OXYGEN AND ITS COMPOUNDS

Oxygen is the most abundant element on earth. It constitutes 21% by volume of atmospheric air.

 Occurrence: It occurs as free elements in nature and in combined states.

GENERAL PROPERTIES OF OXYGEN FAMILY

Elements in group VI include: Oxygen (O), Sulphur (S), Selenium (Se), Tellurium (Te), and Polonium (Po).their properties are as follows:

1. They are non-metals and exist as solid at room temperature except for oxygen
2. They are electron acceptors and oxidizing in nature.
3. They do not react with water in any form. But oxygen and sulphur combine directly with hydrogen to yield water and hydrogen sulphide respectively.

ELECTRONIC STRUCTURE AND BONDING IN OXYGEN

Oxygen has an atomic number of 8; hence its electronic configuration is 1s²2s²2p⁴. This shows that oxygen needs two electrons in order to attain an octet configuration.

Oxygen atom has six valence electrons and can acquire a stable octet configuration by:

1. Accepting two electrons from electropositive elements like metals to form negative oxide ion, O^2-. Example

Ca²⁺ +  O^2- → CaO

2. Entering into covalent bond formation with non-metals by covalently sharing two out of its six outer electrons. Example formation of carbon (IV) oxide molecule.

O            C            O

3. Forming covalent bond with itself.

O             O

LABORATORY PREPARATION OF OXYGEN

1. By the thermal decomposition of potassium trioxochlorate (V) in the presence of MnO₂ as catalyst

2KClO_3(s)^heat      ——-    2KCl_2(s) +  3O_2(g)

2. By decomposition of hydrogen peroxide, H₂O₂ in the presence of MnO₂ as catalyst

2H₂O_2(aq) ^heat                2H₂O_(l)  +  O_2(g)

Hydrogen peroxide also reacts with acidified KMnO₄ in the cold to produce oxygen

5H₂O_2(aq)   +  2KMnO_4(aq)      +     3H₂SO_4(aq)      ——-    K₂SO_4(aq) + 2MnSO_4(aq)  +   8H₂O_(l)  +  50_2(g)

NOTE: If the gas is required dry, it is passed through anhydrous calcium chloride or concentrated tetraoxosulphate (VI) acid and collected over mercury.

INDUSTRIAL PREPARATION

1. Electrolysis of water
2. Fractional distillation of liquid air: This preparation involves two main processes:
3. Liquefaction of air

Air is first passed through caustic Soda, NaOH_(aq) to remove CO₂.  It is then subjected through series of compressions, expansions and cooling until liquid air is obtained at -200^oC.

1. Fractional distillation of liquid air

The liquid air is led to a fractional distillation column.  On distillation, nitrogen with a lower boiling point of -196⁰C is evolved first, leaving behind liquid oxygen.  Further heating converts the liquid oxygen to a gas at -183⁰C

 EVALUATION

1. Explain the laboratory preparation of oxygen
2. By what process is oxygen obtained from air

PHYSICAL PROPERTIES

1. It is colourless , odorless and tasteless diatomic gas
2. It is neutral to litmus
3. It is slightly soluble in water
4. Gaseous oxygen is denser than air
5. Gaseous oxygen liquefies at -183⁰C

CHEMICAL PROPERTIES

1. 1. Reaction with metals: Oxygen combines directly with most metals to form basic oxides

2Ca      +          O₂                    2CaO

4K        +          O₂                    K₂O

The oxides of very electropositive metals, K, Na, Ca dissolves in water to form alkalis

2 K₂O   +          2 H₂O              4KOH

2. Reaction with non-metals: Non-metals burn in oxygen to acidic oxides. These are known as acid anhydrides as they dissolve in water to form acids.

