Hydrocarbons

Question 1

Draw the structure of displayed and 3D formula of the alkane ethane

Question 2

Draw the general formula of an alkane = CnH2n + 2

Question 3

What shape do the covalent bonds around each carbon atom form

Answer 3

A tetrahedral shape

Question 4

What are the bond angles of a covalent bond around a carbon atom

Answer 4

109.5 degrees

Question 5

What are the bonds found within alkanes

Answer 5

Sigma bond

Question 6

Explain a sigma bond

Answer 6

Sigma bonds form when electrons orbitals from adjacent atoms directly overlap .

A sigma bond contains a pair of electrons one from each atom on either side of the bond.

The pair of electrons in sigma bond lie directly between the bonding atoms.

Question 7

What is a key feature of sigma bonds

Answer 7

They are fully rotational so carbon atoms can rotate relative to each other.

The covalent bonds in alkanes are also relatively strong and take a lot for energy to break.

Question 8

Explain how alkanes are a non-polar molecule

Answer 8

The similar electronegativity between a carbon and hydrogen means that alkanes are non-polar molecules

Question 9

Explain how alkanes are insoluble in water

Answer 9

Water molecule will form hydrogen bonds with each other. However, since alkanes have no permanent dipoles they cannot form hydrogen bonds. Therefore, alkanes cannot dissolve in water

Question 10

Explain how alkanes are generally unreactive

Answer 10

The strong covalent bonds within an alkane molecule works to prevent alkanes from reacting. However, will under certain conditions

Question 11

Explain the pattern in the boiling point of alkanes

Answer 11

Short chain alkanes have low boiling points and are gases at room temperature.However, longer chain alkanes have higher boiling points.

As the carbon chain increases we find liquid and solid alkanes at room temperature

Question 12

How can the increasing boiling point of increasing number of carbons in alkanes be explained

Answer 12

Alkanes are non-polar molecules thus the intermolecular forces acting between alkanes molecules are induced dipole-dipole interactions.

This is also known as London or dispersion forces

London forces are weak and do not take much energy to break thus short chain hydrocarbons have low boiling points.However, once carbon chain increases the strength of London forces also increase thus longer chain hydrocarbons have a greater boiling point.

Question 13

What are the two reason why London forces also increase when chain length increases

Answer 13

Longer chain alkanes have more electrons than shorter chain alkanes. The strength of London forces increase as the number of electron increases.

Longer chain alkanes have a greater surface area than shorter chain alkanes (greater surface area). This means that there are many points along the molecule where they can form London forces

Question 14

Explain why branched chain alkanes have a lower boiling point than straight chain alkanes

Answer 14

Branches prevent alkane molecules from getting close together

London forces are the strongest over short distances. Thus in branched chains the London forces are reduced.

Question 15

What are the uses of alkanes

Answer 15

Alkanes are used for:

Fuels in vehicles

Starting materials for the production of a whole range of organic molecules including pharmaceutical

Question 16

What is crude oil

Answer 16

Crude oil is a fossil fuel that has formed underground. Over millions of years , heat and pressure convert the chemicals in these remains into crude oil.

Question 17

Why is crude oil considered renewable

Answer 17

Crude oil is being used at a faster rate then is being formed

Question 18

What is crude oil composed of

Answer 18

Mixture of straight chain and branched chain alkanes with other chemical such as sulfur

Question 19

Explain the stages of fractional distillation of crude oil

Answer 19

Firstly, the crude oil is heated in a furnace. The temperature of the furnace is hot enough to boil a lot of the alkanes in the crude oil converting them into a gas

The crude oil vapors and liquid pass into the fractionating column. The column is hotter at the bottom and becomes progressively cooler going upwards.

The crude oil vapors makes it way up the column.

There are collection trays at different levels of the fractionating column. These trays have bubble caps which allow vapor to pass upwards

AS each alkane moved up the column it will reach a temperature which is cooler than its boiling point

It will then condense back into a liquid and pass out of the column

Short chain hydrocarbons (low b.p) collected at the bottom. Long chain hydrocarbons (high b.p) collected at the the top

Alkanes with very long chains form a thick liquid called bitumen collected at the bottom

Very short alkanes are collected at the top of the column as gases

Question 20

Misconceptions of fractional distillation

Answer 20

Fractional distillation does not separate each individual alkane. Instead, each fraction contain a number of of alkanes with similar boiling points.

