NCERT Class 10 Science Chapter 4: Carbon and its Compounds - A Complete Guide

Introduction to Carbon and its Compounds

Welcome, students! Prepare to delve into one of the most fascinating and fundamental chapters of Class 10 Chemistry: Carbon and its Compounds. Carbon is an element that is truly the backbone of life. From the food we eat and the clothes we wear to the fuels that power our world, carbon is everywhere. This field of chemistry, which deals with carbon compounds, is so vast that it's a separate branch called Organic Chemistry. In this chapter, we will explore the unique properties of carbon that allow it to form millions of compounds, understand the types of bonds it forms, and learn about some of its important compounds that we use in our daily lives.

Why is carbon so special? What makes it different from all other elements? This chapter will answer these questions by exploring its unique bonding nature and its ability to form long, stable chains with itself and other elements. Let's begin our journey into the world of carbon!

Bonding in Carbon – The Covalent Bond

To understand carbon's versatility, we must first understand how it forms bonds with other atoms. Most elements form bonds to achieve a stable electron configuration, usually by gaining, losing, or sharing electrons to have a full outermost shell (the octet rule). Carbon has an atomic number of 6, which means its electronic configuration is 2, 4. It has 4 electrons in its outermost shell (the L shell).

To achieve a stable, noble gas configuration, carbon has two theoretical options:

• Gain 4 electrons: It could gain four electrons to form a C⁴⁻ anion. However, it would be extremely difficult for a nucleus with only 6 protons to hold onto 10 electrons.
• Lose 4 electrons: It could lose four electrons to form a C⁴⁺ cation. This would require an immense amount of energy to remove four electrons from the atom.

Since both gaining and losing four electrons are energetically unfavorable, carbon adopts a brilliant strategy: it shares its valence electrons with other atoms. This sharing of electrons to form a chemical bond is known as a covalent bond.

What is a Covalent Bond?

A covalent bond is a chemical bond formed by the mutual sharing of one or more pairs of electrons between two atoms. The shared pair of electrons belongs to both atoms and helps them both achieve a stable electronic configuration. Compounds formed by covalent bonds are called covalent compounds.

Formation of Simple Covalent Molecules

Let's see how covalent bonds are formed in some simple molecules using electron-dot structures.

1. Hydrogen Molecule (H₂): A hydrogen atom has 1 electron. It needs one more to fill its K shell. Two hydrogen atoms share one electron each to form a pair of shared electrons. This results in a single covalent bond between them.

H• + •H → H:H or H—H

2. Oxygen Molecule (O₂): An oxygen atom has 6 valence electrons. It needs two more to complete its octet. Two oxygen atoms share two pairs of electrons (a total of four electrons) to form a stable O₂ molecule. This is a double covalent bond.

O::O or O=O

3. Nitrogen Molecule (N₂): A nitrogen atom has 5 valence electrons and needs three more. Two nitrogen atoms share three pairs of electrons, forming a very strong triple covalent bond.

N:::N or N≡N

4. Methane Molecule (CH₄): A carbon atom shares its four valence electrons with four hydrogen atoms. Each hydrogen atom shares its single electron with carbon. This way, carbon completes its octet, and each hydrogen atom completes its duet, forming a stable methane molecule with four single covalent bonds.

Properties of Covalent Compounds

Covalent compounds exhibit distinct properties due to the nature of their bonds:

• Low Melting and Boiling Points: The forces of attraction between the molecules of covalent compounds (intermolecular forces) are weak. Less energy is required to break these forces, resulting in low melting and boiling points. For example, methane (CH₄) has a boiling point of -162°C.
• Poor Conductors of Electricity: Since electrons are shared between atoms and are not free to move, covalent compounds do not have charged particles (ions). Therefore, they are generally poor conductors of electricity in both solid and molten states.

The Versatile Nature of Carbon

The number of known carbon compounds runs into the millions, far outnumbering the compounds formed by all other elements combined. This remarkable ability is due to two key properties of carbon:

1. Tetravalency

As we've seen, carbon has a valency of four. This means a single carbon atom can form covalent bonds with four other atoms (which can be carbon or atoms of other elements like hydrogen, oxygen, nitrogen, sulphur, and halogens). This property allows for the formation of a vast array of compounds with different properties depending on the elements attached to the carbon.

2. Catenation

Perhaps the most unique property of carbon is catenation — the ability to form strong covalent bonds with other carbon atoms. This self-linking property allows carbon to form long, stable chains, branched chains, and even closed rings. No other element exhibits catenation to the extent that carbon does. The carbon-carbon bond is very strong and stable, which is the reason for the stability of these large molecules.

