A maximum of three team members may participate in each event. Also see other events for both Level I and Level II teams listed at the end.
The Balancing Act
A variety of equations will be completed and balanced by each team.
Advance Preparation: None, beyond any desired practice.
The Event:
Teams consist of three or fewer students, as at all events. Given reactants either by names or formulas, and sometimes products, each team will be challenged to write within 30 minutes a number of balanced equations. The balancing method(s) can be freely selected by the team–only the results matter. Notes are not allowed. The winning team will be the one with the greatest number of correct, complete, and balanced equations.
Concurrent event is Beat the Clock.
LeChatelier's Lunacy
Advance Preparation:None, beyond any desired practice.
The Event:
Any required materials except calculators and writing instruments will be provided. The team will solve a series of chemical equilibrium problems using a nonprogrammable calculator. These problems could involve Kc, Kp, Ksp, Ka, or Kb. At least one problem will involve LeChatelier's Principle. The team will turn in its calculations as well as its final answer and both may be used to determine the final rankings.
A draft sample problem is available. For additional practice problems, it is suggested that teams try old AP Chemistry Exam equilibrium free response questions – there are plenty to choose from since there is one every year (it has been Question 1 for the last several years). At the College Board's AP Central website the last several years' exams are available as .pdf documents. You will have to become a registered user if you are not already one, but it is a free service.
Concurrent event is Chemysterie.
Beat the Clock
Each team will bring its own iodine clock reaction stock solutions. The team is asked to adjust the solutions during the event so the color change takes place in a specific time–from 15 seconds to 2 minutes.
The iodine clock reaction is based on iodate and meta-bisulfite ions reacting in solution to produce dissolved molecular iodine (I2) which combines with starch indicator to form a deep blue color. The time required for the appearance of the blue starch-iodine complex depends on the concentration of the reactants and the temperature of the system.
Advance Preparation: The team (with teacher supervision) should prepare stock solutions of the two "iodine clock reaction" reactants–0.02 M potassium iodate (KIO3) and 0.001 M sodium meta-bisulfite (Na2S2O5)/starch suspension–according to the procedure suggested below. The team should also bring a small amount (less than 50 mL) of 1 M sulfuric acid. The other equipment and distilled water required for the event will be provided at the Chemathon. The team should bring its own burets and pipets.
The potassium iodate stock solution can be prepared by dissolving 4.3 g potassium iodate in enough distilled water to make one liter of solution. To prepare the sodium meta-bisulfite/starch stock solution, first make a thin paste by stirring 4 g soluble starch into a small amount of distilled water, add the starch paste to roughly 500 mL of boiling distilled water, and allow the suspension to cool. (As an alternative to soluble starch, you may wish to try either spray starch or starch-based biodegradable packaging "peanuts" dissolved in water.) To the starch suspension, add 0.2 g sodium meta-bisulfite and enough distilled water to bring the total volume to 995 mL. To prepare the solution for use, add 5 mL of 1 M sulfuric acid (with stirring) immediately before conducting the clock reaction, or no more than 24 hours in advance. These stock solutions, if mixed in equal volumes, produce a very fast reaction time. The team (with teacher supervision) should determine the dilutions needed to obtain reasonable clock reaction times. Results are best if the potassium iodate solution is diluted rather than the starch solution. The team should conduct preliminary time trials–see below–in advance of the Chemathon to determine a strategy for adjusting the solutions to yield a pre-specified time interval.
The Event:
The judge will assign the team a random time from 15 seconds to 2 minutes. The team will have 10 minutes to dilute the concentrations (or adjust temperatures, if desired) of either or both solutions to obtain the specified reaction time. The team may not actually mix the solutions until the official trial begins. When the team is ready (within the 10-minute time limit) the judge signals "start." At that time, the team must pour the solutions together into a 250-mL beaker on a magnetic stirrer, and the official time for the reaction (when the first blue color appears) is noted. Each team may complete up to three trials. The trial time closest to the specified time will be the team's official event result.
The team achieving the reaction time closest to the requested time will be declared the winner.
Brought by the team:
- goggles, aprons, shoes without open spaces
- solutions for clock reaction
- burets and pipets
- a nonprogrammable calculator or pre-prepared graph may be used
- tables and spreadsheets are not allowed
Provided by Chemathon:
- 250-mL beaker
- 10-mL and 50-mL graduated cylinder
- ring stands and buret clamps
- funnel for filling burets
- thermometer
- magnetic stirrer and stirring bar
- distilled water
- paper towels
Concurrent event is The Balancing Act.
Make My [Fara]day
Using electrolysis, each team will plate out a specified mass of metal using Faraday's law to determine the amount of time needed to carry out this process.
Advance Preparation: The team will need to bring about 300 mL of a 1 M CuSO4 solution (the addition of about 1 mL of 6 M sulfuric acid to this solution may improve the adhesion of the plated copper). The team will also need to bring 2 copper electrodes suitably marked as cathode and anode, a six-volt battery, a 0-5 A ammeter, a variable resistor, 4 insulated wire leads, and 8 alligator clips. Beakers (250 mL) will be provided.
We understand getting 6V lantern batteries is becoming more difficult. So we will allow any combination of dry cell batteries (AAA,AA,C, or D) not to exceed 6V. A single dry cell battery has a voltage of 1.5V. You can connect up to 4 AAA-D batteries in series and use that in place of the 6V Lantern battery. Power supplies are still not allowed for this event.
