In the previous paragraph, we looked at the graph of ice melting and solidification. The graph shows that while the ice is melting, its temperature does not change (see Fig. 18). And only after all the ice has melted does the temperature of the resulting liquid begin to rise. But even during the melting process, the ice receives energy from the fuel burning in the heater. And from the law of conservation of energy it follows that it cannot disappear. What is fuel energy spent on during melting?
We know that in crystals the molecules (or atoms) are arranged in in strict order. However, even in crystals they are in thermal motion (oscillate). When the body heats up average speed molecular movement increases. Consequently, their average also increases kinetic energy and temperature. On the graph this is section AB (see Fig. 18). As a result, the range of vibrations of molecules (or atoms) increases. When the body heats up to the melting temperature, the order in the arrangement of particles in the crystals is disrupted. Crystals lose their shape. A substance melts, passing from a solid to a liquid state.
Consequently, all the energy that a crystalline body receives after it has already been heated to the melting point is spent on destroying the crystal. In this regard, the body temperature stops rising. On the graph (see Fig. 18) this is the BC section.
Experiments show that to transform various crystalline substances of the same mass into liquid at the melting point, it is required different quantities warmth.
A physical quantity showing how much heat must be imparted to a crystalline body weighing 1 kg in order to completely transform it into a liquid state at the melting point is called the specific heat of fusion.
The specific heat of fusion is denoted by λ (Greek letter “lambda”). Its unit is 1 J / kg.
The specific heat of fusion is determined experimentally. Thus, it was found that the specific heat of fusion of ice is 3.4 10 5 -. This means that to transform a piece of ice weighing 1 kg, taken at 0 °C, into water of the same temperature, 3.4 10 5 J of energy is required. And to melt a block of lead weighing 1 kg, taken at its melting temperature, you will need to expend 2.5 10 4 J of energy.
Therefore, at the melting temperature internal energy substances in liquid state greater internal energy of the same mass of matter in the solid state.
To calculate the amount of heat Q required for melting crystalline body mass t, taken at its melting point and normal atmospheric pressure, you need to multiply the specific heat of fusion λ by the body mass m:
From this formula it can be determined that
λ = Q / m, m = Q / λ
Experiments show that during hardening crystalline substance exactly the same amount of heat is released that is absorbed during its melting. Thus, when water weighing 1 kg solidifies at a temperature of 0 °C, an amount of heat is released equal to 3.4 10 5 J. Exactly the same amount of heat is required to melt ice weighing 1 kg at a temperature of 0 °C.
When a substance hardens, everything happens in reverse order. The speed, and therefore the average kinetic energy of molecules in a cooled molten substance decreases. Attractive forces can now hold slow-moving molecules close to each other. As a result, the arrangement of particles becomes ordered - a crystal is formed. The energy released during crystallization is spent on maintaining constant temperature. On the graph this is the EF section (see Fig. 18).
Crystallization is facilitated if some foreign particles, such as dust particles, are present in the liquid from the very beginning. They become centers of crystallization. Under normal conditions, there are many crystallization centers in a liquid, around which the formation of crystals occurs.
Table 4.
Specific heat melting of certain substances (at normal atmospheric pressure)
During crystallization, energy is released and transferred to surrounding bodies.
The amount of heat released during the crystallization of a body of mass m is also determined by the formula
The internal energy of the body decreases.
Example. To prepare tea, the tourist put 2 kg of ice at a temperature of 0 °C into a pot. What amount of heat is needed to turn this ice into boiling water at a temperature of 100 °C? The energy spent on heating the boiler is not taken into account.
What amount of heat would be needed if, instead of ice, a tourist took water of the same mass at the same temperature from an ice hole?
Let's write down the conditions of the problem and solve it.
Questions
- How to explain the process of melting a body based on the theory of the structure of matter?
- What is fuel energy used for when melting a crystalline body heated to the melting temperature?
- What is the specific heat of fusion called?
- How to explain the solidification process based on the theory of the structure of matter?
- How is the amount of heat required to melt a crystalline solid taken at its melting point calculated?
- How to calculate the amount of heat released during the crystallization of a body that has a melting point?
Exercise 12
Exercise
- Place two identical tin cans on the stove. Pour water weighing 0.5 kg into one, put several ice cubes of the same mass into the other. Note how long it takes for the water in both jars to boil. Write short report about your experience and explain the results.
- Read the paragraph “Amorphous bodies. Melting amorphous bodies" Prepare a report on it.
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 into the ice-water mixture and, consequently, 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, the increase in internal energy during melting is an increase in the potential energy of the molecules.
Experience shows that the above is true for all crystals. When melting a crystal, it is necessary to continuously increase the internal energy of the system, while the temperature of the crystal and the melt remains unchanged. Typically, 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, for example by friction. So, the internal energy of a melt is always greater than the internal energy of the same mass of crystals at the same temperature. This means that the ordered arrangement of particles (in the crystalline state) corresponds to lower energy than the disordered arrangement (in the melt).
