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

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

Question 2: Explain why alkenes are considered unsaturated hydrocarbons.

Answer: Because they contain at least one double bond between carbon atoms, allowing for additional atoms to be added via reactions, unlike saturated hydrocarbons which have only single bonds.

Question 3: Describe the mechanism of the addition of bromine (Br₂) to an alkene.

Answer: The addition of bromine to an alkene occurs via electrophilic addition. The double bond electrons are attracted to the bromine molecule, forming a bromonium ion intermediate. A bromide ion then attacks the more positive carbon, resulting in a dibromoalkane.

Question 1: An alkene has 8 carbon atoms. Calculate its molecular formula.

Answer: C8H16

Question 2: If a molecule of alkene C5H10 reacts with bromine, how many bromine atoms are needed to fully brominate the molecule? Show your working.

Answer: Since C5H10 has a double bond, one molecule reacts with one Br₂ molecule (containing 2 Br atoms). Therefore, 2 bromine atoms are needed to add across the double bond.

Question 1: Outline the safety precautions that should be taken when adding bromine to an alkene in the laboratory.

Answer: Wear safety goggles and gloves, perform the reaction in a well-ventilated fume cupboard, and handle bromine with care as it is corrosive and toxic.

Question 2: Describe a simple experimental setup to demonstrate the addition of bromine to an alkene.

Answer: Place the alkene solution in a test tube, add a few drops of bromine water, and observe the colour change from orange-brown to colourless, indicating addition across the double bond.

Question 1: A sample of an alkene reacts with bromine, resulting in a colourless product. What does this indicate about the structure of the alkene?

Answer: It indicates that the alkene contains a double bond that reacts with bromine, confirming it is unsaturated.

Question 2: A student measures the mass of an alkene before and after bromination and notices a mass increase of 80 grams. If the original alkene had a mass of 120 grams, what is the molecular formula of the brominated product?

Answer: The increase of 80 grams corresponds to 4 bromine atoms (each approximately 80 g/mol). The original alkene formula is CnH2n, and after addition, it becomes CnH2nBr4.

Question 1: Explain how alkenes are used in the manufacturing of plastics such as poly(ethene).

Answer: Alkenes like ethene are polymerised through addition polymerisation to produce plastics such as poly(ethene), which is used in packaging, bottles, and containers.

Question 2: Discuss the importance of understanding alkene reactions for developing new technologies in sustainable chemistry.

Answer: Understanding alkene reactions enables chemists to design efficient pathways for producing bio-based plastics, renewable fuels, and environmentally friendly chemicals. It also aids in developing catalysts that minimise waste and energy consumption, contributing to sustainability.