The total amount of heat. Solving problems for calculating the amount of heat required to heat the body or released by it during cooling

The internal energy of a thermodynamic system can be changed in two ways:

committing over system work,
through thermal interaction.

The transfer of heat to a body is not connected with the performance of macroscopic work on the body. In this case, the change in internal energy is caused by the fact that individual molecules of the body with a higher temperature do work on some molecules of the body, which has a lower temperature. In this case, thermal interaction is realized due to thermal conduction. The transfer of energy is also possible with the help of radiation. The system of microscopic processes (pertaining not to the whole body, but to individual molecules) is called heat transfer. The amount of energy that is transferred from one body to another as a result of heat transfer is determined by the amount of heat that is transferred from one body to another.

Definition

warmth called the energy that is received (or given away) by the body in the process of heat exchange with the surrounding bodies (environment). Heat is denoted, usually by the letter Q.

This is one of the basic quantities in thermodynamics. Heat is included in the mathematical expressions of the first and second laws of thermodynamics. Heat is said to be energy in the form of molecular motion.

Heat can be communicated to the system (body), or it can be taken from it. It is believed that if heat is imparted to the system, then it is positive.

The formula for calculating heat with a change in temperature

The elementary amount of heat is denoted as . Note that the element of heat that the system receives (gives off) with a small change in its state is not a total differential. The reason for this is that heat is a function of the process of changing the state of the system.

The elementary amount of heat that is reported to the system, and the temperature changes from T to T + dT, is:

where C is the heat capacity of the body. If the body under consideration is homogeneous, then formula (1) for the amount of heat can be represented as:

where is the specific heat of the body, m is body mass, is the molar heat capacity, is the molar mass of the substance, is the number of moles of the substance.

If the body is homogeneous, and the heat capacity is considered independent of temperature, then the amount of heat () that the body receives when its temperature increases by a value can be calculated as:

where t 2 , t 1 body temperature before and after heating. Please note that when finding the difference () in the calculations, temperatures can be substituted both in degrees Celsius and in kelvins.

The formula for the amount of heat during phase transitions

The transition from one phase of a substance to another is accompanied by the absorption or release of a certain amount of heat, which is called the heat of the phase transition.

So, to transfer an element of matter from a solid state to a liquid, it should be informed of the amount of heat () equal to:

where - specific heat melting, dm is the element of body mass. In this case, it should be taken into account that the body must have a temperature equal to the melting point of the substance in question. During crystallization, heat is released equal to (4).

The amount of heat (heat of vaporization) required to convert liquid to vapor can be found as:

where r is the specific heat of vaporization. When steam condenses, heat is released. The heat of evaporation is equal to the heat of condensation of equal masses of matter.

Units for measuring the amount of heat

The basic unit for measuring the amount of heat in the SI system is: [Q]=J

An off-system unit of heat that is often found in technical calculations. [Q]=cal (calorie). 1 cal = 4.1868 J.

Examples of problem solving

Example

The task. What volumes of water should be mixed to obtain 200 liters of water at a temperature of t=40C, if the temperature of one mass of water is t 1 =10C, the second mass of water is t 2 =60C?

Solution. We write the heat balance equation in the form:

where Q=cmt - the amount of heat prepared after mixing water; Q 1 \u003d cm 1 t 1 - the amount of heat of a part of water with temperature t 1 and mass m 1; Q 2 \u003d cm 2 t 2 - the amount of heat of a part of water with temperature t 2 and mass m 2.

Equation (1.1) implies:

When combining cold (V 1) and hot (V 2) parts of water into a single volume (V), we can accept that:

So, we get a system of equations:

Solving it, we get:

The task 81.
Calculate the amount of heat that will be released during the reduction of Fe 2O3 metallic aluminum if 335.1 g of iron was obtained. Answer: 2543.1 kJ.
Solution:
Reaction equation:

\u003d (Al 2 O 3) - (Fe 2 O 3) \u003d -1669.8 - (-822.1) \u003d -847.7 kJ

Calculation of the amount of heat that is released upon receipt of 335.1 g of iron, we produce from the proportion:

(2 . 55,85) : -847,7 = 335,1 : X; x = (0847.7 . 335,1)/ (2 . 55.85) = 2543.1 kJ,

where 55.85 is the atomic mass of iron.

