CARBON & ITS COMPOUNDS - Questions & Answers

EXERCISE

Introduction

1. Differentiate between the two branches of chemistry - organic chemistry & inorganic chemistry with suitable examples.

Organic Chemistry: It is the study of specific carbon compounds which include compounds mainly of carbon, hydrogen, and oxygen. Examples: Hydrocarbons like methane (CH₄) and ethane (C₂H₆), and acids like acetic acid (CH₃COOH).
Inorganic Chemistry: It includes the study of oxides of carbon, carbonates, bicarbonates, metallic carbides, etc. Examples: Carbon monoxide (CO), carbon dioxide (CO₂), and calcium carbonate (CaCO₃).

2. State how carbon occurs in the free state and in the combined state.

Free state: Carbon occurs free in nature in crystalline forms as diamond and graphite, and in amorphous forms as coal.
Combined state: Carbon occurs as carbon dioxide in the atmosphere; as hydrocarbons in natural gas and petroleum; in the solid state as wood (cellulose); and in plants and animals as carbohydrates.

Allotropes - of carbon

3. Define the term 'allotropy'. Give a reason why carbon exhibits allotropy.

Allotropy: It is the existence of an element in more than one physical form having different physical properties but the same chemical properties. Carbon exhibits allotropy due to the difference in the atomic arrangement of carbon atoms in its crystal structure.

4. Name two crystalline and four amorphous allotropes of carbon.

Two crystalline allotropes: Diamond and Graphite.
Four amorphous allotropes: Coal, Coke, Lampblack (soot), and Charcoal (Wood/Bone/Sugar charcoal).

Crystalline allotropes - diamond & graphite

5. Compare the structure of the crystal of diamond & graphite with special reference to the reason for diamond being the hardest natural substance while graphite one of the softest. Compare the electrical & thermal conductivity of the two crystalline allotropes of carbon.

Structure and Hardness: Diamond is built from a three-dimensional tetrahedral unit where each carbon atom is linked to four other carbon atoms by strong covalent bonds, creating a rigid lattice that makes it the hardest substance. Graphite is built from two-dimensional hexagonal units arranged in parallel layers held by weak Van der Waals forces; these layers slide over each other, making graphite soft.
Conductivity: Diamond is a non-conductor of electricity (due to lack of free electrons) and a very poor conductor of heat. Graphite is a good conductor of both electricity (due to the presence of mobile electrons) and heat.

6. With reference to the structure of the two crystalline allotropes of carbon, state why diamond is inert or unreactive while graphite is comparably more reactive.

Diamond is chemically inert because of its extremely compact and rigid three-dimensional structure where all valency electrons are tightly bound. Graphite is comparatively more reactive because it has a more open, layered structure that is more prone to chemical attack.

7. State the reasons for a] Use of diamond - as an item of jewellery. b] Use of graphite - i] as a lubricant for heated machine parts, ii] as a lining for crucibles used in manufacture of high grade steel iii] as an electrode in electroplating.

a] Diamond in jewellery: Because it has a sparkling brilliance due to its high refractive index.
b] Graphite uses:
i] Lubricant: Because its layers slide over one another making it slippery, and it does not char at high temperatures.
ii] Crucible lining: Because it can withstand very high temperatures (high melting point) and is a good conductor of heat.
iii] Electrode: Because it is a good conductor of electricity and is relatively inert (inactive with acids).

Amorphous allotropes

8. State in brief the transformation of vegetable matter to different types of coal varying in carbon content. State two uses each of a] coal b] coke c] lampblack or soot

Coal is formed by the slow bacterial decomposition of vegetable matter buried under heat, pressure, and limited air over millions of years. The types are Peat (60% C), Lignite (65% C), Bituminous (85% C), and Anthracite (90% C).
a] Coal uses: Cheap fuel, manufacture of coke/coal gas.
b] Coke uses: Fuel (burns without smoke), reducing agent in iron production.
c] Lampblack uses: Making printers ink/black shoe polish, filler in rubber tyres.

9. 'Wood charcoal - an amorphous allotrope of carbon reduces heated metallic oxides to metals'. Give a balanced equation to support the statement.

