What is melt heat 1

Big Bang 5 RG, textbook

Thermodynamics 129 Expansion, Diffusion and Phase Transitions 13 In order to melt or evaporate a substance, one needs heat of fusion or heat of evaporation. Conversely, solidification heat or condensation heat is released during solidification or condensation. In Table 13.2 you can see examples of some substances. Ice has a very high melting heat. The same amount of energy is required to melt 1 kg of ice and to heat 1 kg of water from 0 ° C to 80 ° C (see Fig. 13.18). Even in a fully opened oven it would take 45 minutes for the block in Fig. 13.17 to melt. The heat of evaporation is a lot bigger! To evaporate 1 kg of water you need almost 7 times as much energy as to melt 1 kg of ice (Fig. 13.18, F17). Info: ice block Why is the heat of evaporation almost always much higher than the heat of fusion? After melting, the particles still touch their closest neighbors. When evaporating, however, the coupling between the particles is practically canceled, and more energy is required for this. Substance heat of fusion or solidification heat kJ / kg evaporation heat or condensation heat kJ / kg spec. Heat capacity kJ / (kg K) Ice 334 - 2.1 Water - 2256 4.2 Water vapor - - 1.9 Iron 268 6364 0.47 Tin 59 2450 0.22 Lead 23.2 921 0.13 Tab. 13.2: Specific heat capacity and amount of heat for phase transitions (see Fig. 13.18). Lead and tin have a very low melting point and a very low heat of fusion and are therefore well suited for “lead pouring” (F18). Note that the specific heat capacity for ice, water and water vapor is different. The values ​​apply to normal pressure. Fig. 13.18: Heat required to completely evaporate 1 kg of ice at 0 ° C. First you have to add 334 kJ to melt the ice. To heat the water from 0 ° C to 100 ° C, 420 kJ are necessary. In order to completely evaporate the water, you need 2256 kJ (see Tab. 13.2)! Summary In the case of phase transitions, the energy supplied is required to overcome the binding energy. Therefore, there is no increase in temperature. One speaks of latent heat. 13.5 Melting anoraks Melting and solidification In this section we take a closer look at examples of the transition between solid and liquid phase. The transition from the solid to the liquid phase is called melting, the transition from the liquid to the solid phase is called solidification or freezing. The phase transitions can be triggered either by changing the pressure, the temperature or a combination of both. In the case of a substance with an anomaly such as water, liquefaction can be generated by increasing the pressure, and in the case of a substance without an anomaly by reducing the pressure. The reason lies in the position of the phase boundary (Fig. 13.19). In order to melt an ice cube at 0 ° C into water at 0 ° C, melting heat must be added. Conversely, this means that when water freezes to ice, heat is released, the solidification heat. Z Ice block The ice block in Fig. 13.17 weighs 25 kg. The required melting heat is therefore 334 kJ · 25 ≈ 8.4 · 10 6 J. Melting takes over 8 million joules! An oven has a maximum output of 3000 W. It delivers 3000 joules of heat per second. It takes 8.4 · 10 6 / (3 · 10 3) seconds to melt the ice block, i.e. 2800 seconds or around 45 minutes. So you have to leave the block in the tube with full juice for three quarters of an hour until it is completely melted. You can therefore imagine that a block of ice can hold out for a long time at room temperature (F19). i When fruit trees and vines sprout in spring, frost often occurs. To prevent the young shoots from freezing, they are sprayed with water, which freezes very quickly (Fig. 13.21). What is the purpose of that? When black ice threatens on the streets, the salt litter ban is sometimes lifted. But why can you actually prevent black ice with salt? F20 S2 F21 S2 For testing purposes only - property of publisher öbv

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