P block elements

5/28/2018 P block elements

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Group 18 ElementsGroup 18 consists of six elements: helium, neon, argon, krypton, xenon and radon.

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OccurrenceAllthe noble gases except radonoccur in the atmosphere.ARGONis the major

constituent.

HELIUM AND NEONare found in radioactive mineralse.g., pitchblende,

monazite, cleveite.

The maincommercial source of HELIUMisnatural gas.Xenon and radon israre and Radon is obtained as a radioactive decay product of 226Ra.

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Why are the elements of Group 18 known as noble gases?

The elements present in Group 18 have theirvalence shell orbitals completely

filled,Due to stable electronic configuration

it is difficult to remove electrons or accept the electron

and therefore these gases exhibitvery high ionization enthalpy and have largepositive values of electron gain enthalpy.

Therefore, react with a few elements only under certain conditions______________________________________________________________________________

Physical PropertiesAll the noble gases are monatomic.They are colorless, odorless and tasteless.

They are sparingly soluble in water.

They have very low melting and boiling points because the only type of interatomic

Interaction in these elements is weak dispersion forces.Helium has the lowest

boiling point (4.2 K) of any known substance. It has an unusual Property of

diffusingthrough most commonly used laboratory materials Such as rubber, glass orplastics.

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Chemical PropertiesIn general, noble gases are least reactive. Their chemical inertness

is attributed to the following reasons:

(i) except helium (1s2) all have completely filled ns2np6electronic configurationin their valence shell.

(ii) They have high ionization enthalpy.

INVESTIGATING THE REACTIVITY OF NOBLE GASESThough noble gas elements have generally very low reactivity (due to their very

high ionization and electron gain enthalpies), the heavier noble gases, Krypton

and Xenon tend to form some compounds. (REASON: These heavier elements

have more electron shells than the lighter ones. Hence, the outermost

electrons experience a shielding effect from the inner electrons that makesthem more easily ionized. This results in an ionization energy low enough to

form stable compounds with the most electronegative elements like fluorine

and oxygen.)Ever since their discovery, the reactivity of the noble gases has

been investigated sporadically ever, but all early attempts to coerce them into

compound formation were unsuccessful. Until the 1960s the only known

compounds were the unstable diatomic species such as He2-and Ar2-,

In March 1962, Neil Bartlett, then at the University of British Columbia,

observed the reaction of Xenon, a noble gas.Bartletts motivation for studyingxenon was based on the observations that the highly oxidizing compound, PtF6,

can oxidize O2, to give the red solid O2+PtF6.As the ionisation energy of O2to O+2 (1165 kJ mol

1) is nearly equal to the

ionisation energy of Xe to Xe+(1170 kJ mol1), he tried the reaction of Xe with

PtF6. This yielded a red crystalline product, xenon

hexafluoroplatinate,Xe+[PtF6].

The compounds of other noble gases are fewer. Only the difluoride (KrF2) and

(RnF2) has been identified. No true compounds of Ar, Ne or He are yet known.


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Xenon fluoridesXeF2, XeF4 and XeF6 are colorless crystalline solidsand sublime readily at 298 K.They are powerful fluorinating agents.

The structures of the three xenon fluoridescan be deducedfrom VSEPR.XeF2 is linear and XeF4 is square planar.XeF6has 7 electron pairs 6 bonding pairs and 1 lone pair)and would, thus, haveadistorted octahedralstructure.Preparation:Xenon forms three binary fluorides, XeF2, XeF4 and XeF6 bythe direct reaction of elements under appropriate experimentalconditions.