• Straight Chains: ...-C-C-C-C-... (e.g., Butane)
• Branched Chains: Carbon atoms forming branches off a main chain (e.g., Isobutane)
• Rings: Carbon atoms arranged in a closed loop (e.g., Cyclohexane)

The combination of tetravalency and catenation gives rise to the immense diversity of organic compounds.

Allotropes of Carbon

Allotropes are different structural forms of the same element in the same physical state. Despite being made of only carbon atoms, allotropes can have vastly different physical properties because the arrangement of atoms is different. Carbon has several well-known allotropes.

1. Diamond

• Structure: In a diamond, each carbon atom is covalently bonded to four other carbon atoms, forming a rigid three-dimensional tetrahedral network.
• Properties: This rigid structure makes diamond the hardest natural substance known. It has a very high melting point and is a poor conductor of electricity because there are no free electrons. It is transparent and has a high refractive index, giving it its characteristic sparkle.
• Uses: Used in jewelry, cutting tools for glass and rocks, and in surgical instruments.

2. Graphite

• Structure: In graphite, each carbon atom is bonded to three other carbon atoms in the same plane, forming flat hexagonal rings. These rings stack on top of each other in layers. The layers are held together by weak van der Waals forces.
• Properties: Because the layers can slide over one another, graphite is soft, slippery, and greasy. One of the four valence electrons of each carbon atom is free to move between the layers, making graphite a good conductor of electricity (a rare property for a non-metal).
• Uses: Used as a lubricant, in pencil ‘lead’, and as electrodes in batteries and industrial electrolysis.

3. Buckminsterfullerene (C-60)

• Structure: This allotrope consists of carbon atoms arranged in the shape of a hollow sphere. The most famous one is C-60, which has 60 carbon atoms arranged like a soccer ball (with 20 hexagons and 12 pentagons).
• Properties: They are dark solids at room temperature and are a new class of materials with potential applications in nanotechnology and medicine.

Saturated and Unsaturated Carbon Compounds

Carbon compounds that contain only carbon and hydrogen atoms are called hydrocarbons. They are the simplest organic compounds and are classified based on the type of bonds between their carbon atoms.

Saturated Compounds (Alkanes)

These are hydrocarbons in which the carbon atoms are connected only by single covalent bonds. They are called 'saturated' because each carbon atom is bonded to the maximum possible number of hydrogen atoms, and no more atoms can be added. They are also known as alkanes.

• General Formula: CₙH₂ₙ₊₂ (where n = 1, 2, 3, ...)
• Examples: Methane (CH₄), Ethane (C₂H₆), Propane (C₃H₈).

Unsaturated Compounds (Alkenes and Alkynes)

These are hydrocarbons that contain one or more double or triple covalent bonds between carbon atoms. They are 'unsaturated' because they have fewer hydrogen atoms than the corresponding alkane, and more atoms can be added across the double or triple bond.

Alkenes

• Bonding: Contain at least one carbon-carbon double bond (C=C).
• General Formula: CₙH₂ₙ (where n = 2, 3, 4, ...)
• Examples: Ethene (C₂H₄), Propene (C₃H₆).

Alkynes

• Bonding: Contain at least one carbon-carbon triple bond (C≡C).
• General Formula: CₙH₂ₙ₋₂ (where n = 2, 3, 4, ...)
• Examples: Ethyne (C₂H₂), Propyne (C₃H₄).

Homologous Series

A homologous series is a series of organic compounds which has the same functional group and similar chemical properties, in which the successive members differ by a –CH₂ group.

Characteristics of a Homologous Series:

• All members can be represented by a general formula (e.g., CₙH₂ₙ₊₂ for alkanes).
• Each successive member differs from the next by one carbon atom and two hydrogen atoms (a –CH₂ group).
• The difference in molecular mass between two consecutive members is 14 u.
• They show similar chemical properties due to the presence of the same functional group.
• They show a gradual change in physical properties (like melting point, boiling point, and density) as the molecular mass increases.

For example, the alcohol series: Methanol (CH₃OH), Ethanol (C₂H₅OH), Propanol (C₃H₇OH), etc., form a homologous series.

Nomenclature of Carbon Compounds

With millions of organic compounds, a systematic method for naming them is essential. The system used is called the IUPAC (International Union of Pure and Applied Chemistry) nomenclature. The IUPAC name of a compound is built from three main parts: a word root, a suffix, and a prefix.

Step 1: Identify the number of carbon atoms (Word Root).

The number of carbon atoms in the longest continuous chain determines the word root.

