In Chemistry, students are discovering the relationships and rules of equilibrium through data analysis and mathematical derivations, rather than textbook readings and lectures.
This block, we are focusing on equilibrium in Chemistry class. During the last Build Week, students did four labs to collect data we’ve been analyzing all block. This block, my goal has been to have students figure out all the relationships and rules for themselves, partly by working through data analysis and partly by doing their own derivations. For example, when I went to review equilibrium in a standard General Chemistry textbook, I decided that rather than assigning the material as reading, I would have the students figure out most of it for themselves. I turned most of three textbook chapters into three problem sets. Without reading assignments or lecture, the students were able to work out the results and relationships for themselves, especially since most of it involved mathematical manipulation.
The first of our four labs involved making multiple dilutions of stock solutions and measuring the conductivity of each. The solutions included strong and weak acids and bases and salts. We then analyzed the data to develop understanding of strong and weak electrolytes: solutes that dissociate into ions in solution. Since the weak electrolytes split up more when diluted more, this also gave us a way into the idea of equilibrium.
Our second experiment focused on complexation reactions with color changes. These provided a fun way to visualize what happens when an equilibrium is perturbed. For example, when we heat a purple mixture of red and blue cobalt complexes in equilibrium, does it get more red or more blue? Later students worked on a problem set teasing out the relationships between their observations and the chemical processes underneath, leading into a statement of Le Chatelier’s Principle.
The third experiment was intended to provide evidence that equilibrium constants are constant. First, students prepared dilutions of a colored solution and measured the absorbance spectra. (They later analyzed these results graphically to find Beer’s Law, which relates absorbance to concentration, and is commonly used to measure concentrations of species in solution.) Then they prepared various combinations of colorless iron nitrate and potassium thiocyanate solutions, which turned orange when mixed due to formation of an iron thiocyanate complex. When students find the concentration of the colored product at equilibrium using absorbance spectra and plug this into the equilibrium constant expression, they should get the same answer for each mixture. In fact, this provided an opportunity to consider the size of experimental error and whether the values were the same within error.
Finally, we are discussing acid-base equilibria, a topic of great biological importance. In the last week or so of the block, we will analyze the results of an experiment showing how buffered solutions differ from plain water, and also pH titrations of polyprotic weak acids.
-- Emily Eames