Whether you are introducing collision theory or something more demanding like reaction order, the reaction between sodium thiosulfate—Na2S2O3 and hydrochloric acid can provide a consistent, accurate, and engaging opportunity for investigating these topics.
A few weeks ago, I was looking for a new reaction that could be used to investigate how concentration affects reaction time. In the past, I had always used traditional reactions such as magnesium and hydrochloric acid or Alka-Seltzer and hydrochloric acid. Though both served their purpose, there would always be groups that didn’t quite get data that was consistent with what I knew the relationship to be. In most cases, this was due to ambiguous and inconsistent timing methods or simply a matter of experimental error like not ensuring the magnesium stayed in the acid without floating to the top. I wanted a reaction that would be more likely to produce consistent results from group to group, easy to execute, and was a bit more exciting than waiting for magnesium or Alka-Seltzer to disappear.
Eventually, I came across a Flinn1 experiment which focused on the reaction between sodium thiosulfate and hydrochloric acid.
Na2S2O3 (aq) + HCl (aq) → 2NaCl (aq) + S (s) + H2O (l) + SO2 (g)
What I liked most about this reaction was the easy and consistent timing mechanism it provided my students with, which could eliminate the ambiguity and differences in timing approaches that lab groups had used in the past.
Here’s how: As the reaction proceeds, one of the products is sulfur. As more sulfur gets produced, the solution becomes more and more cloudy until eventually the solution is opaque. Because of this, the moment that you can no longer see through the solution can be used as a consistent way to stop time. When I asked my students how we would all consistently decide on when the solution is opaque, many of them suggested to place some sort of object on the other side of the beaker so that we would all stop the timer when the object was no longer visible. This naturally progressed to the idea of drawing something on the beaker itself (an X on the bottom in this case) and applying the same reasoning.
This reaction and the implementation of this natural clock can be seen below in a Flinn video2.
Even though it is just a matter of changing from visible to opaque, I noticed that the anticipation of waiting for that X to disappear had nearly all my students hovering over their beakers anxiously waiting to stop their timer. It even got to a point where different groups started to use their phones to make time lapse videos of their reaction beakers. You can see one below. As a teacher, it was fun to watch their level of excitement over something so seemingly simple.
Though I used this experiment to primarily investigate collision theory and different factors that affect the time it takes for a reaction to complete, it could easily be used to determine something more complex like reaction order (see the entire Flinn video from which the above clip is taken).
I also found this lab to serve as a great opportunity for my students to play a larger role in the creation of the experimental setup since there wasn’t much complexity to it. I facilitated the design of the experiment by asking my students a series of questions that were meant to feel like it was a genuine conversation happening between scientists interested in answering a question. The PowerPoint that I used to help facilitate this discussion can be found as Supporting Information at the bottom of this post if you are logged in to ChemEd X, but the general theme followed these questions:
- What is our independent variable? How should we go about changing this?
- Should the total volume of each beaker be the same or different? Why?
- What is our dependent variable?
- Are there any variables that we should control?
- How should we go about timing our reaction?
- How should we record and organize our data?
- How are we going to figure out our concentrations in terms of Molarity?
- How should we record and organize our data?
- What are we going to do with our data once we have it? Graph it?
I don’t include students in things like this often enough and it’s important that I continue to remind myself the beneficial experience this can provide for students to get a more accurate understanding of how science operates.
However you decide to do it, the general approach to this experiment goes something like this:
1) Using a Sharpie, draw a black X on the bottom (outside) of each beaker.
2) A stock solution of 0.15 M Na2S2O3 is used to make 5 different concentrations using different amounts of distilled water, though our tap water worked just fine too. The total volume of each solution should be the same in each beaker.
3) Add 5 mL of 2 M HCl to your first beaker to start the reaction. You can give it an initial stir to uniformly distribute the HCl. The timer starts after this initial swirl.
4) While looking down at the beaker, stop the timer the moment you see the X completely disappear from sight.
5) Do this for all your samples and start analyzing your data
After everyone had finished the experiment and analyzed their results, I was thrilled to see that the data from each group produced a graph that displayed the relationship I was looking for. Not a single group had one weird outlier or a graph with seemingly random points all over the place! Some of the groups even paid close enough attention to the fact that each beaker had different levels of “opaqueness” to them. This provided a great opportunity to talk about the benefits of qualitative evidence as well. I attribute these consistent results to two primary things:
1) Consistent timing mechanism that each group can easily reproduce
2) It is almost impossible to mess up this reaction—you’re just pouring HCl into your Na2S2O3 solution. Minimizing chances for experimental error was huge.
Though I don’t always shoot for consistent data between groups when we do a lab, I knew that the arguments would vary between groups when trying to explain why their experiment displayed the relationship it did. It is the arguments I am most interested in developing after students complete their data analysis.
Students were tasked with developing their initial argument using a Claim, Evidence, Reasoning (CER) framework. Though most boards had similar claims, they often differed in what evidence they chose to present. They all had access to the same evidence and yet different groups intentionally left out certain pieces of evidence—why? Where their boards differed the most was in their reasoning, which is meant to have them justify why their evidence makes sense based on known scientific principles. I should mention that the students had not been presented anything about collision theory before this lab and yet many of them were able to come up with a valid particle-based explanation while others either circled around ambiguity, lacked detail, or simply displayed some form of misconception. The important part of this was that they tried their best, based on the models they had running around in their heads, to explain the phenomenon and knew that it was up to the scientific community (our class) to act as a filter for sorting out valid explanations from ones that either lacked detail or could not quite account for the evidence. This is the process I love doing the most.
The lab itself took about 30 mins to do but because I involved them in the experimental setup and dedicated time to construct arguments that were presented, debated, and refined, the entire process took 3 periods (1 hr each).
I want to thank Flinn for inspiring the idea for the experiment in the first place and NSTA’s book Argument-Driven Inquiry in Chemistry3 for providing the framework we used to set up and make sense of the investigation.
1 Rate of Reaction of Sodium Thiosulfate and Hydrochloric Acid. N.p.: Flinn Scientific, n.d. Pdf. https://www.flinnsci.com/globalassets/flinn-scientific/all-free-pdfs/dc91860.pdf
2 "Rate of Reaction of Sodium Thiosulfate and Hydrochloric Acid..."20 Dec. 2012, https://www.youtube.com/watch?v=r4IZDPpN-bk. Accessed 17 Jan. 2017.
3 "NSTA Science Store: Argument-Driven Inquiry in Chemistry: Lab ...." 1 Oct. 2014, https://www.nsta.org/store/product_detail.aspx?id=10.2505/9781938946226. Accessed 17 Jan. 2017.