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 internal energy value 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.
Enthalpy vs Entropy
Curiosity is one aspect of a person that helps him discover various phenomena in the world. One person looks up at the sky and wonders how rain is formed. One person looks at the ground and wonders how plants can grow. This is an everyday phenomenon that we encounter in our lives, but those people who are not inquisitive enough never try to find answers why such phenomena exist. Biologists, chemists and physicists are just a few people who are trying to find answers. Our modern world is now integrated with the laws of science such as thermodynamics. "Thermodynamics" is a branch of natural science that involves the study of the internal movements of body systems. This is a study on the relationship of heat with various forms of energy and work. Applications of thermodynamics show up in the flow of electricity and from simply turning and turning a screw and other simple machines. As long as heat and friction are involved, there is thermodynamics. The two most common principles of thermodynamics are enthalpy and entropy. In this article, you will learn more about the differences between enthalpy and entropy.
In a thermodynamic system, the measure of its total energy is called enthalpy. To create a thermodynamic system, internal energy is required. This energy serves as the impetus or trigger for the creation of the system. The units of enthalpy are the joule (International System of Units) and the calorie (British thermal unit). "Enthalpy" is the Greek word "enthalpos" (to infuse heat). Heike Kamerlingh Onnes was the person who coined the word, while Alfred W. Porter was the one who coined the "H" symbol for "enthalpy". In biological, chemical, and physical measurements, enthalpy is the most preferred expression for system energy changes because it has the ability to simplify specific definitions of energy transfer. It is not possible to reach a value for the total enthalpy because the total enthalpy of the system cannot be directly measured. Only enthalpy change is the preferred measure of quantity, not the absolute value of enthalpy. In endothermic reactions, there is a positive change in enthalpy, and in exothermic reactions, a negative change in enthalpy occurs. Simply put, the enthalpy of a system is equivalent to the sum of non-mechanical work and the heat supplied. At constant pressure, enthalpy is equivalent to the change in the internal energy of the system and the work that the system exerted on its environment. In other words, heat can be absorbed or released by a particular chemical reaction under such conditions.
"Entropy" is the second law of thermodynamics. This is one of the most fundamental laws in physics. This is important for understanding life and cognition. This is regarded as the Law of Disorder. In the middle of the last century, "entropy" had already been formulated with extensive efforts by Clausius and Thomson. Clausius and Thomson were inspired by Carnot's observation of the flow that turns the mill wheel. Carnot stated that thermodynamics is the flow of heat from higher to lower temperatures that makes the steam engine work. Clausius was the one who coined the term "entropy". The symbol for entropy is "S", which states that the world is considered inherently active when it acts spontaneously to dissipate or minimize the presence of a thermodynamic force.
"Enthalpy" is the transfer of energy and "entropy" is the Law of Disorder.
Enthalpy takes on the symbol "H" and entropy takes on the symbol "S".
Heike Kamerlingh Onnes coined the term "enthalpy" and Clausius coined the term "entropy".
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 by which a given macrostate can be realized). 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 from the Hess law for calculating the change in the enthalpy of reaction: the change in the enthalpy of 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.
LECTURE №8.
Patterns of chemical reactions
Introduction to thermodynamics. The concept of entropy, enthalpy, Gibbs energy. Possibility of chemical reactions. Enthalpy and entropy factors of processes.
Chemical thermodynamics
The question of whether this or that spontaneous reaction is possible in principle under certain conditions is considered by chemical thermodynamics. For example, the explosion of gunpowder (saltpeter, sulfur and coal) is not possible by itself. Under normal conditions, the reaction does not proceed. To start it, you need t °, or a blow.
Chemical thermodynamics considers the transition of a system from one state to another, completely ignoring the transition mechanism. About how the transition of the starting substances into the reaction products takes place and how the rate depends on the reaction conditions considers chemical kinetics. If a reaction is thermodynamically forbidden, then it is meaningless to consider its rate, this reaction does not proceed spontaneously.
If the reaction is thermodynamically possible, then the rate can be changed, for example, by introducing a catalyst. Theories, laws, numerical characteristics are necessary in order to control reactions: to slow down the processes of corrosion of metals or to compose rocket fuel, etc.
Thermodynamics - the science of the transformation of some types of energy and work into others. There are 3 laws of thermodynamics.
