The specific heat of fusion of ice is J mol. Specific heat of fusion of various substances. Topic: State of aggregation of matter

Density, thermal conductivity and heat capacity of ice depending on temperature

The table shows the values ​​of density, thermal conductivity, specific heat capacity of ice depending on temperature in the range from 0 to -100 ° C.

According to the data in the table, it can be seen that with a decrease in temperature, the specific heat capacity of ice decreases, while the thermal conductivity and density of ice, on the contrary, increase. For instance, at a temperature of 0 ° C, the density of ice has a value of 916.2 kg / m 3, and at a temperature of minus 100 ° C, its density becomes equal to 925.7 kg / m 3.

The specific heat of ice at 0 ° C is 2050 J / (kg · deg). With a decrease in ice temperature from -5 to -100 ° C, its specific heat capacity decreases by 1.45 times. The heat capacity of ice is two times less.

The thermal conductivity of ice when its temperature decreases from 0 to minus 100 ° С increases from 2.22 to 3.48 W / (m · deg). Ice is more thermally conductive than water - it can conduct 4 times more heat under the same boundary conditions.

It should be noted that the density of ice is less, however, with a decrease in temperature, the density of ice increases, and as the temperature approaches absolute zero, the density of ice becomes close to the value of the density of water.

Thermophysical properties of ice and snow

The table shows the following properties of ice and snow:

• ice density, kg / m 3;
• thermal conductivity of ice and snow, kcal / (m · hour · deg) and W / (m · deg);
• specific mass heat capacity of ice, kcal / (kg · deg) and J / kg · deg);
• coefficient of thermal diffusivity, m 2 / hour and m 2 / sec.

The properties of ice and snow are presented depending on the temperature in the range: for ice from 0 to -120 ° С; for snow from 0 to -50 ° C, depending on the compaction (density). The thermal diffusivity of ice and snow in the table is given with a multiplier of 10 6. For example, the thermal diffusivity of ice at a temperature of 0 ° C is 1.08 · 10 -6 m 2 / s.

Ice vapor pressure

The table shows the values ​​of the saturated vapor pressure of ice during sublimation (the transition of ice to vapor, bypassing the liquid phase) depending on the temperature in the range from 0.01 to -80 ° C. The table shows that with decreasing ice temperature, the pressure of its saturated vapor decreases.

Sources:

1. Volkov. A.I., Zharsky. THEM. Great chemical reference book. - M: Soviet School, 2005. - 608 p.

Specific heat of fusion is the amount of heat required to melt one gram of a substance. Specific heat of fusion is measured in joules per kilogram and is calculated as the quotient of dividing the amount of heat by the mass of the melting substance.

Specific heat of fusion for different substances

Different substances have different specific heats of fusion.

Aluminum is a silvery metal. It is easy to process and is widely used in engineering. Its specific heat of fusion is 290 kJ / kg.

Iron is also a metal, one of the most abundant on Earth. Iron is widely used in industry. Its specific heat of fusion is 277 kJ / kg.

Gold is a noble metal. It is used in jewelry, dentistry and pharmacology. The specific heat of fusion of gold is 66.2 kJ / kg.

Silver and platinum are also precious metals. They are used in jewelry making, engineering and medicine. The specific heat is 101 kJ / kg, and that of silver is 105 kJ / kg.

Tin is a gray low-melting metal. It is widely used in solders, tinplate and bronze. The specific heat is 60.7 kJ / kg.

Mercury is a mobile metal that freezes at -39 degrees. This is the only metal that is normal conditions exists in a liquid state. Mercury is used in metallurgy, medicine, technology, and the chemical industry. Its specific heat of fusion is 12 kJ / kg.

Ice is the solid phase of water. Its specific heat of fusion is 335 kJ / kg.

Naphthalene is an organic substance similar in chemical properties With . It melts at 80 degrees and self-ignites at 525 degrees. Naphthalene is widely used in the chemical industry, pharmaceuticals, explosives and dyes. The specific heat of fusion of naphthalene is 151 kJ / kg.

Methane and propane gases are used as energy carriers and are used as raw materials in the chemical industry. The specific heat of fusion of methane is 59 kJ / kg, and 79.9 kJ / kg.

