Carbon and its Compounds-Exercises
Chemistry - Exercise
Q1. Ethane, with the molecular formula \(\ce{C2H6}\) has
(a) 6 covalent bonds.
(b) 7 covalent bonds.
(c) 8 covalent bonds.
(d) 9 covalent bonds.
- The two carbon atoms share one pair of electrons, forming one C–C single covalent bond.
- Each carbon is bonded to three hydrogen atoms, giving six C–H covalent bonds in total.
- 1 C–C bond
- 6 C–H bonds
Correct Answer: (b) 7 covalent bonds.
Q2. Butanone is a four-carbon compound with the functional group
(a) carboxylic acid
(b) aldehyde
(c) ketone
(d) alcohol
Solution:
Butanone is an organic compound made up of four carbon atoms, as suggested by the prefix “but-”. The ending “-one” tells us something important about its nature: it indicates the presence of a ketone functional group.In a ketone, the carbonyl group \(\ce{C=O}\) is attached to carbon atoms on both sides, meaning it is never found at the extreme end of the carbon chain. In butanone, this carbonyl group appears on the second carbon, giving the structure commonly written as 2-butanone.
Because of the presence and position of this carbonyl group, butanone clearly belongs to the ketone family of organic compounds.
Correct Answer: (c) ketone
Q3. While cooking, if the bottom of the vessel is getting blackened on the outside,
it means that
(a) the food is not cooked completely.
(b) the fuel is not burning completely.
(c) the fuel is wet.
(d) the fuel is burning completely.
Solution:
When a fuel such as kerosene, LPG, or wood burns properly, it produces a clean flame that gives off almost no soot. This happens when there is enough oxygen available for the fuel to undergo complete combustion.However, if the flame is yellow and smoky, the fuel is not able to burn fully. Incomplete combustion produces tiny black particles of unburnt carbon, commonly called soot. These carbon particles easily stick to the bottom of cooking vessels, causing the black deposits that we see.
So, if the utensil is getting blackened, it is a clear sign that the fuel is not burning completely, usually because the air supply is insufficient.
Correct Answer: (b) the fuel is not burning completely.
Q4. Explain the nature of the covalent bond using the bond formation in \(\ce{CH3 Cl}\).
Solution:
A covalent bond is formed when two atoms share electrons so that each can achieve a more stable electronic arrangement. Instead of gaining or losing electrons like in ionic bonding, the atoms hold the shared pair between them, which keeps the bond strong and directional.In the molecule chloromethane \(\ce{CH3Cl}\), the bonding happens through this sharing principle:
Bond formation in \(\ce{CH3Cl}\):
Carbon has 4 electrons in its outer shell and needs 4 more to complete its octet.
Each hydrogen atom has 1 electron and needs 1 more to complete its duplet.
Chlorine has 7 valence electrons and needs just 1 more to complete its octet.
To achieve stability:
Carbon shares one electron with each of the three hydrogen atoms, forming three C–H covalent bonds.
Carbon also shares one electron with chlorine, forming a C–Cl covalent bond.
The chlorine atom, after sharing one electron with carbon, completes its octet, while carbon completes all eight electrons in its outer shell.
Nature of the covalent bond in \(\ce{CH3Cl}\):
All the bonds in \(\ce{CH3Cl}\) are formed by sharing of electron pairs, making them covalent.
These bonds are strong and directional, meaning the atoms remain connected in fixed positions.
Unlike ionic compounds, covalent molecules like \(\ce{CH3Cl}\) do not contain charged particles in their structure.
\[ \begin{array}{cccccccccc} &&H\\ &&|\\ \ce{H&-&C&-&Cl}\\ &&|\\ &&H \end{array} \] Thus, the formation of \(\ce{CH3Cl}\) clearly shows that covalent bonds arise from mutual sharing of electrons so that all participating atoms achieve stable electronic configurations.
Q5. Draw the electron dot structures for
(a) ethanoic acid
(b) \(\ce{H2S}\)
(c) propanone
(d) \(\ce{F2}\)
Solution:
Ethanoic acid — \(\ce{CH3COOH}\)
\(\ce{H2S}\)
Propanone
\(\ce{F2}\)
Q6 What is an homologous series? Explain with an example.
Solution:
A homologous series is a family of organic compounds that share the same functional group and show a regular pattern in their structures. Each successive member of the series differs from the previous one by a fixed unit — a \(\ce{CH2}\) group. Because of this uniform increase, the members of a homologous series show gradual changes in physical properties such as boiling point or density, while their chemical properties remain similar due to the same functional group.Why members resemble one another
They contain the same functional group, so they react in similar ways.
Their structures follow a simple increasing pattern, so their formulas are predictable.
Their physical properties change steadily because the size of the molecule grows step by step.
Example: Alcohols
Consider the series of alcohols:
Methanol:
\(\ce{CH3OH}\)
Ethanol:
\(\ce{C2H5OH}\)
Propanol:
\(\ce{C3H7OH}\)
Each member has the –OH functional group and each one differs from the next by a \(\ce{CH2}\) unit. They all show similar reactions because of the –OH group, but their boiling points rise steadily as their molecular size increases.
