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Alkanes Flashcards

1
Q

Is an alkane saturated or unsaturated?

A

• saturated

- they only contain single covalent bonds

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2
Q

What are the 3 types of alkanes?

A
  • Straight
  • Branched
  • Cycloalkanes
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3
Q

Smaller alkanes are what at room temperature?Do they have a high or low boiling point?

A

Gas, and have a low boiling point

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4
Q

Larger alkanes are what at room temperature?

Do they have a high or low boiling point?

A

Liquid, and have a high boiling point

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5
Q

What forces are between molecules of alkanes?

A

Van Der Waals

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6
Q

Why is it the longer the chain, the stronger the VDW

A

Because there’s more molecular surface area and more electrons to interact.
Also as it gets longer, it takes more energy to overcome the VDW and seperate the molecules.

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7
Q

Why does a branch chain isomer have a lower boiling point than straight chain?

A

Because branch chain can’t pack closely together, there’s s smaller molecular surface area so the VDW are reduced

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8
Q

Why are alkanes unreactive?

A
  • strong single bonds

* no polar bond

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9
Q

In plentiful supply of alkanes burn to produce?

A

Carbon dioxide and water
-> it’s an exothermic process bc heat is given out. Oxidation of alkanes is the basis for their use as fuels
• Natural gas = heating purposes
• Diesel = power
- they burn by a free radical reaction in a gas phase, they have to be vapourised before burned

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10
Q

Hetrolytic Fission?

A

Two different substances are formed

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11
Q

Homolytic Fission

A

Two electrically uncharged radicals are formed. A radical is a species with an unpaired electron. Because its unpaired it’s highly reactive

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12
Q

Which stage are free radicals produced?

A

Initiation stage

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13
Q

General formula for an alkane?

A

CnH2n+2

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14
Q

What is Carbon Capture and Storage?

A

Capture : they capture CO2 from fossil fuels, industrial plants or power stations and store it underground rather than releasing it into the air
Storage: underground rocks act as huge sponge that soak up unwanted CO2

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15
Q

How to minimise climate change?

A
  • providing the government with scientific evidence global warming is taking place
  • investigating solutions to environmental problems e.g carbon capture &&storage
  • monotoring progress against initiatives
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16
Q

In limited supply of oxygen, alkanes burn to produce?

A

• carbon monoxide and water

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17
Q

What is the problem with carbon monoxide?

A

• it’s poisonous

  • the oxygen in the blood is carried by haemoglobin, CO is better at binding to haemoglobin than oxygen is
  • if you breathe in air with a high concentration of CO it will bind to haemoglobin in bloodstream before oxygen can
  • means less O2 will reach cells and you suffer symptoms e.g headaches, fatigue and it can be fatal
18
Q

Halogens react with alkanes in what type of reaction?

A

• photochemical, which is started by light ( UV )

19
Q

What is a free radical substitution reaction?

A

• where a hydrogen gets replaced by a chlorine or bromine

20
Q

What is the initation stage?

A
  • sunlight provides enough energy to break the Cl-Cl bond = photodissociation
  • the bond splits equally and each atom keeps one electron
  • atom becomes highly reactive free radical because it’s unpaired
21
Q

What happens in the propagation stage?

A

• the radical attacks a molecule
• the new radical can attack a Cl2/Br2 and produce the product and a radical
( start with a radical, end with a radical )

22
Q

What happens in termination stage?

A

• all the free radicals are used up

23
Q

What is a problem with substitution reactions?

A
  • you don’t just get the chloromethane for e.g but another by product
  • if there is too much chlorine the remaining H2 atoms on the chloromethane will be swapped do chlorine
  • to avoid this, the best way is to use excess methane bc there’s a greater chance of chlorine radical colliding with methane not chloromethane
24
Q

Separating hydrocarbons inside crude oil?

A
  • crude oil must be separated first by F.D to obtain useful bits : technique relies on differences in boiling points of liquids
25
Q

How does F.D work?

A
  • crude oil is vaporised at 350 degrees/ 65OK, enters the column at the bottom and rises up through the trays
  • the largest hydrocarbons don’t vapourise bc the B.P are too high
  • as the crude oil goes up, gets cooler bc of different chain lengths - each fraction condenses @ different temperatures
  • hydrocarbons w/lowest B.P don’t condense ( they’re drawn off as gases )
26
Q

Uses of the fractions?

A
  • fuels

* processed to make petrochemicals

27
Q

What is cracking?

