21. What are the homologous series of carbon compounds? Explain their characteristics.
Answer:
A homologous series is a group of organic compounds that share a general formula, similar chemical properties, and a gradual variation in physical properties. The members of a homologous series differ from one another by a constant unit, typically a CH₂ group. For example, in the alkane series, methane (CH₄), ethane (C₂H₆), propane (C₃H₈), and butane (C₄H₁₀) follow the general formula CₙH₂ₙ₊₂. One key characteristic of a homologous series is that each successive member increases by a molecular mass of 14 units (due to the addition of CH₂). The physical properties, such as melting point and boiling point, show a regular increase or decrease with an increase in molecular size. However, the chemical properties remain largely similar since all members possess the same functional group. Homologous series are important in organic chemistry because they provide a systematic way to study and categorize organic compounds. They also help in predicting the properties of compounds based on their position in the series.
22. What are functional groups in organic chemistry? Provide examples.
Answer:
A functional group is a specific group of atoms within a molecule that is responsible for the characteristic chemical reactions of that molecule. In organic chemistry, functional groups define the reactivity and properties of organic compounds, regardless of the length or nature of the carbon chain to which they are attached. Some common functional groups include:
- Hydroxyl group (-OH), found in alcohols such as ethanol (C₂H₅OH).
- Carboxyl group (-COOH), found in carboxylic acids such as acetic acid (CH₃COOH).
- Aldehyde group (-CHO), found in aldehydes such as formaldehyde (HCHO).
- Ketone group (-C=O), found in ketones such as acetone (CH₃COCH₃).
- Amine group (-NH₂), found in amines such as methylamine (CH₃NH₂).
Functional groups determine how a molecule will react in chemical reactions. For example, alcohols react with acids to form esters, while carboxylic acids react with bases to form salts. The presence of a functional group also influences the physical properties, such as solubility and boiling point, of the compound.
23. What are isomers? Explain with an example.
Answer:
Isomers are compounds that have the same molecular formula but different structural arrangements of atoms, leading to different physical and chemical properties. There are two main types of isomerism: structural isomerism and stereoisomerism. In structural isomerism, the atoms are connected in different ways, forming different structures. For example, butane (C₄H₁₀) has two isomers: n-butane, where all carbon atoms are in a straight chain, and isobutane, where the carbon atoms form a branched chain. Both have the same molecular formula (C₄H₁₀), but their structures are different.
Stereoisomers, on the other hand, have the same structure but differ in the spatial arrangement of atoms. For example, in geometric isomerism, cis and trans forms occur when there is restricted rotation around a double bond. Isomers can have very different properties; for example, n-butane is a gas, while isobutane is a liquid at the same temperature and pressure. The concept of isomerism is essential in organic chemistry because it explains the diversity of organic compounds with the same molecular formula but different behaviors.
24. What is the role of detergents in environmental pollution? Explain their effects on water bodies.
Answer:
Detergents contribute to environmental pollution, particularly in water bodies, due to their non-biodegradable nature and the presence of harmful chemicals. Unlike soaps, which are made from natural fats and oils, many detergents contain synthetic compounds like alkyl benzene sulfonates that do not easily decompose in the environment. When detergents are washed into rivers, lakes, or oceans, they can accumulate over time, forming foam and affecting water quality. The surfactants in detergents reduce the surface tension of water, allowing more oxygen to escape from the water, which can harm aquatic life by depleting the oxygen levels.
Phosphates, commonly added to detergents to enhance their cleaning power, contribute to eutrophication in water bodies. Eutrophication is the excessive growth of algae due to high nutrient levels (especially phosphorus), which eventually leads to the death of aquatic organisms as algae consume the oxygen in the water. Additionally, detergent chemicals can be toxic to fish, amphibians, and other aquatic organisms, disrupting ecosystems. The long-term environmental effects of detergent pollution have led to regulations limiting the use of phosphates and encouraging the development of biodegradable detergents.
25. How are carbon atoms able to form a large number of compounds? Explain the concept of catenation.
Answer:
Carbon atoms have a unique ability to form a large number of compounds due to two key properties: catenation and tetravalency. Catenation is the ability of carbon atoms to form stable bonds with other carbon atoms, allowing them to create long chains, branched chains, and even rings. This property arises because the carbon-carbon bond is strong and stable, making it possible for carbon to form a variety of complex structures, such as straight-chain hydrocarbons, branched hydrocarbons, and cyclic compounds.
Tetravalency refers to carbon’s ability to form four covalent bonds with other atoms, including other carbon atoms, hydrogen, oxygen, nitrogen, and halogens. This gives carbon the flexibility to form single, double, or triple bonds with other elements. The combination of catenation and tetravalency allows carbon to form an incredibly diverse range of compounds, including simple molecules like methane (CH₄) and complex biomolecules like proteins and DNA. This versatility is why carbon forms the backbone of organic chemistry and why it is the basis of life on Earth.
