Quantity of heat. Specific heat

The specific heat of fusion is the amount of heat required to melt one gram of a substance. Specific heat melting is measured in joules per kilogram and is calculated as the quotient of the amount of heat divided 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 silver-colored metal. It is easy to process and is widely used in technology. Its specific heat of fusion is 290 kJ/kg.

Iron is also a metal, one of the most common 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 - also precious metals. They are used in the manufacture of jewelry, technology and medicine. The specific heat is 101 kJ/kg, and that of silver is 105 kJ/kg.

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

Mercury is a mobile metal that freezes at -39 degrees. It is the only metal that 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 - organic matter, similar in chemical properties With . It melts at 80 degrees and spontaneously ignites at 525 degrees. Naphthalene is widely used in the chemical industry, pharmaceuticals, manufacturing explosives and dyes. The specific heat of fusion of naphthalene is 151 kJ/kg.

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

The processes of crystallization and melting describe the same physical quantities. The difference is that during melting, the body requires energy to destroy the lattice, and during crystallization, on the contrary, the body releases energy to the environment.

The concept of specific heat of crystallization

The specific heat of crystallization (melting) is understood as the amount of energy released (consumed) by 1 kg. substances during the transition from liquid to solid (and vice versa). It is important to note that during the process of crystallization (melting), the temperature of the substance does not change and it has already been brought to a value at which the process itself is possible.

The specific heat of crystallization (melting) is measured in J/kg, denoted by the letter of the Greek alphabet λ. A-priory:

where Q is the amount of energy released (consumed) by m kilograms of the substance.

Energy calculations for sequential thermal processes

Let's consider the process of cooling m kilograms of water from a temperature, for example, +20°C to -10°C. Here we are dealing with three thermal processes:

• water cooling from temperature +20°С to 0°С, ∆T1 = - 20°;
• crystallization of water into ice at a temperature of 0°C;
• ice cooling from temperature 0°С to -10°С, ∆T2 = - 10°;

The amount of energy released Q is equal to the sum of the energies in each of these processes:

Q = Q1 + Q2 + Q3;

Q1 = C1 * m * ∆T1;

Q3 = C2 * m * ∆T2;

where C1 and C2 are the specific heat capacity of water and ice, respectively. The “-” sign at Q2 means that the process of energy release during crystallization is underway.

In order to melt a solid substance, it must be heated. And when heating any body, one curious feature is noted

The peculiarity is this: the body temperature rises up to the melting point, and then stops until the entire body passes into a liquid state. After melting, the temperature begins to rise again, if, of course, heating is continued. That is, there is a period of time during which we heat the body, but it does not heat up. Where does the heat energy that we spend go? To answer this question, we need to look inside the body.

In a solid, the molecules are arranged in a certain order in the form of crystals. They practically do not move, only slightly oscillating in place. In order for a substance to turn into a liquid state, the molecules need to be given additional energy so that they can escape the attraction of neighboring molecules in the crystals. By heating the body, we give the molecules this necessary energy. And until all the molecules receive enough energy and all the crystals are destroyed, the body temperature does not increase. Experiments show that for different substances One mass requires different amounts of heat to completely melt it.

That is, there is a certain value on which it depends how much heat does a substance need to absorb to melt?. And this value is different for different substances. This quantity in physics is called the specific heat of fusion of a substance. Again, as a result of experiments, the values ​​of the specific heat of fusion for various substances have been established and collected in special tables from which this information can be gleaned. The specific heat of fusion is denoted by the Greek letter λ (lambda), and the unit of measurement is 1 J/kg.

Formula for specific heat of fusion

The specific heat of fusion is found by the formula:

where Q is the amount of heat required to melt a body of mass m.

Again, it is known from experiments that when substances solidify, they release the same amount of heat that was required to melt them. Molecules, losing energy, form crystals, being unable to resist the attraction of other molecules. And again, the body temperature will not decrease until the entire body hardens, and until all the energy that was expended on its melting is released. That is, the specific heat of fusion shows both how much energy must be expended to melt a body of mass m, and how much energy will be released when a given body solidifies.