Answer: 2543.1 kJ.

Thermal effect of the reaction

Task 82.
Gaseous ethyl alcohol C2H5OH can be obtained by the interaction of ethylene C 2 H 4 (g) and water vapor. Write the thermochemical equation for this reaction, having previously calculated its thermal effect. Answer: -45.76 kJ.
Solution:
The reaction equation is:

C 2 H 4 (g) + H 2 O (g) \u003d C2H 5 OH (g); = ?

The values of the standard heats of formation of substances are given in special tables. Considering that the heats of formation simple substances conditionally accepted zero. Calculate the thermal effect of the reaction, using the consequence of the Hess law, we get:

\u003d (C 2 H 5 OH) - [ (C 2 H 4) + (H 2 O)] \u003d
= -235.1 -[(52.28) + (-241.83)] = - 45.76 kJ

Reaction equations in which their state of aggregation or crystalline modification, as well as the numerical value of thermal effects, are indicated near the symbols of chemical compounds, are called thermochemical. IN thermochemical equations, unless otherwise stated, the values of thermal effects are given at constant pressure Q p equal to change enthalpy of the system. The value is usually given on the right side of the equation, separated by a comma or semicolon. The following abbreviations for the aggregate state of matter are accepted: G- gaseous, well- liquid, to

If heat is released as a result of a reaction, then< О. Учитывая сказанное, составляем термохимическое уравнение данной в примере реакции:

C 2 H 4 (g) + H 2 O (g) \u003d C 2 H 5 OH (g); = - 45.76 kJ.

Answer:- 45.76 kJ.

Task 83.
Calculate the thermal effect of the reduction reaction of iron (II) oxide with hydrogen, based on the following thermochemical equations:

a) EEO (c) + CO (g) \u003d Fe (c) + CO 2 (g); = -13.18 kJ;
b) CO (g) + 1/2O 2 (g) = CO 2 (g); = -283.0 kJ;
c) H 2 (g) + 1/2O 2 (g) = H 2 O (g); = -241.83 kJ.
Answer: +27.99 kJ.

Solution:
The reaction equation for the reduction of iron oxide (II) with hydrogen has the form:

EeO (k) + H 2 (g) \u003d Fe (k) + H 2 O (g); = ?

\u003d (H2O) - [ (FeO)

The heat of formation of water is given by the equation

H 2 (g) + 1/2O 2 (g) = H 2 O (g); = -241.83 kJ,

and the heat of formation of iron oxide (II) can be calculated if equation (a) is subtracted from equation (b).

\u003d (c) - (b) - (a) \u003d -241.83 - [-283.o - (-13.18)] \u003d + 27.99 kJ.

Answer:+27.99 kJ.

Task 84.
During the interaction of gaseous hydrogen sulfide and carbon dioxide, water vapor and carbon disulfide СS 2 (g) are formed. Write the thermochemical equation for this reaction, preliminarily calculate its thermal effect. Answer: +65.43 kJ.
Solution:
G- gaseous, well- liquid, to- crystalline. These symbols are omitted if the aggregate state of substances is obvious, for example, O 2, H 2, etc.
The reaction equation is:

2H 2 S (g) + CO 2 (g) \u003d 2H 2 O (g) + CS 2 (g); = ?

The values of the standard heats of formation of substances are given in special tables. Considering that the heats of formation of simple substances are conditionally taken equal to zero. The thermal effect of the reaction can be calculated using the corollary e from the Hess law:

\u003d (H 2 O) + (CS 2) - [(H 2 S) + (CO 2)];
= 2(-241.83) + 115.28 – = +65.43 kJ.

2H 2 S (g) + CO 2 (g) \u003d 2H 2 O (g) + CS 2 (g); = +65.43 kJ.

Answer:+65.43 kJ.

