i. What alkenes have one or more double bonds between carbon atoms? They have the general formula CₙH₂ₙ. When two carbon atoms share two pairs of electrons, they form a double bond between the carbon atoms. How many electrons are left on each carbon atom?
Answer: a) 2
Explanation: In a carbon-carbon double bond, each carbon atom shares two of its four valence electrons. This leaves each carbon with two remaining valence electrons to form bonds with other atoms (typically hydrogen).
ii. The type of bond present between the carbon atoms in alkynes is:
Answer: c) Triple covalent bond
Explanation: Alkynes are unsaturated hydrocarbons characterized by a carbon-carbon triple bond (C[source symbol unavailable]C), consisting of one sigma and two pi bonds.
iii. Alkenes are classified as:
Answer: b) Unsaturated hydrocarbons
Explanation: Alkenes contain at least one carbon-carbon double bond, which means they have fewer hydrogen atoms than the maximum possible (they are not saturated). Therefore, they are classified as unsaturated hydrocarbons.
iv. Cracking of large alkane molecules requires:
Answer: d) High temperature and high pressure (Note: Catalytic cracking uses a catalyst at moderate temperature and pressure. Thermal cracking uses high temperature and high pressure. Option d is more comprehensive for cracking in general.)
Explanation: Cracking is the process of breaking large hydrocarbon molecules into smaller ones. It requires high temperatures (400-700°C) and often high pressure or a catalyst to break the strong C-C bonds.
v. Which of the following is a product of cracking large alkane molecules?
Answer: b) Alkenes (Note: Hydrogen is also produced, but alkenes are the primary desired product. Option 'b' is the most specific answer.)
Explanation: Cracking produces smaller alkanes and alkenes. It may also produce hydrogen gas. However, the primary purpose of cracking is to produce alkenes (like ethene and propene) for the petrochemical industry. Therefore, alkenes are a key product.
vi. The chemical test to distinguish between alkanes and alkenes involves:
Answer: b) Bromine water
Explanation: Alkenes decolorize bromine water (reddish-brown to colorless) via an addition reaction. Alkanes do not react with bromine water under normal conditions, so the color remains. This is a standard test for unsaturation.
vii. Alkynes contain which type of carbon-carbon bond?
Answer: c) Triple covalent bond
Explanation: Alkynes are defined by the presence of a carbon-carbon triple bond (C[source symbol unavailable]C).
viii. Ethyne is commonly used for:
Answer: b) Welding
Explanation: Ethyne (acetylene) burns in oxygen to produce an extremely hot flame (over 3000°C), which is used for welding and cutting metals.
ix. Fractional distillation separates petroleum into fractions based on:
Answer: b) Boiling point
Explanation: Fractional distillation separates the components of crude oil based on their different boiling points. The fractionating column has a temperature gradient that allows different fractions to condense at different heights.
x. Which property increases as you move from the bottom to the top of the fractionating column?
Answer: a) Volatility
Explanation: As you move from the bottom to the top of the fractionating column, the fractions become lighter (shorter chain length), more volatile (evaporate more easily), less viscous, and have lower boiling points.
xi. The gasoline/petrol fraction from petroleum is primarily used for:
Answer: c) Car fuel
Explanation: The gasoline (petrol) fraction is primarily used as fuel for internal combustion engines in cars.
xii. Doubly bonded carbon atoms form which type of bond with two other atoms.
Answer: b) Double
Explanation: Doubly bonded carbon atoms are connected to each other by a double bond (sharing two pairs of electrons).
i. Describe the bonding in alkenes.
Answer: In alkenes, the carbon atoms involved in the double bond are sp² hybridized. Each carbon atom forms three sigma (σ) bonds: one with the other carbon atom and two with other atoms (usually hydrogen). The remaining unhybridized p-orbital on each carbon overlaps sideways to form a pi (π) bond. The double bond consists of one strong sigma bond and one weaker pi bond. The
pi bond restricts rotation around the C=C bond, which leads to geometric (cis-trans) isomerism.
ii. What are the conditions required for the cracking of alkanes?
