Question

Give reasons for the following:
(I) Covalent compounds are poor conductor of electricity.
(II) Soap does not form lather in hard water.
(III) Carbon shows catenation but silicon does not.

Hint:

(I) Covalent compounds: no free electrons/ions \(\Rightarrow\) no conductivity. (II) Hard water: Ca²⁺/Mg²⁺ react with soap \(\Rightarrow\) insoluble scum \(\Rightarrow\) no lather. (III) Catenation: C-C strong (small size); Si-Si weak (large size).

Answer 1

Part (I): Covalent compounds are poor conductor of electricity
Step 1: Understand electrical conductivity.
For a substance to conduct electricity, it must have free-moving charged particles (electrons or ions).
Step 2: Nature of covalent compounds.
Covalent compounds are formed by sharing of electrons between atoms. They consist of neutral molecules.
Step 3: Absence of free electrons or ions.

• In covalent compounds, all electrons are involved in bonding and are not free to move.
• They do not dissociate into ions (except in some cases like polar covalent compounds in water, but even then conductivity is low).
• No free electrons or ions are available to carry electric current.

Step 4: Exception.
Some covalent compounds like graphite conduct electricity due to delocalized electrons, but generally covalent compounds are insulators.
Step 5: Final answer.
Covalent compounds are poor conductors because they do not have free electrons or ions to carry electric current. They consist of neutral molecules with all electrons involved in bonding. Part (II): Soap does not form lather in hard water
Step 1: Understand soap and hard water.
Soap is sodium or potassium salt of long-chain fatty acids (e.g., sodium stearate, C₁₇H₃₅COONa). Hard water contains dissolved calcium (Ca²⁺) and magnesium (Mg²⁺) ions.
Step 2: Reaction of soap with hard water.
When soap is added to hard water, the calcium and magnesium ions react with soap to form insoluble precipitates called scum (calcium or magnesium salts of fatty acids). \[ 2\text{C}_{17}\text{H}_{35}\text{COONa} + \text{Ca}^{2+} \rightarrow (\text{C}_{17}\text{H}_{35}\text{COO})_2\text{Ca} \downarrow + 2\text{Na}^+ \] (Sodium stearate) + (Calcium ions) \(\Rightarrow\) (Calcium stearate - insoluble scum) + (Sodium ions)
Step 3: Result of this reaction.

• The insoluble scum precipitates out instead of forming lather.
• Soap is wasted in reacting with Ca²⁺ and Mg²⁺ ions.
• No lather is formed until all hardness ions are precipitated.

Step 4: Final answer.
Soap does not form lather in hard water because calcium and magnesium ions in hard water react with soap to form insoluble scum (precipitate), preventing lather formation. Part (III): Carbon shows catenation but silicon does not
Step 1: Define catenation.
Catenation is the property of an element to form long chains, rings, or branched structures by bonding with atoms of the same element. This is most commonly seen in carbon compounds.
Step 2: Reason for carbon's catenation.
Carbon shows extensive catenation due to:

• Strong C-C bond: The carbon-carbon bond is very strong (bond energy ~348 kJ/mol) due to small size and effective overlapping of orbitals.
• Tetravalency: Carbon has four valence electrons, allowing it to form four stable covalent bonds.
• No lone pairs: Carbon has no unshared electron pairs, so it does not repel other atoms.
• Ability to form multiple bonds: Carbon can form single, double, and triple bonds with itself.

Step 3: Reason why silicon does not show catenation.
Silicon does not show significant catenation because:

• Weak Si-Si bond: The silicon-silicon bond is much weaker (bond energy ~222 kJ/mol) due to larger atomic size and less effective orbital overlap.
• Larger atomic size: Silicon atoms are larger, so the orbitals do not overlap effectively to form strong bonds.
• Presence of d-orbitals: Silicon has empty d-orbitals, making it more reactive and prone to attack by other atoms.
• Tendency to form multiple bonds with oxygen: Silicon prefers to form strong Si-O bonds rather than Si-Si bonds (as seen in silicates and silica).

Step 4: Comparison.
Carbon can form long chains (up to thousands of atoms) like in hydrocarbons, while silicon can only form very short chains (up to 6-7 atoms) under special conditions.
Step 5: Final answer.
Carbon shows catenation due to strong C-C bonds, small size, and tetravalency. Silicon does not show catenation because Si-Si bonds are weak due to larger atomic size and poor orbital overlap.