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GCSE Chemistry > Cracking, alkenes > Flashcards

Cracking, alkenes Flashcards

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1
Q

Alkanes only have single _______________-

A

Alkanes only have single covalent bonds between the carbon atoms

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2
Q

describe the structure of alkanes

A

Alkanes only have single covalent bonds between the carbon atoms

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3
Q

long chain hydrocarbon are ___________

A

-hydrocarbon with a lot of carbon atoms

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4
Q

why do long chain hydrocarbons not make good fuels

A

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

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5
Q

Describe what is meant by cracking

A

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

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6
Q

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

A

Used for fuels e.g. in cars

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7
Q

Two ways to carry out cracking

A

Catalytic cracking
Steam cracking

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8
Q

Describe catalytic cracking
Conditions

A

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

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9
Q

Describe steam cracking
Conditions

A

In steam cracking, high temperature and steam is used.

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10
Q

describe the structure of alkenes

A

Alkenes have a double covalent bond between two carbon atoms

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11
Q

Uses of alkenes (produced from cracking)

A

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

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12
Q

Describe what is meant by an alkene

A

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.

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13
Q

which is more reactive, alkenes or alkanes

A

Alkenes are more reactive than alkanes

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14
Q

Describe the test for alkenes

A

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

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15
Q

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

A

C25H52 —> C20H42 + C5H10

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16
Q

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

A

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

17
Q

Molecules that cracking produces

A

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

18
Q

Describe the structure of alkenes

A
19
Q

Explain why alkenes are unsaturated

A

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

20
Q

what is the displayed formula

A

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

21
Q

general formula of alkenes

A

CnH2n

22
Q

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

A

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

23
Q

what is meant by a functional group

A

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)

24
Q

what is meant by a homologous series

A

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

25
Q

members of a homologous series all have the same ________

A

members of a homologous series all have the same functional group

26
Q

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

A

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

27
Q

describe the combustion of alkenes

A

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.

28
Q

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

A

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.

29
Q

describe the reaction of alkenes with hydrogen

what they produce + conditions

A

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

30
Q

why do alkenes all react in the similar way

A

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

31
Q

functional group for alkenes

A

carbon to carbon double bond

C=C

32
Q

describe the reactions of alkenes with water

A

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

33
Q

describe the reactions of alkenes with halogens

A

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

34
Q

propene + chlorine –> ____–

A

propene + chlorine –> dichloropropane

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

35
Q

ethene + iodine –>

A

ethene + iodine –> diiodoethane

36
Q

Worked example

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

A

The structure will be: CH3CHBrCH2Br.

37
Q

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

A

The structure will be: CH2BrCHBrCH2CH3.

GCSE Chemistry (43 decks)

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