26. What is a triple bond, and how does it affect the properties of a molecule? Give an example.
Answer:
A triple bond is a type of covalent bond where three pairs of electrons are shared between two atoms. In organic molecules, a triple bond is commonly found between two carbon atoms, such as in alkynes. The triple bond consists of one sigma (σ) bond and two pi (π) bonds, making it shorter and stronger than single or double bonds. Because of its strength and rigidity, the triple bond restricts the rotation of the bonded atoms, giving the molecule a linear geometry.
An example of a molecule with a triple bond is ethyne (acetylene, C₂H₂), where two carbon atoms are joined by a triple bond, with each carbon atom also bonded to a hydrogen atom. The presence of a triple bond gives ethyne unique properties, such as high reactivity and a high melting and boiling point compared to hydrocarbons with single or double bonds. Alkynes like ethyne are more reactive than alkenes (with double bonds) and alkanes (with single bonds) due to the availability of electrons in the pi bonds for reactions like addition. Ethyne is used in welding and cutting metals because it burns at a very high temperature when combined with oxygen.
27. Explain why carbon dioxide (CO₂) is considered a greenhouse gas. How does it contribute to global warming?
Answer:
Carbon dioxide (CO₂) is considered a greenhouse gas because it has the ability to trap heat in the Earth’s atmosphere, contributing to the greenhouse effect. The greenhouse effect is a natural process that warms the Earth’s surface. When the sun’s energy reaches the Earth, some of it is reflected back to space, but most of it is absorbed and re-radiated as heat. Greenhouse gases like CO₂ trap some of this heat, preventing it from escaping into space, thereby warming the planet.
However, human activities, such as the burning of fossil fuels (coal, oil, and natural gas) and deforestation, have significantly increased the concentration of CO₂ in the atmosphere. This excess CO₂ enhances the greenhouse effect, leading to global warming, which is the long-term increase in Earth’s average temperature. As CO₂ levels rise, they contribute to various climate changes, such as more extreme weather patterns, melting polar ice, and rising sea levels. Carbon dioxide is responsible for about 60% of the human-caused greenhouse effect, making it a major contributor to the current trend of global warming.
28. What is cracking in the petroleum industry? Why is it important?
Answer:
Cracking is a process in the petroleum industry used to break down large, complex hydrocarbon molecules found in crude oil into smaller, more useful molecules like gasoline, diesel, and other valuable products. It is a form of thermal decomposition carried out at high temperatures and sometimes in the presence of a catalyst. There are two main types of cracking: thermal cracking, which uses heat, and catalytic cracking, which uses a catalyst to lower the temperature and pressure required for the reaction.
The importance of cracking lies in the fact that crude oil contains large amounts of long-chain hydrocarbons, which are not very useful in their original form. Cracking transforms these heavy hydrocarbons into lighter products that are in high demand, such as gasoline and jet fuel. Cracking also produces alkenes like ethene and propene, which are used as feedstocks in the chemical industry to produce plastics, synthetic fibers, and other materials. Without cracking, the petroleum industry would not be able to meet the global demand for fuels and chemicals efficiently.
29. Why are hydrocarbons like methane (CH₄) considered important fuels? Discuss their properties.
Answer:
Hydrocarbons like methane (CH₄) are considered important fuels because they are highly efficient energy sources and readily available from natural gas reserves. Methane is the simplest alkane, consisting of one carbon atom bonded to four hydrogen atoms, and is a major component of natural gas. It burns cleanly in oxygen, producing carbon dioxide (CO₂), water (H₂O), and a large amount of energy:
CH4+2O2→CO2+2H2O+EnergyCH₄ + 2O₂ \rightarrow CO₂ + 2H₂O + EnergyCH4+2O2→CO2+2H2O+Energy
This high energy output makes methane a valuable fuel for heating, electricity generation, and as a fuel for vehicles in the form of compressed natural gas (CNG). Methane is also favored because it produces fewer pollutants than other hydrocarbons, such as coal or oil, when combusted. It emits lower levels of carbon monoxide (CO), sulfur dioxide (SO₂), and nitrogen oxides (NOₓ), which contribute to air pollution and acid rain. Additionally, methane is relatively abundant and cheaper to extract compared to other fuels. Its properties, such as being odorless and lighter than air, make it a convenient and clean fuel source in various industries and households.
30. What is the significance of carbon compounds in living organisms?
Answer:
Carbon compounds play a fundamental role in living organisms, as carbon is the primary element in organic molecules that form the basis of life. Organic molecules such as carbohydrates, proteins, lipids, and nucleic acids (DNA and RNA) are all carbon-based. Carbon’s ability to form stable covalent bonds with other elements, including itself, enables the formation of complex molecules that are essential for biological processes.
For example, carbohydrates like glucose (C₆H₁₂O₆) are a primary source of energy for living cells. Proteins, composed of carbon-containing amino acids, are crucial for building and repairing tissues and are involved in almost every biological function. Lipids (fats and oils) store energy and make up the cell membranes, while nucleic acids carry genetic information necessary for reproduction and functioning of all living organisms. Carbon compounds also play a role in metabolic reactions, enzyme activity, and the transfer of energy in living cells. The versatility of carbon chemistry is what allows the diversity and complexity of life forms on Earth.
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