Chemical Bondings

Question 3.1a: Give reason for the differences in observed values and theoretical values of bond energies of halogens with hydrogen.

Answer.

Theoretical bond energies are based on only covalent nature of bond, while observed bond energies are based on both covalent as ionic nature of bonds.

In H-X, X atom has higher electronegativity than hydrogen atom. As a result bond pair of electron is attracted more towards halogen atom that gets partial negative charge and hydrogen atom that gets partial positive charge. Due to polar nature, H - X bond gets an extra ionic character which further increases the bond strength.

Ionic character is much greater in H - F than other H-X bonds due to highest electronegativity of F as compared to other halogens. Similarly, following decreasing ionic character among remaining HX is H-Cl> HBr >HI due to decreasing order of electronegativity of respective halogens, i.e. Cl>Br>I.

Question 3b: What is the effect of bond length on the bond energy? Give reason.

Answer. Bond energy increases as bond length decreases. Reason is that, in shorter bonded length, electrons are tightly held between bonded nuclei due to effective overlapping of atomic orbitals.

On the other hand, bond energy decreases as bond length increases. Further nuclei are from bonded electron, less tightly bond electrons are held.

Question 3c: What is the effect of the ionic character on the bond energy? Give reason.

Answer. Generally, ionic bond is much stronger than covalent bond, so, increase in ionic character further increases bond strength, hence bond energy is increased. Furthermore, ionic character is developed due to difference in electronegativities of bonded atoms. For example, in HX, electronegativity of X is greater than H. So, X attracts a share pair of electrons more towards itself and gets partial negative charge while H gets partial positive charge. Due to this polarization of bond, ionic character is developed, and bond between H and X become further stronger.

Question 3.2 : The melting points, boiling points, heat of vaporization and heats of sublimations of electrovalent compounds are higher as compared to those of covalent compound. Explain

Answer. All physical properties of compounds are such as melting points, boiling points, heat of vaporization and heats of sublimations depend upon forces of attractions among particles (molecules or ions). In electrovalent compounds, cations and anions are held together by stronger electrostatic force of attractions. On the other hand, in covalent compounds, small molecules are held together by weaker vender wall forces.

Therefore, melting points, boiling points, heat of vaporization and heats of sublimations of electrovalent compounds are higher as compared to those of covalent compound.

Also Read: Distinguish between a sigma bond and a Pi bond

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