Cracking, alkenes

Question 1

Alkanes only have single _______________-

Answer 1

Alkanes only have single covalent bonds between the carbon atoms

Question 2

long chain hydrocarbon are ___________

Answer 2

-hydrocarbon with a lot of carbon atoms

Question 3

why do long chain hydrocarbons not make good fuels

Answer 3

Long chain hydrocarbons are not very flammable
This is one reason why long-chain hydrocarbons do not make good fuels

Question 4

Describe what is meant by cracking

Answer 4

In cracking, a long-chain alkane is broken down (cracked) to produce smaller, more useful molecules

Question 5

what are shorter chain alkanes (which have been produced by cracking) used for

Answer 5

Used for fuels e.g. in cars

Question 6

Two ways to carry out cracking

Answer 6

Catalytic cracking
Steam cracking

Question 7

Describe catalytic cracking
Conditions

Answer 7

In catalytic cracking we use high temperature and a catalyst
The job of the catalyst is to speed up the reaction

Question 8

Describe steam cracking
Conditions

Answer 8

In steam cracking, high temperature and steam is used.

Question 9

describe the structure of alkenes

Answer 9

Alkenes have a double covalent bond between two carbon atoms

Question 10

Uses of alkenes (produced from cracking)

Answer 10

Alkenes are used to make chemicals called polymers
Alkenes are also used as the starting material for the production of many other useful chemicals

Question 11

Describe what is meant by an alkene

Answer 11

An alkene is a hydrocarbon which has a double bond between two of the carbon atoms in their chain

Alkenes are hydrocarbons with a double carbon-carbon bond.

Question 12

which is more reactive, alkenes or alkanes

Answer 12

Alkenes are more reactive than alkanes

Question 13

Describe the test for alkenes

Answer 13

Alkenes are more reactive than alkanes
We can use this to test for alkenes

We test for alkenes using bromine water, which is orange
If we add the alkene and shake our alkene with bromine water, then the bromine water turns colourless.
The bromine water is decolourised

When bromine water is added to an a saturated compound, like an alkane, no reaction will happen and it will stay bright orange

Question 14

Balance chemical equations to show cracking
C25H52 –> C20H42 + C……H……..
we are cracking a long chain hydrocarbon into two shorter molecules

Answer 14

C25H52 —> C20H42 + C5H10

Question 15

why is cracking needed - why do long chain hydrocarbons need to be converted to short chain hydrocarbons

Answer 15

Long chain hydrocarbons are not very flammable
This is one reason why long-chain hydrocarbons do not make good fuels

That is a problem as there is a high demand for short-chain hydrocarbons to be used as fuels
To solve this problem, long chain hydrocarbons are converted into shorter chain hydrocarbons. To do that, we use a process called cracking

Question 16

Molecules that cracking produces

Answer 16

Cracking produces shorter chain alkanes and another type of hydrocarbon called alkenes

Question 17

Describe the structure of alkenes

Question 18

Explain why alkenes are unsaturated

Answer 18

Alkene molecules are unsaturated
That is because alkenes have two fewer hydrogen atoms than the alkane with the same number of carbon atoms

both ethane and ethene have 2 carbon atoms
however ethane has six hydrogen atoms
whereas ethene only has 4 hydrogen atoms
alkenes have two fewer hydrogen atoms than the equivalent alkane

Question 19

what is the displayed formula

Answer 19

The displayed formula shows us all of the atoms and the covalent bonds

Question 20

general formula of alkenes

Answer 20

CnH2n

Question 21

name and draw displayed formula and write structural formula of first four aljenes

Answer 21

https://bam.files.bbci.co.uk/bam/live/content/z86nw6f/medium

https://content.connect.collins.co.uk/Content/Live/Infuze/COL/GCSE_Science_Core_SB_OCR_Gateway/content/images/GAT_Sci_0112__art_r2.png

Question 22

what is meant by a functional group

Answer 22

Functional group - the part of the molecule that determines how it reacts

a group of atoms that are responsible for the chemical properties of a compound

Question 23

what is meant by a homologous series

Answer 23

molecules such as alkenes are a homologous series

A group of chemicals that react in a similar way because they have the same functional group

Question 24

members of a homologous series all have the same ________

Answer 24

members of a homologous series all have the same functional group

Question 25

what does each molecule in the homologous series have (COMPARED TO THE PREVIOUS MOLECULE IN THE homologous SERIES

Answer 25

Each molecule in the homologous series has an extra carbon atom and two extra hydrogen atoms

Question 26

describe the combustion of alkenes

Answer 26

when we combust alkenes, we produce carbon dioxide and water
combusting alkene also produces unburnt carbon particles.
That is because of incomplete combustion
This means that alkenes burn in air with a smoky flame

Alkenes react with oxygen in combustion reactions in the same way
as other hydrocarbons, but they tend to burn in air with smoky flames because of incomplete combustion.

Question 27

in the combustion of alkenes why do alkenes tend to burn in air with smoky flames

Answer 27

Alkenes react with oxygen in combustion reactions in the same way
as other hydrocarbons, but they tend to burn in air with smoky flames because of incomplete combustion.

Question 28

describe the reaction of alkenes with hydrogen

Answer 28

Reaction of alkenes with hydrogen is called hydrogenation

ethene + hydrogen -> ethane
This reaction requires a temperature of around 150 degrees and also a nickel catalyst (to speed up the reaction)

In this reaction, the two hydrogen atoms add across the double bond
The carbon to carbon double bond is converted /becomes to a carbon to carbon single bond
Reacting an alkene with hydrogen produces an alkane

Question 29

why do alkenes all react in the similar way

Answer 29

Because of the double bond, alkenes all react in a similar way.
This double bond is the alkene functional group

Question 30

functional group for alkenes

Answer 30

carbon to carbon double bond

C=C

Question 31

describe the reactions of alkenes with water

Answer 31

the reactions of alkenes with water is called hydration
when we hydrate ethene we make ethanol which is an alcohol

ethene + water (steam) -> ETHANOL (can be reversible)

In hydration, the water must be in the form of steam
The temperature must be around 300 degrees celcius
The pressure around 70 atmospheres
Phosphoric acid is used as a catalyst to speed up the reaction

this reaction is reversible
to increase the yield of ethanol, any unreacted ethene and steam are passed back through the catalyst

when alkenes react with steam, water is added across teh double bond and an alcohol is formed

Question 32

describe the reactions of alkenes with halogens

Answer 32

When alkenes react in addition reactions with halogens, the molecules formed are saturated, with the C=C each becoming bonded to a halogen atom

The two chlorine atoms in the chlorine molecule have added across the double bond
The double carbon to carbon double bond has turned into a single carbon to carbon double bond

ethene + chlorine -> dichloroethane

https://encrypted-tbn0.gstatic.com/images?q=tbn:ANd9GcQ6GIzSkVkdQ3MI2FKJ9ZjVlz7GmrXw6ibZbcBLe51WB7538cGV

Question 33

propene + chlorine –> ____–

Answer 33

propene + chlorine –> dichloropropane

https://www.passmyexams.co.uk/GCSE/chemistry/images/alkanes/eq-ethene-halogens-021.gif

Question 34

ethene + iodine –>

Answer 34

ethene + iodine –> diiodoethane

Question 35

Worked example

Propene, CH3CH=CH2, reacts with bromine. Predict the structure of the product formed.

Answer 35

The structure will be: CH3CHBrCH2Br.

Question 36

But-1-ene, CH2=CHCH2CH3, reacts with bromine. Predict the structure of the product formed.

Answer 36

The structure will be: CH2BrCHBrCH2CH3.