A

•breaking long chain alkanes into smaller hydrocarbons, involves breaking the C-C bond
• this is bc the shorter alkanes are more in demand
• heavier fractions are passed over a catalyst at high temps and moderate pressure
• this breaks them into smaller molecules
• using a catalyst cuts costs because the reaction is done at lower temps+ pressure and speed up reactions
• alkanes and alkenes are formed
- the products of cracking are more valuable than the starting materia

28
Q

What is the bond angle around the saturated hydrocarbons?

A

109.5 - the shape is a tetrahedral as a result of the 4 bond pairs of electrons equally repelling

29
Q

The problem with straight chain alkanes?

A

• they auto ignite; when fuel/ air mixture is compressed they explode without being ignited by spark. This extra explosion causes ‘knocking’ in the engine
- to get rid of the knocking and make combustion efficient, shorter branched, Cycloalkanes + arenes are added to petrol which increases octane rating

30
Q

How do manufacturers covert straight chain alkanes into branched chain alkanes and cyclic hydrocarbons?

A
  • isomerism

* reforming

31
Q

What is isomerism?

A
  • heat straight chain alkanes with a catalyst stick on inert Al3O2
  • alkanes break up and join back together as branched isomers.
  • molecular sieve used to seperate isomers ( zeolite )
32
Q

What is reforming?

A
  • converts alkanes into cyclic hydrocarbons
  • uses a catalyst made of platinum
  • catalyst = rhodium/ platinum
33
Q

Fossil fuels?

A
  • combustion of F.F is exothermic - gives lots of energy which is used to generate most of worlds power stations
  • F.F they are important raw materials in chemical industry for plastics, lubes
34
Q

Relying on fossil fuels?

A
  • we rely on F.F to produce energy for transport, heating etc
  • finite amount of F.F and they’re running out so its not sustainable to keep using F.F
  • burning F.F contribute to global warming/climate change bc it releases CO2 = greenhouse gas
35
Q

Alternative sources?

A
  • Bioethanol : produced from plants made by fermentation of sugar from crops e.g maize - has no overall emission into atmosphere bc all CO2 that’s released when fuel is burned is removed by crops when they grow
  • Biodiesel : comes from plants, can be used in Diesel engines. Made by refining renewable fats/oils
36
Q

Problem with using crops?

A
  • possible developed countries they will create a huge demands, poorer countries may see this as a way of making money and will convert farming land to producing crops for fuel so they won’t grow food to eat
  • forests are being cleared for room for the biofuel crops but they absorb less CO2 than forests so it defeats the main object for growing the crops
37
Q

Greenhouse effect keeping us alive?

A
  • gases H20, ozone, CO2, CH4, nitrous oxide absorb/ remit IR. Effect occurs when solar radiation is absorbed by Earths surface, converted to heat/emitted into atmosphere as IR.
  • some IR escapes into outer space, some absored/re-emitted back towards earths surface by G.H.G. Returing waves of IR radiation warm troposphere/earths surface
  • G.H.E creates equilibrium. Earths surface/ atmospheric gases absorb energy same rate as it radiates energy maintaining steady temp
38
Q

Greenhouse effect?

A
  • IR strikes a molecule, causes bonds to bend/vibrate. IR is absorbed by bending and stretching of bonds
  • molecule gains K.E by this absorption of IR radiation and this K.E may be transmitted to other molecules e.g O2 which causes general heating of atmosphere

• IR is absorbed by C=O and C-H bonds in H20, CO2/CH4 -> these contribute to global warming. The effect of these is dependent on how long they stay in atmosphere and ability to absorb IR

39
Q

Minimising Climate change?

A

CarbonCapture + Storage
• Capture : CO2 from the F.F’s, power stations/ industrial plants then store underground rather than releasing into atmosphere
• Storage : rocks underground can act as a huge sponge to soak up unwanted CO2 or reacting with metal oxides to form stable carbonate minerals

40
Q

Minimising climate change

A
  • providing scientific evidence to government that global warming is taking place
  • investigating solutions to environmental problems e.g CCS
  • monitoring the progress against initiatives e.g Kyto protocol
41
Q

Why is it important to control global warming?

A
  • it can lead to sea levels rising by the melting of ice caps which leads to flooding
  • global warming means there’s more heat energy in system so it could lead to stormier and less predictable weather
  • some places could have less rainfall which could bring drought and crop failure

Revision (20 decks)

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