For example, the specific heat of fusion of water in the solid state, that is, the specific heat of fusion of ice is 3.4 * 105 J/kg. These data allow you to calculate how much energy is required to melt ice of any mass. Knowing also the specific heat capacity of ice and water, you can calculate exactly how much energy is required for a particular process, for example, melting ice weighing 2 kg and temperature - 30˚C and bringing the resulting water to a boil. Such information for various substances is very necessary in industry to calculate the real energy costs in the production of any goods.

In physics, melting is the transition of a substance from solid state into liquid. Classic examples of the melting process are the melting of ice and the transformation of a solid piece of tin into liquid solder when heated with a soldering iron. The transfer of a certain amount of heat to a body leads to a change in its state of aggregation.

Why does a solid become liquid?

Heating solid leads to an increase kinetic energy atoms and molecules, which at normal temperature are clearly located in the nodes of the crystal lattice, which allows the body to maintain a constant shape and size. When certain critical speed values ​​are reached, atoms and molecules begin to leave their places, bonds are broken, the body begins to lose its shape - it becomes liquid. The melting process does not occur abruptly, but gradually, so that for some time the solid and liquid components (phases) are in equilibrium. Melting refers to endothermic processes, that is, those that occur with the absorption of heat. The opposite process, when a liquid solidifies, is called crystallization.

Rice. 1. Transition of the solid, crystalline state of a substance into the liquid phase.

It was discovered that until the end of the melting process the temperature does not change, although heat is constantly supplied. There is no contradiction here, since the incoming energy during this period of time is spent on breaking the crystalline bonds of the lattice. After the destruction of all bonds, the influx of heat will increase the kinetic energy of the molecules, and consequently, the temperature will begin to rise.

Rice. 2. Graph of body temperature versus heating time.

Determination of specific heat of fusion

The specific heat of fusion (designated by the Greek letter “lambda” - λ) is called physical quantity equal to the amount of heat (in joules) that must be transferred to a solid body weighing 1 kg in order to completely convert it into the liquid phase. The formula for the specific heat of fusion looks like this:

$$ λ =(Q \over m)$$

m is the mass of the melting substance;

Q is the amount of heat transferred to the substance during melting.

Values ​​for different substances are determined experimentally.

Knowing λ, we can calculate the amount of heat that must be imparted to a body of mass m for its complete melting:

In what units is the specific heat of fusion measured?

Specific heat of fusion in SI (International System) is measured in joules per kilogram, J/kg. For some tasks, a non-systemic unit of measurement is used - kilocalorie per kilogram, kcal / kg. Let us remember that 1 kcal = 4.1868 J.

Specific heat of fusion of certain substances

Information on specific heat values ​​for a particular substance can be found in book reference books or in electronic versions on Internet resources. They are usually presented in table form:

Specific heat of fusion of substances

One of the most refractory substances is tantalum carbide - TaC. It melts at a temperature of 3990 0 C. TаC coatings are used to protect metal molds in which aluminum parts are cast.

Rice. 3. Metal melting process.

What have we learned?

We learned that the transition from solid to liquid is called melting. Melting occurs through the transfer of heat to a solid. The specific heat of fusion shows how much heat (energy) a solid substance weighing 1 kg is needed to transform it into a liquid state.

Test on the topic

Evaluation of the report

average rating: 4.7. Total ratings received: 217.

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 when the temperature decreases, i.e., 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 temperaturetions .

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

The figure shows a graph of the temperature of a crystalline body (ice) versus heating time (from the point A to the point D) and cooling time (from point D to the point K). It shows time along the horizontal axis, and temperature along the vertical axis.

