Classic Chem II
Unit IV Project – Uncle Joe’s Pool
A key aim of learning science is being able to apply scientific models to explain real life situations. Another key aim is to be able to explain scientific phenomena to other people. This project asks you to do both.
Background
As large bodies of water, swimming pools left on their own tend to become like ponds - algae grow on the sides and scum forms on the top. In addition, people tend to have bacteria on their bodies that end up in the water, which could infect other people. To address both of these concerns, people add chemicals ("chlorine") to pools to kill the bacteria and algae. Adding elemental chlorine (Cl2) is impractical (because it is a gas) and unwise (because it is very toxic). Instead, people add chemicals that result in the formation of hypochlorous acid (HOCl), the chemical that does the killing work in pools.
Several different sets of chemicals can give rise to hypochlorous acid in pools. One method is to add solid calcium hypochlorite, which dissolves to give aqueous calcium and hypochlorite (OCl-) ions. The hypochlorite ions are the conjugate weak base of hypochlorous acid, and reversibly react with water to form hypochlorous acid.
Scenario
Suppose that you find yourself at a large family reunion and you get into a conversation with your third cousin's godfather, Joe, about his swimming pool. Joe is a well-educated guy, but doesn't really remember high school chemistry, and never took any chemistry in college. He wants to understand more about his pool, how to maintain it, and what really is happening at the molecular level. He has the following questions:
1. Everything I read about pools says that you have to keep the pH just right. What is pH?
2. I get that I add this calcium hypochlorite stuff to get hypochlorous acid, but where does the hypochlorous acid come from? How and why does its concentration depend on the pH of the pool?
3. I have this pool chemistry testing set that I use every week. I take a sample of pool water, add a couple drops of one chemical, and then add another chemical drop by drop until the sample changes color. Then the number of drops tells me how much of these other chemicals I need to add to my pool. What's happening when I do this test, and how does my result relate to the amount of those chemicals I need to add?
(Note: see the next page for a snapshot from your textbook about pool chemistry.)
Project description
Your assignment for this project is to answer Joe's three questions. Your answers must address the concepts of chemical equilibrium, and acid-base reactions. You need to explain what is happening at the particle level and connect these explanations with macroscopic effects, including pool water quality. Joe needs to be able to understand all of your explanations, so think about what concepts you have to explain for him to understand the stories you are telling.
Your presentation to Joe can take a variety of forms including: an illustrated letter, a webpage, a presentation/slide show, a movie, etc. Your work will be evaluated in the following areas:
● Accurate discussion of chemical equilibrium and Le Chatelier's principle
● Accurate discussion of acids, bases, acid-base reactions, auto-ionization of water (Kw), pH, and pOH
● Visual representation of interactions and transformations at the molecular level
● Correct symbolic representation (chemical formulas, reaction equations)
● Clarity of presentation (from Joe's perspective)
From your textbook: