Carbon And Its Properties

CONTENT
• Occurrence and Allotropes
• Properties

OCCURRENCE

1. It occurs naturally as diamond and graphite.
2. It occurs in an impure form as coal.
3. It occurs in the combined state as petroleum, wood and natural gases.
4. It occurs in minerals such as limestone (CaCO3) and dolomite (MgCO3)
5. It occur in the atmosphere (air) as CO2
6. It is an essential constituent of all forms of plant and animal life.

ALLOTROPES OF CARBON
Allotropy is the phenomenon whereby an element exists in two or more different forms in the same physical state. The different forms of the elements are known as allotropes. They have the same chemical properties but different physical properties.
Carbon exists in several allotropic forms:
(1). Crystalline Allotropes e.g Diamond and graphite
(2). Non-crystalline Allotropes/Amorphous carbon e.g coal, charcoal, coke, lampblack and carbon black (soot)

Crystalline Allotropes of carbon
Diamond: Diamond is the purest form of carbon. The diamond crystal is octahedral in shape. It is actually a giant molecule in which the carbon atoms are closely packed and held together by strong covalent bonds.

Basic Tetrahedral Shape in Diamond Crystals

PROPERTIES OF DIAMOND
(1) Diamond is extremely hard and strong with high melting point because of strong covalent bond. Diamond is the hardest substance known in the world.
(2) It has a high density because of its compactness of crystal.
(3) It is a very resistant to chemical action and temperature because all four valence electrons are saturated bonded.
(4) It is a non-conductor of electricity because there are no free valence electrons in the crystal lattice.
(5) Transparent and highly refractive, hence it is used as a jewel and sparkling substance.

USES
(1) They are used industrially in drills for in mining since they are dense and hard.
(2) They are used to sharpen very hard tools.
(3) They are used for cutting glass and metals.
(4) They are also used as pivot supports in precision instruments and as dies for drawing wires
(5) It is valuable in making jewellery (i.e. its high refractive index and dispersion power give it a sparkling brilliance when it is cut and polished).
Artificial diamond: They are made by subjecting graphite to a very high temperature and pressure for several hours in the presence of nickel or rhodium catalyst.

GRAPHITE: The graphite crystal is hexagonal in shape. The carbon atoms in graphite form flat layers. These layers are arranged in parallel, one above the other to form a crystal lattice.

PROPERTIES OF GRAPHITE
(1) Graphite is soft and slippery because of weak forces holding its layers. Each layer can slide over one another. Hence, graphite acts as a lubricant.
(2) It is less dense and prone to chemical attack due to its open structures in layers.
(3) It is a good conductor of electricity because of the presence of free delocalized electrons (mobile electron) in the crystal lattice.
(4) It is inert and used to absorb radiations in nuclear station in atomic pile.

USES
(1) It is usually used on bicycle chains and for the bearings of some motor cars.
(2) It is used as a non greasy lubricant (i.e combining it with oil makes a high temperature lubricant).
(3) It is used as electrodes in electroplating and in dry cells (since it is a good conductor of electricity and relatively inert).
(4) Graphite can be used to make a non-conductor conductive by coating with it.
(5) It is used to line crucibles for making high-grade steel and other alloys (since it can withstand high temperature).
(6) It is used in making lead pencils i.e. combining it with clay makes lead in pencils.
(7) It is used as a black pigment in paints.
(8) It is used as a neutron moderator in atomic piles.

INDUSTRIAL PREPARATION OF GRAPHITE
Graphite is produced industrially by heating coke in an electric furnace to a very high temperature for about 20 to 30 hours. This process is called the Acheson process. Acheson process is a process of producing graphite from coke at high temperature. Air is excluded by covering the coke with sand. The graphite produced is very pure and free from grit.