The graph shows that observation of the process began from the moment when the ice temperature was -40 ° C, or, as they say, the temperature at the initial moment of time tbeginning= -40 °С (point A on the graph). With further heating, the temperature of the ice increases (on the graph this is the section AB). The temperature increases to 0 °C - the melting temperature 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 is melted), the temperature of the ice 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 does the temperature begin to rise again (section CD). After the water temperature reaches +40 °C, the burner is extinguished and the water begins to cool, i.e., heat is removed (to do this, you can place a vessel with water in another, larger vessel with ice). The water temperature begins to decrease (section DE). When the temperature reaches 0 °C, the water temperature stops decreasing, despite the fact that heat is still removed. This is the process of water crystallization - ice formation (horizontal section E.F.). Until all the water turns into ice, the temperature will not change. Only after this does the ice temperature begin to decrease (section FK).

The appearance of the considered graph is explained as follows. Location on AB Due to the heat supplied, 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 ice crystal lattice: the ordered spatial arrangement of its molecules is replaced by a disordered one, the distance between the molecules changes, i.e. The molecules are rearranged in such a way that the substance becomes liquid. The average kinetic energy of the molecules does not change, so the temperature remains unchanged. 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 drops - the temperature decreases, the water cools. At 0°C (horizontal section E.F.) molecules begin to line up in a certain order, forming crystal lattice. Until this process is completed, the temperature of the substance will not change, despite the heat being removed, which means that when solidifying, the liquid (water) releases energy. This is exactly the energy that the ice absorbed, turning into liquid (section Sun). The internal energy of a liquid is greater than that of a solid. Upon melting (and crystallization) internal energy bodies change abruptly.

Metals that melt at temperatures above 1650 ºС are called refractory(titanium, chromium, molybdenum, etc.). Tungsten has the highest melting point among them - about 3400 ° C. Refractory metals and their compounds are used as heat-resistant materials in aircraft construction, rocket production and space technology, nuclear energy.

Let us emphasize once again that when melting, a substance absorbs energy. During crystallization, on the contrary, it gives it away to environment. Receiving a certain amount of heat released during crystallization, the medium heats up. This is well known to many birds. No wonder they can be seen in winter in frosty weather sitting on the ice that covers rivers and lakes. Due to the release of energy when ice forms, the air above it is several degrees warmer than in the trees in the forest, and birds take advantage of this.

Melting of amorphous substances.

Availability of a certain melting points- This important sign crystalline substances. It is by this feature that they can be easily distinguished from amorphous bodies, which are also classified as solids. These include, in particular, glass, very viscous resins, and 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 easily be 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 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 transition of a solid into a liquid here, since it does not exist.

The reasons for this lie in the fundamental difference in the structure of amorphous bodies from the structure of crystalline ones. Atoms in amorphous bodies are arranged randomly. Amorphous bodies resemble liquids in their structure. Already in solid glass, the atoms are arranged randomly. This means that increasing the temperature of glass only increases the range of vibrations of its molecules, giving them gradually greater and greater 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 disorderly one.

Heat of fusion.

Heat of Melting- this 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 transform it from a solid crystalline state to a liquid. The heat of fusion is equal to the amount of heat that is released during the crystallization of a substance from the liquid state. During melting, all the heat supplied to a substance goes to increase the potential energy of its molecules. The kinetic energy does not change since melting occurs at a constant temperature.

By experimentally studying the melting of various substances of the same mass, one can notice that different amounts of heat are required to transform them into liquid. For example, in order to melt one kilogram of ice, you need to expend 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 complete combustion of fuel.

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

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

Physical quantity showing how much heat is released during complete combustion of fuel weighing 1 kg is called specific heat of combustion of fuel.The specific heat of combustion is designated by the letterqand is measured in joules per kilogram (J/kg).

Quantity of heat Q released during combustion m kg of fuel is determined by the formula:

To find the amount of heat released during complete combustion of a fuel of an arbitrary mass, the specific heat of combustion of this fuel must be multiplied by its mass.