Cracking, alkenes Flashcards
Alkanes only have single _______________-
Alkanes only have single covalent bonds between the carbon atoms
describe the structure of alkanes
Alkanes only have single covalent bonds between the carbon atoms
long chain hydrocarbon are ___________
-hydrocarbon with a lot of carbon atoms
why do long chain hydrocarbons not make good fuels
Long chain hydrocarbons are not very flammable
This is one reason why long-chain hydrocarbons do not make good fuels
Describe what is meant by cracking
In cracking, a long-chain alkane is broken down (cracked) to produce smaller, more useful molecules
what are shorter chain alkanes (which have been produced by cracking) used for
Used for fuels e.g. in cars
Two ways to carry out cracking
Catalytic cracking
Steam cracking
Describe catalytic cracking
Conditions
In catalytic cracking we use high temperature and a catalyst
The job of the catalyst is to speed up the reaction
Describe steam cracking
Conditions
In steam cracking, high temperature and steam is used.
describe the structure of alkenes
Alkenes have a double covalent bond between two carbon atoms
Uses of alkenes (produced from cracking)
Alkenes are used to make chemicals called polymers
Alkenes are also used as the starting material for the production of many other useful chemicals
Describe what is meant by an alkene
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.
which is more reactive, alkenes or alkanes
Alkenes are more reactive than alkanes
Describe the test for alkenes
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
Balance chemical equations to show cracking
C25H52 –> C20H42 + C……H……..
we are cracking a long chain hydrocarbon into two shorter molecules
C25H52 —> C20H42 + C5H10
why is cracking needed - why do long chain hydrocarbons need to be converted to short chain hydrocarbons
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
Molecules that cracking produces
Cracking produces shorter chain alkanes and another type of hydrocarbon called alkenes
Describe the structure of alkenes
Explain why alkenes are unsaturated
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
what is the displayed formula
The displayed formula shows us all of the atoms and the covalent bonds
general formula of alkenes
CnH2n
name and draw displayed formula and write structural formula of first four aljenes
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
what is meant by a functional group
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 (e.g. the -OH group in an alcohol)
what is meant by a homologous series
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
members of a homologous series all have the same ________
members of a homologous series all have the same functional group
what does each molecule in the alkane and alkene homologous series have (COMPARED TO THE PREVIOUS MOLECULE IN THE homologous SERIES
Each molecule in the homologous series has an extra carbon atom and two extra hydrogen atoms
describe the combustion of alkenes
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.
in the combustion of alkenes why do alkenes tend to burn in air with smoky flames
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.
describe the reaction of alkenes with hydrogen
what they produce + conditions
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
why do alkenes all react in the similar way
Because of the double bond, alkenes all react in a similar way.
This double bond is the alkene functional group
functional group for alkenes
carbon to carbon double bond
C=C
describe the reactions of alkenes with water
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
describe the reactions of alkenes with halogens
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
propene + chlorine –> ____–
propene + chlorine –> dichloropropane
https://www.passmyexams.co.uk/GCSE/chemistry/images/alkanes/eq-ethene-halogens-021.gif
ethene + iodine –>
ethene + iodine –> diiodoethane
Worked example
Propene, CH3CH=CH2, reacts with bromine. Predict the structure of the product formed.
The structure will be: CH3CHBrCH2Br.
But-1-ene, CH2=CHCH2CH3, reacts with bromine. Predict the structure of the product formed.
The structure will be: CH2BrCHBrCH2CH3.
