Concentration Dependence

Source: Smaa Koraym at Johns Hopkins University, MD, USA

1. Concentration DependenceExpand

   In this experiment, you will combine varying concentrations of aqueous HCl and sodium thiosulfate to make solid sulfur, which rapidly gathers into opaque yellow particles at a certain sulfur concentration. Since the reaction solution starts clear and colorless, you can easily tell when it reaches that concentration.

   You'll use the same test tube and total volume each time, so it will take the same amount of sulfur to make each solution completely opaque. Thus, you'll measure the reaction progress by timing how long that takes. You'll then use that data to estimate the reaction orders of the individual reactants and of the overall reaction.

   Before starting the lab, make a table listing the reactant concentrations, the time to solution opacity, and the solution temperature for the trials.

   Table 1: Reaction progress

   Trial	[Na2S2O3] (M)	[HCl] (M)	Time (s)	Temp (°C)
   1	0.1 M	3.0 M
   2	0.1 M	3.0 M
   3	0.2 M	3.0 M
   4	0.15 M	3.0 M
   5	0.05 M	3.0 M
   6	0.1 M	6.0 M
   7	0.1 M	4.5 M
   8	0.1 M	1.5 M

   Click Here to download Table 1

   We want to make sure that the reactions are all taking place at room temperature. You'll perform two benchmark trials, three trials with different sodium thiosulfate concentrations, and three trials with different HCl concentrations. You will vary the concentrations by diluting stock solutions of sodium thiosulfate and HCl, as shown in the following tables.

   Table 2: Sodium thiosulfate dilutions

   Target Concentration	Volume of 0.2 M sodium thiosulfate	Volume of DI water
   0.05 M	5 mL	15 mL
   0.10 M	10 mL	10 mL
   0.15 M	15 mL	5 mL
   0.20 M	20 mL	0 mL

   Click Here to download Table 2

   Table 3: HCl Dilutions

   Target Concentration	Volume of 6.0 M HCl	Volume of DI water
   1.5 M	2.5 mL	7.5 mL
   3.0 M	5.0 mL	5.0 mL
   4.5 M	7.5 mL	2.5 mL
   6.0 M	10 mL	0 mL

   Click Here to download Table 3

   Remember to use caution when handling HCl, which is toxic and highly corrosive. Sulfur dioxide, which is a gaseous product of this reaction, is also toxic. You will leave the reaction waste in the fume hood overnight to let the sulfur dioxide escape harmlessly.