In this lesson, we will explore the concept of "specific heat of fusion". This value characterizes the amount of heat that must be communicated to 1 kg of a substance at the melting temperature so that it passes from a solid state to a liquid (or vice versa).

We will study the formula for finding the amount of heat that is necessary for melting (or released during crystallization) of a substance.

Topic: State of aggregation of matter

Lesson: Specific Heat of Fusion

This lesson is devoted to the main characteristic of melting (crystallization) of a substance - the specific heat of fusion.

In the last lesson, we raised the question: how does the internal energy of a body change during melting?

We found out that when heat is supplied, the internal energy of the body increases. At the same time, we know that the internal energy of a body can be characterized by such a concept as temperature. As we already know, the temperature does not change during melting. Therefore, a suspicion may arise that we are dealing with a paradox: the internal energy increases, but the temperature does not change.

The explanation for this fact is quite simple: all energy is spent on the destruction of the crystal lattice. Similarly, in the reverse process: during crystallization, the molecules of a substance are combined into a single system, while the excess energy is given up and absorbed by the external environment.

As a result of various experiments, it was possible to establish that for the same substance, a different amount of heat is required to transfer it from a solid to a liquid state.

Then it was decided to compare these amounts of heat with the same mass of matter. This led to the appearance of such a characteristic as the specific heat of fusion.

Definition

Specific heat of fusion- the amount of heat that must be imparted to 1 kg of a substance heated to the melting point in order to transfer it from a solid state to a liquid.

The same value is released during the crystallization of 1 kg of the substance.

Specific heat of fusion is indicated (Greek letter, read as "lambda" or "lambda").

Units: . In this case, there is no temperature in the dimension, since the temperature does not change during melting (crystallization).

To calculate the amount of heat required to melt a substance, use the formula:

Heat quantity (J);

Specific heat of fusion (which is searched for according to the table;

The mass of the substance.

When a body crystallizes, it is written with a "-" sign, as heat is released.

An example is the specific heat of melting of ice:

... Or the specific heat of fusion of iron:

.

The fact that the specific heat of melting of ice is higher than the specific heat of melting of iron should not be surprising. The amount of heat that a particular substance needs to melt depends on the characteristics of the substance, in particular, on the energy of bonds between the particles of a given substance.

In this lesson, we examined the concept of specific heat of fusion.

In the next lesson, we will learn how to solve problems on heating and melting crystalline bodies.

Bibliography

1. Gendenshtein L. E, Kaidalov A.B., Kozhevnikov VB Physics 8 / Ed. Orlova V.A., Royzen I.I. - M .: Mnemosina.
2. Peryshkin A.V. Physics 8. - M .: Bustard, 2010.
3. Fadeeva A.A., Zasov A.V., Kiselev D.F. Physics 8. - M .: Education.

1. Physics, mechanics, etc. ().
2. Cool physics ().
3. Internet portal Kaf-fiz-1586.narod.ru ().

Homework

We have seen that a vessel of ice and water brought into a warm room does not heat up until all the ice has melted. In this case, water is obtained from ice at the same temperature. At this time, heat flows to the ice-water mixture and, therefore, the internal energy of this mixture increases. From this we must conclude that the internal energy of water at is greater than the internal energy of ice at the same temperature. Since the kinetic energy of molecules, water and ice is the same at the same, the increment in internal energy during melting is an increment in the potential energy of the molecules.

Experience shows that what has been said is true for all crystals. When the crystal melts, it is necessary to continuously increase the internal energy of the system, while the temperature of the crystal and the melt remains unchanged. Usually, an increase in internal energy occurs when a certain amount of heat is transferred to the crystal. The same goal can be achieved by doing work, such as friction. So, the internal energy of the melt is always greater than the internal energy of the same mass of crystals at the same temperature. This means that an ordered arrangement of particles (in a crystalline state) corresponds to a lower energy than a disordered arrangement (in a melt).

The amount of heat required to transfer a unit mass of a crystal to a melt of the same temperature is called the specific heat of fusion of a crystal. It is expressed in joules per kilogram.

When a substance solidifies, the heat of fusion is released and transferred to the surrounding bodies.

