Ethylene glycol-water mixtures, freezing

Dielectric Constant (D) and Freezing Point of Ethylene Glycol—Water Mixtures"... 

Some practical applications involving colligative properties are (1) use of salt-ice mixtures to provide low freezing temperatures for homemade ice cream, (2) use of sodium chloride or calcium chloride to melt ice from streets, and (3) use of ethylene glycol-water mixtures as antifreeze in automobile radiators (ethylene glycol also raises... 

Table 3.11 Freezing point of different ethylene glycol — water mixtures (for Question 27)...

Depression of the Freezing Point in a Mixture of Ethylene glycol-Water... 

An antifreeze solution is prepared containing 50.0 cm3 of ethylene glycol, C2H602 (d = 1.12 g/cm3), in 50.0 g of water. Calculate the freezing point of this 50-50 mixture. 

The depression of freezing point is dependent on concentration. A mixture of glycol in water will lower the freezing point with an increasing amount of ethylene glycol, as given in Table 14-2. 

Ethylene glycol is not as active in depression of the freezing point as methanol, but it has a very low vapor pressure evaporation loss in a coolant system is due more to the evaporation of water than to the evaporazation of ethylene glycol. Furthermore, the flammability problem is literally eliminated. 1 1 mixtures of ethylene glycol and water do not exhibit a flash point at all. 

The key feature of ethylene glycol (EG) is the hydroxyl group, -OH, one on each of the two carbon atoms. The hydroxyls are responsible for its reactivity EG is a monomer used in the production of polyester polymers. The hydroxyls also give EG its most important physical property its solubility in water. That, linked with its low freeze point, makes EG suitable as an antifreeze and as a deicer. When EG is sprayed on ice, it combines with the water crystals and lowers the freeze point. This causes the mixture to melt and effectively keeps it in the liquid state. 

Antifreeze. Any compound or mixture that lowers the freeing point of water can be referred to as antifreeze. The preponderant, commercial antifreeze is ethylene glycol diluted with water. Other organic compounds used occasionally are methanol, ethanol, and propylene glycol. The primary application is to protect automotive cooling systems from freezing. 

A mole of either glycol in water is as effective as the other in reducing the freezing point of water however, on a molar basis the diethylene glycol is twice as expensive as the ethylene glycol. So we want to maximize ethylene glycol, and minimize the diethylene glycol in the mixture. 

It is often important to lower the temperature at which a liquid freezes. Think about the following three examples A chemical called ethylene glycol (antifreeze) is added to the water in car radiators so the water will not freeze in the winter. Salt is sprinkled on icy pavements to melt ice. Salt is added to ice and water to make a mixture that is colder than 32 degrees. This cold salt water is used to freeze homemade ice cream. 

This principle can be used in a number of practical ways. One application is the use of antifreeze in car radiators. Since water expands on freezing, the water in a car s cooling system can crack the engine block of a parked car when the outside temperature falls below 0°C. The addition of a common antifreeze, ethylene glycol, prevents this because the freezing point is depressed and the water-ethylene glycol mixture freezes at a much lower temperature. 

Of course, all the calculations here are approximate. In particular, we have used UNIFAC, which is not meant to be applicable to liquid-liquid equilibrium, and further, —25°C is below the temperature at which the UNIFAC parameters were obtained, so. the results we have obtained are not expected to be accurate. Therefore, all the conclusions should be checked against experimental data. As examples of the uncertainty of such predictions, from experimental data it is found that the amount of ethylene glycol required to result in a mixture freezing point of —25°C is closer to 45 wt % than the - 57 wt % estimated here, and that -pentanol is actually only soluble in water to about 2 j wt %, rather than the complete miscibility found here. This should serve as a warning concerning the use of any completely predictive method, even the UNIFAC model, which is currently the best. 

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