Xe g) + F2 g) XeF2 s) excess)Xe g) + 2F2 g XeF4 s) 1:5 ratio)Xe g) + 3F2 g) XeF6 s) 1:20 ratio)XeF6 can also be prepared by the interaction of XeF4 and O2F2. XeF4 +O2F2XeF6 + O2

Key points:Xenon fluorides are strong oxidizing agentsXenon fluorides react with strong Lewis acids

They are readily hydrolyzedeven by traces of WATER.For example, XeF2 ishydrolyzed to give Xe, HF and O2.2XeF2 s) + 2H2O l) 2Xe g) + 4 HF + O2

As with the interhalogens, the xenon fluorides react with strong Lewis acidstoform xenon fluoride cations:

An example is the reaction with FLUORIDE ION ACCEPTORSto form cationicspecies and fluoride ion donors to form fluoroanions.XeF2 + PF5 [XeF]+[PF6]XeF4 + SbF5 [XeF3]+[SbF6]XeF6 + MF M+[XeF7] M = Na, K, Rb or Cs)


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Xenon-oxygen compoundsPreparation:Xenon oxides are endergonic compounds and cannot beprepared by direct interaction of the elements, so we need to look for an

indirect method. The oxides and oxofluorides are prepared by thehydrolysis of xenon fluorides:

Hydrolysis of XeF4 and XeF6 with watergives Xe03.6XeF4 + 12 H2O 4Xe + 2Xe03 + 24 HF + 3 O2XeF6 + 3 H2O XeO3 + 6 HFPartial hydrolysis of XeF6gives oxyfluorides, XeOF4 and XeO2F2.XeF6 + H2O XeOF4 + 2 HFXeF6 + 2 H2O XeO2F2 + 4HF

XeO3 is a colorless explosive solid and has a pyramidal molecular structure

XeOF4 is a colorless volatile liquid and has a square pyramidal molecular

structure

XeOF5 is a pentagonal pyramid.

USES Heliumis a non-inflammable and light gas. Hence, it is used in fillingballoons for meteorological observations. It is also used ingas-cooled nuclear

reactors.Liquid helium (b.p. 4.2 K) finds use as cryogenic agent for carrying outvarious experiments at low temperatures. It is used to produce and sustain

powerful superconducting magnets which form an essential part of modern NMRspectrometers and Magnetic Resonance Imaging (MRI) systems for clinical

diagnosis. It is used as a diluent for oxygen in modern diving apparatus becauseof its very low solubility in blood.

Neonis used in discharge tubes and fluorescent bulbs for advertisement displayArgonis used mainly to provide an inert atmosphere in high temperature

Metallurgical processes arc welding and for filling electric bulbs.

It is also used in the laboratory for handling substances that are air-sensitive.


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Group 17ElementsFluorine, chlorine, bromine, iodine and astatine are members of Group 17. They are called

halogens, meaning: salt producers). There is a regular gradation in their physical and

chemical properties._____________________________________________________________________________________________

OCCURRENCEGenerally, these occur as metal halides in rocks and sea water.

Fluorineis present as insoluble fluorides fluorspar CaF2, cryolite Na3AlF6 andfluoroapatite 3Ca3 PO4)2.CaF2)Chlorineis seen as sodium chloride and carnallite, KCl.MgCl2.6H2O.Iodineis present in sea weeds and Chile saltpetre sodium iodate)_____________________________________________________________________________________________

Halogen Fluorine Chlorine Bromine Iodine

Electronegativity 4 3.2 3 2.6

Electron

Affinity

328 349 325 295

Bond Enthalpy 158 242 192 151

The electronic configuration ns 2np 5)showsthat theelements are one electron short of the next noble gas.

This is the reason for their maximum effective nuclearchargein the periods, which further renders them these properties: Smallestatomic radiiin their respective periods Very highionization enthalpy Maximum negativeelectron gain enthalpy

(NOTE: Electron gain enthalpy of the elements of the group becomes less negativedown the group. However,the negative electron gain enthalpy of fluorine is less

than that of chlorine. Due to small size of fluorine atom, there are strong interelectronicrepulsions in the relatively small 2p orbitals of fluorine and thus, the incoming electron does not

experience much attraction. This anomaly is also reflected in theX-X bond dissociation

enthalpies, where the same reason applies.)