Chemical is called thermodynamics considering the transformation of energy and work in chemical reactions. For this you need to know state function.
Status function called such a variable characteristic of the system, which does not depend on the prehistory of the system and the change in which during the transition of the system from one state to another does not depend on how this change was made.
(Sisyphus, mountain,
ΔЕ of a stone on a mountain is a state function)
ΔE - potential energy
ΔE \u003d mg (h 2 -h 1)
In order to be able to use state functions, the states themselves must be declared.
State Options
|
P - pressure |
|
|
V - volume |
part of the space occupied by the system. |
|
ν is the number of moles |
|
|
T - temperature |
For an ideal gas, T = 273.16 K for the triple point of water. |
|
Т˚ - standard t˚ |
Т˚ = 25˚С = 298.16 K |
|
Р˚ - standard Р |
P˚ \u003d 1 atm \u003d 760 mm Hg. = 101.3 kPa |
;
;Status functions
|
U - internal energy |
|
|
H - enthalpy |
|
|
S - entropy |
|
|
G is the Gibbs energy |
|
A and Q, i.e. work and heat are two functions that thermodynamics is devoted to, but which are not state functions.
Any system whose transition from one state to another is considered by thermodynamics can have:
I constant volume(i.e., for example, a sealed ampoule), V - const.
Processes that occur at constant volume are called isochoric, (isochoric).
II constant pressure. isobaric processes (isobaric), P – const.
III constantt˚. isothermal processes, T - const.
The processes occurring in the system under conditions when there is no heat exchange between the system and the external environment are called adiabatic.
The heat received by the system is considered positive, and the heat given off by the system to the external environment is considered negative. Heat is defined by the number J (kJ).
First law of thermodynamics. Enthalpy.
I law of thermodynamics - the law of conservation and transformation of energy.
the change in the internal energy of the system is equal to the difference between the amount of heat received by the system from the environment and the amount of work done by the system on the environment.
ΔU - in a chemical reaction - is the change in the internal energy of the system as a result of the conversion of a certain number of moles of the starting substances into a certain number of moles of the reaction products.
(the difference between the energies of the final and initial states).
Then 
If the reaction is isochoric, then V-const and 
(i.e. the amount of heat received or given away by the system).
If the reaction is isobaric, then it takes place at constant external pressure:
Then
Most chemical reactions take place under isobaric conditions, i.e. it is necessary to determine Q P and the expansion (compression) work.
To simplify the situation in thermodynamics, a new function has been adopted - enthalpy N.
The enthalpy change in the reaction will be:
Taking into account equation (1), we obtain
and since the reaction proceeds under isobaric conditions, then P = const 
.
, but we know that 
, let's substitute:
, Then
, i.e. the difference between the thermal effects of the same reaction, measured at constant pressure and constant volume, is equal to the work of expansion. Thus, the change in enthalpy is uniquely related to the amount of heat received or given away by the system during the isobaric transition, and the change in enthalpy ΔH is usually taken as a measure of the heat effect of a chemical reaction.
The warmth of a fire, the burning of limestone, the photosynthesis of plants, and electrolysis are examples of the exchange of various forms of energy.
The heat effect of a chemical reaction is the change in energy during the isobaric transition of a certain number of moles of the starting substances into the corresponding number of moles of the reaction products(in J or kJ).
It is measured by the change in enthalpy during the transition of the system from the state of the starting materials to the reaction products. At the same time, the term exo and endothermic reaction is retained. measured with a calorimeter. The thermal effects of reactions proceeding in the forward and reverse directions are equal in magnitude and opposite in sign.
H 2 + Cl 2 \u003d 2HCl ΔH \u003d - 184 kJ
2HCl \u003d H 2 + Cl 2 ΔH \u003d + 184 kJ
The fundamental law of thermochemistry was formulated by Hess in 1840.
T 
The heat effect of a reaction depends only on the state of the initial and final substances and does not depend on the number of intermediate stages.
To obtain 1 mol of CO 2, 1 mol of C (tv) and 1 mol of O 2 (g) are needed.
