All topics

In Class XI, you have learnt that the p-block elements are placed in groups 13 to 18 of the periodic table. Their valence shell electronic configuration is ns²np^1–6 (except He which has 1s² configuration). The properties of p-block elements like that of others are greatly influenced by atomic sizes, ionisation enthalpy, electron gain enthalpy and electronegativity. The absence of d- orbitals in second period and presence of d or d and f orbitals in heavier elements (starting from third period onwards) have significant effects on the properties of elements. In addition, the presence of all the three types of elements; metals, metalloids and non-metals bring diversification in chemistry of these elements.

Having learnt the chemistry of elements of Groups 13 and 14 of the p-block of periodic table in Class XI, you will learn the chemistry of the elements of subsequent groups in this Unit.

Group 15 ELEMENTS

Group 15 includes nitrogen, phosphorus, arsenic, antimony, bismuth and moscovium. As we go down the group, there is a shift from non- metallic to metallic through metalloidic character. Nitrogen and phosphorus are non-metals, arsenic and antimony metalloids, bismuth and moscovium are typical metals.

Occurrence

Molecular nitrogen comprises 78% by volume of the atmosphere. In the earth’s crust, it occurs as sodium nitrate, NaNO₃ (called Chile saltpetre) and potassium nitrate (Indian saltpetre). It is found in the form of proteins in plants and animals. Phosphorus occurs in minerals of the apatite family, Ca₉(PO₄)₆. CaX₂ (X = F, Cl or OH) (e.g., fluorapatite Ca₉ (PO₄)₆. CaF₂) which are the main components of phosphate rocks. Phosphorus is an essential constituent of animal and plant matter. It is present in bones as well as in living cells. Phosphoproteins are present in milk and eggs. Arsenic, antimony and bismuth are found mainly as sulphide minerals. Moscovium is a synthetic radioactive element. Its symbol is Mc, atomic number 115, atomic mass 289 and electronic configuration [Rn] 5f ¹⁴6d¹⁰7s²7p³. Due to very short half life and availability in very little amount, its chemistry is yet to be established.

Here, except for moscovium, important atomic and physical properties of other elements of this group along with their electronic configurations are given in Table 7.1.

Table 7.1: Atomic and Physical Properties of Group 15 Elements

Trends of some of the atomic, physical and chemical properties of the group are discussed below.

Electronic ConfiguratioN

The valence shell electronic configuration of these elements is ns²np³. The s orbital in these elements is completely filled and p orbitals are half-filled, making their electronic configuration extra stable.

Atomic AND Ionic Radii

Covalent and ionic (in a particular state) radii increase in size down the group. There is a considerable increase in covalent radius from N to P. However, from As to Bi only a small increase in covalent radius is observed. This is due to the presence of completely filled d and/or f orbitals in heavier members.

Ionisation Enthalpy

Ionisation enthalpy decreases down the group due to gradual increase in atomic size. Because of the extra stable half-filled p orbitals electronic configuration and smaller size, the ionisation enthalpy of the group 15 elements is much greater than that of group 14 elements in the corresponding periods. The order of successive ionisation enthalpies, as expected is \(\Delta\)_iH₁ < \(\Delta\)_iH₂ < \(\Delta\)_iH₃ (Table 7.1).

Electronegativity

The electronegativity value, in general, decreases down the group with increasing atomic size. However, amongst the heavier elements, the difference is not that much pronounced.

PHYSICAL Properties

All the elements of this group are polyatomic. Dinitrogen is a diatomic gas while all others are solids. Metallic character increases down the group. Nitrogen and phosphorus are non-metals, arsenic and antimony metalloids and bismuth is a metal. This is due to decrease in ionisation enthalpy and increase in atomic size. The boiling points, in general, increase from top to bottom in the group but the melting point increases upto arsenic and then decreases upto bismuth. Except nitrogen, all the elements show allotropy.

Chemical Properties

Oxidation states and trends in chemical reactivity

The common oxidation states of these elements are –3, +3 and +5. The tendency to exhibit –3 oxidation state decreases down the group due to increase in size and metallic character. In fact last member of the group, bismuth hardly forms any compound in –3 oxidation state. The stability of +5 oxidation state decreases down the group. The only well characterised Bi (V) compound is BiF₅. The stability of +5 oxidation state decreases and that of +3 state increases (due to inert pair effect) down the group. Besides +5 oxidation state, nitrogen exhibits + 1, + 2, + 4 oxidation states also when it reacts with oxygen. However, it does not form compounds in +5 oxidation state with halogens as nitrogen does not have d-orbitals to accommodate electrons from other elements to form bonds. Phosphorus also shows +1 and +4 oxidation states in some oxoacids. In the case of nitrogen, all oxidation states from +1 to +4 tend to disproportionate in acid solution. For example, 

3HNO₂  \(\to\)  HNO₃ + H₂O + 2NO

Similarly, in case of phosphorus nearly all intermediate oxidation states disproportionate into +5 and –3 both in alkali and acid. However

+3 oxidation state in case of arsenic, antimony and bismuth becomes increasingly stable with respect to disproportionation. 

