1. The warm weather climates necessary for citrus orchards (like peaches) genera
ID: 774983 • Letter: 1
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1. The warm weather climates necessary for citrus orchards (like peaches) generally minimize the risk of frost conditions. Nevertheless, occasionally measures are needed to protect the trees and their fruit from freezing temperatures. Citrus crops become threatened when temperatures fall below 28 degree F for 4 hours or more. Heating is the most effective protection against frost, and farmers often use heaters to warm the air temperature in the orchard. However, spraying the crops with water at conditions where freezing occurs also provides sufficient protection from the cold temperatures. a. Using the following information, explain why spraying citrus crops with water at conditions where freezing occurs is successful for protecting the crops from cold temperatures. . The pressure in the orchard is constant (i.e., assume that atmospheric pressure does not change during the spraying of water and the formation of frost). . The enthalpy of fusion for water is equal to -6.01 kJ/mol. . The temperature of the water that is sprayed is about 32 degree F (i.e., T water > 32 degree F). b. Would spraying the crops with warm water (i.e. T water > 32 degree F) be a more effective or less effective way to protect the crops from cold temperatures? Why? hint: the heat capacity for liquid water at 20 degree C is 4.2 Jg^-1 K^-1.Explanation / Answer
a. From the thermodynamic point of view, spraying the crops with water increases the partial pressure of water vapor in the orchard. This increased water content in the air, when when forms frost or condenses, leaves of heat equivalent to the heat of fusion or heat of condensation. The heat increases the temperature of surrounding or orchard temporarily and protects the crops from freezing conditions.
b. spraying warm water may not be as effective as spraying cold water, because it takes time (warm water will have to cool) to condense. Although it appears that warm water may heat up the surroundings better than cold water does, the amount of heat released from warm water is much smaller than it is released from heat of condensation. The heat released from a mole of water at 20C is 18*4.2*(293-273) = 1.512 kJ, which is about 4 times lower than the heat released from heat of condensation/fusion 6.01 kJ.
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