Answer: Cracking of alkanes requires:
High Temperature: Typically between 400°C and 700°C. High Pressure: For thermal cracking, high pressure is used. For catalytic cracking, moderate pressure is used. Catalyst (for catalytic cracking): Zeolites or aluminium silicates are used to speed up the reaction at lower temperatures.
These conditions provide the energy needed to break the strong C-C and C-H bonds in large alkane molecules.
iii. Why is the cracking of large alkane molecules important in the petroleum industry?
Answer: Cracking is important because:
Matches Demand: It converts low-demand heavy fractions into high-demand lighter fractions like gasoline (petrol). Produces Alkenes: It produces alkenes (e.g., ethene, propene), which are essential raw materials for making plastics, fibers, and other chemicals. Increases Octane Number: It improves the quality of gasoline by producing branched and unsaturated hydrocarbons with higher octane numbers.
iv. How can you test for the presence of an unsaturated hydrocarbon?
Answer: Two standard tests can be used:
Bromine Water Test: Add bromine water (reddish-brown) to the sample. If it decolorizes rapidly, the hydrocarbon is unsaturated (contains a double or triple bond). Acidified KMnO₄ Test: Add dilute acidified potassium manganate(VII) solution (purple) to the sample. If the purple color
disappears, the hydrocarbon is unsaturated.
v. Write the symbol equation for the preparation of ethene from ethanol.
Answer: C₂H₅OH [source symbol unavailable] C₂H₄ + H₂O (Conditions: Heat with concentrated H₂SO₄ at 170°C, or pass over hot Al₂O₃ at 350°C.)
vi. What type of bond is present in alkynes?
Answer: Alkynes contain a carbon-carbon triple bond (C[source symbol unavailable]C). This bond consists of one sigma (σ) bond and two pi (π) bonds. The carbons are sp hybridized.
vii. Name two uses of ethene.
Answer: Two major uses of ethene:
1. Production of Polyethene: Ethene is polymerized to make polythene, a widely used plastic.
2. Ripening Fruits: Ethene is used as a plant hormone to artificially ripen fruits.
viii. What is the principle behind the fractional distillation of petroleum?
Answer: The principle is that different hydrocarbons in crude oil have different boiling points. When heated, they vaporize at different temperatures. In the fractionating column, the vapors rise and cool. Fractions with higher boiling points condense at lower, hotter trays, while those with lower boiling points condense at higher, cooler trays. This allows for separation based on boiling point.
ix. How does the boiling point of fractions change from the bottom to the top of the fractionating column?
Answer: The boiling point of fractions decreases from the bottom to the top of the fractionating column. The bottom is the hottest part, where heavy fractions with high boiling points condense. The top is the coolest part, where light fractions with low boiling points condense.
x. List two uses of the naphtha fraction obtained from petroleum.
Answer: Naphtha is used for:
1. Chemical Feedstock: It is a major source of raw materials for the petrochemical industry (e.g., for making plastics).
2. Gasoline Blending: It is used as a component in the production of high-octane gasoline.
xi. How can you distinguish between propane and propene?
Answer: Use bromine water. Propene is an alkene (unsaturated) and will rapidly decolorize the reddish-brown bromine water via an addition reaction. Propane is an alkane (saturated) and will not react with bromine water under normal conditions, so the color remains unchanged.
xii. In a refinery, large alkane molecules are cracked to produce smaller alkenes and other hydrocarbons.
Answer: Alkenes are unsaturated because they contain a carbon-carbon double bond, meaning they have fewer hydrogen atoms than the maximum possible for their carbon chain length. Conditions for cracking: High temperature (400-700°C), high pressure, and/or a catalyst (e.g., zeolites).
Answer: C₁₀H₂₂ [source symbol unavailable] C₈H₁₈ + C₂H₄
Answer: To meet market demand for lighter fractions like petrol and to produce alkenes for making plastics.
Answer: Use a catalyst (catalytic cracking) to lower the temperature and pressure required, saving energy and increasing the reaction rate.
xiii. Ethyne (Acetylene) is a hydrocarbon, used as a fuel for welding and in artificially ripening fruits.
Answer: Ethyne contains a carbon-carbon triple bond, which is a highly unsaturated structure with fewer hydrogens than alkanes or alkenes.
