CHEM 251 Laboratory

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CHEM 251 Laboratory

• Chemical Kinetics Lab

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Agenda

• For the week of November 29th
• Experiment Iodine Clock, handout
• Prelab
• Quiz Material in the handout

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Overview

• When it comes to experiments involving kinetics,
  the first thing one must do is to determine the
  order of a reaction.
• Order of reaction the sum of the exponents on
  the concentration terms for a rate law.
• Rate kAxBy, where reaction order xy
• What the reaction order tells us
• It illustrates the functional relationship
  between concentration and rate.
• Ex. a reaction order of three means that the rate
  of reaction increases as the cube of the
  concentration while a reaction order of -1 means
  the compound actually slows down the rate of
  reaction.

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Overview, cont

• The reaction order becomes a part of the rate law
  for the reaction
• Rate -DA/Dt kAx
• The rate law tells of the change in the quantity
  of a reactant or product that takes place in a
  period of time.
• In this experiment, you will monitor by
  spectroscopy the disappearance of color of one of
  the reactants and use that information to
  determine the reactant behavior.

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Overview, cont

• You can assume that the disappearance of color is
  due to the consumption of the reactant and the
  appearance of the product.
• The intense color of crystal violet is due to the
  system of alternating single and double bonds of
  the 3 benzene rings and the central carbon atom
  in the dye structure.
• This is called conjugation.
• When the dye is added to NaOH, the OH group is
  added to the central C-C double bond, breaking it
  and disrupting the conjugation between the rings.
• This is why the product is colorless.
• You will determine the order of reaction with
  respect to this consumed reactant, and from
  there, determine the rate law for the reaction.
• You will first have to find the order of the
  reaction with respect to dye by graphical
  determination, then use the method of integrated
  rate laws to find the order of the reaction with
  respect to NaOH.

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Methodology

• You will first have to make three graphs of your
  data
• Absorbance vs. time (zero order)
• Ln(absorbance) vs. time (first order)
• 1/absorbance vs. time (second order)
• Only one of these will be linear, and the one
  that is linear determines the order of your
  reaction with respect to dye.

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Methodology, cont

• Using the integrated rate laws, you can find the
  reaction with respect to NaOH.
• Because the NaOH is in such great access, its
  concentration will not change much during the
  reaction. Therefore, the rate law can be
  rewritten in the following manner
• Rate k1dyem where k1 kOH-n
• In order to establish the reaction order with
  respect to the NaOH concentration, we have to
  take into account the effect of different NaOH.

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Methodology, cont

• Therefore, we get the two equations below
• k1A kOH1-An
• k1B kOH1-Bn
• where k1A and k1B are the slopes of the lines
• Therefore, in order to determine the value of n,
  you can use the two above equations to obtain the
  ratio between k1A and k1B.
• Hint solve for n using logarithms!

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Methodology, cont

• Using logarithms, your two equations become
• lnkA lnk nlnOH-A
• lnkB lnk nlnOH-B
• Using basic algebra and subtracting B from A, you
  get the simplified version of this equation
• ln(kA/kB) mln(OH-A/OH-B)
• By solving for m, you can determine the reaction
  order with respect to NaOH.
• Afterwards, you can plug in this value into any
  of the two above equations and solve for k.

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Procedure

• Turn on the Spectronic 20 and set the wavelength
  to 540 nm. Take a clean cuvette and fill it 2/3
  full with one of the two NaOH solutions to use as
  a blank to calibrate the machine.
• When you are ready to start the reaction, add one
  single drop of crystal violet dye solution,
  invert to mix the solution, and place inside the
  spectrometer (this is time zero). One minute
  after you placed it in the spec, you will begin
  taking the absorbance values every minute for 15
  minutes.
• Once you have taken the 15th absorbance value,
  remove the sample tube from the instrument and
  re-zero the spec with the second NaOH solution.
  Repeat steps 1-2 with the second NaOH solution.
• When you finish collecting the data, turn off the
  instrument, discard samples down the drain, and
  clean the test tubes you used.

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Outside of class

• Include the following in your lab report
• Sample calculations on how you found the reaction
  order and the k value.
• Make three graphs
• Absorbance vs. time
• ln (absorbance) vs. time
• 1/absorbance vs. time
• For your discussion conclusion, answer the
  following questions along with the usual error
  and data analysis
• Prior to placing the sample inside the
  spectrometer, it takes a bit of time to
  completely mix the reactants. Would a delay of
  2-3 minutes cause an error in the data? If so,
  how would it influence the graph of absorbance
  vs. time?
• If the temperature was increased while the
  reaction was occurring, how would this effect the
  graph of absorbance vs. time?
• Why do you think initial rates are used to study
  a chemical reaction?
• Sometimes linearity could not be seen in any of
  the three graphs. Why do you think this happens?
  What do you think are some possible sources of
  error that could have occurred in this experiment?

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Before you leave

• Clean up your workstations before you leave
• Dont forget to turn in your carbon copies to me
  before you go
• There will be no class data this week
• You are responsible for your data only