Step 2: Identify the type of C-C bonds (Primary Suffix).

• If all bonds are single: Suffix is ‘-ane’ (e.g., Prop + ane = Propane).
• If there is a double bond: Suffix is ‘-ene’ (e.g., Prop + ene = Propene).
• If there is a triple bond: Suffix is ‘-yne’ (e.g., Prop + yne = Propyne).

Step 3: Identify the functional group (Prefix or Secondary Suffix).

A functional group is an atom or a group of atoms that replaces hydrogen in a hydrocarbon chain and is responsible for the characteristic chemical properties of the compound.

Chemical Properties of Carbon Compounds

Let's explore some of the key chemical reactions that carbon compounds undergo.

1. Combustion

Combustion means burning. Most carbon compounds, especially hydrocarbons, burn in the presence of oxygen to produce carbon dioxide, water, heat, and light. This is an exothermic reaction.

• For saturated hydrocarbons (like methane): They generally burn with a clean, blue flame. CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(l) + Heat and Light
• For unsaturated hydrocarbons: They often burn with a yellow, sooty flame due to incomplete combustion.

2. Oxidation

Oxidation is the addition of oxygen or removal of hydrogen. Alcohols can be oxidized to carboxylic acids using oxidizing agents like alkaline potassium permanganate (KMnO₄) or acidified potassium dichromate (K₂Cr₂O₇).

CH₃CH₂OH (Ethanol) + [O] → CH₃COOH (Ethanoic Acid) + H₂O (In the presence of alkaline KMnO₄ + Heat)

Here, the oxidizing agent provides the oxygen for the reaction.

3. Addition Reaction

Unsaturated hydrocarbons (alkenes and alkynes) undergo addition reactions where an atom or group is added across the double or triple bond, converting them into saturated compounds. A common example is hydrogenation.

Vegetable oils (unsaturated fats) are converted into solid fats like vanaspati ghee (saturated fats) by adding hydrogen in the presence of a catalyst like nickel or palladium.

R₂C=CR₂ (Unsaturated hydrocarbon) + H₂ → R₂CH-CHR₂ (Saturated hydrocarbon) (In the presence of a Ni catalyst)

4. Substitution Reaction

Saturated hydrocarbons are relatively unreactive. However, in the presence of sunlight, they undergo substitution reactions, where one or more hydrogen atoms are replaced by another atom (like a halogen).

For example, methane reacts with chlorine in the presence of sunlight.

CH₄ + Cl₂ → CH₃Cl + HCl (in the presence of sunlight)

This can be a chain reaction, with further hydrogens being substituted.

Some Important Carbon Compounds

Let's study the properties and uses of two commercially important carbon compounds: ethanol and ethanoic acid.

Ethanol (C₂H₅OH)

Ethanol, commonly known as alcohol, is the active ingredient in all alcoholic beverages.

• Properties: It is a colorless liquid at room temperature with a pleasant smell and a burning taste. It is an excellent solvent and is soluble in water in all proportions.
• Reactions of Ethanol:
  • Reaction with Sodium: Ethanol reacts with active metals like sodium to produce hydrogen gas and sodium ethoxide. 2Na + 2CH₃CH₂OH → 2CH₃CH₂ONa + H₂
  • Dehydration: When ethanol is heated with excess concentrated sulphuric acid (a dehydrating agent) at 443 K, it gets dehydrated to form ethene. CH₃CH₂OH → CH₂=CH₂ + H₂O (with hot conc. H₂SO₄)
• Uses: Used in alcoholic drinks, as a solvent in medicines (like tincture of iodine) and paints, and as a fuel in some countries (mixed with petrol).
• Harmful Effects: Consumption of even a small quantity of pure ethanol (absolute alcohol) can be lethal. Long-term consumption damages the liver and nervous system.

Ethanoic Acid (CH₃COOH)

Ethanoic acid is commonly known as acetic acid. A 5-8% solution of acetic acid in water is called vinegar.

• Properties: It is a colorless liquid with a sour taste and a pungent smell. Its melting point is 290 K (17°C), so it often freezes during winter in cold climates, giving it the name 'glacial acetic acid'.
• Reactions of Ethanoic Acid:
  • Esterification Reaction: Ethanoic acid reacts with an alcohol (like ethanol) in the presence of an acid catalyst to form a sweet-smelling compound called an ester. CH₃COOH + CH₃CH₂OH ⇌ CH₃COOCH₂CH₃ + H₂O (in the presence of acid)
  • Reaction with a Base: Being an acid, it reacts with a base like sodium hydroxide (NaOH) to form a salt (sodium ethanoate) and water. CH₃COOH + NaOH → CH₃COONa + H₂O
  • Reaction with Carbonates and Hydrogencarbonates: It reacts with carbonates and hydrogencarbonates to produce a salt, carbon dioxide, and water. 2CH₃COOH + Na₂CO₃ → 2CH₃COONa + H₂O + CO₂
• Uses: Used as vinegar for preserving food, as a laboratory reagent, and in the manufacture of various chemicals like esters and plastics.

