A pesticide P in a shallow lake is removed by two independent pathways at : •…
CHEMISTRY · Multi-part · CBSE · Chemistry · Chemical Kinetics · Chemical Kinetics
Problem
A pesticide P in a shallow lake is removed by two independent pathways at 298K:
• Hydrolysis: first-order in P with rate constant kh=2.1×10−4day−1
• Photolysis: second-order in P with rate constant kp=1.5×10−3Lmol−1day−1 (sunlight intensity remains constant)
Immediately after application the concentration of P in the lake water is 2.0×10−4molL−1. Assume the lake is well mixed.
(a) Assuming hydrolysis is the only significant pathway initially, calculate the number of days required for the concentration of P to fall to 1.0×10−4molL−1.
(b) Using the same assumption as in part (a), determine the concentration of P predicted after 120\,days.
(c) At 120\,days, calculate the instantaneous rate of removal of P by hydrolysis and the instantaneous rate of removal by photolysis. Based on these values, evaluate whether the assumption used in parts (a) and (b) remains justified at this time.
(d) Suppose environmental regulations require that [P] must be below 5.0×10−5molL−1. Explain, using quantitative reasoning, which pathway will dominate the removal of P as this target concentration is approached, and discuss how this influences the prediction of the time needed to meet the regulatory limit.