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Chemistry A-level > Hydrocarbons > Flashcards

Hydrocarbons Flashcards

1
Q

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

A
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2
Q

Draw the general formula of an alkane

A

Cn H2n + 2

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3
Q

What shape do the covalent bonds around each carbon atom form

A

A tetrahedral shape

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4
Q

What are the bond angles of a covalent bond around a carbon atom on an alkane

A

109.5 degrees

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5
Q

What are the bonds found within alkanes

A

Sigma bond

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6
Q

Explain a sigma bond

A

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.

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7
Q

What is a key feature of sigma bonds

A

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 of energy to break.

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8
Q

Explain how alkanes are a non-polar molecule

A

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

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9
Q

Explain how alkanes are insoluble in water

A

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

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10
Q

Explain how alkanes are generally unreactive

A

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

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11
Q

Explain the pattern in the boiling point of alkanes

A

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

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12
Q

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

A

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.

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13
Q

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

A

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

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14
Q

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

A

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.

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15
Q

What are the uses of alkanes

A

Alkanes are used for:

Fuels in vehicles

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

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16
Q

What is crude oil

A

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.

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17
Q

Why is crude oil considered non-renewable

A

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

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18
Q

What is crude oil composed of

A

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

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19
Q

Explain the stages of fractional distillation of crude oil

A

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

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20
Q

Misconceptions of fractional distillation

A

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

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21
Q

What is the a problem with the proportion of long chain hydrocarbons and short chain hydrocarbon in crude oil

A

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

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22
Q

Why is there a high demand for short chain hydrocarbons

A

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.

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23
Q

what are the benefits of cracking

A

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.

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24
Q

Explain the process of thermal cracking

A

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

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25
Q

Explain the benefits of thermal cracking

A

A high percentage of alkenes is formed in the products. (Useful molecule due to their high reactivity)

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26
Q

How is a radical created during thermal cracking

A

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

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27
Q

Explain the process of catalytic cracking

A

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 produce cyclic alkanes and aromatic hydrocarbons such as benzene

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28
Q

What is a free radical

A

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

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29
Q

Draw the word equation for the reaction between methane and bromine

A

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

30
Q

Explain what happens in the reaction of methane and bromine

A

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:

Step 1)
A bromine free radical reacts with a methane molecule. The bromine free radical take a hydrogen atom plus one electron from the methane molecule. This produces hydrogen bromide and a methyl free radical.

Step2 )

The methyl free radicals now reacts with the bromine molecule. This produces our end product bromoethane plus another bromine free radical

Propagation step 1 and 2 can form a chain reaction. (The bromine free radical formed in propagation step 2 can go back an react with methane in propagation step 1) . This reaction continue unilateral termination.

Termination:

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

31
Q

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

A

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

32
Q

Explain what are alkenes

A

Alkenes are unsaturated hydrocarbons with the general formula of CnH2n

33
Q

Explain planar molecules

A

Planar molecules are flat with all of the atoms lying on the same plane

When looking at molecular shapes we treat both single and double bonds as bonding regions

There are three bonding regions around each carbon atom. These repel as far away as possible

34
Q

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

A

The bond angles around the carbon atom are 120

35
Q

Explain boding regions in alkenes

A

When looking at molecular shapes we treat both single and double bonds as bonding regions

There are three bonding regions around each carbon atom. These repel as far away as possible

36
Q

Explain how a pi bond appears in a double bond

A

When the double bond forms there is one sigma bond between the carbon atom. Each carbon atom will have one electron left in the p-orbital .

These lie above and below the plane of the molecule

The two p-orbitals now overlap sideways. They form a covalent bond called a pi bond (both above and below the sigma bond)

37
Q

Explain how the double bonds can cause differences in reactivity and structure of alkenes

A

Pi bonds cannot rotate- rotation will reduce the overlap of p-orbitals. The structure across the double bond is effectively locked. Thus are able to form stereoisomers

Alkenes are very reactive. The bond enthalpy of the pi bond is less than the sigma bond. That’s because pi bond is a sideways overlap of orbitals whereas in a sigma bond, the orbitals directly overlap. Thus the pi bond is easier to break as it takes less energy resulting in it to be more likely to be used in reactions

The double bond contains two pairs of electrons ( a pair in the pi bond and sigma bond). The double bond is a region of high energy electron density. This makes it a highly reactive molecule.