Thermochemical reaction equation

Task 85.
Write the thermochemical equation for the reaction between CO (g) and hydrogen, as a result of which CH 4 (g) and H 2 O (g) are formed. How much heat will be released during this reaction if 67.2 liters of methane were obtained in terms of normal conditions? Answer: 618.48 kJ.
Solution:
Reaction equations in which their state of aggregation or crystalline modification, as well as the numerical value of thermal effects, are indicated near the symbols of chemical compounds, are called thermochemical. In thermochemical equations, unless it is specifically stated, the values of thermal effects at constant pressure Q p are indicated equal to the change in the enthalpy of the system. The value is usually given on the right side of the equation, separated by a comma or semicolon. The following abbreviations for the aggregate state of matter are accepted: G- gaseous, well- something to- crystalline. These symbols are omitted if the aggregate state of substances is obvious, for example, O 2, H 2, etc.
The reaction equation is:

CO (g) + 3H 2 (g) \u003d CH 4 (g) + H 2 O (g); = ?

\u003d (H 2 O) + (CH 4) - (CO)];
\u003d (-241.83) + (-74.84) - (-110.52) \u003d -206.16 kJ.

The thermochemical equation will look like:

22,4 : -206,16 = 67,2 : X; x \u003d 67.2 (-206.16) / 22? 4 \u003d -618.48 kJ; Q = 618.48 kJ.

Answer: 618.48 kJ.

Heat of Formation

Task 86.
The thermal effect of which reaction is equal to the heat of formation. Calculate the heat of formation of NO from the following thermochemical equations:
a) 4NH 3 (g) + 5O 2 (g) \u003d 4NO (g) + 6H 2 O (g); = -1168.80 kJ;
b) 4NH 3 (g) + 3O 2 (g) \u003d 2N 2 (g) + 6H 2 O (g); = -1530.28 kJ
Answer: 90.37 kJ.
Solution:
The standard heat of formation is equal to the heat of formation of 1 mol of this substance from simple substances under standard conditions (T = 298 K; p = 1.0325.105 Pa). The formation of NO from simple substances can be represented as follows:

1/2N 2 + 1/2O 2 = NO

Given the reaction (a) in which 4 moles of NO are formed and the reaction (b) is given in which 2 moles of N2 are formed. Both reactions involve oxygen. Therefore, to determine the standard heat of formation of NO, we compose the following Hess cycle, i.e., we need to subtract equation (a) from equation (b):

Thus, 1/2N 2 + 1/2O 2 = NO; = +90.37 kJ.

Answer: 618.48 kJ.

Task 87.
Crystalline ammonium chloride is formed by the interaction of gaseous ammonia and hydrogen chloride. Write the thermochemical equation for this reaction, having previously calculated its thermal effect. How much heat will be released if 10 liters of ammonia were consumed in the reaction in terms of normal conditions? Answer: 78.97 kJ.
Solution:
Reaction equations in which their state of aggregation or crystalline modification, as well as the numerical value of thermal effects, are indicated near the symbols of chemical compounds, are called thermochemical. In thermochemical equations, unless it is specifically stated, the values of thermal effects at constant pressure Q p are indicated equal to the change in the enthalpy of the system. The value is usually given on the right side of the equation, separated by a comma or semicolon. The following are accepted to- crystalline. These symbols are omitted if the aggregate state of substances is obvious, for example, O 2, H 2, etc.
The reaction equation is:

NH 3 (g) + HCl (g) \u003d NH 4 Cl (k). ; = ?

\u003d (NH4Cl) - [(NH 3) + (HCl)];
= -315.39 - [-46.19 + (-92.31) = -176.85 kJ.

The thermochemical equation will look like:

The heat released during the reaction of 10 liters of ammonia in this reaction is determined from the proportion:

22,4 : -176,85 = 10 : X; x \u003d 10 (-176.85) / 22.4 \u003d -78.97 kJ; Q = 78.97 kJ.

Answer: 78.97 kJ.

Heat capacity is the amount of heat absorbed by the body when heated by 1 degree.

The heat capacity of the body is indicated by a capital Latin letter FROM.

What determines the heat capacity of a body? First of all, from its mass. It is clear that heating, for example, 1 kilogram of water will require more heat than heating 200 grams.

