We all know that water in a kettle boils at 100°C. But have you noticed that the temperature of water does not change during the boiling process? The question is - where does the generated energy go if we constantly keep the container on fire? It goes into converting liquid into steam. Thus, for the transition of water into a gaseous state, a constant supply of heat is required. How much it is needed to convert a kilogram of liquid into steam of the same temperature is determined by a physical quantity called the specific heat of vaporization of water.
Boiling requires energy. Most of it is used to break the chemical bonds between atoms and molecules, resulting in the formation of vapor bubbles, and the smaller part is used to expand the vapor, that is, so that the formed bubbles can burst and release it. Since the liquid puts all its energy into the transition to the gaseous state, its "forces" run out. For constant renewal of energy and prolongation of boiling, more and more heat must be brought to the container with liquid. A boiler, gas burner or any other heating device can provide its inflow. During boiling, the temperature of the liquid does not increase, the process of formation of steam of the same temperature takes place.
Different liquids require different amounts of heat to turn into vapor. Which one - shows the specific heat of vaporization.
You can understand how this value is determined from an example. Take 1 liter of water and bring it to a boil. Then we measure the amount of heat needed to evaporate all the liquid, and we get the value of the specific heat of vaporization for water. For other chemical compounds, this indicator will be different.
In physics, the specific heat of vaporization is denoted by the Latin letter L. It is measured in joules per kilogram (J / kg). It can be derived by dividing the heat expended on evaporation by the mass of the liquid:
This value is very important for production processes based on modern technologies. For example, they are guided by it in the production of metals. It turned out that if iron is melted and then condensed, with further hardening, a stronger crystal lattice is formed.
What is equal to
The value of specific heat for various substances (r) was determined in the course of laboratory studies. Water at normal atmospheric pressure boils at 100 °C, and the heat of vaporization of water is 2258.2 kJ/kg. This indicator for some other substances is given in the table:
| Substance | boiling point, °C | r, kJ/kg |
|---|---|---|
| Nitrogen | -196 | 198 |
| Helium | -268,94 | 20,6 |
| Hydrogen | -253 | 454 |
| Oxygen | -183 | 213 |
| Carbon | 4350 | 50000 |
| Phosphorus | 280 | 400 |
| Methane | -162 | 510 |
| Pentane | 36 | 360 |
| Iron | 2735 | 6340 |
| Copper | 2590 | 4790 |
| Tin | 2430 | 2450 |
| Lead | 1750 | 8600 |
| Zinc | 907 | 1755 |
| Mercury | 357 | 285 |
| Gold | 2 700 | 1 650 |
| Ethanol | 78 | 840 |
| Methyl alcohol | 65 | 1100 |
| Chloroform | 61 | 279 |
However, this indicator can change under the influence of certain factors:
- Temperature. With its increase, the heat of evaporation decreases and can be equal to zero.
t, °C r, kJ/kg 2500 10 2477 20 2453 50 2380 80 2308 100 2258 200 1940 300 1405 374 115 374,15 - Pressure. As the pressure decreases, the heat of vaporization increases, and vice versa. The boiling point is directly proportional to pressure and can reach a critical value of 374 °C.
p, Pa bp, °C r, kJ/kg 0,0123 10 2477 0,1234 50 2380 1 100 2258 2 120 2202 5 152 2014 10 180 1889 20 112 1638 50 264 1638 100 311 1316 200 366 585 220 373,7 184,8 Critical 221.29 374,15 - - The mass of the substance. The amount of heat involved in the process is directly proportional to the mass of the resulting steam.
The ratio of evaporation and condensation
Physicists have found that the reverse evaporation process - condensation - steam spends exactly the same amount of energy as was spent on its formation. This observation confirms the law of conservation of energy.
Otherwise, it would be possible to create an installation in which the liquid would evaporate and then condense. The difference between the heat required for evaporation and the heat sufficient for condensation would result in the accumulation of energy that could be used for other purposes. In fact, a perpetual motion machine would be created. But this is contrary to physical laws, and therefore impossible.
