Chemistry

This is a Short Answer Type Questions as classified in NCERT Exemplar

Ans: The average bond enthalpy is defined as the ratio of total bond dissociation enthalpy to the number of bonds broken in the structure.

The identical O- H bonds in water molecule does not have the same bond enthalpies. According to the structure of a water molecule, there are two O- H bonds, but there is a change in the breaking of the first O- H bond than the second because of the different charge.

Hence in water molecule the average bond enthalpy will be:

The average O-H bond enthalpy = $\frac{502+427}{2}$ = 464 mol^-1

In ethanol C₂H₅OH  the bond enthalpy of O-H is different because the electronic environment around oxygen atom is different. In ethanol the O-H is attached to the carbon atom and in water molecule the O-H is attached to the hydrogen atom.

This is a Short Answer Type Questions as classified in NCERT Exemplar

Ans: (i) According to the electronic configuration, a total of 8 electrons must be present in the valence shell of an element. Element X has 4 valence electrons, so it will share the remaining 4 electrons for the formation of the bond, the molecular formula will be XH₄ . The element Y has 5 valence shell electrons, so it will form 3 bonds and the formula will be YH₃ . The element Z has 7 valence shell electrons, so it will form one bond with hydrogen and has the molecular formula H-Z .

(ii) Elements X, Y and Z having, 5 and 4 7 valence electrons respectively belongs to the second period and group 14^th ,15^th and 17^th . The electronegativity of the elements increases on moving across the period and the dipole moment is calculated by the difference in the electronegativity of the atoms in a molecular structure. Higher the difference in the electronegativity will be the dipole moment. So, the dipole moment of H - Z will be the highest.

This is a Short Answer Type Questions as classified in NCERT Exemplar

Ans: Ionic bonds are such bonds in which complete transfer of electrons from one atom to another atom. Due to complete transfer positive and negative ions are formed in this bond. The ions in this bond are held together by electrostatic force of attraction. The formation of calcium fluoride CaF₂ leads to the formation of an ionic bond.

Ca→  Ca²⁺ + e ^-   The electronic configuration of  Ca= [Ar] 4s²   and Ca²⁺= [Ar]

F +e ^-→F^- The electronic configuration of F= [He] 2p⁵  and F = [He ]2p⁶

Thus Ca²⁺ +2F^- → CaF₂

A covalent bond is formed due to mutual sharing of electrons between two atoms of a non-metal. This mutual sharing of electrons between the two non-metal species leads to the formation of a covalent bond. The making of Cl₂ leads to a formation of a covalent bond between two chloride ions.