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
