Chemical Properties of Carbon Compounds for CBSE Class 10 Science

Most of the fuels we use to generate heat, run motor vehicles, and power electricity grids—such as wood, coal, natural gas (methane), LPG (butane), petrol, and diesel—are either pure carbon or carbon compounds. Carbon compounds undergo a rich variety of chemical reactions that make them indispensable as fuels, synthetic reagents, and industrial materials.

In CBSE Class 10 Science, Chapter 4 (Carbon and its Compounds) categorizes these transformations into four fundamental chemical reaction types: Combustion, Oxidation, Addition, and Substitution.

What You Will Learn

• Combustion Reactions: Saturated vs. unsaturated flames and why soot forms
• Clean fuels vs. smoky fuels and heating efficiency
• Oxidation Reactions: Role of oxidizing agents (alkaline $\text{KMnO}_4$ and acidified $\text{K}_2\text{Cr}_2\text{O}_7$)
• Addition Reactions: Hydrogenation of vegetable oils into vanaspati ghee
• Substitution Reactions: Photochemical chlorination of alkanes
• Board examination chemical equations, conditions, and common mistakes

1. Combustion Reactions (Burning in Air)

Carbon in all its allotropic forms, and all carbon compounds, burn in oxygen to produce carbon dioxide, water vapour, and large quantities of heat and light:

$\text{Carbon Compound} + \text{Oxygen} \longrightarrow \text{Carbon Dioxide} + \text{Water} + \text{Heat} + \text{Light}$

Fundamental Combustion Equations:

1. Combustion of Carbon: $\text{C}(s) + \text{O}_2(g) \longrightarrow \text{CO}_2(g) + \text{Heat and Light}$
2. Combustion of Methane (Natural Gas): $\text{CH}_4(g) + 2\text{O}_2(g) \longrightarrow \text{CO}_2(g) + 2\text{H}_2\text{O}(g) + \text{Heat and Light}$
3. Combustion of Ethanol: $\text{CH}_3\text{CH}_2\text{OH}(l) + 3\text{O}_2(g) \longrightarrow 2\text{CO}_2(g) + 3\text{H}_2\text{O}(g) + \text{Heat and Light}$

Saturated vs. Unsaturated Hydrocarbon Flames (NCERT Activity 4.3)

                            Combustion Flames
                                    |
       +----------------------------+----------------------------+
       |                                                         |
Saturated Hydrocarbons                                    Unsaturated Hydrocarbons
(Alkanes: C-C Single Bonds)                              (Alkenes & Alkynes: C=C, C≡C)
       |                                                         |
Complete Combustion in Clean Air                          Incomplete Combustion / High % Carbon
Clean, Non-Sooty BLUE Flame                               Yellow, Smoky, SOOTY Flame
(Leaves cookware clean)                                   (Leaves black soot on cookware)

1. Saturated Hydrocarbons (Alkanes):
   • Burn with a clean, blue, non-sooty flame in an adequate supply of air.
   • The percentage of carbon is relatively low, permitting complete combustion.
   • Domestic Gas Stoves: Gas burners on your kitchen stove have air inlets. When properly adjusted, methane/LPG mixes with sufficient oxygen to burn completely with a blue flame, leaving the bottom of cooking vessels completely clean.
   • If the air inlets become blocked with dirt, oxygen supply is choked, resulting in incomplete combustion that produces a yellow, smoky flame that blackens the bottom of pots.
2. Unsaturated Hydrocarbons (Alkenes and Alkynes):
   • Burn with a yellow, sooty flame accompanied by black smoke.
   • The percentage of carbon in alkenes and alkynes is significantly higher than in alkanes. Atmospheric oxygen is insufficient to completely burn all the carbon atoms, leaving unburnt carbon particles that glow yellow and deposit as soot.

2. Oxidation Reactions

While combustion is a violent, total oxidation of carbon to carbon dioxide, controlled chemical oxidation converts carbon compounds into other functional families.

