THE CHEMISTRY OF THERMAL CRACKING

Let us consider the cracking of n-hexane to propane, C₃H₈, and propene, C₃H₆ at room temperature (298 K). In chemistry there are two factors that determine whether a reaction can occur.

• Thermodynamics, and
• Activation Energy (generally known as the kinetic factor).

We will always deal with the thermodynamics of the reaction first, because if it says that the reaction cannot occur, it will not happen. A reaction can occur only if the change in Free Energy of the system from reactants to products, ΔG, is negative.

ΔG = ΔH − TΔS   where ΔH is the change in enthalpy for the reaction and ΔS is the change in entropy for the reaction.

When ΔG < 0 ; then we consider whether the energy of the system is sufficient for the reagents to "climb" over the activation energy barrier to become the product. What we are saying is; when ΔG < 0, it is like a car parked on a level ground on top of a hill. The only obstacle that prevented the car from rolling down the hill is a bump in front of the car. If some one has enough strength to push the car across the bump the car should roll down the hill.

THERMODYNAMICS	n−hexane	propane	propene	C6H14(g) � C3H8(g)+ C3H6(g)
ΔH˚ / kJ mol‾� S˚ / kJ K‾� mol‾� ΔG˚ / kJ	− 167 + 0.389	− 104 + 0.270	21 + 0.267	(− 104 + 21) − (− 167) = 84 (0.270 + 0.267) − 0.389 = 0.148 84 − (298)(0.148) = 40
ΔG > 0, the reaction is not possible.

Let us now increase the temperature to about 400˚C (700 K).

THERMODYNAMICS	n-hexane	propane	propene	C6H14(g) � C3H8(g) + C3H6(g)
ΔfH�(700 K) / kJ mol‾� S�(700 K) / J K‾� mol‾� ΔG˚ / kJ	−141 406	−90.2 278	31.3 276	82.1 0.148 82.1 − (700 x 0.148) = − 21.5

Thermodynamics says that this reaction is possible. The change in entropy of the system is now sufficient to overcome the change in enthalpy. At 700 K the system is of sufficient energy to cross the activation energy barrier. So this reaction will take place.

At lower temperatures the chains preferred to break up in the middle. At higher temperature it is more likely to be broken off at the ends. To obtain smaller molecules, cracking is conducted at high temperature of above 800˚C (that is 1100 K).

Ethylene and propylene are obtained from the cracking of ethane and propane, which are by-products of the various cracking processes to yield gasoline. Ethylene and propylene are the raw materials for 50 - 60% of all organic chemicals.

MECHANISM OF CRACKING ETHANE

CH3CH3 CH3� + CH3CH3 CH3CH2� H� + CH3CH3

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CH3� + CH3� CH4 + CH3CH2� CH2=CH2 + H� H2 + CH3CH2�

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In thermal cracking the by-products are hydrogen and methane. The alkenes formed are more likely to be reduced by hydrogen to give alkanes. So a short residence time would give a better yield of alkenes.

Tutorial 2.7

• Which are the initiation and propagation steps in the free radical chain reaction outlined above.
• Propose a termination reaction(s) for this chain reaction.
• What do you understand by residence time? Answer

The important by-reactions in the cracking of ethane are;

CH3� + CH2=CH2 2 CH3CH2CH2�

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CH3CH2CH2� CH3CH=CH2 + CH3CH2CH3

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