Chlorinated solvent 1,2-dichloroethane (DCE) in lake water decomposes under sunlight by a reaction that is first order with respect to DCE. Under full summer sunlight, the measured half-life is 14.1d. The initial concentration of DCE in the lake is 3.50×10−5M.
(a) Determine the rate constant for the photodecomposition of DCE under full sunlight. Include units in your final answer.
(b) Calculate the time required for the DCE concentration to fall to the EPA drinking-water standard of 5.0×10−7M, assuming constant full sunlight.
(c) During an extended period of cloudy weather the effective rate constant drops to 60% of the value you obtained in part (a), and this reduced rate persists for the entire degradation period. Recalculate the time required to reach the EPA standard under these conditions.
(d) Discuss two specific environmental factors, other than sunlight intensity, that could cause deviations from the constant half-life you assumed in parts (a)–(c). Explain qualitatively how each factor might increase or decrease the observed half-life.