DIFFERENCES IN PROPERTIES BETWEEN GRAPHITE AND DIAMOND
Graphite Diamond

1. It has a density of 2.3gcm-3 1. It has a density of 3.5gcm-3
2. It is a black, opaque solid 2. It is a colourless, transparent solid
3. It is very soft, marks paper 3. It is the hardest known substance.
4. It is a good conductor of electricity 4. It is a non-conductor of electricity
5. Attacked by potassium trioxochlorate (v) and trioxonitrate (v) acid together. 5. Not attacked by these reagents.
   Note: Diamond is transparent to x-rays while glass is almost opaque.

EVALUATION

1. (a) What is allotropy? (b) Mention two allotropes of carbon
2. Compare the properties of these allotropes of carbon.

AMORPHOUS CARBON
Carbon also occurs in a number of other forms which has no definite crystalline structure. These non-crystalline structures which are not considered to be true allotropes include:

CHARCOAL: This is made by burning wood, bones, sugar etc in a limited amount of air. Charcoal is used to remove colour from substances. Wood charcoal is used in absorbing poisonous gases while animal charcoal is used in absorbing colours.

CARBON BLACK AND LAMP BLACK: Lamp black is obtained by burning the wick of an oil lamp excessively so that it leaves a deposit of soot on the lamp-glass and cover; while carbon black is obtained from burning coal gas, natural gas or petroleum. Carbon black and lamp black are used as an additive to rubber tyres. They are also used in making printer’s ink, carbon paper, black shoe polish, type writing ribbons e.t.c

PHYSICAL PROPERTIES OF CARBON
(1) All the different allotropes of carbon are black or greyish-black solids except diamond and they are odourless and tasteless.
(2) They have a high melting point of about 35000C.
(3) They are insoluble in all common solvents like water, alkalis, acids, petrol and carbon (iv) sulphide (CS2). This is the reason carbon deposits inside motor engines have to be removed mechanically. This is known as decarbonisation of motor engines.

CHEMICAL PROPERTIES

1. Reaction with water: Carbon (iv) oxide is not very active chemically. It dissolves in water to form trioxocarbonate (iv) acid (Soda water). This is a weak, dibasic acid which ionizes slightly.

(a)  CO_2(g)             +     H₂O_(l)  — >   H₂CO_3(aq)

(b)  H₂CO_3(aq)     +     H₂O_(l) — >    H₃O⁺_(aq)       +    HCO₃^–_(aq)

(c)  HCO₃^– _(aq)       +     H₂O_(l)  — >   H₃O⁺_(aq)     +    CO₃^2-_(aq)

On heating, trioxocarbonate (iv) acid decomposes to form H₂O_(l) and CO_2(g).

2. Reaction with alkalis: It reacts directly with alkalis to yield trioxocarbonate (iv)

CO_2(g)     +  2NaOH_(aq) — >   Na₂CO_3(aq)       +       H₂O_(l)

Limited

Excess CO₂ reacts with alkalis to produce Hydrogen trioxocarbonate (iv) salt.

CO_2(g)       +  NaOH_(aq) — >     NaHCO_3(aq)

Excess.

3.  Reaction with burning Na, K or Mg: CO₂ is reduced to carbon by burning Na, K or Mg.

CO_2(g)     +      2Mg_(s) — >    C_(s)      +       2MgO_(s)

Note: CO₂ does not support combustion.

4.  Reaction with red hot carbon: CO₂ is reduced to CO, If the gas is passed over red hot carbon.

CO_2(g)      +     C_(s)   — >    2CO_(g)

The reaction is of great importance in the blast furnace and in the manufacture of gaseous fuels.

Test for CO₂: Bubble the unknown gas through a solution of lime water (Calcium hydroxide)if the lime water turn milky due to the formation of insoluble calcium trioxocarbonate (iv), then the unknown gas is CO₂

Ca(OH)_2(aq)        +      CO_2(g) — >  CaCO_3(s)     +       H₂O_(l).

If the gas is bubbled in excess, the milkiness disappears and turns to a clear solution due to the formation of soluble calcium hydrogen trioxocarbonate (iv).

