ENCE 433 Dr. Alba Torrents

ENVIRONMENTAL ENGINEERING ANALYSIS Spring 2001

Homework Set # 2 Out: February 13 Due: February 20

1. Many attempts are being made to develop efficient methods of using solar energy for heating houses. One such method uses crushed sodium sulfate decahydrate (Na₂SO₄.10H₂O) in an enclosed bin on the roof of the house. This bean stores heat collected from the sun during the day. The heat storage involves the following endothermic reaction:

This reaction is reversible and is completely to the right at temperatures above 32.4^oC. At night, when the temperature falls below 32.4^oC the reaction is reversible and completely to the left. The heat liberated from the bin is then circulated throughout the house by a system of fans.

Assuming that the chemical reactions and the heat transfer are all 100% efficient, how much heat could a 322-kg charge of Na₂SO₄.10H₂O provide to the solar house by night?

Compound	Mol. wt.	Standard heat of formation at 298 K (kcal/mole)
Na2SO4.10H2O(s) Na2SO4(s) H2O(l)	322 142 18	-1033.5 -330.9 -68.3

2. Compute the thermodynamic equilibrium constant for the following reaction:

Ca⁺² + 2HCO₃^- = CaCO₃ (s) + H₂O(aq) + CO₂ (aq) T= 25^oC

If we assume that [CO_2(aq)] is constant with T; How does [Ca⁺²] changes when we decrease T? (Qualitative answer)