Reaction kinetics 

background [prelab assignment (15 points) due at the beginning of the lab]

how could you use a colorimeter to measure the [reactant] as a function of time ?

how could you use graphical analysis of the preceding data (i.e. not using the rate of a reaction) and the below experimental methods to determine the parameters (i.e. α & β) in equation 1 (shown below) ? 

purpose / background

• determine the rate law (i.e. value of α & β, & k, in equation 1) for the following reaction (expressed as its net ionic equation):

C₂₅H₃₀N₃⁺  +  OH^-  -->  C₂₅H₃₀N₃OH

The rate law of the above reaction is  [units of rate  =  concentration /  time] 

rate = k [Cv⁺] ^α [OH^-] ^β                                                                    [equation 1]

When [OH^-] >> [Cv⁺], the psudeo-rate law is

rate = k* [Cv⁺] ^α, where k* = k [OH^-] ^β.

The value of β and k may be obtained by varying the [OH^-].

• determine the rate (k & α) of the burning of a candle

  rate = k (mass of candle) ^α ;  units of rate = mass / time                                 [equation 2]

• graphical analysis:  use of linearized data (for nerds:  potential issue) & R²

materials

colorimeter, cuvette, data collection device, watch (provide your own)

crystal violet (Cv⁺; 25 µM);  NaOH (100 mM);  water

pipet;  candle, electronic balance

methods [delegate work]

crystal violet reaction

part 1:  gather data to generate calibration curve for crystal violet

• review Beer's law lab
  • use of colorimeter & its calibration
  • determine best λ in the colorimeter to use in this experiment

part 2:  gather data to determine the reaction's rate law

quickly mix solutions of Cv⁺, NaOH, and water into the cuvette, then put it into the colorimeter

trail 1:  4 mL Cv⁺, 2 mL NaOH, 0 mL water

trail 2:  4 mL Cv⁺, 1 mL NaOH, 1 mL water  [ignore effect of solution's ionic strength]

start timer upon mixing the chemicals.   record absorbance at 30 seconds intervals for 30 minutes (or absorbance < 20% initial absorbance; whichever is shorter).  

burning a candle

 student design

 

Content of lab report  [35 points]

graphical analysis  [20 points]

• fading of cyrstal violet  [6 graphs & calibration curve; 2 points / graph = 14 points]
• burning a candle  [3 graphs; 2 points / graph = 6 points]

calculation of rate law, i.e. parameters in equations 1 & 2; refer to preceding graphs    [15 points]

• burning a candle  [6 points]
• fading of crystal violet  [9 points]

 

resources:

lab activity source (examples): site 1  & site 2 [access 4 / 2009]

video:  data analysis hint [access 1 / 2010]