prepared by mr. amulya kumar samal pgt(chemistry) kendriya vidyalaya no-2,crpf,bhubaneswar phone...
TRANSCRIPT
PREPARED BYMR. AMULYA KUMAR SAMALPGT(CHEMISTRY)KENDRIYA VIDYALAYA NO-
2,CRPF,BHUBANESWARPHONE [email protected]
Group 15 Elements
OBJECTIVES:-
After studying this unit, the students will be able to :
1.appreciate general trends in the chemistry of elements of groups 15
2.describe the properties of group-15.
3.Give answer the reasoning question.
The p-block elements
The p-block elements are placed in groups 13 to 18 of the periodic table.Their valence shell electronic configuration is ns2np1–6 (except He which has 1s2 configuration)
Group 15 Elements
Nitrogen(N) Phosphorus(P)Arsenic(As)Antimony(Sb)
Bismuth(Bi)
Metalloids
Typical Metal
Non Metals
Trends in properties
Electronic Configuration
The s orbital in these elements is completely filled and p orbitals are half-filled, making their electronic configuration extra stable.
The valence shell electronic configuration of these elements is ns2np3.
Atomic and Ionic RadiiN
P
As
Sb
Bi
considerable increase
small increase
due to the presence of completely filled d and/or f orbitals in heavier members.sss
Ionisation Enthalpy Δi HN
P
As
G-14
Bi
Δi H Decreases.
Sb
G-15
Δi H IncreasesBecause of the
extra stable half-filled p orbitals electronicconfiguration and smaller size
Due to gradual increase in atomic size
ΔiH1 < ΔiH2 < ΔiH3
Electronegativity
N
P
As
Bi
E.N Decreases.
Sb
Due to gradual increase in atomic size
3.0
2.1
2.0
1.9
1.9
Physical PropertiesAll the elements of this group are polyatomic.Dinitrogen is a diatomic gas while all others
are solids.Metallic character increases down the group.The b.p, in general, increase from top to
bottom in the group but the m.p increases up to As and then decreases up to Bi.
Except nitrogen, all the elements show
allotropy.
Chemical Properties Oxidation states and trends in chemical reactivity
The common o.s of these elements are –3, +3
and +5. Group-
15Tendency to exhibit –3 o.s decreases due to increase in size and metallic character.
The stability of +5 o.s decreases and that of +3 state increases (due to inert pair effect) down the group.
Nitrogen exhibits + 1, + 2, + 4 O.S also when it reacts
with oxygen. In the case of nitrogen, all O.S from +1 to +4
tend to disproportionate in acid solution. For example,3HNO2 → HNO3 + H2O +
2NOSimilarly, in case of phosphorus nearly all intermediate
O.S disproportionate into +5 and –3 both in alkali and acid. Nitrogen is restricted to a maximum
covalency of 4 .Because only four (one s and three p) orbitals are available for bonding. The heavier elements have vacant d orbitals in the outermost shell which can be used for bonding (covalency) and hence, expand their covalence as in PF6
–.
Anomalous properties of nitrogen
Reasons:1.Its small
size2.high electronegativity3.high ionisation enthalpy4.non-availability of d orbitals.
Some anomalous properties of Nitrogen
1.State:Nitrogen is a gas whereas the others elements are solids and have allotropic forms.2.Atomicity:Dinitrogen is diatomic and involves triply bonded nitrogen N N . Other members have tetra-atomic molecules such P4,As4,Sb4 and complicated arrangements such as red and black phosphrous.
3.Ability to form pπ -pπ multiple bonds: Nitrogen has unique ability to form pπ -pπ
multiple bonds with itself and with other elements having small size and high electronegativity (e.g., C, O). Heavier elements of this group do not form pπ -pπ bonds as their atomic orbitals are so large and diffuse that they cannot have effective overlapping.Thus, nitrogen exists as a diatomic molecule with a triple bond (one s and two p) between the two atoms. Consequently, its bond enthalpy (941.4 kJ mol–1) is very high. On the contrary, phosphorus, arsenic and antimony form single bonds as P–P, As–As and Sb–Sb while bismuth forms metallic bonds in elemental state.
4.Catenation: The single N–N bond is weaker than the
single P–P bond because of high interelectronic repulsion of the non-bonding electrons, owing to the small bond length. As a result the catenation tendency is weaker in Nitrogen.5) Reactivity towards
hydrogen:
The hydrides are Lewis bases due to the presence of lone electron pair on central atom. The basic strength decreases in the order:NH3>PH3>AsH3>SbH3>BiH3
The thermal stabilities decrease from NH3 towards BiH3 as the thermal stability is inversely proportional to the M-H bond length.
The reducing nature of the hydrides is expressed in terms hydrogen giving tendency and it is found to increase from NH3 to BiH3
The boiling point of ammonia is more than that of phosphine (and also AsH3) because of the presence of intermolecular hydrogen bonding in the molecules.
(6) Reactivity towards oxygen: All these elements form two types of oxides: E2O3 and E2O5. The oxide in the higher oxidation state of the element is more acidic than that of lower oxidation state.
Their acidic character decreases down the group. The oxides of the type E2O3 of nitrogen and phosphorus are purely acidic,that of arsenic and antimony amphoteric and those of bismuth predominantly basic.
*Nitrogen does not form pentahalide due to non-availability of the d orbitals in its valence shell. Pentahalides are more covalent than trihalides
*Dinitrogen is rather inert at room temperature because of the high bond enthalpy of N N bond. Reactivity, however, increases rapidly with rise in temperature.
Self AssesmentEXAMINATION DECODEDImportant question with answerGive Reason for each of the following :- Q.1# NCl5 does not exist but NCl3 exits BUT both PCl3 & PCl5 exists .
Ans:-Due to absence of vacant d-orbitals in N, it cannot extend its co-valency.Q.2# PCl5 is ionic in nature in the solid state. OR, Solid PCl5 exists as an ionic solid.
Ans:- Due to unequal bond length of equatorial and axial bond . The close range in solid state makes it stabilized by transferring one Cl- from one PCl5 to other PCl5 and exist as ionic solid, [PCl4] + [PCl6]־ .Q.3# NH3 acts as ligand or good complexing agent.OR, NH3 has higher H+ affinity than PH3.
Ans:-Due to presence of lone pair electrons.
(4)Arrange the following in increasing order of the property indicated:NH3, PH3, AsH3, SbH3, BiH3 (Thermal stability)NH3, PH3, AsH3, SbH3, BiH3 (Bond dissociation enthalpy)NH3, PH3, AsH3, SbH3, BiH3 (Reducing character)NH3, PH3, AsH3, SbH3, BiH3 (Basic character)Ans: (a)NH3> PH3 > AsH3 >SbH3 >BiH3
(b) NH3< PH3 < AsH3 <SbH3 <BiH3
(c) NH3< PH3 < AsH3 <SbH3 <BiH3 (d) NH3> PH3 > AsH3 >SbH3 >BiH3