ZnO + C → Zn + CO
(or Fe₂O₃ + 3C → 2Fe + 3CO)

Carbon dioxide

10. Carbon dioxide occurs both in the free state and in the combined state. State three methods how carbon dioxide is added to the atmosphere.

1. Respiration in living organisms.
2. Burning of carbon compounds (e.g., wood, petroleum).
3. Decay and decomposition of organic matter.

11. In the laboratory preparation of carbon dioxide by action of a dilute acid on a metallic carbonate give - a] A balanced equation for the preparation b] A reason for i] use of a washer bottle containing conc. H₂SO₄ in the preparation ii] Not collecting the prepared gas over water iii] Not using dilute sulphuric acid as a reactant in the preparation.

a] Equation: CaCO₃ + 2HCl [dil.] → CaCl₂ + H₂O + CO₂
b] Reasons:
i] Conc. H₂SO₄ is used to absorb moisture from the gas.
ii] It is not collected over water because it is fairly soluble in water.
iii] Dilute sulphuric acid reacts with calcium carbonate to form insoluble calcium sulphate, which forms a coating on the marble chips and stops the reaction.

12. How would you prove experimentally that a] Carbon dioxide does not support combustion b] Is slightly acidic in nature.

a] Support of combustion: Lower a burning candle into a jar of CO₂; the candle is immediately extinguished.
b] Acidic nature: Introduce a moist blue litmus paper into a jar of CO₂; it turns pink or scarlet.

13. Starting from carbon dioxide how would you obtain a] A weak acid b] A fertilizer c] A highly poisonous gas d] Black particles of carbon. [Give balanced equations for the same]

a] Weak acid: CO₂ + H₂O → H₂CO₃ (Carbonic acid)
b] Fertilizer: 2NH₃ + CO₂ → (NH₂)₂CO + H₂O (Urea)
c] Poisonous gas: CO₂ + C → 2CO (Carbon monoxide)
d] Black carbon: CO₂ + 2Mg → 2MgO + C

14. State how you would convert carbon dioxide to a metallic carbonate using a basic oxide e.g. sodium oxide. [Give a balanced equation]

Na₂O + CO₂ → Na₂CO₃ (Sodium carbonate)

15. 'When carbon dioxide is bubbled into lime water, the lime water turns milky and when bubbled in excess the milkiness disappears'. Give balanced equations to support the statement.

Turning milky: Ca(OH)₂ + CO₂ → CaCO₃↓ + H₂O
Disappearing milkiness: CaCO₃ + H₂O + CO₂ → Ca(HCO₃)₂ (Soluble)

16. Explain the term 'dry ice'. State its application. Give three reasons why carbon dioxide finds application in fire extinguishers.

Dry ice: It is solid carbon dioxide, formed when CO₂ is cooled to -78°C. It is used as a refrigerant for food preservation.
Fire extinguisher reasons:
1. It is non-combustible.
2. It is a non-supporter of combustion.
3. It is heavier than air and covers the fire like a blanket, cutting off oxygen.

17. Using a magnesium ribbon, how would you prove that a given gas jar contains carbon dioxide.

A burning magnesium ribbon continues to burn in a jar of CO₂ and deposits black particles of carbon on the walls of the jar. This specific reaction proves the presence of carbon dioxide.

Carbon monoxide

18. State the function of conc. sulphuric acid in the laboratory preparation of carbon monoxide from oxalic acid.

Conc. sulphuric acid acts as a dehydrating agent, removing a molecule of water from the acid to liberate carbon monoxide.

19. Give reasons why carbon monoxide is considered a highly poisonous gas. State why it is dangerous i] To sleep in a closed room with a fuel burning ii] To be in the vicinity of smokers.

Carbon monoxide is poisonous because it combines with haemoglobin to form stable carboxyl-haemoglobin, preventing oxygen transport and causing asphyxia (suffocation).
i] Closed room: Burning fuel in limited air produces CO, which can lead to death by suffocation.
ii] Smokers: Tobacco fumes contain traces of CO, which is harmful to inhale.