Thus, a homologous series is like a well-organized chain of compounds that grow in a predictable manner while retaining the same core chemical behaviour.
Q7. How can ethanol and ethanoic acid be differentiated on the basis of their physical and chemical properties?
Solution:
Ethanol and ethanoic acid are both carbon compounds, but they belong to different functional groups and therefore show clear differences in their physical and chemical behaviour. These differences make it easy to distinguish one from the other.| Property / Test | Ethanol \( \ce{(C2H5OH)} \) | Ethanoic Acid \( \ce{(CH3COOH)} \) |
|---|---|---|
| Odour | Mild, spirit-like smell. | Strong, vinegar-like smell. |
| Taste | Slight burning sensation. | Sour taste due to acidic nature. |
| Reaction with Sodium Metal |
Reacts slowly, producing hydrogen gas: \[ \ce{2C2H5OH + 2Na -> 2C2H5ONa + H2} \] |
Reacts vigorously with sodium: \[ \ce{CH3COOH + Na -> CH3COONa + \tfrac12 H2} \] |
| Reaction with Carbonates / Bicarbonates | No reaction with \( \ce{Na2CO3} \) or \( \ce{NaHCO3} \). |
Produces brisk effervescence of \( \ce{CO2} \): \[ \ce{CH3COOH + NaHCO3 -> CH3COONa + CO2 + H2O} \] |
| Esterification Reaction | Forms esters when heated with acids and other alcohols. |
Reacts with ethanol to form a fruity-smelling ester: \[ \ce{CH3COOH + C2H5OH <=> CH3COOC2H5 + H2O} \] |
| Freezing / Boiling Behaviour | Lower freezing point; boils relatively easily. | Higher freezing point; may solidify in cold (glacial acetic acid). |
| Simple Lab Distinguishing Tests |
|
|
Q8. Why does micelle formation take place when soap is added to water? Will a micelle be formed in other solvents such as ethanol also?
Solution:
When soap is added to water, its molecules arrange themselves in a very special way because each soap molecule has two contrasting ends:
- a hydrophobic tail (water-repelling, usually a long hydrocarbon chain), and
- a hydrophilic head (water-loving, usually containing –COO⁻).
In water, the hydrophobic tails try to stay away from the water molecules, while the hydrophilic heads try to remain in contact with water. To satisfy both tendencies, the soap molecules automatically gather into tiny spherical groups known as micelles.
Inside a micelle, the hydrophobic tails point inward, hiding from water, and the hydrophilic heads face outward, touching the surrounding water. This structure helps lift and trap oily dirt inside the micelle, allowing the dirt to be washed away.
Will micelles form in other solvents like ethanol?
No, micelles do not form in ethanol or similar solvents.
Micelle formation requires a strong difference between a hydrophilic medium (like water) and the hydrophobic tails of soap. Ethanol is not as polar as water and can dissolve both the hydrophilic and hydrophobic parts of the soap molecule. Because of this:
- the tails no longer need to hide from the solvent,
- the heads do not need to form an outward shell,
so the driving force for micelle formation disappears.
Therefore, micelles form only in highly polar solvents such as water, not in ethanol.
Q9. Why are carbon and its compounds used as fuels for most applications?
Solution:
Carbon and its compounds are widely used as fuels because they possess a combination of properties that make them excellent sources of energy. Whether it is wood, coal, petrol, diesel, LPG, or natural gas, all of them contain carbon-based materials that burn easily and release heat.
-
High Energy Release on Combustion
Carbon compounds react with oxygen to form carbon dioxide and water, releasing a large amount of heat. This high energy output makes them suitable for heating, cooking, transportation, and industrial use. -
Controlled and Steady Burning
Most carbon fuels burn in a controlled manner, producing a steady flame. This makes them convenient for applications like cooking and powering engines, where a constant supply of energy is needed. -
Easy Availability and Abundance
Carbon-based fuels like coal and petroleum are naturally occurring and widely available. Their availability in large quantities has made them the primary energy source for households and industries. -
Ease of Handling and Transportation
Many carbon fuels (such as LPG, kerosene, and petrol) can be stored, transported, and used safely with proper equipment. Their physical forms—solid, liquid, or gas—make them adaptable to different needs. -
Formation of Useful By-products
Some carbon fuels, when processed, also yield important by-products like kerosene, wax, lubricants, and petrochemicals, adding to their usefulness.
Q10. Explain the formation of scum when hard water is treated with soap.
Solution:
Hard water contains dissolved calcium \((\ce{Ca2+})\) and magnesium \((\ce{Mg2+})\) salts. When soap is added to this water, the soap molecules do not behave the way they normally do in soft water. Instead, the soap reacts with these metal ions and forms insoluble salts, which appear as a greyish-white, sticky substance called scum.
How scum is formed
- Soap contains sodium or potassium salts of long-chain fatty acids.
