1) Leucomethylene blue is known to be re-oxidized by dissolved O 2 (from the air

Question

1)    Leucomethylene blue is known to be re-oxidized by dissolved O2 (from the air) back to methylene blue. Under acidic conditions the oxidation of leucomethylene blues is slow while the reduction of methylene blue is fast. This ensures that the complication of leucomethylene blue oxidation is largely avoided in our experiments (which were performed at low pH). Nevertheless, briefly discuss how the observed kinetics of methylene blue reduction would be affected if the reverse reaction (HMB oxidation) was significant (e.g., at higher pH). Suggest how the rate law (equation 3 which is -d[MB+]/dt=kobs [MB+ ]a would need to be modified to take this into account, assuming that the oxidation is first order in HMB ad that dissolved O2 is in large excess over HMB.

2) Draw an energy level diagram that illustrates Rayleigh, Stokes, and anti-Stokes light scattering processes. Use arrows between energy levels to depict photon absorption and emission.

3) What is the Gross selection rule for Raman scattering?

4) What is a rate law? Define it in general mathematical terms.

5) At what approximate wavenumber values are the imine C=N stretch of benzylideneaniline (benzan) and the C=O stretch of benzaldehyde expected to appear?

Explanation / Answer

Ans 3: In moleules the polarizability many be anisotropic (direction dependent) and depends on the rotational and vibrational coordinates. this give rise to Raman Scattering. Gross Selection rule: to be a Raman active a molecule must have anisotropic polarizability/. Less restrictive than the need for a dipole moment symmetric molecules can be Raman active.

Ans 4: For all forward, irresversible reaction the rate is proportional to the product of the concentrations of the reactant each raised to an exponent, i,e

rate = k [A]m [B]n

this expression is the rate law for the general reaction where k is rate constant.

Ans5: Benzylidene aniline display their C=N stretching vibration at about 1600 cm-1 and Benzaldehyde C=O shows to band at 1730 and 1709 cm-1

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