Why is fluorine a better oxidizing agent than chlorine, in spite of its lower

electron gain enthalpy?It is due to

(i) low enthalpy of dissociation of F-F bond

(ii) high hydration enthalpy of F____________________________________________________________________________________________________________________________________________________________


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Oxidation states and trends in chemical reactivity__________________________________________________________________

All the halogens exhibit1 oxidation state. Chlorine, bromine and iodine exhibit + 1, +3,

+ 5 and + 7 oxidation states also. (Fluorine doesnt exhibit other oxidation states primarilybecause it is the most electronegative element and also because it cant expand its octet due

to lack of d orbitals.)The higher oxidation states of chlorine, bromine and iodine are observed in their

compounds with the small and highly electronegative fluorine and oxygen atoms. e.g.,

in interhalogens, oxidesand oxoacids.F2 is the strongest oxidizing halogen and it oxidizes other halide ionsin solution. Theirhighly +standard electrode red.) potentialsalso illustrate this.Their reactions with water also show this:

Fluorine oxidizes water to oxygen Chlorine and bromine react with water to form corresponding hydrohalic andhypohalous acids. Iodine is too weak an oxidizer to react with water. In fact, Ican be oxidized byoxygen in acidic mediumwhich is just the reverse of the reaction observed with

fluorine.

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Anomalous behavior of fluorine Ionization enthalpy Electronegativity, Enthalpy of bond dissociation

andelectrode potentials are allHIGHER

Ionic andcovalent radii, Melting and boiling points Electron gain enthalpyareLOWER

This anomaly is due to:

Small size, highest Electronegativity, low F-F bond dissociation enthalpy, Non availability of d orbitals in valence shell. Form exothermic compounds(due to the small and strong bond formed

by it with other elements).

Forms only one oxoacid: HOF(hypofluorous acid), while other halogensform a number of oxoacids.


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HYDROGEN HALIDESHFis a liquid due to strong hydrogen bonding. Others are gases.

They all react with hydrogento give hydrogen halidesThe ACIDIC STRENGTH(Ka) of the acids is in the order: HF < HCl < HBr < HI.So, STABILITYis in the reverse order (dissHand bond length).

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HALOGEN OXIDESHalogens form many oxides with oxygen but most of them are unstable.

Stability of oxides formed by halogens, is in the order: I > Cl > Br.The higher oxidesof halogens tend to be more stablethan the lower ones.

Fluorineforms two oxides OF2and O2F2. Only OF2 is thermally stable. O2F2is used to remove plutonium from spent nuclear fuelby oxidizing it

toPuF6

Chlorineoxides, Cl2O, ClO2, Cl2O6and Cl2O7 are explosively oxidizing. ClO2is used as a bleaching agentin paper industry and in water

treatment.

Bromineoxides,Br2O, BrO2,and BrO3are the least stable halogen oxides. Iodineoxides, I2O4, I2O5, and I2O7decompose on heating. I2O5 is used in the estimation of carbon monoxide.

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METAL HALIDESThe ionic characterof the halides: MF >MCl > MBr > MIThe halides in higher oxidation state will be more covalentthan the one inlower oxidation state.

For e.g., SnCl4, UF6 are more covalent than SnCl2,UF4._____________________________________________________________________________________________________________________


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CHLORINEPreparation

By heating HCl on MnO2.4NaCl + MnO2 + 4H2SO4 MnCl2 + 4NaHSO4 + 2H2O + Cl2 By the action of HCl on KMnO4.2KMnO4 + 16HCl 2KCl + 2MnCl2 + 8H2O + 5Cl2

Manufacture of chlorine eacons process:oxidation of HClin the presence of CuCl2 Electrolysis of brine: Chlorine is liberated at anode.