20. Convert carbon monoxide to carbon dioxide using two different methods.

1. Burning in air: 2CO + O₂ → 2CO₂
2. Using a reducing agent (reducing a metallic oxide): ZnO + CO → Zn + CO₂

21. State how carbon monoxide finds application in i] The metallurgy of iron ii] Preparation of an alcohol. [Give balanced equations for the same]

i] Metallurgy: It acts as a reducing agent. Fe₂O₃ + 3CO → 2Fe + 3CO₂
ii] Alcohol: It reacts with hydrogen to form methyl alcohol. CO + 2H₂  ZnO, 450°C → CH₃OH

CRITICAL THINKING OBJECTIVE TYPE QUESTIONS

Q.1 Name the following:

1. A crystalline allotrope of carbon built up from a hexagonal unit. Graphite
2. An allotrope of carbon used for the manufacture of coke. Coal (Bituminous)
3. An amorphous allotrope of carbon which floats on water. Wood charcoal
4. An acid formed when carbon dioxide is dissolved in water under pressure. Carbonic acid
5. The gaseous product obtained when carbon monoxide is passed over heated iron [III] oxide. Carbon dioxide

Q.2 Select the correct answer from A, B, C, D & E for each statement given below:
A: Anthracite   B: Diamond   C: Carbogen   D: Urea   E: Lampblack

1. An allotrope of carbon used as a tip for deep boring drills. B: Diamond
2. The type of coal with the highest carbon content. A: Anthracite
3. An allotrope of carbon, obtained by burning kerosene oil in a limited supply of air. E: Lampblack
4. A nitrogenous fertilizer obtained from carbon dioxide. D: Urea
5. A compound which finds use for a victim of carbon monoxide poisoning. C: Carbogen

Q.3 Give a balanced equation for the following conversions: [In one or two steps]

1. Coke to water gas. C + H₂O [steam]  1000°C → (CO + H₂)
2. Calcium bicarbonate to calcium nitrate using a dilute acid. Ca(HCO₃)₂ + 2HNO₃ → Ca(NO₃)₂ + 2H₂O + 2CO₂
3. Lime water [soln. of calcium hydroxide] to calcium bicarbonate. Step 1: Ca(OH)₂ + CO₂ → CaCO₃ + H₂O; Step 2: CaCO₃ + H₂O + CO₂ [excess] → Ca(HCO₃)₂
4. Carbon dioxide to carbon. CO₂ + 2Mg → 2MgO + C
5. A metallic oxide to calcium carbonate. CaO + CO₂ → CaCO₃

Q.4 Complete the statements by filling the blanks with the correct word from the bracket.

1. The crystal of graphite [diamond/graphite] is opaque to light and is a good conductor of heat.
2. A graphite-clay mixture baked at high temperature is used in making refractory crucibles [lubricants/refractory crucibles]
3. Adsorption capacity of wood charcoal is increased by passing steam [carbon dioxide/steam] over wood charcoal, at high temperatures.
4. Organic [organic/inorganic] Chemistry is the Chemistry of carbon compounds mainly of 'Carbon', 'Hydrogen' & 'Oxygen'.
5. Sodium oxide combines with carbon dioxide to give sodium carbonate [sodium carbonate/sodium bicarbonate].

Q.5 Give reasons for the following :

1. Diamond & graphite are allotropic modifications of carbon.
Because they are both made of the same element, carbon, but have different atomic arrangements resulting in different physical properties.
2. It is dangerous to stand behind a running engine of a vehicle.
Exhaust fumes from cars contain carbon monoxide (due to incomplete combustion), which is a highly poisonous gas.
3. Both CO₂ & SO₂ turn lime water milky. Moist potassium permanganate paper, helps in distinguishing the two gases.
SO₂ turns moist purple potassium permanganate paper colourless, whereas CO₂ has no effect on it.
4. Carbon monoxide and not carbon dioxide is a highly poisonous gas.
CO combines with haemoglobin to block oxygen transport, whereas CO₂ is a normal product of respiration and only dangerous in extremely high concentrations by displacing oxygen.
5. Lime water finds application for testing both carbon dioxide & carbon monoxide gas individually.
CO₂ turns lime water milky directly. CO does not, but if CO is ignited, it burns to form CO₂, which then turns lime water milky.