Equilibrium conversions Iced temperature

Melting is the conversion of a solid to the liquid state. The normal melting point of a solid is the temperature at which solid and liquid are in equilibrium under a pressure of 1 atm. The normal melting point of ice is 0.00°C, thus liquid water and ice coexist indefinitely (are in equilibrium) at this temperature at a pressure of 1 atm. If the temperature is reduced by even a small amount, then all the water eventually freezes if the temperature is raised infinitesimally, all the ice eventually melts. The qualifying term normal is often omitted in talking about melting points because they depend only weakly on pressure. 

At what temperature T would the water —> ice conversion no longer be spontaneous The condition of equilibrium, AG = AH — T AS = 0, is satisfied when... 

If, when we calculate AG for a process, such as the conversion of ice to water at a particular temperature, we find that AG = 0, we know at once that the system is at equilibrium (in this example, that the ice and water are in equilibrium with each other). The following chapters build on this enormously important relation. 

Case I a temperature at which the vapor pressure of the solid is greater than that of the liquid. At this temperature the solid requires a higher pressure of vapor than the liquid to be in equilibrium with the vapor. Thus, as vapor is released from the solid to try to achieve equilibrium, the liquid absorbs vapor in an attempt to reduce the vapor pressure to its equilibrium value. The net effect is a conversion from solid to liquid through the vapor phase. In fact, no solid can exist under these conditions. The amount of solid steadily decreases and the volume of liquid increases. Finally, there is only liquid in the right compartment, which comes to equilibrium with the water vapor, and no further changes occur in the system. The temperature for Case 1 must be above the melting point of ice, since only the liquid state can exist. 

One of the early triumphs of the study of thermodynamics was the demonstration that there is an absolute zero of temperature. However, there are several different temperature scales, for historical reasons. All you need to know about this is that the kelvin scale (named after William Thompson, Lord Kelvin) has an absolute zero of OK and a temperature of 273.16K at the triple point where water, ice, and water vapor are at equilibrium together. The melting point of ice at one atmosphere pressure is 0.01 degrees less than this, at 273.15 K (Figure 3.1). The Celsius scale (named after Anders Celsius, a Swedish astronomer) has a temperature of 0 °C at the ice point (273.15 K) and absolute zero at -273.15°C. This gives almost exactly 100 C between the freezing and boiling points of water at one atmosphere, so water boils at 100 "C (373.15 K). Thus the numerical conversion between the two scales is... 

The spontaneity of a process can depend on temperature. Consider, for example, the endothermic process of melting ice under atmospheric pressure. WhenT > 0°C, ice melts spontaneously the reverse process, liquid water turning into ice, is not spontaneous at these temperatures. However, when T < 0°C, the opposite is true. Liquid water converts into ice spontaneously, and the conversion of ice into water is not spontaneous (Figure 19.3 ). What happens at T = 0°C, the normal melting point of water At the normal melting point of a substance, the solid and liquid phases are in equilibrium. (Section 11.6) At this particular temperature the two phases are interconverting at the same rate, and there is no preferred direction for the process Both the forward and reverse processes occur with equal preference, and the process is not spontaneous in either direction. 

M Figure 19.3 The spontaneity of a process can depend on the temperature. At r > 0°C ice melts spontaneously to liquid water. At T < 0°C the reverse process, water freezing to ice, is spontaneous. At T = 0°C the two states are in equilibrium and neither conversion occurs spontaneously. 

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