Introduction:

Our experiment was focused on the chemical iodine clock reaction between potassium iodate and sodium metabisulfite. These two initially clear liquids are combined, with the addition of starch, to result in a dark brown liquid. We explored the impact of different temperatures on reaction time; we hypothesized that as temperature increased, the amount of time it would take the solution to change color would decrease.

Background Information:

In the outside world, sodium metabisulfite is often used in dough for cookies and crackers as a reducing agent or in wine, dried fruit, or jams as a preservative. Potassium iodate, similarly, can be used in baking to strengthen gluten bonds; however, it is often commonly used as a thyroid blocking agent after nuclear exposure. The reacting of these two molecules is commonly performed in scientific laboratories to visually demonstrate reaction rate and kinetics.

Methods:

First, we created a one-liter stock solution of .10M potassium iodate and .25M sodium metabisulfite, and a half-liter stock solution of 1% starch. To create the .10M potassium iodate we dissolved 21.4 of KIO₃ into 800 ml of distilled water which we poured into a 1L volumetric flask. We diluted the solution with distilled water until it reached a volume of 1L. To create the .25M sodium metabisulfite we dissolved 24g of NaS₂O₅ into 600 ml of distilled water and diluted it in a 1L volumetric flask until it reached a volume of 1L. We diluted 5% starch to a 1% solution by adding 100ml of the starch to a .5L volumetric flask and then added distilled water until we reached .5L.

We conducted the experiment in 6 different temperatures and did 5 trials in each. We measured out 10ml of KIO₃, 5ml of starch, and 20ml of distilled water into a 50ml beaker. We found in preliminary testing that we need to dilute the reaction more for it to be slow enough to time at higher temperatures. We then measured out 2ml of metabisulfite and 23 ml of water into an 80ml beaker. We started our timer before starting the reaction and filmed the timer and reaction because we found in preliminary testing the reaction happened instantly for us to accurately stop the timer. Once the timer and video had started we poured the contents of the 50ml beaker into the 80ml beaker. We used the video to get an accurate read on time. We repeated this 5 times at each temperature. For 0℃ we placed the beakers in an ice bath and used a thermometer to determine when the solutions reached a temperature of 0℃. Once they reached the desired temperature we reacted the solutions in the ice bath to maintain the temperature. For 12℃ we placed the solution in a fridge for 10 mins. Once the temperatures read 12℃ we reacted to the solutions within the fridge. For 20℃ we reacted to the solution at room temperatures. For 40°C, 50℃, and 60 ℃ we used a hot plate to bring the solutions to the desired temperature once they reached the desired temperature we removed them from the hot plate and reacted immediately. We found in preliminary testing if we left the solutions on the hot plate during the reaction it would continue to heat up increasing our uncertainty.

Discussion:

As we increased temperature we observed that the speed of our reaction increased; this observation was in agreement with our initial hypothesis. As we increased the temperature, the molecules in our sodium metabisulfite and potassium iodate started to move faster–as the average kinetic energy is increased–, causing a faster reaction. As seen in Figure 1, the R^2 value so close to 1, we can confidently infer that the relationship between our temperature and time is exponential.

In the Arrhenius equation: k=Ae-Ea/RTas we increase temperature, the rate constant, “k,” also increases. This correlation can more easily be seen if we take the natural logarithm of both sides of the equation, yielding, ln(k)=ln(A)-Ea/RT. If we graph ln(k) versus 1/T, we see that the slope equals -Ea/R.

The relationship between reaction rate and T is exponential because of T’s relationship to k in the Arrhenius equation. We could then plug k into our reaction rate formula (Reaction rate = k[A][B] where A is sodium metabisulfite and B is potassium iodate). As we kept the concentrations constant, since we would only be increasing k by increasing temperature, reaction rate would increase directly.