How is it measured
- The specific heat of vaporization of water is measured experimentally in physical laboratories. For this, calorimeters are used. The procedure is as follows:
- A certain amount of liquid is poured into the calorimeter.
From §§ 2.5 and 7.2 it follows that during vaporization the internal energy of a substance increases, and during condensation it decreases. Since during these processes the temperatures of the liquid and its vapor can be equal, the change in the internal energy of the substance occurs only due to a change in the potential energy of the molecules. So, at the same temperature, a unit mass of a liquid has less internal energy than a unit mass of its vapor.
Experience shows that the density of a substance in the process of vaporization greatly decreases, and the volume occupied by the substance increases. Therefore, during vaporization, work must be done against the forces of external pressure. Therefore, the energy that must be imparted to a liquid to turn it into vapor at a constant temperature goes partly to increase the internal energy of the substance and partly to doing work against external forces in the process of its expansion.
In practice, heat is supplied to the liquid to convert it into vapor during heat exchange. The amount of heat required to convert a liquid to vapor at a constant temperature is called the heat of vaporization. When a vapor turns into a liquid, an amount of heat must be removed from it, which is called the heat of condensation. If the external conditions are the same, then with equal masses of the same substance, the heat of vaporization is equal to the heat of condensation.
With the help of a calorimeter, it was found that the heat of vaporization is directly proportional to the mass of liquid converted into vapor
Here - coefficient of proportionality, the value of which depends on the type of liquid and external conditions.
The value that characterizes the dependence of the heat of vaporization on the type of substance and on external conditions is called the specific heat of vaporization. The specific heat of vaporization is measured by the amount of heat required to convert a unit mass of liquid into steam at a constant temperature:
In SI, the specific heat of vaporization of such a liquid is taken as a unit, for the transformation into steam of 1 kg of which at a constant temperature, 1 J of heat is required. (Show this with formula (7.1a).)
As an example, we note that the specific heat of vaporization of water at a temperature of (100°C) is equal to
Since vaporization can occur at different temperatures, the question arises: will the specific heat of vaporization of a substance change in this case? Experience shows that as the temperature rises, the specific heat of vaporization decreases. This is because all liquids expand when heated. In this case, the distance between molecules increases and the forces of molecular interaction decrease. In addition, the higher the temperature, the greater the average energy of the liquid molecules and the less energy they need to add so that they can fly out of the surface of the liquid.
goal of the work
Assimilation and consolidation of theoretical material on the topic of the thermodynamics course "Water vapor", as well as mastering the methods of experiment and processing of the data obtained, familiarization with the tables "Thermophysical properties of water and steam".
1. Study the scheme of the experimental setup, turn it on and bring it to a given stationary thermal regime.
2. Carry out the experiment in accordance with the guidelines, fill in table 1.
3. Determine the specific heat spent on the vaporization of water in the experiment.
4. For the isobaric process of vaporization, determine the tabular values of the parameters of water on the saturation line and dry saturated steam, as well as the specific heat of vaporization.
5. Calculate the internal energy of the liquid on the vapor saturation line for the conditions of the experiment.
6. Calculate the error of the found value of the specific heat of vaporization in relation to the table.
7. Depict the processes occurring in the Dewar vessel in P-v and T-s-diagrams.
8. Make a conclusion on the work.
METHODOLOGICAL INSTRUCTIONS
The transition of a substance from a liquid state to a gaseous state is called vaporization, the reverse transition is called condensation. Boiling a liquid is a process of vaporization inside a liquid that occurs at a strictly defined temperature t n, ° C, determined by pressure. If a gaseous phase exists with a liquid phase of the same substance, then it is called vapor. The gaseous phase of the system is dry saturated steam, and the liquid phase is a liquid that retains the state corresponding to the beginning of vaporization.
