Q1. Explain with suitable examples how combustion reactions of carbon compounds are important in our daily lives.
Answer:
Combustion of carbon compounds means burning them in air to produce carbon dioxide, water, and energy.
We use combustion every day when we cook food using cooking gas (LPG, main component: propane or butane).
Methane (mainly in natural gas) is also used for heating and electricity generation as it burns cleanly, producing less smoke.
Petrol and diesel are burnt in vehicles to run engines; both undergo combustion reactions releasing energy for movement.
Ethanol is used as a spirit fuel; when it burns, it gives a clean blue flame with little smoke, used in labs and stoves.
The heat and light from burning candles or even burning wood in bonfires are due to the combustion of carbon compounds.
Q2. What is the difference between complete and incomplete combustion? How does each affect the environment?
Answer:
Complete combustion happens when there is plenty of oxygen, forming only CO₂ and H₂O with a lot of energy released.
Example: CH4+2O2→CO2+2H2O+energy
Incomplete combustion occurs with less oxygen, producing carbon monoxide (CO) and sometimes soot (carbon), which is harmful.
Example: CH4+O2→C+2H2O or 2CH4+3O2→2CO+4H2O
CO is poisonous and affects health if breathed in.
Incomplete burning produces pollution and less energy, while complete combustion is cleaner but still adds CO₂ (greenhouse gas) to the air.
Q3. Describe an oxidation reaction of alcohols with a balanced equation and state its significance.
Answer:
Oxidation of an alcohol means adding oxygen or removing hydrogen, usually with an oxidizing agent.
For example, ethanol turns into ethanoic acid (acetic acid) with potassium permanganate (KMnO₄) (an oxidizing agent):
C2H5OH[O],KMnO4CH3COOH+H2O
This reaction is used to make vinegar from alcohol, showing its importance in the kitchen and food industry.
The mild nature of the reaction is also used in making preservatives and medicines.
It demonstrates how alcohols can form acids, which is a key property for identifying different compounds.
Combustion is a fast version of this, but in oxidation, the process is controlled and stepwise.
Q4. How does the addition reaction help in turning liquid vegetable oils into solid fats? Name the catalyst used.
Answer:
Liquid vegetable oils (like sunflower or soybean oil) have double bonds (unsaturated), making them liquid at room temperature.
In an addition reaction, hydrogen is added to these double bonds under pressure and temperature.
The catalyst used is usually nickel (Ni), and the process is done at around 473 K.
The reaction is:
Vegetable oil+H2Ni,473KVegetable ghee (solid fat)
This process is called hydrogenation.
Solid fats like vanaspati ghee are made this way, which are used in cooking and bakery products.
It illustrates the use of chemistry in food processing.
Q5. What is the bromine water test for unsaturated carbon compounds and how does it work?
Answer:
Bromine water is orange-brown in color and is used to test for unsaturated compounds like alkenes (double bonds).
When an alkene is added, it reacts with bromine, and the solution becomes colorless as bromine adds across the double bond (addition reaction):
CH2=CH2+Br2→CH2Br−CH2Br
Alkanes (saturated compounds) do not react and hence the color stays the same.
This test is simple and helps quickly identify unsaturated compounds in labs and industries.
The loss of brown color means the presence of a double or triple bond.
It’s an important test for purity check and quality control in petrochemicals and food oils.
High Complexity (Analysis & Scenario-based)
Q6. A car using petrol emits smoke containing both CO₂ and black soot. Identify which type(s) of combustion is/are occurring and discuss the reasons and risks.
Answer:
When a car engine runs on petrol, it ideally undergoes complete combustion, producing only CO₂ and H₂O with maximum energy.
If the engine is not well-maintained, there may not be enough oxygen in some parts—incomplete combustion then takes place.
This leads to the release of carbon monoxide (CO) and black soot (carbon particles), along with CO₂.
The reasons include poor air supply, engine tuning issues, or impure fuel.
Risks:
CO is deadly if inhaled, can cause suffocation and health hazards.
Soot can cause respiratory problems and pollutes the air, dirtying buildings and harming plants.
Thus, it's important to maintain engines and use good quality fuel to minimize pollution and maximize energy efficiency.
Q7. Consider a scenario where alcohol is exposed to the open air and eventually smells sour. Explain the chemistry behind this change.
Answer:
Alcohol like ethanol, when left open, slowly reacts with oxygen in the air—a process called oxidation.
Over time, with the help of bacteria or mild oxidizing agents present in the environment, ethanol gets converted into ethanoic acid (acetic acid):
C2H5OH+O2→CH3COOH+H2O
Ethanoic acid has a sharp, vinegar-like (sour) smell.
This is similar to how wine or liquor can turn into vinegar if left uncapped.
The process shows how organic substances change under natural conditions, and why proper storage is needed to avoid spoilage.
This is an example of a slow oxidation reaction occurring at room temperature.
Q8. Why do saturated hydrocarbons not undergo addition reactions, but unsaturated hydrocarbons do? Explain with examples.
Answer:
Saturated hydrocarbons (alkanes) have only single bonds between carbon atoms, so the structure is fully filled with hydrogen atoms; there’s no space for more atoms to add without breaking these bonds.
Unsaturated hydrocarbons like alkenes (with double bonds) and alkynes (with triple bonds) have extra bonds, which can easily break and provide sites for new atoms (like H₂, Br₂) to be added.
Example:
C2H4(ethene)+H2NiC2H6(ethane)
Alkanes can only undergo substitution (swap H for another atom), not addition.
Because of this, unsaturated compounds are more reactive and are easily tested using addition reactions like bromine water.
In daily life, this property is used in hydrogenating oils and in chemical industries.
Q9. In a laboratory, methane is exposed to chlorine gas in sunlight and a series of products is formed. Describe the stepwise sequence of reactions and the principle behind them.
Answer:
Methane (CH₄) and chlorine (Cl₂), under sunlight, undergo a series of substitution reactions:
CH4+Cl2→CH3Cl+HCl (methyl chloride)
CH3Cl+Cl2→CH2Cl2+HCl (methylene chloride)
CH2Cl2+Cl2→CHCl3+HCl (chloroform)
CHCl3+Cl2→CCl4+HCl (carbon tetrachloride)
In each step, one hydrogen atom in methane is replaced by a chlorine atom—a process called substitution.
The reaction needs UV light to start, which helps break Cl₂ into reactive chlorine atoms (free radicals).
The principle here is that alkanes are generally unreactive, but can react with halogens under specific conditions.
This series is used to produce chemicals used as solvents, in refrigeration, and for laboratory tests.
It’s an example of a chain reaction in organic chemistry.
Q10. An industrialist wants to convert ethene obtained from natural gas into ethanol and then into acetic acid. What sequence of reactions should he follow, and what chemical principles are at work?
Answer:
Step 1:Ethene (C₂H₄) to Ethanol (C₂H₅OH) is an addition reaction—water is added across the double bond in the presence of a catalyst.
C2H4+H2OH3PO4,300∘C,60atmC2H5OH
Step 2:Ethanol is then oxidized to acetic acid (CH₃COOH) using an oxidizing agent like acidified potassium dichromate (K₂Cr₂O₇) or potassium permanganate (KMnO₄):
C2H5OH[O],K2Cr2O7CH3COOH+H2O
The chemical principles are:
Addition reaction for saturation of the double bond in ethene.
Oxidation reaction for converting alcohol to acid.
These methods are widely used in industry to make important chemicals from cheap raw materials.
The sequence highlights how organic reactions are connected and used for industrial production of useful compounds.