PropertiesGreenish yellow gas

Heavier than air

Soluble in water

Pungent smelling

AmmoniaWith excess ammonia, chlorine gives N2 and NH4Cl8NH3 + 3Cl2 6NH4Cl + N2With excess chlorine, NCl3 explosive)formsNH3 + 3Cl2 NCl3 + 3HClAlkali

With cold and dilutealkalis: chloride and hypochlorite2NaOH + Cl2 NaCl+ NaOCl + H2O

With hot and concentratedalkalis: gives chloride and chlorate.6 NaOH + 3Cl2 5NaCl +NaClO3 +3H2O

With dry slaked limeit gives bleaching powder.2 Ca OH)2+ 2Cl2 Ca(OCl)2 + CaCl2 + 2H2O

The composition of bleaching powder is Ca OCl)2.CaCl2.Ca OH)2.2H2O.


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Chlorine waterOn standingloses its yellow colordue to the formation of HCl and HOCl.

Bleaching propertiesis due to the release of nascent (O)as HOCl decomposes

Oxidizing properties:(i) ferrous to ferric(ii) sulphite to sulphate,(iii) sulphur dioxide to sulphuric acid(iv) iodine to iodic acid (HIO3)(v) Colored substance Colorless substance

USES bleaching wood pulp (manufacture of paper) bleaching cotton extraction of gold and platinum preparation of poisonous gases :

phosgene COCl2),tear gas (CCl3NO2), mustard gas__________________________________________________________________

HYDROGEN CHLORIDEPreparationNaCl + H2SO4 NaHSO4 + HClNaHSO4 + NaCl Na2SO4 + HClAqua regia(3:1of HCl and HNO3) dissolves gold, platinum.Except copper, silver, goldand those metals below hydrogen in the reactivityseries, all other metals react with HClIron in HCl forms FeCl2 and not FeCl3Hydrochloric acid decomposes salts of weaker acids

NaHCO3 + HCl NaCl + H2O + CO2

Na2SO3 + 2HCl 2NaCl + H2O + SO2USES:Manufacture of glucose (from corn starch)

Extracting glue from bones and purifying bone black,


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OXOACIDSHalic I) acid(Hypohalous acid)

Halic III) acid(Halous acid)

Halic V) acid(Halic acid)

Halic VII) acid(Perhalic acid)

HOF ------------ ------------- ------------HOCl HOCIO HOCIO2 HOCIO3

HOBr ------------ HOBrO2 HOBrO3

HOI ------------ HOIO2 HOIO3

INTERHALOGEN COMPOUNDSXX, XX3 , XX5and XX7

Preparation By direct combinationor By the action of halogenon lower interhalogenscompounds.I2 + Cl2 2ICl equimolar)I2 + 3Cl2 2ICl3 excess)Cl2 + F2 2ClF equal volume)Cl2 + 3F2 2ClF3 excess)Br2 + 3F2 2BrF3 diluted with water)Br2 + 5F2 2BrF5 excess)

Properties Are diamagneticin nature. They are volatile solids or liquids except CIFwhich is a gas. Interhalogens compounds are more reactive than halogens(except

fluorine). (This is because XXbond in interhalogens is weaker than XX

bond in halogens except FF bond.)

Hydrolysis

These undergo hydrolysis giving halide ion derived from the smaller halogenandhypohalite ( when XX),

halite ( whenXX3),

halate (when XX5) or

perhalate (when XX7) derived from the larger halogen.e.g. 2 XX + H O HX + HOX


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USES:These compounds can be used as non aqueous solvents.Interhalogen compounds are very useful fluorinating agents. ClF3 and BrF3 are

used for the production of UF6 in the enrichment of 235U.


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Group 16 Chalcogens: ore-forming

ElementsOxygen, sulphur, selenium, tellurium and polonium constitute Group 16.

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OccurrenceOrder of elements in the earthscrust: O Si Al Fe Ca Na K MgMost copper minerals contain either oxygenor sulphur

Oxygen is the most abundant of all the elements on earth (46.6% by mass)Combined sulphur exists primarily asSulphates:

gypsum CaSO4.2H2O, Epsom salt MgSO4.7H2O, baryte BaSO4

Sulphides: galena PbS, zinc blende ZnS, Copper pyrites CuFeS2.