During vaporization according to the isobaric-isothermal process, according to the first law of thermodynamics, the specific heat of phase transformation (specific heat of vaporization) r, J / kg,
r \u003d u "- u" + p (v "-v"), (1)
r = i" - i" , (2)
where u", i", v" - respectively, internal energy, enthalpy, J / kg, and the specific volume of dry saturated steam, m 3 / kg;
u", i", v" - respectively, the internal energy, enthalpy, J / kg, and the specific volume of the liquid in the state of saturation, m 3 / kg.
The pressure p, Pa, is not marked with special indices, since it does not change during the entire phase transition and is equal to the saturation pressure.
Thus, the specific heat of vaporization includes a change in the internal energy of a substance and the work of a change in volume during a phase transition.
The specific heat of vaporization is functionally related to the state parameters. For most substances used in practice, the properties of liquid and vapor on the saturation line are determined and tabulated. These tables give the values of p and t on the saturation line and the corresponding values of v", v", i", i", r, s", s". The internal energy of the liquid on the saturation line u", J / kg, and dry saturated steam u", J / kg, is determined respectively by the equations
u"=i"-pv"(3)
u"=i"-pv" (4)
EXPERIMENTAL SETUP
Picture. Scheme of the experimental setup
The experimental setup (figure) consists of a Dewar vessel 1 with an electric heater 2, into which a portion of distilled water is poured from a container 3, regulated by a valve 4. The resulting vapor in the condenser 5, through which tap water passes, turns into a liquid. The water flow is regulated by the valve 7 according to the control lamp 8. The resulting condensate is collected in a measuring cylinder 9. On the control panel are: switch "NETWORK" 10, voltmeter 11, ammeter 12, mode switch 13; 6 - glass funnel.
EXPERIMENTAL TECHNIQUE
1. Turn the unit on by turning switch 10 to position "1".
2. Check the filling of the Dewar vessel 1 by setting the mode switch 13 to the "FILLING" position. If at the same time the green signal lamp "Vessel is full" lights up, you can start the experiment. Otherwise, the vessel is filled with distilled water, for which valve 4 is opened. After the green signal lamp lights up, close the vessel tightly.
3. Move switch 13 to the "HEATING" position.
4. Turning the knob of the autotransformer 14, set the value of the voltage on the heater U, V (and the current strength I, A) set by the teacher.
5. Supply cooling water to the condenser 5 by opening the valve 7 and adjust the water flow according to the control lamp 8.
6. When a stationary mode of water boiling in a Dewar vessel is established (15-20 cm of condensate will accumulate in measuring cylinder 9), make a control collection of condensate in the amount indicated by the teacher (V, m 3). The duration of the control collection t, s, is determined by the stopwatch.
7. Using a barometer, determine the atmospheric pressure P a, mm Hg.
8. Enter the measurement data into the table of observations and sign it with the teacher.
9. Turn on the unit by turning the switch "0", close valve 7, turn the handle of the autotransformer counterclockwise until it stops, drain the condensate into container 3.
Table 1
|
Measurement number |
mm. rt. Art. | ||||
PROCESSING OF EXPERIMENTAL DATA
1. Calculate the amount of heat spent on vaporization of 1 kg of water r op, J / kg:
r op = (W - Q) / (Vr),
where W = UI - heater power, W;
Q = 0.04W - heat losses, W;
r is the density of the condensate, kg / m 3. We accept r \u003d 1000 kg / m 3.
2. Assuming that water boils at atmospheric pressure, determine from the tabular values of the water parameters on the saturation line and dry saturated steam, which are entered in Table 2.
Table 2
|
i", kJ/kg |
S", kJ/(kgK) |
i", kJ/kg |
S", J/(kgK) | |||
3. Calculate the values of the internal energy of water on the saturation line u" and dry saturated steam u", kJ/kg, using formulas (3) and (4).
4. Calculate the error, %, of the found value of the specific heat of vaporization r op, kJ / kg, in relation to the tabular r, kJ / kg, according to the formula:
D \u003d (r op - r) 100 / r.