The Event:
Team members will use an electrolysis cell consisting of a copper anode, a copper cathode, and 1 M CuSO4 in a 250-mL beaker to plate out an assigned mass of copper onto the cathode. Notes on calculations or procedures, etc., are not allowed. The judge will pre-mass the cathode and assign a mass, between 0.20 g and 1.00 g, to plate onto it. The power source will be a 6 V lantern battery (preferably one that has not been stored for a long time or used much) and the circuit will also contain an ammeter to measure the current flow. Please see invitation. The current can be controlled with a variable resistor, or by adjusting the separation of the electrodes. The students should select a current of no more than 2.5 A and using Faraday's law, calculate the time needed to plate out the specified mass of copper. The distance between the electrodes may be varied to control resistance, but the electrodes must remain in the 250-mL beaker provided. The electrolysis should be carried out to verify the calculations. The judge will mass the final cathode. The team score will depend on the lowest percent error after one continuous run.
Brought by the team:
- goggles, aprons, shoes without open spaces
- copper electrodes
- 1 M copper sulfate solution
- meter, resistor, wires and clips
- 6-volt battery or (see invitation)
- nonprogrammable calculator; pen or pencil
- notes on procedures and calculations are not allowed
Provided by Chemathon:
- 250-mL beaker
- distilled water
- blank paper for calculations
- paper towels
Concurrent event is Avogadro's Tiebreaker.
Traveling Electrons
The team will use an oxidation-reduction titration to determine the percent by mass of sodium hypochlorite in chlorine-based liquid laundry bleach provided at the event.
Advance Preparation: Prior to the Chemathon, the team (with teacher supervision) must prepare 500 mL of approximately 0.1 M sodium thiosulfate solution, Na2S2O3(aq). The actual concentration of the solution should then be determined (to the nearest 0.001 M or better). This can be accomplished either by dissolving a known mass of Na2S2O3.5H2O (248.19 g/mol) in enough distilled water to make 500 mL of solution, using a 500-mL volumetric flask; or else by titrating the solution against a primary standard such as potassium iodate or oxalic acid dihydrate, using the method described below for titrating NaOCl. The second method is more accurate but less convenient.
We have found that 6.0% NaOCl Bleach is becoming difficult to purchase. For the Chemathon we will be purchasing the concentrated 8.25% NaOCl bleach and then diluting it to be between 5-7%. The diluted bleach will be tested by the Chemathon committee against a Primary standard in order to get the actual concentration. Your teams should expect ~6% NaOCl and should practice accordingly.
The Event:
Using their own standardized sodium thiosulfate solution and their own 50-mL burets, team members will determine the moles of sodium hypochlorite, NaOCl, in weighed samples of the bleach solution provided at the event. The team will supply three burets which they can fill with the standardized sodium thiosulfate solution so that all three team members can conduct independent titrations. As many titrations as desired may be completed within the 30-minute time limit. No electronic pipets or burets may be used. Notes on calculations or procedures, etc., are not allowed; however, the container of sodium thiosulfate solution may be labeled with the concentration.
The team will report only one value of the percent of sodium hypochlorite, with an appropriate number of significant digits, for judging. Supporting data and calculations must be provided. The unknown percent by mass of NaOCl will be between about 5 and 6 percent. The team must bring to the Chemathon three 50-mL burets and its standardized sodium thiosulfate solution. Other glassware and supplies will be provided at the event.
Liquid laundry bleach such as Chlorox brand bleach (see invitation) is an aqueous solution of sodium hypochlorite, NaOCl. The unknown bleach will be available in plastic dispensers. The suggested titration procedure is as follows. A 250-mL Erlenmeyer flask is rinsed, dried on the outside, and weighed to the nearest milligram. A 2.0-2.5 mL bleach sample is dispensed into the flask, which is then reweighed to determine the sample mass. About 100 mL distilled water is poured down the inside wall of the flask to dilute the bleach. Then 10 mL 6 M acetic acid and 8 mL 2 M KI is added. The NaOCl oxidizes the iodide ion to molecular iodine:
2I- + OCl- + 2H+ –> I2 + Cl- + H2O
The molecular iodine is then promptly titrated with the standard sodium thiosulfate solution:
I2 + 2S2O32- –> 2I- + S4O62-
When the solution has changed to a gold-orange and then to a faint yellow color, twenty drops (1 mL) of starch indicator are added to produce the deep blue color of the starch-iodine complex. (By not adding the starch until near the end point, the progress of the titration can be followed by the fading of the yellow color of the molecular iodine.) The end point is reached when the blue color vanishes with the addition of one final drop of titrant. From the mass of the sample and the moles of NaOCl in it, the percent by mass of NaOCl is calculated.
The team reporting the percent mass value closest to the "known" value will be declared winner.
Brought by the team:
- goggles, aprons, shoes without open spaces
- standardized sodium thiosulfate solution
- three burets
- nonprogrammable calculator; pen or pencil
- notes on procedures and calculations are not allowed
Provided by Chemathon:
- unknown bleach solution
- 2 M KI solution
- 6 M acetic acid
- starch indicator solution
- wash bottle and distilled water
- digital balances weighing to 0.001 g
- 3 250-mL Erlenmeyer flasks
- 3 ringstands and buret clamps
- graduated cylinders, funnel (for filling burets), dropper pipets
- blank paper for calculations
- paper towels