Answer: Ethyne burns in oxygen to produce a flame with an extremely high temperature (over 3000°C), sufficient to melt metals.
Answer: Calcium carbide (often used to produce ethyne for ripening) may contain traces of arsenic and phosphorus, which can be toxic. Ethyne itself may also cause respiratory irritation.
Answer: Use ethene gas, a natural plant hormone, which is safe for ripening fruits.
xiv. Why alkenes are considered unsaturated hydrocarbons.
Answer: Alkenes are unsaturated because they contain at least one carbon-carbon double bond, which means they do not have the maximum possible number of hydrogen atoms per carbon atom.
xv. Write the symbol equation for the cracking of decane (C₁₀H₂₂).
Answer: C₁₀H₂₂ [source symbol unavailable] C₈H₁₈ + C₂H₄ (Decane [source symbol unavailable] Octane + Ethene)
xvi. Why the refinery gas fraction is more volatile than the fuel oil fraction.
Answer: Refinery gas consists of very short-chain hydrocarbons (C₁-C₄), while fuel oil consists of long-chain hydrocarbons (C₂₀+). Shorter chains have weaker van der Waals forces between molecules, so they have lower boiling points and are more volatile (evaporate more easily).
1. Explain the process of fractional distillation of petroleum, including how the properties of fractions change from the bottom to the top of the fractionating column.
Answer: Fractional distillation is the primary process used to separate crude oil (petroleum) into its various useful components, called fractions. Crude oil is a complex mixture of hydrocarbons with different chain lengths and boiling points.
The Process:
1. Heating: Crude oil is first heated in a furnace to a very high temperature (around 400°C). This converts most of the liquid into a mixture of hot vapors and liquids.
2. Entering the Column: This hot mixture is fed into the bottom of a tall, vertical steel tower called a fractionating column.
3. Separation: The fractionating column has many horizontal trays or plates with bubble caps, and a temperature gradient exists
inside it. It is hottest at the bottom and coolest at the top.
As the hot vapors rise up the column, they cool down. When a vapor reaches a tray where the temperature is at or below its boiling point, it condenses back into a liquid. The liquid is collected on that tray and drawn off as a fraction.
4. Collection: Fractions with high boiling points condense at the bottom, while those with low boiling points condense at the top.
How Properties Change from Bottom to Top:
Boiling Point: Decreases from bottom to top. Chain Length (Number of Carbon Atoms): Decreases from bottom to top. Viscosity (Thickness): Decreases from bottom to top. Volatility (Ease of Evaporation): Increases from bottom to top. Flammability: Increases from bottom to top.
In summary, fractional distillation separates crude oil into fractions based on their boiling points, with lighter, more volatile fractions rising to the top and heavier, thicker fractions remaining at the bottom.
2. Discuss the significance of cracking in the petroleum industry and how it helps in meeting the demand for various hydrocarbons.
Answer: Cracking is a vital chemical process in the petroleum industry that breaks down large, complex hydrocarbon molecules into smaller, more useful ones. Its significance is immense.
The Need for Cracking:
Supply vs. Demand: The demand for lighter fractions like gasoline (petrol) and alkenes (e.g., ethene) is much higher than the demand for heavier fractions like fuel oil. Crude oil contains a large proportion of heavy fractions.
Creating Useful Products: Cracking converts these low-value heavy fractions into high-value lighter fractions, matching market demand.
Types of Cracking:
Thermal Cracking: Uses high temperature and high pressure to break bonds. Catalytic Cracking: Uses a catalyst (e.g., zeolites) at a lower temperature and pressure to speed up the reaction.
Significance:
1. Increases Gasoline Yield: Cracking significantly boosts the production of gasoline, the most important fuel for vehicles.
2. Produces Alkenes: Cracking produces a large amount of alkenes like ethene (C₂H₄) and propene (C₃H₆). These are essential monomers for making plastics (polyethene, polypropene), fibers, and other chemicals.
3. Produces Hydrogen: It also produces hydrogen gas, which is used in processes like ammonia production and hydrogenation.
