Cracking 

In a nutshell  

Cracking is a process in which long-chain alkanes are broken down into short-chain hydrocarbons, which are more useful. This process is carried out at an industrial level to supply the demand of short-chain hydrocarbons. The products of cracking include both alkanes and alkenes.

Cracking conditions

There are two types of cracking - steam cracking and catalytic cracking.

• Steam cracking conditions: steam and temperatures over $$800$$​$$\degree C$$ ​
• Catalytic cracking conditions: zeolite catalyst and temperatures between $$470-550$$​$$\degree C$$​ 
  
• Zeolite catalysts are made up of aluminium oxide and silicon oxide 

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Cracking equation 

Longer-chain alkanes are cracked into multiple shorter-chain hydrocarbons. 

​$$long-chain ~alkane \rightarrow hydrocarbon ~ A + hydrocarbon~ B+ unknown \space hydrocarbon$$​​

PROCEDURE

Example

Pentadecane is an alkane with the formula: $$C_{15}H_{32}$$​. Pentadecane is cracked to give octane ($$C_8H_{18}$$​), ethene ($$C_2H_4$$​) and one other product. Write out the balanced equation for this process.

Add the number of carbon atoms from octane and ethene: 

$$8 + 2 = 10\ carbon\ atoms$$​

Subtract this from the number of carbon atoms in pentadecane: 

$$15 - 10 = 5\ carbon\ atoms$$

Add the number of hydrogen atoms from octane and ethene: 

$$18 + 4 = 22\ hydrogen\ atoms$$​

Subtract this from the number of hydrogen atoms in pentadecane:

$$32 - 22 = 10\ hydrogen\ atoms$$​​

This gives the formula for the other product:

 $$C_5H_{10}$$

The equation is:

$$pentadecane \rightarrow octane + ethene +pentene\\ \ \\C_{15}H_{32}\rightarrow C_8H_{18} + C_2H_4 + C_5H_{10}$$​

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Supply vs. demand

The industrial supply of long-chain hydrocarbons, such as fuel oil, is higher than consumer demand. Conversely, the supply of short-chain hydrocarbons is lower than consumer demand. This is the reason long-chain hydrocarbons are cracked into shorter-chain hydrocarbons.