Rate Law and Activation Energy Methyl Blue Determining the Rate Law using the Time Dependent Rate.

Rate Law and Activation Energy Methyl Blue

Determining the Rate Law using the Time Dependent Rate

The Reaction

The Reaction

• Reaction takes about 30 s to a few minutes depending on the initial concentrations

• Can use a continuous monitoring method

• Because there is a color change we can use spectroscopy

The Reaction

• In this experiment, the initial concentration of the hydroxide is at least 1000 times larger than the concentration of the MB

• This means that

• And

Figuring out x

• To figure out the order with respect to the MB+ we will observe [MB+] vs t

• To do this we use Beer’s Law

• Plot At vs t, LN(At) vs t and 1/At vs t to see if the reaction is zeroth order, first order or second order wrt MB+

Absorbance

Constants

The MathZeroth Order First Order

Second Order• The slope of the linear curve will get you k’=k[OH-]o

y to within a constant

• Repeating the experiment at a second [OH-] will get you access to how slope depends on [OH-] and get y

Figuring out x

At LN(At) 1/At

Which one gives a straight line?

If straight x = 0 If straight x = 1 If straight x = 2

Part A: Set Up Spectrometer*• First we need to Calibrate the Spectrometer by placing an empty cuvette in the

cuvette holder (follow instructions on the first page of the spectroVis handout sheet)

• Once Calibrated we need to record the absorption spectrum of Methyl Blue, by placing a cuvette with MB in it in the spectrometer and collecting absorbance vs wavelength

• Find the wavelength λmax of maximal absorption make a note in you notebook and print the graph for each group member

• Set up the spectrometer to record absorbance vs time at λmax (see page 3 on the handout)

* Follow procedures set out in the Vernier

handout

Part B: Determining x the order of reaction with respect to the Methyl Blue

Methyl Blue 0.1 M NaOH

Beaker 1Measure 10 mL

Methyl Blue

Beaker 2Measure 10 mL

NaOH

Transfer contents of beaker 2 into beaker 1

Fill Cuvette and transfer to spectrometerrecord absorbance 3 mins

• Follow instructions in the lab manual to plot A vs t, LN(A) vs t and 1/A vs t

• Fit each graph to a linear fit and determine which gives the best straight line this will tell you if x = 0, x = 1 or x = 2

• Record data in your notebook• Do 5 trials and use the average to determine k’ = k[OH]y

Part B: Determining x the order of reaction with respect to the Methyl Blue

• Repeat part B with 0.5M NaOH• Measure the absorbance vs time and determine k’= k[OH]y

• Repeat 5 times to improve your statistics• Following the method shown in the sample report by taking

the ratio of k[OH]y for the 2 concentrations y can be determined

Part C: Determining y the order of reaction with respect to the hydroxide

We need to determine the rate constant at different temperatures

Each group needs to perform – 2 runs at a temperatures below room temperature– 2 runs at a temperatures above room temperature– 2 runs at room temperature– Post of the board the temperature (oC) and the slope of the LN(A) vs t

graph

Part D: Determining Ea and A

Part D: Determining Ea and A

ignition tubes

hotplate

methyl blue0.1 M NaOH

thermometer

• place NaOH in one ignition tube and the MB in the other sit them in a beaker of warm water sitting on the hot plate allow them time to reach thermal equilibrium record Tbefore

• Then pour the contents from one tube into the other, leaving it in the water bath, stir with the thermometer and transfer some of the mixture into the cuvette

• Record A vs t for 20 seconds• Measure the temperature of the solution in the cuvette after Tafter

Calculating Ea and A• Using your 6 trials, calculate k, ln(k), T(K) and 1/T• Make an Arrhenius plot ln(k) vs. 1/T

0.00335 0.0034 0.00345 0.0035 0.00355 0.0036

-4.5

-4

-3.5

-3

-2.5

-2

-1.5

-1

-0.5

0

f(x) = − 4707.99519198426 x + 12.8915959476479R² = 0.998760932500059

ln(k) vs 1/T

1/T (1/K)

ln(k

)