Nitrogen is restricted to a maximum covalency of 4 since 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 PF\(% MathType!MTEF!2!1!+- % feaagKart1ev2aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaWaa0baaSqaai % aaiAdaaeaacqGHsislaaaaaa!37D0! _6^ - \).

Anomalous properties of nitrogen

Nitrogen differs from the rest of the members of this group due to its small size, high electronegativity, high ionisation enthalpy and non-availability of d orbitals. Nitrogen has unique ability to form p\(\pi\) -p\(\pi\) 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 p\(\pi\) -p\(\pi\) 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. However, 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. Another factor which affects the chemistry of nitrogen is the absence of d orbitals in its valence shell. Besides restricting its covalency to four, nitrogen cannot form d\(\pi\) –p\(\pi\) bond as the heavier elements can e.g., R₃P = O or R₃P = CH₂ (R = alkyl group). Phosphorus and arsenic can form d\(\pi\) –d\(\pi\) bond also with transition metals when their compounds like P(C₂H₅)₃ and As(C₆H₅)₃ act as ligands.

(1) Reactivity towards hydrogen: All the elements of Group 15 form hydrides of the type EH₃ where E = N, P, As, Sb or Bi. Some of the properties of these hydrides are shown in Table 7.2. The hydrides show regular gradation in their properties.

        Table 7.2: Properties of Hydrides of Group 15 Elements                                                             

The stability of hydrides decreases from NH₃ to BiH₃ which can be observed from their bond dissociation enthalpy. Consequently, the reducing character of the hydrides increases. Ammonia is only a mild reducing agent while BiH₃ is the strongest reducing agent amongst all the hydrides. Basicity also decreases in the order NH₃ > PH₃ > AsH₃ > SbH₃ > BiH₃. Due to high electronegativity and small size of nitrogen, NH₃ exhibits hydrogen bonding in solid as well as liquid state. Because of this, it has higher melting and boiling points than that of PH₃.

(2) 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.

(3) Reactivity towards halogens: These elements react to form two series of halides: EX₃ and EX₅. Nitrogen does not form pentahalide due to non-availability of the d orbitals in its valence shell. Pentahalides are more covalent than trihalides. This is due to the fact that in pentahalides +5 oxidation state exists while in the case of trihalides +3 oxidation state exists. Since elements in +5 oxidation state will have more polarising power than in +3 oxidation state, the covalent character of bonds is more in pentahalides. All the trihalides of these elements except those of nitrogen are stable. In case of nitrogen, only NF₃ is known to be stable. Trihalides except BiF₃ are predominantly covalent in nature.

(4) Reactivity towards metals: All these elements react with metals to form their binary compounds exhibiting –3 oxidation state, such as, Ca₃N₂ (calcium nitride) Ca₃P₂ (calcium phosphide), Na₃As (sodium arsenide), Zn₃Sb₂ (zinc antimonide) and Mg₃Bi₂ (magnesium bismuthide).

EXAMPLE 1

Though nitrogen exhibits +5 oxidation state, it does not form pentahalide. Give reason.

SOLUTION

Nitrogen with n = 2, has s and p orbitals only. It does not have d orbitals to expand its covalence beyond four. That is why it does not form pentahalide.

EXAMPLE 2

PH₃ has lower boiling point than NH₃. Why?

SOLUTION

Unlike NH₃, PH₃ molecules are not associated through hydrogen bonding in liquid state. That is why the boiling point of PH₃ is lower than NH₃.

INTEXT QUESTION

Why are pentahalides of P, As, Sb and Bi more covalent than their trihalides?

Why is BiH₃ the strongest reducing agent amongst all the hydrides of Group 15 elements ?

Dinitrogen

Preparation

Dinitrogen is produced commercially by the liquefaction and fractional distillation of air. Liquid dinitrogen (b.p. 77.2 K) distils out first leaving behind liquid oxygen (b.p. 90 K).

In the laboratory, dinitrogen is prepared by treating an aqueous solution of ammonium chloride with sodium nitrite.

NH₄CI(aq) + NaNO₂(aq) \(\to\) N₂(g) + 2H₂O(l) + NaCl (aq)

Small amounts of NO and HNO₃ are also formed in this reaction; these impurities can be removed by passing the gas through aqueous sulphuric acid containing potassium dichromate. It can also be obtained by the thermal decomposition of ammonium dichromate.

Very pure nitrogen can be obtained by the thermal decomposition of sodium or barium azide.

