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

)