38
Q

Explain why the double bond has a high region of electron density

A

A double bond contains a sigma bond and pi bond which both contain a pair of electrons

The double bond in alkenes actually consist of four electrons.

This mean that the double one is a region of high electron density.

39
Q

Key idea behind halogen atom during the electrophilic addition of hydrogen halides

A

Halogen atoms are more electronegative than hydrogen. This means that hydrogen atoms molecules have a permanent dipole.

The hydrogen atom has a small positive charge whereas the halogen atom has a small negative charge.

When a hydrogen halide is added to a symmetrical alkene, we can only make one product.

When an asymmetrical alkenes reacts with a hydrogen halide molecule we can form two different isomers. This depends on what cabin atom has the positive charge in the carbocation intermediate.

We do not create an equal amounts of product e.g (2-bromobutane is a major product whereas 1-bromobutane is a minor product)

40
Q

Explain the reaction mechanism of the electrophilic addition of hydrogen halide to alkenes

A

The hydrogen halide molecule approaches the alkene.

The positive hydrogen on the hydrogen halide is attracted to the high electron density of the double bond. This is referred to as an electrophile (positive ion/molecule attracted to a region of high electron density)

The positive charge on the hydrogen atom attracts the pair of electrons in the pi bond of the alkene.This pair of electrons move toward the hydrogen atom. (Curly arrow is used to show this)

A covalent bond forms towards the hydrogen atom and hydrogen halide. This creates a problem as a hydrogen atom only has one covalent bond. At the same time the pair of electrons between the hydrogen and halide now move onto the halide atom. (This is shown as a curly arrow). When covalent bonds break like this, with both electrons going to one atom this is called heterocyclic fission.

A positively charged intermediate molecule is formed from the alkene. This is called the carbocation intermediate. This contains a positively charged carbon atom as it has lost its pair of the electron in the pi bond.

A negatively charged halide ion. Both the electrons that were in the covalent bond were now on the bromide ion.

The electron pair on the halide ion are attracted to the positive carbon atom in the carbocation. ( shown through a curly arrow).

Lastly a product is formed

41
Q

Key point about the reaction of a hydrogen halide to an alkene

A

Key points- covalent bonds is a pair of electrons

when a curly arrow is used make sure it starts where the electrons are moving from and ends where they are moving to.

The hydrogen halide is added to the alkene. This reaction is electrophilic addition.

The alkene in this reaction is Ethene which is symmetrical molecule. Thus only one product can be formed. However, if the reaction was carried out with an asymmetric alkene then two products can be formed

42
Q

Key ideas behind asymmetric alkeness

A

When an asymmetric alkene reacts with a hydrogen halide molecule we can form two different isomers.

This depends on which carbon atom has the positive charge in the carbocation intermediate.

Equal amount of products between isomers are not created (one isomers is the major product and the other is the minor product)

43
Q

Explain how are major and minor products formed from asymmetric alkene isomers

A

Carbocation intermediate is an unstable molecule which only exists for a short period of time.

The stability of the carbocation depends on how many alkyl groups are bonded to the carbon atoms with the positive charge.

If the positive carbon atom is bonded to only one alkyl group then we call this a primary carbocation. However, if it is bonded to two alkyls it is called a secondary carbocation

Secondary carbocation are more stable than a primary carbocation. Thus secondary carbocations exist for longer and is more likely to form a product.

44
Q

what is Markownikoff’s rule

A

When a hydrogen halide reacts with an asymmetric alkene, the hydrogen atom of the hydrogen halide is more likely to bond to the carbon atom which is attached to the greater number of hydrogen atoms

45
Q

Explain Markownikoff’s rule

A

Secondary carbocation are more stable than primary carbocation. This is because the electrons in alkyl groups can shift towards the positive charge. In order to stabilise the positive charge . (Positive inductive effect)

In primary carbocations, the positive charge is stabilized by only one alkyl group whereas in secondary carbocations, the positive charge is stabilised by two alkyl groups. Therefore, secondary carbocations are more stable and the major product is formed.

46
Q

Describe the reaction mechanism between an alkene and a halogen

A

In the first stage of the reaction, the halogen molecule approaches the alkene molecule. The halogen does not have a permanent dipole. However, the double bond of the alkene is a region of high electron density.This high electron density repels the electron pair of the covalent bond in the halogen molecule. This means that the halogen molecule has an induced dipole.

