Question

What happens when?
(1) \[PC{l_5}\] is heated
(2) ${H_3}P{O_3}$ is heated?
Write the reaction involved.

Answer 1

Hint: Try and draw the structure of $PC{l_5}$ and focus on the bonds and also the fact that on heating $PC{l_5}$ undergoes an equilibrium reaction .
For ${H_3}P{O_3}$ focus on the various oxidation states possible for P atoms.

Complete step by step answer:
(1)$PC{l_5}$ is heated:
The phosphorus atom in $PC{l_5}$ has a hybridisation of $s{p^3}d$ and thus it has a triangular bipyramidal structure.

                                                               

In this type of structure 3 bonds are equatorial (in a plane) and 2 are axial bonds (one above the plane and other below). The 2 axial bonds are at an angle of ${90^ \circ }$ to the 3 equatorial bonds. While the 3 equatorial bonds are at an angle of ${120^ \circ }$ to each other. In this way the axial bonds are nearer to the equatorial bonds. Hence axial bonds experience greater repulsion from both sides leading to the elongation of these axial bonds.

So, axial bonds (219pm) are longer than the equatorial bonds (204pm). Due to the repulsion experienced by the axial bonds they have lesser stability than the equatorial bonds.
So when $PC{l_5}$ is heated the 2 unstable axial bonds break, causing $PC{l_5}$ to decompose into $PC{l_3}$ and $C{l_2}$ .
This reaction can be written as:
                                  $PC{l_5}\xrightarrow{\Delta }PC{l_3} + C{l_2}$

(2)${H_3}P{O_3}$ is heated:
When ${H_3}P{O_3}$ is heated it undergoes a disproportionation reaction and forms ${H_3}P{O_4}$ and$P{H_3}$.
The oxidation state of P in ${H_3}P{O_3}$ is +3, in ${H_3}P{O_4}$it is +5 and in $P{H_3}$ it is -3. Here we can see that the oxidation number of P is both decreasing and increasing which means that it undergoes both reduction and oxidation.
So, ${H_3}P{O_3}$ undergoes a disproportionation reaction.
This reaction can be written as:
                      $4{H_3}P{O_3}\xrightarrow{\Delta }3{H_3}P{O_4} + P{H_3}$

Note: While doing such questions always be careful while describing the structures and bonds. Also the oxidation states should be calculated with precision and also do not get confused between phosphoric acid (${H_3}P{O_4}$) and orthophosphorous acid (${H_3}P{O_3}$).