What is the specific heat of crystallization? Quantity of heat. Specific heat
The amount of heat that must be reported to the body in equilibrium isobaric-isothermal. process to transfer it from TV. (crystalline) into liquid (the same amount of heat is released during crystallization of the substance). T.p. is a special case of the heat of phase transition.
There are beats. T. p. (measured in J/kg, kcal/kg) and molar (molar) T. p. (J/mol). In table the values of beats are given. T. p. Lmelt at atm. pressure 760 mm Hg. Art. (or 101,325 Pa) and melting temperature Tmel.
Physical encyclopedic dictionary. - M.: Soviet Encyclopedia. . 1983 .
The amount of heat that must be imparted to a substance in isobaric-isothermal equilibrium. process to completely convert it from solid crystalline. state into liquid. T.p. is equal to the amount of heat released during the crystallization of a substance from the liquid phase. T. L pl for certain substances at normal pressure (1013.25 hPa) and melting temperature T pl.
Physical encyclopedia. In 5 volumes. - M.: Soviet Encyclopedia. Editor-in-chief A. M. Prokhorov. 1988 .
See what “HEAT OF MELTING” is in other dictionaries:
The amount of heat that must be imparted to a crystalline solid at constant pressure in order to completely transform it into a liquid state. The heat of fusion of a unit mass of a substance is called the specific heat of fusion. * * *… … encyclopedic Dictionary
The amount of heat that must be imparted to a solid crystalline substance at constant pressure in order to completely transform it into a liquid state. The heat of fusion of a unit mass of a substance is called the specific heat of fusion... Big Encyclopedic Dictionary
heat of fusion- the amount of heat required by a substance in an equilibrium isobaric isothermal process to transition from a solid (crystalline) state to a liquid (the same amount of heat is released during crystallization of the substance). Warmth... ... Encyclopedic Dictionary of Metallurgy
The amount of heat that must be imparted to a substance in an equilibrium process in order to transfer it from a solid (crystalline) state to a liquid (the same amount of heat is released during crystallization of the substance). Heat of fusion... ... Metallurgical dictionary
heat of fusion- lydymosi šiluma statusas T sritis Standartizacija ir metrologija apibrėžtis Šilumos kiekis, reikalingas medžiagai išlydyti. atitikmenys: engl. fusion heat; melting heat vok. Schmelzwärme, f rus. heat of fusion, f pranc. chaleur de fusion, f… Penkiakalbis aiškinamasis metrologijos terminų žodynas
heat of fusion- lydymosi šiluma statusas T sritis chemija apibrėžtis Šilumos kiekis, reikalingas medžiagai išlydyti. atitikmenys: engl. fusion heat; melting heat rus. heat of fusion... Chemijos terminų aiškinamasis žodynas
heat of fusion- lydymosi šiluma statusas T sritis fizika atitikmenys: engl. fusion heat; melting heat vok. Schmelzwärme, f rus. heat of fusion, f pranc. chaleur de fusion, f… Fizikos terminų žodynas
heat of fusion- lydymosi šiluma statusas T sritis Energetika apibrėžtis Šiluma, reikalinga kietai kristalinei medžiagai paversti skysčiu, esant pastoviai lydymosi temperatūrai. Būna savitoji ir molinė lydymosi šiluma. Jų matavimo vienetai – džaulis kilogramui… … Aiškinamasis šiluminės ir branduolinės technikos terminų žodynas
The amount of heat that must be imparted to a substance in an equilibrium isobaric isothermal process in order to transfer it from a solid (crystalline) state to a liquid (the same amount of heat is released during crystallization... ... Great Soviet Encyclopedia
The amount of heat must be reported to the TV. crystalline in wu at post. pressure to completely transform it into a liquid state. Etc. units of mass in va are called. specific T. p... Natural science. encyclopedic Dictionary
Books
- Mechanical properties of liquid metals. Extreme properties of minimal metal single crystals, O. S. Nikolaev. This book consists of two parts. The first part describes a thermal method for assessing the mechanical properties of liquid metals. It is applicable to bodies in any of three states. Received...
The specific heat of fusion is the amount of heat required to melt one gram of a substance. The specific heat of fusion 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 are also noble 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, under 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 to. It melts at 80 degrees and spontaneously 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 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.
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 a 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. During melting (and crystallization), the internal energy of the body changes 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, rocketry and space technology, and nuclear energy.