To separate each individual alkane would require further rounds of fractional distillation

Question 21

What is the a with crude oil

Answer 21

Crude oils contains a higher proportion of longer chain hydrocarbons than short chain hydrocarbons

Question 22

Why is there a high demand for short chain hydrocarbons

Answer 22

Short chain hydrocarbons are used for fuel for vehicles.

Therefore, there is an economic benefit for cracking long chain hydrocarbons which are less in demand in order to create short chain hydrocarbon which are high in demand. This process is called cracking.

Question 23

what are the benefits of cracking

Answer 23

Firstly, cracking converts long chain hydrocarbons into shorter chain hydrocarbons.

Cracking produces alkanes and alkenes which are highly reactive and are used as major feedstock (raw materials;) for the chemical industry to make a range of products e.g polymers.

Question 24

Explain the process of thermal cracking

Answer 24

Thermal cracking requires both high temperature and high pressure (450-900c) and the pressure is 70atm.

In thermal cracking, long chain alkanes form both shorter chain alkanes and alkenes.

Hydrogen can also be one of the products

Question 25

Explain the benefits of thermal cracking

Answer 25

We created a high percentage of alkenes in the products. (Useful molecule due to their high reactivity)

Question 26

How is a radical created during thermal cracking

Answer 26

During thermal cracking a covalent bond spilts to form intermediate molecules.

When the covalent bond splits, both of the intermediate molecules now have one unpaired electrons.

This is called a free radical

Question 27

Explain the process of catalytic cracking

Answer 27

Catalytic cracking also require a high temperature (450c). However, does not require high pressure.

In fact the pressure for catalytic cracking is 1-2 atmospheres.

Catalytic cracking uses a zeolite catalyst which contains a mixture of aluminum oxide and silicon dioxide.

Zeolite has large surface area thus supports to make it an effective catalyst

When a product undergoes catalytic cracking the product are often branched chained alkenes which are especially useful for petrol as they combust very easily

Catalytic cracking can last produce cyclic alkanes and aromatic hydrocarbons such as benzene

Question 28

What is a free radical

Answer 28

A free radical is any species with an unpaired electron. They are highly reactive

Question 29

Draw the word equation for the reaction between methane and bromine

Answer 29

Methane + Bromine -UV light—> Bromoethane + Hydrogen bromide

Question 30

Explain what happens in the reaction of methane and bromine

Answer 30

The is the initiation stage: The mixture of methane and bromine have ultraviolet light shined into the reaction mix. The energy of ultraviolet light causes the single covalent bond between the two bromine atoms to break.

A single covalent bond consist of a pair of electrons. When a bond breaks like this, one electron now goes to each bromine atom because these now have an unpaired electron, these are now bromine free radicals. This is called homolytic fission. This is called the initiation stage.

The propagation stage starts where a bromine free radical reacts with a methane molecule. The bromine free radical take a hydrogen atom plus one electron from the methane molecule.

The hydrogen atom on the methane molecule has been substituted with a bromine atom because this reaction involves free radicals. This reaction is an example of free radicals substitution.

This produces both hydrogen bromide and a methyl free radical

The methyl free radical now reacts with a bromine molecule. This produces the end product bromomethane plus another bromine free radical

Propagation step 1 and 2 can form a chain reaction. (Bromine free radical can react with the methane in propagation step 1). This reaction will continue until termination

In termination , the two free radical react together to form a molecule with no unpaired electrons. This is now a stable molecule and no longer takes part in the reaction.

There are three different reaction which can take place:

Two bromine free radicals can form a bromine molecule

Two methyl free radicals can form a molecule of ethane

A methyl free radicals and bromine free radicals can form a molecule of bromoethane

Question 31

Explain why there is one big problem with free radical substitution of alkanes

Answer 31

We get a whole range of side products.

E.g-if a bromine free radical reacts with a molecule of bromoethane then we make dibromoethane

A huge range of products can be formed including isomers. Therefore, at the end of this reaction, we need to separate out our product molecules

Question 32

Explain what are alkenes

Answer 32

Alkenes are unsaturated hydrocarbons with the general formula of CnH2n