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The latent heat of fusion is the amount of heat needed to cause a phase change between solid and liquid. The latent heat of vaporization is the amount of heat needed to cause a phase change between liquid and gas.
Heat steam from 100 °C to 120 °C. The heat needed to change the temperature of a given substance (with no change in phase) is: q = m × c × Δ T (see previous chapter on thermochemistry). The heat needed to induce a given change in phase is given by q = n × Δ H.
The heat Q required to change the phase of a sample of mass m is given by \(\mathrm{Q=mL_f}\) (melting or freezing) and \(\mathrm{Q=mL_v}\) (evaporating or condensing), where \(\mathrm{L_f}\) and \(\mathrm{L_v}\) are the latent heat of fusion and the latent heat of vaporization, respectively.
Summary. Fusion, vaporization, and sublimation are endothermic processes, whereas freezing, condensation, and deposition are exothermic processes. Changes of state are examples of phase changes, or phase transitions. All phase changes are accompanied by changes in the energy of a system.
During phase changes, heat absorbed or released is given by: \[Q=m L, \nonumber\] where \(L\) is the latent heat coefficient.
• Vaporizing a liquid involves an input of energy to increase the separation between molecules. Here, the latent heat involves a change in internal energy, plus work in expanding the volume. Examples of latent heats: material fusion, L f (J / kg) vaporization, L v (J / kg) water 33.5 x 104 22.6 x 105 ethyl alcohol 10.8 x 104 8.55 x 105
If heat were added at a constant rate to a mass of ice to take it through its phase changes to liquid water and then to steam, the energies required to accomplish the phase changes (called the latent heat of fusion and latent heat of vaporization ) would lead to plateaus in the temperature vs time graph.
