9 min readConcept sheet1-mark MCQ and assertion-reason; 3-mark questions on each type of reaction; often a 5-mark answer or a case-based question

Chemical properties of carbon compounds

Carbon compounds burn to give CO₂, water, heat and light. They can be oxidised. Unsaturated ones take part in addition, while saturated ones show substitution.

CombustionCH₄ + 2O₂ → CO₂ + 2H₂O + heat and light

Oxidising agentsAlkaline KMnO₄ or acidified K₂Cr₂O₇

Addition (hydrogenation)Unsaturated + H₂ → saturated, with a nickel or palladium catalyst

SubstitutionCH₄ + Cl₂ → CH₃Cl + HCl, in sunlight

Combustion (burning)

C(s) + O₂(g) → CO₂(g) + heat and light
CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(l) + heat and light
CH₃CH₂OH(l) + 3O₂(g) → 2CO₂(g) + 3H₂O(l) + heat and light
Saturated hydrocarbons usually burn with a clean blue flame. Unsaturated ones give a yellow flame with black smoke.
If the air holes of a gas stove are blocked, the fuel burns incompletely. The flame turns yellow, fuel is wasted, and the bottoms of vessels turn black.

Oxidation of ethanol

1
Warm about 3 mL of ethanol in a test tube in a water bath.
2
Add a 5% solution of alkaline potassium permanganate drop by drop.
The purple colour disappears at first, because it is used up in oxidising the ethanol
3
Keep adding it. When no ethanol is left, the purple colour stays.
4★
CH₃CH₂OH → CH₃COOH
Write the conditions over the arrow: alkaline KMnO₄ + heat, or acidified K₂Cr₂O₇ + heat. These are oxidising agents: they add oxygen to the ethanol

★ key step

Addition reaction

Unsaturated hydrocarbons add hydrogen across their double bond and become saturated. This needs a catalyst such as nickel or palladium: CH₂=CH₂ + H₂ → CH₃-CH₃. A catalyst speeds up a reaction without being used up itself. This hydrogenation is used to turn vegetable oils into solid fats such as vanaspati.

Substitution reaction

Saturated hydrocarbons are fairly unreactive. In sunlight, chlorine replaces their hydrogen atoms one by one: CH₄ + Cl₂ → CH₃Cl + HCl. With more chlorine, the other hydrogen atoms can also be replaced, giving a mixture of products.

Addition vs substitution

AdditionHappens in unsaturated compounds (C=C or C≡C). Atoms are added, and no atom leaves. Adding hydrogen needs a catalyst such as nickel. Example: hydrogenation of oil.

SubstitutionHappens in saturated compounds. One atom replaces another. Chlorination needs sunlight. Example: CH₄ + Cl₂ → CH₃Cl + HCl.

Your textbook says that animal fats are mostly saturated and are said to be harmful for health. Oils with unsaturated fatty acids are better for cooking.

In real life

Vanaspati is made by hydrogenating vegetable oil with a nickel catalyst. Your textbook advises cooking with oils that have unsaturated fatty acids, rather than saturated animal fats.

Exam angle

2 marks: “The gas flame in a kitchen has turned yellow, and the vessels are getting a black coat of soot. What is wrong, and how can it be put right?” The air holes of the burner are blocked, so the fuel burns incompletely with a sooty flame and fuel is wasted. Cleaning the air holes lets in enough air for a clean blue flame.

3 marks: “What is an addition reaction? Write the equation for adding hydrogen to ethene, and give one industrial use.” An unsaturated compound adds atoms across its double bond. CH₂=CH₂ + H₂ → CH₃-CH₃, with a nickel catalyst. Used to hydrogenate vegetable oils.

3 marks: “What is a substitution reaction? Write the equation for the chlorination of methane.” A reaction in which one atom is replaced by another. CH₄ + Cl₂ → CH₃Cl + HCl, in sunlight.

Remember it

Unsaturated ADD, saturated SWAP. A blue flame is clean (saturated); a yellow, sooty flame means unsaturated or too little air.