What about the kind of substance? Let's do an experiment. Let's take two identical vessels and, pouring 400 g of water into one of them, and into the other - vegetable oil weighing 400 g, we will start heating them with the help of identical burners. By observing the readings of thermometers, we will see that the oil heats up quickly. To heat water and oil to the same temperature, the water must be heated longer. But the longer we heat the water, the large quantity heat it receives from the burner.

Thus, to heat the same mass of different substances to the same temperature, different amounts of heat are required. The amount of heat required to heat a body and, consequently, its heat capacity depend on the kind of substance of which this body is composed.

So, for example, to increase the temperature of water with a mass of 1 kg by 1 ° C, an amount of heat equal to 4200 J is required, and to heat the same mass of sunflower oil by 1 ° C, an amount of heat equal to 1700 J is required.

The physical quantity showing how much heat is required to heat 1 kg of a substance by 1 ºС is called specific heat this substance.

Each substance has its own specific heat capacity, which is denoted by the Latin letter c and is measured in joules per kilogram-degree (J / (kg ° C)).

Specific heat the same substance in different aggregate states (solid, liquid and gaseous) is different. For example, the specific heat capacity of water is 4200 J/(kg ºС), and the specific heat capacity of ice is 2100 J/(kg ºС); aluminum in the solid state has a specific heat capacity of 920 J / (kg - ° C), and in the liquid state - 1080 J / (kg - ° C).

Note that water has a very high specific heat capacity. Therefore, the water in the seas and oceans, heating up in summer, absorbs from the air a large number of heat. Due to this, in those places that are located near large bodies of water, summer is not as hot as in places far from water.

Calculation of the amount of heat required to heat the body or released by it during cooling.

From the foregoing, it is clear that the amount of heat necessary to heat the body depends on the type of substance of which the body consists (i.e., its specific heat capacity) and on the mass of the body. It is also clear that the amount of heat depends on how many degrees we are going to increase the temperature of the body.

So, to determine the amount of heat required to heat the body or released by it during cooling, you need to multiply the specific heat of the body by its mass and by the difference between its final and initial temperatures:

Q= cm (t 2 -t 1),

where Q- quantity of heat, c- specific heat capacity, m- body mass, t1- initial temperature, t2- final temperature.

When the body is heated t2> t1 and hence Q >0 . When the body is cooled t 2and< t1 and hence Q< 0 .

If the heat capacity of the whole body is known FROM, Q is determined by the formula: Q \u003d C (t 2 - t1).

22) Melting: definition, calculation of the amount of heat for melting or solidification, specific heat of melting, graph of t 0 (Q).

Thermodynamics

A branch of molecular physics that studies the transfer of energy, the patterns of transformation of some types of energy into others. Unlike the molecular-kinetic theory, thermodynamics does not take into account the internal structure of substances and microparameters.

Thermodynamic system

This is a collection of bodies that exchange energy (in the form of work or heat) with each other or with environment. For example, the water in the teapot cools down, the exchange of heat of the water with the teapot and of the teapot with the environment takes place. Cylinder with gas under the piston: the piston performs work, as a result of which the gas receives energy and its macro parameters change.

Quantity of heat

This energy, which is received or given by the system in the process of heat exchange. Denoted by the symbol Q, measured, like any energy, in Joules.

As a result of various heat transfer processes, the energy that is transferred is determined in its own way.

Heating and cooling

This process is characterized by a change in the temperature of the system. The amount of heat is determined by the formula

The specific heat capacity of a substance with measured by the amount of heat required to heat up mass units of this substance by 1K. Heating 1 kg of glass or 1 kg of water requires a different amount of energy. Specific heat capacity is a known value already calculated for all substances, see the value in physical tables.

Heat capacity of substance C- this is the amount of heat that is necessary to heat the body without taking into account its mass by 1K.

Melting and crystallization

Melting is the transition of a substance from a solid to a liquid state. The reverse transition is called crystallization.

The energy spent on the destruction of the crystal lattice of a substance is determined by the formula

The specific heat of fusion is a known value for each substance, see the value in the physical tables.