Selenium and tellurium are also found as metal selenides and tellurides in sulphide ores.

Polonium occurs in nature as a decay product of thorium and uranium minerals.

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Physical propertiesThe elements of Group16 have ns2np4general electronic configuration.

Elements of this group have lower ionization enthalpy valuescompared to those ofGroup15 or Group17[reason: This is due to the fact that Group 15 elements have extra stable half-filled

p orbitals electronic configurations.]

Oxygen has the highest electronegativity value; the metallic character increases from

oxygen to polonium.

The size of oxygen atom is very small and has the leastnegative electron gain enthalpy.Order: O


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Anomalous behavior of oxygenDue to its small size, high electronegativity and absence of d orbitals. The effects are:

Hydrogen bondingin H2O which is not found in H2S. The absence of d orbitals in oxygen limits its covalencyto four

Hydrides of the type H2ETheir acidic character(diss, constant Ka) H2O < H2S < H2Se < H2Te < H2Po[The increase in acidic character can be explained in terms of decrease in bond (HE)

dissociation enthalpy down the group.]

So,Thermal stabilityof hydrides: H2O > H2S > H2Se > H2Te > H2PoReducing property and this character increases from H2S to H2Te.Bond angle () H2O > H2S > H2Se > H2Te > H2Poa 1.81016 1.3107 1.3104 2.3103

Oxidesof the type EO2 and EO3Both types of oxides are acidicin nature.Ozone (O3) and sulphur dioxide (SO2) are gasesSelenium dioxide SeO2) is solid.Reducing propertyof dioxide decreases from SO2 to TeO2;SO2 is reducing while TeO2 is an oxidising agent

Halidesof the type, EX6, EX4 and EX2The stability of the halidesdecreases in the order F> Cl> Br> IHexahalidesHexafluorides are the only stable halides. All are gaseous.

SF6 is exceptionally stable for steric reasons.

TetrafluoridesSF4 is a gas, SeF4 a liquid and TeF4 a solid.

sp3d hybridization andtrigonal bipyramidal structure and see-sawgeometry.

DihalidesSelenium cannotform dihalidessp3hybridization andtetrahedralstructure.

MonohalidesDimericin nature. Egs: S2F2, S2Cl2, S2Br2, Se2Cl2 and Se2Br2.

Undergo disproportionation:2Se2Cl2SeCl4 + 3Se


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DioxygenPreparationin the laboratory:

By heating oxygen rich salts such as chlorates, nitrates and permanganates.

By the thermal decomposition of the oxides of metals low in the electrochemicalseries and higher oxides of some metals.

o 2Ag2O s) 4 Ag s) + O2g)o 2HgO s)2 Hg l) + O2g)o 2PbO2s)2 PbO s) + O2g)

Decomposition of Hydrogen peroxide by catalysts such as finely divided metals andmanganese dioxide. 2H2O2(aq) 2H2O(1) + O2(g)

Electrolysis of water Fractional distillationProperties

Dioxygen is a colourless and odourless gas.Its solubility in water is to the extent of 3.08 cm3 in 100 cm3 water at 293 K which is just

sufficient for the vital support of marine and aquatic life.

It liquefies at 90 K and freezes at 55 K.

It has three stable isotopes: 16O, 17O and 18O.

Molecular oxygen, O2 is unique in being paramagnetic (in spite of having even no. of e-)

bond dissociation enthalpy of oxgyen-oxygen double bond is high (493.4 kJ mol1).

Oxides can be simple (e.g., MgO, Al2O3 ) or mixed (Pb3O4, Fe3O4).

acidic oxide (e.g., SO2, Cl2O7, CO2, N2O5 ).

non-metal oxides are acidic but oxides of some metals in high oxidation state also have

acidic character (e.g., Mn2O7, CrO3, V2O5).

basic oxides (e.g., Na2O, CaO, BaO). In general, metallic oxides are basic.

amphoteric oxides or neutral oxides are CO, Al2O3, NO and N2O.

