Crack! Many of us have dropped something fragile on the floor before, be it a glass, a jar of jam, a plate, or an egg. The object smashes - it breaks into many tiny pieces.
But cracking doesn't have to be a bad thing. Sure, smashing a plate isn't much fun, but unless you break open an egg, you'll never be able to make an omelette with the tasty white and yolk inside. Cracking an object into smaller pieces to make it more useful is much like chemical cracking.
- This article is about cracking in chemistry.
- We'll define cracking before looking at its purpose and products.
- We'll then explore two common types of cracking, including their processes.
- Finally, we'll learn how to balance cracking equations.
Cracking: Definition
When we crack a Christmas cracker, we snap it in half to reveal a silly joke and paper hat. When we crack a coconut, we are rewarded with a feast of refreshingly cool coconut water. In both cases, we break a larger object apart into smaller pieces to make it more useful. This also happens in cracking in chemistry.
Chemical cracking is the process of breaking down large molecules into smaller, more useful ones. It is a type of thermal decomposition reaction.
The term cracking is most commonly used to describe the breakdown of long-chain hydrocarbon fractions from crude oil into shorter-chain alkanes and alkenes. This involves breaking a C-C single bond. The cracking of hydrocarbons will be the main focus of our article today.
Cracking: Purpose
Firstly, why do we crack hydrocarbons? It is to make them more economically valuable.
You might know from the article Fractional Distillation that many of our hydrocarbons come from distilling crude oil. Fraction distillation produces multiple different hydrocarbon fractions, each with different purposes and demands.
- Shorter-chain hydrocarbons are less viscous and have low boiling points. They are more useful and so have a higher economic demand than longer-chain hydrocarbons. For example, there is a huge demand for naphtha, a fraction containing C6 - C12 hydrocarbons. We use it for petrol and in the chemical industry.
- Longer-chain hydrocarbons are more viscous and have high boiling points. They are less useful than shorter-chain hydrocarbons and so have a lower economic demand. For example, longer-chain alkanes are too thick to be used as petrol and so must be used in aircraft instead.
The usefulness of different crude oil fractions creates a problem: Their supply doesn't meet their demand! For example, crude oil from the North Sea typically contains over 88% long-chain C10+ hydrocarbons1. We end up with a lot of spare longer-chain hydrocarbons and not much to do with them!
C10+ hydrocarbons have chains that are at least ten carbon atoms long. C6 - C12 hydrocarbons, such as those in naphtha, have chains that are between six and twelve carbon atoms long.
However, there is a solution. To make surplus longer-chain fractions more economically valuable, we can crack them into more useful products. The hydrocarbons produced have a much higher demand and are a lot more useful to us than the original longer-chain hydrocarbons, making cracking an economically significant reaction. Let's explore the products of cracking now.
Cracking: Products
Chemical cracking breaks down longer-chain hydrocarbons into two types of smaller hydrocarbons:
The process is random, meaning that we can't control exactly which molecules we end up with. However, it doesn't matter so much - both types of products are much more useful to us than the original longer-chain hydrocarbons. With cracking, we can turn relatively useless molecules that we probably wouldn't otherwise use into relatively useful molecules that massively enhance our lives.
Short-chain alkanes
Cracking firstly produces short-chain alkanes. These, as we already know, have a higher demand than longer-chain hydrocarbons. They're used primarily as fuels, but also in cigarette lighters, aerosols, and more.
Alkenes
Cracking also produces alkenes.
Alkenes are unsaturated hydrocarbons that contain at least one C=C double bond.
Alkenes are considered more useful than the original longer-chain hydrocarbons for multiple reasons:
- Alkenes are more reactive than alkanes.
- They are a good chemical feedstock, which means that they are used in industry as the starting materials to make further products.
- We can react multiple alkene molecules together to form polymers, such as those used in plastic bags, bottles, and nylon clothing.
- We also use alkenes as a base for solvents, paints, and medicines.
Fig. 1 - The simplest alkene, known as ethene
To find out more about these hydrocarbons, check out Alkenes.
Cracking: Types and process
So, we know why cracking is important and what it produces. We'll now move on to discuss the different types of cracking.
There are two different methods of cracking commonly used to split hydrocarbons:
- Thermal cracking.
- Catalytic cracking.
Because hydrocarbons like alkanes are relatively unreactive due to their strong, non-polar C-C and C-H bonds, both types of cracking require particularly harsh conditions to break them down. However, they also have their differences. Let's look at types of cracking now.
Thermal cracking
Thermal cracking involves putting the hydrocarbon alkanes under extreme heat and pressure for a brief period of time, usually only one second. We typically use a very high temperature of 700-1200 K and a high pressure of 7000 kPa. The alkane splits homolytically, meaning one electron from the bonded pair goes to each of the new molecules formed. This forms two free radicals.
A free radical is an extremely reactive molecule with an unpaired outer shell electron.
Free radicals react further to produce various hydrocarbons, but especially alkenes. However, maintaining such extreme conditions requires a lot of fuel. Therefore, thermal cracking has a large economic and environmental footprint.
Catalytic cracking
Catalytic cracking stands out from thermal cracking because it uses a catalyst.
A catalyst is a substance that increases the rate of reaction by lowering the activation energy needed for a reaction to occur.
