Question

The charges and sizes of the ions in an ionic compound affect the strength of the electrostatic attraction holding that compound together. Based on ion charges and relative ion sizes, rank these ionic compounds by their expected melting points from highest to lowest

a.SrCl2 ,b.CsBr ,c.RbCl ,d.SrS

Answer 1

Decreasing order of melting point:

SrS >
`SrCl_2` > RbCl > CsBr

Step-by-step explanation:

Born–Landé equation explained the relation between lattice energy, charges and sizes of ions present in a compound.

`U=-k(Q_1Q_2)/(r_0)`

Here, U is internal energy,
`Q_1\ and\ Q_2` are charges and
`r_0` is the size.

More the lattice energy more will be the energy to break ionic lattice in order to melt.

Therefore, more the lattice energy, more will be melting point.

Moreover, lattice energy is proportional to charge and inversely proportional to size.

Consider the compounds given

In
`SrCl_2,\ charges\ are\ Sr^(2+),\ Cl^-`

In CsBr, charges are
`Cs^+,\ Br^-`

In RbCl charges are
`Rb^+,\ Cl^-`

In Srs, charges on ions are
`Sr^(2+),\ S^(2-)`

Order of the sizes are as follows:

Cs > Rb

Br > Cl

Cs > Sr

Therefore, decreasing order of melting point:

SrS >
`SrCl_2` > RbCl > CsBr