Chapter 9, Problem 1DE

(a)

Interpretation Introduction

To determine: The experiment that determines the energy absorbed by the given compounds.

Answer to Problem 1DE

Solution: The experiment that determines the energy absorbed by the given compounds is UV-visible spectrometry.

Explanation of Solution

The energy absorbed by the given compound is calculated by the formula,

$\text{Energy}=\frac{\text{Plank's}\text{constant}\times \text{speed}\text{of}\text{the}\text{light}}{\text{wavelength}\text{of}\text{light}}$

Plank’s constant and speed of the light are constant values and therefore, the wavelength of the light is to be experimentally determined.

The experiment that makes possible to measure the wavelength of the light absorbed by the compound is UV-visible spectrometry.

The maximum wavelength absorbed by the solution of the compound is determined and substituted in the above formula.

Thus, the energy absorbed by the compound is determined.

Conclusion

The experiment that determines the energy absorbed by the given compounds is UV-visible spectrometry.

(b)

Interpretation Introduction

To determine: The dependency of the excitation energy on the length of the conjugated system.

Answer to Problem 1DE

Solution: The increase in the length of the conjugated system causes decreases in the excitation energy.

Explanation of Solution

The $\text{π}$ electrons of the conjugated system absorb energy and are excited to the high energy level. The $\text{π}$ electrons jumps back to the ground state by the loss of energy. If the energy emitted by the electron falls within the visible spectrum, it gives color to the compound.

The, distance between the ground state and the excited state determines the energy needed for the excitation.

As the length of the conjugated system increases, the distance between the atom in the chain decreases and hence the distance between the ground state and the excited state decreases. Therefore, the excitation energy decreases.

Thus, the increase in the length of the conjugated system causes decreases in the excitation energy.

Conclusion

The increase in the length of the conjugated system causes decreases in the excitation energy.

(c)

Interpretation Introduction

To determine: The additional molecule required to test the energy dependency on the length of the conjugated system.

Answer to Problem 1DE

Solution: The additional molecule required to test the energy dependency on the length of the conjugated system are $1,3,5-\text{heptatriene}$ , phenolphthalein, lycopene.

Explanation of Solution

The dependency of the excitation energy on the length of the conjugated system is tested by measuring the wavelength of light absorbed by the conjugate compounds with different lengths. Therefore, the examples are chosen with different length of the conjugated system.

The examples are,

$1,3,5-\text{heptatriene}$

Figure 1

Phenolphthalein

Figure 2

Lycopene

Figure 3

The above examples absorb light of different wavelength, depending upon their conjugation length.

Conclusion

Phenolphthalein, $1,3,5-\text{heptatriene}$ and lycopene absorb light of different wavelength, depending upon their conjugation length.

(d)

Interpretation Introduction

To determine: The dependency of the absorption of energy on the delocalization of the electron.

Answer to Problem 1DE

Solution: The dependency of the absorption of energy on the delocalization of the electron is explained using phenolphthalein.

Explanation of Solution

The excitation of the $\text{π}$ electrons takes place only when the electrons are delocalized.

The structure of the phenolphthalein is,

Figure 2

Phenolphthalein is acidic in nature due to the presence of the phenolic hydroxide group. Thus, in the acidic solution, the hydroxide group is not ionised. In the basic solution, the hydoxide group ionizes to form oxide ion that takes part in the resonance and stabilizes the aromatic ring.

Phenolphthalein in the acidic solution is colorless, while that in the basic solution is colored. The reason for the same is that, the delocalisation of the electrom takes place in the basic solution and is absent in the acidic solution. Hence, unless the electrons are not delocalised, excitation of the electrons does not take place.

Conclusion

The dependency of the absorption of energy on the delocalization of the electron is explained using phenolphthalein.