OXYGEN AT A GLANCE

 

OXYGEN AND ITS COMPOUNDS

Oxygen is the 8^th element on the periodic table. It has an atomic number of 8 and a mass number of 16  (₁₆⁸O). it has an electronic configuration of 1s²2s²2p⁴. It exhibits oxidation states of -2, -1,0, and It exists in isotopic mixtures it also has two allotropic forms that is, molecular O₂ and ozone O₃. It was discovered separately by Carl. W Scheele in 1972 and Priestley 1974 but it was named by Antoine Lavoisier in 1777.

OCCURRENCE

It occurs  freely as molecular oxygen in the atmosphere (O₂) about 21% of the atmosphere is Oxygen. It also occurs in the combined state as Trioxosilicate (IV) (Al₂(SiO₃)₃), trioxocarbonates (IV) e.g  (CaCO₃), it is present most oxides that make up rocks and clays as well as in water (H₂O).

Laboratory Preparation

Oxygen is prepared in the laboratory by several methods the commonest being

1.  The decomposition of Potassium trioxochlorate V

2KClO₃  →^heat 2KCl + 3O₂

2.  Oxidation of Hydrogen peroxide using MnO₂ or acidified potassium tetraoxomanganate VII (KMnO₄) as the oxidizing agent.

      2H₂O₂  →^MnO₂     2H₂ + O₂            

     MnO₂ here is acting as a catalyst

5H₂O_2(aq) + 2KMnO_4(aq) + 3H₂SO_4(aq)  →K₂SO_4(aq) + MnSO_4(aq) + H₂O_(l) + O_2(g)

Industrial preparation of Oxygen

Oxygen is prepared industrially by 

1. Electrolysis of acidified water and 

2. Fractional distillation of liquid air

By electrolysis: hydrogen is discharged at the cathode while oxygen is discharged at the anode.

FRACTIONAL DISTILLATION OF LIQUEFIED AIR

1. Air is first passed through caustic soda to remove CO₂. The air is then subjected to a series of conditions which includes high pressures, low temperature and expansions which causes the air to liquefy (that is become a liquid).

2. The liquid air is then passed into the fractionating column and heated, Nitrogen with a lower boiling point of -196⁰C distills first followed by Oxygen with a boiling point of -183⁰C. Oxygen distils over as a gas and is collected, dried and stored in steel cylinders.

    Physical properties

1.  Oxygen  is a colourless gas 

2. it is an odourless 

3. it is a tasteless gas

4. It is slightly soluble in water

5. It is neutral to litmus 

Chemical properties 

Oxygen combines readily with almost all substances  as discussed below

Reaction with

1.  metals: - most metals burn in Oxygen to yield basic oxides

            2Mg_(s) + O_2(g) →2MgO_(s)

2. Non-metals: - non-metals burn in oxygen to yield acidic oxides

          S(s)  + O2(g) → SO2(g)

3. Reaction with organic compounds: - most organic compounds burn in oxygen to yield CO2, H2O and the oxide of any other element except oxygen present in the compound. E.g 

i.  C2H6 +O2  → CO2 + H2O

ii.  

   

  TEST FOR OXYGEN

When oxygen gas is brought close to a dying flame it rekindles the flame 

USES OF OXYGEN

1.      It is used for breathing by divers and mountain climbers

2.      It is combined with ethyne by welders to produce very hot flames

3.      Liquid oxygen and fuels are used as propellants for space rockets.

                             

                        OXIDES

Oxides are binary compounds formed when elements burn in oxygen.

They are classified as

1.      Basic oxides: - these are oxides formed when metals burn in oxygen. 

Examples of basic oxides are Na2O, CaO, MgO, K2O

2Mg_(s) + O_2(g) →2MgO_(s)

Properties of basic oxides

a.      They are mainly solids

b.      Soluble oxides dissolve in water to form Alkalis

      Na₂O_(s) + H₂O_(l) → 2NaOH_(aq) + H₂O_(l)

c.       They react with acids to form salt and water

      K₂O_(s) + HCl_(aq) →KCl_(aq) + H₂O_(l)  

2.      Acidic Oxides: - these are oxides of non-metals; they are formed when non-metals burn in oxygen. Examples of acidic oxides are SO2, SO3, CO2, NO2, P2O5

            S_(s) + O_2(g) → SO_2(g)

Properties of acidic oxides

a.      They dissolve in water to form corresponding acids ( also called acid anhydride)

SO_2(g) +H₂O_(l) → H₂SO_3(aq)

b.      They react with bases to form salt and water.