Ba(N₃)₂ \(\to\) Ba + 3N₂

Properties

Dinitrogen is a colourless, odourless, tasteless and non-toxic gas. Nitrogen atom has two stable isotopes: ¹⁴N and ¹⁵N. It has a very low solubility in water (23.2 cm³ per litre of water at 273 K and 1 bar pressure) and low freezing and boiling points (Table 7.1).

Dinitrogen is rather inert at room temperature because of the high bond enthalpy of N \(% MathType!MTEF!2!1!+- % feaagKart1ev2aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaeyyyIOlaaa!37BF! \equiv \) N bond. Reactivity, however, increases rapidly with rise in temperature. At higher temperatures, it directly combines with some metals to form predominantly ionic nitrides and with non-metals, covalent nitrides. A few typical reactions are:

It combines with hydrogen at about 773 K in the presence of a catalyst (Haber’s Process) to form ammonia:

Dinitrogen combines with dioxygen only at very high temperature (at about 2000 K) to form nitric oxide, NO.

Uses: The main use of dinitrogen is in the manufacture of ammonia and other industrial chemicals containing nitrogen, (e.g., calcium cyanamide). It also finds use where an inert atmosphere is required (e.g., in iron and steel industry, inert diluent for reactive chemicals). Liquid dinitrogen is used as a refrigerant to preserve biological materials, food items and in cryosurgery.

EXAMPLE 3

Write the reaction of thermal decomposition of sodium azide.

SOLUTION Thermal decomposition of sodium azide gives dinitrogen gas.

2NaN₃ \(\to\) 2Na + 3N₂

INTEXT QUESTION

Why is N₂ less reactive at room temperature?

Ammonia

Preparation

Ammonia is present in small quantities in air and soil where it is formed by the decay of nitrogenous organic matter e.g., urea.

On a small scale ammonia is obtained from ammonium salts which decompose when treated with caustic soda or calcium hydroxide.

2NH₄Cl + Ca(OH)₂ \(\to\) 2NH₃ + 2H₂O + CaCl₂

(NH₄)₂ SO₄ + 2NaOH \(\to\) 2NH₃ + 2H₂O + Na₂SO₄

On a large scale, ammonia is manufactured by Haber’s process.

N₂ (g) + 3H₂ (g) Ö 2NH₃ (g); 

In accordance with Le Chatelier’s principle, high pressure would favour the formation of ammonia. The optimum conditions for the production of ammonia are a pressure of 200 × 10⁵ Pa (about 200 atm), a temperature of ~ 700 K and the use of a catalyst such as iron oxide with small amounts of K₂O and Al₂O₃ to increase the rate of attainment of equilibrium. The flow chart for the production of ammonia is shown in Fig. 7.1. Earlier, iron was used as a catalyst with molybdenum as a promoter.

Fig. 7.1. Flow chart for the manufacture of ammonia

Properties

Ammonia is a colourless gas with a pungent odour. Its freezing and boiling points are 198.4 and 239.7 K respectively. In the solid and liquid states, it is associated through hydrogen bonds as in the case of water and that accounts for its higher melting and boiling points than expected on the basis of its molecular mass. The ammonia molecule is trigonal pyramidal with the nitrogen atom at the apex. It has three bond pairs and one lone pair of electrons as shown in the structure.

Ammonia gas is highly soluble in water. Its aqueous solution is weakly basic due to the formation of OH^– ions.

It forms ammonium salts with acids, e.g., NH₄Cl, (NH₄)₂ SO₄, etc. As a weak base, it precipitates the hydroxides (hydrated oxides in case of some metals) of many metals from their salt solutions. For example,

ZnSO₄ (aq) + 2NH₄OH (aq )  \(\to\) Zn (OH)₂ (s) + (NH₄)  SO₄ (aq)

                                                        ( white ppt )     