Converting Alcohols to Carboxylic Acids

When an alcohol like ethanol is treated with specific oxidizing agents, it is converted into ethanoic acid:

$\text{CH}_3\text{CH}_2\text{OH} \text{ [Ethanol]} \xrightarrow[\text{or Acidified } \text{K}_2\text{Cr}_2\text{O}_7 + \Delta]{\text{Alkaline } \text{KMnO}_4 + \Delta} \text{CH}_3\text{COOH} \text{ [Ethanoic acid]}$

What are Oxidizing Agents?

Substances that are capable of adding oxygen to other substances are known as oxidizing agents.

In this reaction:

• Alkaline Potassium Permanganate ($\text{KMnO}_4 + \text{heat}$) OR
• Acidified Potassium Dichromate ($\text{K}_2\text{Cr}_2\text{O}_7 + \text{heat}$) act as powerful oxidizing agents that deliver nascent oxygen to oxidize the alcohol group ($-\text{CH}_2\text{OH}$) into a carboxylic acid group ($-\text{COOH}$).

3. Addition Reactions (Hydrogenation)

Unsaturated hydrocarbons contain double or triple bonds between carbon atoms. Because these multiple bonds can break open, unsaturated hydrocarbons can add other atoms (such as hydrogen, halogens) across the multiple bond to become saturated.

Definition

An addition reaction is a reaction in which an unsaturated hydrocarbon combines with another substance across its multiple bond to form a single, saturated product.

The Hydrogenation of Vegetable Oils (CBSE High-Frequency Question)

Vegetable oils (such as sunflower oil, groundnut oil, mustard oil) contain long, unsaturated carbon chains and are healthy liquids at room temperature. When hydrogen gas is bubbled through vegetable oil in the presence of a nickel ($\text{Ni}$) or palladium catalyst, hydrogen adds across the double bonds, converting the unsaturated liquid oil into a solid saturated fat (commonly known as vanaspati ghee):

$\begin{matrix} R \\ & \backslash \\ & & C = C \\ & / \\ R \end{matrix} \begin{matrix} R \\ / \\ \\ \\ R \end{matrix} + \text{H}_2 \xrightarrow{\quad \text{Ni Catalyst} \quad} \begin{matrix} R \\ & \backslash \\ & & \text{CH} - \text{CH} \\ & / \\ R \end{matrix} \begin{matrix} R \\ / \\ \\ \\ R \end{matrix}$

Health Advisory in NCERT: <u>Vegetable oils containing unsaturated fatty acids are healthy and good for the heart. Saturated fats (like vanaspati ghee and animal fats) contain saturated fatty acids that are harmful to cardiovascular health!</u>

4. Substitution Reactions

Saturated hydrocarbons (alkanes) are typically unreactive and inert toward most laboratory reagents (acids, bases, oxidizers) because their $C-C$ and $C-H$ single bonds are strong and stable. However, in the presence of sunlight, alkanes undergo rapid reactions with halogens.

Definition

A substitution reaction is a reaction in which one or more atoms (or groups of atoms) of a compound are directly replaced by other atoms.

Photochemical Chlorination of Methane:

In the presence of sunlight, chlorine replaces hydrogen atoms of methane one by one:

$\text{CH}_4 + \text{Cl}_2 \xrightarrow{\quad \text{Sunlight} \quad} \text{CH}_3\text{Cl} \text{ [Chloromethane]} + \text{HCl}$

On continued exposure to chlorine in sunlight, successive substitution reactions yield dichloromethane ($\text{CH}_2\text{Cl}_2$), chloroform ($\text{CHCl}_3$), and carbon tetrachloride ($\text{CCl}_4$).

5. Summary and Examination Tips

Exam Tip: If a board question asks how to chemically distinguish between cooking oil (unsaturated) and butter (saturated), state the Bromine Water Test: Cooking oil decolourizes red-brown bromine water (via an addition reaction), whereas butter does not decolourize bromine water!

Common Mistake: Forgetting the catalyst in addition reactions. Writing an addition reaction without mentioning Nickel (Ni) or Palladium (Pd) catalyst loses half a mark!