   • First, put on a lab coat, splash-proof safety glasses, and nitrile gloves.
   • Fill a plastic wash bottle with deionized water and get a test tube marked with an X.
   • Clamp the test tube over the center of a small stir plate. The label must be on the opposite side from you. Make sure that you can clearly see the X. Place a small magnetic stir bar in the test tube.
   • Use a thermometer clamp to secure a glass thermometer in the lower third of the tube, keeping it clear of the X and the stir bar.
   • Cut a sheet of about 20 squares of plastic paraffin film from a roll and bring the sheet back to your hood.
   • Lay paper towels in the fume hood as a clean surface for labware to be reused.
   • Label a 600-mL beaker as ‘waste’ for the reaction, a 100-mL beaker as ‘0.2 M sodium thiosulfate’, and a 50-mL beaker as ‘6 M HCl’.
   • Fill the 100-mL beaker with 0.2 M sodium thiosulfate from the stock bottle in the dispensing hood.
   • After that, bring the 50-mL beaker and a watch glass to the acid-dispensing hood to obtain 50 mL of 6 M HCl. Cover the filled beaker with the watch glass and bring your portion of HCl back to your hood.
   • Prepare 20 mL of 0.1 M sodium thiosulfate for the first benchmark trial. Use a 10-mL volumetric pipette to measure 10 mL of 0.2 M sodium thiosulfate and dispense it into a 20-mL volumetric flask. Fill the 20-mL volumetric flask with deionized water to the mark.
   • Then, cut a square of plastic paraffin film, seal the flask, and invert it several times to thoroughly mix the solution.
   • Remove the film and pour the solution into the test tube.
   • Prepare 10 mL of 3 M HCl. Pipette 5 mL of 6 M HCl into a 10-mL graduated cylinder. Add deionized water to fill the graduated cylinder to the 10-mL line.
   • Seal the top of the graduated cylinder with plastic paraffin film and thoroughly mix the HCl solution by inverting it several times.
   • Now, start the stir motor and get a reset stopwatch. Confirm that you can see the X through the solution in the test tube.
   • Simultaneously start the stopwatch and pour the 3 M HCl into the test tube.
   • Watch the X through the solution, which will start becoming cloudy when enough sulfur has been produced to saturate it. Stop the stopwatch exactly when the solution becomes so opaque that the X disappears.
   • Record the time in your lab notebook for benchmark trial 1 and then reset the stopwatch. Also, record the temperature of the solution.
   • Now, turn off the stir motor, remove the thermometer, and rinse it with deionized water into the waste beaker.
   • Then, empty the test tube into the waste beaker, retrieve the stir bar with long forceps, and rinse the test tube and stir bar with deionized water.
   • Thoroughly clean the test tube with a test tube brush. Briefly rinse the volumetric flask and graduated cylinder with deionized water as well. Dry everything with paper towels.
   • Repeat the trial in the exact same way for the second benchmark time. If the time is not within 3 - 5 s of the first trial, repeat the benchmark trial until you have two consistent trials.
   • Follow the same overall procedure for each subsequent trial, changing the dilutions as indicated in your table and cleaning your glassware between each trial. Use a 5-mL volumetric pipette as needed when measuring the sodium thiosulfate stock solution in trials four and five.
   • Once you have performed all eight trials, rinse your glassware and tools one last time and neutralize the reaction waste with baking soda. Use caution because sulfur dioxide is one of the reaction products.
   • Transfer the neutralized reaction waste to the designated waste container. Then, pour any remaining sodium thiosulfate solution into its labeled waste container and rinse the beaker with deionized water.
   • Neutralize any remaining HCl with baking soda before flushing it down the drain with tap water.
   • Finally, clean and put away your labware according to your standard practices and throw out any trash remaining in your hood. Your instructor will dispose of the reaction waste later.
2. ResultsExpand
   • Now, estimate the reaction order of sodium thiosulfate and HCl. Calculate the starting concentration of sodium thiosulfate in the reaction mixture for trials 1 through 5. Looking at the concentration of sodium thiosulfate and the reaction times, we see that as the concentration doubles, the time to opacity is halved.

     Table 4: Estimating reaction order sodium thiosulfate

     Trial	[Na2S2O3] added	Na2S2O3 volume (mL)	Total volume (mL)	[Na2S2O3] in mixture	Time (s)	Inverse time (s-1)
     1, 2	0.1
     3	0.2
     4	0.15
     5	0.05

     Click Here to download Table 4
   • Remember that each trial used the same volume, so it took the same amount of sulfur to make each solution opaque. Thus, the reaction is twice as fast when the sodium thiosulfate concentration is doubled.
   • When the concentration increases by a factor of 4, the reaction rate also increases by a factor of 4. This one-to-one relationship suggests that the reaction was first order for sodium thiosulfate. If so, we would expect a plot of rate versus sodium thiosulfate concentration to be linear.
   • We didn't measure how much sulfur it took to turn the solution opaque, but we assume that it was the same for each trial. This lets us use the inverse of time to opacity as a stand-in for the rate. The slope won't be equal to k, but that's OK for this lab.
   • Plot inverse reaction time as a function of sodium thiosulfate concentration. This confirms the linear relationship between sulfur production and thiosulfate concentration. Thus, the reaction seems to be first order with respect to thiosulfate.
   • Use the same method to estimate the reaction order of HCl. First, calculate the starting concentration of HCl in the benchmark trials and in trials 6 through 8.

     Table 5: Estimating reaction order HCl

     Trial	[HCl] added	HCl volume (mL)	Total volume (mL)	[HCl] in mixture	Time (s)
     1, 2	0.1
     3	0.2
     4	0.15
     5	0.05

     Click Here to download Table 5
   • Then, look at the concentration of HCl and the reaction times for these trials. The reaction times are nearly identical, so the HCl concentration has no effect on the reaction rate. This means that HCl seems to be zeroth order for this reaction.
   • Lastly, add up the reaction orders of the reactants to get the overall reaction order.