6. (a) Several processes are used in oil refining to convert less useful hydrocarbons into more useful ones - Leaving Cert Chemistry - Question 6 - 2012
Question 6
6. (a) Several processes are used in oil refining to convert less useful hydrocarbons into more useful ones. For each conversion, (i), (ii), (iii), name the process ... show full transcript
Worked Solution & Example Answer:6. (a) Several processes are used in oil refining to convert less useful hydrocarbons into more useful ones - Leaving Cert Chemistry - Question 6 - 2012
Step 1
(i) C₈H₁₈ → 4H₂ +
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This reaction typically involves the process of dehydrocyclization (or cyclodehydrogenation), which rearranges the carbon structure of hydrocarbons through the removal of hydrogen, leading to the formation of cyclic compounds.
Step 2
(ii) C₈H₁₈ → C₈H₁₄ + X
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The process used here is catalytic cracking, which breaks larger hydrocarbons into smaller ones, resulting in the formation of alkenes and other hydrocarbons. The hydrocarbon X could be but-1-ene (C₄H₈) or but-2-ene (C₄H₈).
Step 3
(iii) C₈H₁₈ → C₆H₁₄ + C₂H₄ +
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This process is known as isomerization, where the original hydrocarbon is rearranged to create different structural isomers, typically enhancing octane ratings for fuel.
Step 4
Name a hydrocarbon that X could be and draw its structural formula.
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A possible hydrocarbon for X is:
H H
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H - C - C - H
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H H
This represents but-2-ene, which is suitable due to the given structural requirements.
Step 5
Name the product of process (ii) and state its octane number.
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The product from process (ii) is 2,2,4-trimethylpentane (also known as iso-octane), which has an octane number of 100.
Step 6
Explain why the substance MTBE (methyl tert-butyl ether) is sometimes added to motor fuel.
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MTBE is utilized in motor fuel to enhance octane ratings and improve combustion quality, thereby reducing engine knocking and increasing efficiency. It also serves to reduce carbon monoxide emissions during combustion.
Step 7
Name the substance previously added to motor fuel for the same purpose and state why its use was discontinued.
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The previously used substance was lead (tetraethyl lead), but its use was discontinued due to serious health risks associated with lead exposure, which can cause neurological damage and other health problems.
Step 8
Define heat of combustion.
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The heat of combustion refers to the amount of energy released as heat when a substance undergoes complete combustion with oxygen under standard conditions. It is typically measured in kJ/mol.
Step 9
Name the laboratory apparatus used to measure the heats of combustion of fuels and foodstuffs.
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The laboratory apparatus used for this measurement is called a 'bomb calorimeter'.
Step 10
Calculate the heat of formation of the hydrocarbon (C₈H₁₈) produced in process (ii).
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To calculate the heat of formation, we can use the formula:
