Name the oil refining process shown in the diagram - Leaving Cert Chemistry - Question 6 - 2016

The physical property that allows hydrocarbons to be isolated as naphtha in this process is the boiling (condensing) point (temperature).

A major use for kerosene is as aviation (airplane, jet) fuel.

A major use for residue is in roofing, road making, or waterproofing.

The oil refining process that converts octane into 2,2,4-trimethylpentane is called isomerisation.

The oil refining process that converts octane into ethylene is called dehydrocyclisation/reforming.

These conversions are desirable because they improve octane rating (number), which reduces the tendency of fuels to cause knocking. This is achieved by producing shorter chained hydrocarbons, which are more branched and therefore more efficient.

To calculate the heat of reaction, we apply Hess's Law. We can calculate it using the equation:

$\Delta H_{reaction} = \Sigma \Delta H_{f} (products) - \Sigma \Delta H_{f} (reactants)$

Given that:

• For C₈H₁₈, ( \Delta H_f = -250.1 \text{ kJ mol}^{-1} )
• For C₈H₄, ( \Delta H_f = -12.5 \text{ kJ mol}^{-1} )

We have:

$\Delta H_{reaction} = (-12.5) - (-250.1) = 237.6 \text{ kJ}$

Since the two known hydrocarbons are octane (C₈H₁₈) and propane (C₃H₈), we can find the molecular formula of the third molecule (C₁₈H₃₈ - C₈H₁₈ - C₃H₈) which results in:

$C_{18}H_{38} - C_{8}H_{18} - C_{3}H_{8} = C_{7}H_{12}$

Thus, the molecular formula of the third molecule is C₇H₁₂.

The molecular structure of octane (C₈H₁₈) is:

       H   H   H   H   H   H   H
       |   |   |   |   |   |   |
  H - C - C - C - C - C - C - C - C - H
       |   |   |   |   |   |   |
       H   H   H   H   H   H   H

Octane has a straight-chain structure.

The molecular structure of 2,2,4-trimethylpentane (C₈H₁₈) is:

       H   H   H
       |   |   |  
  H - C - C - C - H
       |   |   |  
       H   C - C - H
           |   |  
           H   H  
           |  
           H  
           

It has a branched structure.

The molecular structure of ethylbenzene (C₈H₁₀) is:

          H   H
          |   |
      H - C - C - H
          |     |   
          C - C
          |   |
          H  H  

Ethylbenzene consists of a benzene ring with an ethyl group.