\(% MathType!MTEF!2!1!+- % feaagKart1ev2aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGceaqabeaacaWGgb % GaamyzaiaadoeacaWGSbWaaSbaaSqaaiaaiodaaeqaaOWaaeWaaeaa % caWGHbGaamyCaaGaayjkaiaawMcaaiabgUcaRiaad6eacaWGibWaaS % baaSqaaiaaisdaaeqaaOGaam4taiaadIeadaqadaqaaiaadggacaWG % XbaacaGLOaGaayzkaaGaeyOKH4QaamOraiaadwgadaWgaaWcbaGaaG % OmaaqabaGccaWGpbWaaSbaaSqaaiaaiodaaeqaaOGaaiOlaiaadIha % caWGibWaaSbaaSqaaiaaikdaaeqaaOGaam4tamaabmaabaGaam4Caa % GaayjkaiaawMcaaiabgUcaRiaad6eacaWGibWaaSbaaSqaaiaaisda % aeqaaOGaam4qaiaadYgadaqadaqaaiaadggacaWGXbaacaGLOaGaay % zkaaaabaGaaGPaVlaaykW7caaMc8UaaGPaVlaaykW7caaMc8UaaGPa % VlaaykW7caaMc8UaaGPaVlaaykW7caaMc8UaaGPaVlaaykW7caaMc8 % UaaGPaVlaaykW7caaMc8UaaGPaVlaaykW7caaMc8UaaGPaVlaaykW7 % caaMc8UaaGPaVlaaykW7caaMc8UaaGPaVlaaykW7caaMc8UaaGPaVl % aaykW7caaMc8UaaGPaVlaaykW7caaMc8UaaGPaVlaaykW7caaMc8Ua % aGPaVlaaykW7caaMc8UaaGPaVlaaykW7caaMc8UaaGPaVlaaykW7ca % aMc8UaaGPaVlaaykW7caaMc8UaaGPaVlaaykW7caaMc8UaaGPaVlaa % ykW7caaMc8UaaGPaVlaaykW7caaMc8UaaGPaVlaaykW7caaMc8UaaG % PaVlaaykW7caaMc8UaaGPaVlaaykW7caaMc8UaaGPaVlaaykW7daqa % daqaaiaadkgacaWGYbGaam4BaiaadEhacaWGUbGaaGPaVlaaykW7ca % WGWbGaamiCaiaadshaaiaawIcacaGLPaaaaaaa!D5C1! \begin{array}{l} FeC{l_3}\left( {aq} \right) + N{H_4}OH\left( {aq} \right) \to F{e_2}{O_3}.x{H_2}O\left( s \right) + N{H_4}Cl\left( {aq} \right)\\ \,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\left( {brown\,\,ppt} \right) \end{array}\)

The presence of a lone pair of electrons on the nitrogen atom of the ammonia molecule makes it a Lewis base. It donates the electron pair and forms linkage with metal ions and the formation of such complex compounds finds applications in detection of metal ions such as Cu²⁺, Ag⁺:

\(% MathType!MTEF!2!1!+- % feaagKart1ev2aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaamyqaiaadE % gadaahaaWcbeqaaiabgUcaRaaakmaabmaabaGaamyyaiaadghaaiaa % wIcacaGLPaaacqGHRaWkcaWGdbGaamiBamaaCaaaleqabaGaeyOeI0 % caaOWaaeWaaeaacaWGHbGaamyCaaGaayjkaiaawMcaaiabgkziUkaa % dgeacaWGNbGaam4qaiaadYgadaqadaqaaiaadohaaiaawIcacaGLPa % aaaaa!4B23! A{g^ + }\left( {aq} \right) + C{l^ - }\left( {aq} \right) \to AgCl\left( s \right)\)

(colourless)                  (white ppt)

AgCl (s) + 2NH₃ (aq) \(\to\) [Ag (NH₃)₂ ] Cl (aq)

(white ppt)                     (colourless)

Uses: Ammonia is used to produce various nitrogenous fertilisers (ammonium nitrate, urea, ammonium phosphate and ammonium sulphate) and in the manufacture of some inorganic nitrogen compounds, the most important one being nitric acid. Liquid ammonia is also used as a refrigerant.

EXAMPLE 4

Why does NH₃ act as a Lewis base ?

SOLUTION

INTEXT QUESTION

Mention the conditions required to maximise the yield of ammonia.

How does ammonia react with a solution of Cu²⁺?

Oxides of Nitrogen

Nitrogen forms a number of oxides in different oxidation states. The names, formulas, preparation and physical appearance of these oxides are given in Table 7.3.

                             Table 7.3: Oxides of Nitrogen

Lewis dot main resonance structures and bond parameters of oxides are given in Table 7.4.

                Table 7.4: Structures of Oxides of Nitrogen

EXAMPLE 5

Why does NO₂ dimerise ?

SOLUTION

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.

INTEXT QUESTION

What is the covalence of nitrogen in N₂O₅ ?

Nitric Acid

Nitrogen forms oxoacids such as H₂N₂O₂ (hyponitrous acid), HNO₂

(nitrous acid) and HNO₃ (nitric acid). Amongst them HNO₃ is the most important.

Preparation

In the laboratory, nitric acid is prepared by heating KNO₃ or NaNO₃ and concentrated H₂SO₄ in a glass retort.

NaNO₃ + H₂SO₄ \(\to\) NaHSO₄ + HNO₃

On a large scale it is prepared mainly by Ostwald’s process.

This method is based upon catalytic oxidation of NH₃ by atmospheric oxygen.

Nitric oxide thus formed combines with oxygen giving NO₂.

2NO (g ) + O₂ (g) 2NO₂ (g)

Nitrogen dioxide so formed, dissolves in water to give HNO₃.

