169 Determining the Equilibriwm Constant for Bromodhymol Blue and Bromophenol Bl

Question

169 Determining the Equilibriwm Constant for Bromodhymol Blue and Bromophenol Blue 9. When molecules with r bonds (e.g. bromothymol blue) absorb light, r electrons undergo a transition from a t to a a molecular orbital. The wavelength of the absorbance depends on the energy gap between these MOs. a) For the molecules in the table, calculate the absorbance wavelength. 1 ev 1.602x10-19J Show one sample calculation. AE (ev) 2 (nm) Molecule Ethylene CH2-CH2 1,3-Butadiene CH2 CH-CH-CH2 1.3,5-Hexatriene CH,-CH-CH-CH-CH-CH 7.72 5.70 4.79

Explanation / Answer

Hi we can solve these problems by using equation E = hv,

where h is planck's constant = 6.626 ×10–34 J·s

v = frequency also equals to v = c / where c = speed of light = 2.99 ×108 m/s; = wavelength

1). Ethylene E = 7.72 eV = 7.72 x 1.602 x 10-19 J = 1.24 x 10-18 J

E = hv = hc / ==> = hc / E

= [(6.626 ×10–34 J·s) x (2.99 ×108 m/s)] / (1.24 x 10-18 J)

= (1.98 x 10-25 J.m) / (1.24 x 10-18 J)

= 1.6 x 10-7 m , we know that 1 meter = 109 nm

= 1.6 x 10-7 x 109 nm

= 160 nm

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2). 1,3-butadiene E = 5.70 eV = 5.70 x 1.602 x 10-19 J = 9.13 x 10-19 J

E = hv = hc / ==> = hc / E

= [(6.626 ×10–34 J·s) x (2.99 ×108 m/s)] / (9.13 x 10-19 J

= (1.98 x 10-25 J.m) / (9.13 x 10-19 J

= 2.2 x 10-7 m , we know that 1 meter = 109 nm

= 2.2 x 10-7 x 109 nm

= 220 nm

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3). 1,3,5-hexatriene E = 4.79 eV = 4.79 x 1.602 x 10-19 J = 7.67 x 10-19 J

E = hv = hc / ==> = hc / E

= [(6.626 ×10–34 J·s) x (2.99 ×108 m/s)] / (7.67 x 10-19 J)

= (1.98 x 10-25 J.m) / (7.67 x 10-19 J)

= 2.6 x 10-7 m , we know that 1 meter = 109 nm

= 2.6 x 10-7 x 109 nm

= 260 nm

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Hope this helped you!

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