We are being asked to prepare a hypothetical experiment - \"Prepare an experimen

Question

We are being asked to prepare a hypothetical experiment -

"Prepare an experimental plan that would determine the activation energy of the reaction between crystal violet and sodium hydroxide."

We have the materials of:

Crystal violet, CV+, 8.00 x 10^-5 M

Sodium hydroxide, NaOH, 0.1M

Graduated pipet, 10 mL (2)

Pipet bulb

Volumetric flask, 50 mL (2)

Vernier Ssystem, SpectroVis and Cuvettes Parafilm

Digital Thermometer

- The Vernier system is what we used in lab to measure concentration through the color of the solution. Can anyone help/explain the step I should take?

Explanation / Answer

The so called Vernier UV-VIS Spectrometer is used to measure absorbance of UV radiations by the coloured compounds.

When radiation is passed through a colored substance, a characteristic portion of the mixed wavelengths of colors is absorbed. Also according to beer Lambert’s law, the absorbance of light of a certain wavelength by any given sample of compound is proportional to the number of absorbing molecules or its concentration. So a sample with higher concentration would have a higher absorbance than its lower concentrations, for a given wavelength of light.

The Vernier Spectrometer allows you to instantly collect emissions/absorption spectra from light bulbs and capture data and analyze it on your computer.

Now moving to your experiment,

You need to determine the rate law and the activation energy for the reaction of sodium hydroxide with crystal violet.As crystal violet undergoes reaction with sodium hydroxide, the product formed is colorless.

Using the characteristic radiation of 565 nm(wavelength) light source of your spectrophotometer ,you need to monitor the absorbance of the crystal violet solution with time.You need to take absorbance reading for a given sample at different time intervals

Absorbance(A)      Time

1)

2)

3)

Assume that the absorbance is proportional to the concentration of crystal violet .By determining the molar absorptivity of crystal violet ,calculate the concentrations from the measured absorbance.

A=e CL(e= molar absorptivity of crystal violet, c= concentration,l=cuvette length)

The rate law for the reaction CV+ +OH-=CVOH(colourless product)

Rate=k[CV+]^n [ OH-]^m

Where, n= order of the reaction with respect to crystal violet,

m= order of the reaction with respect to sodium hydroxide

plot three graphs,1) between concentration of CV (C) vs time, linear plot indicates zero order with respect to CV

2)ln C vs time, linear plot indicates first order with respect to CV

3) 1/C vs time, linear plot indicates second order with respect to CV

Calculate the rate constant of the reaction.using the order and concentration of CV from the rate law,assuming NaOH to be constant.

Once you determine the order in CV ,you need to determine the order in hydroxide ion. It can be done by obtaining data in the same manner but at different concentrations of hydroxide ion.

Rate constant    initial [naoh]

Relative ratio method is used to determine the order of reaction with respect to NaOH

Now you have order of reaction with respect to both NaOH and CV.

To measure activation energy, Repeat the experiment with some constant concentration of sodium hydroxide and crystal violet for two other temperatures(heating the sample) and noting the temperature.

Calculate rate constant(k) for three different temperatutes.

Use Arrhenius equation to calculate the activation energy.

K=A exp(-E/RT).

lnK=lnA + E/RT

plot graph between lnK and 1/T

the slope of the graph=E/R,where R=universal gas constant and E is the activation energy.

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