Lab 11 - Chemical Kinetics
PurposeTo determine the rate law for the reaction
2 I− + S2O82− → I2 + 2 SO42−.
1To relate changes in reactant concentrations to changes in reaction rate.
2To calculate a rate constant from experimental data.
IntroductionChemical kinetics deals with the speed, or rate, of a reaction and the mechanism by which the reaction occurs. We can think of the rate as the number of events per unit time. The rate at which you drive (your speed) is the number of miles you drive in an hour (mi/hr). For a chemical reaction the rate is the number of moles that react in a second. In practice, we usually monitor how much the concentration (the number of moles in a liter) changes in a second. Reaction rates are usually expressed in units of moles per liter per second, or molarity per second (M/s).
Reaction RatesThe reaction to be studied in this experiment is a redox reaction between iodide ions and persulfate anions:
( 1 )
2 I− + S2O82− → I2 + 2 SO42−
( 2 )
( 3 )
Rate = −
( 4 )
Δ[S2O82− ] = [S2O82− ]final − [S2O82− ]initial < 0
( 5 )
Rate = −
The Chemical System Being StudiedThe three reactions of importance in this experiment are:
( 6 )
2 I− + S2O82− → I2 + SO42−
( 7 )
I2 + 2 S2O32− → 2 I− + S4O62−
( 8 )
I2 + starch indicator → blue-black product
Method of Initial RatesThe change in concentration of a chemical species usually depends on the concentration of that species when the reaction begins. At the beginning of the reaction, its concentration is high, and a substantial amount of reactant will disappear in a second. Near the end of the reaction, its concentration is low, and a much smaller amount of reactant will disappear in a second. A standard way to compare reaction rates under different conditions is to measure the time it takes for the first small percentage of reaction to occur. This gives the initial rate. In this experiment, you will find the time it takes for the first small percentage of Reaction 6 to occur. But we do it in an indirect way. The time it takes for Reaction 7 to occur will actually be measured. The experiment will be set up such that the amount of thiosulfate (S2O32-) is only about one percent of the amounts of iodide and persulfate (S2O82-) ions. During the first percent or so of Reaction 6, the elemental iodine produced is consumed by thiosulfate in Reaction 7. No iodine is left to react with the starch indicator. When the S2O32- is used up, Reaction 6 is still going (it still has about 99% to go!), but the iodine produced then begins to react with the starch, giving a color change. Thus, the color change indicates the total consumption of thiosulfate, S2O32-. Knowing the amount of thiosulfate that reacted and measuring the time that it takes to react gives Δ[S2O32- ]/Δt. This is related to the rate of Reaction 6 (what we want!) by the stoichiometry of the reactions,
( 9 )
Rate = −
Rate Constants and Reaction OrdersA simple question about reaction rates is this: does the rate you measure depend on the amount of reactants you start with? It seems as if it should, but this is not always the case. The answer to the question depends on the mechanism of the reaction. A reaction mechanism is the series of steps that the reaction follows. The slowest step determines the measured reaction rate, and is called the rate-determining step. Look again at the reaction whose rate is to be measured:
( 10 )
2 I− + S2O82− → I2 + 2 SO42−
1The rate-determining step has two iodide ions coming together. In this case, the rate depends only on the initial concentration of iodide.
2The rate-determining step involves a persulfate ion decomposing. In this case, the rate depends only on the initial concentration of persulfate.
3The rate-determining step has an iodide ion and a persulfate ion coming together. In this case, the rate depends on the concentrations of both ions.
( 11 )
Rate = k[I− ]x[S2O2− ]y
250 mL burets
250 mL graduated cylinders
4100 mL beakers
450 mL beakers
1magnetic stir plate
1magnetic stir bar
110.00 mL volumetric pipet
1ring stand with buret clamp
1deionized water squirt bottle
~90 mL0.200 M KI
~90 mL0.100 M (NH4)2S2O8
~60 mL0.0050 M Na2S2O3
~30 mL0.200 M KCl
~30 mL0.100 M (NH4)2SO4
~30 mL3% starch solution
SafetySodium persulfate is an oxidizing agent. Oxidizing agents can supply their own source of oxygen and may react violently with organic liquids or other reducing agents. The dilute persulfate solutions used in this experiment may be handled safely with other aqueous solutions.
Waste DisposalAll of the solutions can be flushed down the sink with plenty of water.
Prior to ClassPlease read the following sections of Lab Safety and Practices: Please review the following videos under Instructional Videos: Please complete WebAssign prelab assignment. Check your WebAssign Account for due dates. Students who do not complete the WebAssign prelab are required to bring and hand in the prelab worksheet.
Lab ProcedurePlease print the worksheet for this lab. You will need this sheet to record your data.
1Five different solutions with similar names are used in this experiment. Label the glassware so you will not mix things up. For example,
- 50 mL buret for KI
- 100 mL beaker for obtaining KI
- 50 mL beaker for KI dispensed from buret
- 50 mL buret for (NH4)2S2O8
- 100 mL beaker for obtaining (NH4)2S2O8
- 50 mL beaker for (NH4)2S2O8 dispensed from buret
- 10.00 mL pipet for Na2S2O3
- graduated cylinder for KCl
- graduated cylinder for (NH4)2SO4
- 100 mL beaker in which to run the reaction
2The following procedure applies for each run. Make sure the reaction beaker is rinsed and dried between each run.
aUse your KI buret to measure the appropriate amount of 0.200 M KI into the labeled beaker.
bUse your (NH4)2S2O8 buret to measure the appropriate amount of 0.100 M (NH4)2S2O8 into the labeled beaker.
cUse a pipet to measure 10.00 mL of Na2S2O3 into the reaction beaker. Add 3 or 4 drops of starch solution.
dUse a graduated cylinder to add KCl or (NH4)2SO4 solution to the reaction beaker if it is required for the run. These solutions are added to maintain a constant concentration of ions in all the runs.
eAdd the magnetic stir bar to the reaction beaker and mix the contents of the reaction beaker.
fThis step needs to be carefully coordinated. While the contents of the reaction beaker are being stirred, add the KI and (NH4)2S2O8 solutions simultaneously to the reaction beaker. This is the initial time for the reaction.
gWatch the reaction and note the time at which the blue-black color of the starch-iodine complex appears. Record the elapsed time in Data Table A.
Table A: Determination of Rate Law:
2 I− + S2O82− → I2 + 2SO42−
Question 1: What are the initial concentrations of iodide, persulfate, and thiosulfate in the first run? Account for dilution and show your work.
Question 2: What is Δ[S2O32- ] at the time of the color change in the first run?
Question 3: What is the rate of the reaction in the first run? Show your work.
3Record the values from these calculations in Data Table B. Repeat the calculations for the other four runs.
Table B: Calculations for Determination of Rate Law
Question 4: Inspect the data in Data Table B. What is the order of the reaction with respect to iodide ions? Explain how you arrived at your answer.
Question 5: Inspect the data in Data Table B. What is the order of the reaction with respect to persulfate ions? Explain how you arrived at your answer.
Question 6: Write the rate law for this reaction, showing the proper exponents.
Question 7: Calculate the rate constant for the first run. Include units. Show your work, and record the result in Data Table B.
4Calculate the rate constants for the other four runs and record the rate constants in Data Table B.
5When you are finished, rinse all of your glassware with water, dry it and return it to the set-up area where you found it.
6Before leaving, enter your results in the in-lab assignment. If all results are scored as correct, log out. If not all results are correct, try to find the error or consult with your lab instructor. When all results are correct, note them and log out of WebAssign. The in-lab assignment must be completed by the end of the lab period. If additional time is required, please consult with your lab instructor.