CaCO_3(s)    +   H₂O_(l)    +   CO_2(g) — >   Ca(HCO₃)_(aq)

Finally, if the clear solution is heated, the milkiness reappears due to the decomposition of soluble Ca(HCO₃)₂ to form insoluble CaCO₃

Ca(HCO₃)_2(aq)  — >  CaCO_3(s)    +  H₂O_(l)   +   CO_2(g)

Uses of carbon (iv) oxide

1.  It is used as fire extinguishers since it does not support combustion.

2.  It gives carbonated (aerated) drinks their refreshing taste. Beer, cider and champagne contains CO₂

3.  It is used in the manufacture of Na₂CO₃ (washing soda) by the Solvay process.

4.  It is used as a leavening agent in the baking of bread. Yeast and baking powder produces CO₂ which make the dough of bread to rise.

5.  It is used in the manufacture of fertilizer (such as urea and (NH₄)₂SO₄.

6.  Solid CO₂ (i.e dry ice) is used as a refrigerant for perishable goods e.g ice cream. (It sublimes on warming and provides a lower temperature).

7.  Gaseous CO₂ is used to preserve fruits.

8.  CO₂ is also used as a coolant in nuclear reactors.

EVALUATION

1.  Describe the laboratory preparation of dry carbon (iv) oxide.

2.  Write balanced equation to show the following reactions of CO₂:

(a)  Reaction with sodium hydroxide

(b)  Reaction with burning magnesium

CARBON (II) OXIDE

LABORATORY PREPARATION

1.  Carbon (ii) oxide can be prepared by passing Carbon (iv) oxide through red-hot carbon while the Carbon (iv) oxide is itself reduced to Carbon (ii) oxide. The gaseous mixture is passed through concentrated NaOH to remove the excess Carbon (iv) oxide.

CO_2(g)            +       C_(s) — >   2CO_(g)

The pure Carbon (ii) oxide is collected over water.

2.  Carbon (ii) oxide can also be prepared by the dehydration of methanoic (formic) acid or ethanedioic (oxalic) acid, using concentrated tetraoxosulphate (vi) acid.

HCOOH_(l)          — >^{Conc. H2SO4}    CO_(g)   +   H₂O

Methanoic acid

Note: The gaseous mixture is passed through concentrated NaOH to remove the CO₂.

Caution: The preparation of CO must be done in a fume cupboard as the gas is poisonous.

The major air pollutants that can result from smoky vehicles are Carbon (ii) oxide and Carbon particles.

When CO is breath in for any length of time, even 1% of it in the air may cause death, which makes it clear how dangerous it can be to run a car engine in a closed garage or a generator in a closed room. Very often the victim collapse without warning so insidious is its effect.

PHYSICAL PROPERTIES OF CO

(1) CO is a poisonous, colourless, tasteless and odourless gas.

(2) It is insoluble in water, but dissolves in a solution of ammoniacal copper (i) chloride.

(3) It is neither lighter nor heavier than air.

(4) It is neutral to litmus.

CHEMICAL PROPERTIES OF CO

(1) As a reducing agent: CO is a strong reducing agent. It reduces some metallic oxides to the metals and it is oxidized to CO₂.

PbO_(s)   +    CO_(g)   — >Pb_(s)         +       CO_2(g)

Fe₂O_3(s)  +   3CO_(g— > 2Fe_(s)        +       3CO_2(g)

CuO_(s)    +    CO_(g) — >  Cu_(s)         +       CO_2(g)

2.  Combination reaction

(a). With oxygen: CO burns in air with a faint pale blue flame to form CO₂ .

2CO_(g)      +      O_2(g)   — >       2CO_2(g)

(b).  With haemoglobin: CO combine irreversibly with haemoglobin in the  red blood cells to form carboxy-haemoglobin thereby preventing the red corpuscle from acting as oxygen carrier.

