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Alkanes

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Chemistry

Alkanes are everywhere. Take a quick look around - you are sure to find many products containing or derived from alkanes. The surface of that road outside your house was made from long-chain alkanes, and it’s highly likely that the fuel you put in your car is made from alkanes too. The plastic in your toothbrush is a type of polymer made up of chains of alkanes, and alkanes form the base of many chemicals, such as your toothpaste and soap. But what actually are they?

An alkane is a saturated hydrocarbon.

Let’s break that term down a little. First:

A hydrocarbon is an organic molecule that contains only hydrogen and carbon atoms.

Next:

Saturated molecules contain only carbon-carbon (C-C) and carbon-hydrogen (C-H) single bonds.

In contrast, an unsaturated hydrocarbon contains at least one carbon=carbon (C=C) double bond. Unsaturated hydrocarbons are known as alkenes, and we’ll take a quick look at them later.

The alkane homologous series

Alkanes form a homologous series with the general formula . Remember that a homologous series is a group of molecules that share the same chemical characteristics and general formula. In fact, they only differ in their chain length and arrangement. For example, ethane () and propane () are two of the simplest alkanes. Their structures are shown below. You can see that propane is very similar to ethane - it simply contains an extra group between the two end carbons.

Alkanes, ethane propane, StudySmarterEthane, left, and propane, right. Anna Brewer, StudySmarter Originals

Let’s take a quick look at propane. You can see that it has 3 carbon atoms. If we take the general formula for an alkane, , then n is clearly 3. According to the formula, alkanes have hydrogen atoms. Substituting n for 3 gives the following:

Therefore, propane must have 8 hydrogen atoms. A quick count confirms this.

How are alkanes formed?

Alkanes are formed from dead plant and animal matter that has been squashed under high temperatures and pressures over a long, long period of time. Cast your mind back 400 million years or so, to a world completely different to Earth as we know it. The first vertebrates were only just starting to emerge on land, giant mushrooms eight metres tall were a common sight, and oceans covered the vast majority of the planet. When creatures living in these oceans died, their remains fell to the ocean floor and were buried in layers of silt and sand. Over millions of years, the layers built up higher and higher, creating a high-pressure, high-temperature anaerobic environment. This allowed the dead organisms’ remains to slowly start turning into a substance called crude oil. The process is known as carbonation.

Crude oil is a complex mixture of hydrocarbons of different lengths.

When mined from the sea bed, crude oil is our primary source of alkanes.

Alkanes, crude oil formation, StudySmarterCrude oil formation. The process takes millions of years.Anna Brewer, StudySmarter Originals

Naming alkanes

Alkanes are probably the simplest type of organic molecule to name. They follow all the basic nomenclature laws, including those involving root names and side chains (see Organic Compounds for a quick recap). Their functional group is indicated by the suffix -ane. The following alkane is a good example:

Alkanes, 2,3-dimethylpentane, StudySmarterCan you name this alkane? Anna Brewer, StudySmarter Originals

First, identify the longest carbon chain in the molecule. Sometimes this chain is hard to spot as it could be part of what looks like a side branch. Here, the longest chain is 5 carbon atoms long. If we take a look at the table of root names, shown below, we know that this molecule must be based on pentane.

Alkanes, root names, StudySmarterA table showing the root names of alkanes. Anna Brewer, StudySmarter Originals

Next look at the side chains. There are 2 methyl groups () attached to 2 of the carbons, and so the prefix dimethyl- will be used. But which carbons are they joined to? To find out, number the carbons from both ends of the chain. We've shown this down below.

Alkanes, 2,3-dimethylpentane nomenclature, StudySmarterOur mystery molecule, with its carbon chain numbered. Anna Brewer, StudySmarter Originals

The methyl groups are either attached to carbons 3 and 4 if you count from the right, or 2 and 3 if you count from the left. However, as you know from Organic Compounds, the numbers of the carbons with the extra side chains and functional groups must add up to the lowest total possible. Therefore, in this molecule we count the carbons from the left. This gives us the overall name of 2,3-dimethylpentane.

Isomers of alkanes

Look at the alkane . This could represent multiple different molecules. For example, it could be either butane or 2-methylpropane:

Alkanes, alkane isomers butane 2-methylpropane, StudySmarterButane, left, and 2-methylpropane, right. Anna Brewer, StudySmarter Originals

Count the carbons and hydrogens to be sure. Both molecules have 4 carbon and 10 hydrogen atoms. These molecules are known as isomers.

Isomers are molecules with the same molecular formula but different arrangements of atoms.

Alkanes can show a type of structural isomerism called chain isomerism, as explored below.