S_(s)        +          O_2(g)                 SO_2(g)

SO_2(g)    +          H₂O_(l)       H₂SO_3(aq)

P_4(s)        +        O_2(g)                 P₄O_6(g)

                                P₄O₆        +        H₂O_(l)   4H₃PO_3(aq)

3. Most hydrocarbon and compounds of carbon, hydrogen and oxygen burn in oxygen to give CO₂ and H₂O

C₂H₅OH_(l)         +          3O_4(g)               2CO_2(g) +   3H₂O_(l)

USES OF OXYGEN

1. It is used in oxy-ethylene flame
2. It is required for respiration
3. It is used in steel industry for the removal of C, S and P impurities from pig iron
4. Liquid oxygen and fuel are used as propellant for space rockets
5. Oxygen is used in the manufacturer of tetraoxosulphate (VI), trioxonitrate (VI) acid and ethanoic acid.

TEST FOR OXYGEN

When a glowing splinter is inserted into a gas jar containing an unknown gas and the glowing splinter is rekindled, then the gas is likely oxygen gas or dinitrogen (I) oxide gas.

If the gas is colourless and reacts with nitrogen (II) oxide to produce reddish-brown fumes of nitrogen (IV) oxide, then the gas is confirmed to be oxygen gas.

 EVALUATION

1. State two physical properties of oxygen
2. Using equation only, state two chemical properties of oxygen

OXIDES

Oxides are binary compounds formed when oxygen combines with other elements

Types of oxides (classification)

1. Basic Oxides: These are oxides of metals e.g. Na₂O, K₂O, MgO, CaO etc. They react with acids to form salt and water only. Example

Na₂O_(s)            +          2HCl_(aq)                        2NaCl_(s)            + H₂O_(l)

2. Acidic Oxides: These are oxides of non-metals which dissolves in water to form acids e.g. CO₂, SO₂, NO₂ etc. They react with alkali to form a salt and water only e.g. CO_2(g) +   2NaOH_(aq)                   Na₂CO_3(aq)   + H₂O_(l)
3. Amphoteric Oxides: These are oxides of metals which can react with both acids and alkalis to form salt and water only. They include the oxides of Al, Zn, Pb and Sn. Example

ZnO_(s)    +   H₂SO₄                     ZnSO_4(aq)  +     H₂O_(l)

ZnO(s)    +    2NaOH_(aq)    +  H₂O                      Na₂Zn(OH)_4(aq)

4. Neutral Oxides: These are oxides of non-metals which are neither acidic nor basic. They are neutral to litmus.  They include CO₂H₂O and N₂O
5. Peroxides: These are oxides which contain higher proportion of oxygen than ordinary oxides e.g Na₂O₂, CaO₂ and BaO₂. They react with dilute acid to produce hydrogen peroxide, H₂O₂

GENERAL EVALUATION/REVISION

1. Mention four air pollutants and state their effects
2. State two air pollution control measures
3. Mention four types of oxides and give two examples each.
4. Describe the electrolysis CuSO_4(aq) using platinum electrodes
5. Using equations only, state the products of the electrolysis of brine

 WEEKEND ASSIGNMENT

SECTION A: Write the correct option ONLY

1. The most abundant element on earth is a. nitrogen b. helium c. silicon d. oxygen
2. Determination of the proportion of oxygen in air can be done by passing a given volume of air through a. lime water b. alkaline pyrogallol solution c. FeSO₄ solution
3. concentrated H₂SO₄
4. Which one of the following is an amphoteric oxide? a. SiO₂ b. Al₂O₃ c. CuO d. K₂O
5. Most acid anhydrides react with water to form acids. Which of these is a mixed anhydride? a. N₂O b. NO c. NO₂ d. SO₂
6. A gas which is neutral to litmus and rekindles a glowing splinter is
7. O₂ b. O₃ c. H₂O₂ d. H₂

 SECTION B

1. Using diagram ONLY, describe the preparation of dry oxygen gas in the laboratory
2. Why is oxygen collected over mercury?

See also

CHEMICAL EQUILIBRIUM | STATIC, DYNAMIC, REVERSIBLE REACTION, PROPERTIES, FACTORS & EFFECTS

VOLUMETRIC ANALYSIS

Test for Anions

INTRODUCTION TO QUALITATIVE ANALYSIS

METALS AND THEIR COMPOUND