The pair of electrons from in the pi bond of the alkene are attracted to the positive halogen. The positive halogen acts as an electrophile. The electron pair then forms a covalent bond to the halogen atom.At the same time, the covalent bond in the halogen molecule now breaks and the pair of electrons move onto the other halogen atom. (When a covalent bond breaks like this with both electron going to the same atom- this is an example of heterolytic fission). We then have a carbocation intermediate and a halogen ion.

The electron pair in the halogen is attracted to the positive carbon atom in the carbocation intermediate. This electron pair now forms a covalent bond and we have our final product. (The halogen atom would add across the double bond. This means that the two halogen atoms end up on two adjacent carbon atoms-both cannot be on the same carbon)

47
Q

Key point of electrophilic addition of alkene and halogens

A

Halogen molecules add across the double bond. This is because halogen molecules do not have a permanent dipole.

48
Q

Key differences between adding a halogen to an alkene and a hydrogen halide to one

A

hydrogen haldie molecules have a permanent dipole whereas halogens have induced dipoles

When a hydrogen halide is added to an asymmetric alkene. This makes major and minor product. However, when a halogen is added to an asymmetric alkene, we only make one product.

49
Q

What can the reaction with a halogen be used for

A

To test for the presence of an unsaturated molecule such as an alkene.

50
Q

Describe how you can test for unsaturated molecules

A

We use bromine water which has an orange/brown colour.

To test if a substance is unsaturated. We add drops of bromine water and gently shake the test tube.

If our substance is unsaturated, then the bromine will add across the double bond and the product of the reaction will be colourless.

The orange bromine water will decolourlise. If the test substance is saturated, then the bromine will not react and the bromine water will remain orange.

51
Q

What is a hydration reaction

A

The electrophillic addition of water to alkenes to make alcohols

52
Q

Key points in hydration reactions

A

In hydration water is in the form of steam

Phosphoric acid is used as a catalyst

The temperature is 300c and the pressure is 60 atmospheres

53
Q

Key points in hydration reactions

A

In hydration water is in the form of steam

Phosphoric acid is used as a catalyst

The temperature is 300c and the pressure is 60 atmospheres

54
Q

Explain the reaction mechanism of a hydration reaction

A

Phosphoric acid has three hydrogen atoms each bonded to oxygen which is a strongly
electronegative element. Therefore, it is a polar molecule.
(Oxygen = - / hydrogen = +)

Stage 1)

The pair of electron in the up bond of the alkene is attracted to the positive charge of the hydrogen atoms in the phosphoric acid. (Positive hydrogen acts as an electrophile). The pair of electrons in the pi bond form a covalent bond to the positive hydrogen atom. At the same time, the covalent bond between the hydrogen and oxygen breaks.The pair of electrons completely move to the oxygen atom. (covalent bond breaks with both electron going to the same atom- heterolytic fission)

A carbocation intermediate is formed with a positively charged carbon atom and a dihydrogen phosphate ion with a negatively charged oxygen atom with a negatively charge oxygen atom. (Phosphoric acid is a catalyst therefore needs to be regenerated at a later stage)

Stage 2)

The carbocation intermediate reacts with a molecule of water (steam). The oxygen atom in a water molecule has two lone pairs of electrons. One lone pair forms a covalent bond between the oxygen and the positive carbonate atom on the carbocation intermediate

We then have an intermediate molecule containing a positive charged oxygen atom.
(The oxygen is positive as the lone pair of electron have formed a covalent bond)

Stage 3)

Dihydrogen phosphate ion forms a covalent bond to a hydrogen in the intermediate molecule. At the same time, the covalent bond in the intermediate now breaks by heterolytic fission and the pair of electrons in the bond now move completely onto the oxygen.

A product is made and phosphoric acid is regenerated.

55
Q

Explain what happens when we hydrate asymmetric alkenes

A

We apply Markownikoff’s rule to determine which carbon atom will bond to the hydrogen atom of the water and which will bond to the PH group.

The hydrogen is more likely to bond to the carbon atom which is already bonded to the greater number of hydrogen atoms.

