Chemical Properties of Carbon Compounds – Long Answer Questions
Medium Level (Application & Explanation)
Q1. Explain complete and incomplete combustion of carbon compounds with examples, observations, and safety measures.
Answer:
Combustion is the reaction of a carbon compound with oxygen, releasing carbon dioxide (CO₂), water (H₂O), and energy. In complete combustion, there is sufficient oxygen and the flame is typically blue. Example: CH₄ + 2O₂ → CO₂ + 2H₂O + energy (methane in cooking gas).
In incomplete combustion, oxygen is limited, producing carbon monoxide (CO) and soot (fine carbon particles). The flame may appear yellow and smoky. Soot deposition can be seen by holding a glass plate above a candle flame. This shows unburnt carbon due to oxygen shortage.
Observations in activity: water droplets in a cold beaker indicate water formation; black marks show soot.
Safety: Ensure good ventilation, avoid using flames in closed rooms, and check for a blue flame in gas stoves to prevent CO buildup, which is poisonous.
Q2. Describe how alcohols undergo oxidation to form acids. Explain the role of oxidizing agents with reactions and observations.
Answer:
Oxidation is the addition of oxygen or removal of hydrogen. In carbon chemistry, alcohols convert to acids using oxidizing agents like acidified potassium permanganate (KMnO₄) or potassium dichromate (K₂Cr₂O₇).
Example: Ethanol (C₂H₅OH) oxidizes to ethanoic acid (CH₃COOH): C₂H₅OH ⟶ CH₃COOH (in presence of [O]).
For methanol (CH₃OH), oxidation occurs in steps: CH₃OH ⟶ HCHO ⟶ HCOOH (methanol → formaldehyde → formic acid).
Observation: The pink color of KMnO₄ gets decolorized during oxidation, indicating it has reacted and the alcohol is oxidized.
Importance: This process is used in making vinegar (ethanoic acid) from alcohols (e.g., souring of wine). Note that combustion is also a form of rapid oxidation, but it fully converts the compound to CO₂ and H₂O with heat and light.
Q3. What is hydrogenation of vegetable oils? Explain the conditions, the chemistry involved, and changes in properties.
Answer:
Hydrogenation is an addition reaction where hydrogen (H₂) is added across the double bonds of unsaturated oils (alkenes), converting them into saturated fats (semi-solids like ghee or margarine).
Reaction: Vegetable oil + H₂ ⟶ Vegetable ghee, in presence of a nickel (Ni) catalyst at about 473 K.
This reaction reduces unsaturation, making the product harder, more stable, and with a higher melting point than the original oil.
It is widely used in food industries to produce margarine and vegetable ghee from liquid oils.
The underlying chemistry: addition of hydrogen breaks the double bonds in alkenes to form alkanes (e.g., C₂H₄ + H₂ ⟶ C₂H₆). The reaction is catalyzed and requires controlled temperature.
Practical note: Unsaturation can be tested using bromine water, which decolorizes with oils before hydrogenation but not after.
Q4. Explain substitution reactions in alkanes with the example of methane chlorination. Why is sunlight or UV light essential?
Answer:
Substitution reactions occur in alkanes (saturated hydrocarbons), where one atom (usually hydrogen) is replaced by a halogen like chlorine or bromine.
Example series with methane (CH₄) under sunlight/UV:
CH₄ + Cl₂ → CH₃Cl + HCl (methyl chloride)
CH₃Cl + Cl₂ → CH₂Cl₂ + HCl (methylene chloride)
CH₂Cl₂ + Cl₂ → CHCl₃ + HCl (chloroform)
CHCl₃ + Cl₂ → CCl₄ + HCl (carbon tetrachloride)
Sunlight/UV light provides the energy to split Cl₂ into reactive chlorine radicals, which initiate the chain reaction. Without UV, the reaction is very slow.
These products are important industrially; substituted compounds serve as useful chemicals and feedstocks (e.g., for making materials related to PVC production).
Safety: Chlorination should be conducted in a controlled setup due to the formation of HCl gas and the reactivity of intermediates.
Q5. Describe the bromine water test for unsaturation. State the principle, steps, observations, and interpretations with equations.
Answer:
Principle: Unsaturated compounds (alkenes/alkynes) undergo addition reactions with bromine (Br₂), causing decolorization of brown bromine water. Saturated compounds (alkanes) generally do not decolorize bromine water in the dark.
Steps:
Take bromine water in a test tube.
Pass ethene (CH₂=CH₂) gas through it or add a drop of vegetable oil.
Gently shake and observe the color change.
Observation: The brown color becomes colorless, indicating an addition reaction at the double bond.
