ENVS 602 Spring 2003 Exam 1 Due Thursday, March 6 in class This exam is open book, open notes. You may use other resources in addition to the course text if you desire (especially a general chemistry text). You may not receive assistance from any other student, faculty member or collegue. Please contact me if you need clarification of exam questions. Responses to numerical/equation questions do not need to be typed, but please make sure that your responses are clearly legible (without scratch-outs). Show all work for full credit. 1. Explain what is meant by the term “speciation”. What kinds of variables can affect speciation? Speciation calculations assume that the system is at equilibrium. Explain what “equilibrium” means in this context. 2. One form of sulfur in the environment is the polyatomic ion sulfite (SO32-). When fully protonated, it is known as sulfurous acid (H2SO3). The acid dissociation constants for sulfurous acid are as follows: Ka1 = 1.72 x 10-2 Ka2 = 5.81 x 10-13 a. Write the chemical reactions that correspond to Ka1, Ka2 and 2 for the sulfite system. b. Instead of dissociation reactions, rewrite the reactions as formations (forming HSO3and H2SO3) starting from SO32-. c. What are the formation constants that describe each of the reactions in part b? 3. Cu2+ can form complexes with the small molecule ammonia (NH3). The formation constants for the stepwise formations of three copper species are as follows: log Kf1 = 4.34 log Kf2 = 3.66 log Kf3 = 3.07 a. Write chemical reactions that correspond to Kf1, Kf2, Kf3, 1, 2 and 3 for the copperammonia systems. b. Write an equation that describes the total concentration of Cu in the system (CT). c. Write an equation for CT that only has the parent material, the ligand and constants. d. Write expressions that contain only parent material, ligand and constants for each of the species that are present. e. Using the following information, determine the concentration of each species in the system at equilibrium. CCu = 3.2 x 10-5 M CNH3 = 7.5 x 10-3 M 4. a. Explain, in detail, why anaerobic environments are often associated with wetlands. b. For each of the following pairs of substances, choose the one that is most likely to appear in an anaerobic environment. Explain your reasoning. i. ii. iii. AsO43Fe2+ N2 AsO33Fe3+ NO3- 5. A platinum electrode and a calomel reference electrode are used to measure the redox potential of an aquifer. The measured potential is -0.541 V. What is the pE in this soil? 6. Nitrous oxide (N2O) is produced in wetlands by microbial activity and is a potent greenhouse gas in the atmosphere. The partial pressure of N2O in the atmosphere is approximately 0.032 Pa. KH = 0.025 mol/(L*atm) a. What would the concentration of N2O be in a water body that is in equilibrium with the atmosphere? b. In a wetland system, the concentration of N2O is found to be 6.2 x 10-8 M. Is this concentration in equilibrium with the atmosphere? If not, will the wetland water release N2O to the atmosphere or absorb N2O from the atmosphere? Explain your answer. 7. Why is it difficult to determine overall chemical properties (such as ability to bind metals) for humic substances?