Chain isomerism

Structural isomers are molecules that have the same molecular formula but different structural formulas. Specifically, chain isomers differ in their arrangement of the carbon chain.

For example, pentane and 2-methylbutane both have the same number of carbon and hydrogen atoms, but whilst pentane has a single long chain that is 5 carbons in length, 2-methylbutane has a 4-carbon chain with a methyl group side chain. Therefore, these molecules are chain isomers.

Alkanes, alkane isomers chain isomers, StudySmarterPentane, left, and 2-methylbutane, right. Anna Brewer, StudySmarter Originals

To find out more about the other types of isomerism, take a look at Isomerism.

Properties of alkanes

Alkanes are saturated hydrocarbons, consisting of C-C and C-H bonds only. These bonds are relatively strong, and because carbon and hydrogen have similar electronegativities, the bonds are also non-polar (see Polarity for further information). This means that the only forces between alkane molecules are van der Waal forces, which are also known as temporary or induced dipole forces.

Electrons in a molecule are constantly moving randomly, and at any one point could be anywhere in the molecule. Some might be clustered together, and some might be further apart. This creates a small dipole that is constantly changing in location and strength. Dipoles in one molecule then attract or repel neighbouring molecules, inducing dipoles in them as well, and this attraction holds the molecules together.

However, the attraction is relatively weak, giving alkanes the following properties:

Solubility

Alkanes are insoluble in water. This is because their non-polar C-C and C-H bonds cannot easily bind to polar water molecules. However, alkanes are soluble in other non-polar solvents and are good solvents themselves.

Combustibility

Alkanes are readily combustible and have high negative enthalpies of combustion, which is why we commonly use them as fuels such as petrol. They burn in excess oxygen to produce carbon dioxide and water.

Volatility

If you ever fill up your car at a petrol station, you’ll notice the stark warning signs: no lighters, no cigarettes. This is because short-chain alkanes are highly volatile and the surrounding air is likely to be saturated with their vapours. Any small spark could cause a devastating explosion. Their volatility decreases as they increase in length.

Reactivity

Alkanes are generally unreactive due to the strength of their non-polar C-H and C-C bonds. These bonds require a lot of energy to overcome, and most reactions simply can’t provide that. However, they can react with chlorine or bromine in UV light; this reaction is further explored in Chlorination. They can also be cracked to produce alkenes, and we’ll look at this in more detail in Cracking.

Melting and boiling points

Alkanes have relatively low melting and boiling points. This is because the only forces between alkane molecules are weak van der Waal forces, due to their C-C and C-H bonds being non-polar.

As the chain length of alkanes increases, their boiling points increase. A larger molecule has more electrons and so at any one time, its temporary dipole could be larger. It will therefore experience greater van der Waal attraction than a smaller molecule. However, as the number of branches increases, an alkane’s boiling point will decrease. This is because the molecules can’t pack together as tightly. Imagine packing strands of spaghetti in a jar - they can all fit together in neat rows in the same orientation. Now imagine if the spaghetti is branched, like twigs. The pieces can’t line up, so there are large gaps between the strands, forcing them further apart from each other and wasting space. Van der Waal forces between molecules are not very strong over long distances, and so the attraction between molecules is weaker. See Intermolecular Forces for a further explanation on van der Waal forces.

Alkanes vs alkenes

We know that alkanes are saturated hydrocarbons. Take propane, for example. Take off 1 hydrogen atom from each carbon and use the 2 free electrons to form another bond between the 2 carbons, and you should get something like this:

Alkanes, propane propene alkanes vs alkenes, StudySmarterPropane, left, and propene, right. Anna Brewer, StudySmarter Originals

This molecule is known as propene and is a type of alkene. We’ll explore alkenes in a later article, but for now you should know that they are unsaturated hydrocarbons that contain a C=C double bond. This bond alters their properties, making them more reactive than alkanes.

Alkanes - Key takeaways

  • Alkanes are saturated hydrocarbons.
  • They are named using standard nomenclature rules and the suffix -ane.
  • The bonds within alkanes are relatively strong and non-polar, making alkanes insoluble in water, readily combustible, and giving them low melting and boiling points.
  • Alkenes differ from alkanes by having one or more C=C double bonds.

Alkanes

Alkanes have n carbon atoms, and 2n+2 hydrogen atoms.

Alkanes are saturated hydrocarbons.

Alkanes are saturated, as they only have C-C and C-H single bonds.

Alkenes are unsaturated, meaning they contain at least one C=C double bond, whereas alkanes are saturated and contain only C-C and C-H single bonds.