- In hard water, the calcium or magnesium ions replace the sodium/potassium part of the soap molecule.
- This reaction forms calcium or magnesium fatty acid salts, which are insoluble in water.
- These insoluble salts separate out and settle as scum on the surface of water or on utensils.
Why this is a problem
- The scum formed prevents soap from producing enough lather.
- More soap is required to achieve proper cleaning because part of the soap gets wasted in forming scum instead of removing dirt.
Q11. What change will you observe if you test soap with litmus paper (red and blue)?
Solution:
Soap is made from the salts of long-chain fatty acids, and these salts are basic in nature. Because of this, soap solutions show clear and predictable behaviour when tested with litmus paper.
Effect on Red Litmus Paper
When a drop of soap solution is placed on red litmus paper, the paper changes color from red
to
blue.
This happens because the basic components of soap release hydroxide ions \((\ce{OH−})\) in
water, which turn red litmus blue.
Effect on Blue Litmus Paper
When soap solution is tested with blue litmus paper, there is no color change.
Basic substances do not affect blue litmus, so the paper remains blue throughout.
Q12. What is hydrogenation? What is its industrial application?
Solution:
Hydrogenation is a chemical process in which hydrogen gas \((\ce{H2})\) is added to
unsaturated organic compounds, such as oils that contain double bonds.
This reaction usually takes place in the presence of a metal catalyst like nickel, which
helps the hydrogen attach to the carbon atoms.
During hydrogenation, the double bonds in the unsaturated compound break, and hydrogen atoms
get added, converting it into a saturated compound. In simple terms, an unsaturated oil
becomes more solid and stable after hydrogenation.
Industrial Application
One of the most important industrial uses of hydrogenation is in the manufacture of vanaspati ghee.
- Natural vegetable oils are generally unsaturated and remain liquid at room temperature.
- When these oils are hydrogenated in the presence of a nickel catalyst, they become semi-solid, forming vanaspati ghee.
- This process improves their texture and shelf life, making them more suitable for cooking and storage.
Q13. Which of the following hydrocarbons undergo addition reactions:
\(\ce{C2H6,\; C3H8,\; C3H6,\; C2H2 \text{ and }CH4}\)
Solution:
Addition reactions are characteristic of unsaturated hydrocarbons. These are compounds that contain double bonds (alkenes) or triple bonds (alkynes). Because their carbon atoms do not have the maximum number of attached hydrogen atoms, they can “add” extra atoms across the multiple bond.
Now let’s examine each hydrocarbon:
- \(\ce{C2H6}\) – Ethane → saturated (alkane) → no addition reaction
- \(\ce{C3H8}\) – Propane → saturated (alkane) → no addition reaction
- \(\ce{CH4}\) – Methane → saturated (alkane) → no addition reaction
- \(\ce{C3H6}\) – Propene → unsaturated (alkene) → undergoes addition reactions
- \(\ce{C2H2}\) – Ethyne → unsaturated (alkyne) → undergoes addition reactions
Thus, only the unsaturated ones participate in addition reactions.
Q14. Give a test that can be used to differentiate between saturated and unsaturated hydrocarbons.
Solution:
A common test to distinguish between saturated and unsaturated hydrocarbons is the bromine water test.
- Unsaturated hydrocarbons (such as alkenes and alkynes) contain double or triple bonds. When a few drops of bromine water are added to them, the reddish-brown colour of bromine disappears. This happens because the unsaturated hydrocarbon adds bromine across the multiple bond.
- Saturated hydrocarbons (alkanes) do not react with bromine water. Therefore, the reddish-brown colour remains unchanged.
Thus, decolourisation of bromine water indicates an unsaturated hydrocarbon, while no change in colour shows a saturated hydrocarbon.
Q15. Explain the mechanism of the cleaning action of soaps.
Answer:
Soap cleans by lifting oily dirt from surfaces and allowing it to mix with water so it can be washed away. This happens because a soap molecule has two different ends, each with its own behaviour in water.
-
Structure of a Soap Molecule
Hydrophobic tail:
A long hydrocarbon chain that repels water but is strongly attracted to oil and grease. Hydrophilic head:
The ionic end (usually \(\ce{COO−})\) that attracts water. -
How soap works on oily dirt
When soap is added to water containing grease:- The hydrophobic tails push themselves into the oily dirt because they do not dissolve in water.
- The hydrophilic heads remain outside the greasy drop and interact with water molecules.
- This arrangement breaks the large grease droplet into many smaller droplets.
-
Formation of Micelles
As more soap molecules surround the tiny grease droplets, they form small spherical structures called micelles.- Inside the micelle → hydrophobic tails trap the oil.
- Outside → hydrophilic heads stay in contact with water.
Because the oil is now enclosed by soap molecules, the grease behaves like it is “dissolved” in water.
-
Removal of Dirt
When the fabric or surface is scrubbed and rinsed:
- the micelles remain suspended in water,
- the trapped dirt is carried away,
- the surface becomes clean.
