Chemical Bonding

Question. Explain with reason why are𝛑 bonds more diffused than 𝛔 bond. 

Answer. As in 𝛔 bond electron density is maximum between the nuclei and symmetrical about line joining the two nuclei. On the other hand in 𝛑 bond electron density lies above and below the nuclei. So, 𝛑 bond is more diffused (soft) than 𝛔 bond. 

Question. Define bond energy or enthalpy. Explain the various parameters which determine its strength. 

Answer. Bond energy is the average amount of energy required to break one mole of a specific bond in gaseous substance into gaseous atoms. It is also called bond enthalpy which is measured under standard conditions i.e 298 K and 1 atm pressure. 

Factors: Following are factors of bond energy.

1. The electronegativity difference of bonded atoms. Greater the electronegativity difference, greater is bond enthalpy. 

2. Sizes of the atoms: Larger the size of atoms, smaller is bond enthalpy. 

3. Bond length: longer the bond length, weaker is bond enthalpy. 

Question. How do you compare the bond strengths of (i) Polar and non polar molecules (ii) 𝛔 and 𝛑 bond 

Answer. Polar molecule and Non-polar molecule: The molecule which contains atoms of different elements is called polar molecule The molecule which contains atoms of same element is called non polar molecule. In non polar molecule, the bonding electron pair is equally shard between the atoms having same electronegativities. Due to even (equal) distribution of electrons, bonded atoms remain electrically neutral. Whereas in a polar molecule, the bonding pair is not equally shard between atoms having different electronegativities. Due to uneven (unequal) distribution of electrons, one end of molecule is partially positive charge and an other partially negative charge. So polar molecule has an additional strength due to this ionic character. So polar molecule is more stronger then non polar molecule.

Sigma Bond (𝛔-bond) and Pi-bond( 𝛑 bond): In 𝛔 bond, electron density is maximum between the nuclei and symmetrical about line joining the two nuclei. So 𝛔 electrons are tightly held between the nuclei. On the other hand in 𝛑 bond electron density lies above and below the nuclei. So, 𝛑 bond is more diffused (soft) than 𝛔 bond. That is why 𝛑 bond is weaker than 𝛔 bond. 

Question. Calculate the bond energy of H-Br.The bond energy of H-H is 436 Kjmol-1 and Br-Br is 193 kJmol-1. 

Answer. Bond energy of H-Br = ?

 Bond energy of one mole H-H = 436 kJmol-1 

Bond energy of one molecule of H-H bond = (436 )/(6.023 x 10^23 ) kJ/molecule = 72.42 x 10^-23 kJ/molecule 

Bond energy of one atom of H-H bond= (72.42 x 10^-23 )/2kJ/atom = 36.21 x 10-23 Kj /atom 

Bond energy of one mole of Br-Br bond = 193 kJmol-1 

Bond energy of Br-Br = (193 )/(6.023 x 10^23 ) kJ/molecule = 32.04 x 10-23 kJ/molecule

 Bond energy of Br-Br = (72.42 x 10^(-23 ))/2kJ/atom = 16.02 x 10-23 Kj /atom

Bond energy of H-Br = 36.21 x 10^-23 Kj /atom of H + 16.02 x 10-23 Kj /atom of Br

                                   = 52.23 x 10-23 Kj / molecule of H-Br

                                    = 52.23 x 10-23 x 6.023x1023 kJ /mole  

                                    = 3.14.5 kJ/mol 

Dipole Moment 

Question. Define dipole moment. Give its various units. Find the relationship between Debye and mC. Answer.

Definition. The product of electric charge (q) and distance (r)between the positive and negative centers.                  

 Mathematically,

                          μ = q x r 

Units: mC and Debye Relationship between Debye and mC: 1D =3.336 X 10^-30 mC 

Question.  The bond length of H-Br is 1.4 x 10^-10 m. Its observed dipole moment is 0.79 D.

 Find the percentage ionic character of the bond. Unit positive charge = 1.6022 x 10^-19 and 1 D =3.336 X 10^(-30) mC 

Solution. % age ionic character = (μ obs/(μ ionic ) x 100 ) μobs=0.79 D

                μ ionic = q X r = 1.6022 x 10^-19 x 1.4 x 10^-10 

                            = 2.243 x 10-29 m C 

               μ ionic   =(2.243 X 10^(-29) mC )/(3.336 X 10^(-30) mC)

                              = 6.723 D 

             Since 1D  =3.336 X 10^(-30) mC 

           % age ionic character  = μ obs./ μ ionic

                                                = ((0.79)/(6.732 ) x 100 ) 

                                                = 11.7%

 Question. Dipole moment of CO2 is zero but that of CO is 0.12D. Why?

 Answer. Dipole moment of CO2 is zero but that of CO is 0.12 D. Reason is that CO2 has a linear structure, where the dipoles are equal and opposite, cancel each other effect. Whereas CO has single dipole moment directed from carbon and oxygen. μ = 0 D μ = 1.61 D 

Question. The dipole moment of CO2 is zero and that of H2O is 1.85 D. Why? 

Answer. Dipole moment of CO2 is zero but that of H2O is 1.85 D. Reason is that CO2 has a linear structure, where the dipoles are equal and opposite, cancel each other effect. Whereas dipole moment of water cancel the linear structure of molecule. A linear H2O molecule could have zero dipole moment. Calculations show that water has a bent or angular structure with bond angle of 104.5O. μ = 0 D μ = 1.49 D 

Question. NH3 and BF3 are both tetra atomic molecules, but the dipole moment of BF3 is zero, while that of NH3 is 1.49D. Justify it. 

Answer. Dipole moment of BF3 is zero but that of NH3 is 1.49 D. Reason is that BF3 has a triangular planer structure, where the dipoles are equal and opposite, cancel each other effect. Whereas dipole moment of NH3 cancel the triangular planer structure of molecule. A triangular planer NH3 molecule could have zero dipole moment. Calculations show that NH3 has a pyramidal structure, with net dipole moment directed towards the direction of lone pair. μ = 1.49 D μ = 0 D 

Question. How the %age of ionic character of the polar bond be determined? 

Answer. For this purpose we require to know actual dipole moment μobs of molecule and bond length, which can be determine experimentally. The dipole moment μionic. of 100% ionic compound is calculated by multiplying full charge of electron or proton with bond length. % age ionic character can determined by following formula. % age ionic character = □((μobs )/(μionic ) x 100 ) 

Question. Why the dipole moment of CH4 is zero? 

Answer. Reason is that CH4 has a regular tetrahedral structure, where the dipoles are equal and opposite, cancel each other effect. 

Question. Why many of ionic compounds are not soluble in water? 

Answer. Although, most of ionic compounds are soluble in water but still there are many of ionic compounds are insoluble in water. Reason is that ionic compounds are only soluble in water when their hydration energy is greater than lattice energy. So the ionic compounds which are not soluble in water have lower hydration energy than lattice energy. 

Question. Why covalent compounds dissolve easily in non-polar solvent? 

Answer. Covalent compounds are only soluble in non polar solvent when attractive forces of solvent molecules with covalent compounds are enough to overcome the intermolecular forces of attraction. 

Question. Ionic compounds do not show isomerism and covalent compounds show isomerism. Why? Answer. As ionic compound involve just electrostatic forces between oppositely charged ions. Therefore, such bonds are non rigid and non directional. Due to this ionic compounds do not show isomerism. On the other hand, covalent compounds involve overlapping of atomic orbitals . therefore such bonds are rigid and directional. Due to this covalent compounds show isomerism.