March 2020 | Resource Feature
Designing a Greener Le Châtelier's Principle Lab
By Cassidy Javner
Risk = Exposure x Hazard
Experiments are an essential component of the chemistry curriculum, and as a chemistry teacher it is my job to make the laboratory a safe learning environment. Traditionally, this involves reducing the exposure to hazardous substances by showing students how to correctly use safety equipment and ensuring the use of proper personal protective equipment (PPE). Protocols, procedures, and rules are implemented to make sure the laboratory is a safe place to learn and to reduce the incidence and severity of accidents. However, accidents and unintended outcomes can still happen in the laboratory, especially with inexperienced high school chemistry students.
Risk, or the probability of a harmful event, is a function of both exposure and hazard, and green chemistry plays a role in improving laboratory safety by addressing the hazard component of the function. Green chemistry’s approach to reducing risk — through the elimination of hazards and toxic chemicals — is more effective than trying to lower exposure through the use of PPE.
Classroom and laboratory safety can be improved by reducing the use of hazardous materials by replacing traditional chemistry labs with greener alternatives. These greener replacement labs teach the same skills and address the same standards as traditional experiments, but utilize methods and chemicals that don’t expose our students to unnecessary risk, and are safer for the environment. The first green replacement lab that I implemented in my classroom was Beyond Benign's Equilibrium/Le Chatelier’s Principle lab.
Traditional Le Châtelier’s Principle experiments
Le Châtelier’s Principle is a favorite topic of my students because of the dramatic colors involved in the reactions. The qualitative observations of the color changes allow students to predict and interpret various shifts in equilibrium systems as the rates of the forward and reverse reactions change until equilibrium is reestablished. There are many different systems that illustrate this effect, including iron (III) thiocyanate and cobalt chloride complex, among others. These systems (detailed in Figure 1) are traditionally studied as part of an investigation of Le Châtelier’s Principle.
Fe3+(aq) + SCN- (aq) ⇄FeSCN2+(aq)
Fe3+(aq) is yellow, SCN- (aq) is colorless, and FeSCN2+(aq) is red-orange.
Add iron (III) nitrate and potassium thiocyanate and heat/cool the solution.
Iron (III) nitrate solution may be a skin and body tissue irritant.
Potassium thiocyanate is toxic by ingestion and emits a toxic gas if strongly heated.
[Co(H2O)6]2+(aq) + 4Cl- (aq) + Heat ⇄ [CoCl4]2-(aq) + 6H2O (l)
[Co(H2O)6]2+(aq) is pink and [CoCl4]2-(aq) is blue.
The equilibrium system of cobalt chloride is violet, and students observe color shifts from blue to pink as they add silver nitrate, concentrated hydrochloric acid, calcium chloride, water, and heat/cool the solution.
Cobalt chloride solution is a flammable liquid and moderately toxic by ingestion.
Concentrated (6M) hydrochloric acid solution is toxic by ingestion and inhalation and is corrosive to skin and eyes.
Figure 1. Summary of traditional equilibrium systems, color changes, and safety warnings.
I think it is always important to question the purpose of the learning activity or experiment. Ask yourself: What is it that I want to teach my students? What lessons do I want students to take away? What level of risk is associated with this laboratory exercise? Is the purpose to impress or excite students with color changes, or is it to teach students how to connect observations made at the macroscopic level to what is happening at the molecular level as an equilibrium system responds to stressors?
In this experiment, students will explore Le Châtelier’s Principle and the phenomena of equilibrium by using materials that are non-toxic and non-hazardous for human health or the environment, while still visualizing the equilibrium shifts through color changes. Further, the materials used in this experiment provide a great cost-savings to the teacher, as everything is easy to acquire at a grocery store or on Amazon. This experiment works well for a general level chemistry class and can also be adjusted for an AP class. This experiment utilizes two different systems to visually explore the effects of changing concentration and temperature on equilibrium.
A greener replacement: Starch-iodine complex
The first system involves a starch-iodine complex to demonstrate the effect of temperature on equilibrium. The formation of the complex is an exothermic reaction and results in a deep purple/blue-black color.
|Iodine (aq) + starch (aq)
||⇄||starch-iodine complex (aq) ΔH < 0
In the above reaction, the shifts in equilibrium position produced by temperature changes are in accordance with Le Châtelier’s Principle. Enthalpy (ΔH) is negative (exothermic) in the forward direction as written above. Cooling the system causes a shift toward the products, forming more blue-black starch-iodine complex (Figure 2). Adding heat to the system favors the reactants, and the starch-iodine complex dissociates, causing the system to become colorless (Figure 2).
Note for AP teachers: In an AP class, it is important to emphasize that changing temperature is changing the value of the equilibrium constant, not just the position of equilibrium. You may also want to check out this article about the use of the language shift when discussing equilibrium systems.1
|Figure 2. Results of cooling and heating the starch iodine solution.
Spray starch or a starch solution works best for the experiment. If you are purchasing spray starch, be sure to check the ingredients list to ensure starch is one of the primary ingredients. Simply spray the starch solution several times into a 250 mL beaker so the bottom of the beaker is covered, and then fill with deionized water (Figure 3). You could also use biodegradable packing peanuts, which are made from starch, and dissolve a few (3-5) in water. Cornstarch works in a pinch, but students need to constantly stir the solution to keep it dissolved.
For the iodine reactant, tincture of iodine or another iodine solution found at the grocery or drug store works well. I have found that the percent iodine in the solution doesn’t matter for the experiment, as long as the solution appears light blue/purple in color when mixed with the starch (Figure 4).