Uses Oxyacetylenewelding, Manufacture of steel. Rocket fuel: hydrazine in liquid oxygen


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Ozone - An allotrope of oxygen

PreparationSilent electric discharge of oxygen

[the formation of ozone from oxygen is an endothermicprocess]PropertiesO-O bond lengths are identical (128pm), bond angle is 117o.Pure ozone is a pale bluegas, dark blueliquid and violet-blacksolid.Ozone has a characteristic smellAbove 100 ppm, breathing difficulty, headache and nausea results.

Ozone is thermodynamically unstablewith respect to oxygen (explosive).As it liberates atoms of nascent oxygen, it acts as a powerful oxidizing agent

Lead sulphide to lead sulphate

Iodide ions to iodine

Estimating O3gas:Iodine liberated, when ozonereacts with potassium iodidesolution can be titrated against

a standard solution of sodium thiosulphate.

Depletion of ozone:Nitric oxide from the exhaust systems of supersonic jets

Freons, which are used in aerosol sprays and as refrigerants

Uses Germicide, disinfectant For sterilizing water. bleaching oils, ivory, flour, starch, Oxidizing agent in the manufacture of potassium permanganate.


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SulphurAllotropic Forms

The allotropes exist in equilibrium @ 369K(transition temperature)Rhombic sulphur -sulphur)

Yellowin color The stable format rtp Prepared by by evaporating the

solution of roll sulphur in CS2

Monoclinic sulphur -sulphur) It is soluble in CS2. prepared by melting rhombicsulphurand cooling, till crust is

formed as needle shaped

Both rhombic and monoclinic sulphur have S8molecules. Shape-puckered crown.Both are insoluble in water but soluble in organic solvents

Cyclo-S6, has chairformAt high temperatures, sulphur exists as S2and is paramagneticlike O2.

[Two unpaired electrons in the antibonding * orbitals]Sulphur Dioxide

Preparation Burn sulphur in air Treat sulphite with acid (protons

from acid are oxidized to water) Roasting sulphide ores

Properties Colorless gas Pungent smell Highly soluble in water

It reacts readily with sodium hydroxidesolution, forming sodium sulphite, and thensodium hydrogen sulphite.Sulphur dioxide reacts with chlorinein the presence of charcoal(which acts as acatalyst) to give sulphuryl chloride, SO2Cl2.When moist, sulphur dioxidebehaves as a reducing agent.

Converts iron (III) ions to iron (II) ions

Decolorizes KMnO4solution SO2DetectionUses:

refining petroleum and sugar bleaching wool and silk anti-chlor Disinfectant and preservative.


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Oxoacids of Sulphur

ManufactureManufactured by the Contact Process low temperature and high pressure) 2bar /720 K.

Burning of sulphur or sulphide ores in air to generate SO2. conversion of SO2to SO3 in the presence of (V2O5), Absorption of SO3 in H2SO4to give Oleum (H2S2O7).

PropertiesThe acid forms two series of salts: normal sulphates and acid sulphates

(hydrogen sulphate).

Concentrated sulphuric acid is a strong dehydrating agent.moderately strong oxidizing agent.

Uses: Manufacture of fertilizers(e.g., ammonium sulphate, superphosphate). petroleum refining manufacture of pigments, paints and dyestuff detergent industry cleansing metals storage batteries nitrocelluloseproducts


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Group 15

ElementsGroup 15 includes nitrogen, phosphorus, arsenic, antimony and bismuth.

N and P are non-metals, Ar and Sb are metalloids and Bi is a typical metal.

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OccurenceNitrogen occurs as NaNO3 (Chile saltpetre) and KNO3 (Indian saltpetre)

Phosphorus occurs in apatite minerals: Ca9(PO4)6. CaX2 (e.g., fluoroapatite Ca9 (PO4)6. CaF2)

Arsenic, antimony and bismuth are found mainly as sulphide minerals.