3NO₂ (g) \(\to\) H₂O (l) \(\to\) 2HNO₃ (aq) + NO (g)

NO thus formed is recycled and the aqueous HNO₃ can be concentrated by distillation upto ~ 68% by mass. Further concentration to 98% can be achieved by dehydration with concentrated H₂SO₄.

Properties

It is a colourless liquid (f.p. 231.4 K and b.p. 355.6 K). Laboratory grade nitric acid contains ~ 68% of the HNO₃ by mass and has a specific gravity of 1.504.

In the gaseous state, HNO₃ exists as a planar molecule with the structure as shown.

In aqueous solution, nitric acid behaves as a strong acid giving hydronium and nitrate ions.

\(% MathType!MTEF!2!1!+- % feaagKart1ev2aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaamisaiaad6 % eacaWGpbWaaSbaaSqaaiaaiodaaeqaaOGaaiikaiaadggacaWGXbGa % aiykaiabgUcaRiaadIeadaWgaaWcbaGaaGOmaaqabaGccaWGpbGaai % ikaiaaigdacaGGPaGaeyOKH4QaamisamaaBaaaleaacaaIZaaabeaa % kiaad+eadaahaaWcbeqaaiabgUcaRaaakiaacIcacaWGHbGaamyCai % aacMcacqGHRaWkcaWGobGaam4tamaaDaaaleaacaaIZaaabaGaeyOe % I0caaOWaaeWaaeaacaWGHbGaamyCaaGaayjkaiaawMcaaaaa!52B9! HN{O_3}(aq) + {H_2}O(1) \to {H_3}{O^ + }(aq) + NO_3^ - \left( {aq} \right)\)

Concentrated nitric acid is a strong oxidising agent and attacks most metals except noble metals such as gold and platinum. The products of oxidation depend upon the concentration of the acid, temperature and the nature of the material undergoing oxidation.

3Cu + 8 HNO₃(dilute)  \(\to\) 3Cu(NO₃)₂ + 2NO + 4H₂O

Cu + 4HNO₃(conc.)\(\to\)Cu(NO₃)₂ + 2NO₂ + 2H₂O

Zinc reacts with dilute nitric acid to give N₂O and with concentrated acid to give NO₂.

4Zn + 10HNO₃(dilute) \(\to\)4 Zn (NO₃)₂ + 5H₂O + N₂O

Zn + 4HNO₃(conc.) \(\to\)Zn (NO₃)₂ + 2H₂O + 2NO₂

Some metals (e.g., Cr, Al) do not dissolve in concentrated nitric acid because of the formation of a passive film of oxide on the surface. Concentrated nitric acid also oxidises non–metals and their compounds. Iodine is oxidised to iodic acid, carbon to carbon dioxide,

sulphur to H₂SO₄, and phosphorus to phosphoric acid.

I₂ + 10HNO₃ \(\to\)2HIO₃ + 10NO₂ + 4H₂O

C + 4HNO₃ \(\to\)CO₂ + 2H₂O + 4NO₂

S₈ + 48HNO₃ \(\to\) 8H₂SO₄ + 48NO₂ + 16H₂O

P₄ + 20HNO₃ \(\to\)4H₃PO₄ + 20NO₂ + 4H₂O

Brown Ring Test: The familiar brown ring test for nitrates depends on the ability of Fe²⁺ to reduce nitrates to nitric oxide, which reacts with Fe²⁺ to form a brown coloured complex. The test is usually carried out by adding dilute ferrous sulphate solution to an aqueous solution containing nitrate ion, and then carefully adding concentrated sulphuric acid along the sides of the test tube. A brown ring at the interface between the solution and sulphuric acid layers indicates the presence of nitrate ion in solution.

