CARBON AND ITS COMPOUND -II

 

OXIDES OF CARBON

When carbon is heated in air it forms two types of oxides. They are: carbon (iv) oxide, CO2 and

carbon(ii)oxide, CO. If oxygen is in excess supply, CO2 is formed. If the oxygen is in limited

supply CO is formed.

C + O2        CO2  Excess supply; 2C + O2      2CO limited supply

Carbon (iv) oxide is one of the naturally accuring compound of carbon. It is one of the gases that make up the air around us. Air contains 0.03% of CO2. Carbon(iv)oxide does not support combustion. It is important to green plants for making their food by the process of photosynthesis.

 Laboratory Preparation of Carbon (iv) oxide

(a) upward delivery or downward displacement of air over water

(b) downward delivery of upward displacement of air.

Carbon(iv)oxide or carbon dioxide is prepared in the laboratory by the action of dilute HCl or HNO3 on marble or limestone.

1. CaCO3(s) + 2HCl(aq)          CaCl2(aq) + CO2(aq) + H2O(l);     2. CaCO3 + 2HNO2               Ca(NO3)2 + H2O + CO2

it is also prepared by the action of the above named acids on hydrogen trioxocarbonate(iv) of sodium or potassium.   NaHCO3(aq) + HNO3(aq) NaNO3(aq) + H2O(l) + CO2(g)

Method: Place some limestone or marble into a flat bottomed flask. Pour dil. HCl or HNO3 through the thistle funnel.

Observation: There will be rapid effervescence and a colourless gas is produced. The gas is then collected over water or by downward delivery.

Note: Dil. H2SO4 is not used in this reaction because it coats the marble with a layer of insoluble calcium tetraoxosulphate(iv), which will terminate or stop the reaction after some seconds.

Other reactions that can result to the liberation of CO2 are as follows:

i. Fermentation of glucose: C6H12O6                            2C2H5OH + 2CO2 + Energy

ii. Thermal decomposition of marble          CaCO3 + Heat        CaO + CO2

iii. Combustion of hydrocarbons   CH4 + 2O2             CO2 + 2H2O

iv. Reduction of metallic oxides by carbon(ii)oxide  Fe2O3 + 3CO         2Fe + 3CO2

Properties of Carbon(iv)oxide

a. CO2 is a colourless, odourless and testless gas.

b. It is denser than air

c. It is slightly soluble in water. It dissolve in water to form trioxocarbonate (iv) acid. It is therefore an acid anhydride of trioxocarbonate (iv) acid.

d. It is an acidic oxide. It turns wet blue litmus paper faint red or claret red.

e. It can either be solidified or liquefied. It solidifies at 78oC and the solid form is known as dry ice.

f. It neither burns or supports combustion.

g. Caustic soda solution absorbs CO2 to form sodium troxocarbonate (iv) and water.

2NaOH + CO2           Na2CO3 + H2O

Burning magnesium continues to burn in a jar of CO2 because the flame is so hot that it can decompose CO2 into carbon and oxygen and the latter supports the burning. 2Mg + CO2                 2MgO + C

Test For CO2

CO2 turns lime water milky   CO2 + Ca(OH)2            CaCO3 + H2O

When excess CO2 is bubbled into lime water for a long time, the milky solution becomes clear again due to the formation of soluble calcium hydrogen trioxocarbonate(iv). CaCO3 + HCO+ CO2                 Ca(HCO3)2

Uses of CO2

1. It is used in fire extinguishers.

2. Green plants use it in the making of their food.

3. It is used in the manufacture of these compounds NaHCO3, Na2CO3, PbCO3, (NH4)2SO4

4. In the baking of bread, it is used as a leavening agent.

Carbon Cycle

Carbon cycle is the process by which the atmospheric CO2 is removed and re-introduced into the atmosphere. It is the process by which CO2 circulates in nature.The volume of CO2 in the air remains almost constant because the rate at which CO2 is removed from air by the process of photosynthesis and by its dissolution in rain water is equal to the rate at which the gas is  introduced into the environment by plant, animal, burning of carbon compounds and decomposition of carbonates in the soil.

CO2 dissolves in rain water to form a weak acid known as trioxocarbonate(iv)acid. CO2 + H2O       H2CO3

Trioxocarbonate (iv) acid

The weak acid on entering the soil, it reacts with the metals, to form the metallic trioxocarbonate (iv).

