Discuss the trends in chemical reactivity of ...

NCERT Class XII Chemistry
Chapter - The p-Block Elements
Questions with Solutions

Question : 3 of 74

Marks: +1, -0

Discuss the trends in chemical reactivity of group 15 elements.

Solution:

Nitrogen has very low reactivity due to unavailability of vacantd-orbital and high bond dissociation energy of N

≡

N bond.

(a) Hydrides : General formula for hydrides is

MH_3, e.g., NH_3, PH_3,

AsH_3,SbH_3, BiH_3

. All these hydridesarecovalent in nature and have pyramidalstructure (

sp^{3}

hybridized).

Property	Down thegroup	Reason
Basicstrength $^{3}$	decreases	The size of central atom increases, electrondensity decreases.
Thermalstability of MH $^{3}$	decreases	The size of the central atom increases,its tendency to form stable M – H bondsdecreases.
Reducingcharacter	increases	The stability of hydrides decreases, thus thereducing character increases.
Melting andboiling point	increases(except in N)	$NH_3$ has high melting point and boilingpoint than $PH_3$ due to hydrogen bonding.As the molecular size increases van derWaals forces increases.

(b) Halides : Elements of group 15 form two types of halides

viz

. trihalidesand pentahalides. The halides are predominantly basic (Lewis bases) innature and have lone pair of electrons (central atom is

sp^{3}

hybridized).The pentahalides arethermallyless stable than the trihalides.

Property	Gradation	Reason
Stability of trihalidesof nitrogen	$NF_3 > NCl_3> NBr_3$	Large size difference between Nand the halogens
Lewis base strength	${NF}_{3} < \ {NCl}_{3}$ $< \ {NBr}_{3} < \ {NI}_{3}$	Decreasing electronegativity ofhalogens
Bond angle amongthe halides ofphosphorus	$\ {PF}_{3} < \ {PCl}_{3}<$ $\ {PBr}_{3} < \ {PI}_{3}$	Due to decreased bond pair-bondpair repulsion

i.e.,M_2O_3

and

M_2O_5

and are called trioxides and pentoxides.

Property	Gradation	Reason
Acidic strengthof trioxides	$\ {N}_{2} {O}_{3}> {P}_{2} {O}_{3}>$ $\ {As}_{2} {O}_{3}$	Electronegativity of central atomdecreases
Acidic strengthof pentoxide	${N}_{2} {O}_{5}>\ {P}_{2} {O}_{5}>$ $\ {As}_{2} {O}_{5}>\ {Sb}_{2} {O}_{5}>$ $\ {Bi}_{2} {O}_{5}$	Electronegativity of central atomdecreases.
Acidic strengthof oxides ofnitrogen	$\ {N}_{2} {O} < \ {NO} < \ {N}_{2} {O}_{3}$ $< {N}_{2} {O}_{4}< {N}_{2} {O}_{5}$	Oxidation state of central atomincreases
Stability ofpentoxide	$\ {P}_{2} {O}_{5}>\ {As}_{2} {O}_{5}>$ $\ {Sb}_{2} {O}_{5}>\ {N}_{2} {O}_{5}>$ $\ {Bi}_{2} {O}_{5}$	Stability of oxides of a higheroxidation state $i.e., M_2O_5$ decreaseswith increasing atomic number

(d) Oxoacids : The elements of this group form a number of oxoacids out of which those of N and P are more common.

Oxoacids of N :

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