy) PV. It is a composite state parameter, with the definition H = U + PV. Enthalpy is denoted by the symbol H, and its units are J or kJ. The enthalpy of 1 kilogram of a working substance is called specific enthalpy, denoted by the symbol h; its unit is J/kg or kJ/kg. Thus, specific enthalpy is given by h = u + pv. Since enthalpy is a composite quantity composed of the state parameters u, p, and v, for a given state of the working fluid, u, p, and v all have definite values; therefore, the value of u + pv is also completely determined. Therefore, enthalpy is a state parameter that depends on the state of the working fluid, and it possesses all the characteristics of a state parameter.