The amount of heat required to transform a unit mass of a crystal into a melt of the same temperature is called the specific heat of melting of the crystal. It is expressed in joules per kilogram.
When a substance solidifies, the heat of fusion is released and transferred to surrounding bodies.
Determining 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 using a calorimeter. Pouring a certain amount of water at a certain temperature into the calorimeter and throwing it into it known mass ice that has already begun to melt, i.e., has a temperature, 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 will draw up a 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 - , the specific heat of water - , the initial temperature of water - , the final temperature - , the specific heat of melting of ice - . Equation heat balance looks like
.
In table Table 16 shows the specific heat of fusion of some substances. Noteworthy is the high heat of melting of ice. This circumstance is very important, as it slows down the melting of ice in nature. If the specific heat of fusion were much lower, spring floods would be many times stronger. Knowing the specific heat of fusion, we can calculate how much heat is needed to melt any body. If the body is already heated to the melting point, then heat must be expended only to melt it. If it has a temperature below the melting point, then you still need to spend heat on heating.
Table 16.
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Substance |
Substance |
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Melting point chemically pure iron is 1539 o C. Technically pure iron, obtained as a result of oxidative refining, contains a certain amount of oxygen dissolved in the metal. For this reason, its melting point drops to 1530 o C.
The melting point of steel is always lower than the melting point of iron due to the presence of impurities in it. Metals dissolved in iron (Mn, Cr, Ni. Co, Mo, V, etc.) lower the melting point of the metal by 1 - 3 o C per 1% of the introduced element, and elements from the group of metalloids (C, O, S, P and etc.) at 30 – 80 o C.
For most of total duration melting, the melting point of the metal changes mainly as a result of changes in carbon content. At a carbon concentration of 0.1 - 1.2%, which is typical for finishing smelting in steel-smelting units, the melting temperature of the metal can be estimated with sufficient accuracy for practical purposes from the equation
Heat of fusion of iron is 15200 J/mol or 271.7 kJ/kg.
Boiling point of iron in publications recent years is given as 2735 o C. However, research results have been published according to which the boiling point of iron is much higher (up to 3230 o C).
Heat of vaporization of iron is 352.5 kJ/mol or 6300 kJ/kg.
Pressure saturated steam gland(P Fe , Pa) can be estimated using the equation
where T is the metal temperature, K.
Results of calculating the saturated vapor pressure of iron at different temperatures, as well as the dust content in the oxidizing gas phase above the metal ( X, g/m 3) are presented in Table 1.1.
Table 1.1– Saturated vapor pressure of iron and dust content of gases at different temperatures
According to existing sanitary standards, the dust content in gases emitted into the atmosphere should not exceed 0.1 g/m3. From the data in Table 1.1 it is clear that at 1600 o C the dust content of gases above the open surface of the metal is higher acceptable values. Therefore, it is necessary to purify gases from dust, consisting mainly of iron oxides.
Dynamic viscosity. Coefficient dynamic viscosity liquid () is determined from the relation
where F is the force of interaction between two moving layers, N;
S – contact area of layers, m2;
– velocity gradient of liquid layers normal to the direction of flow, s -1.
The dynamic viscosity of iron alloys usually varies in the range of 0.001 - 0.005 Pa s. Its value depends on temperature and the content of impurities, mainly carbon. When the metal is overheated above the melting point above 25 - 30 o C, the influence of temperature is not significant.
Kinematic viscosity fluid is the rate of momentum transfer in a unit mass flow. Its value is determined from the equation
where is the density of the liquid, kg/m3.
The value of the dynamic viscosity of liquid iron is close to 6 10 -7 m 2 /s.
Iron Density at 1550 - 1650 o C is equal to 6700 - 6800 kg/m 3. At the crystallization temperature, the density of the liquid metal is close to 6850 kg/m3. The density of solid iron at the crystallization temperature is 7450 kg/m3, at room temperature - 7800 kg/m3.
From ordinary impurities greatest influence Carbon and silicon influence the density of iron melts, lowering it. Therefore, liquid cast iron of ordinary composition has a density of 6200–6400 kg/m3, solid cast iron at room temperature has a density of 7000–7200 kg/m3.
The density of liquid and solid steel is intermediate position between the densities of iron and cast iron and are respectively 6500 - 6600 and 7500 - 7600 kg/m 3.
Specific heat liquid metal is practically independent of temperature. In estimation calculations, its value can be taken equal to 0.88 kJ/(kg K) for cast iron and 0.84 kJ/(kg K) for steel.
Surface tension of iron has maximum value at a temperature of about 1550 o C. In the region of higher and low temperatures its size decreases. This distinguishes iron from most metals, which are characterized by a decrease surface tension as the temperature rises.
The surface tension of liquid iron alloys varies significantly depending on chemical composition and temperature. Typically it varies between 1000 – 1800 mJ/m2 (Figure 1.1).
