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Question 7.1:

Discuss the general characteristics of Group 15 elements with reference to

their electronic configuration, oxidation state, atomic size, ionisation enthalpy

and electronegativity.

•   Answer 

General trends in group 15 elements

(i) Electronic configuration: All the elements in group 15 have 5 valenceelectrons. heir general electronic configuration is ns! np".

(ii) Oxidation states: All these elements have 5 valence electrons and

re#uire three more electrons to complete their octets. $owever, gaining

electrons is very difficult as the nucleus will have to attract three more

electrons. his can ta%e place only with nitrogen as it is the smallest in sizeand the distance &etween the nucleus and the valence shell is relatively

small. he remaining elements of this group show a formal oxidation state of

'" in their covalent compounds. (n addition to the '" state, ) and * also

show '1 and '! oxidation states.

All the elements present in this group show +" and +5 oxidation states.

$owever, the sta&ility of +5 oxidation state decreases down a group,

whereas the sta&ility of +" oxidation state increases. his happens &ecause

of the inert pair effect.

(iii) Ionization energy and electronegativity 

irst ionization decreases on moving down a group. his is &ecause ofincreasing atomic sizes. As we move down a group, electronegativity

decreases, owing to an increase in size.

(iv) Atomic size: -n moving down a group, the atomic size increases. his

increase in the atomic size is attri&uted to an increase in the num&er ofshells.

Question 7.2:

hy does the reactivity of nitrogen differ from phosphorus/

•   Answer 

)itrogen is chemically less reactive. his is &ecause of the high sta&ility of its

molecule, )!. (n )!, the two nitrogen atoms form a triple &ond. his triple&ond has very high &ond strength, which is very difficult to &rea%. (t is

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&ecause of nitrogen0s small size that it is a&le to form p'p &onds with

itself. his property is not exhi&ited &y atoms such as phosphorus. hus,phosphorus is more reactive than nitrogen.

Question 7.3:

Discuss the trends in chemical reactivity of group 15 elements.

•   Answer 

General trends in cemical properties of group ! 15

(i) "eactivity to#ards ydrogen:

he elements of group 15 react with hydrogen to form hydrides of type 2$",

where 2 3 ), *, As, 4&, or i. he sta&ility of hydrides decreases on moving

down from )$" to i$".

(ii) "eactivity to#ards oxygen:

he elements of group 15 form two types of oxides: 2!-" and 2!-5, where 2

3 ), *, As, 4&, or i. he oxide with the element in the higher oxidation state

is more acidic than the other. $owever, the acidic character decreases onmoving down a group.

(iii) "eactivity to#ards alogens:

he group 15 elements react with halogens to form two series of salts: 26" 

and 265. $owever, nitrogen does not form )65 as it lac%s the d 7or&ital. All

trihalides 8except )6"9 are sta&le.

(iv) "eactivity to#ards metals:

he group 15 elements react with metals to form &inary compounds in whichmetals exhi&it '" oxidation states

Question 7.4:

hy does )$" form hydrogen &ond &ut *$" does not/

•   Answer 

)itrogen is highly electronegative as compared to phosphorus. his causes a

greater attraction of electrons towards nitrogen in )$" than towardsphosphorus in *$". $ence, the extent of hydrogen &onding in *$" is very less

as compared to )$".

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Question 7.5:

$ow is nitrogen prepared in the la&oratory/ rite the chemical e#uations of

the reactions involved.

Answer 

An a#ueous solution of ammonium chloride is treated with sodium nitrite.

)- and $)-" are produced in small amounts. hese are impurities that can

&e removed on passing nitrogen gas through a#ueous sulphuric acid,

containing potassium dichromate.

Question 7.6:

$ow is ammonia manufactured industrially/

•   Answer 

•

•   Discussion 

Ammonia is prepared on a large7scale &y the $a&er0s process.

he optimum conditions for manufacturing ammonia are:

(i) *ressure 8around ! ; 15 *a9

(ii) emperature 89

(iii) ?atalyst such as iron oxide with small amounts of Al!-" and >!-

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Question 7.7:

(llustrate how copper metal can give different products on reaction with

$)-".

•   Answer 

?oncentrated nitric acid is a strong oxidizing agent. (t is used for oxidizing

most metals. he products of oxidation depend on the concentration of theacid, temperature, and also on the material undergoing oxidation.

Question 7.8:

Give the resonating structures of )-! and )!-5.