         2NaOH_(aq) + SO_2(g) → Na₂SO₃ + H₂O_(l)

3.      Amphoteric Oxide: - these are oxide of metals that behave both as acidic and basic oxides. They are oxides of Al, Sn Pb and Zn. Examples of Amphoteric oxides are PbO₂, ZnO, Al₂O₃, SnO₂

1. With acids they form salt and water only

ZnO(aq) + HCl(aq)  →ZnCl2(aq)  +  H2O(l)

2. With alkalis they form complex salts 

ZnO + 2KOH + H2O → K2Zn(OH)4

4.      Neutral Oxides: - these are non-metallic oxides that are neutral to litmus.

 E.g carbon II oxide (CO), dinitrogen (I) oxide (N₂O) and water (H₂O) which is the only neutral oxide that is liquid at room temperature.

5.      Peroxides: - these are oxides that contain a higher proportion than the usual oxides. E.g sodium peroxide Na₂O₂, H₂O₂,

Objective Questions 

1. Amphoteric oxides are oxides which 

a) react with water to form acids 

b) react with water to form alkali

c) show neither acid nor basic properties 

d) react with both acids and alkalis 

2.  The component of air that is removed when air is bubbled through alkaline pyrogallol solution is 

a) Carbon (IV) oxide 

b) oxygen

 c) water vapour 

d) nitrogen

3.  When the trioxonitrate (V) salt of an alkali metal Y is heated, the formula of the residue is 

 a) Y₂O 

b) YNO₂ 

c) Y₂O₃ 

d) Y(NO₂)₂

3.   Which of the following oxides is amphoteric 

a) Na₂O 

b) Fe₂O₃ 

c)Al₂O₃

d) CuO.

5.      The following oxides react with both acids and bases to form salts except

 a) zinc oxide 

b) lead (II) oxide

 c) aluminum oxide

 d) tin (IV) oxide.

6.      The following oxides reacts with water except 

a) Na₂O

 b) SO₃ 

c) NO₂ 

d) CuO

7.      If X is a group III element, its oxide would be represented as 

a) X₃O₂ 

b) X₂O

 c) X₂O₃ 

d) XO₃

8.      Which of the following elements is diatomic? 

a) Iron 

b) Neon 

c) Oxygen 

d) Sodium.

9.       Which of the following substances is mainly responsible for the depletion of the ozone layer?

 a)  Chlorofluorocarbon

 b) Carbon (IV) oxide

 c) Nitrogen

 d) Oxygen

10.    Which of the following oxides is ionic

a) P₄O₁₀ 

b) MgO

 c) Al₂O₃

d)SO₂

11. What term is used to describe an oxide whose aqueous solution turns red litmus blue

a. Strong electrolyte

b. Acid anhydride

c. Amphoteric oxide

d. Basic oxide

 THEORY

1.  (a)(i) what are acidic oxides? 

(ii). give one example of each of the following oxides I. acidic oxide II. Basic oxide III. Amphoteric oxide IV. Neutral oxide

b (i) Explain what is meant by acid anhydride 

(ii) give one example of the oxide mentioned in b(ii) above

2.(a).Draw a well labelled diagram for the laboratory preparation of oxygen.

(b). Write the formulae of three different oxides of period 3 elements that react with water.

3a(I). Classify each of the following oxides as acidic, basic, neutral or amphoteric.         

(a)(i). I. ZnO  (II) CO (III) NO₂

(b).  Consider the following oxides: CaO, SiO₂, CO, NO₂ and ZnO. Which of the oxide(s)

 (i). is an acidic oxide that is insoluble in water?

 (ii). Reacts with water to give alkaline solution

 (iii). Is amphoteric? 

 (iv). Is neutral 

(v) is/are gaseous at room temperature.

4.   ZnO is an amphoteric oxide. Write equations to illustrate this statement