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The table shows the octane numbers of four hydrocarbons - Leaving Cert Chemistry - Question 6 - 2006
Question 6
The table shows the octane numbers of four hydrocarbons. (a) (i) What is meant by the octane number of a fuel? (ii) Hexane has the lowest octane number of the fo... show full transcript
Worked Solution & Example Answer:The table shows the octane numbers of four hydrocarbons - Leaving Cert Chemistry - Question 6 - 2006
Step 1
What is meant by the octane number of a fuel?
Answer
The octane number of a fuel is a measure of its tendency to auto-ignite. It indicates how well the fuel resists knocking, pre-ignition, and other premature ignition events in an engine. The octane rating is based on a scale where iso-octane (2,2,4-trimethylpentane) is assigned a rating of 100, and heptane is assigned a rating of 0. Therefore, a higher octane number indicates better resistance to engine knocking.
Step 2
What structural feature of hexane contributes to its low octane number?
Answer
Hexane has a straight-chain structure. This structure contributes to its lower octane number as it has a more linear arrangement of carbon atoms, which allows for easier auto-ignition under the conditions present in an engine, compared to branched or cyclic structures that have higher stability.
Step 3
Identify the structural feature of cyclohexane, benzene, and 2,2,4-trimethylpentane that contributes to having high octane numbers.
Answer
Cyclohexane contains a ring structure, which increases stability and reduces the likelihood of knocking. Benzene is an aromatic compound, which also offers greater stability due to its resonance structures. 2,2,4-trimethylpentane is a branched hydrocarbon, which minimizes the likelihood of pre-ignition compared to straight chains, leading to a higher octane rating.
Step 4
Name the process carried out in an oil refinery that converts hexane to compounds such as cyclohexane and benzene. Why is the use of benzene in petrol strictly controlled?
Answer
The process used is known as dehydrocyclization or cyclodehydration. The use of benzene in petrol is strictly controlled because benzene is a known carcinogen and poses serious health risks, including toxicity to humans.
Step 5
Give two reasons why oxygenates such as MTBE are added to petrol.
Answer
- Oxygenates like MTBE increase the octane rating of petrol, which helps to improve engine performance and reduce knocking.
- They also produce cleaner combustion, which leads to reduced emissions of harmful pollutants, contributing to better environmental standards.
Step 6
Calculate the heat of combustion of cyclohexane using the given balanced equation.
Answer
To find the heat of combustion ( ΔH_combustion ) of cyclohexane, use the formula:
The equation shows:
- For the products: 6 mol of CO2 [-394 kJ/mol] and 6 mol of H2O [-286 kJ/mol],
- For the reactants: 1 mol of C6H12 [-156 kJ/mol] and 9 mol of O2 [0 kJ/mol].
Calculating:
Following the calculation yields:
Thus, the heat of combustion of cyclohexane is -3924 kJ.