Group 16. Sulfur

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Group 15 Nitrogen

(Section 22.7, p.962)

Nitrogen, N

2

, - diatomic gas containing N

N triple bond.

Nitrogen compounds show wide range of

O.S.’s and coordination numbers (C.N.),

N

2

O, NO, NH

3

, NH

4

Cl, NaNO

3

.

Note: C.N. of an element in a covalent compound is defined as the no. of other atoms or groups covalently bonded to it. e.g. C.N. of N in NH

3

is 3; C.N. of N in

NaNO

3

is also 3.

One of the most important compounds of nitrogen is ammonia, NH

3

. Industrially made by HABER process:

N

2

+ 3H

2

2NH

3

Several nitrogen oxides known since nitrogen can have variety of O.S.’s, e.g. N

2

O, NO and NO

2

. Different reaction conditions give different oxides.

Note: O always has O.S. –2 in these compounds (Why?)

N oxides are covalent oxides, hence react with water giving acidic solutions (section

3.6). For example:

Reaction of nitrogen dioxide and water gives the products monoxide. nitric acid and nitrogen

Question:

Write the balanced equation for this reaction.

NO

2

+ H

2

O

HNO

3

+ NO (Not balanced!)

Give the O.S. of each element in this equation and hence say whether this is a redox reaction.

You use nitric acid in the practical course. It is a strong acid and also a strong oxidising agent i.e. it oxidises compounds and is itself reduced. e.g.

C + 4HNO

3

CO

2

+ 4NO

2

+2H

2

O

When nitric acid reacts with a base, (acidbase reaction), the products are the nitrate salt and water. e.g.

2HNO

3

+ Ca(OH)

2

Ca(NO

3

)

2

+ 2H

2

O

(acid) (base) (salt) (water)

Nitrates metabolised by most plants so useful as fertilisers.

Group 15 Phosphorus

(Section 22.8, p.967)

P exists as a molecular compound, P

4

, with the P atoms arranged in a tetrahedral fashion.

P

P P

P

PH

3

(the P analogue of NH

3

) is known. Is a toxic, colourless gas which smells of fish.

Cannot be formed by direct reaction between P

4

and H

2

. Possible reason: P & H have same electronegativity value (2.1).

Instead to make PH

3

, react an acid with an ionic phosphide. e.g. Na

3

P + 3HCl

3NaCl + PH

3

(base) (acid)

P

4

reacts with oxygen to form phosphorus oxides. Unlike nitrogen, only two oxides are stable - P

4

O

6

and P

4

O

10

.

Formed by direct reaction of P

4

and O

2

.

Nature of product depends on the stoichiometry (ratio) of the reactants.

P

4

+ 3O

2

P

4

O

6

P

4

+ 5O

2

P

4

O

10

Structures:

(O.S. of P is +3)

(O.S. of P is +5)

O

O

P

O

P

O O

P

O

P

O

O

O

P

O

P

O

O O

P

P

O

(3-D structures on CD,

ch. 22

)

O

O

P oxides are covalent oxides and so react with water giving acidic solutions (section

3.6).

Either

P

4

O

6

+ 6H

2

O

4H

3

PO

3

H

3

PO

3

is phosphor

ous

acid. O.S.(P) +3.

Or

P

4

O

10

+ 6H

2

O

4H

3

PO

4

H

3

PO

4

is phosphor

ic

acid. O.S. (P) +5.

Phosphoric acid used as a preservative, e.g. in soft drinks (Coke), detergents, pharmaceuticals etc.

Note: H

3

PO

3

and H

3

PO

4

are

oxy

acids of phosphorus.

P

4

reacts with Group 17 (the halogens) to give compounds of the type PX

3

and PX

5

(X

= halogen, i.e. F, Cl, Br). Product obtained depends on ratio of reactants.

P

4

+ 6Cl

2

4PCl

3

P

4

+ 10Cl

2

4PCl

5 i.e. Cl

2

will oxidise P to +3 oxidation state if low ratio of Cl

2

present, and to +5 oxidation state if high ratio Cl

2

present.

PCl

3

and PCl

5

undergo hydrolysis reactions to form oxyacids.

PCl

3

+ 3H

2

O

3HCl + H

3

PO

3

PCl

5

+ 4H

2

O

5HCl + H

3

PO

4

Group 16.

Sulfur

(Section 22.6, p.958)

Sulfur – exists as a molecular compound, with the atoms arranged in a crown fashion.

S

S

S

S

S

S

S

S

(3-D model on CD,

ch. 22

)

H

2

S (the S analogue of H

2

O) is known. Best made by reaction of an ionic sulfide with acid, e.g.

2HCl + Na

2

S

2NaCl + H

2

S

(acid) (base)

Note: Cannot make H

2

S by direct reaction of

S

8

with H

2

; these elements do not react readily together.

Sulfur oxides:

2 common oxides

(a) SO

2

(sulfur dioxide) made by combination reaction:

S

8

+ 8O

2

8SO

2

SO

2

used to sterilise dried fruit and wine as it is toxic to microorganisms.

(b) SO

3

(sulfur trioxide) made by combination of SO

2

and O

2

. Reaction is slow, so need catalyst, e.g. V

2

O

5

(an oxide of the d-block element, vanadium) to speed it up.

2SO

2

+ O

V

2

O

5

2

2SO

3

Catalyst –substance that changes rate of reaction but is itself not used up in reaction.

S oxides are covalent oxides and so react with water giving acidic solutions.

SO

2

+ H

2

O

H

2

SO

3

H

2

SO

3

is sulfur

ous

acid. S in O.S. +4.

SO

3

+ H

2

O

H

2

SO

4

H

2

SO

4

is sulfur

ic

acid. S in O.S. +6.

Sulfuric acid - strong acid and moderately good oxidising agent.

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