3.  CO mixed with NaOH at 150⁰C under 50atm pressure to form Sodium formate.

CO_(g)       +     NaOH_(aq)                   HCOONa_(aq)

This product decomposes into Sodium oxalate and H₂ at 400⁰C

                                                                                                            COONa

2HCOONa_(aq)      +    H_2(g)  ^4000C  — >COONa

4.  CO combined with certain metals (e.g Ni, Fe, Cobalt) when hot to form a volatile carbonyl.

Ni_(s)    +     4CO_(g) — >     Ni(CO)_4(aq)

5. CO is absorbed readily by a solution of Copper (i) Chloride (CuCl) in aqueous NH₃ or conc. HCl forming a compound, CuCl.CO.2H₂O. This reaction is used to separate CO from a mixture of gases.

CO_(g)  +  CuCl_(s)   +  2NH₄OH_(aq) — > CuCl.CO.2H₂O_(aq)    +   2NH_3(g)

6.  CO combined with Chlorine gas when expose to ultra-violet light or passed over a catalyst of activated charcoal at 150⁰C to form carbonyl chloride.

CO_(g)     +       Cl_2(g) — >    COCl_2(g)

This product, COCl₂, is also known as Phosgene and was employed as a poisonous gas in the First World War. It is now use in the manufacture of dyestuff.

Test for Carbon (ii) oxide

When a lighted splint is inserted into a test tube containing CO_(g) and some lime water added to it and shaken, the CO_(g) burn with a pale-blue flame and turn lime water milky after burning but not before burning.

Uses of Carbon (ii) oxide

(1) CO is used in the extraction of metals from their ores.

(2) It is also an important constituent of gaseous fuels like producer gas and water gas.

(3) CO gas is used in the manufacture of methyl alcohol, synthetic petrol, carbonyl chloride, oxalate and formate.

EVALUATION

1.  Describe the laboratory preparation of Carbon (ii) oxide.

2.  Explain why Carbon (ii) oxide cannot be collected by any method of delivery

3.  Write two equations to show the chemical properties of Carbon (ii) oxide

GENERAL EVALUATION/REVISION

1. What is the pH of a solution whose H⁺ concentration is 3.0 x 10^-5moldm^-3?

2. Mention the separation technique that be used to recover a soluble salt that (a) decompose on heating (b) does not decompose on heating

3. How many molecules are there in 14g of Nitrogen gas at s.t.p? [N=14, N_A=6.0 x 10²]

4. What is the number of oxygen atoms in 32g of the gas?

5. State Avogadro’s law

READING ASSIGNMENT

New school chemistry for senior secondary schools by O.Y Ababio Page 128-133

WEEKEND ASSIGNMENT

1. Kipp’s apparatus is important in the laboratory because it (a) allows intermittent supply of gases. (b) is used for preparing poisonous gases. (c) is used to prepare light gas. (d) is used to prepare sensitive gas

2. Gas prepared by the reaction between methanoic acid and concentrated tetraoxosulphate (vi) acid is (a) SO₂           (b) CO              (c) CO₂           (d) H₂S.

3. Gas which dissolves in ammoniacal copper (i) chloride but insoluble in water is

(a) NH₃ (b) CO (c) N₂O (d) CO₂.

4. Where else is CO₂ found in free state apart from the atmosphere?

(a) In carbonated drinks. (b) Dissolved form in water. (c) In corals. (d) In limestone region

5. It is dangerous to stay in a badly ventilated room which has a charcoal fire because of the presence of (a) carbon (ii) oxide (b) carbon (iv) oxide (c) hydrogen sulphide (d) producer gas.

THEORY

1(a) Why is the laboratory preparation of carbon (ii) oxide done in a fume chamber?

(b) State the property of CO₂ that makes it to be used in (i) carbonated drinks (ii) fire extinguishers

2(a) Why it is not advisable to stay in a closed garage for a long time when racing a car engine.

(b). State what is observed when (i) excess CO₂ is bubbled through lime water. (ii) the solution in b(i) above is heated.