Let us emphasize once again that when melting, a substance absorbs energy. During crystallization, on the contrary, 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. 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 is an important feature of 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.
Melting is the transition of a body from a crystalline solid state to a liquid state. Melting occurs with the absorption of specific heat of fusion and is a first-order phase transition.
The ability to melt refers to the physical properties of a substance
At normal pressure, tungsten has the highest melting point among metals (3422 °C), simple substances in general - carbon (according to various sources, 3500 - 4500 °C) and among arbitrary substances - hafnium carbide HfC (3890 °C). We can assume that helium has the lowest melting point: at normal pressure it remains liquid at arbitrarily low temperatures.
Many substances at normal pressure do not have a liquid phase. When heated, they immediately transform into a gaseous state by sublimation.
Figure 9 - Ice melting
Crystallization is the process of phase transition of a substance from a liquid to a solid crystalline state with the formation of crystals.
A phase is a homogeneous part of a thermodynamic system separated from other parts of the system (other phases) by an interface, during the transition through which the chemical composition, structure and properties of the substance change abruptly.
![](https://i1.wp.com/studbooks.net/imag_/16/178451/image010.jpg)
Figure 10 - Crystallization of water with the formation of ice
Crystallization is the process of isolating the solid phase in the form of crystals from solutions or melts; in the chemical industry, the crystallization process is used to obtain substances in their pure form.
Crystallization begins when a certain limiting condition is reached, for example, supercooling of a liquid or supersaturation of steam, when many small crystals - crystallization centers - appear almost instantly. Crystals grow by attaching atoms or molecules from a liquid or vapor. The growth of crystal faces occurs layer by layer; the edges of incomplete atomic layers (steps) move along the face as they grow. The dependence of the growth rate on crystallization conditions leads to a variety of growth forms and crystal structures (polyhedral, lamellar, needle-shaped, skeletal, dendritic and other forms, pencil structures, etc.). During crystallization, various defects inevitably arise.
The number of crystallization centers and the growth rate are significantly affected by the degree of supercooling.
The degree of supercooling is the level of cooling of the liquid metal below the temperature of its transition to the crystalline (solid) modification. It is necessary to compensate for the energy of the latent heat of crystallization. Primary crystallization is the formation of crystals in metals (and alloys) during the transition from a liquid to a solid state.
Specific heat of fusion (also: enthalpy of fusion; there is also an equivalent concept of specific heat of crystallization) - the amount of heat that must be imparted to one unit of mass of a crystalline substance in an equilibrium isobaric-isothermal process in order to transfer it from a solid (crystalline) state to a liquid (then the same amount of heat is released during crystallization of a substance).
Amount of heat during melting or crystallization: Q=ml
Evaporation and boiling. Specific heat of vaporization
Evaporation is the process of transition of a substance from a liquid state to a gaseous state (steam). The evaporation process is the reverse of the condensation process (transition from a vapor state to a liquid state. Evaporation (vaporization), the transition of a substance from a condensed (solid or liquid) phase to a gaseous (vapor); first-order phase transition.
There is a more developed concept of evaporation in higher physics
Evaporation is a process in which particles (molecules, atoms) fly out (break off) from the surface of a liquid or solid, with Ek > Ep.
![](https://i2.wp.com/studbooks.net/imag_/16/178451/image011.png)
Figure 11 - Evaporation over a mug of tea
Specific heat of evaporation (vaporization) (L) is a physical quantity indicating the amount of heat that must be imparted to 1 kg of a substance taken at the boiling point in order to transfer it from a liquid to a gaseous state. The specific heat of evaporation is measured in J/kg.
Boiling is the process of vaporization in a liquid (the transition of a substance from a liquid to a gaseous state), with the appearance of phase separation boundaries. The boiling point at atmospheric pressure is usually given as one of the main physicochemical characteristics of a chemically pure substance.
Boiling is a first-order phase transition. Boiling occurs much more intensely than evaporation from the surface, due to the formation of centers of vaporization, determined both by the achieved boiling temperature and the presence of impurities.
The process of bubble formation can be influenced using pressure, sound waves, and ionization. In particular, it is on the principle of boiling of microvolumes of liquid from ionization during the passage of charged particles that the bubble chamber operates.
![](https://i0.wp.com/studbooks.net/imag_/16/178451/image012.png)
Figure 12 - Boiling water
Amount of heat during boiling, evaporation of liquid and condensation of steam: Q=mL
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