1. What is catenation? How does carbon exhibit catenation?
Answer:
Catenation is the ability of an element to form bonds with atoms of the same element, leading to the formation of long chains, rings, or branched structures. Carbon exhibits catenation due to its small size and the ability to form strong covalent bonds with other carbon atoms. This unique property allows carbon to form a vast variety of compounds, ranging from simple hydrocarbons to complex biomolecules like proteins and DNA. Carbon atoms can bond to form chains of varying lengths and shapes, such as straight chains, branched chains, and cyclic compounds. The strength of the carbon-carbon bond is high, which adds to the stability of these compounds. Carbon also forms single, double, and triple bonds, contributing to its versatility in creating a variety of organic compounds. This property is a major reason for the existence of millions of organic compounds. Silicon, a neighboring element, also shows catenation but not as extensively as carbon. The catenation property of carbon is central to organic chemistry, the branch of chemistry dealing with carbon compounds. This ability of carbon is why it’s referred to as the backbone of life.
2. Explain the formation of covalent bonds in carbon compounds.
Answer:
Covalent bonds are formed when atoms share electrons to achieve a stable electronic configuration, usually the noble gas configuration. Carbon has an atomic number of 6, with an electronic configuration of 2,4. It has four electrons in its outermost shell, and thus, needs four more electrons to complete its octet. Instead of losing or gaining electrons, carbon forms covalent bonds by sharing its four valence electrons with other atoms. For example, in methane (CH₄), carbon shares one electron with each of the four hydrogen atoms, forming four single covalent bonds. These bonds are strong and directional, leading to the formation of distinct molecular geometries. The shared electrons allow both atoms in the bond to have a stable electronic configuration. Covalent bonding can also involve multiple pairs of electrons, leading to the formation of double or triple bonds. This ability to form strong covalent bonds is what makes carbon so versatile in forming various compounds. Carbon’s covalent bonding with elements like hydrogen, oxygen, nitrogen, and other carbon atoms forms the basis of organic chemistry.
3. What are hydrocarbons? Classify them and give examples.
Answer:
Hydrocarbons are organic compounds composed entirely of carbon and hydrogen atoms. They can be classified into two main categories: saturated and unsaturated hydrocarbons. Saturated hydrocarbons, also called alkanes, contain only single bonds between carbon atoms. An example of a saturated hydrocarbon is methane (CH₄), where each carbon atom forms four single bonds. Unsaturated hydrocarbons include alkenes and alkynes. Alkenes contain at least one double bond between carbon atoms, such as ethene (C₂H₄), while alkynes have at least one triple bond between carbon atoms, such as ethyne (C₂H₂). Hydrocarbons can also be classified based on their structure into acyclic (open-chain) and cyclic (ring) hydrocarbons. Cyclohexane (C₆H₁₂) is an example of a cyclic hydrocarbon. Hydrocarbons are the basic structural units for more complex organic compounds and serve as the backbone for the petroleum industry. They are used as fuels, solvents, and in the synthesis of numerous chemical products.
4. What are isomers? Explain with an example.
Answer:
Isomers are compounds that have the same molecular formula but different structural formulas, meaning they differ in the arrangement of atoms within the molecule. This phenomenon is known as isomerism. There are different types of isomerism, such as structural isomerism and stereoisomerism. Structural isomers differ in the connectivity of their atoms. For example, butane (C₄H₁₀) has two isomers: normal butane, where the carbon atoms are arranged in a straight chain, and isobutane, where the carbon atoms form a branched chain. Although these two isomers have the same molecular formula, they differ in their physical and chemical properties. Isomerism is important because it demonstrates that molecules with the same atoms can have different structures and, consequently, different functions. This property is especially important in organic chemistry, where the structure of a compound can significantly influence its reactivity and behavior in biological systems. Stereoisomers, on the other hand, have the same connectivity but differ in the spatial arrangement of atoms.
5. Describe the structure and bonding of methane. Why is methane a tetrahedral molecule?
Answer:
Methane (CH₄) is the simplest alkane and consists of one carbon atom bonded to four hydrogen atoms. The carbon atom in methane has four valence electrons, which are shared with four hydrogen atoms to form four covalent bonds. Since all four bonds are identical, the repulsion between the pairs of electrons leads to a symmetrical arrangement. Methane adopts a tetrahedral geometry where the hydrogen atoms are positioned at the corners of a tetrahedron with the carbon atom at the center. The bond angle between the hydrogen atoms in methane is 109.5°, which is characteristic of a tetrahedral structure. This tetrahedral geometry minimizes electron-pair repulsion according to the Valence Shell Electron Pair Repulsion (VSEPR) theory. The symmetrical distribution of the hydrogen atoms around the carbon atom ensures that methane is a non-polar molecule, even though the C-H bond is slightly polar. Methane is a stable molecule due to the strength of the C-H covalent bonds and is commonly found in natural gas.
6. What is a homologous series? List its characteristics.
Answer:
A homologous series is a group of organic compounds with the same functional group and similar chemical properties, in which each successive member differs by a CH₂ unit. For example, the alkanes form a homologous series with the general formula CₙH₂ₙ₊₂, such as methane (CH₄), ethane (C₂H₆), propane (C₃H₈), and so on. The characteristics of a homologous series include the following:
- All members have the same general formula.
