Chemical Bonding
Part-5
5. Effect of Temperature. Formation of a solution may be exothermic or endothermic process
and may be represented as follows :
Exothermic : Solute + Solve
Solution + Heat ....(1)
Endothermic : Solute + Solvent + Heat
Solution ....(2)
The addition of heat (i.e., a rise in temperature) in equation (2) causes more of the solute to
dissolve. This is the case with most of the solid- liquid solutions, where the solubility increases
with rise in temperature. In equation (1) the solublility decreases with the rise in temperature.
For example, the solubilities of KNO3, NaNO3, KCl, NH4Cl etc. increases with the rise in
temperature whereas that of Na2SO4 decreases with the rise in temperature.
6. Hydrogen bonding. It is very minor factor which control solubility of compounds. In the
system where solute and solvent particals shows association with H-Bonding are more soluble
than other combination. For example NH3 is more soluble in H2O than PH3, ROH are more
soluble in H2O than RSH due to association. In case of ROH & ROR their solubility in water is
almost same because both show H-Bonding with H2O. Diols and triols are much more soluble
than mono hydroxyderivatives because they form more effective hydrogen bonding.
(1) Types of Covalent Bonds
It has already been discussed that the formation of a covalent bond involves the overlapping of
half-filled atomic orbitals. The covalent bonds can be classified into two different categories
depending upon the type of overlapping. These are :
(a) Sigma covalent bond ,
(b) Pi covalent bond.
(a) Sigma ( ) bond. This type of covalent bond is formed by the axial overlapping of halffilled atomic orbitals. The atomic orbitals overlap along the inter-nuclear axis and involve end
to end or head on overlap. The electron cloud formed as a result of axial overlap is
cylindrically symmetrical about inter-nuclear axis. The electrons constituting sigma bond are
called sigma electrons. There can be three types of axial overlap as discussed below :
(i) s-s overlap. It involves mutual overlap of half-filled s-orbitals of the atoms approaching to
form a bond. The bond formed is called s-s bond.
(ii) s-p overlap. It involves mutual overlap of half-filled s-orbital of the one atom with halffilled p-orbital of the other. The bond so formed is called s-p bond.
(iii) p-p overlap. It involves mutual overlap of half-filled p-orbitals of the two atoms. The bond
so formed is called p-p bond.
The s-s, s-p and p-p overlaps have been shown diagramatically in Figure below.
Strength of three types of sigma bonds. The strength of three types of sigma bonds varies as
follows :
p-p > p-s > s-s
It is because of the fact, that p-orbitals allow overlap to a greater extent as compared to p-s
which is larger as compared to s-s overlap.
(b) Pi ( ) Bond. This type of covalent bond is formed by the lateral or sidewise overlap of the
atomic orbitals. The orbitals overlap takes place in such a way that their axes are parallel to
each other but perpendicular to the internuclear axis. The pi bond consists of two charge
clouds above and below the plane of the atoms involved in the bond formation. The electrons
involved in the p-bond formation are called -electrons.
It may be noted that :
(i) Sigma bond is stronger than pi bond. It is because of the fact that overlapping of atomic
orbitals can take place to a greater extent during the formation of sigma bond whereas
overlapping of orbitals occurs to a smaller extent during the formation of pi bond.
(ii) Pi bond between the two atoms is formed only in addition to a sigma bond. It is
because of the fact that the atoms constituting a single bond prefer to form a strong sigma
bond rather than a weak pi bond. Thus, pi bond is always present in molecules having multiple
bonds, i.e., double or triple bond. In other words, a single bond cannot be a pi bond.
(iii) The shape of molecule is controlled by the sigma frame work (orientations of sigma bonds)
around the central atom. Pi bonds are superimposed on sigma bonds hence they simply modify
the dimensions of the molecule.
Compraison between sigma and pi bonds. The various points of distinction between sigma and
pi bonds are given in Table below.
Table below. Comparison of Sigma and Pi Bonds
he formation of sigma and pi bonds, in oxygen (O2) molecule.
Oxygen atom (8O) has two half-filled p-orbitals in its valence shell as is evident from its
electronic configuration (1s2, 2s2, 2px2, 2py1>, 2pz21). One of the half-filled p-orbital overlaps
axially with half-filled p-orbital of the other oxygen atom to form s bond. The other half-filled
p-orbitals of the two atoms overlap sidewise to form a bond which is denoted as p -p bond.
The formation of molecule is shown in Fig. below.
Thus, O = O bond consists of one s bond and one p bond.
Bonding Parameters
Covalent bonds are characterised by the following parameters, bond energy, bond length and
bond angle.
(a) Bond Energy
It has already been pointed out that the formation of a bond occurs as a result of decrease of
energy. Therefore, same amount of energy is required to break the bond between the two
atoms. For example, the energy released during the formation of bonds between the gaseous
hydrogen atoms to form one mole of hydrogen moleculs is 433 kJ mol -1. This energy involved in
making or breaking of bonds is referred to as bond energy. Thus, bond energy may be defined
as the amount of energy required to break one mole of bonds of same kind so as to
separate the bonded atoms in the gaseous state.
The magnitude of bond energy reflects the strength of the bond. Its magnitude depends upon
the following factors :
(i) Size of the participating atoms. Larger the size of the atoms involved in bond formation,
lesser is the extent of overlapping and consequently, smaller is the value of bond energy.
For example, bond energy of Cl—Cl bond is 237 kJ mol-1 whereas that of H—H bond is 433 kJ
mol-1.
(ii) Multiplicity of bonds. The magnitude of bond energy increases with the multiplicity of
bonds even though the atoms involved in the bond formation are same. It is because of the fact
that with the multiplicity of bonds the number of shared electrons between the atoms
increases. As a result, the attractive force between nuclei and electrons also increases and
consequently, the magnitude of bond energy increases. For example, bond energy of C — C
bond is 348 kJ/mol-1 but that of C = C bond is 619 kJ mol-1. The average bond energies of some
bonds are given in Table below.
(iii) Number of lone pairs of electrons. Greater the number of lone pair of electrons present
on the bonded atoms, greater is the repulsive interactions between them and smaller is the
bond energy. Let us compare the bond energies of some of the single bonds
Table below. Bond Energies of Some Common Bonds