Alkenes are more reactive than alkanes due to their C=C double bond.

Final Alkanes Quiz

Question

What is an alkane?


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Answer

A saturated hydrocarbon.

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Define saturated.

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Answer

Containing only C-H and C-C single bonds.

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Question

Give the general formula for an alkane.


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Give three properties of a homologous series.


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  • They have similar chemical properties.
  • They can be represented by a general formula.
  • They differ only in chain length and arrangement.


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How do alkanes differ from alkenes?


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Answer

  • Alkanes are saturated and consist of single C-C and C-H bonds only.
  • Alkenes are unsaturated and contain one or more C=C double bonds.


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Question

Hexane has 6 carbon atoms. How many hydrogen atoms does it have?


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Answer

14

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Where do alkanes come from?

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Answer

Crude oil reserves in sea beds formed from dead plant and animal matter over millions of years.

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Why is crude oil considered a finite resource?


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The formation of crude oil is so slow that it is essentially non-renewable.

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Give five uses of alkanes.


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  • Fuels such as petrol and diesel.
  • Gas canisters for camping stoves.
  • Plastics.
  • Road surfaces.
  • As the basis for many chemicals.

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Question

Why are alkanes relatively unreactive?


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Answer

They contain only C-C and C-H bonds, which are strong and non-polar.


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What is the strongest type of force found between alkane molecules?


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Answer

Van der Waals forces.

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Question

Which molecule will have a higher boiling point?

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Answer

Pentane

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Which of the following properties do alkanes have?

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Answer

Soluble in water.

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Question

Which molecule has a higher boiling point?

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Answer

Pentane.

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Question

What is the root name of an alkane with a chain length of 3?

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Answer

Meth-

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What is the root name of an alkane with a chain length of 2?

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Answer

Eth-

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Question

Name the following molecule:


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Answer

2-methylpentane

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Name the following molecule:

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Answer

2,2-dimethylpropane

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Question

How long is the longest carbon chain in the following molecule?

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Answer

5 carbon atoms long.

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Question

What is crude oil?

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Answer

A mixture of hydrocarbons of different chain lengths and other organic impurities.

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Question

How is crude oil formed?

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Answer

Through the breakdown of plant and animal matter deep within the Earth’s crust under high pressures and temperatures, over millions of years.

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Question

What is fractional distillation?

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Answer

A process that involves separating a mixture into fractions, which are samples containing molecules with similar boiling points.

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Question

How is crude oil distilled?


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Answer

  • Crude oil is heated so it evaporates. 
  • The vapours rise up a fractionating column that has a temperature gradient, until they reach a temperature cool enough for them to condense into trays.
  • The condensed vapours are piped off to be collected.

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Why are longer-chain hydrocarbons collected lower down in the fractionating column?


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Answer

They have a higher boiling point than shorter chain hydrocarbons, and so will condense at a higher temperature.

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Propane () and octane () are both hydrocarbons. Which will be collected higher up in the fractionating column?


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Answer

Propane

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Question

Give five uses of fractions obtained from crude oil fractional distillation.


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Answer

  • Fuels. 
  • Plastics.
  • Chemical feedstock.
  • Camping stove gas canisters.
  • Road surfaces.

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Predict a use for decane, 


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Answer

Fuel for planes.

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Where is the hottest part of the fractionating column?


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Answer

The bottom.

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Question

 Define renewable resource.


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A resource that is produced naturally at the same rate as it is used.

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Crude oil is formed from the remains of dead organisms. This is a natural process, so why is crude oil not a sustainable resource?


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Answer

Crude oil takes millions of years to form. We are using it at a faster rate than it is being created.

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Question

What are the products of hydrocarbon combustion?


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Answer

Carbon dioxide and water vapour.

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Name an impurity often found in crude oil.


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Answer

​Sulfur.

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Give three arguments against extracting and refining crude oil.


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  • It is non-renewable.
  • It burns to produce a greenhouse gas.
  • It contains damaging impurities that have negative environmental impacts when burnt.

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Question

What is cracking?

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The process of breaking down longer-chain fractions from the fractional distillation of crude oil into shorter lengths.

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What are the products of cracking?


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  • Shorter chain alkanes.
  • Alkenes.
  • Cyclical alkanes.

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Why do we crack hydrocarbons?


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Answer

Longer-chain hydrocarbons are in low demand, so we crack them to produce more economically valuable shorter-chain hydrocarbons.

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Question

Give five uses of the alkane .


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Answer

  •  In cigarette lighters.
  • In aerosols.
  • In fridges.
  • As a food additive.
  • In petrol.