\(% MathType!MTEF!2!1!+- % feaagKart1ev2aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGceaqabeaacaWGob % Gaam4tamaaDaaaleaacaaIZaaabaGaeyOeI0caaOGaey4kaSIaaG4m % aiaadAeacaWGLbWaaWbaaSqabeaacaaIYaGaey4kaScaaOGaey4kaS % IaaGinaiaadIeadaahaaWcbeqaaiabgUcaRaaakiabgkziUkaad6ea % caWGpbGaey4kaSIaaG4maiaadAeacaWGLbWaaWbaaSqabeaacaaIZa % Gaey4kaScaaOGaaGOmaiaadIeadaWgaaWcbaGaaGOmaaqabaGccaWG % pbaabaWaamWaaeaacaWGgbGaamyzamaabmaabaGaamisamaaBaaale % aacaaIYaaabeaakiaad+eaaiaawIcacaGLPaaadaWgaaWcbaGaaGOn % aaqabaaakiaawUfacaGLDbaadaahaaWcbeqaaiaaikdacqGHRaWkaa % GccqGHRaWkcaWGobGaam4taiabgkziUoaadmaabaGaamOraiaadwga % daqadaqaaiaadIeadaWgaaWcbaGaaGOmaaqabaGccaWGpbaacaGLOa % GaayzkaaWaaSbaaSqaaiaaiwdaaeqaaOWaaeWaaeaacaWGobGaam4t % aaGaayjkaiaawMcaaaGaay5waiaaw2faamaaCaaaleqabaGaaGOmai % abgUcaRaaakiabgUcaRiaadIeadaWgaaWcbaGaaGOmaaqabaGccaWG % pbaabaGaaGPaVlaaykW7caaMc8UaaGPaVlaaykW7caaMc8UaaGPaVl % aaykW7caaMc8UaaGPaVlaaykW7caaMc8UaaGPaVlaaykW7caaMc8Ua % aGPaVlaaykW7caaMc8UaaGPaVlaaykW7caaMc8UaaGPaVlaaykW7ca % aMc8UaaGPaVlaaykW7caaMc8UaaGPaVlaaykW7caaMc8UaaGPaVlaa % ykW7caaMc8UaaGPaVlaaykW7caaMc8UaaGPaVlaaykW7caaMc8UaaG % PaVlaaykW7caaMc8UaaGPaVlaaykW7caaMc8UaaGPaVlaaykW7caaM % c8UaaGPaVlaaykW7caaMc8UaaGPaVlaaykW7caaMc8UaaGPaVlaayk % W7caaMc8+aaeWaaeaacaWGIbGaamOCaiaad+gacaWG3bGaamOBaaGa % ayjkaiaawMcaaaaaaa!CCB5! \begin{array}{l} NO_3^ - + 3F{e^{2 + }} + 4{H^ + } \to NO + 3F{e^{3 + }}2{H_2}O\\ {\left[ {Fe{{\left( {{H_2}O} \right)}_6}} \right]^{2 + }} + NO \to {\left[ {Fe{{\left( {{H_2}O} \right)}_5}\left( {NO} \right)} \right]^{2 + }} + {H_2}O\\ \,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\left( {brown} \right) \end{array}\)

USES: 

The major use of nitric acid is in the manufacture of ammonium nitrate for fertilisers and other nitrates for use in explosives and pyrotechnics. It is also used for the preparation of nitroglycerin, trinitrotoluene and other organic nitro compounds. Other major uses are in the pickling of stainless steel, etching of metals and as an oxidiser in rocket fuels.

Phosphorus — Allotropic Forms

Phosphorus is found in many allotropic forms, the important ones being white, red and black.

White phosphorus is a translucent white waxy solid. It is poisonous, insoluble in water but soluble in carbon disulphide and glows in dark (chemiluminescence). It dissolves in boiling NaOH solution in an inert atmosphere giving PH₃.

P₄ + 3NaOH + 3H₂O \(\to\) PH₃ + 3NaH₂PO₂

(sodium hypophosphite)

White phosphorus is less stable and therefore, more reactive than the other solid phases under normal conditions because of angular strain in the P₄ molecule where the angles are only 60°. It readily catches fire in air to give dense white fumes of P₄O₁₀.

P₄ + 5O₂ \(\to\) P₄O₁₀

It consists of discrete tetrahedral P₄ molecule as shown in Fig. 7.2.

Fig. 7.2 White phosphorus

Red phosphorus is obtained by heating white phosphorus at 573K in an inert atmosphere for several days. When red phosphorus is heated under high pressure, a series of phases of black phosphorus is formed. Red phosphorus possesses iron grey lustre. It is odourless, non- poisonous and insoluble in water as well as in carbon disulphide. Chemically, red phosphorus is much less reactive than white phosphorus. It does not glow in the dark.

It is polymeric, consisting of chains of P₄ tetrahedra linked together in the manner as shown in Fig. 7.3.

                  Fig.7.3: Red phosphorus

Black phosphorus has two forms  \(\alpha\)-black phosphorus and  \(\beta\)-black phosphorus. \(\alpha\)-Black phosphorus is formed when red phosphorus is heated in a sealed tube at 803K. It can be sublimed in air and has opaque monoclinic or rhombohedral crystals. It does not oxidise in air. \(\beta\)-Black phosphorus is prepared by heating white phosphorus at 473 K under high pressure. It does not burn in air upto 673 K.

Phosphine

Preparation

Phosphine is prepared by the reaction of calcium phosphide with water or dilute HCl.

Ca₃P₂ + 6H₂O  \(\to\) 3Ca(OH)₂ + 2PH₃

Ca₃P₂ + 6HCl \(\to\) 3CaCl₂ + 2PH₃

In the laboratory, it is prepared by heating white phosphorus with concentrated NaOH solution in an inert atmosphere of CO₂.

