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The Vault
RowQ
The Vault
CBSE Class 10 Chemistry · 11 questions · 26 marks
Carbon makes up less than one percent of Earth's crust, yet it forms millions of known compounds because of its unique ability to bond with itself. This chapter explores covalent bonding, the versatility of carbon chains and rings, and the chemistry of everyday organic compounds like ethanol and acetic acid.
Why does carbon form covalent compounds rather than ionic compounds?
Answer
Because carbon has 4 valence electrons and sharing electrons is more energetically favourable than losing or gaining 4 electrons — forming C4+ or C4- ions would require too much energy, so carbon achieves a stable octet by sharing electrons through covalent bonds.
Which of the following is the correct general formula for the alkane homologous series?
Answer
CnH2n+2 — alkanes are saturated hydrocarbons with only single bonds, and each member follows this general formula, e.g. methane CH4 (n=1) and ethane C2H6 (n=2).
What is observed when ethanoic acid is added to sodium bicarbonate solution?
Answer
A brisk effervescence of carbon dioxide gas, which turns lime water milky — CH3COOH + NaHCO3 -> CH3COONa + H2O + CO2, and this reaction is a standard test to confirm the presence of the -COOH functional group.
Soap fails to lather well in hard water because:
Answer
Calcium and magnesium ions in hard water react with soap to form an insoluble scum — these ions combine with the soap's fatty acid anions to form insoluble calcium/magnesium salts, wasting soap and reducing lather until all the hardness ions are used up.
Assertion (A): Diamond and graphite are both made entirely of carbon atoms, yet diamond is extremely hard while graphite is soft and slippery. Reason (R): The arrangement of carbon atoms and the type of bonding differ between diamond (rigid three-dimensional network) and graphite (layered hexagonal sheets that can slide over each other).
Answer
Both A and R are true and R is the correct explanation of A. Diamond and graphite are allotropes of carbon with identical composition but very different structures, and it is this difference in atomic arrangement and bonding that explains their contrasting physical properties.
What is catenation? Explain briefly why carbon shows an exceptionally strong tendency for catenation compared to other elements.
Answer
Catenation is the property of an element to form bonds between atoms of the same element, creating chains, branched chains, or rings. Carbon shows an exceptionally strong tendency for catenation because the carbon-carbon covalent bond is very strong and stable, due to carbon's small atomic size, which allows for close, effective orbital overlap; this strong self-linking ability is a major reason why carbon forms such a huge number of compounds.
Write the chemical equation for the combustion of ethanol, and state whether the reaction is exothermic or endothermic.
Answer
The combustion of ethanol is: C2H5OH + 3O2 -> 2CO2 + 3H2O + heat and light. It is an exothermic reaction because it releases a large amount of heat and light energy, which is why ethanol is used as a fuel.
Differentiate between saturated and unsaturated hydrocarbons, giving one example and one chemical test to distinguish them.
Answer
Saturated hydrocarbons contain only single covalent bonds between carbon atoms, are relatively less reactive, and burn with a clean blue flame; an example is ethane, C2H6. Unsaturated hydrocarbons contain at least one double or triple bond between carbon atoms, are more reactive, and tend to burn with a yellow, sooty flame; an example is ethene, C2H4. They can be distinguished using bromine water: unsaturated hydrocarbons decolourise bromine water (an addition reaction) instantly, whereas saturated hydrocarbons do not decolourise it under normal conditions.
Explain the structures and physical/chemical properties that distinguish covalent compounds like methane from ionic compounds like sodium chloride.
Answer
Covalent compounds such as methane (CH4) are formed by the mutual sharing of electrons between atoms, resulting in discrete molecules held together by covalent bonds; ionic compounds such as sodium chloride (NaCl) are formed by the complete transfer of electrons from a metal to a non-metal, resulting in oppositely charged ions held by strong electrostatic forces of attraction in a crystal lattice. Because of these structural differences, covalent compounds generally have low melting and boiling points (since only weak intermolecular forces need to be overcome), are often gases, liquids, or soft solids, and do not conduct electricity since they lack free ions or electrons. Ionic compounds, in contrast, generally have high melting and boiling points (since the strong lattice of ions must be broken), are usually hard, brittle solids, and conduct electricity when molten or dissolved in water because their ions become free to move and carry charge. This is why methane is a gas at room temperature and a non-conductor, while sodium chloride is a high-melting solid that conducts electricity only in molten or aqueous form.
Describe the preparation of soap through saponification, and explain, with a labelled description of a soap molecule's structure, how soap removes oily dirt from clothes.
Answer
Soap is prepared by the process of saponification, in which a fat or oil (an ester of long-chain fatty acids with glycerol) is heated with concentrated sodium hydroxide solution; this hydrolyses the ester bonds, producing soap (the sodium salt of the fatty acid) and glycerol as a by-product. A soap molecule has two distinct parts: a long hydrocarbon chain that is hydrophobic (water-repelling but attracted to oil and grease) and a charged ionic head, such as -COO-Na+, that is hydrophilic (water-attracting). When soap is used in water containing oily dirt, the hydrocarbon tails of many soap molecules bury themselves inside the oil droplet while their ionic heads remain on the outside facing the water, forming a spherical cluster called a micelle. Because the outer surface of the micelle is charged, it stays suspended and dispersed in water rather than clumping together, allowing the trapped oily dirt to be rinsed away with water, which is how soap cleans greasy dirt that water alone cannot remove.
A student performs an experiment where she adds a few drops of concentrated sulphuric acid to a mixture of ethanoic acid and ethanol, and gently warms the test tube. After a few minutes, she notices a distinct sweet, fruity smell coming from the mixture, quite different from the sharp, vinegar-like smell of ethanoic acid she started with. Based on this, answer: (a) Name the reaction taking place and write its balanced chemical equation. (b) What is the role of concentrated sulphuric acid in this reaction? (c) Name the product responsible for the sweet, fruity smell, and state one common use of such products.
Answer
(a) This is an esterification reaction: CH3COOH + C2H5OH -> CH3COOC2H5 + H2O (in presence of concentrated H2SO4, on warming). (b) Concentrated sulphuric acid acts as a catalyst and dehydrating agent, speeding up the reaction and helping to drive it toward ester formation by removing water. (c) The product responsible for the sweet, fruity smell is ethyl ethanoate, an ester; esters like this are commonly used in making perfumes and as flavouring agents in the food industry because of their pleasant fragrance.
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