Depending on time, you can also have students explore what happens when iodine is placed on starch-based foods such as bread, flour, rice, and pasta. This would allow students to connect chemistry to materials they are familiar with and make their own conclusions about the interaction of iodine and starch before beginning the equilibrium experiment.
of starch solution using laundry starch spray (left).
Figure 4. Equilibrium starch-iodine solution (right).
A greener replacement: Butterfly pea tea
In the second half of the experiment, students evaluate the effect of pH on the equilibrium system of butterfly pea tea. There are several vendors of dried butterfly flower pea tea on Amazon, and the cost ranges between $10 and $15 for a bag that can continue to be used for several years. The molecules in butterfly pea tea being considered in this experiment are a group of polyphenols called theaflavins. There are four major theaflavins found in tea, but we will only consider the reaction of theaflavin (TF-1). Theaflavin is highly stable in acidic solution, but will degrade to theanaphthoquinone via the mechanism proposed by Jhoo et al. 2 This experiment allows students to readily observe shifts in equilibrium by increasing and decreasing the concentrations of the reactants, and still involves beautiful color changes.
|Tea (aq) + H+ (aq)||↔||Tea-H+(aq)|
To brew the tea, simply place a small handful of the flowers into hot water. Let sit for a few minutes until it turns a deep blue color, and then remove the flowers (Figure 5). When an acid is added to the tea, the color of the solution lightens and changes from blue to purple (Figure 6). There are many acidic solutions I have utilized in the experiment, largely dependent on what I have in stock in my classroom. Vinegar, lemon juice, and citric acid all have worked successfully. When a base, such as baking soda, is added to the tea, the solution darkens and turns a green/blue color (Figure 6). Black tea can be substituted for butterfly pea tea, and results in a color change of light brown to dark brown.
| Figure 5. Butterfly pea tea flowers and brewed butterfly pea tea (left).
Figure 6. Color changes of the butterfly pea tea system with vinegar and baking soda(right).
As the chemicals are non-hazardous, I encourage students to engage in their own additional investigations — or what I like to call “scientific play.” It is during this time that students often engage in deeper thinking about equilibrium as they explore their own ideas: Can we continue to switch the system back and forth? What happens if we use a different acidic source? What if we add both an acid and a base at the same time? Is there a limit to the number of shifts involved in equilibrium? What happens if we heat the tea or cool it down? These types of questions encourage curiosity and allow students to explore their own hypotheses and challenge their ideas about equilibrium.
Principles of green chemistry
Authors Anastas and Warner define green chemistry as the “design of chemical products and processes that reduce and/or eliminate the use or generation of hazardous substances.”3 By switching to greener materials, teachers can avoid ordering, transporting, storing, and monitoring the use of hazardous chemicals. Using greener labs also reduces costs by preventing the creation of hazardous waste in the first place.
Anastas and Warner have identified 12 Principles of Green Chemistry that guide chemists in creating more sustainable chemicals, processes, and products, outlined in this graphic created by Compound Interest. I like to introduce these principles at the beginning of the year and reference the specific principles addressed in each experiment with my students. Furthermore, it can be very powerful to share with students your rationale for switching to a more sustainable lab. This experiment incorporates several green chemistry principles, including #1: Prevention of Waste and #12: Safer Chemistry for Accident Prevention. The concluding questions in the activity ask students to analyze the “greenness” of the experiment compared to the traditional Le Châtelier’s Principle experiments.
Finding green inspiration
My journey with green chemistry began with a partnership with Beyond Benign, a non-profit organization whose mission is to “develop and disseminate green chemistry and sustainable science educational resources that empower educators, students and the community at large to practice sustainability through chemistry.”4
The Lead Teacher Program at Beyond Benign has provided me with the skills and tools needed to implement green chemistry and collaborate with teachers to spread the word throughout my school, state, and region. Lead Teachers work to develop, test, and refine new sustainable experiments and curriculum (including “A Greener Le Châtelier’s” Principle experiment), and also lead workshops and presentations. The Beyond Benign website has additional replacement lab options for you to try, and Lead Teachers are hard at work to make these labs applicable for your classroom.
So why implement greener replacement labs in your classroom? Doing so in my classroom shows my students that science can be a solution to the many problems facing society and the environment. Green chemistry is a current, relevant, and important topic in science and incorporates critical thinking, creativity, and problem solving. In addition, incorporating green chemistry in your labs:
- Connects topics that are new, modern, and relevant for you and your students
- Utilizes less toxic materials and generates less hazardous waste
- Exposes students to fewer hazards
- Uses materials that are often less costly to obtain and dispose of
- Shows students how careers in STEM fields can have an impact on a sustainable future
I am grateful for being part of a collaboration that began with a Beyond Benign Green Chemistry workshop in 2008. That collaboration led to the original creation of this lab, as well as the many edits made over the years from curriculum specialists, Lead Teachers, and workshop participants.
- Carroll, D. Taking Inspiration from the AP Chemistry Reading. Chemistry Solutions, Nov 2018. Available online at https://teachchemistry.org/periodical/issues/november-2018.
- Jhoo, J.; Lo, C.; Li, S.; Sang, S.; Ang, C.; Heinze, T.; Ho, C. Stability of Black Tea Polyphenol, Theaflavin, and Identification of Theanaphthoquinone as Its Major Radical Reaction Product, Journal of Agricultural and Food Chemistry, 2005, 53 (15), 6146-6150. DOI: 10.1021/jf050662d.
- Anastas, P.J.; Warner, J.C. Green Chemistry: Theory and Practice; Oxford University Publishing: New York, 1998.
- Beyond Benign “Reaction Rates & Equilibrium” page. https://www.beyondbenign.org/curriculum_topic/hs-reaction-rates-equilibrium/ (accessed Feb 1, 2020).
Photo credit (article cover): Malp/Bigstockphotos.com