56
Q

Write a word equation for the hydrogenation of ethene

A

Ethene + hydrogen —(nickel catalyst) —> ethane

57
Q

Key points of hydrogenation

A

It is carried out at a temperature of 150c

The two hydrogen atoms in the hydrogen molecule add across the double bond. Therefore, when we hydrogenate an alkene which is an unsaturated molecule we make an alkane which is a saturated molecule.

The reaction also uses a solid catalyst (metal nickel)l

58
Q

Important uses of hydrogenation

A

In order to make margarine. This is produced by hydrogenation vegetable oil.

59
Q

What categories are polymer divide into

A

Addition polymers

Condensation polymers

60
Q

What are polymers

A

Polymers are large molecules- they are joined together by thousands of small identical molecules called monomers.

61
Q

Rules for naming polymers

A

The polymer is named by using the world poly followed by the name of the monomer in brackets. E.g: when we polymerise ethene we make poly(ethene).

62
Q

What is polyethene used for

A

To create plastic bags and bottles

63
Q

Key ideas of addition polymerisatioon

A

When we form addition polymers, the monomers are alkenes

polymerisation requires high temperature and pressure as well as a catalyst.

When an alkene forms a polymer the double bond in the alkene open up to join one monomer to another

The product polymer has no double bond in the carbon backbone.This means that despite being formed from an alkene ,addition polymers are actually alkanes
​
Addition polymers contain a lot of carbon-hydrogen and carbon-carbon bonds. These bonds are non-polar and relatively strong, which makes them difficut to break. Therefore, are unreactive resulting in polymers to survive in environments for a long time.
​
At the end of polymer molecules there are carbon atoms at either end with only three covalent bonds. However, during polymerisation other molecules are used to cap the ends of the polymer chain

64
Q

How to identify a repeating unit:

A

We take any two adjacent carbon atoms on the main chain.

We then draw those carbon atoms and any either above or below.

Then we draw square brackets around them and draw covalent bonds which extend through the square brackets.

We also show a repeating unit at the bottom right which shows the number of monomers in the polymer.

65
Q

How to identify a monomer from a polymer:

A

We take two adjacent carbon atoms on the main chain with its attached atoms above and below.

We then replace the single bond between the carbon aotms with a double bond.

66
Q

What dos the repeating unit represent

A

The repeating union shows the arrangement of atoms that are repeated in the polymer chain

67
Q

Difference between the monomer and the repeating unit

A

The monomer always has a double bond between the two carbon atoms whereas the repeating unit never has a double bond between the two carbon atoms.

68
Q

What is poly(phenylethene) used for

A

To create foam food containers and drink cups

69
Q

Problems of polymers on the environment

A

The lack of reactivity of polymers make it useful for products such as containers for food and drinks.

However, their lack of reactivity also present an environmental problem as additiion polymers are non-biodegradeable (cannot be broken down by microorganisms in the environemnt). Therefore, can pollute the environment for decades or longers. This can cause extreme harm to wildlife.

Additionally, even when crude oils are use responsibly they still involve the use of crude oil which is as non-renewable resource.

Addition polymers also need to be transported and refined which requires energy and cause spaces in landfill causing major effect on the environment.

70
Q

How we can reduce the environmental affects of addiiton polymers

A

Rather than sending waste polymers to landfill we can send it to be combusted, generating energy.( Around 14% are recycled this way)
.However, combusting polymers can release harmful chemicals. (PVC- when combusted HCl is released which is corrosive)

Polymer waste can be sorted into different polymers and recycled into new products. By recycling polymers, we reduce the use of crude oil and the amount of waste in landfills.

We can also use feedstock recycling where waste polymers are converted back to simpler hydrocarbons. These can be cracked and converted into different polymers. (An advantage of this is that the waste polymers do not need sorting and allows us to convert one polymer into a different polymer)

71
Q

Explain the future of polymers

A

Biodegradable:

Scientists also are working to create new polymers which are biodegradeable ( based on biological material). The benefit of these polymers are that they naturally break down due to the action of microorganisms and manufactured from plant material (renewable). E.g- bioplastic made from plant, starch ,cellulose ,plaint oil ,proteins)

Biodegradable polymers may contain OH groups which can form hydrogen bonds with water and break down

Biodegradable polymers are not addition polymers

Photodegradable:

Scientist are working on an oil based polymer known as photodegradable polymers which contain additive such as C=O which absorb light. These bond vibrate when light is absorbed and weakens the polymer starting the degradation process.

Chemistry A-level (35 decks)

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