Vaporization (evaporation or boiling) and condensation

Vaporization is the transition of a substance from a liquid (solid) state to a gaseous state. The reverse process is called condensation.

The specific heat of vaporization is a known value for each substance, see the value in the physical tables.

Combustion

The amount of heat released when a substance burns

The specific heat of combustion is a known value for each substance, see the value in the physical tables.

For a closed and adiabatically isolated system of bodies, the heat balance equation is satisfied. Algebraic sum the amount of heat given and received by all bodies participating in heat exchange is equal to zero:

Q 1 +Q 2 +...+Q n =0

23) The structure of liquids. surface layer. Surface tension force: examples of manifestation, calculation, surface tension coefficient.

From time to time, any molecule can move to an adjacent vacancy. Such jumps in liquids occur quite often; therefore, the molecules are not tied to certain centers, as in crystals, and can move throughout the entire volume of the liquid. This explains the fluidity of liquids. Due to the strong interaction between closely spaced molecules, they can form local (unstable) ordered groups containing several molecules. This phenomenon is called short-range order(Fig. 3.5.1).

The coefficient β is called temperature coefficient volume expansion . This coefficient for liquids is ten times greater than for solids. For water, for example, at a temperature of 20 ° C, β in ≈ 2 10 - 4 K - 1, for steel β st ≈ 3.6 10 - 5 K - 1, for quartz glass β kv ≈ 9 10 - 6 K - one .

The thermal expansion of water has an interesting and important anomaly for life on Earth. At temperatures below 4 °C, water expands with decreasing temperature (β< 0). Максимум плотности ρ в = 10 3 кг/м 3 вода имеет при температуре 4 °С.

When water freezes, it expands, so the ice remains floating on the surface of the freezing body of water. The temperature of freezing water under ice is 0°C. In denser layers of water near the bottom of the reservoir, the temperature is about 4 °C. Thanks to this, life can exist in the water of freezing reservoirs.

Most interesting feature liquids is the presence free surface . Liquid, unlike gases, does not fill the entire volume of the vessel into which it is poured. An interface is formed between the liquid and the gas (or vapor), which is in special conditions compared to the rest of the liquid mass. It should be borne in mind that, due to the extremely low compressibility, the presence of a more densely packed surface layer does not lead to any noticeable change in the volume of the liquid . If the molecule moves from the surface into the liquid, the forces of intermolecular interaction will do positive work. On the contrary, in order to pull a certain number of molecules from the depth of the liquid to the surface (i.e., increase the surface area of the liquid), external forces must do a positive work Δ A external, proportional to the change Δ S surface area:

It is known from mechanics that the equilibrium states of a system correspond to the minimum value of its potential energy. It follows that the free surface of the liquid tends to reduce its area. For this reason, a free drop of liquid takes on a spherical shape. The fluid behaves as if forces are acting tangentially to its surface, reducing (contracting) this surface. These forces are called surface tension forces .

The presence of surface tension forces makes the liquid surface look like an elastic stretched film, with the only difference that the elastic forces in the film depend on its surface area (i.e., on how the film is deformed), and the surface tension forces do not depend on the surface area of the liquid.

Some liquids, such as soapy water, have the ability to form thin films. All well-known soap bubbles have the correct spherical shape - this also manifests the action of surface tension forces. If a wire frame is lowered into the soapy solution, one of the sides of which is movable, then the whole of it will be covered with a film of liquid (Fig. 3.5.3).

Surface tension forces tend to shorten the surface of the film. To balance the moving side of the frame, an external force must be applied to it. If, under the action of the force, the crossbar moves by Δ x, then the work Δ A ext = F ext Δ x = Δ Ep = σΔ S, where ∆ S = 2LΔ x is the increment in the surface area of both sides of the soap film. Since the moduli of forces and are the same, we can write:

Thus, the surface tension coefficient σ can be defined as modulus of the surface tension force acting per unit length of the line bounding the surface.