P₄ + 3NaOH + 3H₂O \(\to\) PH₃ + 3NaH₂PO₂

                                             (sodium hypophosphite)

When pure, it is non inflammable but becomes inflammable owing to the presence of P₂H₄ or P₄ vapours. To purify it from the impurities, it is absorbed in HI to form phosphonium iodide (PH₄I) which on treating with KOH gives off phosphine.

PH₄I + KOH \(\to\) KI + H₂O + PH₃

properties

It is a colourless gas with rotten fish smell and is highly poisonous. It explodes in contact with traces of oxidising agents like HNO₃, Cl₂ and Br₂ vapours.

It is slightly soluble in water. The solution of PH₃ in water decomposes in presence of light giving red phosphorus and H₂. When absorbed in copper sulphate or mercuric chloride solution, the corresponding phosphides are obtained.

3CuSO₄ + 2PH₃ \(\to\) Cu₃P₂ + 3H₂SO₄

3HgCl₂ + 2PH₃ \(\to\) Hg3P₂ + 6HCl

Phosphine is weakly basic and like ammonia, gives phosphonium compounds with acids e.g.,

PH₃ + HBr \(\to\) PH₄Br

USES: The spontaneous combustion of phosphine is technically used in Holme’s signals. Containers containing calcium carbide and calcium phosphide are pierced and thrown in the sea when the gases evolved burn and serve as a signal. It is also used in smoke screens.

EXAMPLE 6

In what way can it be proved that PH₃ is basic in nature?

SOLUTION

PH₃ reacts with acids like HI to form PH₄I which shows that it is basic in nature. 

PH₃ + HI \(\to\) PH₄I

Due to lone pair on phosphorus atom, PH₃ is acting as a Lewis base in the above reaction.

INTEXT QUESTION

(a) Bond angle in PH\(% MathType!MTEF!2!1!+- % feaagKart1ev2aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaWaa0baaSqaai % aaisdaaeaacqGHRaWkaaaaaa!37C3! _4^ + \) is higher than that in PH₃. Why?

(b)  What is formed when PH₃ reacts with an acid?

What happens when white phosphorus is heated with concentrated NaOH solution in an inert atmosphere of CO₂ ?

Phosphorus Halides

Phosphorus forms two types of halides, PX₃ (X = F, Cl, Br, I) and PX₅ (X = F, Cl, Br).

Phosphorus Trichloride

Preparation

It is obtained by passing dry chlorine over heated white phosphorus.

P₄ + 6Cl₂ \(% MathType!MTEF!2!1!+- % feaagKart1ev2aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaWaa0baaSqaai % aaisdaaeaacqGHRaWkaaaaaa!37C3! _4^ + \) 4PCl₃

It is also obtained by the action of thionyl chloride with white phosphorus.

P₄ + 8SOCl₂ \(% MathType!MTEF!2!1!+- % feaagKart1ev2aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaWaa0baaSqaai % aaisdaaeaacqGHRaWkaaaaaa!37C3! _4^ + \) 4PCl₃ + 4SO₂ + 2S₂Cl₂

Properties

It is a colourless oily liquid and hydrolyses in the presence of moisture.

PCl₃ + 3H₂O \(% MathType!MTEF!2!1!+- % feaagKart1ev2aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaWaa0baaSqaai % aaisdaaeaacqGHRaWkaaaaaa!37C3! _4^ + \) H₃PO₃ + 3HCl

It reacts with organic compounds containing –OH group such as CH₃COOH, C₂H₅OH.

\(% MathType!MTEF!2!1!+- % feaagKart1ev2aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGceaqabeaacaaIZa % Gaam4qaiaadIeadaWgaaWcbaGaaG4maaqabaGccaWGdbGaam4taiaa % d+eacaWGibGaey4kaSIaamiuaiaadoeacaWGSbWaaSbaaSqaaiaaio % daaeqaaOGaeyOKH4QaaG4maiaadoeacaWGibWaaSbaaSqaaiaaioda % aeqaaOGaam4qaiaad+eacaWGdbGaamiBaiabgUcaRiaadIeadaWgaa % WcbaGaaG4maaqabaGccaWGqbGaam4tamaaBaaaleaacaaIZaaabeaa % aOqaaiaaiodacaWGdbWaaSbaaSqaaiaaikdaaeqaaOGaamisamaaBa % aaleaacaaI1aaabeaakiaad+eacaWGibGaey4kaSIaamiuaiaadoea % caWGSbWaaSbaaSqaaiaaiodaaeqaaOGaeyOKH4QaaG4maiaadoeada % WgaaWcbaGaaGOmaaqabaGccaWGibWaaSbaaSqaaiaaiwdaaeqaaOGa % am4qaiaadYgacqGHRaWkcaWGibWaaSbaaSqaaiaaiodaaeqaaOGaam % iuaiaad+eadaWgaaWcbaGaaG4maaqabaaaaaa!65F7! \begin{array}{l} 3C{H_3}COOH + PC{l_3} \to 3C{H_3}COCl + {H_3}P{O_3}\\ 3{C_2}{H_5}OH + PC{l_3} \to 3{C_2}{H_5}Cl + {H_3}P{O_3} \end{array}\)

​​​​​​​

Phosphorus Pentachloride

It has a pyramidal shape as shown, in which phosphorus is sp³ hybridised.