5. Present graphically the processes occurring in the Dewar vessel in P-v and T-s-diagrams.
6. Make a conclusion on the work.
QUESTIONS FOR SELF-EDUCATION
1. Vaporization of liquid; the essence of the processes of boiling and evaporation of a liquid.
2. Isobaric process of transition of liquid into superheated steam in P-v and T-s-diagrams.
3. Boundary curves with the degree of dryness x = 0 and x = 1, the critical state of the substance
4. Concepts: liquid on the saturation line, wet saturated steam, dry saturated steam, superheated steam.
5. Specific heat of vaporization of liquid.
6. The degree of dryness, the degree of humidity of the steam.
7. Tables of thermophysical properties of water and water vapor, their meaning.
8. Determination of wet steam parameters.
9. i-s-diagram of water vapor, its purpose.
10. Steam thermodynamic processes in P-v, T-s, i-s-diagrams.
REFERENCES
1. Heat engineering / Ed. A.P. Baskakova.- M.: Energoizdat, 1991.- 224 p.
2. Nashchokin V.V. Technical thermodynamics and heat transfer. - M .:: Higher School, 1980. - 496 p.
3. Yudaev B.N. Technical thermodynamics. Heat transfer. - M .: Higher school, 1998. - 480 p.
4. Rivkin S.L., Alexandrov A.A. Tables of thermophysical properties of water and steam.- M.: Energy, 1980.- 408 p.
Instruments and accessories used in the work:
2. Steam line (rubber tube).
3. Calorimeter.
4. Electric stove.
5. Thermometer.
6. Technical scales with weight.
7. Beaker.
Objective:
To learn experimentally to determine the specific heat of vaporization of water.
I. THEORETICAL INTRODUCTION.
In the process of energy exchange between matter and the environment, the transition of matter from one state of aggregation to another (from one phase state to another) is possible.
The transition of a substance from a liquid state to a gaseous state is called vaporization.
Vaporization occurs in the form of evaporation and boiling.
Vaporization that occurs only from the free surface of a liquid is called evaporation .
Evaporation occurs at any temperature of the liquid, but with increasing temperature, the rate of evaporation of the liquid increases.
The evaporating liquid can be cooled if heat is not intensively supplied to it from the outside, or it can be heated, heat is supplied from the outside intensively.
Vaporization, which occurs throughout the volume of a liquid and at a constant temperature, is called boiling.
The boiling point depends on the external pressure on the surface of the liquid.
The boiling point of a liquid at normal atmospheric pressure is called boiling point this liquid.
During vaporization, the internal energy of a substance increases, therefore, to convert a liquid into vapor, heat must be supplied to it in the process of heat transfer.
The amount of heat required to change a liquid to vapor at a constant temperature is called heat of vaporization.
The value is directly proportional to the mass of the liquid turned into vapor:
The value g, which characterizes the dependence of the heat of vaporization on the type of substance and on external conditions, is called specific heat of vaporization . The specific heat of vaporization is measured by the amount of heat required to convert a unit mass of liquid into steam at a constant temperature:
In SI, the specific heat of vaporization is measured in .
The value depends on the temperature at which vaporization occurs. Experience shows that as the temperature rises, the specific heat of vaporization decreases. The graph (Fig. 1) shows the dependence on for water.
In this paper, the specific heat of vaporization of water is determined using the boiling process, using the heat balance equation for the condensation of water vapor. To do this, take a calorimeter (K) (see Fig. 2), in which there is water at a temperature, water vapor, having a boiling point, is introduced from the flask through the steam line P into the cold water of the calorimeter, where it condenses.
After some time, the steam pipeline tube is removed and the temperature established in the calorimeter is measured and the mass of the steam introduced into the calorimeter is determined.
Then the heat balance equation is drawn up.
When steam condenses, heat is released.
where is the specific heat of condensation (it is also the specific heat of vaporization). The condensed steam turns into water at a temperature , which then cools down to a temperature and releases heat .
(4)
The heat released during the condensation of steam and cooling of hot water is received by the calorimeter and the water in it. Due to this, they are heated from temperature to temperature . The heat received by the calorimeter and cold water is calculated in it by the formula:
The heat balance equation is compiled in accordance with the law of conservation of energy during heat transfer.