Due to the action of surface tension forces in liquid droplets and inside soap bubbles overpressure occurs Δ p. If we mentally cut a spherical drop of radius R into two halves, then each of them must be in equilibrium under the action of surface tension forces applied to the boundary of a cut with a length of 2π R and overpressure forces acting on the area π R 2 sections (Fig. 3.5.4). The equilibrium condition is written as

If these forces are greater than the forces of interaction between the molecules of the liquid itself, then the liquid wets the surface of a solid body. In this case, the liquid approaches the surface of the solid body at some acute angle θ, which is characteristic of the given liquid-solid pair. The angle θ is called contact angle . If the interaction forces between liquid molecules exceed the forces of their interaction with solid molecules, then the contact angle θ turns out to be obtuse (Fig. 3.5.5). In this case, the liquid is said to does not wet the surface of a solid body. At complete wettingθ = 0, at complete non-wettingθ = 180°.

capillary phenomena called the rise or fall of fluid in small diameter tubes - capillaries. Wetting liquids rise through the capillaries, non-wetting liquids descend.

On fig. 3.5.6 shows a capillary tube of a certain radius r lowered by the lower end into a wetting liquid of density ρ. The upper end of the capillary is open. The rise of the liquid in the capillary continues until the force of gravity acting on the liquid column in the capillary becomes equal in absolute value to the resulting F n surface tension forces acting along the boundary of contact of the liquid with the surface of the capillary: F t = F n, where F t = mg = ρ hπ r 2 g, F n = σ2π r cos θ.

This implies:

With complete nonwetting, θ = 180°, cos θ = –1 and, therefore, h < 0. Уровень несмачивающей жидкости в капилляре опускается ниже уровня жидкости в сосуде, в которую опущен капилляр.

Water almost completely wets the clean glass surface. Conversely, mercury does not completely wet the glass surface. Therefore, the level of mercury in the glass capillary falls below the level in the vessel.

24) Vaporization: definition, types (evaporation, boiling), calculation of the amount of heat for vaporization and condensation, specific heat of vaporization.

Evaporation and condensation. Explanation of the phenomenon of evaporation based on ideas about the molecular structure of matter. Specific heat of vaporization. Her units.

The phenomenon of liquid turning into vapor is called vaporization.

Evaporation - the process of vaporization occurring from an open surface.

Molecules in a liquid move with different speeds. If any molecule is at the surface of the liquid, it can overcome the attraction of neighboring molecules and fly out of the liquid. The escaping molecules form vapor. The velocities of the remaining liquid molecules change upon collision. In this case, some molecules acquire a speed sufficient to fly out of the liquid. This process continues, so liquids evaporate slowly.

*Evaporation rate depends on the type of liquid. Those liquids evaporate faster, in which the molecules are attracted with less force.

*Evaporation can occur at any temperature. But at higher temperatures, evaporation is faster .

*Evaporation rate depends on its surface area.

*With wind (air flow), evaporation occurs faster.

During evaporation, the internal energy decreases, because. during evaporation, fast molecules leave the liquid, therefore, the average speed of the remaining molecules decreases. This means that if there is no influx of energy from outside, then the temperature of the liquid decreases.

The phenomenon of the transformation of vapor into liquid is called condensation. It is accompanied by the release of energy.

Vapor condensation explains the formation of clouds. Water vapor rising above the ground forms clouds in the upper cold layers of air, which consist of tiny drops of water.

Specific heat of vaporization - physical. a quantity indicating how much heat is required to turn a liquid of mass 1 kg into vapor without changing the temperature.

Oud. heat of vaporization denoted by the letter L and is measured in J / kg

Oud. heat of vaporization of water: L=2.3×10 6 J/kg, alcohol L=0.9×10 6

The amount of heat required to turn a liquid into steam: Q = Lm

As you know, during various mechanical processes, there is a change in mechanical energy W meh. The measure of change in mechanical energy is the work of forces applied to the system:

\(~\Delta W_(meh) = A.\)

During heat transfer, a change in the internal energy of the body occurs. The measure of change in internal energy during heat transfer is the amount of heat.

Quantity of heat is a measure of the change in internal energy that the body receives (or gives away) in the process of heat transfer.