Determination of the specific heat of fusion of refractory bodies (bodies with a high melting point) is not an easy task. The specific heat of fusion of a low-melting crystal such as ice can be determined with a calorimeter. Having poured a certain amount of water of a certain temperature into the calorimeter and thrown into it a certain mass of ice that has already begun to melt, that is, has a temperature, let us wait until all the ice has melted and the temperature of the water in the calorimeter takes on a constant value. Using the law of conservation of energy, we compose the heat balance equation (§ 209), which allows us to determine the specific heat of melting of ice.

Let the mass of water (including the water equivalent of the calorimeter) be equal to the mass of ice -, specific heat of water -, initial water temperature -, final -, specific heat of melting of ice -. The heat balance equation has the form

.

Table 16 shows the values ​​of the specific heats of fusion of some substances. The great heat of melting of ice is noteworthy. This circumstance is very important, since it slows down the melting of ice in nature. If the specific heat of fusion was much lower, the spring floods would be many times stronger. Knowing the specific heat of fusion, we can calculate how much heat is needed to melt a body. If the body is already heated to the melting point, then it is necessary to spend heat only to melt it. If it has a temperature below the melting point, then you still need to spend heat on heating.

Table 16.

The transition of a substance from a solid crystalline state to a liquid is called melting. To melt a solid crystalline body, it must be heated to a certain temperature, that is, heat must be supplied.The temperature at which a substance melts is calledmelting point of the substance.

The reverse process - the transition from a liquid to a solid state - occurs with a decrease in temperature, that is, heat is removed. The transition of a substance from a liquid to a solid state is calledhardening , or crystallization . The temperature at which a substance crystallizes is calledcrystal temperaturezations .

Experience shows that any substance crystallizes and melts at the same temperature.

The figure shows a graph of the dependence of the temperature of the crystalline body (ice) on the heating time (from the point A to the point D) and cooling time (from point D to the point K). Time is plotted on the horizontal axis, and temperature is plotted on the vertical axis.

It can be seen from the graph that the observation of the process began from the moment when the ice temperature was -40 ° С, or, as they say, the temperature at the initial moment of time tearly= -40 ° C (point A on the graph). With further heating, the ice temperature rises (on the graph this is a section AB). The temperature increases to 0 ° C - the melting point of ice. At 0 ° C, ice begins to melt, and its temperature stops rising. During the entire melting time (i.e. until all the ice has melted), the ice temperature does not change, although the burner continues to burn and heat is therefore supplied. The melting process corresponds to the horizontal section of the graph Sun . Only after all the ice has melted and turned into water, the temperature starts to rise again (section CD). After the water temperature reaches + 40 ° C, the burner is extinguished and the water begins to cool, that is, the heat is removed (for this, you can place a vessel with water in another, larger vessel with ice). The water temperature begins to drop (section DE). When the temperature reaches 0 ° C, the water temperature stops decreasing, despite the fact that the heat is still removed. This is the process of water crystallization - ice formation (horizontal section EF). Until all the water turns to ice, the temperature will not change. Only after this does the ice temperature begin to decrease (section FK).

The view of the considered graph is explained as follows. Location on AB due to the supplied heat, the average kinetic energy of ice molecules increases, and its temperature rises. Location on Sun all the energy received by the contents of the flask is spent on the destruction of the crystal lattice of ice: the ordered spatial arrangement of its molecules is replaced by disordered ones, the distance between the molecules changes, i.e. there is a rearrangement of molecules in such a way that the substance becomes liquid. In this case, the average kinetic energy of the molecules does not change, therefore the temperature also remains unchanged. A further increase in the temperature of molten ice-water (in the area CD) means an increase in the kinetic energy of water molecules due to the heat supplied by the burner.

When cooling water (section DE) part of the energy is taken away from it, water molecules move at lower speeds, their average kinetic energy decreases - the temperature decreases, the water cools. At 0 ° С (horizontal section EF) molecules begin to line up in a certain order, forming a crystal lattice. Until this process is completed, the temperature of the substance will not change, despite the heat removed, which means that when solidified, the liquid (water) releases energy. This is exactly the energy that the ice absorbed, turning into a liquid (section Sun). The internal energy of the liquid is greater than that of solid... When melting (and crystallizing), the body's internal energy changes abruptly.