•   Answer 

(1)

($)

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Question 7.9:

he $)$ angle value is higher than $*$, $As$ and $4&$ angles. hy/

@%int: ?an &e explained on the &asis of sp" hy&ridisation in )$" and only s−p

&onding &etween hydrogen and other elements of the group.

•   Answer 

$ydride )$" *$" As$" 4&$"

$'B'$ angle 1=C !C 1C C

he a&ove trend in the $'B'$ &ond angle can &e explained on the &asis ofthe electronegativity of the central atom. 4ince nitrogen is highly

electronegative, there is high electron density around nitrogen. his causes

greater repulsion &etween the electron pairs around nitrogen, resulting in

maximum &ond angle. e %now that electronegativity decreases on movingdown a group. ?onse#uently, the repulsive interactions &etween the electron

pairs decrease, there&y decreasing the $'B'$ &ond angle.

Question 7.10:

hy does E"*3- exist &ut E")3- does not 8E 3 al%yl group9/

•   Answer 

) 8unli%e *9 lac%s the d 7or&ital. his restricts nitrogen to expand its

coordination num&er &eyond four. $ence, E")3- does not exist.

Question 7.11:

2xplain why )$" is &asic while i$" is only fee&ly &asic.

•   Answer 

&%' is distinctly asic #ile i%' is feely asic*

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)itrogen has a small size due to which the lone pair of electrons is

concentrated in a small region. his means that the charge density per unitvolume is high. -n moving down a group, the size of the central atom

increases and the charge gets distri&uted over a large area decreasing the

electron density. $ence, the electron donating capacity of group 15 element

hydrides decreases on moving down the group.

Question 7.12:

)itrogen exists as diatomic molecule and phosphorus as *

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(t undergoesspontaneous

com&ustion in air.

(t is relatively less reactive.

(n &oth solid and vapourstates, it exists as a *

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?an *?l5 act as an oxidising as well as a reducing agent/ Fustify.

•   Answer 

*?l5 can only act as an oxidizing agent. he highest oxidation state that * can

show is +5. (n *?l5, phosphorus is in its highest oxidation state 8+59.$owever, it can decrease its oxidation state and act as an oxidizing agent.

Question 7.17:

Fustify the placement of -, 4, 4e, e and *o in the same group of the periodic

ta&le in terms of electronic configuration, oxidation state and hydride

formation.

•   Answer 

he elements of group 1 are collectively called chalcogens.

(i) 2lements of group 1 have six valence electrons each. he general

electronic configuration of these elements is ns! np

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-xygen is smaller in size as compared to sulphur. Due to its smaller size, it

can effectively form  p' p &onds and form -! 8-33-9 molecule. Also, theintermolecular forces in oxygen are wea% van der all0s, which cause it to

exist as gas. -n the other hand, sulphur does not form B! molecule &ut

exists as a puc%ered structure held together &y strong covalent &onds.

$ence, it is a solid.

Question 7.19:

>nowing the electron gain enthalpy values for - → -' and - → -!' as '1

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Discussion 

4ulphuric acid is manufactured &y the contact process. (t involves the

following steps:

.tep (i):

4ulphur or sulphide ores are &urnt in air to form 4-!.

.tep (ii):

y a reaction with oxygen, 4-! is converted into 4-" in the presence of I!-5 

as a catalyst.

.tep (iii):

4-" produced is a&sor&ed on $!4-

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$* 2ven in very low concentrations, 4-! causes irritation in the respiratory

tract. (t causes throat and eye irritation and can also affect the larynx tocause &reathlessness.

'* (t is extremely harmful to plants. *lants exposed to sulphur dioxide for a

long time lose colour from their leaves. his condition is %nown as chlorosis.his happens &ecause the formation of chlorophyll is affected &y the

presence of sulphur dioxide.

Question 7.23:

hy are halogens strong oxidising agents/

•   Answer 

he general electronic configuration of halogens is n p

5

, where n 3 !7. hus,halogens need only one more electron to complete their octet and to attainthe sta&le no&le gas configuration. Also, halogens are highly electronegative

with low dissociation energies and high negative electron gain enthalpies.

herefore, they have a high tendency to gain an electron. $ence, they act as

strong oxidizing agents.

Question 7.24:

2xplain why fluorine forms only one oxoacid, $-.

•   Answer 

•

luorine forms only one oxoacid i.e., $- &ecause of its high

electronegativity and small size.

Question 7.25:

2xplain why inspite of nearly the same electronegativity, oxygen forms

hydrogen &onding while chlorine does not.