Important Questions and Answers

Q1: What are the two properties of carbon which lead to the huge number of carbon compounds we see around us?

Answer: The two properties of carbon that are responsible for the vast number of its compounds are:

1. Tetravalency: Carbon has a valency of four, which enables it to form strong covalent bonds with four other atoms. This allows it to bond with a wide variety of elements (H, O, N, S, halogens), creating diverse compounds.
2. Catenation: This is the unique ability of carbon atoms to link with one another through strong covalent bonds to form long straight chains, branched chains, and closed rings of various sizes. This property, combined with tetravalency, leads to the formation of a massive number of stable organic molecules.

Q2: Draw the electron dot structure for ethanoic acid (CH₃COOH).

Answer: The formula for ethanoic acid is CH₃COOH. To draw the electron dot structure, we must satisfy the valency of each atom: Carbon (4), Oxygen (2), and Hydrogen (1).

1. The structure has a methyl group (CH₃) attached to a carboxyl group (-COOH).
2. The first carbon is bonded to three hydrogen atoms and the second carbon atom.
3. The second carbon atom is double-bonded to one oxygen atom and single-bonded to another oxygen atom.
4. This second oxygen atom is bonded to a hydrogen atom.

Q3: What is a homologous series? Explain with an example.

Answer: A homologous series is a series of organic compounds that have the same functional group and similar chemical properties. The successive members of the series differ by a –CH₂ group (a molecular mass of 14 u). Example: The homologous series of Alkanes. The general formula is CₙH₂ₙ₊₂.

• Methane (CH₄)
• Ethane (C₂H₆)
• Propane (C₃H₈)
• Butane (C₄H₁₀)

Q4: How would you distinguish experimentally between an alcohol and a carboxylic acid?

Answer: We can distinguish between an alcohol (e.g., ethanol) and a carboxylic acid (e.g., ethanoic acid) using the following tests:

1. Litmus Test: Take two test tubes, one with the alcohol and one with the carboxylic acid. Add a few drops of blue litmus solution to each. The carboxylic acid, being acidic, will turn the blue litmus solution red. The alcohol is neutral and will show no change in the colour of the litmus solution.
2. Sodium Bicarbonate Test: Add a pinch of sodium bicarbonate (NaHCO₃) powder to both solutions. The carboxylic acid will react with NaHCO₃ to produce brisk effervescence due to the evolution of carbon dioxide gas. The alcohol will not react and show no effervescence.

   Reaction: CH₃COOH + NaHCO₃ → CH₃COONa + H₂O + CO₂(g)

Q5: Differentiate between saturated and unsaturated hydrocarbons.

Answer:

Chapter Summary

Here are the key takeaways from our exploration of Carbon and its Compounds:

• Carbon's Bonding: Carbon is a non-metal that forms strong covalent bonds by sharing its four valence electrons to achieve a stable octet.
• Versatile Nature: Carbon's ability to form a vast number of compounds is due to its tetravalency (valency of 4) and catenation (self-linking ability).
• Allotropes: Carbon exists in different structural forms called allotropes, such as diamond (hard, insulator), graphite (soft, conductor), and fullerenes.
• Hydrocarbons: Compounds of carbon and hydrogen are called hydrocarbons. They can be saturated (alkanes) with only single bonds or unsaturated (alkenes and alkynes) with double or triple bonds.
• Homologous Series: A series of compounds with the same functional group and similar chemical properties, where successive members differ by a –CH₂ group.
• Functional Groups: Atoms or groups of atoms like -OH (alcohol), -CHO (aldehyde), -COOH (carboxylic acid) that determine the chemical properties of organic compounds.
• IUPAC Nomenclature: A systematic method for naming organic compounds based on their structure.
• Key Chemical Reactions: Carbon compounds undergo several important reactions, including combustion (burning), oxidation (gaining oxygen), addition (in unsaturated compounds), and substitution (in saturated compounds).
• Important Compounds: We studied Ethanol (alcohol) and Ethanoic Acid (carboxylic acid), their key reactions like esterification, and their uses in daily life.