During heat transfer, the sum of the amounts of heat given away by all bodies, in which the internal energy decreases, is equal to the sum of the amounts of heat received by all bodies, in which the internal energy increases:
(6)
In our case, for the heat exchange that occurred in the calorimeter, we assume that there is no heat loss to the environment. Therefore, equation (6) can be written as: or
From this equation we obtain a working formula for calculating the value based on the results of the experiment:
2. PROGRESS OF WORK.
1. Make a table in which the results of measurements and calculations will be entered in the form given at the end of the description.
2. Weigh the inner vessel of the calorimeter, enter the resulting value in the table.
3. Using a beaker, measure 150 200 ml of cold water, pour it into the calorimeter and measure the mass of the inner vessel of the calorimeter with water (m 2). Find the mass of water:
m in \u003d m 2 - m to
Record the mass of cold water in the table.
4. Measure the initial temperature of the calorimeter and water in it Value, write in the table.
5. Immerse the tip of the steam pipe into the water of the calorimeter and let steam in until the temperature of the water rises by 30°K - 35°K (q-temperature after heat exchange).
6. Weigh the inner beaker of the calorimeter and determine the mass of condensed vapor. Record the result in a table. ()
7. The values of the specific heat capacities of water and the substance of the calorimeter (aluminum) and the tabular value of the specific heat of vaporization of water are given in the table of measurement and calculation results.
8. Using formula (7), calculate the specific heat of vaporization of water.
9. Calculate the absolute and relative error of the result obtained relative to the tabular result using the formulas:
; 
10. Make a conclusion about the work done and the result of the specific heat of vaporization of water.
TABLE OF MEASUREMENT AND CALCULATION RESULTS
Do you know what the temperature of the boiled soup is? 100 ˚С. No more, no less. At the same temperature, the kettle boils and the pasta is boiled. What does it mean?
Why does the temperature of the water inside not rise above one hundred degrees when a saucepan or kettle is constantly heated with burning gas? The fact is that when water reaches a temperature of one hundred degrees, all the incoming thermal energy is spent on the transition of water into a gaseous state, that is, evaporation. Up to a hundred degrees, evaporation occurs mainly from the surface, and when it reaches this temperature, the water boils. Boiling is also evaporation, but only over the entire volume of the liquid. Hot steam bubbles are formed inside the water and, being lighter than water, these bubbles break out to the surface, and the steam from them escapes into the air.
Up to a hundred degrees, the temperature of the water rises when heated. After a hundred degrees, with further heating, the temperature of the water vapor will increase. But until all the water boils away at one hundred degrees, its temperature will not rise, no matter how much energy you apply. We have already figured out where this energy goes - to the transition of water into a gaseous state. But if such a phenomenon exists, then there must be the physical quantity that describes this phenomenon. And such a value exists. It is called the specific heat of vaporization.
Specific heat of vaporization of water
The specific heat of vaporization is a physical quantity that indicates the amount of heat required to turn a 1 kg liquid into vapor at the boiling point. The specific heat of vaporization is denoted by the letter L. And the unit of measurement is the joule per kilogram (1 J / kg).
The specific heat of vaporization can be found from the formula:
where Q is the amount of heat,
m - body weight.
By the way, the formula is the same as for calculating the specific heat of fusion, the difference is only in the designation. λ and L
Empirically, the values of the specific heat of vaporization of various substances were found and tables were compiled from which data can be found for each substance. Thus, the specific heat of vaporization of water is 2.3*106 J/kg. This means that for every kilogram of water, an amount of energy equal to 2.3 * 106 J must be spent to turn it into steam. But at the same time, the water should already have a boiling point. If the water was initially at a lower temperature, then it is necessary to calculate the amount of heat that will be required to heat the water to one hundred degrees.
In real conditions, it is often necessary to determine the amount of heat required for the transformation of a certain mass of a liquid into vapor, therefore, more often one has to deal with a formula of the form: Q \u003d Lm, and the values \u200b\u200bof the specific heat of vaporization for a particular substance are taken from ready-made tables.
