Ionic bond:
Ionic bond is
an electrostatic bond that forms between positive and negative ions. In ionic
bond, one or more electrons transfer from the valence shell of an atom to
valence shell of another atom. The atom that loses electrons become positive
ion (cation) and the atom that gains electrons become negative ion (anion).
The positive
ion -cation- loses electrons, cation has a number of protons larger than the
number of electrons. The negative ion -anion- gains electrons, anion has a
number of electrons larger than the number of protons.
Any of those
positive or negative ions tend to attract a large number of opposite charges forming
an ionic solid. The solid normally has a regular, crystalline structure that
allows for the maximum attraction of ions, given their particular sizes.
Na [Ne] 3s1 + Cl [Ne]3s23p5 → Na+([Ne])
+ Cl-([Ne]3s23p6)
A wide variety
of ionic compounds result from the combine of a Group 1A or Group 2A metal with
a halogen or oxygen.
To be able to
understand how ionic compounds form, we can imagine that the formation of ionic
compounds occur in separate steps:
For the
reaction of lithium and fluorine atoms to form:
First the
lithium loses electron to form positive ion (cation):
Li (1s22s1) → Li (1s22s0)
+ e-
The electron
transfer to fluorine atom:
F (1s22s22p5) + e‑→ F (1s22s22p6)
Now that would
lead to joining between both ions:
Li+ + F- → Li+F-
Another
example, the combustion of calcium in presence of oxygen:
2Ca(s) + O2(g) → 2CaO(s)
Ca [Ar]4s2 + O [1s22s22p4]
→ Ca2+ [Ae] O2-[Ne]
Lattice energy of ionic compounds:
Lattice energy
cannot be measured directly. Using Coulomb’s law it is possible to measure
crystal energy if we know the structure and composition of the ionic compound. For
example the crystal lattice energy of LiF:
Eα QLi+QF-
E= k(QLi+QF-)/r
Where QLi+
and QF- are the charges on the Li+ and F-
ions and k is the proportionality constant. E is negative quantity, because QLi+
is positive and QF- is negative so formation of an ionic
bond is exothermic process and the lattice energy of LiF is positive and the bonded
pair of Li+ and F- ions is more stable than separate Li+
and F- ions.
Born-Haber
cycle (the process of determining lattice energy) :
Born-Haber
cycle for calculating the lattice energy of fluorine and lithium:
1. The convert
solid lithium to lithium vapor (the direct conversion of a solid to a gas is
called sublimation): where the enthalpy change is 155.2 (energy needed for
sublimation):→
Li(s) → Li(g) ∆H1o
= 155.2kJ/mol
2- Dissociation
of a half mole of F2 gas into gaseous F atoms (breaking of bond):
½ F2(g) → F(g) ∆H2o = 75.3kJ/mol
3- Ionization
of one mole of gaseous Li atoms:
Li(g) → Li+(g) + e- ∆H3o
= 520kJ/mol
4- Addition of
one mole of electrons to one mole of gaseous F atoms (opposite to electron
affinity):
F(g) + e- → F-(g) ∆H4o
= -328kJ/mol
5- Combination
of 1mole of gaseous Li+ and 1mole of F- forming LiF:
Li+(g) + F-(g) → LiF(s)
∆H5o
= ?
Reverse step 5: Energy + LiF(s) → Li+(g)
+ F-(g)
So,
Li(s) → Li(g) ∆H1o
= 155.2kJ/mol
½ F2(g) → F(g) ∆H2o
= 75.3kJ/mol
Li(g) → Li+(g) + e- ∆H3o
= 520kJ/mol
F(g) + e- → F-(g) ∆H4o
= -328kJ/mol
Li+(g) + F-(g) → LiF(s)
∆H5o = ?
Sum: Li(s) + ½ F2(g)
→ LiF(s) ∆Hoverallo = -594.1kJ/mol
According to
Hiss’s law the total enthalpies equals the sum of enthalpies of each steps:
∆Hoverallo = ∆H1o + ∆H2o
+ ∆H3o + ∆H4o + ∆H5o
-594.1kJ/mol = 155.2kJ/mol +75.3kJ/mol +520kJ/mol + 328kJ/mol + ∆H5o
∆H5o = -1017kJ/mol
Steps 1, 2 and
3 require energy input were 4 and 5 release energies.
∆H5°
is a large negative quantity while the lattice energy of LiF is a large
positive quantity. According to Coulomb’s law, the separation of ions is endothermic
and for that, the lattice energy is always a positive quantity.
No comments:
Post a Comment