PropertyThe valence shell e.c. is ns 2np 3.The p orbitals are half-filled, giving it extra stability.Except nitrogen, all the elements show allotropy.The melting pointincreases up to arsenicand then decreases up to bismuthOxidation states and trends in chemical reactivityThe common oxidation states are3, +3 and +5.

3 Oxidation state becomes less common down the group due to decreased

electronegativity. Bismuthdoesnthave compounds with3oxidation state.The stability of +5 oxidation state decreases and that of +3 state increases (due to inert

pair effect) down the group.

In the case of nitrogen, all oxidation states from +1 to +4tend to disproportionate in acidsolution. 3HNO2 HNO3 + H2O + 2NOIn case of phosphorus nearly all intermediate oxidation states disproportionate into +5 and3both in alkali and acid.

Anomalous properties of nitrogen Nitrogen has unique ability to form p-p multiple bondswith itself and others.

Thus, nitrogen exists as a diatomic molecule with a triple bond 941.4 kJ mol1.

The single NN bond is weaker than the single PP bond because of highinterelectronic repulsion of the non-bonding electrons, owing to the small bond

length.

Due to the absence of d orbitals in its valence shell:

covalency is restricted to 4 Cannot form d p bond as the heavier elements can e.g., R3P = O or R3P

= CH2 or d d bondas in P(C2H5)3 and As(C6H5)3.


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HydridesThe stabilityof hydrides decreases from NH3to BiH3 (bond dissociation enthalpy)So, the reducing characterof the hydrides increases. BiH3 is the strongest reducing agent.Basicityalso decreases in the order NH3 > PH3 > AsH3 > SbH3 > BiH3.Oxides: E2O3 and E2O5.

The oxide in the higher oxidation state is more acidic.N2O3 and P2O3 are acidic; As2O3 and Sb2O3 areamphoteric; BiO3 is basicHalides: EX3 and EX5.Pentahalides are more covalentthan trihalides.Only stabletrihalide of nitrogen is NF3Only ionic halide is BiF3

Dinitrogen

Preparation Fractional distillation of air. Treating an aqueous solution of ammonium chloridewith sodium nitrite.NH4CI+ NaNO2 N2+ 2H2O+ NaCl Thermal decomposition of ammonium dichromate.

(NH4)2Cr2O7

N2 + 4H2O + Cr2O3

Very pure nitrogen -thermal decomposition of sodium or barium azide.Ba N3)2 Ba + 3N2PropertiesForms ionic nitrides with metals and covalent nitrides with non metals.

AmmoniaPreparation

Treating ammonium saltswithNaOH/CaO

Habers process:200 atm~ 700 KCatalyst such as iron oxidewith small amounts of K2Oand Al2O3

PropertiesIts aqueous solution is weakly basic due

to the formation of OHions.

It IS a Lewis base. It forms linkage with

metal ions in complex compounds

Used in the detection of metal ions such

as Cu2+, Ag+:

Cu2+ (blue) + 4 NH3[Cu(NH3)4]2(deep blue)


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Oxides of Nitrogen


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Why does NO2 dimerise ?

NO2 contains odd number of valence electrons. It behaves as a typical

odd molecule. On dimerisation, it is converted to stable N2O4 molecule

with even number of electrons.

Nitric Acid

Preparation heating Nitrateand concentratedH2SO4 Ostwalds process: Platinum-Rhodiumcatalyst, NH3NONO2HNO3

PropertiesPlanar moleculeConcentrated nitric acid is a strong oxidising agent:I2+ 10 HNO32 HIO3+ 10 NO2 + 4 H2OC+ 4 HNO3CO2+ 2 H2O + 4 NO2S8 + 48 HNO3 (conc.)8 H2SO4+ 48 NO2 + 16 H2OP4 + 20 HNO3 (conc.)4 H3PO4+ 20 NO2 + 4 H2OThe products of oxidation depend upon the concentration of the acid,

temperature and the nature of the material undergoing oxidation.Copper reacts withdilute nitric acidto giveNO and withconcentrated acid to giveNO2Zinc reacts withdilute nitric acid to giveN2O and withconcentrated acid to giveNO2.Cr and Aldo not reactdue to the formation of a passive oxidefilmon the surface.