Metals melting at temperatures above 1650 ºС are called refractory(titanium, chromium, molybdenum, etc.). The highest melting point among them is for tungsten - about 3400 ° C. Refractory metals and their compounds are used as heat-resistant materials in aircraft construction, rocketry and space technology, and nuclear power.

Let us emphasize once again that a substance absorbs energy when it melts. On the other hand, during crystallization, it releases it into the environment. Receiving a certain amount of heat released during crystallization, the medium heats up. This is well known to many birds. Not so long ago they can be seen in winter in frosty weather sitting on the ice that covers rivers and lakes. Due to the release of energy during the formation of ice, the air above it is several degrees warmer than in the forest in the trees, and birds take advantage of this.

Melting amorphous substances.

The presence of a certain melting point Is an important feature of crystalline substances. It is on this basis that they can be easily distinguished from amorphous bodies, which are also referred to as solids. These include, in particular, glasses, highly viscous resins, plastics.

Amorphous substances(unlike crystalline ones) do not have a specific melting point - they do not melt, but soften. When heated, a piece of glass, for example, first becomes soft from hard, it can be easily bent or stretched; at a higher temperature, the piece begins to change its shape under the influence of its own gravity. As it heats up, the thick, viscous mass takes the shape of the vessel in which it lies. This mass is at first thick, like honey, then - like sour cream and, finally, becomes almost the same low-viscosity liquid as water. However, it is impossible to indicate a certain temperature of the transition of a solid to a liquid here, since it does not exist.

The reasons for this lie in the fundamental difference between the structure of amorphous bodies and the structure of crystalline ones. The atoms in amorphous bodies are randomly arranged. Amorphous bodies are similar in structure to liquids. Already in solid glass, atoms are arranged randomly. This means that an increase in the temperature of the glass only increases the range of oscillations of its molecules, gradually gives them more and more freedom of movement. Therefore, the glass softens gradually and does not exhibit a sharp "solid-liquid" transition, characteristic of the transition from the arrangement of molecules in a strict order to a disordered one.

Heat of fusion.

Heat of fusion Is the amount of heat that must be imparted to a substance at constant pressure and constant temperature equal to the melting point in order to completely transfer it from a solid crystalline state to a liquid state. The heat of fusion is equal to the amount of heat that is released during the crystallization of a substance from a liquid state. When melting, all the heat supplied to a substance is spent on increasing the potential energy of its molecules. The kinetic energy does not change, since melting occurs at a constant temperature.

Studying experimentally the melting of various substances of the same mass, one can notice that different amounts of heat are required to transform them into a liquid. For example, in order to melt one kilogram of ice, you need to spend 332 J of energy, and in order to melt 1 kg of lead - 25 kJ.

The amount of heat released by the body is considered negative. Therefore, when calculating the amount of heat released during the crystallization of a substance with a mass m, you should use the same formula, but with a minus sign:

Heat of combustion.

Heat of combustion(or calorific value, calorie content) Is the amount of heat released during the complete combustion of the fuel.

To heat bodies, the energy released during the combustion of fuel is often used. Common fuels (coal, oil, gasoline) contain carbon. When burning, carbon atoms combine with oxygen atoms in the air, resulting in the formation of carbon dioxide molecules. The kinetic energy of these molecules turns out to be greater than that of the original particles. The increase in the kinetic energy of molecules during combustion is called energy release. The energy released during the complete combustion of the fuel is the heat of combustion of this fuel.

The heat of combustion of fuel depends on the type of fuel and its weight. The greater the mass of the fuel, the greater the amount of heat released during its complete combustion.

The physical quantity that shows how much heat is released during the complete combustion of fuel weighing 1 kg is called specific heat of combustion of fuel.Specific calorific value is denoted by the letterqand is measured in joules per kilogram (J / kg).

Quantity of heat Q combustion emission m kg of fuel, determined by the formula:

To find the amount of heat released during the complete combustion of fuel of arbitrary mass, you need to multiply the specific heat of combustion of this fuel by its mass.