In summary, cracking is essential for balancing the supply of petroleum fractions with market demand, and for producing the vital raw materials for the petrochemical industry.
3. Describe the preparation of alkenes by elimination reactions from halogenoalkanes and alcohols, providing balanced chemical equations for each process.
Answer: Alkenes can be prepared by elimination reactions from halogenoalkanes and alcohols. In these reactions, two atoms or groups are removed from adjacent carbon atoms to form a carbon-carbon double bond.
1. Preparation of Alkenes from Halogenoalkanes (Dehydrohalogenation):
When a halogenoalkane is heated with a strong base (like alcoholic KOH or NaOH), a hydrogen halide (HX) is eliminated, forming an alkene.
General Equation: R-CH₂-CH₂-X + KOH (alc) [source symbol unavailable] R-CH=CH₂ + KX + H₂O Example (Bromoethane to Ethene):
Equation: CH₃-CH₂-Br + KOH (alc) [source symbol unavailable] CH₂=CH₂ + KBr + H₂O Conditions: Heat under reflux with alcoholic potassium hydroxide.
2. Preparation of Alkenes from Alcohols (Dehydration):
When an alcohol is heated with a strong acid catalyst (like concentrated H₂SO₄) or passed over hot alumina (Al₂O₃), a water molecule is eliminated, forming an alkene.
General Equation: R-CH₂-CH₂-OH [source symbol unavailable] R-CH=CH₂ + H₂O Example (Ethanol to Ethene):
Equation: CH₃-CH₂-OH [source symbol unavailable] CH₂=CH₂ + H₂O Conditions: Heat with concentrated H₂SO₄ at 170°C, or pass over Al₂O₃ at 350°C.
In summary, both methods involve the elimination of a small molecule (HX or H₂O) from adjacent carbons to create an alkene.
1. Compare the economic benefits of producing alkenes via cracking versus extracting them directly from crude oil.
Answer: Producing alkenes via cracking is economically far more beneficial than extracting them directly from crude oil. Here is a comparison:
Extraction from Crude Oil:
Availability: Crude oil naturally contains very few alkenes. It is primarily composed of alkanes. Process: Separating any trace alkenes would require complex and expensive separation techniques. Economic Value: This method is not economically viable because alkenes are not naturally abundant in crude oil. It would be like trying to extract gold from seawater.
Production via Cracking:
Availability: Cracking can convert the abundant, low-value heavy alkane fractions into high-demand alkenes. Process: Cracking is a well-established and efficient industrial process. Catalytic cracking is particularly cost-effective. Economic Value: This method is highly profitable. It adds value to low-cost raw materials by converting them into high- demand products used to make plastics, fibers, and other chemicals. The demand for alkenes (like ethene and propene) is massive, driving the profitability of the cracking process.
In summary, cracking is the economically superior method because it creates valuable alkenes from cheap, abundant raw materials, whereas extracting them directly from crude oil is impractical and unprofitable.
2. Describe the chemical properties of ethyne that make it suitable for welding.
Answer: Ethyne (acetylene, C₂H₂) is widely used for welding and cutting metals due to its unique chemical properties.
1. High Heat of Combustion:
The most important property is that ethyne burns in oxygen to produce an extremely hot flame. The reaction is highly exothermic. Equation: 2C₂H₂ + 5O₂ [source symbol unavailable] 4CO₂ + 2H₂O + Heat The oxy-acetylene flame can reach temperatures of over 3000°C, which is hot enough to melt most metals.
2. Unsaturation and Reactivity:
Ethyne is an unsaturated hydrocarbon containing a carbon-carbon triple bond. This makes it highly reactive. It undergoes addition reactions readily.
3. Incomplete Combustion:
When burnt in a limited supply of oxygen, ethyne produces a sooty flame (due to incomplete combustion). However, in a pure oxygen environment, it produces a clean, intensely hot flame ideal for welding.
In summary, the high heat of combustion and the intensely hot flame produced when burned in oxygen make ethyne the fuel of choice for welding and cutting metals.
3. In a laboratory, you need to distinguish between a sample of hexane and hexene. Describe the test to distinguish between them.
