Freezing, spontaneous

The sign of AG depends on the temperature. When T > 0°C, AG < 0, since ice will spontaneously melt. When T < 0°C, AG > 0, since liquid water will spontaneously freeze and when T= 0°C, AG = 0, since that is the melting point of water and the reaction is at equilibrium. 

If water behaved in a similar fashion to most other materials and possessed a positive value of AVm, then water would spontaneously freeze when pressure was applied, rather than solid ice melting under pressure. Furthermore, a positive value of AVm would instantly remove the problems discussed above, caused by vehicles travelling over black ice, because the ice would remain solid under pressure and remember that the slipperiness occurs because liquid water forms on top of solid ice. 

A dog dish full of water is placed outside and left undisturbed on a cold Wisconsin night. The water spontaneously freezes. Clearly, the final state (ordered crystalline ice) has lower entropy than the initial state (disordered liquid water), so AS < 0, proving that the second law is invalid. 

Haymet and coworkers used an automated lag-time apparatus (ALTA) to obtain statistical data on the supercooling point (SCP, also known as freezing temperature) of water freezing to ice (Wilson et al., 2003) and water/tetrahydrofuran (THF) freezing to hydrate/ice (Wilson et al., 2005). The SCP is the temperature of spontaneous freezing of a solution (Zachariassen, 1982). A small sample (300 rL) was cooled linearly (at 4.5 K/min) until the sample froze. The frozen sample was melted, and then refrozen. This freezing-melting cycle was repeated over 300 times on the same sample. 

The second law of thermodynamics states that for all spontaneous processes the entropy of the universe will always increase. This is often misunderstood to mean that the entropy of all parts of the system will increase. For example, if a small container of water is placed into a freezer, it will spontaneously freeze. Although the entropy of the water in the container decreased, a number of processes had to occur for that change to take place. The processes that occurred in the freezer that allowed the water to freeze (such as the movement of the compressor, the evaporation and condensation of the refrigerant, and the warming of the air around the container), all combine to produce a net increase in the entropy of the universe. 

At temperatures below 0°C, water spontaneously freezes and at temperatures above 0°C, ice spontaneously melts. 

We can now understand why spontaneity is often dependent on temperature and thus why water spontaneously freezes below 0°C and melts above 0°C. The term ASsurr is temperature-dependent. Since... 

There is a close relationship between the reversibility of a process and whether it is spontaneous or at equilibrium. Recall Figure 19.3, in which we showed the spontaneous melting of ice at T > 0°C and tiie spontaneous freezing of hquid water at T < 0°C. Bofli of these processes are irreversible. At T = 0°C ice and water are in equilibrium, and they can convert back and forth reversibly. These observations are examples of two very important concepts regarding reversible and irreversible processes ... 

A spontaneous process accompanied by an entropy decrease must also be accompanied by a compensating energy decrease. Thus, when water spontaneously freezes at 0°C, the energy loss compensates for the entropy decrease that oeeurs as the water molecules assume the well-ordered arrangement in iee. 

The first result is obtained by considering the reversible path —10 C-> 0 C->—10 C for the transformation water->ice. The second result is obtained by first calculating the heat given to the environment in the spontaneous freezing at —10 C. This heat is 6666 J mol and the corresponding entropy change of the environment is... 

We can probably think of spontaneous processes in which entropy appears to decrease (that is, in which energy appears to become less dispersed). For example, on a cold winter s day, a bucket of water left outdoors will spontaneously freeze the water molecules go from a disorderly liquid (in which energy is more highly dispersed) to an orderly solid (in which energy is less dispersed). The water goes from a state of high entropy (the liquid) to a state of low entropy (the solid). However, on closer inspection, we find that it is only within the system, the bucket of water, that entropy decreases. In the surroundings, entropy increases because heat... 

We can now understand why water spontaneously freezes at low temperature but not at high temperature. For the freezing of liquid water into ice, the change in entropy of the system is negative at all temperatures ... 

Explain why water spontaneously freezes to form ice helow 0 °C even though the entropy of the water decreases during the state transition. Why is the freezing of water not spontaneous above 0 °C ... 

Everyday experience tells us that water spontaneously freezes at 263.15 K. Let s use equation (13.10) to demonstrate that the freezing of super-cooled water is spontaneous at 263.15 K, and the reverse process is nonspontaneous. 

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