Summing up the stages and enthalpies of all stages, we find that:
This process is called a cycle. In order to calculate the thermal effect of a reaction, it is necessary to know the enthalpies of decomposition of the initial substances and the enthalpies of formation of reaction products from simple substances. But they are equal in magnitude and different in sign, so it is enough to know one enthalpy. Because enthalpy depends on its state and conditions, then all states and conditions are referred to the same, which are called standard.
t˚ = 25˚С, P = 101.3 kPa
t˚ the effect of a chemical reaction is differences the sum of the heats of formation of the reaction products and the sum of the heats of formation of the starting materials.
The transition from the standard state to any other is accompanied by an increase in enthalpy, i.e. endothermic thermal effect.
simple substances are equal to zero.
It is called the standard enthalpy (heat of formation).
(˚) - means that all substances are in standard states.
The enthalpy of formation of a complex substance from simple substances is the heat effect of the reaction of formation of a given substance from simple substances in standard states, related to 1 mole of the resulting substance. . (f- formation - education).
Entropy
Entropy (S) is proportional to the logarithm of the thermodynamic probability (W) of the state of the system.
H - Boltzmann's constant
Entropy is a measure of the disorder of a system. Entpropy is introduced as a state function, the change of which is determined by the ratio of the amount of heat received or given away by the system at t - T.
If the system receives a certain amount of heat at a constant t˚, then all the heat goes to increase the random, chaotic motion of particles, i.e. increase in entropy.
II The second law of thermodynamics
The second law of thermodynamics states that only such processes can spontaneously occur in an isolated system that lead to an increase in entropy.(disordered system).
The evaporation of ether from the hand proceeds spontaneously with an increase in entropy, but the heat for such a transition is taken away from the hand, i.e. the process is endothermic.
III The third law of thermodynamics
The entropy of an ideal crystal at absolute zero is zero. This is the third law of thermodynamics.
S˚ 298 is the standard entropy, J/(k mol).
If ΔH is large, then ΔS is small. But it is not always the case. Gibbs introduced a new state function into thermodynamics - the Gibbs energy - G .
G=H-TS or ∆G = ∆H – T∆S
In any closed system at constant P and T, such a spontaneous process is possible, which leads to a decrease in the Gibbs energyΔG and enthalpy. ... Gibbs. enthalpy And entropy factors, their influence on leakage reactions at low and high temperatures. 18. Grade possibilities and conditions leaks reactions ...
This manual can be used for independent work by students of non-chemical specialties.
Documentprevailing factor a. Energy Gibbs serves as a criterion for spontaneous leaks chemical reactions with isobaric - isothermal processes. Chemical reaction fundamentally possible, If energy Gibbs decreasing...
Guidelines Training sessions for the course "Theoretical Foundations of Chemistry" consist of lectures, seminars, laboratory work, term papers and homework
Guidelines... concept about entropy, absolute entropy substances (S°t) and entropy processes(S°t). Energy Gibbs as a measure chemical affinity. Change energy Gibbs in different processes, entropy And enthalpy factors. Calculation of G°298 and S °298 processes ...
... H reactions. concept about entropy. Absolute entropy and its dependence on the structure of matter. Change entropy in different processes. Energy Gibbs, its connection with entropy And enthalpy. enthalpy And entropy factors process ...
The program of the entrance examination to the master's program in the direction 050100 Science education
Program... processes. Energy and direction chemical processes Chemical thermodynamics. Main concepts thermodynamics: system, process, parameter, state. System Status Functions: Internal energy And enthalpy ...
Enthalpy, Also thermal function And heat content- thermodynamic potential characterizing the state of the system in thermodynamic equilibrium when choosing as independent variables pressure , entropy and the number of particles.
Simply put, enthalpy is that energy that is available for conversion into heat at a certain temperature and pressure.
If the thermomechanical system is considered as consisting of a macrobody (gas) and a piston with an area S with a load of weight P = pS, balancing gas pressure R inside the vessel, then such a system is called extended.
