Kinetics-- a Clock Reaction

Chem. 128

October 4, 2002

2 I- + S₂O₈2- ---> I₂ + 2 SO₄2-

The next goal is to determine the order of the reaction with respect to Iodide and with respect to peroxydisulfate. You do this by repeating the reaction with different starting concentrations of the two reactants. This allows you to determine how the rate changes with concentration.

The last goal is to see how the rate of the reaction changes with temperature. This is done by repeating an earlier run at several different temperatures. This allows you to compute the Energy of Activation for the reaction.

• I₂ + 2 S₂O₃2- -----> 2 I^- + S₄O₆2-

• You should also note that the amount of thiosulfate is quite small compared to the other reagents. For example one reaction mixture will be 0.0667 M KI, 0. 033 M peroxydisulfate and 0. 00167 M thiosulfate. When the thiosulfate is used up the peroxidisulfate concentration will still be 0.032M. This justifies saying that we observe the initial rate of this reaction.
• don't confuse the various sulfur compounds
  • S₂O₈2- peroxydisulfate ion, S₂O₃2- thiosulfate ion
• We will also be adding Potassium Chloride or Ammonium Sulfate to some of our reaction mixture. These materials are completely inert in this reaction. Their role is to keep the total concentration of ions the same from run to run. There is a small effect from ionic strength and we are trying to eliminate it as being responsible for any of our observations.

Experimental (Work in pairs on this experiment)

The following reagents are available. These will be provided in bottles with automatic dispensers that deliver a fixed volume of reagent.

• Potassium Iodide, 0.200 M (10 ml Repipet dispenser)
• Ammonium Peroxydisulfate, 0.100M (10 ml Repipet dispenser)
• Sodium Thiosulfate, 0.005 M (10 ml Repipet dispenser)
• Ammonium Sulfate 0.100M (10 ml Repipet dispenser)
• Potassium Chloride 0.100M (10 ml Repipet dispenser)
• Hydrogen Chloride 0.100 M (10 ml Repipet dispenser)
• Copper Sulfate ).1 M
• Starch, 1% in water

• These chemicals offer no significant hazards at the concentrations involved.
  • Of course, safety goggles are to be worn at all times.
• All materials and reaction products can be discarded by rinsing down the sink.

• Place 20 ml of Ammonium Peroxydisulfate into a clean, dry flask or beaker
  • (this is two Repipet portions)
  • measure and record the temperature of this solution; the others will be at the same temperature
• Place 20 ml of Potassium Iodide in another beaker
  • (this is also two Repipet portions)
• Place 10 ml of Sodium Thiosulfate and 3 drops of starch in a third beaker
• Quickly mix all three solutions together and note the exact time of mixing.
  • Wait until the solution turns blue and note the time that elapsed
• Repeat, varying the concentrations

  run #	beaker 1	beaker 2	beaker 3
  1 (as above)	20 ml Peroxydisulfate	20 ml Iodide (KI)	10 ml thiosulfate 3 drops starch
  2	10 ml Peroxydisulfate 10 ml (NH4)2SO4	20 ml Iodide (KI)	10 ml thiosulfate 3 drops starch
  3	20 ml peroxydisulfate	10 ml Iodide (KI) 10 ml KCl	10 ml thiosulfate 3 drops starch
  4	20 ml peroxydisulfate	10 ml Iodide (KI) 10 ml HCl	10 ml thiosulfate 3 drops starch
  5	20 ml peroxydisulfate	20 ml Iodide (KI) 1 drop 0.1M CuSO4	10 ml thiosulfate 3 drops starch

  • Notice that Runs 1 and 2 differ only in the concentration of Peroxydisulfate.
  • Runs 1 and 3 differ only in the concentration of Iodide Ion.
  • Runs 3 and 4 have identical concentrations of both reactants; the added HCl will let us see if H+ acts as a catalyst in this reaction. (H+ is a common catalyst.)
  • Runs 1 and 5 differ only in the addition of Cu²⁺, a known catalyst

In the first case, place all three beakers into a bath of water 10-15 degrees above room temperature. Wait about 5 minutes for the solutions to reach temperature. Measure and record the temperature in any of the solutions. Mix and time the reaction. (Keep the mixed solution in your water bath to keep the temperature constant.)

In the second case, place all three beakers into a bath cooled about 10 degrees below room temperature. Proceed as above. (It would be simpler to cool the solutions with ice, but at that temperature the reaction is too slow to be convenient.)

Calculations:

• Compute the starting concentrations of Iodide, Thiosulfate and Peroxydisulfate for each reaction mixture.
  • Remember the reaction volume is 60 ml, so the starting materials are diluted.
  • for example: we added 20 ml of 0.200M KI
    • after mixing, the concentration is 0.0167 M KI
• Compute the rate of reaction, based on the amount of Iodide Ion consumed
  • thiosulfate concentration: 0.00167 M
  • this reacted with 0.000833 M of I₂
  • that was produced by the reaction of 0.000833 M of Peroxydisulfate
• If the reaction occurred in 47.0 seconds,
  • the rate is 0.000833M / 47.0 (moles/liter/second)

• Algebraically, our formula is rate = k * (I-)^x (S₂O₈2-)^y
  • x and y are the order of the reaction with respect to the two reactants
  • our goal is to determine x and y, hopefully as a simple integer (0, 1, 2)
• This is easiest if you compute a ratio for two of the reactions
• (the next expression might get garbled as it moves to the Web site)
  • rate 1 ₌ k * (I, _run1)^x (S₂O₈2-, _run1 )^y ₌ (2/1)^x
  • rate 2 k * (I, _run2 )^x (S₂O₈2-, _run2 )^y
  • ln (rate1/rate2) / ln(2) = x

• Once you determine x and y, you can compute k for each run; it is the only unknown in the equation
  • rate = k * (I-)^x (S₂O₈2-)^y

• Prepare a graph
  • on Y axis plot ln(rate) or ln (rate constant) or ln (1/time)
  • on the X axis plot 1/ (Temp +273.2) that is, 1/T_Kelvin
• The Arrhenius equation predicts a straight line
  • the slope = - E_A/R
    • where R= 8.314 Joules/K mol
• Measure the slope and evaluate E_A

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• kinetics experiment, overview