Ca + H2CO3           CaCO3 + H2      (Calcium trioxocarbonate (iv) )

2Na + H2CO3        Na2CO3 + H2     (Sodium trioxocarbonate (iv) )

When the soil gets heated by the sun, the temperature of these trioxocarbonate (iv) and trioxocarbonate (iv) and hydrogen trioxocarbonate (iv) is raised and they decompose to release CO2 into the atmosphere.

CaCO3           CaO + CO2 ;       Ca(HCO3)2                 CaO + H2O + CO2

 

 Carbon(II)Oxide (CO)

CO is prepared by dehydrating methanoic acid or ethanedoic acid with concentrated H2SO4.

HCOOH  H2SO4       CO + H2O ;      C2H2O4   Conc H2SO4             CO + CO2 + H2O

Laboratory preparation of CO.

The apparatus used is shown above. Some crystals of Ethanedoic acid (H2C2O4.2H2O) is placed in a strong round bottomed flask. Concentrated H2SO4 is poured down the thistle funnel. The mixture is warmed gently. As the crystals dissolve, effervescence occurs. A mixture of equal volume of CO and CO2 is envolved. The gas mixture is passed through KOH or NaOH which dissolves CO2 to form trioxocabonate (iv) salt of potassium or sodium, CO is collected over water.

CO OH

                                     CO2(g) + CO(g)

COO H

Ethandoic acid

    Conc. H2SO4 is not a catalyst in this reaction. It acts as a dehydrating agent.

Properties of CO

1. It is a colourless, odourless and tasteless gas       2. It is lighter than air       3. It is a neutral oxide

4. It is insoluble in water       5. It is soluble in ammonical cppoer(i) chloride

6. It is a strong reducing agent. Fe2O3(s) + 3CO(g) 2Fe(s) + 3CO2(g)

7. It is oxidized to CO2 when it burns in air with a blue flame. 2CO(g) + O2(g) 2CO2(g)

8. It combines readily with haemoglobin to form carboxy-haemoglobin (a stable compound), which prevents haemoglobin from doing its usual work of carrying oxygen.

Haemoglobin reacts with oxygen to form oxy-haemoglobin (an unstable compound). The formation of carboxy-haemoglobin when CO is inhaled in a large dose, makes one to die of suffocation.

O2 + Haemoglobin            Oxy-haemoglobin ;  CO + Haemoglobin Carboxy-haemoglobin

O2 + Carboxy-haemoglobin No reaction

Uses of Carbon (ii) oxide, CO.

1. It is used as fuel                     2. It is used a reducing agent

 Trioxocarbonate (IV) Salt

H2CO3 acid is dibasic and forms two types of salts. The salts are trioxocarbonate (iv).

H2CO3 + 2KOH K2CO3 + 2HO

Normal Salt

H2CO3 + KOH KHCO3 + H2O

Acid Salt

Trioxocarbonate (iv) salt is formed when H2CO3 reacts with free metal, metallic hydroxide,

metallic oxide or other metallic salts

H2CO3 + Zn ZnCO3 + H2

H2CO3 + Zn(OH)2 ZnCO3 + 2H2O

H2CO3 + ZnO ZnCO3 + H2O

H2CO3 + ZnCl2 ZnCO3 + HCl(aq)

Properties of Trioxocarbonate (iv) Salts

1. All trioxocarbonate (iv) salts are insoluble in water except Na2CO3, K2CO3 and (NH4)2CO3.

2. All common trioxocarbonate (iv) break down on heating to liberate CO2 with the exception of Na2CO3, K2CO3 and BaCO3.   ZnCO3(s)             ZnO(s) + CO2(g)

3. Na2CO3, K2CO3 and (NH4)2 CO3 dissolved in water to form the corresponding hydroxide.

K2CO3(s) + 2H2O             2KOH(aq) + H2CO3(aq)

4. All trioxocarbonate (iv) react with any dilute acid to form CO2, water and salt.

ZnCO3 + 2HCl             ZnCl2 + H2O + CO2

This equation can be represented ionically as:

CO2-3(s) + 2H+(aq)         H2O(l) + CO2(g)

5. Sodium, ammonium and potassium trioxocarbonate (iv) Dissolves in water to form a weak acid and a strong base.

K2CO3(s) + H2O     2KOH(g) + H2CO3 etc; KOH     K+(aq) + OH-  ;      H2CO3        2H+ + CO2-3(aq)

KOH is a strong base but H2CO3 is a weak acid. The resultant solution is alkaline to litmus because the number of OH- is greater than that of the hydrogen ions.