- They exhibit similar chemical properties because they contain the same functional group.
- Physical properties such as boiling point, melting point, and solubility show a gradual change with an increase in molecular mass.
- Each successive compound differs from the previous one by a -CH₂ group, which corresponds to an increase in molecular weight.
- The chemical reactivity of the members is similar, though their physical properties vary due to differences in molecular size.
- Homologous series provide a systematic and predictable way to study organic compounds.
Homologous series help in the classification of organic compounds and provide insights into their structure and reactivity.
7. What are functional groups? Give examples.
Answer:
A functional group is an atom or group of atoms that imparts specific chemical properties to an organic compound and defines its characteristic reactions. The presence of a functional group in an organic molecule determines its reactivity and behavior in chemical reactions. Some common functional groups include:
- Hydroxyl group (-OH) in alcohols, such as ethanol (C₂H₅OH).
- Carbonyl group (>C=O) in aldehydes and ketones, such as formaldehyde (HCHO) and acetone (CH₃COCH₃).
- Carboxyl group (-COOH) in carboxylic acids, such as acetic acid (CH₃COOH).
- Amino group (-NH₂) in amines, such as methylamine (CH₃NH₂).
- Halogen groups (-Cl, -Br, -I) in haloalkanes, such as chloromethane (CH₃Cl).
- Nitro group (-NO₂) in nitro compounds, such as nitrobenzene (C₆H₅NO₂).
Functional groups are crucial because they determine how an organic molecule will react with other substances. Organic compounds with the same functional group exhibit similar chemical reactions.
8. Explain the combustion of carbon compounds. What are the products of combustion?
Answer:
Combustion is a chemical reaction in which a substance reacts rapidly with oxygen, releasing energy in the form of heat and light. Carbon compounds, particularly hydrocarbons, undergo combustion when they are exposed to oxygen and ignited. The products of combustion depend on the availability of oxygen. In complete combustion, when there is sufficient oxygen, carbon compounds burn to produce carbon dioxide (CO₂) and water (H₂O). For example, the combustion of methane (CH₄) can be represented by the equation:
CH₄+2O₂→CO₂+2H₂O+energy\text{CH₄} + 2\text{O₂} \rightarrow \text{CO₂} + 2\text{H₂O} + \text{energy}CH₄+2O₂→CO₂+2H₂O+energy
In incomplete combustion, when oxygen supply is limited, carbon monoxide (CO) or even carbon (in the form of soot) may be produced along with water. The incomplete combustion of methane can produce carbon monoxide and water:
2CH₄+3O₂→2CO+4H₂O2\text{CH₄} + 3\text{O₂} \rightarrow 2\text{CO} + 4\text{H₂O}2CH₄+3O₂→2CO+4H₂O
Combustion of carbon compounds is an exothermic process, releasing a large amount of energy, which is why hydrocarbons are widely used as fuels. However, incomplete combustion can be dangerous, as carbon monoxide is a toxic gas.
9. What are soaps and detergents? How do they work to clean surfaces?
Answer:
Soaps and detergents are cleansing agents used to remove dirt and grease from surfaces. Both are composed of molecules with a hydrophilic (water-attracting) head and a hydrophobic (water-repelling) tail. Soaps are sodium or potassium salts of long-chain fatty acids, while detergents are synthetic compounds derived from petrochemicals. The cleaning action of soaps and detergents is based on their ability to emulsify oily dirt. When added to water, the hydrophobic tail of the soap or detergent molecule attaches to the grease or oil, while the hydrophilic head remains in the water. This creates a micelle, which is a spherical structure where the oil is trapped inside and surrounded by water molecules. The micelles are suspended in water, and the grease is removed from the surface. Soap works well in soft water but forms scum in hard water due to the reaction with calcium and magnesium ions. Detergents, on the other hand, do not form scum and are effective even in hard water.
10. Differentiate between saturated and unsaturated hydrocarbons. Give examples.
Answer:
Saturated hydrocarbons are organic compounds in which all the carbon-carbon bonds are single bonds, and each carbon atom is fully “saturated” with hydrogen atoms. They follow the general formula CₙH₂ₙ₊₂ and are also called alkanes. An example of a saturated hydrocarbon is methane (CH₄), where all four hydrogen atoms are singly bonded to a single carbon atom. Unsaturated hydrocarbons, on the other hand, contain one or more double or triple bonds between carbon atoms. Alkenes, such as ethene (C₂H₄), have at least one double bond, while alkynes, such as ethyne (C₂H₂), contain at least one triple bond. The presence of multiple bonds in unsaturated hydrocarbons reduces the number of hydrogen atoms attached to the carbon skeleton. This difference in bonding leads to distinct physical and chemical properties. Saturated hydrocarbons tend to be less reactive, whereas unsaturated hydrocarbons are more reactive due to the presence of the double or triple bonds, which can easily undergo addition reactions. Saturated hydrocarbons are commonly found in natural gas and petroleum, while unsaturated hydrocarbons are often used in the production of polymers and other chemicals.