Brown Ring Test:Based on the ability ofFe2+to reduce nitrates to nitric oxide,which reacts with Fe2+to form a brown colored complex.

Uses: fertilizers and explosives nitroglycerin, trinitrotoluene pickling of stainless steel, etching of metals oxidizer in rocket fuels


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Phosphorus

Allotropic Forms-white, red and blackWhite phosphorusTranslucent whitewaxy solidIt is poisonousInsoluble in water but soluble in CS2Glowsin darkIt gives PH3and Na2HPO2 withNaOH

Less stable/more reactivebecauseof angular strainin P4 molecule 60Catches firein air to give densewhite fumesofP4O10.It consists of discrete P4 molecules

Red phosphorusPossesses iron grey lustreNonpoisonousInsolublein water as well as in CS2It does not glow in the dark.When red phosphorus is heated,black phosphorusis formed.

Red phosphorus is much lessreactivethan white phosphorusBy heating white phosphorus@ 573KPolymeric-P4tetrahedra linked

together

Black phosphorus has two forms -blackphosphorus and -blackphosphorus.red phosphorus ---heat@803K -black phosphorus.white phosphorusheat@473K-Black phosphorus

PhosphinePreparation

reaction of calcium phosphidewith water or dilute HCl. heating white phosphorus with concentrated NaOH solution

It is non inflammable when pure, but becomes inflammable owing to the presence of

P2H4 orP4vapors.To purify it from the impurities, it is absorbed in HI to form (PH4I) treated with KOH


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Properties Colorless gas with a rotten fish smelland is highly poisonous. It explodesin contact with oxidizing agents It is slightly soluble in water. PH3 in water decomposesin presence of light giving red

phosphorus and H2.

With copper sulphateor mercuric chloridesolution, their phosphidesform Phosphine is weakly basic like ammonia

Uses:

Holmess signals. calcium carbide CaC2 and calcium phosphide Ca3P2 It is also used in smoke screens.

Phosphorus Halides

Phosphorus TrichloridePreparation passing dry chlorineover heated white phosphorus.

P4 + 6Cl24PCl3 action of thionyl chloridewith white phosphorus.P4 + 8SOCl2 4PCl3 + 4SO2 + 2S2Cl2Properties colourless PCl3 fume in moisture as it hydrolyses giving fumes of HCl and H3PO3

Phosphorus PentachloridePreparation passing excessdry chlorineover heated white phosphorus.

P4 + 10Cl2 4PCl5 action of SO2Cl2with white phosphorus.P4 + 10SO2Cl2 4PCl5 + 10SO2Properties yellowish whitepowder hydrolyses to POCl3and then to H3PO4 When heated, it gives PCl3 and Cl2 Finely divided metals on heating with PCl5 give corresponding chlorides In gaseous and liquid phases, it has a trigonal bipyramidalstructure. The twoaxial bonds are longer than equatorial bonds. (Repulsion) In the solid state it exists as an ionic solid, [PCl4]+[PCl6],

with tetrahedral cation, [PCl4]+ and octahedral anion, [PCl6]


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Oxoacids of Phosphorus

The compositions of the oxoacids are in terms of loss or gain of H2O or O-atom In oxoacids phosphorus is tetrahedrally surroundedby other atoms. All these acids contain one P=O and at least one POH bond.The PP and PH (cant be found together. Acids in +3 oxidation tend to disproportionateto higher and lower oxidation

states. For example, orthophophorous acid (or phosphorous acid) on heating

disproportionate to give orthophosphoric acid (or phosphoric acid) andphosphine.

The acids which contain PH bond have strong reducing properties. Compare Basicity of these acids

Date post:	18-Oct-2015
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P block elements

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