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Why is it not economically viable to crack octane ?


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Answer

It is already useful as it is. It would be a waste of fuel and money.

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Question

What temperature is needed for thermal cracking?


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Answer

700-1200K

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What pressure is needed for thermal cracking?

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Answer

7000 kPa

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Question

What catalyst is used in catalytic cracking?


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Answer

A zeolite catalyst.

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Question

What is the catalyst used in catalytic cracking made of?

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Answer

A lattice of aluminium, silicon, and oxygen.

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Question

Describe the structure of a zeolite catalyst.

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Answer

A complex lattice made from aluminium, silicon, and oxygen, with a honeycomb structure to increase its surface area.


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Compare and contrast the two common types of cracking.


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Answer

  • Both catalytic and thermal cracking use high temperatures.
  • Thermal cracking uses a high pressure and no catalyst.
  • Catalytic cracking uses a slightly raised pressure and a zeolite catalyst. 
  • Thermal cracking can be done on larger alkanes and produces predominantly alkenes. 
  • Catalytic cracking cannot be done on very large hydrocarbons and produces mostly branched or cyclic alkanes.

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Question

Balance the following equation to show some of the possible products of cracking  .



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Which method of cracking produces a high proportion of alkenes?


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Answer

Thermal.

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How can you tell that the products of cracking are shorter-chain molecules than the reactants?


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Answer

The products have lower boiling points than the reactants.

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Question

One molecule of alkane Y is cracked to produce one molecule of pentane and two molecules of butene. Identify alkane Y.


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Why does using the safety flame of a Bunsen burner cause sooty deposits?


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Answer

Using the safety flame encourages incomplete combustion. One of the products is carbon, which accumulates as soot.


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Write an equation to show the complete combustion of ethane.

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Write an equation to show the incomplete combustion of ethane to produce carbon monoxide and water only.


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Which of the following two alkanes releases more energy when burnt? Their enthalpies of combustion are given:


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Answer

Butane.


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Name four pollutants produced in combustion.


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Answer

  • Sulfur dioxide.
  • Nitrous oxides.
  • Carbon.
  • Carbon monoxide.
  • Carbon dioxide.

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Hydrocarbon fuels can contain sulfur impurities. What are the negative effects caused by these impurities?


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Answer

Sulfur burns to form sulfur dioxide, which can cause acid rain and breathing difficulties.


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Question

What are catalytic converters made of?


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Answer

Platinum and rhodium.

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Question

Write an equation to show the reaction of a nitrous oxide, , with an unburnt hydrocarbon , in a catalytic converter to form carbon dioxide, water, and nitrogen.


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Discuss the pros and cons of biofuels.

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Answer

  • Biofuels are cheap, easy to grow and are carbon-neutral.
  • However, they are relatively inefficient to burn.
  • They take up valuable growing space that could be used for food crops. 
  • They may also require processing before use, which uses energy, and can be big and bulky to transport.

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Question

What is a free radical?

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Answer

A molecule with an unpaired outer shell electron.

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What is a substitution reaction?


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Answer

A reaction in which one functional group, atom, or group of atoms on a molecule is replaced by another.

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What is free radical substitution?


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A reaction in which an atom on a molecule is replaced by a free radical.

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Give the structural formulae of a propane free radical, using a dot to represent the unpaired electron.


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What are the four stages of chlorination using free radical substitution?


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Answer

  • Initiation
  • Propagation I
  • Propagation II 
  • Termination

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Question

What are the conditions needed for the free radical substitution reaction of ethane with chlorine?

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Answer

UV light.

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Question

 What is the overall equation for the primary free radical substitution of ethane with chlorine?


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Question

A molecule of chloroethane reacts with a chlorine free radical in the propagation step. What are the two possible products?


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Answer

1,1-dichloroethane and 1,2-dichloroethane.

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Question

What is homolytic fission?


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Answer

A molecule splits into two identical smaller molecules.

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Question

A pentane molecule reacts with chlorine in free radical substitution. Draw the three possible products.


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Question

Write an equation for the termination reaction between an ethane radical and a chlorine radical.


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Question

Write two equations showing the propagation reactions between butane, a chlorine radical and chlorine to form 1-chlorobutane.


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Question

What happens in the initiation step of chlorination?

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Answer

Chlorine splits into two free radicals.

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Question

What happens in the propagation I step of chlorination?


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Answer

A chlorine free radical reacts with an alkane.


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Question

What happens in the propagation II step of chlorination?

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Answer

An alkyl free radical reacts with chlorine.

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Question

What happens in the termination step of chlorination?

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Answer

Two free radicals react together.

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