Petrol is mostly a mixture of hydrocarbons, obtained from the fractional distillation of crude oil, and is in high demand as a fuel for cars - Leaving Cert Chemistry - Question 6 - 2020
Question 6
Petrol is mostly a mixture of hydrocarbons, obtained from the fractional distillation of crude oil, and is in high demand as a fuel for cars. To avoid knocking in th... show full transcript
Worked Solution & Example Answer:Petrol is mostly a mixture of hydrocarbons, obtained from the fractional distillation of crude oil, and is in high demand as a fuel for cars - Leaving Cert Chemistry - Question 6 - 2020
Step 1
Explain the underlined terms.
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Answer
Knocking refers to the unwanted pre-ignition or premature ignition of the air-fuel mixture in the engine cylinder, which can cause engine damage. It occurs when the fuel ignites before the spark plug fires, leading to a loss of power and efficiency. The octane number is a measure of a fuel's ability to resist knocking; higher octane numbers indicate a greater resistance, making it suitable for high-performance engines.
Step 2
Name the type of oil refining reaction in each of the three equations above.
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Answer
Dehydrocyclization
Cracking
Isomerization.
Step 3
Deduce the formula of hydrocarbon X.
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Answer
The hydrocarbon X in the reaction can be deduced by balancing the equation. The balanced equation suggests that X must be C₄H₁₀.
Step 4
Give the systematic IUPAC name for the product in the third equation.
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Answer
The systematic IUPAC name for the product is 2,2,4-trimethylpentane.
Step 5
Give an example of an oxygenate used in petrol.
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Answer
An example of an oxygenate used in petrol is ethanol.
Step 6
Why are oxygenates added to petrol?
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Oxygenates are added to petrol primarily to improve the combustion efficiency, reduce harmful emissions, and enhance the octane rating of the fuel, thereby minimizing knocking in the engine.
Step 7
Define heat of formation.
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Answer
The heat of formation is the amount of energy released or absorbed when one mole of a compound is formed from its elements in their standard states.
Step 8
The heats of formation of propane, carbon dioxide and water are –104.3, –393.5 and –285.8 kJ mol⁻¹ respectively. Write a balanced equation for the combustion of propane in an excess supply of oxygen and use the data provided to calculate the heat of combustion of propane.
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Answer
The balanced equation for the combustion of propane (C₃H₈) is:
ightarrow 3 ext{CO}_2 + 4 ext{H}_2 ext{O}$$
To calculate the heat of combustion, we can use the formula:
$$ ext{ΔH}_{comb} = ext{ΣΔH}_{formation ext{ (products)}} - ext{ΣΔH}_{formation ext{ (reactants)}}$$
This leads us to:
$$ ext{ΔH}_{comb} = [3(-393.5) + 4(-285.8)] - [-104.3] = -2219.0 ext{ kJ mol}^{-1}$$
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