Question

S- block elements are very reactive in nature they form various compounds. The order of thermal stability / solubility in water of different compounds is given. Choose the incorrect order.
A. $BeO > MgO > CaO > SrO > BaO$ (thermal stability)
B. $L{i_2}C{O_3} > N{a_2}C{O_3} > {K_2}C{O_3} > R{b_2}C{O_3} > C{s_2}C{O_3}$ (solubility in water)
C. $BeS{O_4} > MgS{O_4} > CaS{O_4} > SrS{O_4}$ (solubility in water)
D. None of these

Answer 1

Hint: S-block elements are group1 and group2 elements. Answer the given question based on the lattice energies, hydration energies and atomic sizes of the elements. Atomic sizes increase, hydration energy and lattice energy decreases down the group.

Complete Step by step answer: The elements of the group1 are alkali metals and group2 are alkaline earth metals which have ns1 and ns2 outermost electronic configuration respectively. They are highly reactive metals with low ionization enthalpies.
So, due to the high reactivity they are never found pure in nature with the exception of lithium and beryllium.
Group2 elements are: Be, Mg, Ca, Sr, Ba and Ra.
The thermal stability decreases down the group due to the increase in size as thermal stability is inversely proportional to the size.
Therefore $BeO > MgO > CaO > SrO > BaO$ is the correct order.
Group 2 metal sulphates are less soluble in water as we go down the column. This is due to the decrease in hydration energy which is more pronounced than the lattice energy. Hence solubility decreases down the group.
Therefore $BeS{O_4} > MgS{O_4} > CaS{O_4} > SrS{O_4}$ is the correct order.
Group1 metals are: Li, Na, K, Rb, Cs and Fr
The Solubility of these carbonates in water increases down the group because the lattice energies decrease more rapidly than the hydration energies.
Therefore $L{i_2}C{O_3} < N{a_2}C{O_3} < {K_2}C{O_3} < R{b_2}C{O_3} < C{s_2}C{O_3}$ is the right order.
But Option B is $L{i_2}C{O_3} > N{a_2}C{O_3} > {K_2}C{O_3} > R{b_2}C{O_3} > C{s_2}C{O_3}$ which is incorrect.

Hence, the answer is Option B.

Note: Although lattice energy and hydration energy are directly proportional to each other, they are different. They both increase only when the charge of ions increases and size decreases. Sometimes hydration energy can be greater than lattice energy and vice versa.