Interpretation: Decolorization confirms unsaturation. If there is no change, the sample is likely saturated (e.g., alkanes).
Application: This test helps distinguish between alkenes and alkanes in simple lab settings, and it supports understanding of addition reactions in organic chemistry.
High Complexity (Analytical & Scenario-Based)
Q6. A student warms a beaker over a candle flame and finds water droplets inside it and soot on a nearby glass plate. Analyze the chemistry and suggest how to improve the combustion.
Answer:
The water droplets inside the beaker are from condensed water vapor, produced during combustion of the candle wax (paraffin, approx. C₂₅H₅₂): C₂₅H₅₂ + O₂ → CO₂ + H₂O + energy.
The black soot on the glass plate indicates incomplete combustion, due to a limited oxygen supply or cool zones around the flame. Incomplete combustion also risks forming carbon monoxide (CO).
Candle flames have zones; placing objects too close disrupts airflow, causing sooty, yellow flames.
Improvements:
Ensure free air circulation; avoid covering the flame too closely.
Increase the oxygen supply (do not enclose the flame; avoid drafts that destabilize but ensure ventilation).
For burners, adjust to get a steady blue flame, which signals complete combustion.
Conclusion: Observations demonstrate both products of combustion (H₂O) and effects of oxygen deficiency (soot), reinforcing the need for proper ventilation.
Q7. Design an experiment to oxidize ethanol to ethanoic acid in the lab and explain how you would confirm that oxidation has occurred.
Answer:
Setup:
In a test tube, add a small amount of ethanol (C₂H₅OH).
Add a few drops of acidified potassium permanganate (KMnO₄) and dilute H₂SO₄ as the acidic medium.
Warm gently in a water bath for safety.
Chemical change:
Ethanol is oxidized to ethanoic acid (CH₃COOH): C₂H₅OH ⟶ CH₃COOH in presence of [O].
The pink color of KMnO₄ disappears, indicating it has been consumed as an oxidizing agent.
Confirmation:
Note the sharp, vinegar-like smell, typical of ethanoic acid.
Compare with a control test tube containing ethanol without oxidant to show no change in smell or color.
Optional simple indicator check: use pH/universal indicator to observe that the product is more acidic than ethanol.
Inference: Loss of KMnO₄ color plus the characteristic odor strongly supports oxidation of alcohol to acid.
Q8. An industry wants to convert vegetable oil to ghee efficiently. Explain how to optimize the hydrogenation process and avoid undesirable outcomes.
Answer:
Goal: Maximize hydrogenation of unsaturated oils to produce semi-solid ghee/margarine without over-hardening.
Optimization factors:
Use a nickel (Ni) catalyst at around 473 K for effective addition of H₂ across double bonds.
Maintain moderate hydrogen pressure and proper mixing to ensure uniform contact between oil, H₂, and catalyst.
Control reaction time carefully; prolonged hydrogenation can lead to excessive saturation, making the product too hard.
Ensure clean oil (free from impurities) to prevent catalyst poisoning and maintain consistent reaction rates.
Quality checks:
Monitor the degree of unsaturation (e.g., by periodic bromine water decolorization tests on sampled product).
Observe melting behavior and texture to match desired ghee consistency.
Outcome: Proper control gives a stable, spreadable product through a classic addition reaction without compromising quality.
Q9. You have three samples: A decolorizes bromine water, B does not decolorize bromine water but reacts with chlorine under sunlight, and C burns with a smoky flame leaving soot. Identify A, B, and C with reasons and possible reactions.
Answer:
Sample A: Decolorizes bromine water; thus it is unsaturated (likely an alkene). Reaction: CH₂=CH₂ + Br₂ ⟶ CH₂Br–CH₂Br. The loss of brown color indicates an addition reaction at a double bond.
Sample B: Does not decolorize bromine water in the dark but reacts with Cl₂ under sunlight, so it is a saturated alkane showing a substitution reaction. Example with methane: CH₄ + Cl₂ (sunlight) ⟶ CH₃Cl + HCl.
Sample C: Burns with a smoky flame and leaves soot, indicating incomplete combustion or a compound prone to soot formation (e.g., heavy hydrocarbons like paraffin wax, C₂₅H₅₂). Soot signals oxygen deficiency and unburnt carbon. General combustion: CₓHᵧ + O₂ ⟶ CO₂ + H₂O (complete) or CO and C (soot) if incomplete.
Conclusion: A = alkene, B = alkane, C = carbon-rich compound showing incomplete combustion.
Q10. Compare and contrast addition and substitution reactions using ethene, ethane, and methane as examples. Include conditions, products, and how to distinguish them experimentally.
