Cracking hydrocarbons

Cracking breaks large hydrocarbon molecules into smaller ones. It is useful because long-chain hydrocarbons obtained from crude oil are often less useful or less in demand than smaller molecules.

The hydrocarbon is vaporised and heated so that some of its carbon–carbon bonds break. The products are a mixture of smaller hydrocarbons, including alkanes, which have only carbon–carbon single bonds, and alkenes, which contain a carbon–carbon double bond.

Methods of cracking

In catalytic cracking, hydrocarbon vapour passes over a hot catalyst. The reaction takes place at a high temperature with the help of the catalyst.

In steam cracking, hydrocarbon vapour is mixed with steam and heated to a very high temperature.

Equations for cracking

A cracking equation shows one possible set of products. For example, a molecule of dodecane can crack to form a molecule of octane and a molecule of butene:

The equation is balanced: each side has 12 carbon atoms and 26 hydrogen atoms. Atoms are rearranged during cracking, so the total number of each type of atom must be the same on both sides of the equation.

Uses of cracking products

Cracking breaks large hydrocarbon molecules into smaller molecules. It is useful because some large hydrocarbons are less useful, while many of the smaller products are in greater demand.

Cracking products as fuels

Small hydrocarbon molecules can be used as fuels. Burning them releases energy for transport, heating and other uses. Cracking therefore helps increase the supply of fuels that people need.

Alkenes as manufacturing materials

Cracking also produces alkenes. These are valuable raw materials because they can be used to make polymers, including plastics. Alkenes are also starting materials for making many other chemicals used in everyday products.

Hydrocarbons therefore supply more than energy: through products such as alkenes, they also provide raw materials for manufacturing.

Alkenes and the bromine water test

Alkenes are hydrocarbons: compounds containing only carbon and hydrogen. They contain a carbon–carbon double bond, whereas alkanes contain only carbon–carbon single bonds.

Alkenes are more reactive than alkanes because their double bond can react more readily than the single bonds in alkanes. Alkenes can be produced by cracking, in which larger hydrocarbon molecules are broken down into smaller molecules, including alkenes.

Testing for an alkene with bromine water

Bromine water is orange. When it is added to an alkene, bromine reacts at the carbon–carbon double bond. The bromine is used up, so the orange colour disappears and the mixture becomes colourless. This is called decolourisation.

Under normal test conditions, an alkane does not react with bromine water, so the orange colour remains. The colour change therefore distinguishes an alkene from an alkane.

Balancing chemical equations

In a chemical reaction, atoms are rearranged to form new substances, but they are not created or destroyed. A balanced chemical equation therefore has the same number of atoms of each element on the reactant side as on the product side.

A number placed before a formula is called a coefficient. It gives the relative number of particles of that substance and multiplies every atom count in the formula. For example, $$\ce{3H2O}$$ represents three water molecules, containing six hydrogen atoms and three oxygen atoms in total. When no coefficient is written, it is understood to be one.

Balancing an equation

To balance an equation, count the atoms of each element on both sides. Adjust the coefficients, then recount until the totals match for every element. Use the simplest whole-number coefficients.

Do not change the small numbers within the given formulae, called subscripts. Changing a subscript changes the substance represented; changing a coefficient changes only the number of its particles.

For example, start with hydrogen reacting with oxygen to form water:

There are two oxygen atoms on the left but only one on the right. Place a coefficient of two before water to give two oxygen atoms on the right. This also gives four hydrogen atoms on the right, so place a coefficient of two before hydrogen:

Recounting confirms four hydrogen atoms and two oxygen atoms on each side. The coefficients are in their simplest whole-number ratio: $$2:1:2$$.

Balancing cracking equations

In a cracking equation, the carbon and hydrogen atoms in the hydrocarbon reactant must equal the total carbon and hydrogen atoms in the hydrocarbon products. For example:

On the right, $$\ce{C4H10}$$ contains four carbon atoms and ten hydrogen atoms. The coefficient in $$\ce{3C2H4}$$ gives six carbon atoms and twelve hydrogen atoms. Together, the products contain ten carbon atoms and twenty-two hydrogen atoms, matching the reactant.