Exp 13: The Rate of an Iodine Clock Reaction Intro: Reaction Kinetics Some reactions proceed so rapid, that they appear to occur instantaneously Many reaction proceed much slower and take minutes to years to occur Can you think of examples? Differences in reaction rates are dependent on molecular characteristics of the reactants Factors that affect the reaction rate are Concentration of reactants Temperature Presence of a catalyst

Exp 13: The Rate of an Iodine Clock Reaction Purpose Determine the rate law for the iodine clock reaction Evaluate the effect of a catalyst on the rate law See: Chang, Chapter 13

Exp 13: The Rate of an Iodine Clock Reaction Series of redox reactions Oxidation of iodide to iodine by potassium persulfate Reaction of iodine with starch gives a dark-blue color See: Chang, Chapter 13 Rxn 1 S2O82- + 2I-  2SO42- + I2 Rxn 2 I2 + 2S2O32-  2I- + S4O62- Rxn 3 I2(aq) + starch (aq)  blue color Modified Landolt Reaction

Iodine Starch Clock Reaction S2O82-(aq) + 2 I-(aq)  I2(aq) + 2 SO42-(aq) I2(aq) + 2 S2O32-(aq)  2 I-(aq) + S4O62-(aq) (3) I2(aq) + starch (aq)  blue color The reaction can be described by the following mechanism Reaction step 1: Potassium persulfate reduces sodium iodide to form iodine and sulfate This reaction proceeds very slowly. It is the rate determining step. Reaction step 2: To study the rate of the reaction, we need to be able to determine how fast one of the products is formed or how fast one of the reactants is consumed. The timing method used in this experiment makes this reaction a "clock reaction". To time the rate of reaction (1), the reaction (2) will occur simultaneously in the same reaction tube as reaction (1) Reaction step 3: Iodine liberated is instantaneously retransformed by to iodide and tetrathionate. This is an "immeasurable fast" reaction. The blue starch iodine complex becomes visible after complete consumption of the thiosulfate ions, since then a reduction of the iodine formed cannot take place according to equation (3) any longer.

EXPERIMENTAL PROCEDURE S2O82- + 2I-  2SO42- + I2 (slow, rate-determining reaction) I2 + 2S2O32-  2I- + S4O62- (very fast reaction) excess I2 + starch  dark-blue complex (very fast) Test 3 different concentrations of reactants See reaction table (next slide) Do reaction at increased temperature (35-37oC) Do reaction in the presence of a catalyst (Cu2+ ions)

EXPERIMENTAL PROCEDURE Record temperature in the lab Mark 3 test tubes Rinse pipets twice with 1-mL of NaI solution Pipet required volume of NaI in each tube (see reaction table, next slide) Add NaCl to required tubes Add Na2S2O3 to required tubes Add starch to required tubes Add K2SO4 to required tubes DO NOT ADD K2S2O8 to the test tubes until you are ready to start the reaction as this will start the reaction!!

EXPERIMENTAL PROCEDURE Reaction Mixture Table Solution used 0.20 M NaI NaCl 0.010 M Na2S2O3 2% starch K2SO4 K2S2O8 Reacting ion: I- “ions”  S2O3-2 “indicator” SO4-2  S2O8-2 Tube # Volume 1 2.0 mL 1.0 mL 2.0 ml 2 0 mL 4.0 ml 3 4.0 mL 4 (at 35oC) 5 (with Cu2+) 5.0 mL Total volume per group 12 8 10 5 17 Repeat Do Once Total volume in each tube: 11.0 mL

EXPERIMENTAL PROCEDURE Note time or use stopwatch Add 2.0 mL K2S2O8 to tube #1 this will start the reaction!! Quickly cover tube with Parafilm and invert ~ 3-5 times to mix Record the time that the dark color appears The color appears suddenly!! Exp #1 should take about 4-8 minutes Experiment #2 Repeat the experiment with tube #2 Add 4.0 mL K2S2O8 to tube #2 Mix as before and record how long it takes for the solution to turn blue Experiment #3 Repeat the experiment with tube #3 Add 2.0 mL K2S2O8 to tube #3 Clean and dry the tubes and repeat the experiments Differences in time should be less than 10 s

EXPERIMENTAL PROCEDURE Experiment #4: temperature effect Put warm water from the faucet in a beaker Pipet ~ 5 mL of K2S2O8 in tube #0 Pipet the volumes in tube #4 as indicated in the reaction table, except K2S2O8 Put the tubes in the warm water bath Allow to equilibrate for ~ 5 min Start the reaction by adding 2 mL K2S2O8 from tube #0 into tube #4 Note time and mix as before Put tube back into water bath Record time to turn blue Experiment #5 Fill tube #5 with the same amounts as tube #1, except K2S2O8 (see reaction table) Add 1 drop of 0.2 M CuSO4 solution Shake gently to mix Initiate reaction by adding 2.0 mL K2S2O8 (room temp, not warm!!) Mix as before and record how long it takes for the solution to turn blue

EXPERIMENTAL PROCEDURE Reaction Mixture Table Solution used 0.20 M NaI NaCl 0.010 M Na2S2O3 2% starch K2SO4 K2S2O8 Reacting ion: I-  S2O32- SO42-  S2O82- Trial # Concentration  1 3.6 x 10-2 1.82 X 10-3 1.0 mL 2 0 mL 7.3 x 10-2 3 4 (at 35oC) 5 (with Cu2+)

Reaction Rate and Product Concentration In general, for the reaction aA + bB cC + dD rate = 1 a - = - [A] t b [B] c [C] = + d [D] The numerical value of the rate depends upon the substance that serves as the reference The rest is relative to the balanced chemical equation

Reaction Rate and Product Concentration S2O82- + 2I-  2SO42- + I2 I2 + 2S2O32-  2I- + S4O62- Net Reaction: S2O82- + 2S2O32-  2SO42- + S4O62- For every molecule of S2O82- that is consumed, 2 molecules of S2O32- are consumed or D [S2O82-] = ½ D[S2O32-]  D[S2O32-] = 2 D [S2O82-] Look at the reaction concentration table: [S2O32-] = 1.82 X 10-3 M: limiting reactant compared to [S2O82-] = 3.6 x 10-2 M

Next Week: Exp 14A – Le Chatelier’s Principle Lab Report for Exp 13” “The Rate of an Iodine Clock Reaction” Due Monday Oct 15: Data Sheets, calculations Post-lab question 1a-e, 3 Post-lab question 2 (5 bonus points) Prelab Preparations for Exp 14A: “Le Chatelier’s Principle” Due: Prelab questions: #1, 2, 3, 4, 5 For more info, see Chang, Chapter 14 Lab preparations as usual: Read and look up the following Introduction and purpose of the experiment Experimental procedure Properties of chemicals Ammonia, NH3 . Nickel nitrate, Ni(NO3)2 . Sodium hydroxide, NaOH . Cobalt nitrate, Co(NO3)2 Hydrochloric acid, HCl . Calcium nitrate,Ca(NO3)2 Next Week: Exp 14A – Le Chatelier’s Principle