Chemistry Important Questions

Q1. Which of the following molecules are IR active?

           H-Br, HCN, O₂, O₃, CO₂

Ans:   All molecules except homonuclear diatomic molecules is IR active

              H-Br, HCN, O₃, CO₂  are IR active.

 Q2. What is Beer-Lamberts law? Explain the terms involved. 

Ans. When a monochromatic light is passed through a dye solution, the absorbance is proportional to product of concentration and thickness of solution.   A= e c t   [ e is molar absorptivity, c is concentration and t is thickness of solution. 

Q3. What are the various types of energy transitions (levels) possible in molecule?

Electronic Energy levels.  The energy associated with distribution of electrons in a molecule. It is quantized. The energy gap between these levels falls in UV-visible region. Transitions between electronic energy levels is possible.

Vibrational Energy levels. The bonds in a molecule always vibrate. Here during vibration the centre of gravity remains constant while the position of atoms changes. The energy associated with vibrational motion is vibrational energy. It is also quantized. The energy gap falls in IR region. Transition between vibrational energy levels is possible.

Rotational Energy levels. Gaseous molecule will always rotate in space about an axis passing through the centre of mass. The energy associated with rotation is called rotational energy levels.  This is also quantized. Therefore it can be represented as levels. The energy gap between rotational energy levels corresponds to microwave region.  Transition between rotational energy levels is possible.

Q4. What are the various types of electronic transitions possible in organic molecule?

Ans: Four types of transitions are possible in an organic molecule.

n-π* transitions :  These types of transitions are related to the promotion of an electron from a non-bonding orbital to π* antibonding orbital. These transitions are shown by unsaturated molecules which contain hetero atoms like N, O, Cl, Br, S etc. Example, in aliphatic aldehydes and ketones n-π* transitions generally occurs in the wavelength range 270 to 300 nm.

π-π* transitions :  These types of transitions are related to the promotion of an electron from a π-bonding orbital to π* antibonding orbital. The unsaturated organic molecules like alkenes have π-bonding orbital as HOMO and π* antibonding orbital as LUMO shows this type of transitions. For example ethylene molecule gives an absorption maximum at 169nm (which is not scanned by commercial spectrometer). Extending the conjugation, further red-shift the absorption maximum.

n-s* transitions :  These types of transitions are related to the promotion of an electron from a non-bonding orbital to σ* antibonding orbital. These are forbidden transitions and therefore are weak intense. The saturated organic molecules with hetero atoms like N, O, S, Cl etc shows this type of transitions. Example ethers, chloroform, methanol, etc. Usually absorption takes place below 200nm, therefore commercial spectrometers does not scan this.

σ-σ* transitions :  These types of transitions are related to the promotion of an electron from a σ-bonding orbital to σ* antibonding orbital. This type of transitions is common for saturated organic molecules without hetero-atoms. These are allowed transitions. Usually this transition occurs below 150 nm. These transitions cannot be observed in commercial spectrometers (200 nm – 750 nm).

Q5. What are the advantages of MRI imaging technique over others?

Ans: MRI technique is used to image soft tissues. The electromagnetic radiation used is RF which is harmless. MRI can cover a wide area of body at a time. It can differentiate injured and non-injured tissues.

Q5.  With a suitable example make clear the Chemical Shift in H-NMR spectroscopy.

Ans:  Consider the low resolution NMR spectra of 1,1,2-trichloroethane.

It has two types of protons, Namely Ha and Hb.  Ha –type is connected to two chlorine so more deshielded than Hb protons. 

Q6.  With a suitable example make clear the SPIN-SPIN splitting in H-NMR spectroscopy.

Ans:  Consider the low resolution NMR spectra of 1,1,2-trichloroethane.

It has two types of protons, Namely Ha and Hb.  Ha –type is connected to two chlorine so more deshielded than Hb protons. 

7. Predict the High resolution NMR spectrum of CH₃-CH₂-CHCl₂ (High resolution means spin-spin splitting).

 

Q8. Write four applications of Electronic Spectroscopy.

Qualitative analysis of Conjugated dienes and aromatics

Characterization of dyes.

Quantitative analysis using Beer lamberts law.

Study the kinetics of reaction by monitoring absorbance vs concentration

Determination of impurities in solvents.

Q9. Which of the following nuclei shows NMR activity?

                                  H¹, H², He⁴, C¹², C¹³, O¹⁸            

To be NMR active, Nuclei should have non-zero spin.

 H¹, H², C¹³     [He⁴ has 2 protons and two neutrons I = 0,  C¹² has 6 protons and 6 neutrons therefore I =0,  O¹⁸ has 8 protons and 10 neutrons  therefore I =0]       

            

Q10. A dye solution of concentration 0.04 M shows absorbance of 0.045 at 530 nm; while a solution of same dye shows absorbance of 0.022 under same conditions. Find the concentration of the test solution?

Soln:      A  =   e  c  t          Here e and t  are constants, therefore

   A1/A2     =    C1/C2

           0.045/0.022    =    0.04/C2

         c₂  =   0.0195 M


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