Frances Pauline U. OntingBS Chemistry III

HW 8

1. Describe the seasonal process of overturn in lakes

Fig. 1 Stratification of Water

A lake is a non-flowing body of water that can form distinct layers due to waters temperature-density relationship as shown in Figure 1. During summer, there is a surface layer (epilimnion) heated, thus floats due to the low density. The bottom layer is known as the hypolimnion. The layer between epilimnion and hypolimnion, is called the thermocline. The phenomenon is called thermal stratification. Because of the temperature difference between the epilimnion and the hypolimnion, they do not mix and have very different chemical and biological properties. The epilimnion may have a heavy growth of algae due to the exposure to the atmosphere. It also has a relatively higher level of dissolved oxygen due to the photosynthetic activity of algae during the day. In the hypolimnion, there is biodegradation of organic material due to the bacterial action that may result to a lack of dissolved oxygen. Thus, chemical species tend to predominate in the hypolimnion.

During the autumn (cooler temperature), the epilimnion cools until the temperature is equal to the hypolimnion. Thus, there is a disappearance of the stratification resulting to a mixing of the entire entire body of water which is known as overturn.An overturn ALSO occurs in the spring, when the temperature of the epilimnion increases after winter and is equal to the hypolimnion. There is no overturn in winter since the surface layer freezes due to the low temperature, creating and trapping a warmer bottom layer. During the overturn, the chemical and physical characteristics of the body of water become much more uniform, and a number of chemical, physical, and biological changes may result. Biological activity may increase from the mixing of nutrients.

2. Assuming a bicarbonate ion concentration [HCO3- ] of 1.00 x 10-3 M and a value of 3.5 x 10-11 for the solubility product of FeCO3 , what would you expect to be the stable iron species at pH 9.5 and pE -8.0 as shown in the pE/pH diagram of iron species. 3. Surface water in an open area of the Indian Ocean adjacent to Kenya has an alkalinity of 2320 mol L-1 and a total carbonate concentration of 2.03 x 10-3 mol L-1 . Calculate the concentrations of HCO3-1 and CO32- and also the pH of this sea water.