Chemistry 104 Laboratory

 

 

This Week

Answer Keys

Gen Chem Lab

Periodic Table

 

 

 

á       Expt 3- Kinetics of the Fading of Phenolphthalein

o   Preliminary Assignment due

á       Quiz 1:  Expt 2 (Parts A,B); Expt 7

 

Next Week

á       Expt 1: Brita, Scum and Scale (Part A)

 

Notes for Today

The Phenolphthalein Fading Reaction:  

 

P2-

+

OH-

->

POH3-

red

 

colorless

 

colorless

 

rate = k[OH-]m[P2-]n

rate = k1[P2-]n     when      k1 = k[OH-]m

 

  • Overall Goals
    • Find the rate law (concentration dependence)
    • Understand the molecularity of each step (how rxn occurs)
    • Find the activation energy

 

  • Part A: Find n and k1

Find n

    • Monitor [P2-] vs. time (at constant [OH-], temperature)
    • Check fit of data to
      • Zero order plot: if [P2-] vs time is linear, then n = 0
      • First order plot: if ln[P2-] vs time is linear, then n = 1
      • Second order plot: if 1/[P2-] vs time is linear, then n = 2

 

    • Example:  Do the [P2-] vs. time data shown below obey zero order kinetics?

             

Find k1

    • Use linear plot to find k1
    • What information from the plot yields k1?

 

  • Part C: Find EA
    • Monitor k1 vs. temperature (at constant [OH-])
    • Use the Arrhenius relationship to find EA

 

     ln k1 = (-EA/R) (1/TK) + constant

 

          y   =       m        x      +       b

 

    • What information from the plot yields EA?

 

  • Safety 
    • Goggles required during solution preparation

 

More About Molecularity and Reaction Mechanism
  • Many rxns are multistep
    • Steps sum to give overall rxn
    • One step will be the rate-controlling step (slow step)
    • Other steps do not affect the overall rate

 

 

  • Rxn orders are (usually) the coefficients of the slow step (not those of the overall rxn)

Example:  If experiment shows that n=0, m=1

    • no P2- is involved in the slow step ([P2-]0 = 1
    • one OH- is probably involved in slow step
    • slow step is unimolecular  (molecularity = n + m = 1)

 

 

  • Molecularity gives clues about the reaction mechanism

Example:  If experiment shows that n=0, m=1

    • Slow step might be: 1OH- -> intermediate (slow step)
    • one OH- is probably involved in slow step
    • Overall reaction must involve additional steps
    • One possible two-step mechanism:
      • (slow) 1OH- -> intermediate
      • (fast) intermediate + P2- -> POH3-