Thus, both work and the amount of heat characterize the change in energy, but are not identical to energy. They do not characterize the state of the system itself, but determine the process of energy transition from one form to another (from one body to another) when the state changes and essentially depend on the nature of the process.

The main difference between work and the amount of heat is that work characterizes the process of changing the internal energy of the system, accompanied by the transformation of energy from one type to another (from mechanical to internal). The amount of heat characterizes the process of transfer of internal energy from one body to another (from more heated to less heated), not accompanied by energy transformations.

Experience shows that the amount of heat required to heat a body with a mass m temperature T 1 to temperature T 2 is calculated by the formula

\(~Q = cm (T_2 - T_1) = cm \Delta T, \qquad (1)\)

where c- specific heat capacity of the substance;

\(~c = \frac(Q)(m (T_2 - T_1)).\)

The SI unit of specific heat is the joule per kilogram-Kelvin (J/(kg K)).

Specific heat c is numerically equal to the amount of heat that must be imparted to a body of mass 1 kg in order to heat it by 1 K.

Heat capacity body C T is numerically equal to the amount of heat required to change the body temperature by 1 K:

\(~C_T = \frac(Q)(T_2 - T_1) = cm.\)

The SI unit of heat capacity of a body is the joule per Kelvin (J/K).

To change a liquid into a vapor at a constant temperature, the amount of heat required is

\(~Q = Lm, \qquad (2)\)

where L- specific heat of vaporization. When steam condenses, the same amount of heat is released.

In order to melt a crystalline body with a mass m at the melting point, it is necessary for the body to report the amount of heat

\(~Q = \lambda m, \qquad (3)\)

where λ - specific heat of fusion. During the crystallization of a body, the same amount of heat is released.

The amount of heat that is released during the complete combustion of fuel mass m,

\(~Q = qm, \qquad (4)\)

where q- specific heat of combustion.

The SI unit of specific heats of vaporization, melting, and combustion is joule per kilogram (J/kg).

Literature

Aksenovich L. A. Physics in high school: Theory. Tasks. Tests: Proc. allowance for institutions providing general. environments, education / L. A. Aksenovich, N. N. Rakina, K. S. Farino; Ed. K. S. Farino. - Mn.: Adukatsia i vykhavanne, 2004. - C. 154-155.

The concept of the amount of heat was formed in the early stages of the development of modern physics, when there were no clear ideas about internal structure matter, about what energy is, about what forms of energy exist in nature and about energy as a form of movement and transformation of matter.

The amount of heat is understood as a physical quantity equivalent to the energy transferred to the material body in the process of heat exchange.

The obsolete unit of the amount of heat is the calorie, equal to 4.2 J, today this unit is practically not used, and the joule has taken its place.

Initially, it was assumed that the carrier of thermal energy is some completely weightless medium that has the properties of a liquid. Numerous physical problems of heat transfer have been and are still being solved based on this premise. The existence of a hypothetical caloric was taken as the basis for many essentially correct constructions. It was believed that caloric is released and absorbed in the phenomena of heating and cooling, melting and crystallization. The correct equations for heat transfer processes were obtained from incorrect physical concepts. There is a known law according to which the amount of heat is directly proportional to the mass of the body involved in heat exchange and the temperature gradient:

Where Q is the amount of heat, m is the mass of the body, and the coefficient from- a quantity called specific heat capacity. Specific heat capacity is a characteristic of the substance involved in the process.

Work in thermodynamics

As a result of thermal processes, purely mechanical work. For example, when heated, a gas increases its volume. Let's take a situation as in the figure below:

In this case, the mechanical work will be equal to the gas pressure force on the piston multiplied by the path traveled by the piston under pressure. Of course, this is the simplest case. But even in it, one difficulty can be noticed: the pressure force will depend on the volume of the gas, which means that we are not dealing with constants, but with variables. Since all three variables: pressure, temperature and volume are related to each other, the calculation of work becomes much more complicated. There are some ideal, infinitely slow processes: isobaric, isothermal, adiabatic and isochoric - for which such calculations can be performed relatively simply. A plot of pressure versus volume is plotted, and work is calculated as an integral of the form.