Enthalpy or energy of an expanded system E is equal to the sum of the internal energy of the gas U and potential energy of the piston with the load E sweat = psx = pV
Thus, the enthalpy in this state is the sum of the internal energy of the body and the work that must be expended so that the body volume V enter into a pressurized environment R and in equilibrium with the body. System enthalpy H- similarly to internal energy and other thermodynamic potentials - has a well-defined value for each state, i.e. it is a state function. Therefore, in the process of changing the state
Examples
| Chemical compound | Phase (substances) | Chemical formula | Δ H f 0 kJ/mol |
|---|---|---|---|
| Ammonia | solvated | NH3 (NH4OH) | −80.8 |
| Ammonia | gaseous | NH3 | −46.1 |
| Sodium carbonate | solid | Na2CO3 | −1131 |
| Sodium chloride (salt) | solvated | NaCl | −407 |
| Sodium chloride (salt) | solid | NaCl | −411.12 |
| Sodium chloride (salt) | liquid | NaCl | −385.92 |
| Sodium chloride (salt) | gaseous | NaCl | −181.42 |
| Sodium hydroxide | solvated | NaOH | −469.6 |
| Sodium hydroxide | solid | NaOH | −426.7 |
| sodium nitrate | solvated | NaNO 3 | −446.2 |
| sodium nitrate | solid | NaNO 3 | −424.8 |
| Sulfur dioxide | gaseous | SO2 | −297 |
| Sulfuric acid | liquid | H2SO4 | −814 |
| Silica | solid | SiO2 | −911 |
| nitrogen dioxide | gaseous | NO 2 | +33 |
| nitrogen monoxide | gaseous | NO | +90 |
| Water | liquid | H2O | −286 |
| Water | gaseous | H2O | −241.8 |
| Carbon dioxide | gaseous | CO2 | −393.5 |
| Hydrogen | gaseous | H2 | 0 |
| Fluorine | gaseous | F2 | 0 |
| Chlorine | gaseous | Cl2 | 0 |
| Bromine | liquid | Br2 | 0 |
| Bromine | gaseous | Br2 | 0 |
Invariant enthalpy in relativistic thermodynamics
When constructing relativistic thermodynamics (taking into account the special theory of relativity), usually the most convenient approach is to use the so-called invariant enthalpy - for a system located in a certain vessel.
In this approach, the temperature is defined as a Lorentz invariant. Entropy is also an invariant. Since the walls affect the system, the most natural independent variable is pressure, and therefore it is convenient to take enthalpy as the thermodynamic potential.
For such a system, the “usual” enthalpy and momentum of the system form a 4-vector , and the invariant function of this 4-vector is taken to determine the invariant enthalpy, which is the same in all frames of reference:
The basic equation of relativistic thermodynamics is written in terms of the invariant enthalpy differential as follows:
Using this equation, one can solve any problem of thermodynamics of moving systems, if the function is known.
see also
Sources
- Bolgarsky A. V., Mukhachev G. A., Shchukin V. K., “Thermodynamics and heat transfer”, Ed. 2nd, revised. and additional Moscow: Higher School, 1975, 495 p.
- Kharin A. N., Kataeva N. A., Kharin L. T., ed. prof. Kharina A. N. "Course of Chemistry", M .: "Higher School", 1975, 416 p.
Notes
Wikimedia Foundation. 2010 .
Synonyms:See what "Enthalpy" is in other dictionaries:
Enthalpy- (from the Greek enthalpo I heat), the state function of a thermodynamic system, the change of which at constant pressure is equal to the amount of heat supplied to the system, therefore enthalpy is often called a heat function or heat content. ... ... Illustrated Encyclopedic Dictionary
- (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 a constant p change ... ... Big Encyclopedic Dictionary
- (designation H), the amount of thermodynamic (thermal) energy contained in the substance. In any system, enthalpy is equal to the sum of internal energy and the product of pressure and volume. Measured in terms of change (usually increase) in the amount of ... ... Scientific and technical encyclopedic dictionary
Heat content Dictionary of Russian synonyms. enthalpy noun, number of synonyms: 1 heat content (1) ASIS Synonym Dictionary … Synonym dictionary
ENTHALPY- (from the Greek enthalpo I heat) ecosystems, the functional state of the ecosystem, which determines its heat content. Enthalpy is an extensive property of an ecosystem. Ecological encyclopedic dictionary. Chisinau: The main edition of the Moldavian Soviet ... ... Ecological dictionary
enthalpy- Function of the state of a thermodynamic system, equal to the sum of internal energy and the product of volume and pressure. Note Enthalpy is a characteristic function if entropy and pressure are independent parameters. [Compilation… … Technical Translator's Handbook
- (from the Greek enthalpo I heat) (heat content, Gibbs thermal function), thermodynamic potential characterizing the state of the macroscopic. systems in thermodynamic equilibrium when choosing as the main independent variables the entropy S and ... ... Physical Encyclopedia
