18-Env-B9 Environmental Chemistry and Microbiology · May 2013
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
National Exams — May 2013 — 04-Env-B9, Environmental Chemistry/Microbiology. 3 hours duration; closed-book exam (approved Casio or Sharp calculator only). The paper has two sections — Section 1: Chemistry (11 questions, 50 marks) and Section 2: Microbiology (14 questions, 50 marks) — twenty-five questions constitute the complete exam and all are answered below. Total examination mark 100.
Reference texts. Davis & Cornwell, Introduction to Environmental Engineering (6th ed.) (water chemistry, disinfection, water/wastewater microbiology, indicator organisms); Metcalf & Eddy (Tchobanoglous, Stensel, Tsuchihashi & Burton), Wastewater Engineering: Treatment and Resource Recovery (5th ed.) (chemical unit processes, chemical phosphorus precipitation, biomass stoichiometry, activated-sludge microbiology); Guidelines for Canadian Drinking Water Quality (Health Canada); MWH's Water Treatment: Principles and Design (3rd ed.) (chlorine disinfection, contact-tank sizing).
Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.
The stoichiometric equation of a reaction states only the mass balance between reactants and products — it says nothing about the mechanism by which A is actually consumed. Reaction order is an experimentally determined quantity, obtained by measuring how the rate depends on reactant concentration (e.g. from initial-rate data or a concentration-vs-time plot fitted to integrated first-, second-, or zero-order forms); it cannot in general be read off the balanced equation, because most reactions proceed through a multi-step mechanism whose slowest (rate-determining) step may involve a different stoichiometry than the overall equation shows.
The one exception is an elementary reaction — one that genuinely occurs in a single step exactly as written. If A → C + D is elementary, its molecularity is unimolecular (one A molecule reacting), and for an elementary step molecularity equals order, so the reaction would be first order, with rate law $r=-\dfrac{d[A]}{dt}=k[A]$. Without being told (or being able to verify from kinetic data) that the reaction is elementary, the only defensible answer is that the order cannot be deduced from stoichiometry alone and must be measured; if an elementary, single-step decomposition is assumed (the usual textbook convention when no other information is supplied), the order is first order in A.
