Chemical Kinetics Lab: The formaldehyde clock reaction

1. Chemical Kinetics Lab: The formaldehyde clock reaction Summer Math Institute 2012 Amber M. Hupp Assistant Professor, Chemistry College of the Holy Cross Modified from P.W.W. Hunter, “Chemistry Laboratory Manual (CHEM 162)”, Michigan State University

2. Thermodynamics or Kinetics? • Thermodynamics – if a reaction occurs • Kinetics – how fast the reaction occurs and route the reaction takes • Examples : • how fast unwanted chemical substances break down (environmental) • how long before chemicals like drugs metabolize in the body(biological) • how fast will this fuel ignite to run my car (energy!)

3. Reaction Mechanisms • Tells us how the reaction proceeds • Chemical reactions occur through a series of steps (intermediate reactions) • Some steps are fast, some slow • Slowest step in mechanism determines rate • Can’t go any faster than slowest step!

4. Clock Reactions • A visible change occurs when a certain point in the reaction is reached (typically the end point) • Time is inversely proportional to rate • The longer a reaction takes, the slower the rate must be • Something we can measure in the lab!

5. Clock Reaction Demonstration • This is a complicated clock reaction called the Briggs-Rauscher Reaction • Goes through 10 to 15 cycles • Chemical oscillator is due to changes in iodine and iodide ion concentrations in solution (several chemical equations) • B.Z. Shakhashiri, Chemical Demonstrations: A handbook for Teachers of Chemistry, V2, 1983, p248

6. Formaldehyde-Bisulfite Reaction • A simpler example of a clock reaction • Step 1: • HSO3- + H2O ↔ SO32- + H3O+ Fast • Step 2: • H2O + HCHO + SO32- → CH2(OH)SO3- + OH- Slow • Produce OH- (basic!) Can use pH indicator phenolphthalein • Colorless to pink at certain pH • Modified from P.W.W. Hunter, “Chemistry Laboratory Manual (CHEM 162)”, Michigan State University • B.Z. Shakhashiri, Chemical Demonstrations: A handbook for Teachers of Chemistry, V4, 1983, p70

7. Rate of F-BS reaction • Rates depend on initial concentrations of reactants • A change in either one will affect reaction rate • Rate = k [formaldehyde]a [bisulfate-sulfite]b • The order of each reactant (a & b) add to give the overall reaction order

8. Orders of Reactions – what do they mean? • For the general reaction X  Z • Say a reaction is first order in X • Then as the concentration of X doubles, the rate also doubles • Say a reaction is second order in X • Then as the concentration of X doubles, the rate quadruples • Shows concentration dependence on rate

9. How to calculate the order of the reaction? • Rate = k [formaldehyde]a [bisulfate-sulfite]b • Hard to understand what to change and what to hold constant… so let’s rearrange by taking log of both sides log(rate) = log(k) + a log[formaldehyde] + b log[bisulfate-sulfite] • If the concentration of F is held constant, then (a log[F] ) is constant and we can plot log(rate) vs (b log[BS]) to get order with respect to BS

10. Experimental Set-up The concentrations of each reactant will be varied, while holding the other reactant constant. The time for each reaction to go to completion (in seconds), via a color change of phenolphthalein, will be recorded in the table. • T. Cassen, J. Chem. Ed. 53(3), 1976, pg 197

11. Experimental Results

12. Graphs

13. Graphs

14. Conclusions • Order of overall reaction is one. • Reaction depends directly on concentration of formaldehyde, but does not depend on bisulfite B.Z. Shakhashiri, Chemical Demonstrations: A handbook for Teachers of Chemistry, V4, 1983, p70

15. Points of interest • Dependent vs independent variables • Calculation of slope • Using log function to “simplify” expression • Significant figures/digits • Fluctuation in measurement/results • Can perform experiment if desired and use discovery style