Question 1: Define an alkene and describe its general molecular structure.

Answer: An alkene is a hydrocarbon containing at least one carbon-carbon double bond, with the general formula CnH2n.

Question 2: Explain the mechanism of addition reactions in alkenes.

Answer: Addition reactions involve the breaking of the double bond in alkenes, allowing atoms or groups to add across the carbons, resulting in saturated compounds.

Question 3: What is the main difference between alkenes and alkanes?

Answer: Alkenes have at least one double bond, while alkanes are saturated hydrocarbons with only single bonds.

Question 1: A 50.0 g sample of ethene (C2H4) reacts completely with bromine (Br2). Calculate the mass of bromine required for the reaction. (Atomic masses: C=12.01 g/mol, H=1.008 g/mol, Br=79.90 g/mol)

Answer: The molar mass of C2H4 = (2×12.01) + (4×1.008) = 28.05 g/mol. Moles of C2H4 = 50.0 / 28.05 ≈ 1.78 mol. Bromine needed = 1.78 mol × 79.90 g/mol ≈ 142.4 g.

Question 2: If 10 mL of ethene gas (density = 1.178 g/L at room temperature) reacts with excess hydrogen, what is the mass of ethene used?

Answer: Mass = volume × density = 0.010 L × 1.178 g/L = 0.01178 g.

Question 1: Describe a basic experimental procedure to test whether a hydrocarbon is an alkene using bromine water.

Answer: Add a few drops of bromine water to the hydrocarbon sample in a test tube and observe the color change from orange-brown to colorless, indicating the presence of a double bond. Ensure proper ventilation and wear gloves and goggles for safety.

Question 2: Identify the independent and dependent variables in this experiment.

Answer: Independent variable: presence or absence of the hydrocarbon sample; Dependent variable: color change of bromine water.

Question 1: In an experiment, a 20 mL sample of an unknown hydrocarbon causes bromine water to decolorize in 30 seconds. When the same volume of a known alkene is used, the decolorization occurs in 15 seconds. What can you infer about the reactivity of the unknown hydrocarbon?

Answer: The unknown hydrocarbon has a lower reactivity towards bromine compared to the known alkene, possibly indicating a saturated hydrocarbon or fewer double bonds.

Question 1: Explain how the presence of a double bond in alkenes influences their chemical reactivity compared to alkanes. Include in your explanation the concept of pi bonds.

Answer: The double bond in alkenes consists of a sigma bond and a pi bond. The pi bond is less stable and more reactive because electrons are more exposed, making alkenes more susceptible to addition reactions. This increased reactivity contrasts with alkanes, which only have single sigma bonds and are less reactive.

Question 2: Discuss the importance of alkene reactions in the production of plastics and other synthetic materials. Provide specific examples.

Answer: Alkene reactions are fundamental in the manufacture of plastics such as polyethylene and polyvinyl chloride (PVC). Through addition polymerization, monomers like ethene or propene link together to form long-chain polymers used in packaging, construction, and everyday products. These reactions enable large-scale, cost-effective production of synthetic materials essential in modern industry.