The P Block Elements

Chemistry

NCERT

1   Discuss the general characteristics of Group $15$ elements with reference to their electronic configuration, oxidation state, atomic size, ionization and electronegativity.

Solution :

General trends in group $15$ elements (i) Electronic configuration: All the elements in group $15$ have $5$ valence electrons. Their general electronic configuration is $ns^2 \ np^3 $.$\\$ (ii) Oxidation states: All these elements have $5$ valence electrons and require three more electrons to complete their octets. However, gaining electrons is very difficult as the nucleus will have to attract three more electrons. This can take place only with nitrogen as it is the smallest in size and the distance between the nucleus and the valence shell is relatively small. The remaining elements of this group show a formal oxidation state of $-3$ in their covalent compounds. In addition to the $-3$ state, $N$ and $P$ also show $-1$ and $-2$ oxidation states. All the elements present in this group show $+3$ and $+5$ oxidation states. However, the stability of $+5$ oxidation state decreases down a group, whereas the stability of $+3$ oxidation state increases. This happens because of the inert pair effect.$\\$ (iii) Ionization energy and electronegativity: First ionization decreases on moving down a group. This is because of increasing atomic sizes. As we move down a group, electronegativity decreases, owing to an increase in size.$\\$ (iv) Atomic size: On moving down a group, the atomic size increases. This increase in the atomic size is attributed to an increase in the number of shells.

2   Why does the reactivity of nitrogen differ from phosphorus?

Solution :

Nitrogen is chemically less reactive. This is because of the high stability of its molecule, $N_2$ . In $N_2$ , the two nitrogen atoms form a triple bond. This triple bond has very high bond strength, which is very difficult to break, It is because of nitrogen’s small size that it is able to form $p \pi - p \pi$ bonds with itself. This property is not exhibited by atoms such as phosphorus. Thus, phosphorus is more reactive than nitrogen.

3   Discuss the trends in chemical reactivity of group $15$ elements.

Solution :

General trends in chemical properties of group $15$$\\$ (i) Reactivity towards hydrogen:$\\$ The elements of group $15$ react with hydrogen to form hydrides of type $EH_3$ , where $E = N, P$ As, Sb, or Bi The stability of hydrides decreases on moving down from $NH_3$ to $BiH_3$ .$\\$ (ii) Reactivity towards oxygen:$\\$ The elements of group $15$ form two types of oxides: $E_{ 203}$ and $E_{205}$ , where $E = N, P,$ As, Sb, or Bi.$\\$ The oxide with the element in the higher oxidation state is more acidic than the other. However, the acidic character decreases on moving down a group.$\\$ (iii) Reactivity towards halogens:$\\$ The group $15$ elements react with halogens to form two series of salts: $EX_3$ and $EX_5$ . However, nitrogen does not form $NX_5$ as it lacks the d-orbital. All trihalides (except $NX_3$ ) are stable,$\\$ (iv) Reactivity towards metals:$\\$ The group $15$ elements react with metals to form binary compounds in which metals exhibit $-3$ oxidation states.

4   Why does $NH_3$ form hydrogen bond but $PH_3$ does not?

Solution :

Hydrogen bond is always formed between highly electronegative atom and $H$ atom. Nitrogen is highly electronegative compared to phosphorus as electronegativity decreases down the group. Hence, the extent of hydrogen bonding in $PH_3$ is very less as compared to $NH_3$ .

5   How is nitrogen prepared in the laboratory? Write the chemical equations of the reactions involved.

Solution :

An aqueous solution of ammonium chloride is treated with sodium nitrite.$\\$ $NH_4Cl_{(aq)}+NaNO_{2(aq)} \to N_{2(g)}+2H_2O_{(l)}NaCl_{(aq)}$$\\$ $NO$ and $HNO_3$ are produced in small amounts. These are impurities that can be removed on passing nitrogen gas through aqueous sulphuric acid, containing potassium dichromate.

6   How is ammonia manufactured industrially?

Solution :

Ammonia is prepared on a large-scale by the Haber’s process.$\\$ $N_{ 2( g )} + 3 H_{ 2( g )} \to 2 NH _{3( g )} \Delta _f H ^o =- 46.1 kJ /$ mol$\\$ The optimum conditions for manufacturing ammonia are:$\\$ (i) Pressure (around $200 ×105$ Pa)$\\$ (ii) Temperature $(4700 K)$ $\\$ (iii) Catalyst such as iron oxide with small amounts of $A1_ 2 O_ 3$ and $K _2 O$

7   Illustrate how copper metal can give different products on reaction with $ HNO _3$ .

Solution :

Concentrated nitric acid is a strong oxidizing agent It is used for oxidizing most metals. The products of oxidation depend on the concentration of the acid, temperature, and also on the material undergoing oxidation.$\\$ $3 Cu + 8 HNO_ {3( dilute )} \to 3 Cu ( NO_ 3 ) _2 + 2 NO + 4 H_ 2 O$ $\\$ $Cu + 4 HNO_ {3( conc )} \to Cu ( NO _3 )_ 2 + 2 NO_ 2 + 2 H_ 2 O$

8   Give the resonating structures of $NO_ 2$ and $N _2 O _5$

Solution :

9   The $HNH $ angle value is higher than $ HPH, HAsH$ and $HSbH $angles. Why?$\\$ [Hint: Can be explained on the basis of sp $3$ hybridisation in $NH_ 3$ and only s-p bonding between hydrogen and other elements of the group].

Solution :

Hydride $NH_ 3 \quad PH _3 \quad AsH _3 \quad SbH_ 3 \quad H-M-H $ angle $107^o\quad 92^o\quad 91^o\quad 90^o$ $\\$ The above trend in the $ H-M-H$ bond angle can be explained on the basis of the electronegativity of the central atom. Since nitrogen is highly electronegative, there is high electron density around nitrogen. This causes greater repulsion between the electron pairs around nitrogen, resulting in maximum bond angle. We know that electronegativity decreases on moving down a group. Consequently, the repulsive interactions between the electron pairs decrease, thereby decreasing the $H-M-H$ bond angle.

10   Why does $R _3 P = 0$ exist but $R _3 N = 0$ does not ($R $= alkyl group)?

Solution :

$N $ (unlike $P$) lacks the d-orbital. This restricts nitrogen to expand its coordination number beyond four. Hence, $R_ 3 N = 0$ does not exist.