•

  Answer 

oth chlorine and oxygen have almost the same electronegativity values, &ut

chlorine rarely forms hydrogen &onding. his is &ecause in comparison to

chlorine, oxygen has a smaller size and as a result, a higher electron densityper unit volume.

Question 7.26:

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rite two uses of ?l-!.

•   Answer 

/ses of 0lO$:

(i) (t is used for purifying water.

(ii) (t is used as a &leaching agent.

Question 7.27:

hy are halogens coloured/

•   Answer 

Almost all halogens are coloured. his is &ecause halogens a&sor& radiationsin the visi&le region. his results in the excitation of valence electrons to a

higher energy region. 4ince the amount of energy re#uired for excitation

differs for each halogen, each halogen displays a different colour.

Question 7.28:

rite the reactions of ! and ?l! with water.

•   Answer 

(i) 

(ii) 

Question 7.29:

$ow can you prepare ?l! from $?l and $?l from ?l!/ rite reactions only.

•   Answer 

(i) ?l! can &e prepared from $?l &y Deacon0s process.

(ii) $?l can &e prepared from ?l! on treating it with water.

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Question 7.30:

hat inspired ). artlett for carrying out reaction &etween 6e and *t/

•   Answer 

)eil artlett initially carried out a reaction &etween oxygen and *t. his

resulted in the formation of a red compound, .

Hater, he realized that the first ionization energy of oxygen 811=5 %FLmol9

and 6e 811= %FLmol9 is almost the same. hus, he tried to prepare a

compound with 6e and *t. $e was successful and a red7coloured

compound, was formed. 

Question 7.31:

hat are the oxidation states of phosphorus in the following:

(i) $"*-" (ii) *?l" (iii) ?a"*!

(iv) )a"*-

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(iii) ?a"*!

(iv) )a"*-<

(v) *-"

Question 7.32:

rite &alanced e#uations for the following:

(i) )a?l is heated with sulphuric acid in the presence of Bn-!.

(ii) ?hlorine gas is passed into a solution of )a( in water.

•   Answer 

(i) 

(ii)

Question 7.33:

$ow are xenon fluorides 6e!, 6e

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6e!, 6e

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Question 7.36:

Arrange the following in the order of property indicated for each set:

(i) !, ?l!, r!, (! 7 increasing &ond dissociation enthalpy.

(ii) $, $?l, $r, $( 7 increasing acid strength.

(iii) )$", *$", As$", 4&$", i$" ' increasing &ase strength.

•   Answer 

(i) ond dissociation energy usually decreases on moving down a group as

the atomic size increases. $owever, the &ond dissociation energy of ! islower than that of ?l! and r!. his is due to the small atomic size of

fluorine. hus, the increasing order for &ond dissociation energy among

halogens is as follows:

(! M ! M r! M ?l!

(ii) $ M $?l M $r M $(

he &ond dissociation energy of $76 molecules where 6 3 , ?l, r, (,decreases with an increase in the atomic size. 4ince $7( &ond is the wea%est,

$( is the strongest acid.

(iii) i$" N 4&$" M As$" M *$" M )$"

-n moving from nitrogen to &ismuth, the size of the atom increases while the

electron density on the atom decreases. hus, the &asic strength decreases.

Question 7.37:

hich one of the following does not exist/

(i) 6e-

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Give the formula and descri&e the structure of a no&le gas species which is

isostructural with:

(i)

(ii)

(iii)

•   Answer 

(i)

6e

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Question 7.39:

hy do no&le gases have comparatively large atomic sizes/

•   Answer 

)o&le gases do not form molecules. (n case of no&le gases, the atomic radii

corresponds to van der aal0s radii. -n the other hand, the atomic radii of

other elements correspond to their covalent radii. y definition, van der

aal0s radii are larger than covalent radii. (t is for this reason that no&le

gases are very large in size as compared to other atoms &elonging to thesame period.

Question 7.40:

List the uses of Neon and argon gases.

•   Answer 

/ses of neon gas:

(i) (t is mixed with helium to protect electrical e#uipments from high voltage.

(ii) (t is filled in discharge tu&es with characteristic colours.

(iii) (t is used in &eacon lights.

/ses of Argon gas:

(i) Argon along with nitrogen is used in gas7filled electric lamps. his is

&ecause Ar is more inert than ).

(ii) (t is usually used to provide an inert temperature in a high metallurgicalprocess.

(iii) (t is also used in la&oratories to handle air7sensitive su&stances.

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