Answer: To distinguish between hexane (an alkane) and hexene (an alkene), you can use two simple chemical tests.
Test 1: Bromine Water Test (Decolourization Test):
Procedure: Add a few drops of bromine water (a reddish-brown solution) to separate test tubes containing hexane and hexene. Shake the test tubes gently. Observation:
Hexane: The reddish-brown color of bromine water remains unchanged (or the orange color persists). Hexane does not react with bromine under normal conditions. Hexene: The reddish-brown color disappears rapidly (the solution becomes colorless). This is because hexene undergoes an addition reaction with bromine, breaking the double bond and forming a colorless dibromohexane.
Equation: CH₃CH₂CH₂CH₂CH=CH₂ + Br₂ [source symbol unavailable] CH₃CH₂CH₂CH₂CHBrCH₂Br
Test 2: Acidified Potassium Manganate(VII) Test (Baeyer's Test):
Procedure: Add a few drops of acidified potassium manganate(VII) solution (KMnO₄) to separate test tubes containing hexane and hexene. Shake gently. Observation:
Hexane: The purple color of KMnO₄ remains unchanged. No reaction occurs. Hexene: The purple color disappears (decolorized). The alkene is oxidized by the KMnO₄, forming a diol (e.g., hexane-1,2-diol).
Conclusion: If the bromine water or acidified KMnO₄ is decolorized, the sample is hexene. If the color persists, it is hexane.
Choose saturated and unsaturated compounds from the following:
(i) CH₃—CH₂—CH₃
Answer: Saturated (Alkane - Propane)
(ii) CH₃—C[source symbol unavailable]CH
Answer: Unsaturated (Alkyne - Propyne)
(iii) CH₃—CH=CH₂
Answer: Unsaturated (Alkene - Propene)
(iv) CH₂=CH—CH=CH₂
Answer: Unsaturated (Diene - Buta-1,3-diene)
Complete the following reactions.
1. CH₃—CH₂—CH₂OH [source symbol unavailable] (Conc. H₂SO₄, 170°C) [source symbol unavailable]
Answer: CH₃—CH=CH₂ + H₂O (Propene)
2. CH₃—CH₂—CH₂Br + KOH (Alcohol) [source symbol unavailable]
Answer: CH₃—CH=CH₂ + KBr + H₂O (Propene)
3. Which alkene is formed by the dehydration of the following alcohol? CH₃—CH₂—CH₂OH
Answer: Propene (CH₃—CH=CH₂)
Complete the following reactions:
1. CH₃—CH=CH₂ + Br₂ [source symbol unavailable]
Answer: CH₃—CHBr—CH₂Br (1,2-Dibromopropane)
2. CH₃—CH=CH₂ + KMnO₄ + H₂O [source symbol unavailable]
Answer: CH₃—CH(OH)—CH₂(OH) (Propane-1,2-diol)
3. CH₃—CH₃ + KMnO₄ + H₂O [source symbol unavailable]
Answer: No reaction (Alkanes do not react with KMnO₄ under normal conditions)
Complete the following reactions
1. CH₃—CH=CH₂ + H₂ [source symbol unavailable] (Ni, 200-300°C) [source symbol unavailable]
Answer: CH₃—CH₂—CH₃ (Propane)
2. CH₃—CH=CH—CH₃ + H₂ [source symbol unavailable] (Ni, 200-300°C) [source symbol unavailable]
Answer: CH₃—CH₂—CH₂—CH₃ (Butane)
1. List names of fractions obtained by the fractional distillation of petroleum.
Answer: Refinery gas, Gasoline (Petrol), Naphtha, Kerosene, Diesel oil, Lubricating oil, Fuel oil, Bitumen.
3. List one use of each petroleum fraction.
Answer: Refinery gas (LPG for cooking), Gasoline (car fuel), Naphtha (chemical feedstock), Kerosene (jet fuel), Diesel (truck fuel), Lubricating oil (lubricants), Fuel oil (ship fuel), Bitumen (roads).
4. How is petroleum obtained?
Answer: Petroleum is obtained by drilling deep into the Earth's crust, both on land and under the ocean floor, to extract crude oil from underground reservoirs.