Preparation

Phosphorus pentachloride is prepared by the reaction of white phosphorus with excess of dry chlorine.

P₄ + 10Cl₂ \(\to\) 4PCl₅

It can also be prepared by the action of SO₂Cl₂ on phosphorus.

P₄ + 10SO₂Cl₂ \(\to\) 4PCl₅ + 10SO₂

properties

PCl₅ is a yellowish white powder and in moist air, it hydrolyses to POCl₃ and finally gets converted to phosphoric acid.

PCl₅ + H₂O \(\to\) POCl₃ + 2HCl

POCl₃ + 3H₂O \(\to\) H₃PO₄ + 3HCl

When heated, it sublimes but decomposes on stronger heating.

​​​​​​​

​​​​​​​

It reacts with organic compounds containing –OH group converting them to chloro derivatives.

C₂H₅OH + PCl₅ \(\to\) C₂H₅Cl + POCl₃ + HCl

CH₃COOH+ PCl₅ \(\to\) CH₃COCl + POCl₃ +HCl

Finely divided metals on heating with PCl₅ give corresponding chlorides.

2 Ag + PCl₅ \(\to\) 2AgCl + PCl₃

Sn + 2PCl₅ \(\to\) SnCl₄ + 2PCl₃

It is used in the synthesis of some organic compounds, e.g., C₂H₅Cl, CH₃COCl.

In gaseous and liquid phases, it has a trigonal bipyramidal structure as shown. The three equatorial P–Cl bonds are equivalent, while the two axial bonds are longer than equatorial bonds. This is due to the fact that the axial bond pairs suffer more repulsion as compared to equatorial bond pairs.

EXAMPLE 7

Why does PCl₃ fume in moisture ?

SOLUTION

PCl₃ hydrolyses in the presence of moisture giving fumes of HCl.

PCl₃ + 3H₂O \(\to\) H₃PO₃ + 3HCl

EXAMPLE 8

Are all the five bonds in PCl₅ molecule equivalent? Justify your answer.

SOLUTION

PCl₅ has a trigonal bipyramidal structure and the three equatorial P-Cl bonds are equivalent, while the two axial bonds are different and longer than equatorial bonds.

Intext Questions

What happens when PCl₅ is heated?

Write a balanced equation for the reaction of PCl₅ with water.

Oxoacids of Phosphorus

Phosphorus forms a number of oxoacids. The important oxoacids of phosphorus with their formulas, methods of preparation and the presence of some characteristic bonds in their structures are given in Table 7.5.

                                          Table 7.5: Oxoacids of Phosphorus

The compositions of the oxoacids are interrelated in terms of loss or gain of H₂O molecule or O-atom. The structures of some important oxoacids are given next.

In oxoacids phosphorus is tetrahedrally surrounded by other atoms. All these acids contain at least one P=O bond and one P–OH bond. The oxoacids in which phosphorus has lower oxidation state (less than +5) contain, in addition to P=O and P–OH bonds, either P–P (e.g., in H₄P₂O₆) or P–H (e.g., in H₃PO₂) bonds but not both. These acids in +3 oxidation state of phosphorus tend to disproportionate to higher and lower oxidation states. For example, orthophophorous acid (or phosphorous acid) on heating disproportionates to give orthophosphoric acid (or phosphoric acid) and phosphine.

        4H₃PO₃ + 3H₃PO₄ \(\to\) PH₃

Fig.7.4. Structures of some important oxoacids of phosphorus

The acids which contain P–H bond have strong reducing properties. Thus, hypophosphorous acid is a good reducing agent as it contains two P–H bonds and reduces, for example, AgNO₃ to metallic silver. 

4 AgNO₃ + 2H₂O + H₃PO₂ \(\to\) 4Ag + 4HNO₃ + H₃PO₄

These P–H bonds are not ionisable to give H⁺ and do not play any role in basicity. Only those H atoms which are attached with oxygen in P–OH form are ionisable and cause the basicity. Thus, H₃PO₃ and H₃PO₄ are dibasic and tribasic, respectively as the structure of H₃PO₃ has two P–OH bonds and H₃PO₄ three.

EXAMPLE 9

How do you account for the reducing behaviour of H₃PO₂ on the basis of its structure ?

SOLUTION

In H₃PO₂, two H atoms are bonded directly to P atom which imparts reducing character to the acid.

INTEXT QUESTION

What is the basicity of H₃PO₄?

What happens when H₃PO₃ is heated?​​​​​​​