Catalytic cracking takes place at 700K and a pressure only slightly above atmospheric levels, but uses a zeolite crystal catalyst. This is a complex lattice made from aluminium, silicon, and oxygen, with a honeycomb structure to increase its surface area. Unfortunately, larger hydrocarbons can’t be cracked in this way because they are too large to fit in the catalyst.
Catalytic cracking produces a high proportion of shorter-chain, branched, and cyclic alkanes, as well as aromatic compounds such as benzene. It also has much lower fuel requirements than thermal cracking.
An aromatic compound contains a ring of delocalised electrons, often known as a benzene ring. Check out Aromatic Chemistry for more.
Comparing types of cracking
The following table will help you summarise your knowledge by comparing thermal and catalytic cracking.
| Name | Thermal | Catalytic |
| Temperature | Very high (700 - 1200 K) | High (700 K) |
| Pressure | High (7000 kPa) | Slightly raised |
| Catalyst | None | Zeolite crystal |
| Products | Mostly alkenes | Mostly shorter-chain/branched alkanes |
| Advantages | Can crack larger hydrocarbonsProduces a large proportion of alkenes | Low fuel costsLow environmental impact |
| Disadvantages | High fuel costsLarge environmental impact | Cannot be used for larger hydrocarbons |
Cracking: Equations
Cracking is a largely random process. It is impossible to predict exactly which molecules will be produced. This means there are multiple different equations and potential products for each reaction, and your examiner could test you in various ways. This will typically involve finding an unknown hydrocarbon reactant or product. However, it's easy enough to 'crack' cracking equations! The important thing to remember is that the equation has to be balanced: the numbers of carbon atoms and hydrogen atoms on each side of the equation must be the same.
Here's a rough method to get you started:
- Write out the basic cracking equation, using CxHy to represent the unknown hydrocarbon.
- Balance the equation in terms of carbon atoms to find the value of x.
- Balance the equation in terms of hydrogen atoms to find the value of y.
- Substitute your values of x and y into the equation to reach your final answer.
Let’s have a go.
Decane (C10H22) can be cracked to produce octane (C8H18) and one other molecule.
- Write a balanced equation for the reaction.
- Name the second product.
For part a, we'll start by writing out our equation, using CxHy to represent the unknown product. It currently looks like this:
$$C_{10}H_{22}\rightarrow C_8H_{18}+C_xH_y$$
Here, x and y represent unknown quantities of carbon and hydrogen atoms respectively. However, we can work these values out by balancing the equation.
To find x, consider carbon. We have ten carbon atoms on the left-hand side of the equation, and so there must be ten carbon atoms on the right. We already have eight carbon atoms in the first product, octane, and so there are \(10-8=2\) carbons remaining. Therefore, \(x=2\).
We can carry out the same process with hydrogen. This will give us a value for y. There are twenty-two hydrogen atoms on the left-hand side, and so we need twenty-two on the right. We already have eighteen hydrogen atoms in the first product, octane, and so there are \(22-18=4\) hydrogens remaining. Therefore, \(y=4\).
Now we just substitute our values for x and y into the equation. Here's our final answer:
$$C_{10}H_{22}\rightarrow C_8H_{18}+C_2H_4$$
Part b asks us to name the second product. C2H4 is an alkene known as ethene.
Don't worry if you don't know how to name alkenes just yet - you'll cover it later on in the article Alkenes.
Here is another example:
One mole of alkane X is cracked to produce one mole of heptane (C7H16) and two moles of propene (C3H6). Deduce the formula of X.
Once again, we'll start by writing an equation with what we know. Note that we produce two moles of propene:
$$C_xH_y\rightarrow C_7H_{16}+2C_3H_6$$
Now we can balance the equation. The only reactant on the left-hand side of the equation is X, our unknown, and so we know that x and y must equal the total number of carbon atoms on the right-hand side. Likewise, y must equal the total number of hydrogen atoms on the right-hand side.
Looking at carbon, there are \(7+2(3)=13\) carbon atoms on the right-hand side of the equation, meaning that x equals 13. Similarily, there are \(16+2(6)=28\) hydrogen atoms on the right, meaning that y equals 28. If we substitute these values into the equation, we get out final answer:
$$C_{13}H_{28}\rightarrow C_7H_{16}+2C_3H_6$$
Not feeling so confident at writing balanced equations? Check out the article Balancing Equations for more worked examples and helpful tips.
Cracking - Key takeaways
- Cracking is a chemical technique used to turn longer-chain hydrocarbons into more economically valuable short-chain alkanes and alkenes.
- Short-chain alkanes are primarily used fuels.
- Alkenes are used to make polymers and as a chemical feedstock.
- Cracking can be thermal or catalytic. Both types of cracking require harsh conditions to break the strong bonds within the longer-chain hydrocarbons.
- Thermal cracking uses a very high temperature of 700-1200 K and a high pressure of 7000 kPa.
- Catalytic cracking uses a high temperature of 700 K, a slightly raised pressure, and a zeolite crystal catalyst.
- Cracking is random and produces a mixture of products.
References
- Aly A Hamouda, Sidra Chughtai, 'Miscible CO2 Flooding for EOR in the Presence of Natural Gas Components in Displacing and Displaced Fluids', Energies 11(2):391 (February 2018)
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