Preparation of Baking Soda (NaHCO3)

Baking powder is a common name for NaHCO3, but its IUPAC nomenclature is sodium hydrogen trioxocarbonate (iv). NaHCO3 can easily be prepared in the laboratory by bubbling excess CO2 into conc. NaOH solution. NaHCO3 is then precipitated as a white powder.

2NaOH(aq) + CO2(g)          Na2CO3(aq) + H2O(l) ;   Na2CO3(aq) + H2O(l) + CO2(g)                      2NaHCO3(s)

Or

NaOH(aq) + CO2(g)               NaHCO3(g)

Excess

NaHCO3 is slightly soluble in water; it is filtered, washed with a little cold distilled water and dried between filter papers.

Uses of NaHCO3

1. It is used in manufacturing effervescent tablets used to relieve indigestion (or constipation)

2. It is used in the production of baking powder, which comprises NaHCO3 and tartaric acid. The two ingredients react when they liberate CO2, which causes the dough to rise during the baking process.

 Laboratory Preparation Of Soda Ash (Na2CO3)

Sodium trioxocarbonate (iv), Na2CO3 is prepared in the laboratory by thermal decomposition of NaHCO3.

2NaHCO3                    Na2CO3 + H2O + CO2

Laboratory Preparation Of Washing Soda Na2CO3. 10H2O

Sodium trioxocarbonate (iv) decahydrate commonly known as the washing soda is prepared in the laboratory by dissolving Na2CO3 in water, concentrating the solution and allowing crystallization to take place.

Na2CO3 + 10H2O            Na2CO3. 10H2O

 Industrial Preparation of Soda Ash (NaCO3) [Solvay process]

Na2CO3 is manufactured in a large quantity by the solvey process. First of all, a mixture known as the ammoniacal brine is made by saturated a concentrated solution of sodium chloride (brine) with ammonia gas or by bubbling in enough quantity of NH3 gas into brine. The ammoniacal brine (NaCl solution + NH3) is made to trickle down a solvey tower. CO2 is at the same time forced into the tower and made to move upwards.

The perforated dome-shaped baffled-plates, slow down the flow of the ammoniacal brine and gas (CO2). They also create greater surface areas for reaction. These reactions which occur in the Solvay process is exothermic and so the tower has to be cooled.

NH3 + H2O + CO2             NH4HCO3 ;    NH4HCO3 + NaCl            NaHCO3 + NH4Cl

NH3 + H2O + CO2 + NaCl                  NaHCO3 + NH4Cl

NaHCO3 is filtered, rinsed with distilled water and heated to yield an anhydrous salt (Na2CO3) known as soda ash. Na2CO3 is dissolved in water and crystallized to form the washing soda (Na2CO3. 10H2O)

 Economic Advantage And Efficiency of the Solvey Process

The principal raw materials are cheap and are easily obtained as follows:

i. Brine is obtained from sea-water or salt deposits.    

ii. CO2 is obtained from the thermal decomposition of limestone.CaCO3         CaO + CO2      

 About 50% of CO2 required in the process is recovered while heating NaHCO3.

2NaHCO3           Na2CO3 + H2O + CO2

iii. The CaO obtained from number (ii) above reacts with NH4Cl (left after the precipitation of NaHCO3) to liberate NH3 gas.  CaO + 2NH4Cl           CaCl2 + H2O + 2NH3

Uses of Na2CO3

1. It is a useful reagent in the qualitative and volumetric analysis.

2. It is used in the manufacture of glass.

3. It is used in the softening of hard water.

4. It is use in the making of caustic soda (NaOH).

Assignment

1. What are the four stages in the coal making process?

2. Which of the two gaseous coke (producer gas and water gas) has more calorific value? Why?

3. Write an equation only to show the burning of carbon in

( a) a limited supply of air,                                       (b) an excess supply of air.

4. Use equations only to show four reactions that can result to the generation of CO2.

5. Write down five properties of CO2.

6. In the laboratory preparation of CO2, dilute H2SO4 is not made to react with marble, CaCO3. What is the actual reason for this?

7. What are the two properties of (a) CO2 (b) CO

8a. What do you understand by carbon cycle?

 b. Write two processes through which  i. CO2 is removed from the atmosphere.     ii. CO2 is re-introduced into the atmosphere.